chapter 25 how does the human body use electricity? · pdf filechapter 25 how does the human...

Chapter 25 How does the human body use electricity? © John Wiley & Sons Australia, Ltd

CHAPTER 25 How does the human body use electricity?

Contents Electricity and the human body Why do humans conduct electricity?

Conduction and mobility Mobile charge carriers in humans Connecting resistance and mobility Charges, electrical potential and human cells

The nervous system and electrical processes What does a nerve cell look like? The action potential Speeding up the transmission of electrical impulses along axons Sources of stimulus — getting an action potential started A simple model of cells and the action potential The RC circuit The cell as an RC circuit

The heart: an electrically powered pump Measuring heart function — the electrocardiogram Heart beat and the ECG Heart problems — fibrillation and defibrillation The artificial pacemaker

Electrical resistance of the human body Resistance and resistivity Effective resistance of the human body Current and the human body Electrosurgery Galvactivator

Neuroscience — frontiers of human bioelectricity Monitoring electrical activity in the brain Neural stimulation: The bionic eye project Bypassing neural damage: Bionic control devices and activated muscles Retraining neural networks

Chapter review Summary Questions

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CHAPTER 25 How does the human body use electricity?

Imaging electrical activity in the brain — the Glass Brain project (developed by the Neuroscape Lab of the University of California, San Francisco)

REMEMBER Before beginning this chapter, you should be able to:

■ recall that like charges repel and unlike charges attract

■ explain the concept of current and electrical potential difference

■ recall Ohm’s Law, namely V = IR

■ recall that electrical power can be expressed as P = IV

■ recall that the electrical energy, E, transferred during a time interval, t, is related to the power consumed by E = Pt = IVt.

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KEY IDEAS At the end of this chapter, you should be able to:

■ compare mobile charge carriers in the human body with those in metals

■ describe how an electrical potential difference arises across the cell membrane

■ understand capacitance and its relationship to the separation of charge carriers and electrical potential difference

■ model polarising and depolarising of a cell in terms of a simple circuit containing a resistor and capacitor in parallel

■ explain how electrical signals are transmitted by nerve cells

■ describe the chemical transfer of signals between nerve cells via the synapses

■ describe the nature and role of electrical pulses in the function of the heart

■ explain the effect of applying an external potential difference to the human body, including the nature of electric shock, the action of a defibrillator, the cauterisation of wounds, the activation of neural responses and the stimulation of neuroplasticity

■ describe the varying electrical resistance of different organs and tissue types in the body and explain how they contribute to the total effective resistance of the body.

■ explain the difference in the effect of direct and alternating sources of potential difference on the effective resistance of the body

■ describe the detection and interpretation of electrical signals from the body by ECG and EEG machines

■ describe modern developments in artificial transmission of stimuli by a device such as a pacemaker, the cochlear implant or the bionic eye

■ describe modern developments in the reception and amplification of electrical signals at points of damage in the nervous system, enabling remote stimulation of real and artificial body parts.

Electricity and the human body Many important discoveries in the history of electricity came from observations of electrical behaviour in animals and humans. There are recorded instances from 46 AD of the use of electric shock therapy to treat headaches. The source of the electric shock was the electric discharge from the tornado fish! In 1781 Luigi Galvani demonstrated that a frog’s leg could be made to twitch using electrical energy. Galvani’s work inspired Volta to further investigate the source of the electrical energy affecting the frog’s leg, leading to the development of the first batteries. Thomas Green, an anaesthetist in the late nineteenth century, discovered that he could restart a sedated patient’s heart by discharging a 300 V battery across the patient’s chest. Today, knowledge of electrical processes in the human body is leading to medical advances in the treatment of paraplegia and degenerative brain and muscle disease.

In this chapter, you will learn how and why the human body responds to electrical stimuli, and what roles electrical processes perform in essential body systems such as the heart and the nervous system.

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Why do humans conduct electricity?

Conduction and mobility We know that metals are good conductors of electricity. When electrical forces are applied to metals, the valence electrons move in response. The freedom of charged particles to move in response to electrical forces is known as their mobility. The greater the mobility of charged particles in a material, the better the material conducts electricity.

Mobile charge carriers in humans Humans consist of approximately 60% water, with many dissolved ions, of which Na+, K+, Ca2+, Mg2+, PO4

3– and Cl– have critical roles to play in cell function. The mobility of these ions is 50 000 times smaller than that of metal valence electrons in metals. Nevertheless, despite having a smaller mobility and being present in much lower concentrations than valence electrons in metals, the ions in humans enable electrical current to be transported through the body.

Connecting resistance and mobility Current flows in response to a difference in electrical potential, or voltage, between two points that are part of a circuit. If you touch one terminal of a 9 V battery with your left hand and the other terminal with your right hand, the electric potential difference between your hands is now 9 V. Your body and the battery form a complete electrical circuit. The electrical potential difference between your hands causes a current to flow through your body. How big is this current? Writing Ohm’s Law as = V

RI , you can see that the current through your body is equal to the potential difference across the body divided by the electrical resistance of your body. A typical effective resistance of a human body is between 2000 and 20 000 ohms, so that the current flowing between the terminals of the 9 V battery is between 0.5 and 5.0 milliamperes, enough to cause a slight tingling sensation in your fingers.

Resistance is dependent on the mobility and concentration of charge carriers. The epidermis, or outer skin layer, is made up of overlapping dead skin cells in which the ions are much less mobile than in the underlying salty tissue. The epidermis contributes 99% of the body’s effective resistance.

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The battery is a source of electrical potential difference between your hands, causing current to flow through your body.

SAMPLE PROBLEM 25.1 A student touches one terminal of a 9 V battery with his left hand and the other with his right hand.

a. Calculate the current through the student if his resistance is 2000 Ω. b. How much electrical energy would be dissipated in the student’s body if he touched the battery for 10

seconds?

Solution:

a.

9 V2000 4.5 mA

VI

R=

=Ω

=

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b.

4.5 mA 9 V 10 s

405 mJ

E IVt== × ×=

Charges, electrical potential and human cells Rather surprisingly, all human cells in their resting or unstimulated state are slightly negatively charged on the inside of the cell and slightly positively charged on the outside of the cell. There is an important exception – the photoreceptor cells found on the retina at the back of the eye. Their resting state is the stimulated state. This reduces the effect of faulty photoreceptors on visual processing. But how does this come about, and what does it mean for the cell?

