6.2 Nervous Coordination

Question 1

Describe the general structure of a motor neuron.

Answer 1

Cell body: contains organelles & 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 the additional features of a myelinated motor neuron.

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 sheath.

Question 3

Name 3 processes Schwann cells are involved in.

Answer 3

● electrical insulation ● phagocytosis
● nerve regeneration

Question 4

Explain why myelinated axons conduct impulses faster than unmyelinated axons.

Answer 4

Saltatory conduction: Impulse ‘jumps’ from one node of Ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator.
So impulse does not travel along whole axon length.

Question 4

How does an action potential pass along an unmyelinated neuron?

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. Meanwhile, the section behind begins to repolarise.
3. Sequential wave of depolarisation.

Question 5

What is resting potential?

Answer 5

Potential difference (voltage) across neuron membrane when not stimulated (-50 to -90 mV, usually about -70 mV in humans).

Question 6

How is resting potential established?

Answer 6

1. Membrane is more permeable to K+ than Na+.
2. Sodium-potassium pump actively transports
   3Na+ out of cell & 2K+ into cell.
   Establishes electrochemical gradient: cell contents more negative than extracellular environment.

Question 7

Name the stages in generating an action potential.

Answer 7

1. Depolarisation
2. Repolarisation
3. Hyperpolarisation
4. Return to resting potential

Question 8

What happens during depolarisation?

Answer 8

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 9

What happens during repolarisation?

Answer 9

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

Question 10

What happens during hyperpolarisation?

Answer 10

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 & sodium-potassium pump re-establishes resting potential.

Question 11

Explain the importance of the refractory period.

Answer 11

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

Question 12

What is the ‘all or nothing’ principle?

Answer 12

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

Question 13

Name the factors that affect the speed of conductance.

Answer 13

● Myelin sheath ● Axon diameter ● Temperature

Question 14

How does axon diameter affect the speed of conductance?

Answer 14

greater diameter = faster
● Less resistance to flow of ions (depolarisation & repolarisation).
● Less ‘leakage’ of ions (easier to maintain
membrane potential).

Question 15

How does temperature affect speed of conductance?

Answer 15

Higher temperature = faster
● Faster rate of diffusion (depolarisation & repolarisation).
● Faster rate of respiration (enzyme-controlled) = more ATP
for active transport to re-establish resting potential. Temperature too high = membrane proteins denature.

Question 16

Suggest an appropriate statistical test to determine whether a factor has a significant effect on the speed of conductance.

Answer 16

Student’s t-test (comparing means of continuous data)

Question 17

Suggest appropriate units for the maximum frequency of impulse conduction.

Answer 17

Hz

Question 18

How can an organism detect the strength of a stimulus?

Answer 18

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

Question 19

What is the function of synapses?

Answer 19

● Electrical impulse cannot travel over junction between neurons.
● Neurotransmitters send impulses between neurons/ from neurons to effectors.
● New impulses can be initiated in several different neurons for multiple simultaneous responses.

Question 20

Describe the structure of a synapse.

Answer 20

Presynaptic neuron ends in synaptic knob: contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter.
synaptic cleft: 20-30 nm gap between neurons.
Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand-gated Na+ channels).

Question 21

Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another.

Answer 21

1. Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca2+ channels to open.
2. Vesicles move towards & fuse with presynaptic membrane.
3. Exocytosis of neurotransmitter into synaptic cleft.

Question 22

How do neurotransmitters cross the synaptic cleft?

Answer 22

Via simple diffusion

Question 23

Outline what happens in the postsynaptic neuron when an action potential is transmitted from one neuron to another.

Answer 23

1. Neurotransmitter binds to specific receptor on postsynaptic membrane.
2. Ligand-gated 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 neuron contains vesicles of neurotransmitter & only postsynaptic membrane has complementary receptors.
So impulse always travels presynaptic → postsynaptic.

Question 25

Define summation and name the 2 types.

Answer 25

Neurotransmitter from several sub-threshold impulses accumulates to generate action potential:
● temporal summation
● spatial summation
NB no summation at neuromuscular junctions.

Question 26

What is the difference between temporal and spatial summation?

Answer 26

Temporal: one presynaptic neuron releases neurotransmitter several times in quick succession.
Spatial: multiple presynaptic neurons release neurotransmitter.

Question 27

What are cholinergic synapses?

Answer 27

Use acetylcholine as primary neurotransmitter. Excitatory or inhibitory. Located at:
● Motor end plate (muscle contraction).
● Preganglionic neurons (excitation).
● Parasympathetic postganglionic neurons
(inhibition e.g. of heart or breathing rate).

Question 28

Explain the importance of AChE.

Answer 28

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

Question 28

What happens to acetylcholine from the synaptic cleft?

Answer 28

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

Question 29

What happens in an inhibitory synapse?

Answer 29

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

Question 30

Describe the structure of a neuromuscular junction.

Answer 30

Synaptic cleft between a presynaptic neuron and a skeletal muscle cell.

Question 31

Contrast a cholinergic synapse and a neuromuscular junction.

Answer 31

check

Question 32

How might drugs increase synaptic transmission?

Answer 32

● InhibitAChE
● Mimic shape of neurotransmitter

Question 33

How might drugs decrease synaptic transmission?

Answer 33

● Inhibit release of neurotransmitter.
● Decrease permeability of postsynaptic
membrane to ions.
● Hyperpolarise postsynaptic
membrane.