Neuron signaling

Electricity Principles• The ECF contains primarily sodium (Na+) and chloride

ions (Cl-)• The ICF contains lots of potassium (K+) ions and other

molecules that are non-diffusable

Electrical potential

• Separated electrical charges of opposite sign have the potential to do work if they are allowed to come together.

• AKA electrical potential. – It is determined by the the difference in the

amount of charge between the two points, – The units of electrical potential are called volts.

• We will measure in millivolts (mV).

Because the charges attract, they line up on either side of the membrane.

Actual membrane potential

• In an actual nerve cell at rest, – K+ concentration is greater inside– Na+ is greater outside.

• actual resting membrane potential is about -70 mV

there is always a membrane potential

The Na+/ K+ pump

• maintains the concentration gradients for each ion and helps create the gradients. – The pump moves 2 K+ ions

in and 3 Na+ ion out each time it works.

– Because it pumps out more + ions than it brings in, it helps make the ICF more negative.

Action Potentials and Graded Potentials

• changes in the membrane potential from its resting level produce electrical signals. – This is the way that neurons process and transmit

information.

• There are two forms of the signals: – Graded potentials (GP) signal over short distances.

• receptors

– Action potentials (AP) signal over long distances in nerves and muscles.

Graded potential• GP’s occur in a

small area. • When a GP occurs

– charge flows from the origin like a wave.

What actually happens

• The membrane is depolarized by a stimulus– The area nearby becomes less negative because

• + ions will flow in

• + ions already inside the membrane will push away from those flowing in.

Spreading of

stimulusSTIMULUS

Size of the GP• The size of the GP is related to the size of the stimulus. • The stronger the stimulus, the bigger the GP, the farther it travels.

– Because ion channels are always open, the signal will only travel a short distance before it loses strength

– GP currents die out within a few millimeters.

Summation

• If additional stimuli occur before the graded potential has died out, these are added to the first stimuli. – summation plays a very

important role in the senses.

Definitions– Polarized means that the outside and inside of the cell

membrane have different net charges – Depolarized is when the potential is less negative than

at resting potential (closer to 0 mV).– Overshoot is a reversal of the resting membrane

polarity (more positive inside or between 0 mV and +50 mV).

• The inside of the cell becomes positive relative to the outside.

– Repolarizing is when the membrane potential is moving towards the resting value (between +50 mV and -70 mV).

– Hyperpolarized is when the membrane potential is more negative than the resting potential (greater than -70 mV).

3 kinds of transport proteins

• Leak channels– Move ions from high to low concentration– Always open

• Na/ K pump– Creates the resting membrane potential

• Voltage gated channels– Ion channels that open at one voltage and close at

another. – Responsible for the graded and action potential.

Action Potentials

• AP’s are large changes in the membrane potential. – The potential changes

from -70 mV to +30 mV and then back to resting potential.

– rapid, can occur 1000X / second.

Excitable cells

• Excitable cells have membranes capable of producing AP’s. – neurons, muscle cells, endocrine, immune and

reproductive cells

• Their ability to produce action potentials is known as excitability. – All cells can conduct GP’s, only excitable cells

can conduct AP’s

Start of the AP• AP begins when the

membrane depolarizes in response to a stimulus.

• This opens voltage gated Na+ channels. – This increases the number

of Na+ ions flowing into the cell and the cell becomes more and more depolarized until a threshold is reached.

– This triggers the AP.

Threshold, depolarization, repolarization

• Once this threshold is reached more voltage gated Na+ channels open. – The membrane potential

overshoots (becomes more positive on the inside) and reaches about +30 mV.

– At the peak, voltage gated Na+ gates close and voltage gated K+ channels open.

– The membrane potential begins to rapidly repolarize to resting levels.

– Once resting potential is reached the voltage gated K+ channels close.

Why does the action potential move?

All or None principle

• If a stimulus causes depolarization to reach threshold, then an AP will always be generated. – Size of the AP is always the same regardless of

the stimulus strength. – AP is an all or none response to the stimulus.

Speed of the AP… FAST

• Unmyelinated neuron: .5 m/sec

• Large Myelinated neuron: 100 m/sec– At 100 meters/ sec, an AP will travel from the

big toe to the brain in 0.02 seconds.