Human cells come in many shapes and sizes, but they all share the same basic structure illustrated below. The cell boundary is formed by the cell membrane, a constantly renewing double layer of molecules called lipids. The lipids are interspersed with proteins. The proteins are extremely important because they control the passage of molecules across the cell membrane. The proteins create channels through the membrane and control the function of the cell. The interior of the cell contains cytoplasm (a term encompassing the clear, jelly-like fluid inside the cell and any additional organelles) and the cell nucleus.

Basic elements of a human cell

The fact that the inside of the cell membrane has a small negative charge and the outside of the cell membrane has a small positive charge is referred to as the cell’s polarisation. When charged objects are held apart, there is an electrical potential difference between them. Electrical potential difference is described from the point of view of a positive charge. Positively charged ions are attracted to the inside of the cell, so there is a negative potential difference between the outside and inside of the cell.

Digital doc: Investigation 25.1:

Mobility investigation

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Electrical potential difference in cells

The difference in charge and electrical potential across the cell membrane is due to the large difference in the concentration of the ions on the inside and outside of the membrane, combined with the significant difference in the ability of the various ions to pass from one side of the membrane to the other.

For nerve cells, the concentration of Na+ outside a cell is typically 10 times greater than its concentration inside the cell, whereas the opposite is the case for K+. This can be seen in the diagram of a nerve cell from a squid. The natural tendency of the K+ and Na+ ions is to diffuse into regions of lower concentration. Na+ ions, in particular, try to enter the cell, but they are strongly hindered by the cell membrane. Remember, the passage of ions across the membrane is controlled by the protein channels. There are a small number of leaky channels that allow Na+ ions to diffuse in to the cell and K+ ions to diffuse out of the cell. Left alone, the cell would eventually depolarise, meaning that the difference in charge across the membrane would gradually reduce to zero. However, the membrane contains ATP (adinose triphosphate), a molecule that acts as a chemically powered pump, transforming to ADP (adenosine diphosphate) and simultaneously transferring three Na+ ions out of the cell and two K+ ions into the cell. The ATP pump maintains the charge imbalance and resting potential difference of about –70 mV across the membrane. Although –70mV might seem quite significant, the charge imbalance, or polarisation, underlying the potential difference is only about 0.01% of the total concentration of ions present.


More on electrical potential

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Maintaining the resting potential difference across the membrane of a squid nerve cell

Electrical stimulus and cells

Although all cells have a resting electrical potential difference across the membrane, only some cells are excitable. An excitable cells is a cell that responds to stimulus by producing an electrical signal. Excitable cells include muscle and nerve cells. In an excitable cell, some of the protein channels are voltage gated. Voltage-gated channels switch between an open or closed state, depending upon the potential difference across the cell membrane.

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For example, when a sperm fertilises an egg cell, there is a change in the potential of the egg cell’s membrane. The channels allowing sperm across the membrane are voltage gated and close in response to the potential change, ensuring that only one sperm fertilises the egg.

The nervous system and electrical processes The nervous system is an extraordinary information superhighway consisting of the brain, spinal cord and peripheral nerves. The building blocks of the nervous system are nerve cells, also called neurons, and support cells known as glial cells.

There are three types of neurons. Sensory neurons respond to receptors of sensations such as taste and touch. They then transmit an electrical signal along the neuron to the nerve cell body, which sends the signal on for processing in the brain. Motor neurons respond to brain stimuli by transmitting electrical signals along the nerve cell body to muscle cells, causing contraction of muscle fibres. Interneurons handle the communication between the sensory and motor neurons.

What does a nerve cell look like?

The nerve cell consists of dendrites surrounding the cell body, which is in turn connected to a long cylindrical section called an axon.

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A nerve cell or neuron is made up of dendrites, the cell body (or soma) and the axon. Dendrites surround the cell body, which in turn is connected to a long cylindrical section called the axons. Some axons are covered by sections of insulating sheaths known as myelin, which enhance the propagation of electrical signals along the nerve cell. The myelin sheaths are formed from a type of glial cell known as the Schwann cell. The axons are connected to sensory receptors, or via synapses to other neurons or muscle fibres, depending on the neuron function.

Synapses lie between neurons and other neurons or muscle fibres. They enable transfer of chemical signals. The pre-synaptic terminal on one side of the synapse releases neurotransmitters, which cross the synapse to the post-synaptic terminal on the other side of the synapse. The internal transfer of information in nerve cells is electrical.

All nerve cell bodies are located in the spinal cord or the brain, making the axon that gathers sensory input from your big toe over 1 metre long!

Nerves are bundles of axons. The electrical pulse transmitted along the axon is called the action potential and is the key to understanding the electrical transmission of nervous impulses in the body, muscle contraction, and indeed the behaviour of any excitable cell.

The action potential The action potential is an electrical pulse that is produced at the point where the cell membrane is stimulated. The pulse is caused by the charge difference across the membrane changing due to ions flowing in and out of the cell. An action potential event is illustrated in the figure below.

Initially, the cell membrane is at its resting potential, polarised, with the inside of the cell membrane slightly negatively charged with respect to the outside of the cell membrane. The cell is stimulated by a current of Na+ ions entering the cell at a particular point. As the Na+ ions enter the cell, the magnitude of the charge difference and potential difference across the cell decrease, causing the cell to depolarise. If the magnitude of the potential difference decreases by more than about 8 mV, suddenly the channels allowing Na+ to cross the membrane activate, allowing Na+ ions to flood into the cell, which in turn rapidly accelerates the depolarisation of the cell.

As the Na+ ions rush in, the inside of the cell membrane actually becomes positively charged compared to the outside, and the Na+ gates deactivate or close. Meanwhile, the change in cell membrane potential causes voltage-gated K+ channels to open, so that K+ ions move rapidly out of the cell, reducing the excess of positive charge inside the cell and repolarising the membrane. Once the cell membrane potential drops below a threshold value, the K+ gates also close. By the end of repolarisation, the membrane potential difference has overshot the resting membrane potential; however, the ATP pump gradually restores the original state of the cell. The total time taken for the membrane to depolarise and repolarise again at the point of stimulus is about 2 milliseconds for a nerve cell.


