6.5 - nerves (sjhs)

8/3/2019 6.5 - Nerves (SJHS)

http://slidepdf.com/reader/full/65-nerves-sjhs 1/33

Topic 6.5 - Nerves

8/3/2019 6.5 - Nerves (SJHS)


6.5.1

Overview of the Nervous System

Three major functions:

• Sensory input – sensory receptors receivesignal – peripheral nervous system (nerves,eyes, ears, etc.)

• Integration – signal is interpreted andresponse started – central nervous system(brain and spinal cord)

• Motor output – response to stimulus – peripheral nervous system (nerves, muscleor gland cells)

8/3/2019 6.5 - Nerves (SJHS)



8/3/2019 6.5 - Nerves (SJHS)


6.5.2

Neurons

• Function - conduct messages to helpcommunication between parts of nervoussystem.

• Neurons are helped by numerous

supporting cells, which provide structuralsupport, protection, and insulation ofneurons.

8/3/2019 6.5 - Nerves (SJHS)


Neuron Structure

• Cell body – large central part of neuron – Contains nucleus and other organelles

• Processes – fiberlike extensions of neuron

– Dendrites –

receive and move signal from tips to cell body (into neuron)

– Axons – carry signals away from cell body to

tips (out of neuron)

6.5.2

8/3/2019 6.5 - Nerves (SJHS)


6.5.2

Neuron Structure

8/3/2019 6.5 - Nerves (SJHS)


6.5.2

Neuron Structure

• Schwann cells – supporting cells that forminsulating myelin sheath layer.

– Increases speed of signal

• Nodes of Ranvier – spaces in between the

Schwann cells• Synaptic terminal – end of axon where

neurotransmitters are released into synapse

8/3/2019 6.5 - Nerves (SJHS)


6.5.2

Nueron Structure

8/3/2019 6.5 - Nerves (SJHS)


6.5.3

Types of Neurons

• Sensory neurons – communicateinformation about external and internalenvironments to central nervous system(input)

• Interneurons – link sensory response tomotor output.

• Motor neurons – communicate responsefrom central nervous system to effectorcells (motor output)

• All combined, these neurons create a reflexarc, which integrates a stimulus andresponse.

8/3/2019 6.5 - Nerves (SJHS)


A Reflex Arc

8/3/2019 6.5 - Nerves (SJHS)


Membrane Potential

• Membrane potential – the difference inelectrical charge across the plasmamembrane.

• The inside of the cell is negative with

respect to the outside.• Neurons have a resting membrane potential

of -70mV

8/3/2019 6.5 - Nerves (SJHS)


Membrane Potential

• Inside the cell:

– Cations: potassium (K+) and few sodium (Na+)

– Anions: proteins, sulfate, phosphate(collectively A-) and few chloride (Cl-)

• Outside the cell: – Cations: Sodium (Na+) and few potassium (K+)

– Anions: chloride (Cl-)

8/3/2019 6.5 - Nerves (SJHS)


Membrane Potential

8/3/2019 6.5 - Nerves (SJHS)


Membrane Potential – How it’s Created

• The plasma membrane is more permeable (more

membrane channels) to K+ than to Na+. – Therefore, large amounts of K+ are transferred out of

the cell (down the concentration gradient)

– Small amounts of Na+ are transferred into the cell

(down the concentration gradient)• The movement of K+ and Na+ across the

membrane generate a net negative membranepotential (-70mV)

• A sodium-potassium pump is used to move K+back into the cell and Na+ back out of the cell tomaintain the constant concentration gradients.

