PACK 25: NERVES AND IMPULSES

Question 1

What is a neurone?

Answer 1

A specialised cell adapted to carrying nerve impulses quickly from one part of the body to another.

Question 2

What components are found in a typical motor neurone?

Answer 2

• Cell body
• Dendrites
• Axon
• Schwann cells/myelin sheath
• Axon terminals
• Nodes of Ranvier

Question 3

What is the role of Schwann cells?

Answer 3

They wrap around the axon, providing insulation and making up the myelin sheath.

Question 4

What is resting potential?

Answer 4

The electrical potential difference across the axon membrane when the neuron is not transmitting an impulse, typically -65 mV to -70 mV.

Question 5

How is resting potential maintained?

Answer 5

Through the movement of sodium ions (Na+) and potassium ions (K+) in and out of the axon via transport proteins.

Question 6

What are ‘Leak’ channels and ‘Gated’ channels?

Answer 6

‘Leak’ channels allow ions to passively flow, while ‘Gated’ channels open in response to specific signals to allow ion movement.

Question 7

What is the sodium-potassium pump?

Answer 7

A transport protein that actively moves 3 Na+ out of the axon and 2 K+ into the axon.

Question 8

What is an action potential?

Answer 8

A temporary reversal of charges across the axon membrane that occurs when a neuron is stimulated.

Question 9

What is the all or nothing principle?

Answer 9

An action potential is generated only if the initial stimulus exceeds a specific threshold value.

Question 10

What occurs during hyperpolarization?

Answer 10

The axon interior becomes more negative than normal, dropping to about -90 mV.

Question 11

What is the refractory period?

Answer 11

A short time after an action potential during which the neuron cannot generate another action potential.

Question 12

What is saltatory conduction?

Answer 12

The process where depolarization occurs only at the nodes of Ranvier, allowing the impulse to ‘jump’ from node to node.

Question 13

What is a synapse?

Answer 13

A gap between the axon of one neurone and a dendrite of another where neurotransmitters transmit signals.

Question 14

What neurotransmitter is used in a cholinergic synapse?

Answer 14

Acetylcholine (ACh).

Question 15

What happens when an action potential arrives at the synaptic knob?

Answer 15

Calcium ion channels open, allowing calcium ions to diffuse in, triggering the release of ACh.

Question 16

What is summation in synapses?

Answer 16

The process where multiple impulses combine to generate enough neurotransmitter to trigger an action potential.

Question 17

What are the two types of summation?

Answer 17

• Spatial summation
• Temporal summation

Question 18

What is the effect of inhibitory synapses?

Answer 18

They make it less likely for the postsynaptic membrane to depolarize, reducing the likelihood of an action potential.

Question 19

What is the effect of recreational drugs on synapses?

Answer 19

They can stimulate or inhibit the nervous system by altering neurotransmitter activity.

Question 20

How does nicotine affect the nervous system?

Answer 20

It binds to ACh receptors, opening sodium channels and increasing action potential generation.

Question 21

What effect does atropine have at a neuromuscular junction?

Answer 21

It blocks ACh receptors, preventing muscle contraction and causing paralysis.

Question 22

What is the role of GABA?

Answer 22

It inhibits the formation of action potentials in postsynaptic neurons.

Question 23

What effect does atropine have on the muscle at a neuromuscular junction?

Answer 23

Atropine binds to ACh receptors in the postsynaptic membrane, blocking ACh from binding and preventing depolarization.

This results in no action potentials being generated in the muscle cell membrane, causing paralysis.

Question 24

What neurotransmitter inhibits the formation of action potentials?

Answer 24

GABA

GABA acts on postsynaptic neurones to inhibit action potential formation.

Question 25

What condition can result from insufficient GABA activity in the brain?

Answer 25

Epilepsy ## Footnote Insufficient GABA leads to increased neuronal activity, contributing to seizures.

Question 26

What enzyme affects GABA levels on the postsynaptic membrane?

Answer 26

An enzyme that breaks down GABA ## Footnote This enzyme's activity can influence the availability of GABA for neurotransmission.

Question 27

What is Vigabatrin used to treat?

Answer 27

Epilepsy ## Footnote Vigabatrin is designed to increase GABA levels in the brain.

Question 28

How might Vigabatrin be effective in treating epilepsy?

Answer 28

It may act as a competitive inhibitor for the enzyme that breaks down GABA, leading to more GABA available. ## Footnote This increases GABA's ability to inhibit action potentials.

