Nerves

Question 1

State the three types of neurone

Answer 1

a) Motor b) Relay c) Sensory

Question 2

Describe motor neurones

Answer 2

a) Cell body at one end (in brain or spinal cord), motor end plate/neuromuscular junction at other b) Myelinated c) Long

Question 3

Describe relay neurones

Answer 3

a) No myelination b) Very short

Question 4

Describe sensory neurones

Answer 4

a) Cell body is projected (sticks out perpendicular to axon) b) Myelinated c) Very long d) Receptor at one end

Question 5

State the resting potential of neurones

Answer 5

-70mV

Question 6

State the 6 steps of how resting potential is established in neurones

Answer 6

1) At the Na+/K+ pump: 3 Na+ ions OUT for every 2 K+ ions IN. Powered by active transport 2) Therefore high concentration of: K+ ions INSIDE axon and Na+ ions OUTSIDE axon 3) Na+ leakage channel is CLOSED, K+ OPEN 4) Therefore K+ ions diffuse out OUT axon via facilitated diffusion 5) Cell cytoplasm also contains large organic anions –> make the inside more -ve 6) Differential permeability = The membrane is more permeable to K+ than Na+

Question 7

State the 5 stages when a membrane becomes stimulated

Answer 7

1) Stimulus 2) Depolarisation 3) Repolarisation 4) Hyperpolarisation 5) Resting potential

Question 8

Describe stage 1 of ‘When a membrane becomes stimulated’ - Stimulus

Answer 8

Stage 1 - Stimulus a) Na+ voltage-gated channels open b) Membrane becomes more permeable to Na+ c) Na+ ions diffuse into the neurone down the electrochemical gradient

Question 9

Describe stage 2 of ‘When a membrane becomes stimulated’ - Depolarisation

Answer 9

Stage 2 - Depolarisation a) Membrane depolarises if the threshold value is reached (approx -55mV) b) More Na+ ions diffuse rapidly into the neurone

Question 10

Describe stage 3 of ‘When a membrane becomes stimulated’ - Repolarisation

Answer 10

Stage 3 - Repolarisation a) At +40mV, Na+ ion channels CLOSE & K+ ion channels OPEN b) Differential permeability - membrane is more permeable to K+, so they diffuse OUT the neurone, down the concentration gradient

Question 11

Describe stage 4 of ‘When a membrane becomes stimulated’ - Hyperpolarisation

Answer 11

Stage 4 - Hyperpolarisation a) K+ ion channels are too slow to close, so there is an overshoot period where too many K+ ions diffuse out the neurone

Question 12

Describe stage 5 of ‘When a membrane becomes stimulated’ - Resting potential

Answer 12

Stage 5 - Resting Potential a) Ion channels are reset b) Na+/K+ pump returns the membrane to its resting potential until another stimulus arrives

Question 13

The refractory period produces ________ impulses

Answer 13

The refractory period produces DISCRETE impulses

Question 14

The refractory period means that…

(3 points)

Answer 14

a) AP’s don’t overlap, but pass along as discrete (separate) impulses
b) There is a limit to the frequency at which nerve impulses can be transmitted
c) AP’s are unidirectional

Question 15

Describe the refractory period

(2 points)

Answer 15

1) During the refractory period, ion channels are recovering and can’t be opened
2) It acts as a time delay between one AP. and the next

Question 16

Describe the ‘all-or-nothing’ nature of AP’s

Answer 16

1) Once the threshold value is reached, an AP will always fire with the same change in voltage, no matter how big the stimulus is
2) If the threshold isn’t reached, an AP won’t fire
3) A bigger stimulus won’t cause a bigger AP, they will just fire more frequently

Question 17

If a neurone is myelinated, it has ___ ____ wrapped around its axon

Answer 17

If a neurone is myelinated, it has Schwann cells wrapped around its axon

Question 18

The myelin sheath acts as an ___ ___

Answer 18

The myelin sheath acts as an electrical insulator

Question 19

The tiny patches of bare membrane between the Schwann cells are called?

Answer 19

Nodes of Ranvier

Question 20

Na+ ion channels are concentrated at the ___________

Answer 20

Nodes of Ranvier

Question 21

The method of AP propagation in myelinated neurones is called…

Answer 21

Saltatory conduction

Question 22

State the three factors which can affect the speed of conduction of AP’s

Answer 22

1) Myelination
2) Axon diameter
3) Temperature

Question 23

Why do AP’s travel faster when a neurone is myelinated?

Answer 23

As the Na+ channels are concentrated at the nodes of Ranvier, depolarisation only occurs there, meaning it skips from Node to Node (Saltatory conduction)

Question 24

Why do AP’s travel faster when the axon’s diameter is wider?