Parts of a nerve cell quiz

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The process of depolarisation and repolarisation of a cell during an action potential

The action potential requires a certain threshold stimulus to cause the sodium gates to open in the first place. Unless sufficient stimulus is applied, there will be no action potential. Once the action potential has started, increasing the stimulus does not affect the size of the action potential, as the sodium gates are already open. When the sodium gates are in the inactivated phase during repolarisation of the cell, they are unable to be restimulated, creating a rest or refractory period before the membrane can respond to the next stimulus. If the cell receives a stimulus that makes it more negatively charged, it is called hyperpolarised. This means that a larger stimulus will be required to initiate an action potential.

So how does the action potential pass along the cell? Some of the ions that flood into the membrane during depolarisation spread along the length of the axon, stimulating an action potential in the next segment of the axon. The refractory period means that an action potential cannot be stimulated in the previously stimulated axon segment, so the signal only moves in one direction along the axon. The action potential travels along the axon at a speed of about 0.5 m s–1.

Speeding up the transmission of electrical impulses along axons The speed of transmission of an impulse along the axon can be greatly increased if sections of the axon are covered with an insulating sheath. This sheath is made from glial cells called myelin. The myelin stops ions from transferring across the membrane. Instead, the current pulse moves rapidly along the sheathed axon, rather like current in a metal, until it reaches a gap in the myelin called a node of Ranvier. At the node of Ranvier, an action potential takes place, then the pulse is passed into the next sheathed section of axon. The leaping of the current from node to node is called saltatory conduction, and increases the speed of transmission of nerve impulses from the 0.5 ms–1 in unmyelinated axons to 100–150 m s–1. As the current passes along the myelinated sheath, the size of the current

Weblink Simulation of propagation of action potential

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pulse decreases. It is important that the myelinated section is not too long, or the node of Ranvier will be unable to generate a new action potential.

Sufferers of multiple sclerosis have damaged myelin sheaths. This affects the effectiveness of the action potential for regeneration at the next node of Ranvier, resulting in symptoms such as difficulty in walking and vision loss. Research into the condition is focusing on ways to grow cells to replace the damaged myelin, as well as drugs to prevent attacks on myelin in the first place.

Multiple sclerosis causes damage to the myelin sheath surrounding the axon, affecting the transmission of the action potential.

Sources of stimulus — getting an action potential started How is an axon stimulated? The source of the initial action potential depends on the type of neuron concerned.

A sensory neuron respond to sensory receptors, such as touch or taste receptors, that stimulate the axon. The axon then transmits the signal to the cell body, which passes the signal via the dendrites to interneurons.

Touch receptors are connected to large, myelinated axons, allowing rapid transfer of the action potential. There are three different types of touch sensor: two that respond to rapid change s in sensation, allowing us to perceive vibrations, and one that adapts more slowly, giving rise to our sense of the firmness or pressure of the touch.

Temperature receptors divide into hot and cold receptors. Temperature receptors are connected to thinner axons, so that the signal transfers more slowly.

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Pain receptors, also called nociceptors, are actually separate from touch and temperature sensors and also divide into relatively fast and slow responders. Although the fast-responding pain receptors are much slower than touch neurons, they allow the source of pain to be quickly localised. The slow-responding pain receptors influence our emotional response to pain.

The gaps between neurons are called synapses. The neuron-to-neuron transmission across the synapses is chemical rather than electrical. At the pre-synaptic terminals, the dendrites stimulate the release of chemicals called neurotransmitters. The neurotransmitters diffuse across each synapse to the post-synaptic terminals of other neurons. Motor neurons, for example, can be stimulated chemically via the dendrites from sensory neurons. Our motion is often in response to our experience of touch. The individual currents from the dendrites are gathered by the cell body, and if they are above the threshold level, they result in an action potential being passed along axons to muscle cells, activating muscle contraction.

The photoreceptors in the eye are an interesting exception to other receptors, because their resting state is actually a depolarised state. We have two types of photoreceptors: rods and cones. The rods are responsible for detecting light and dark, and shapes and movement. Rods are extremely sensitive to light levels; they can become hyperpolarised upon the absorption of single photon. The cones, which are responsible for our sense of colour, require much higher photon intensities to become hyperpolarised. Cones contain pigments that make them sensitive to the wavelength of the light, and fall into three groups that are more easily stimulated by red, blue and green light respectively. Colour blindness is generally caused by defective pigments in the cones.

Sometimes we experience sensations such as tingling when in fact there has been no stimulus. This is known as paraesthesia and often occurs when the blood supply to a nerve is cut off. However, it can also be caused by inflammation of tissue, for example in carpal tunnel syndrome, a painful condition in which the lining of the channel enclosing the main nerve running through the wrist becomes inflamed, pressing on the nerve. Other causes of paraesthesia include metabolic and nutritional disorders such as diabetes or alcoholism.

Neurons can also be stimulated by an external electrical potential, as Louis Galvani observed in the twitching of frogs’ legs in the presence of lightning. There are many cases of medical technology in which neurons respond to an artificial or external stimulus. The cochlear implant, developed among others by an Australian team led by Graeme Clark, consists of an array of electrodes implanted into the auditory canal. The electrodes receive an electrical signal from a small microphone worn on the person’s head. The electrodes stimulate auditory nerve dendrites. The auditory nerve passes the signal to the auditory processing centre in the brain. Developing the best possible interface between the electrodes and the auditory nerve is a current active research area.

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Electrodes are implanted into the cochlea and stimulate the auditory nerve directly.

A simple model of cells and the action potential In 1952, Alan Lloyd Hodgkin and Andrew Huxley developed a simple electrical model that could explain the response of the cell to stimulus. Hodgkin and Huxley studied axons in squid. The diameter of a squid’s ‘giant axon’ (a particular axon that helps squids move rapidly to escape predators) is so large that an electrode can be inserted directly into the axon, allowing the cell membrane potential to be measured. In 1963 they were awarded a Nobel Prize for their work on the action potential.

