5.1.3 neuronal comminucation

Question 1

What are the key types of receptors?

Answer 1

Mechanoreceptor-pressure
Chemoreceptor-chemical
Thermoreceptor-temperature
Photoreceptor-light

Question 2

What is a transducer?

Answer 2

A transducer transduce the external energy of the stimulus into the code of the nerve impulse.

Question 3

What is the Pacinian corpuscle?

Answer 3

Pacinian corpuscles are sensory neurones that detect mechanical pressure.

Question 4

What is the gross structure of a neurone?

Answer 4

-cell body: contains nucleus and large amount of endoplasmic reticulum and mitochondria (involved in the production of neurotransmitters).
-Dendrons: short extensions from the cell body, divide into smaller extensions called dendrites. Responsible for transmitting electrical impulses towards the cell body.
-Axons: singular, elongated nerve fibres transmitting impulses away from cell body.

Question 5

What are the three types of neurones?

Answer 5

-Sensory neurone.
-Relay neurone.
-Motor neurone.

Question 6

What is the structure of a Sensory neurone?

Answer 6

They have one dendron and one axon, the cell body is in the middle of the neurone.
They also have myelin sheath and node of Ranvier.

Question 7

What is the structure of a Relay neurone?

Answer 7

They have many short axons and dendrons coming off the cell body.

Question 8

What is the structure of a Motor neurone?

Answer 8

One long axon and many short dendrites. The cell body is on the end and also has a myelin sheath and node of Ranvier.

Question 9

What is the usual pathway for an electrical impulse?

Answer 9

Receptor->Sensory neurone->Relay neurone->Motor neurone->Effector cell.

Question 10

What is the structure of the myelin sheath?

Answer 10

Myelin sheath is made of layers of plasma membrane.
Schwann cells produce layers of membrane by growing around the axon.

Question 11

What is the function of the myelin sheath?

Answer 11

Myelin sheath acts as an insulating layer, and allows myelinated neurones to conduct the electrical impulse at a faster speed.

Question 12

What is node of Ranvier?

Answer 12

Node of Ranvier is a gap between each Schwann cell, creates gaps in myelin sheath.

Question 13

What is the function of node of Ranvier?

Answer 13

Allows electrical impulse to ‘jump’ from one node to another=>allows impulse to be transmitted faster.

Question 14

How does a Pacinian corpuscle convert mechanical pressure into a nervous impulse?

Answer 14

1. In its normal state, the stretch-mediated sodium ion channels in the sensory neurone’s membrane are too narrow to allow sodium ion to pass through.
2. When pressure is applied to the Pacinian corpuscle, the corpuscle changes shape, causing the membrane surrounding its neurone to stretch.
3. When the membrane stretches, the sodium ion channels widen, sodium ions can now diffuse into the neurone.
4. The influx of sodium ions changes the potential of the membrane-it becomes depolarised, results in a generator potential.
5. The generator potential creates an action potential that passes along the sensory neurone.

Question 15

What is a resting potential?

Answer 15

The potential difference across a neurone when it is not transmitting an impulse.

Question 16

What causes the creation of a resting potential?

Answer 16

1. Sodium ions are actively transported out of the axon and potassium ions are actively transported into the axon by a sodium-potassium pump. Three Sodium ions pumped out for every two potassium ions pumped in.
2. As a result there are more sodium ions outside the membrane. Therefore sodium ions diffuse back into the axon down its electrochemical gradient, whereas potassium ions diffuse out the axon.
3. However, most of the gated sodium ion channels are closed, whereas the potassium ion channels are open. There are more positively charged ions outside the axon rather than in.

Question 17

What happens during an action potential?

Answer 17

1. Neurone has a resting potential, some potassium ion channels open but sodium voltage-gated channels closed.
2. Energy of the stimulus triggers some sodium voltage-gated channels to open, making the membrane more permeable to sodium ions.
3. The change in charge causes more sodium ion channels to open allowing more sodium ions to diffuse across axon (positive feedback).
4. When the action potential reaches +40 mV the voltage-gated sodium ion channels close, and potassium voltage-gated channels open.
5. Potassium ions diffuse out the axon, reduces the charge.
6. Initially lots of potassium ions diffuse out and the inside of the axon becomes more negative than resting state (hyperpolarisation).
7. Voltage-gated potassium channels close, sodium-potassium pump causes sodium ions to move out the cell and potassium ions in (repolarised).

