Nervous coordination

Question 1

Describe the general composition of a motor neuron.

Answer 1

Cell body, dendron, axon

Question 2

Describe the structure of a cell body

Answer 2

contains organelles & high proportion of RER.

Question 3

Describe the structure of a dendron.

Answer 3

branch into dendrites which carry impulses towards cell body

Question 4

Describe the structure of an axon.

Answer 4

long, unbranched fibre carries nerve impulses

away from cell body.

Question 5

Describe the additional features of a myelinated

motor neuron.

Answer 5

● 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 6

Name 3 processes Schwann cells are involved in.

Answer 6

● electrical insulation
● phagocytosis
● nerve regeneration

Question 7

How does an action potential pass along an

unmyelinated neuron?

Answer 7

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 8

Explain why myelinated axons conduct impulses

faster than unmyelinated axons

Answer 8

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 9

What is resting potential?

Answer 9

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

Question 10

How is resting potential established?

Answer 10

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 11

Name the stages in generating an action potential.

Answer 11

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

Question 12

What happens during depolarisation?

Answer 12

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 13

What happens during repolarisation?

Answer 13

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 14

What happens during hyperpolarisation?

Answer 14

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 15

Explain the importance of the refractory period.

Answer 15

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

Question 16

What is the ‘all or nothing’ principle?

Answer 16

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

Question 17

Name the factors that affect the speed of conductance.

Answer 17

● Myelin sheath
● Axon diameter
● Temperature

Question 18

How does axon diameter affect the speed of

conductance?

Answer 18

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

Question 19

How does temperature affect speed of conductance?

Answer 19

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 20

Suggest an appropriate statistical test to determine

whether a factor has a significant effect on the speed of conductance.

Answer 20

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

Question 21

Suggest appropriate units for the maximum

frequency of impulse conduction.

Answer 21

Hz

Question 22

How can an organism detect the strength of a

stimulus?

Answer 22

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

Question 23

What is the function of synapses?

Answer 23

● 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 24

Describe the structure of a synapse.

Answer 24

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 25

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

Answer 25

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 26

How do neurotransmitters cross the synaptic cleft?

Answer 26

Via simple diffusion

Question 27

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

Answer 27

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 28

Explain why synaptic transmission is unidirectional.

Answer 28

Only presynaptic neuron contains vesicles of neurotransmitter & only postsynaptic membrane has complementary receptors.
So impulse always travels presynaptic → postsynaptic.

Question 29

Define summation and name the 2 types.

Answer 29

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

Question 30

What is the difference between temporal and spatial summation?

Answer 30

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

Question 31

What are cholinergic synapses?

Answer 31

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 32

What happens to acetylcholine from the synaptic cleft?

Answer 32

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 33

Explain the importance of AChE.

Answer 33

● Prevents overstimulation of skeletal muscle cells.

● Enables acetyl and choline to be recycled.

Question 34

What happens in an inhibitory synapse?

Answer 34

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 35

Describe the structure of a neuromuscular junction.

Answer 35

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

Question 36

How might drugs increase synaptic transmission?

Answer 36

● Inhibit AChE

● Mimic shape of neurotransmitter

Question 37

How might drugs decrease synaptic transmission?

Answer 37

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