Neuronal Communication

Question 1

What features are common to all sensory receptors?

Answer 1

● Act as energy transducers which establish a generator potential.
● Respond to specific stimuli.

Question 2

Describe the basic structure of a Pacinian corpuscle.

Answer 2

Single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule.
Stretch-mediated Na+ channels on plasma membrane.
Capillary runs along base layer of tissue.

Question 3

What stimulus does a Pacinian corpuscle respond to? How?

Answer 3

1. Pressure deforms membrane, causing stretch-mediated Na+ ion channels to open.
2. If influx of Na+ raises membrane to threshold potential, a generator potential is produced.
3. Action potential moves along sensory neuron.

Question 4

Describe the features of all neurons.

Answer 4

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

Question 5

Describe the structure and function of a sensory neuron.

Answer 5

IMAGE 29

Question 6

Describe the structure and function of a relay neuron.

Answer 6

IMAGE 30

Question 7

Describe the structure and function of a motor neuron.

Answer 7

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Question 8

Describe the additional features of a myelinated neuron.

Answer 8

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 9

Name 3 processes Schwann cells are involved in.

Answer 9

● electrical insulation ● phagocytosis

● nerve regeneration

Question 10

Explain why myelinated axons conduct impulses faster than unmyelinated axons.

Answer 10

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 11

Where are myelinated and non-myelinated neurons found in the body?

Answer 11

Myelinated: most neurons in central & peripheral nervous systems e.g. those involved in spinal reflex.
Non-myelinated: group C nerve fibres
involved in transmitting secondary pain.

Question 12

What is resting potential?

Answer 12

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

Question 13

How is resting potential established?

Answer 13

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 14

Name the stages in generating an action potential.

Answer 14

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

Question 15

What happens during depolarisation?

Answer 15

1. Stimulus → facilitated diffusion of Na+ 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. (positive feedback
   mechanism).
4. Significant influx of Na+ ions reverses p.d. to +40mV.

Question 16

What happens during repolarisation?

Answer 16

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 17

What happens during hyperpolarisation?

Answer 17

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 18

Explain the importance of the refractory period.

Answer 18

No action potential can be generated in hyperpolarised sections of membrane.
● Ensures unidirectional impulse.
● Ensures discrete impulses.
● Limits frequency of impulse transmission;
larger stimuli have higher frequency.

Question 19

Why is the frequency of impulse transmission significant?

Answer 19

Enables organism to distinguish size of stimulus although all action potentials have same magnitude.
Larger stimuli result in higher frequency of transmission since they overcome hyperpolarisation more quickly.

Question 20

What is the function of synapses?

Answer 20

● Electrical impulse cannot cross junction.
● Neurotransmitters send impulses between neurons/
from neurons to effectors for excitatory or inhibitory
response.
● Summation of sub-threshold impulses.
● New impulses can be initiated in several different
neurons for multiple simultaneous responses.

Question 21

Describe the structure of a synapse.

Answer 21

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 22

What happens in the presynaptic neuron when an action potential is transmitted between neurons?

Answer 22

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 23

How do neurotransmitters cross the synaptic cleft

Answer 23

simple diffusion

Question 24

What happens in the postsynaptic neuron when an action potential is transmitted between neurons?

Answer 24

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 25

What happens in an inhibitory synapse?

Answer 25

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 so no action potential is generated.

Question 26

Define summation and name the 2 types.

Answer 26

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

Question 27

What is the difference between temporal and spatial summation?

Answer 27

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

Question 28

What are cholinergic synapses?

Answer 28

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 29

What happens to acetylcholine from the synaptic cleft?

Answer 29

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.