From an electrical perspective, the cell membrane is a non-conducting layer that acquires an electrical potential difference, V, across the layer because there is an effective charge, –Q, on the inside of the cell and an effective charge, +Q, on the outside of the cell. An object that has that property is called a capacitor. Capacitance, C, is the ratio of the charge stored on the component, Q, to the electrical potential difference across the component, V. The unit for capacitance is the farad, named after one of the founding figures of our modern understanding of electricity, Michael Faraday. The larger the capacitance, the more charge the capacitor requires to achieve a given potential difference.

An important safety point when working with capacitors is to note the maximum potential difference that a capacitor can sustain. Above that potential difference, the internal structure of the capacitor will break down, usually irreversibly, allowing charge to flow freely across the capacitor.

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Capacitors of varying capacitance

SAMPLE PROBLEM 25.2 A potential difference of 9 V is applied across a 10 μF capacitor. How much charge is stored by the capacitor?

Solution:

9V 10 F

90 C

QC

VQ CV

=

== × μ= μ

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The RC circuit When a capacitor and a resistor are connected in series in a circuit, the potential difference across the resistor changes with the charge and discharge of the capacitor, even though the supply voltage may be constant.

In the circuit below, when the switch is closed, current begins to flow in the circuit, causing charge to build up on the capacitor. As charge builds up, the potential difference across the capacitor also increases. When the potential difference across the capacitor is equal and opposite to the potential difference of the voltage source, no current can flow.

(a) An RC circuit (b) The potential difference across a capacitor as it charges

When the switch is closed, the rate of charging of the capacitor is at its highest. This is because the current in the circuit is initially at a maximum and decreases as the share of the voltage across the resistor decreases, thereby decreasing the rate of charging of the capacitor. You can see from the graph that the voltage across the capacitor approaches the supply voltage increasingly slowly, and in theory takes an infinite amount of time to fully charge! When comparing capacitor charging times, a useful measure is the time taken for the capacitor to reach 63% of the source voltage. This time interval, τ, is a characteristic of the circuit and is related to the resistance, R, and capacitance, C, of the circuit, through the formula τ = RC. After a time interval of 5τ, the capacitor is regarded as effectively fully charged.

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(a) A circuit for discharging a capacitor (b) The potential difference across a capacitor as it discharges

In the circuit above, if the switch is open, then ideally no current will flow and the capacitor will hold its charge. If the switch is closed, a current will flow until the capacitor is fully discharged. Again, the characteristic time for the potential difference across the capacitor to drop to 37% of the initial value is equal to RC. After a time interval of 5τ the capacitor has fully discharged.

SAMPLE PROBLEM 25.3 A 0.1 μF capacitor takes 5 milliseconds to be charged to 63% of its capacity. How large is the resistance of the resistor? How long will it be until the capacitor can be considered effectively fully charged?

Solution:

A capacitor charges to 63% of its possible capacity in one characteristic time interval of τ = RC = 5 milliseconds.

3

6

4

5 10 s0.1 10 F5 10

RCτ

−

−

=

×=

×= × Ω

The capacitor is considered fully charged after 5τ = 25 milliseconds.

The cell as an RC circuit How is an RC circuit connected to cell behaviour? A cell membrane is like a capacitor because it can support the separation of positive and negative charges. The protein channels create connections across the membrane and can be modelled as resistive electrical pathways in parallel with the capacitor. The leakage channels can be modelled as a resistor, and the voltage-gated protein channels behave like a battery in series with a variable resistor, actively transporting ions across the membrane once they are activated by the right level of membrane potential.


RC circuit investigation activity

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A model circuit for the action potential

This is the essence of the model that Hodgkin and Huxley developed. Their model was a triumph because it was able to describe the shape and amplitude of the action potential across the cell during depolarisation and repolarisation, the threshold activation and refractory period of the action potential, and the form and speed of the action potential as it travelled along the axon.

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Comparison of the calculated action potential using the Hodgson Huxley model with the action potential measured in the squid axon

The heart: an electrically powered pump The heart is electrical pump controlled by the autonomic nervous system. The autonomic nervous system operates involuntary and reflexive processes such as digestion and the contraction of the iris. The action potential is triggered in heart muscle cells in the same way as for nerve cells. An influx of sodium ions causes the cell to start to depolarise. However, the action potential in each muscle cell lasts up to 300 milliseconds, compared to 3 milliseconds in a nerve cell.

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Conduction system of the heart

The initial action potential starts in the sinus or sinoatrial (SA) node above the right atrium, up in the superior vena cava, where there is a group of autonomically activated cells that generate an action potential approximately 70 times per minute. The depolarisation triggered by the SA node propagates through the atria, or upper two chambers of the heart. The atria cannot directly stimulate the ventricles; instead, the depolarisation in the atria stimulates the atrio-ventricular (AV) node. The AV node then stimulates action potentials along bundles of fibres called either the ‘fibres of His’ (after their discoverer, Wilhelm His) or ‘the common bundle’. The fibres of His propagate the signal across to the ventricles, then separate into the left and right bundles before dividing further into the Purkinje fibres that stimulate the muscle cells in the ventricle walls.

The heart has a couple of back-up mechanisms. If the SA node fails to stimulate the AV node, then the AV node falls back on excitation by cells in the AV node that are directly connected to the autonomic nervous system. These cells produce an action potential approximately 50 times per minute. If the AV node also fails, or if there is failure in the fibres of His, the Purkinje fibres also contain cells connected to the autonomic nervous system that can produce action potentials at 15–30 times per minute.

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Measuring heart function — the electrocardiogram The change in potential difference as the action potentials progress through the heart muscle cells can be detected and displayed on an electrocardiogram.

How the electrical activity of the heart contributes to an electrocardiogram

Measurement electrodes were first placed on a patient’s body by Augustus Waller in 1887. The typical features of the ECG are given the letters P, Q, R, S, T, and U and are matched with the depolarisation and repolarisation of the heart cells in the atria and ventricles.

Heart beat and the ECG Initially the heart is unstimulated and the atrial chambers full of blood. The atrial muscle cells depolarise in response to the trigger from the SA node, causing the atrial muscle wall to contract and pump blood from the atria into the ventricles. The hump due to the depolarisation of the atria in the ECG trace is called the P-wave. The action potential passes relatively slowly into the ventricles (allowing the atrial chambers to completely empty), eventually reaching the Purkinje fibres, where the depolarisation and then contraction of the ventricles begins, marking the start of the QR period. The depolarisation of the ventricular wall causes a sharp rise in the ECG trace, peaking at R, and dropping again as the ventricular wall begins to depolarise. At this point the atria also begin to repolarise, and the atrial chambers refill with blood. The geometry of the heart and the placement of the ECG results in a third hump appearing at T, also associated with the depolarisation of the ventricular muscle. By the end of the TP period, the ventricular chambers are fully relaxed, ready for the next stimulus from the AV node.