Question 29

True or False: Vigabatrin can mimic GABA's action on postsynaptic neurones.

Answer 29

True ## Footnote Vigabatrin may bind to GABA receptors and inhibit action potential initiation.

Question 30

Fill in the blank: GABA is a _______ neurotransmitter.

Answer 30

inhibitory ## Footnote It inhibits action potentials in postsynaptic neurones.

Question 31

What happens to muscle contraction when atropine is present?

Answer 31

Muscle contraction is prevented ## Footnote This is due to the lack of depolarization and action potentials in muscle cells.

Question 32

Explain what occurs during resting potential

Answer 32

· Active transport of 3Na+ out of axon and 2K+ into axon by sodium/potassium pumps · Resulting in more K+ inside the axon than out (and more Na+ outside than in) · The membrane is more permeable to K+ ions than to Na+ ions · K+ ions therefore diffuse back out faster than Na+ ions diffuse back in · Leading to a pd of -65 mV inside compared to out · Membrane is said to be POLARISED

Question 33

Explain what happens to generate an action potential

Answer 33

1. A stimulus causes the gated sodium ion channels to open, making the membrane more permeable to sodium ions. Na+ ions rapidly diffuse into the axon, causing depolarisation. The inside of the membrane becomes charged to +40mV. The gated sodium ion channels close, meaning the axon is again almost impermeable to Na+ ions. 2. The gated potassium ion channels open (i.e. the axon’s permeability to potassium ions increases) and K+ ions diffuse rapidly out of the axon. (repolarisation). 3. So many potassium ions diffuse out of the axon that they cause a more negative potential difference than normal, dropping the axon interior down to about -90mV. This is known as hyperpolarisation and causes the refractory period (see later). 4. The gated potassium ion channels close and the sodium-potassium pump restores the concentration gradients for Na+ and K+. This returns the resting potential of the axon to -65mV. The resting permeability of the axon is restored. The whole process lasts 2-3 milliseconds.

Question 34

Explain what is meant by the term's synaptic cleft, presynaptic neurone, and synaptic knob

Answer 34

Individual neurones do not touch; they are separated from each other by gap called the synaptic cleft. The neurone which releases the transmitter is called the presynaptic neurone. The end of the axon swells to form the synaptic knob (or bouton). The knob contains many mitochondria.

Question 35

Give a rough explanation of what happens during the movement of a neurotransmitter across a synapse

Answer 35

Once manufactured, the transmitter is stored in synaptic vesicles which, when stimulated, will fuse with the presynaptic membrane and release the neurotransmitter into the cleft. Once there, it will diffuse across to the postsynaptic neurone which has specific receptor molecules to bind to the transmitter.

Question 36

Explain the process of Ach being transmitted across a synapse

Answer 36

1. Arrival of an action potential at the synaptic knob opens calcium ion channels in the membrane which allows calcium ions (Ca2+) to diffuse in. 2. Influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane and release ACh by exocytosis into the synaptic cleft. 3. ACh diffuses across the synaptic cleft and binds to receptors on the sodium ion channels in the postsynaptic membrane. 4. This causes the sodium ion channels to open and sodium ions diffuse rapidly into the post synaptic cell, causing depolarisation. If the threshold is reached an action potential will be initiated. 5. To prevent the initiation of more action potentials in the postsynaptic membrane, by the continued presence of ACh, an enzyme, acetylcholinesterase, hydrolyses ACh. 6. The products diffuse back across the cleft and can be reabsorbed into the presynaptic neurone. ACh is re-formed and repackaged into vesicles

Question 37

What is spatial summation

Answer 37

Spatial summation – in which a number of different presynaptic neurones together release enough transmitter at the same time to cause enough depolarisation to exceed the threshold of the post-synaptic neurone and trigger an action potential e.g. rod cells in the eye (see Receptor pack.

Question 38

What is temporal summation

Answer 38

Temporal summation – in which a single presynaptic neurone releases small amounts of neurotransmitter many times in a short period (several impulses arrive in quick succession), which may add up to cause enough depolarisation to exceed threshold and trigger a new action potential in the post synaptic neurone.

Question 39

What are inhibitory synapses

Answer 39

Inhibition –On the postsynaptic membranes of some neurones there are chloride ion (Cl-) channels which can be opened when activated by a certain neurotransmitter. This causes chloride ions to flood into the postsynaptic knob and make it more negative than it normally is at rest (hyperpolarisation). This in turn makes it less likely that the membrane can depolarise and so a new action potential cannot be generated. They are known as inhibitory synapses.