Nerves

Question 1

What is a neurone?

Answer 1

Specialised cells adapted to rapidly carry out nerve impulses from one part of the body to another.

Question 2

What are the 6 main structures of a myelinated neurone?

Answer 2

1. A cell body - contains a nucleus and lots of endoplasmic reticulum
2. Dendrons- Small branched fibres that carry the impulse towards the cell body
3. Axon- Single long fibre that carries impulse away from cell body
4. Schwaan cells- surround the axon and protect it and provide electrical insulation
5. Myelin sheath- covers the axon and is made up of layers of Schwaan cells - rich is myelin and allow impulses to travel faster
6. Nodes of ranvier- gaps between adjacent Schwaan cells where there is no myelin sheath - allows impulse to jump.

Question 3

What is a resting potential?

Answer 3

When the inside of the axon is negatively charged relative to the outside of the axon. In this state the membrane is known as polarised.

Question 4

How is resting potential controlled?

Answer 4

1. Phospholipid bilayer- prevents Na+ and K+ ions diffusing across it
2. Intrinsic proteins- allow ions in when needed
3. Sodium-potassium pump- actively transport sodium and potassium

Question 5

How is the resting potential maintained?

Answer 5

• Sodium ions are actively transported OUT the axon by the sodium potassium pump
• Potassium ions are actively transported INTO the axon by the S-P pump.
• More sodium ions are actively transported - so more potassium inside cell (this creates a chemical gradient)
• Sodium ions begin to diffuse naturally into the axon and potassium ions back out
• However most of the gates that let sodium back in are closed
• As a result the axon membrane is more permeable to potassium and potassium diffuses out faster than sodium diffuses back in- this further increases the potential difference.
• The outside of the membrane becomes more and more positive which makes the potassium want to move back into the axon because they are positive so are attracted to the negativity of the axon.
• An equilibrium is established and there is no net movement of ions in or out of the axon.

Question 6

What is an action potential?

Answer 6

When a stimulus is received by a receptor its energy causes a temporary reversal of the charges on the axon membrane - it becomes around a positive charge of +40mV. This is depolarised.

Question 7

How does an action potential occur?

Answer 7

• At resting potential some of the potassium voltage gated channels are open but the sodium ones are closed.
• The energy of the stimulus causes some of the sodium voltage gated channels to open and sodium diffuses in, being positively charged they cause a reversal in charge
• As more sodium diffuses in more sodium channels open meaning a more influx of sodium.
• Once the charge of around +40mV is reached the sodium channels close and the potassium gates open.
• Potassium ions move out causing repolarisation of the axon and the resting potential of the axon being -65mV is resumed once the potassium pump closes and the S-P pumps resume to their normal activities.

Question 8

What is the passage of an unmyelinated action potential described as?

Answer 8

A Mexican wave polarised-depolarised-repolarised

Question 9

How does the passage of an action potential along a myelinated axon work?

Answer 9

The fatty sheath of myelin around the axon acts as a electrical insulator preventing from action potentials from forming. At intervals there are breaks in the sheath called the nodes of Ranvier- the action potential jumps from node to node (saltatory conduction)
THIS IS MUCH FASTER THAN AN UNMYELINATED AXON

Question 10

What factors affect the speed at which an action potential travels?

Answer 10

• Myelin Sheath- This increases the speed of conductance
• The diameter of the axon- The greater the diameter, the faster the conductance
• Temperature- Affects the rate of diffusion of ions and therefore the higher the temperature the faster the nerve impulse - works by enzymes so enzymes work best at high temperatures until they denature

Question 11

What is the refractory period?

Answer 11

When an further action potential can be created due to the closing of sodium-voltage gated channels.

Question 12

What are the refractory periods three purposes?

Answer 12

1. Ensures an action potential is propagated in one direction only- can only pass from an active region to a resting region, cant pass through an refractory period.
2. Produces discrete impulses- due to the refractory periods it means that an action potential cant be formed immediately behind an one, this ensures that action potentials are separated from one another.
3. It limits the number of action potentials- As action potentials are separated from one another this limits the number of action potentials that can pass along an axon in a given time

Question 13

What is the all or nothing principle?