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When analysing heart performance, medical practitioners consider the timing and amplitude of each of the components of the ECG trace.

Heart problems — fibrillation and defibrillation If the chambers of the heart stop working in sequence, the heart will not pump effectively. Sometimes the sequence of the heart is disrupted and depolarisation and repolarisation begin to occur chaotically, causing the heart to fibrillate. Experiments in the late nineteenth century showed that even relatively small AC currents of 0.1 A could cause a heart to go into fibrillation. However, it was also found that larger currents could stop fibrillation.

Today, portable machines called defibrillators are often found in public places such as sports grounds and shopping centres for use in the case of a suspected heart attack. Such defibrillators use a battery to charge up a large capacitor to approximately 5kV. The capacitor is then connected across a person’s heart using two leads and special skin contact pads, forming a series RC circuit. The capacitor discharges over 2–5 milliseconds, depending on the effective resistance of the patient. The large voltage depolarises a significant amount of the heart muscle, allowing the sinoatrial node to restart normal heart rhythm.

A defibrillator for use in a suspected heart attack

SAMPLE PROBLEM 25.4 A defibrillator machine contains a 20 μF capacitor. If there is a resistance of 500 Ω between the contact points on the chest:

a. how long will it take for the defibrillator to have discharged to 37% of its original value? b. how much charge is stored on the capacitor when it is charged up to 5 kV? c. what is the average current delivered by the capacitor?

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Solution:

a. The capacitor discharges to 37% of original value after one characteristic time period, τ = RC.

500 20 F 1 ms

RCττ

== Ω × μ =

b.

20 F 5 kV

100 mC

QC

VQ

=

= μ ×=

c. The capacitor is considered fully discharged after 5 τ = 5 ms.

100 mC

Avarage current 20A5 ms

Qt

= = =

The artificial pacemaker The first pacemakers were used in the 1950s and were designed to treat Stokes Adam syndrome, a condition where a blockage between the atria and the ventricles stops the ventricles from being stimulated. These pacemakers simply provided a regular electrical pulse to stimulate the ventricles, in imitation of the normal function of the heart. They also required an external power supply. Modern pacemakers consist of a tiny integrated circuit powered by a lithium–ion battery and are implanted inside the patient. The modern pacemaker senses any existing pulse and adjusts accordingly, increasing heart rate in response to physical activity. The sensing mechanism reduces the likelihood of the pacemaker being out of step with any currently functioning heart stimulation and causing defibrillation.

Pacemakers may be single- or dual-chamber devices, depending on whether they stimulate the atria, the ventricles or both. The advantages of a single-chamber pacemaker are that only a single lead needs to be implanted in the heart and less energy is required, so the battery lasts longer. Often the return path to the pacemaker is simply through body tissue. The advantage of a dual-chamber device is that it ensures synchronicity between the atrial and ventricular contractions. Advances in battery life have made dual-chamber devices more attractive.

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Single-lead pacemaker implanted in a patient

The first pacemakers were based on a circuit very similar to the one below. In the circuit below, when the switch is open, current flows in the circuit until the capacitor is fully charged. When the switch is closed, charge from the capacitor flows through the heart (RB) until the switch opens again, at which point the capacitor begins to recharge. This type of circuit is an RC oscillator circuit and delivers a pulse of voltage to a load across the capacitor.

(a) RC oscillator circuit (b) Charging and discharging of capacitor as switch opens and closes

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Another example of such a circuit is found in flashing neon lights. When potential difference across the lamp reaches a certain threshold, the neon gas becomes a plasma and glows brightly. At this stage, the lamp conducts electricity, and current from the capacitor and the battery flows through the lamp. The potential difference across the lamp starts to drop, but the plasma does not immediately revert to ordinary neon gas. When it does, the lamp ceases to glow and no longer conducts electricity, so the capacitor begins to recharge. The potential difference across the capacitor rises again until the plasma threshold is reached, and the cycle begins again, resulting in a regularly flashing lamp.

This simple circuit creates a flashing neon lamp.

Electrical resistance of the human body

Resistance and resistivity Human electrical resistance varies a great deal for different parts of the body and even within the parts of the body. Dead skin cells on the surface of our skin have a resistance hundreds of times greater than the layers of live skin cells covering our muscles and other body tissue.

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The resistance of a body part depends on the concentration of mobile charge carriers in the body part, the mobility of the charge carriers, the length of the body part and the thickness of the body part. Motor neurons transport signals more quickly because the axons are myelinated and have larger diameter s.

Like hardness, taste and smell, electrical resistivity, ρ, is an important physical property of a material. Resistivity is affected by the concentration and mobility of charge carriers in the material. In the body, fat cells have a higher resistivity than muscle cells due to their spherical shape and relative lack of mobile ions. Men generally have more muscle mass than women, and women generally have more fat cells as a proportion of their body than men. This means that males who are exposed to electrical potential difference experience a greater current passing through their body on average than a female.

Effective resistance of the human body What happens when a potential difference is applied across the human body? By far the greatest contribution to the electrical resistance of the human body comes from the dead outer layers of skin. Dry outer skin layers can have electrical resistance as high as 100 000 ohms, whereas pathways through body tissue will generally present a total effective resistance of about 300 ohms.

If skin is cut or deeply abraded through to expose lower skin cells, the total effective resistance of the body drops dramatically. Water on the skin also greatly reduces the electrical resistance of the outer skin layer, thus decreasing the effective resistance of the human body.

From Ohm’s Law, we know that the smaller the resistance, the higher the current that flows in response to an applied potential difference. A human being might inadvertently experience a potential difference when handling a faulty appliance such as a broken toaster or hairdryer. In such a scenario, a person might find that they become part of a path of least resistance for current to flow from the electricity supply to the ground.

SAMPLE PROBLEM 25.5 Cecily, who has an effective resistance of 40 000 ohms, is handling a faulty hairdryer that is plugged into a 230 V power point. She becomes the path of least resistance for the current to flow from the electricity supply to the ground.

