Neurophysiology and Action Potentials

Question 1

What is chemical ion channel gating?

Answer 1

A chemical signalling molecule binds to receptive site on channel resulting in change in shape of channel (i.e. opening it). Indicated through neurotransmitters e.g. ACh

Question 2

What is voltage ion channel gating?

Answer 2

Channels are gated by the voltage across them. Once inactivated, cannot open again until the membrane is depolarised (until potassium leaves cell)

Question 3

What is mechanical ion channel gating?

Answer 3

Channels are gated in response to physical/mechanical forces. Crucial in triggering signals in response to touch & pressure

Question 4

Neuronal distribution in relation to ion channels

Answer 4

Dendrites are where very large no. of chemically gated ion channels are found. Axon is where the voltage gated Na+ and K+ channels are found. Axon terminal is where voltage gated Ca2+ channels are

Question 5

What are local potentials?

Answer 5

Interaction of a neurotransmitter with a receptor on the neuronal cell that results in a localised change in voltage across a membrane

Question 6

What is an excitatory local potential ?

Answer 6

Excitatory postsynaptic potentials (EPSP) bring the neuron’s potential closer to its threshold

Question 7

What is an inhibitory local potential?

Answer 7

Inhibitory postsynaptic potentials (IPSP) change the charge across the membrane to be further from the threshold

Question 8

What does it mean by ‘local potentials are not actively propagated’?

Answer 8

The magnitude of a local potential decreases with distance away from the sites of its initiation

Question 9

What is summation of local potentials?

Answer 9

The effect of local potentials on a cell membrane potential is summed over both time and space

Question 10

What is the initial segment of an axon?

Answer 10

The point where the axon joins the neuron cell body. It is the most sensitive to change in voltage and has a high density of volatile gated Na channels. Action Potential is generated here

Question 11

What is required for an action potential to be generated?

Answer 11

The net voltage change at this point must exceed a minimal depolarisation, the threshold

Question 12

What is the action potential threshold?

Answer 12

Depolarising of local potentials may result in opening of voltage gated Na channels which drives further depolarisation and a sudden large increase in Na influx will occur

Question 13

What is an action potential?

Answer 13

A very brief increase in the permeability of the membrane to Na followed by termination of Na permeability and brief increase in K permeability

Question 14

What is an absolute refractory period in an action potential?

Answer 14

Absolute refractory period means no matter how large the stimulus another AP cannot be generated

Question 15

What is an relative refractory period in an action potential?

Answer 15

An AP can be generated but only in response to a very large stimulus, causing by VG Na channel inactivation, prevents AP propagating backwards

Question 16

Propagation of an unmyelinated axon

Answer 16

The membrane potential at this site depolarises to +30mV, as Na ions enter a graded depolarisation brings membrane to threshold, AP develops in segment 2 and initial segment begins to repolarise. Depolarisation of segment 2 brings segment 3 to threshold and initial segment reaches absolute refractory period

Question 17

What is conduction velocity in action potentials?

Answer 17

APs are transmitted along unmyelinted axons relatively slowly. Slow AP conduction is not adequate for all human needs so myelin increases AP conduction velocity

Question 18

Propagation of myelinated axons

Answer 18

AP develops at initial segment, local current produces a graded depolarisation that bring node 1 to threshold, initial segment begins to repolarise and begins to refract, local current produces graded depolarisation and brings node 2 to threshold