Answer 13

There is a certain stimulus called the threshold value which will trigger an action potential- if this doesn’t happen then there is no action potential.

Question 14

How can an organism perceive the size of a stimulus?

Answer 14

• The number of impulses passing in a given time

- By having different neurones with different threshold values the brain can determine the size of them.

Question 15

What is the part that separates the neurones from each other?

Answer 15

Synaptic cleft

Question 16

What is the neurone that releases the neurotransmitter?

Answer 16

Presynaptic neurone

Question 17

Where is the neurotransmitter stored?

Answer 17

Synaptic vesicles

Question 18

What is the function of a synapse?

Answer 18

Transmit impulses from one neurone to another

Question 19

Where is the neurotransmitter made?

Answer 19

In the presynaptic neurone

Question 20

What happens when an action potential reaches the synaptic knob?

Answer 20

The neurotransmitter is released into the synapse from the synaptic vesicles

Question 21

What happens when the neurotransmitter diffuses across the synapse?

Answer 21

The neurotransmitter binds with the receptor molecules and sets up a new action potential in the post-synaptic neurone

Question 22

What are the main features of a synapse?

Answer 22

• Unidirectionality
• Summation
• Inhibition

Question 23

What is unidirectionality?

Answer 23

Synapses can only pass impulses in one direction from the presynaptic neurone to the postsynaptic neurone - this means they act like a valve

Question 24

What is summation?

Answer 24

When low frequency action potentials cause a build up of neurotransmitter in the synapse so that an action potential has to be triggered- this can be done in two ways

Question 25

What are the two types of summation?

Answer 25

Spatial and temporal

Question 26

What is spatial summation?

Answer 26

A number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the post-synaptic neurone, this then triggers an action potential

Question 27

What is temporal summation?

Answer 27

A single presynaptic neurone releases neurotransmitter many times over a short period and if it exceeds the threshold value of the post synaptic neurone then an action potential is triggered

Question 28

What is an inhibitory synapse?

Answer 28

On the post-synaptic membrane of some synapses the chloride channels can be made to be open which causes an inward leak of chloride ions which make the synapse even more negative at resting potential, this creates hyperpolarisation and makes it less likely that a new action potential will be created.

Question 29

What is a cholinergic synapse?

Answer 29

A synapse where the neurotransmitter is acetylcholine- these are common in the central nervous system and neuromuscular junctions.

Question 30

How does a cholinergic response occur?

Answer 30

1. The arrival of an action potential at the end of a presynaptic neurone causes calcium ion channels to open and calcium enters the synaptic knob.
2. The influx of calcium causes synaptic vesicles to fuse with the presynaptic membrane and this releases acetylcholine into the synaptic cleft.
3. Acetylcholine molecules fuse with receptor sites on the sodium channel in the membrane of the postsynaptic neurone which causes the sodium ion channels to open allowing sodium to diffuse into the synaptic cleft along a concentration gradient
4. The influx of sodium ions generates a new action potential in the postsynaptic neurone.
5. Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid which diffuse back across the synaptic cleft into the presynaptic neurone (recycling) The continual breakdown of acetylcholine also prevents it from generating another action potential.
6. ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine and this is stored in the synaptic vesicles for future use. Sodium ion channels close in the absence of acetylcholine

Question 31

How do drugs act on synapses?

Answer 31

• Stimulate the nervous system by creating more action potentials in the postsynaptic neurones (may inhibit the enzyme that breaks down the neurotransmitter)
• Inhibit the nervous system by creating fewer action potentials in post-synaptic neurones (inhibit the release of the neurotransmitter or blocking the receptors on sodium channels)

Question 32

What will a drug do if it inhibits an excitatory neurotransmitter?

Answer 32

Reduce a particular effect

Question 33

What will a drug do if it inhibits an inhibitory neurotransmitter?

Answer 33

Enhances a particular effect