Find the current that flows through Cecily and the energy deposited in her body if she holds the hairdryer for 5 seconds.

Solution:

2

3 2

230 V40 000 5.75 mA

(5.75 10 ) 40 000 5 J

6.61 J

VI

R

E Pt

I Rt−

=

=Ω

==== × × ×=

A current of 5.75 mA will result in a tingling sensation for Cecily but will not do long term damage.

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SAMPLE PROBLEM 25.6 Bill uses the same hairdryer as Cecily, but he has wet hands, reducing his effective resistance to 400 ohms.

Find the current that flows through Bill and the energy deposited in his body if he holds the hair dryer for 5 seconds.

Solution:

2

2

230 V400 0.575 mA

0.575 400 5 J

661 J

VI

R

E Pt

I Rt

=

=Ω

==== × ×=

This is considerably more serious, mainly because of the AC nature of the voltage supply.

If a person experiences a large potential difference, typically greater than 500 V, the structure of the skin actually breaks down and ionises. This greatly reduces the effective resistance of the person, thus exposing them to higher currents. This is analogous to the occurrence of lightning. In a lightning strike, air, normally a very poor conductor, breaks down and ionises, allowing a current to pass from clouds to the surface of the earth; this occurs when the atmospheric conditions have resulted in the build-up of an extremely large electrical potential difference between clouds and the earth.

Because the body contains so many elements of varied resistance, it is rather like applying a potential difference across a complex parallel circuit. In a parallel circuit, the largest proportion of current flows through the part of the circuit with the least resistance. It is the same for the body. If the human body experiences a potential difference, then the most current will flow through the parts with lowest electrical resistance, namely body fluids and tissue. The effective resistance of the body is further complicated by the capacitative cell membranes. A simplified equivalent circuit for the body can be drawn as in the figure below.

Most current flows through extracellular fluid. However, the capacitance of the cell membranes also affects the resistance to current flow and allows current to pass across the membrane.

The capacitance of the cell membrane is very important when the current is not constant. The effective resistance contributed by a capacitor is affected by the frequency of variation of the current. Resistances that are frequency dependent are called impedances.

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A popular method for determining the ratio of fat cells to other cells in the body is the ‘bioelectric impedance measurement.’ Although such measurements are relatively easy to obtain, they are strongly affected by how hydrated the person is, causing the values obtained to vary considerably.

Current and the human body What effect does current have on humans? The energy deposited in the body is proportional to the square of the current flowing through the body and the time that the human is exposed to the current. Heating of tissue occurs when current flows through the body. In addition, because of the capacitative properties of the cell membranes, the effect of current on humans also depends on whether the current is alternating current (AC) or direct current (DC). A 50 Hz AC current can cause serious physical damage at much lower voltage and current values than a DC current.

Handyman suffering involuntary muscle contraction and possibly heart failure after receiving an electric shock

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DC current

If a constant potential difference is applied to the body, mobile ions such as K+ and Na+ ions will respond, disrupting the electrical balance of the cell. At high enough voltages, this may trigger a nervous response in the case of nerve cells, or a contraction in the case of muscle cells. If the potential difference remains constant, then the repolarising processes in the cell cannot begin, so that contracted muscles are unable to release. This increases the time of exposure to current and further worsens the effect of the shock. The damage to cell function is mainly due to electrical heating caused by the current passing through the cell, or if the potential difference is above 500 V, electrical breakdown of the cell.

AC current and voltage

In Australia, most electricity comes from coal- or gas-fired power stations that generate large alternating potential differences. These potential differences are transmitted along high voltage power lines to individual users, where they are transformed to smaller potential differences varying between about ±250V. The effect of an alternating voltage supply is to produce a current that varies sinusoidally between a positive and negative peak value. The frequency of the variation is 50 Hz in Victoria. Because the current and voltage vary, an average value, the RMS (root mean square) value is used to describe the magnitude of the current and voltage. Calculations of electrical power and energy consumption use the RMS current and voltage values.

Standard varying household supply voltage. The RMS value is indicated as a dashed line.

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High voltage transmission lines carrying alternating current to local areas from the power station

If the applied potential difference is varying, then the electrical balance of the cell will change continuously and trigger a sequence of nervous and/or contraction responses. This is why AC currents are significantly more dangerous than DC currents. A 50 Hz current has a period of 2 milliseconds, meaning that an action potential is being almost continually triggered in nerve cells. In the case of heart muscle cells, fibrillation can occur and lead to heart failure. Breathing can be affected if effective chest wall muscle contractions cease. For very high

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voltages, such as those in high-voltage supply lines, the person will experience strong contraction of all muscles, which often causes them to jerk away from the point of contact.

In a typical Australian household, circuits are rated to carry currents of up to 5 or 15 A, depending on whether they are lighting or power circuits. Table 25.1 shows how deadly such currents are should a person inadvertently form part of a return circuit.

Table 25.1

AC current (60 Hz) Effect 1 mA Barely perceptible

16 mA Maximum current up to which a person can still let go 20 mA Paralysis of respiratory muscles 100 mA Ventricular fibrillation threshold

2 A Cardiac arrest

Electrosurgery In electrosurgery, the heating properties of current passing through the body are used to cut through tissue. The patient lies on a large return electrode, so that where the instrument is applied to the patient, there is a high current density. The current density causes water in the cells to vaporise, cutting through tissue. From the entry point the current spreads out over a large volume, causing little damage to the rest of the patient’s body. Lower voltages are used to stop bleeding by causing the blood to coagulate, a process also called cauterising. (This is not to be confused with the cauterising technique known as electrocautery, in which the body is heated radiatively by a metal tip that has a large current flowing through it. In electrocautery, no current passes from the device to the patient.)

A surgeon has to take care not to inadvertently provide a low-resistance return path for the current when using electrosurgical tools, as this can result in severe burns to the patient and/or the surgeon. If the patient has metal implants, care has to be taken to keep the current path away from those areas.

In previous sections, we saw that action potentials typically last for 200–400 milliseconds, depending on the cell type. In electrosurgery, the triggering of action potentials and disruption of nervous system processes is avoided by using very high frequency currents of 100 khz to 5 MHz, which have periods of microseconds. Remember that frequency = 1

period .