Answer 24

Bigger axon = less resistance to the flow of ions in the axon’s cytoplasm

Question 25

Why do AP’s travel factor when the temperature is higher?

Answer 25

High temperature = ions diffuse faster. The speed only increases up to around 40*C though - after that, the proteins start to denature and the speed decreases

Question 26

State the two types of summation

Answer 26

1) Spatial summation
2) Temporal summation

Question 27

Describe spatial summation

Answer 27

Many neurones synapsing to one neurone.

(Calculates the sum of all EPSP’s and IPSP’s from MANY neurones summating to one neurone)

Question 28

Describe temporal summation

Answer 28

Two+ nerve impulses arrive in quick succession from the same presynaptic neurone. This makes an AP more likely as more neurotransmitter is released into the synaptic cleft

Question 29

What neurotransmitter does a cholinergic synapse use?

Answer 29

Acetylcholine (ACh)

Question 30

Describe the 9 steps of ACh transmitting a nerve impulse across a cholinergic synapse

Answer 30

1. AP arrives at pre-synaptic neurone
2. Voltage-gated Ca²⁺ ion channels in the pre-synaptic neurone to open
3. Ca²⁺ ions diffuse into the synaptic knob (they’re pumped out afterwards by active transport)
4. Synaptic vesicles to move to, then fuse with the presynaptic membrane
5. Vesicles release ACh into the synaptic cleft via exocytosis
6. ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane
7. This causes Na⁺ ion channels in the postsynaptic neurone to open
8. The influx of Na⁺ ions into the postsynaptic membrane causes depolarisation. An AP on the postsynaptic membrane is generated if the threshold value is reached
9. ACh is removed from the synaptic cleft so the response doesn’t keep happening and is broken down by the enzyme acetylcholinesterase (AChE). The products are re-absorbed by the presynaptic neurone and used to make more ACh

Question 31

Excitatory neurotransmitters _____ the postsynaptic membrane

Answer 31

Excitatory neurotransmitters DEPOLARISE the postsynaptic membrane

Question 32

An AP will only fire from the postsynaptic membrane if the ____ ___ is reached

Answer 32

An AP will only fire from the postsynaptic membrane if the THRESHOLD VALUE is reached

Question 33

ACh is an ______ neurotransmitter at cholinergic synapses in the CNS –> It binds to cholinergic receptors to cause an AP in the postsynaptic membrane and at neuromuscular junctions (motor-end plates)

Answer 33

ACh is an EXCITATORY neurotransmitter at cholinergic synapses in the CNS –> It binds to cholinergic receptors to cause an AP in the postsynaptic membrane and at neuromuscular junctions (motor-end plates)

Question 34

Inhibitory neurotransmitters _______ the postsynaptic membrane

Answer 34

Inhibitory neurotransmitters HYPERPOLARISE the postsynaptic membrane (makes it more negative, preventing it from firing an AP)

Question 35

ACh is an ______ neurotransmitter at cholinergic synapses in the heart –> It binds to cholinergic receptors to cause an AP in the postsynaptic membrane and at neuromuscular junctions (motor-end plates)

Answer 35

ACh is an INHIBITORY neurotransmitter at cholinergic synapses in the heart –> When it binds to receptors here, it can cause K+ ion channels to open on the postsynaptic membrane, hyperpolarising it.

Question 36

What is a neuromuscular junction?

Answer 36

A synapse between a motor neurone and a muscle cell

Question 37

Which neurotransmitter do neuromuscular junctions use?

Answer 37

Acetylcholine (ACh)

Question 38

State four ways in which a neuromuscular junction is different to a cholinergic synapse

Answer 38

Neuromuscular junction =

a) Postsynaptic membrane has many folds which contain AChE
b) Larger number of receptors on post-synaptic membrane
c) ACh is always EXCITATORY at a neuromuscular junction, so when a motor neurone fires an AP, it normally triggers a response in a muscle cell. This isn’t the case for a synapse between two neurones.
d) The receptors are called ‘nicotinic cholinergic receptors’

Question 39

Define an ‘Agonist’

Answer 39

The same shape as the specific neurotransmitter, so they mimic their action. Therefore more receptors are activated

Question 40

Define an ‘Antagonist’

Answer 40

Bind to and therefore block receptors so they can’t be activated by neurotransmitters. Therefore fewer receptors can be activated

Question 41

Describe stimulatory drugs and give an example

Answer 41

Stimulate the release of neurotransmitters from the presynaptic neurone so more receptors are activated e.g. amphetamines

Question 42

Describe inhibitory drugs and give an example

Answer 42

INHIBIT the release of neurotransmitters from the presynaptic neurone, so fewer receptors are activated e.g. alcohol