CH15 Nervous coordination

Question 1

Describe general structure of a motor neurone

Answer 1

Cell body - contains organelles and high proportion of RER
Dendrons - branch into dendrites which carry impulses towards cell body
Axon - long, unbranched fibre carries nerve impulses away from cell body

Question 2

Describe additional features of myelinated motor neurone

Answer 2

Schwann cells: wrap around axon many times
Myelin sheath: made from myelin-rich membranes of Schwann cells
Nodes of Ranvier: very short gaps between neighbouring Schwann cells where there is no myelin

Question 3

3 processes Schwann cells are involved in

Answer 3

Electrical insulation
Phagocytosis
Nerve regeneration

Question 4

How does an action potential pass along an unmyelinated neurone

Answer 4

1. Stimulus leads to influx of Na+ ions. First section of membrane depolarises
2. Local electrical currents cause sodium voltage-gated channels further along membrane to open
3. Sequential wave of depolarisation

Question 5

Explain why myelinated axons conduct impulses faster than unmyelinated axons

Answer 5

Saltatory conduction - Impulse jumps from 1 node of Ranvier to another
Depolarisation cannot occur where myelin sheath acts as insulator
Impulse does not travel whole length of axon

Question 6

What is resting potential

Answer 6

Potential difference across neurone membrane when not stimulated (-70mV in humans)

Question 7

How is resting potential established

Answer 7

1. Membrane more permeable to K+ than Na+
2. Sodium-Potassium pump actively transports 3Na+ out of cell and 2K+ into cell
   Establishes an electrochemical fradient - cell contents more negative than extracellular environment

Question 8

Stages in generating action potential

Answer 8

Depolarisation
Repolarisation
Hyperpolarisation
Returning to resting potential

Question 9

Depolarisation

Answer 9

1. Stimulus –> facilitated diffusion of Na+ ions into cell down electrochemical gradient
2. p.d. across membrane becomes more positive
3. If membrane reaches threshold potential (-50mV) voltage gated Na+ channels open
4. Significant influx of Na+ ions reverses p.d. to +40mV

Question 10

Repolarisation

Answer 10

1. Voltage-gated Na+ channels close and voltage-gated K+ channels open
   Facilitated diffusion of K+ ions out of cell down electrochemical gradient
2. p.d. across membrane becomes more negative

Question 11

Hyperpolarisation

Answer 11

1. Overshoot when K+ ions diffuse out = p.d. becomes more negative than resting potential
2. Refractory period - no stimulus is large enough to raise membrane potential to threshold
3. Voltage-gated K+ channels close and sodium-potassium pump re-establishes resting potential

Question 12

Importance of refractory period

Answer 12

No action potential can be generated in hyperpolarised sections of membrane:
Ensures unidirectional impulse
Ensures discrete impulses
Limits frequency of impulse transmission

Question 13

What is all or nothing principle

Answer 13

Any stimulus that causes the membrane to reach threshold potential will generate an action potential
All action potentials have the same magnitude

Question 14

Factors that affect speed of conductance

Answer 14

Myelin sheath
Axon diameter
Temperature

Question 15

How does axon diameter affect speed of conductance

Answer 15

Greater diameter = faster
Less resistance to flow of ions
Less leakage of ions

Question 16

How does temperature affect speed of conductance

Answer 16

Higher temperature = faster
Faster rate of diffusion
Faster rate of respiration = more ATP for active transport to re-establish resting potential
Temperature too high = denaturing of membrane proteins

Question 17

How can an organism detect strength of stimulus

Answer 17

Larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulese

Question 18

What is function of synapses

Answer 18

Electrical impulse cannot travel over junction between neurones
Neurotransmitters send impulses between neurones and from neurones to effectors

Question 19

New impulses can be initiated in several different neurones for multiple simultaneous responses

Question 20

Describe structure of a synapse

Answer 20

Presynaptic neurone ends in synaptic knob - contains lots of mitochondria, ER and vesicles of neurotransmitter
Synaptic cleft - gap between neurones
Postsynaptic neurone - has complementary recpetors to neurotransmitter

Question 21

What happens in presynaptic neurone when action potential is transmitted

Answer 21

1. Wave of depolarisation travels down presynaptic neurone causing voltage-gated Ca2+ channels to open
2. Causes vesicles to move and fuse with presynaptic membrane
3. Exocytosis of neurotransmitter into synaptic cleft

Question 22

How do neurotransmitters cross the synaptic cleft

Answer 22

Simple diffusion

Question 23

What happens in postsynaptic neurone

Answer 23

1. Neurotransmitter binds to specific receptor on postsynaptic membrane
2. Na+ channels open
3. If influx of Na+ ions raises membrane to threshold potential, action potential is generated

Question 24

Explain why synaptic transmission is unidirectional

Answer 24

Only presynaptic neurone contains vesicles of neurotransmitter
Only postsynaptic membrane has complementary receptors

Question 25

Define summation

Answer 25

Neurotransmitter from several sub-threshold impulses accumulates to generate an action potential

Question 26

2 types of summation

Answer 26

Temporal summation | Spatial summation

Question 27

What is temporal summation

Answer 27

1 presynaptic neurone releases neurotransmitter several times in quick succession

Question 28

What is spatial summation

Answer 28

Multiple presynaptic neurones release neurotransmitter

Question 29

What are cholinergic synapses

Answer 29

Uses acetylcholine as primary neurotransmitter

Question 30

What happens to acetylcholine from the synaptic cleft

Answer 30

1. Hydrolysis into acetyl and choline by acetylcholinesterase (AChE) 2. Acetyl and choline diffuse back into presynaptic membrane 3. ATP used to reform acetylcholine for storage in vesicles

Question 31

Importance of AChE

Answer 31

Prevents overstimulation of skeletal muscle cells | Enables acetyl and choline to be recycled

Question 32

What happens in an inhibitory synapse

Answer 32

1. Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane and triggers K+ channels to open 2. Cl- moves in and K+ moves out via facilitated diffusion 3. p.d. becomes more negative - hyperpolarisation

Question 33

Describe structure of neuromuscular junction

Answer 33

Synaptic cleft between presynaptic neurone and a skeletal muscle cell

Question 34

How might drugs increase synaptic transmission

Answer 34

Inhibit AChE | Mimic shape of neurotransmitter