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The Bovie is the tradename of the instrument used in electrosurgery.

Galvactivator Variations in the conductivity of skin are linked to the release of moisture through the eccrine glands located on the palms of the hands and soles of the feet. The eccrine glands are connected to the sympathetic nervous system, the body’s ‘the fight or flight’ response system. Skin conductivity measurements form one component of the polygraph or lie detector. An assumption is made that a person who is lying is psychologically stressed, causing them to sweat more so that their skin becomes more conducting. A research group at MIT has developed a wearable glove called the Galvactivator that has an LED array to indicate the conductivity of the skin at that moment. The researchers are using the glove to better understand how a person’s psychological state is connected to the activity of the eccrine glands. Potentially, such gloves could communicate the mood of a class to the teacher!

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Human skin structure

Neuroscience — frontiers of human bioelectricity The field of neuroscience is making extraordinary advances in the detection, interpretation, stimulation and manipulation of neural activity. The following is a brief outline of some of these frontiers.

Monitoring electrical activity in the brain The electroencephalograph (EEG) detects electrical potential differences across the brain. Like the ECG, it can be used to monitor brain function and diagnose brain conditions such as epilepsy. The EEG detects a signal when large numbers of neurons are activated synchronously. Such signals have been studied for over 100 years, and over time, particular waves with characteristic frequencies that are observed in the EEG have become associated with particular types of brain activity, such as the phases of sleep and wakefulness.

Not all features of EEG images are well understood. The Glass Brain project uses image reconstruction techniques similar to those used in medical imaging techniques such as X-ray CT scans to produce real-time 3D images of brain electrical activity from the potential difference measurements.

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Different characteristic electrical potential difference signals observed in EEG

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Patient with electrodes attached to the scalp recording an EEG

Better understanding of normal brain function will lead to improvements in treatments such as deep brain stimulation. In deep brain stimulation a ‘pacemaker’ for the brain is inserted into the brain. The pacemaker counteracts abnormal electrical signals associated with Parkinson’s disease and Tourette’s syndrome that affect proper motor function.

Direct stimulation of the brain with small electrical currents also seems to show some promising benefits for mood and memory. Nevertheless, the mechanism for and success of these treatments is not yet fully understood. Neuroscientists hope to one day insert feedback devices in the brain that can both detect electrical activity indicating the onset of an epileptic seizure and counteract it with an electrical signal.

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Neural stimulation: The bionic eye project The bionic eye project aims to restore sight to blind people by bypassing the retina and optic nerve, instead directly stimulating the visual cortex. This will help those who are vision impaired because of damage to the optic nerve. In the bionic eye, an array of electrodes is implanted in the back of the brain and receives signals wirelessly from a digital camera mounted on a pair of glasses. The glasses also contain a sensor that tracks eye movement, so that the camera can follow the direction of the eye. The brain is able to learn to interpret the electrical stimulus from the electrode array. Such a device would also be suitable for people who have partial sight, because the electrodes would complement rather than replace the optical signals being received by the brain from the retina. At present, the largest array that has been implanted contains 473 electrodes. One thousand electrodes is thought to be the minimum required to be able to read large-print lettering. The world leaders in this technology are based at Monash University in Melbourne.

The elements of the bionic eye: (1) a camera (2) a sensor that tracks eye movement (3) a wireless transmitter of the electrical signal representing the digital image (4) array of electrodes implanted in the brain that stimulate the visual cortex

Bypassing neural damage: Bionic control devices and activated muscles In the field of myoelectrics, signals passed along motor neurons are detected by sensor electrodes and used to direct electrically powered motors, allowing, for example, amputees to control bionic arms and hands. Motor-assisted prosthetic hands and arms have been available for some time. In 2014, a bionic walking brace was developed that allows people who had previously lost motor control of their legs to walk, stand and sit.

Weblink Bionic eye app

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A myoelectric prosthesis allows better fine motor control for the wearer.

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Bionic leg brace

The same underlying principles can be used when a person has lost function in a limb. Sensor electrodes detect the original signal from the brain, which is then delivered artificially to muscle tissue in the limb, activating the muscle to stimulate movement.

Retraining neural networks An important recent discovery has been the finding that even where a patient may be suffering from paraplegia due to damage to the spinal column, there is often some residual nervous activity. This has led to a new therapy — activity based training, in which patients perform particular exercises that stimulate the strengthening of the remaining neural connections. Remarkably, it has also been found that other neurons will readapt their function to assist the remaining neurons. Some forms of therapy employ a sensor that detects motor neuron signals and then amplifies the signal to drive a motorised device such as a bionic brace to help the limb to move and thereby rebuild neural networks. The ability of neural networks to reorganise themselves in response to stimulation and form new synaptic connections is called neuroplasticity.

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Robotic suit assisting in activity based therapy for patients with paraplegia

Like all scientific advances, moral and ethical questions arise when considering the applications of these technologies to enhancing the human experience. We live in a revolutionary age in which cross-disciplinary fields such as biophysics and neuroscience have the potential to change the world for us all.

Chapter review

Summary ■ Our bodies contain many electrically charged ions that are able to respond to electrical forces

■ Cells have a resting electrical potential difference due to a charge imbalance across the cell membrane. This arises from the difference in concentration of ionic species inside and outside the cell combined with the difference in mobility of the Na+ and K+ ions and the action of the ATP molecule transferring Na+ ions out of the cell and K+ ions into the cell.

■ Neurons respond to electrical stimulus at a point in the cell by allowing sodium ions to enter the cell at that point, altering the local potential difference across the cell membrane. If the influx of sodium ions exceeds a threshold number, a process of depolarisation and repolarisation of the cell occurs, known as the action potential.

■ The occurrence of the action potential at one point along the cell membrane triggers the onset of an action potential at the neighbouring section of the membrane, allowing the transmission in a domino-like fashion of the action potential along the length of the cell.

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■ The sheathing of axons in myelin for short sections increases the velocity of the transfer of the action potential.

■ Neurons communicate with other neurons through synapses, where a chemical transfer takes place via a neurotransmitter. The transfer of electrical signals along axons is much faster than the transfer of chemical signals.

