Nervous Coordination and Synapses

Question 1

Describe resting potential

Answer 1

The inside of the axon has a negative charge relative to the outside, as there are more positive ions outside compared to inside).

Question 2

How is a resting potential is established across the axon membrane in
a neurone

Answer 2

Sodium-Potassium ion pump actively transports 3 Sodium ions out of the axon and 2 Potassium ions into the axon. This creates an electrochemical gradient as there is a higher Potassium concentration inside and a higher Sodium ion concentration outside.

Question 3

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

Answer 3

1. Stimulus: Sodium ion channels open, so membrane permeability to Sodium ions increases. This means that Sodium ions diffuse into the axon down the electrochemical gradient (causing depolarisation)
2. Depolarisation: If the threshold potential reached, an action potential is generated, because more voltage-gated sodium ion channels open, so more sodium ions diffuse in rapidly
3. Repolarisation: The voltage-gated sodium ion channels close, and voltage-gated potassium ions channels open, so potassium ions diffuse out of the axon
4. Hyperpolarisation: Potassium ion channels are slow to close, so there’s a slight overshoot – too many Potassium ions diffuse
   out
5. Resting potential: Restored by Sodium Potassium ion pump

Question 4

Draw / label a graph showing an action potential

Answer 4

https://www.moleculardevices.com/applications/patch-clamp-electrophysiology/media_1c6efa9f5f7a50e4dab68c492527445b7fdabda54.jpeg?width=750&format=jpeg&optimize=medium

Question 5

Describe the all-or-nothing principle

Answer 5

For an action potential to be produced, depolarisation must exceed threshold potential. The action potential produced are always same magnitude / size / peak at same potential. Bigger stimuli instead increase frequency of action potentials.

Question 6

Explain how the passage of an action potential along non-myelinated and myelinated axons results in nerve impulses

Answer 6

Non-myelinated axon
* Action potential passes as a wave of depolarisation
* Influx of Sodium ions in one region increases the permeability of adjoining region to Na+ by causing voltage-gated Na+ channels to open
so adjoining region depolarises

Myelinated axon
* Myelination provides electrical insulation
* The axon is depolarised only at the nodes of Ranvier
* This results in saltatory conduction, so there is no need for depolarisation along the whole length of axon

Question 7

Suggest how damage to the myelin sheath can lead to slow responses and/or jerky movement

Answer 7

There will be less / no saltatory conduction, so depolarisation will occur along whole length of axon, so nerve impulses take longer to reach neuromuscular junction and there is a delay in muscle contraction. Also, ions / depolarisation may pass to other neurones, causing wrong muscle fibres to contract

Question 8

What is the refractory period?

Answer 8

It is the time taken to restore the axon to the resting potential when no further action potential can be generated as Sodium ion channels are closed.

Question 9

Explain the importance of the refractory period

Answer 9

Ensures that discrete impulses are produced (action potentials don’t overlap). Also limits the frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus). Higher intensity stimulus causes higher frequency of action potentials, but only up to certain intensity. Also ensures action potentials travel in one direction – can’t be propagated in a refractory region

Question 10

Describe the factors that affect speed of conductance

Answer 10

• Myelination: The more myelination, the faster the conduction as depolarisation only occurs at nodes of ranvier (saltatory conduction), and impulse doesn’t travel along whole length of the axon
• Axon diameter: Bigger diameter means less resistance to flow of ions in cytoplasm
• Temperature: Higher temperatures increase the rate of diffusion of Sodium and Potassium ions as there is more kinetic energy. However, enzymes could denature at certain temperatures.

Question 11

What are cholinergic synapses?

Answer 11

Synapses that use the neurotransmitter acetylcholine (ACh).

Question 12

Describe transmission across a cholinergic synapse

Answer 12

At the pre-synaptic neurone, the depolarisation of the pre-synaptic membrane causes opening of voltage-gated Calcium ion channels, so Calcium ions diffuse into the pre-synaptic knob. This causes vesicles containing ACh to move and fuse with the pre-synaptic membrane,
releasing ACh into the synaptic cleft by exocytosis.

At the post-synaptic neurone, ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane, causing Sodium ion channels to open, so Sodium ions diffuse into the post-synaptic knob causing depolarisation. If the threshold is met, an action potential is initiated

Question 13

Explain how synapses result in unidirectional nerve impulses

Answer 13

Neurotransmitters are only released from pre-synaptic neurone, and the receptors are only on the post-synaptic membrane

Question 14

Explain summation by synapses and why is it important?

Answer 14

Addition of a number of impulses converging on a single post-synaptic neurone, causing rapid buildup of neurotransmitter, so threshold more likely to be reached to generate an action potential. Important because low frequency action potentials release insufficient neurotransmitter to exceed threshold

Question 15

What is spatial summation?

Answer 15

Many pre-synaptic neurones
share one post-synaptic neurone, so they collectively release sufficient neurotransmitter to reach threshold to trigger an
action potential

Question 16

What is temporal summation?

Answer 16

One pre-synaptic neurone
releases neurotransmitter many
times over a short period of time, and so over time, there can be sufficient neurotransmitter to reach threshold to trigger an action potential

Question 17

Describe the structure of a neuromuscular junction

Answer 17

Very similar to a synapse except receptors are on muscle fibre instead of postsynaptic membrane and there are more of them. Also,
muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter

Question 18

Describe inhibition by inhibitory synapses

Answer 18

Inhibitory neurotransmitters hyperpolarise postsynaptic membrane, Cl- channels open, so Cl- diffuse in. K+ channels open, so K+ diffuse out. More Na+ is
required for depolarisation, reduces likelihood of threshold being met and action potential formation
at post-synaptic membranes

Question 19

Compare transmission across cholinergic synapses and neuromuscular
junctions

Answer 19

• In both, transmission is unidirectional
• Cholinergic synapses join neurones to other neurones, whereas Neuromuscular junctions join neurones to muscles
• In Cholinergic synapses, neurotransmitters can be excitatory or inhibitory, whereas in Neuromuscular junctions, they are always excitatory

Question 20

Explain the effect of drugs on a synapse

Answer 20

Some drugs stimulate the nervous system, leading to more action potentials. They have a similar shape to neurotransmitter, so stimulate the release of more neurotransmitter and inhibit enzyme that breaks down neurotransmitter, so Na+ continues to enter

Some drugs inhibit the nervous system, leading to fewer action potentials.. They inhibit the release of neurotransmitter, preventing the opening of calcium ion channels. They block receptors by mimicking shape of neurotransmitter