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 the 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+ raises membrane to threshold
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 fibres carries nerve impulses away from cell body

Question 5

Describe the structure and function of a sensory neurone

Answer 5

Usually uni polar
Transmits impulses from receptors to CNS

Question 6

Describe the structure and function of a relay neuron

Answer 6

Usually bipolar
Transmits impulses between neurons

Question 7

Describe the structure and function of a motor neuron

Answer 7

Usually multipolar
Transmits impulses from relay neurons in the CNS to effectors

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 process 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 an electrical insulator
So impulse does not travel along whole length

Question 11

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

Answer 11

Myelinated: Most neurons in central and 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 across a neuron membrane when not stimulated (-50V to -90V, usually about -70V in humans)

Question 13

How is resting potential establish

Answer 13

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

Question 14

Name the stages in creating 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 of the 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 7 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 the hyperpolarised section of the 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 the 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 end in synaptic knob: contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter
Synaptic cleft: 20-30 nm gap between neurons
Postsynaptic neuron: has complimentary 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 and fuse with presynaptic membrane
   3.Exocytosis of neurotransmitter into synaptic cleft

Question 23

How do neurotransmitters travel across 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 open 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 so no action potential is generated

Question 26

Define summation and name the two types

Answer 26

Neurotransmitter from several sub-threshold impulses accumulates to generate action potential
- Temporal summation
- Spatial summation
NB no summation at neuromuscular joints

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 and Choline diffuse back into presynaptic membrane
3. ATP is used to reform acetylcholine for storage in vesicles.