■ The behaviour of a cell can be modelled by a simple circuit consisting of a capacitor in parallel with a resistor. The time taken to charge and discharge the cell as the action potential depolarises and repolarises the cell can be modelled as the characteristic time associated with charging and discharging the capacitor.

■ Our bodies are able to conduct electricity. However, excessive currents damage cell tissue and disrupt electrical processes in the heart and nervous system.

■ Alternating current sources pose greater risk to the human body than direct current sources due to the capacitative properties of body tissue.

■ The nervous system can be stimulated artificially as seen in pacemakers, bionic eyes and cochlear devices.

■ Heart and muscle cells also respond to electrical stimulus.

■ Electrocardiograms detect the potential differences between various parts of the heart and are used to diagnose the quality of heart function.

■ Stimulation of the heart generates a regular electrical signal with characteristic features that can be seen in an electrocardiogram.

■ The defibrillator is a device that deliberately disrupts the function of the heart in order to effect a restart of the heart action.

■ Electroencephalographs detect the potential difference between various parts of the brain and are used to monitor brain function.

■ Neural signals are able to be detected and amplified, allowing bionic control of existing of artificial body parts.

Questions

Charge carriers and electrical potential difference in cells

1. List two ions essential to the function of nerve cells and describe their role.

2. When electrical forces are applied to a conducting material, the charge carriers move. What is the term used to describe how easily the charge carriers can move?

3. Electrons in Ag have a mobility of 56 cm2 V–1 s–1 at 27 °C. Na+ ions have a mobility of 5.2 × 10–4cm2 V–1 s–1 in extracellular fluid.

a. Which charge carriers move faster in response to an applied potential difference?

b. The resistivity of Ag is 1.59 × 10–8 Ω m, whereas the resistivity of extracellular fluid is approximately 20 Ω m, or 1000 million times larger. List two factors that result in the significantly smaller resistivity of Ag compared to extracellular fluid.

4. Describe the basic structural elements of a human cell.

5. Give two reasons why the Na+ ions in the extracellular fluid try to enter the unstimulated cell.

6. How do proteins located in the cell membrane affect cell function?

7. Describe the role of the ATP pump in maintaining the resting cell potential difference.

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8. The resting potential difference across cells varies for different types of cells, from -60 mV for smooth muscle cells to 95 mV for skeletal muscle cells. Explain how the resting potential difference across a cell can be made more negative.

The nervous system and electrical processes

9. List the principal components of a nerve cell.

10. What is the role of the synapses in the nervous system?

11. a. Explain the difference between a nerve cell and a nerve.

b. What happens when a neuron is stimulated by 5 mV or less?

12. Describe the phenomenon of hyperpolarisation.

13. When hyperpolarisation occurs, an action potential cannot be established. True or False?

14. The magnitude of the action potential depends upon the strength of the stimulus. True or False?

15. Which is the main direction of current flow as an action potential moves along an axon — across the membrane or along the membrane?

16. Towards the end of the refractory period, the neuron is able to be stimulated again. How does the threshold stimulus compare with the initial stimulus applied to the neuron?

17. List three differences between saltatory conduction and conduction by an unmyelinated axon.

18. Sensory receptors stimulate the sensory neurons. Which sensory receptors are connected to the thickest axons?

19. What is the advantage of a larger diameter axon?

20. If you perceive a tingling sensation in your arm, what is a likely explanation for this?

A simple electrical model of cells and the action potential

21. A circuit contains a 10 μF capacitor. If the voltage across the capacitor is 10 V, how much charge is stored by the capacitor?

22. A 0.50 μF capacitor is in series with a 500 Ω resistor.

a. When attached to a 9.0 V battery, how long does it take for the potential difference across the capacitor to reach 5.7 V?

b. Is the current in this circuit increasing or decreasing with time? Explain.

23. A 10 F capacitor is charged to 5000 V. It is then connected in series with a 500 Ω resistor.

a. How long until the capacitor is regarded as fully discharged?

b. What is the average current that passes through the resistor?

24. Compare the figures illustrating charging and discharging of a capacitor with the action potential measured for the giant squid axon, –70 mV. UNCORRECTED P

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How voltage varies with time when (i) charging and (ii) discharging a capacitor (a) Which figure best describes the depolarising of the membrane? (b) Which figure best describes the repolarising of the membrane? (c) Do you think that the effective resistance of the membrane is the same during depolarising and repolarising?

The heart: an electrically powered pump

25. Describe the electrical activity of the heart as it progress from P through to U on an ECG.

26. What are two important diagnostic criteria for doctors when they look at an ECG?

27. What is the advantage of a dual-chamber pacemaker?

28. Explain how a supply of constant voltage such as a battery can be used in a circuit containing a resistor, a capacitor and a variable conductor such as a neon gas tube to produce a periodic voltage pulse.

29. Fibrillation can be induced by exposure to AC electrical currents as small as 100 mA. Why is the household 50 Hz AC supply particularly dangerous?

Electrical resistance of the human body

30. Compare the resistivity of fat and muscle cells.

31. Why is an alternating source of potential difference more dangerous than a constant voltage source?

32. When using a defibrillator, special skin contact pads are used. If the patient has hair on their chest, it must be shaved off before the pads are applied. Why is this essential?

33. What is the word used to describe a resistance that depends upon the frequency of the current in the circuit?

34. The output power of a Bovie being used in an electrosurgery cutting procedure is 200 W. How much energy is deposited in the cells in 1 minute?

35. A person is wearing a Galvactivator glove powered by a 6 V battery with a light that flashes red when the skin conductivity of the wearer changes by 50%.

a. If the baseline resistance of the person’s skin is 100 000 Ω, what is the current that passes through the person’s hand?

b. When the person is excited, the current rises to 100 μA. What is their skin resistance now?

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Neuroscience: frontiers of human bioelectricity

36. How is the signal monitored by an EEG related to the action potentials generated by individual neurons?

37. Compare the cochlear and bionic eye devices.

38. Describe the role of neuroplasticity in activity based training therapy.

39. Many current advances in biophysics and neuroscience could enhance experience for ordinary humans with normal neural function. Emergency workers such as firefighters could benefit from bionic aids to reduce fatigue. It is also speculated that localised electrical stimulation of the brain can improve both mood and memory. Identify ethical issues that might arise when such devices and techniques are employed on ordinary individuals.

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