8/3/2019 6.5 - Nerves (SJHS)


Membrane Potential

8/3/2019 6.5 - Nerves (SJHS)


6.5.4

Changes in Membrane Potential

• Neurons are excitable cells – a stimuluscan change the neuron’s membrane

potential

• Resting potential – membrane potential of

unexcited neuron (-70mV)• Neurons become “excited,” when a stimulus

opens a gated ion channel and increasesthe movement of K+ or Na+ across the

plasma membrane

8/3/2019 6.5 - Nerves (SJHS)



Hyperpolarization:

• A stimulus opens a K+ ion channel andefflux of K+ out of the cell increases

• Membrane potential becomes more

negative

8/3/2019 6.5 - Nerves (SJHS)


Hyperpolarization

8/3/2019 6.5 - Nerves (SJHS)



Depolarization:

• A stimulus opens a Na+ ion channel andinflux of Na+ into the cell increases

• Membrane potential becomes more positive

6.5.4

8/3/2019 6.5 - Nerves (SJHS)


6.5.4

Depolarization

8/3/2019 6.5 - Nerves (SJHS)


6.5.4

Action Potential

• When depolarization reaches a certainpoint, the threshold potential is achieved.

• When threshold potential is reached, anaction potential is triggered.

– Action potential is a nerve impulse.• Action potentials consist of a rapid

depolarization, a rapid repolarization, andundershoot (hyperpolarization)

8/3/2019 6.5 - Nerves (SJHS)


6.5.2

Action Potential

8/3/2019 6.5 - Nerves (SJHS)


6.5.5

Action Potential

• Caused by voltage-gated channels

– Open and close in response to changes inmembrane potential

– K+ channels – one gate; closed at restingpotential; opens slowly during depolarization

– Na+ channels – two gates:

• Activation gate – closed at resting potential;opens rapidly during depolarization

• Inactivation gate – open at resting potential;closes slowly during depolarization

8/3/2019 6.5 - Nerves (SJHS)


6.5.5

Steps in Action Potential

• Depolarization: Na+ activation gates openand Na+ enters cell.

• Repolarization: Na+ inactivation gatecloses (prevents Na+ influx) and K+ gate

opens and K+ exits cell.• Undershoot: K+ gates remain open and K+

continues to leave cell

• Resting state: All gates closed, Na+/K+pump (active transport) moves Na+ out andK+ in to restore resting potential.

8/3/2019 6.5 - Nerves (SJHS)


Steps in the Action Potential

8/3/2019 6.5 - Nerves (SJHS)


Steps in Action Potential

8/3/2019 6.5 - Nerves (SJHS)


8/3/2019 6.5 - Nerves (SJHS)


6.5.6

Synapses

• Synapse – junction between

two neurons – Transmitting cell –

presynaptic cell

– Receiving cell – postsynapticcell

• Neurons are separated by agap called the synaptic cleft.

• Messages are transmittedacross the synaptic cleft bychemical neurotransmitters.

8/3/2019 6.5 - Nerves (SJHS)


6.5.6

Steps in Synaptic Transmission

1. A nerve impulse reaches end ofpresynaptic neuron.

2. Presynaptic membrane depolarizes,opening voltage-gated Ca2+ channels.

– Ca2+ ions diffuse into presynaptic neuron3. Influx of Ca2+ causes neurotransmitter

vesicles to fuse to presynaptic membraneand release neurotransmitters into the

synaptic cleft (exocytosis)

8/3/2019 6.5 - Nerves (SJHS)


6.5.6


4. Neurotransmitter diffuses across synapticcleft and bind to receptors on postsynapticmembrane.

5. Receptors open gated ion channels in

postsynaptic membrane. – Specific receptors open specific ion channels

– May open Na+, K+, or Cl- channels

– Different ions have different responses

(excitatory or inhibitory)

8/3/2019 6.5 - Nerves (SJHS)


6.5.6


6. Enzymes quickly degradeneurotransmitter, ending its activity.

– E.g. acetylcholine is degraded bycholinesterase.

7. Ca2+ is pumped out of presynaptic cellback into synaptic membrane.

8/3/2019 6.5 - Nerves (SJHS)


Chemical Synapse

6.5 - nerves (sjhs)

Documents