5.1.3 Neuronal communication

Question 1

What are some features of all sensory receptors ?

Answer 1

They act as energy transducers which establish a generator potential
They respond to specific stimuli

Question 2

Describe the basic structure of a Pacinian corpuscle.

Answer 2

A single nerve fibre surrounded by layers of connective tissue, which are separated by a 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 a sensory neurone.

Question 4

Describe the features of all neurones.

Answer 4

Cell body: contains organelles (lots of mitochondria) and high portion of RER
Dendrons: branch into dendrites which carry impulses toward cell body
Axon: long, unbranched fibre carries nerve impulses away from cell body.

Question 5

Describe the structure and function of a sensory neurone.

Answer 5

Usually unipolar
They transmit impulses from receptors cells to CNS

Contain: Dendrites, a long axon with a cell body in the middle, and axon terminals.

Question 6

Describe the structure and function of a relay neurone.

Answer 6

Usually bipolar
They transmit impulses between neurones

Contain: highly branched dendrites, cell body and highly branched axon terminals.

Question 7

Describe the structure and function of a motor neurone.

Answer 7

Usually multipolar
They transmit impulses from relay neurones in the CNS to effectors

Question 8

Describe the additional features of a myelinated neurone.

Answer 8

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

Question 9

!

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 the impulse does not travel down the whole axon length.

Question 11

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

Answer 11

Myelinated: most neurones in CNS and peripheral nervous system
Non-myelinated: group C nerve fibres involved in transmitting some pain / temperature.

Question 12

What is resting potential ?

Answer 12

Potential difference across a neurone membrane when not stimulated
Usually ~ -70mV 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 the cell and 2K+ into the cell
3. This establishes an electrochemical gradient: cell contents are 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 the 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 the cell down their electrochemical gradient
3. Potential difference across membrane becomes more negative.

Question 17

What happens during hyper polarisation ?

Answer 17

1. ‘Overshoot’ when K+ ions diffuse out, potential difference becomes more negative than resting potential
2. Refractory period - no stimulus is large enough to raise membrane potential threshold
3. Voltage gated K+ ion channels close and 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 impulses
-Ensures discrete impulses
-Limits frequency of impulse transmission; larger stimuli have higher frequency.

Question 19

Why is the frequency of impulse transmission important ?

Answer 19

-It enables the organism to distinguish the size of the 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 impulses cannot travel across the cleft between neurones
-Neurotransmitters send action potentials between neurones /from neurones to effectors for an excitatory / inhibitory response
-Summation of sub-threshold impulses
-New impulses can be initiated from several different neurones for multiple simultaneous responses.

Question 21

Describe the structure of a synapse.

Answer 21

• Pre synaptic neurone ends in a synaptic bulb which contains lots of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter (eg acetylcholine)
  -Synaptic cleft: 20-30nm gap between neurones
  -Post synaptic neurone has complementary receptors to pre synaptic neurotransmitter (ligand-gated Na+ channels).

Question 22

What happens in the presynaptic bulb when an action potential is transmitted between neurones ?

Answer 22

1. A wave of depolarisation travels down presynaptic neurone, 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 cross the synaptic cleft ?

Answer 23

Simple diffusion

Question 24

What happens in the postsynaptic neurone when an action potential is transmitter between neurones ?

Answer 24

1. Neurotransmitter binds to specific receptor on postsynaptic neurone
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 system ?

Answer 25

1. Neurotransmitter binds to and opens Cl- channels on post synaptic membrane and triggers K+ channels to open
2. Cl- moves in and K+ moves out of the cell via facilitated diffusion
3. Potential difference becomes more negative: hyper-polarisation so no more action potential is generated.

Question 26

Define summation and give the two types

Answer 26

Neurotransmitters from servers sub-threshold impulses accumulates to generate action potential
•Temporal summation
•Spatial summation

Question 27

What is the difference between temporal and spatial summation ?

Answer 27

Temporal: one presynaptic neurone releases neurotransmitter several times in quick succession
Spatial: multiple presynaptic neurones release neurotransmitters.

Question 28

What are the cholinergic synapses ?

Answer 28

•Use acetylcholine and primary neurotransmitter
•Excitatory or inhibitory.

Question 29

What happens to acetylcholine at the synaptic cleft ?