Myelinated axon cause saltatory conduction

• Myelin is mostly lipids. – So no charge will flow

through this tissue. – When less ions leak, the AP

spreads farther.

• It doesn’t cover the entire cell length. – where there is no myelin,

there are large numbers of voltage gated Na+ channels.

– AP’s only occur here (nodes of Ranvier).

What is the “stimuli” that starts an action potential?

• In afferent neurons (going towards the brain), the initial depolarization is done by a GP (aka receptor potential) generated by sensory receptors

• In all other neurons, – Synaptic potential- GP from

the synaptic input to the neuron (occurs at the synapse).

– Pacemaker potential- a spontaneous changes in the neuron’s membrane potential.

Smell, chemical stimulus

Action Potential

Graded

Potential

Hair, mechano receptors in ear

Current

• The movement of electrical charge is called a current. – The electrical potential between charges tend to

make them flow, producing a current.

Resistance

• The hindrance to electrical charge movement is known as resistance. – If resistance is high, then the current flow will

be low.

Conductors/insulators• Conductors- materials that have a low resistance to

current flow.– Water is a good conductor. (ICF, ECF)– Current flows easily through water.

• Insulators- Materials that have a high electrical resistance and reduce current flow.– Membranes are non-polar and are regions of high resistance. – They separate the ICF and the ECF.

Resting membrane potential

• All cells at rest have a potential difference across their cell membrane. – The inside of the cell is - charged compared to

the outside. – ECF has more + ions than negative ions– the ICF has more negative ions, so the membrane

potential has a negative voltage

Ions present• Ions that matter must be diffusable.

– include: Na+, K+ and Cl-• Na+ and Cl- highest outside cell

• K+ is highest inside cell.

Size of the resting potential

• The size is determined by two factors:– Differences in ion concentrations between the

intracellular and ECF.– the number of open ion channels

• More ion channels open, more permeability

How a membrane potential changes

• The changes in membrane potential occur because of changes in membrane permeability to ions. – Some ion channels are gated, open or closed by

electrical, chemical or mechanical stimuli. – i.e. when a cell receives a chemical signal from a

neighboring cell, some channels will open allowing ions to flow into the cell (chemical messengers, ion channel).

Pacemaker Potential

• Pacemaker potential is a spontaneous generation of an action potential.

• Pacemaker potentials are found in the neurons that control the heartbeat, breathing and peristalsis

• a graded potential is caused by the behavior of some ion channels in the membrane.

Pacemaker Potential

– When the threshold is reached an AP is generated.

– The membrane then repolarizes and again begins to depolarize.

– There is no stable, resting membrane potential.

– The rate at which the membrane depolarizes to threshold determines the AP frequency.

Drugs modify synapses

• Most drugs that act on the nervous system do so by altering the mechanism by which synapse work and changing the strength of the synaptic potential.

Synaptic Mechanisms vulnerable to drug influence

• A- Increase leakage of neurotransmitter from vesicle to cytoplasm, this causes it to be broken down by enzymes.

• B- Increase neurotransmitter release into the synapse.

• C- Blocking neurotransmitter release • D- Inhibit transmitter synthesis, • E block transmitter reuptake• F Block enzymes in the synapse that

break down the neurotransmitter. • G Bind to receptors on the post

synaptic neuron to block (antagonist) or mimic (agonist) transmitter action.

What long term effects can drugs have?

• It is difficult to predict because body adapts to imbalances created by the drugs– It does this through feedback mechanisms that

regulate the affected process. • For example, if a drug interferes with the action of a

neurotransmitter by blocking its synthesis, the cells may respond by producing more of the enzymes involved in synthesis.

Diseases can also affect synapses

• A toxin that cause Tetanus is made by a bacteria. – The toxin destroys the proteins that move the

vesicles containing neurotransmitters into the synapse.

– No neurotransmitter is released. – The neurons that depend on the transmitter are

inhibitory • (without the neurotransmitter, they will not send a signal

to relax).

– the muscles that are affected do not relax and a person can become paralyzed.