Neurophysiology Of The Action Potential

Question 1

What causes the resting membrane potential

Answer 1

A high concentration of Na+ and a low concentration of K+ in the extra cellular fluid
A low concentration of Na+ and a high concentration of K+ in the intracellular fluid
Phospholipid bilayer not penetrable by charged particles
Diffusion and electrostatic forces drive Na+ into cell

Question 2

What is chemical ion channel gating

Answer 2

A chemical signalling molecule (eg neurotransmitters) binds to a receptive site on the ion channel (integral membrane protein), typically resulting in shape change of channel (opening it). Ions are free to move through membrane driven by electrochemical gradient. The signalling molecule unbinds, ion Chanel changes shape, membrane permeability is reduced

Question 3

What is voltage ion channel gating

Answer 3

Common in areas of excitable membranes. Crucial in generation and conduction of action potentials and synaptic transmission. Channels are gated by the voltage across them. May be opened by a change in voltage, then pass to an inactivated state state so they can not re open again until the membrane has been repolarised

Question 4

What is mechanical ion channel gating

Answer 4

Channels are gated in response to a physical/mechanical force. Crucial in triggering signals in response to tough and pressure

Question 5

What type of ion channels are found on the neuron cell

Answer 5

Chemically gated ion channels are located on the dendrites/cell body
Voltage gated Na+ and K+ channels are located from the axon initial segment, down to the axon terminals

Question 6

What are local, graded potentials

Answer 6

Interaction of neurotransmitter on neuron cell results in localised change in voltage across membrane. Magnitude of change is related to stimulus strength. Hence referred to as local and graded

Question 7

What are inhibitory and excitatory potentials

Answer 7

Excitatory post synaptic potentials create a local depolarisation in the membrane in the post synaptic neuron and bring it closer to threshold.
Inhibitory post synaptic potentials create a local hyper polarisation in the membrane in the post synaptic neuron and move it further away from threshold

Question 8

What is meant by local potentials not being actively propagated

Answer 8

The magnitude of a local potential decreases with distance away from the site of its initiation. The magnitude of the local potential will also decrease with time as high local Na+ conc dissipates

Question 9

What is meant my local potential summation

Answer 9

The effect of local potentials on a cell membrane potential is summed over both time (temporal summation) and space (spatial summation) and space (spatial summation)

Question 10

What is the initial segment of a neuron

Answer 10

the point where the axon joins the cell body. Has high density of voltage gated Na+ channels (hence is most sensitive to voltage change). Is the site where the action potential is generated. For action potential to be generated, net voltage change at this point must exceed Minaj depolarisation (10mV ish), I.e. the Threshold

Question 11

Explain how threshold is reached

Answer 11

Depolarising local potentials may result in opening of voltage gated Na channels
Na+ channel opening drives further depolarisation
If sufficient Na+ channels open (at initial segment) the depolarisation reaches a point at which large numbers of channels open, resulting in sudden increase in Na+ influx - Threshold

Question 12

What is an action potential

Answer 12

A reversal of trans membrane voltage that is completed in 2-3ms

Question 13

What is an action potential in terms of Na and K

Answer 13

A very brief increase in permeability of the membrane to Na (inside of cell becomes positive) followed by termination of Na permeability and brief increase in K permeability (restores negate inside)

Question 14

What are the 5 steps of an action potential
DDIMK

Answer 14

1: Depolarising local potentials result in opening of voltage gated Na channels such that threshold is reached
2: Depolarisation triggers more voltage gated Na channels to open, causing rapid depolarisation
3: Inactivation of Na channels, activation of K channels, rapid repolarisation
4: Membrane potential is close to resting potential, Na channels close and reactivates
5: K channels close, cell returns to resting potential

Question 15

What is the absolute refractory period

Answer 15

A period during which, no matter how large the stimulus, another action potential can not be generated
Due to Na channel being inactivated - no Na flow hence no depolarising/action potential

Question 16

What is the relative refractory period

Answer 16

The time during which. An action potential can be generated, but only in response to a very large stimulus
Needs to be large as many voltage gated Na channels not read yet

Question 17

What are refractory periods

Answer 17

Periods preventing, or requiring larger stimulus for action potential to arise.
Caused by voltage gated Na channel inactivation
Prevents action potential propagating “backwards”

Question 18

What are the 4 steps of action potential propagation in Unmyelinated axons

Answer 18

1: Action potential develops at initial segment, membrane at this site depolarises
2: Entered Na ions spread away from open VG channels, local current of Na causes depolarisation of next segment
3: Action potential develops at this segment, initial segment begins repolarisation (unavailability of Na channels prevents reverse propagation)
4: Entered Na ions spread away from open VG channels, local current of Na causes depolarisation of next segment. AP only move forwards, as membrane potential of previous segment is in absolute refractory period of repolarisation.

Question 19

Why can action potentials only propagate forwards

Answer 19

Voltage gated Na channels behind action potential are in absolute refractory period, and cannot be restimulated. Voltage gated Na channels in front of action potential are primed and ready to be used.

Question 20

How is the transmission velocity of action potentials increased

Answer 20

Myelin

Question 21

What is a myelin sheath, and a node of ranvier

Answer 21

Myelin sheath - multiple Schwann (PN S) cells in a line wrapped around the axon of a neuron (oligodendrocytes in CNS)
Nodes of ranvier - gaps between myelin. Increase conduction velocity

Question 22

What are the 4 steps of action potential propagation in myelinated axons

Answer 22

1: Action potential arises at the initial segment
2: A local current produces graded depolarisation that brings axolemma at node 1 to threshold
3: Action potential develops at node 1. Initial segment begins repolarisation (now refractory)
4: A local current produces graded depolarisation that brings axolemma at node 2 to threshold

Question 23

How does myelin increase transmission velocity

Answer 23

As current passively propagates, some current loss due to ionic release (out of ion channels). In myelinated segments, leakage possibility very low, resulting in current being better maintained.