Question 18

What is the ‘All-or-nothing’ principle?

Answer 18

An action potential will only be created if the threshold value is reached.
The same size action potential will be reached no matter the size of the stimulus.

Question 19

What is Saltatory conduction?

Answer 19

Depolarisation of the axon membrane only occurs at the nodes of Ranvier (no myelin present)- Na+ ions can pass through protein channels.
Longer localised circuits therefore arise between adjacent nodes.
The action potential ‘jumps’ from one node to another.

Question 20

What are the advantages of Saltatory conduction?

Answer 20

Faster- reduced number of places where channels open and ions move in.
More energy efficient long-term- repolarisation uses ATP in the Na+ pump, so reduces the amount of repolarisation needed.

Question 21

What affects the speed at which an action potential travels?

Answer 21

Axon diameter- the bigger the axon diameter, the faster the impulse is transmitted (less resistance to the flow of ions in the cytoplasm).
Temperature- the higher the temperature, the faster the nerve impulse (ions diffuse faster at higher temperatures).

Question 22

What affects the speed at which an action potential travels?

Answer 22

Axon diameter- the bigger the axon diameter, the faster the impulse is transmitted (less resistance to the flow of ions in the cytoplasm).
Temperature- the higher the temperature, the faster the nerve impulse (ions diffuse faster at higher temperatures).

Question 23

What are excitatory neurotransmitters?

Answer 23

Excitatory neurotransmitters result in the depolarisation of the postsynaptic neurone.
E.g. Acetylcholine

Question 24

What are inhibitory neurotransmitters?

Answer 24

Inhibitory neurotransmitters result in the hyperpolarisation of the postsynaptic membrane. (Prevents action potential being triggered)
E.g. GABA

Question 25

What do synaptic transmission occur as a result of?

Answer 25

The action potential reaches the end of the presynaptic neurone.
Depolarisation of the presynaptic membrane causes calcium ion channels to open.
Calcium ions diffuse into the presynaptic knob- causing synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane.
Neurotransmitter is released into synaptic cleft by exocytosis- they diffuse across the synaptic cleft and bind with specific receptor molecules on the post synaptic membrane- causing sodium ion channels to open.
Sodium ions diffuse into the postsynaptic neurone triggering an action potential.

Question 26

How does transmission occur across a cholinergic synapse?

Answer 26

Depolarisation of the presynaptic membrane causes calcium ion channels to open.
Calcium ions diffuse in causing an influx of calcium ions which cause vesicles carrying acetylcholine to fuse with the presynaptic membrane by exocytosis.
Acetylcholine diffuses across the synaptic cleft to acetylcholine receptors attached to sodium channels on the postsynaptic membrane.
Sodium channels open when acetylcholine binds to receptors, sodium enters postsynaptic knob allowing for depolarisation.
Acetylcholinesterase breaks acetylcholine into acetyl+choline to stop more sodium ions entering- these products enter the presynaptic knob and using ATP become acetylcholine again.

Question 27

What are the roles of synapses?

Answer 27

Ensure impulses are unidirectional- impulses can only travel from one presynaptic neurone to the postsynaptic neurone.
Allow an impulse form one neurone to be transmitted to a number of neurones at multiple synapses.
A number of neurones may feed into the same synapses with a single postsynaptic neurone- stimuli from different receptors interacting to produce a single result.

Question 28

What is summation?

Answer 28

Summation is when the amount of neurotransmitters builds up sufficiently to reach the threshold value then triggers an action potential.

Question 29

What is spatial summation?

Answer 29

Spatial summation occurs when a number of presynaptic neurones connect to one postsynaptic neurone.

Question 30

What is temporal summation?

Answer 30

Temporal summation occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period.