Potentials and gating

Question 1

Action potential

Answer 1

a nerve cell action potential is a reversal of trans-membrane voltage that is completed in 2-3ms

Question 2

ion channels

Answer 2

regulators of changes in membrane permeability. changes in permeability to specific ions are due to the open or closed status of specific ion channels, opening/closing = gating

Question 3

movement of ions causes…

Answer 3

current flow and change in transmembrane voltage

Question 4

Ion channel gating:

chemical

Answer 4

located on the cell body and dendrites of a neuron, where the neuron receives chemical signals

Question 5

Ion channel gating:

voltage

Answer 5

axon and axon terminals

Question 6

Ion channel gating:

mechanical

Answer 6

exists?

Question 7

local potentials (4 facts)

Answer 7

• opening of ion channels in region of receipt of chemical signal results in a local potential
• release of the chemical from the nerve terminal causes chemical signals to interact with the next cell in the chain (post-synaptic cell) which causes a change in voltage in the post-synaptic cell
• can be inhibitory or excitatory
• not actively propagated
• magnitude of the local potential decreases with distance away from the site of it’s initiation

Question 8

Summation

Answer 8

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

Question 9

Summation process:

Facilitation

Answer 9

• membrane potential sits at -70mV (resting potential)
• chemical stimulus open sodium ion channels
• depolarisation occurs and the membrane potential raises to around -60mV
• repolarisation occurs = stimulus is removed and the excess sodium ions are transported out of the cytosol. membrane potential returns to -70mV

Question 10

Summation process:

Inhibited

Answer 10

• chemical stimulus opens potassium ion channels
• hyperpolarisation occurs and the membrane potential lowers to around -80mV
• chemical stimulus is removed and the membrane potential raises back up to the resting potential (-70mV)

Question 11

Summation process:

summation

Answer 11

Both stimuli applied at the same time, summation occurs, stimuli removed

Question 12

Initial segment:

axon hillock

Answer 12

• the point where the axon joins the neuron cell body
• where an action potential is generated
• high density of voltage-gated Na channels

Question 13

For an action potential to be generated:

Answer 13

the net voltage change at the axon hillock (summed local potentials) must exceed a minimal depolarisation (typically ~10mV), the threshold

Question 14

Threshold

Answer 14

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

Question 15

Action potentials: refractory periods

Absolute refractory period

Answer 15

no matter how large the stimulus, another AP cannot be generated

Question 16

Action potentials: refractory periods

Relative refractory period

Answer 16

an AP can be generated, but only in response to a very large stimulus

Question 17

Propagation occurs in…

Answer 17

Unmyelinated axons

myelinated axons

Question 18

Propagation:

stage 1

Answer 18

As an action potential develops at the initial segment, the membrane potential at this site depolarises to +30mV

Question 19

Propagation:

stage 2

Answer 19

As the sodium ions entering at spreadt away from the open voltage-gated channels, a graded depolarisation quickly brings the membrane in segment 2 to threshold

Question 20

Propagation:

stage 3

Answer 20

An action potential develops in segment 2. the initial segment begins repolarisation (and is now refractory)

Question 21

Propagation:

stage 4

Answer 21

As the sodium ions entering at segment 2 spread laterally, a graded depolarisation quickly brings the membrane in segment 3 to threshold. The action potential can only move forward, not backward, because the membrane at the initial segment is in the absolute refractory period of repolarisation

Question 22

Conduction velocity (4 facts)

Answer 22

• AP’s are transmitted along unmyelinated axons relatively slowly - approximately 1-5 m/s
• Given our large bodies and need for long axons (e.g. distance from lower motoneuron in spinal cord to muscles in foot is often > 1m), slow AP conduction velocity is not adequate for all our needs
• Nature has evolved an efficient way of dramatically increasing AP conduction velocity along many of our axons - myelin
• in myelinated axons, AP conduction velocity is typically in the range of 20-100 m/s

Question 23

Synaptic transmission:

step 1

Answer 23

Action potential triggers the opening of voltage-gated calcium channels

Question 24

Synaptic transmission:

step 2

Answer 24

Calcium ions diffuse into the axon terminal and trigger synaptic vesicles to release ACh by exocytosis

Question 25

Synaptic transmission:

step 3

Answer 25

ACh diffuses across synaptic cleft, binds to ACh-gated sodium ion channels, and produces a graded depolarisation

Question 26

Synaptic transmission:

step 4

Answer 26

Depolarisation ends as ACh is broken down into acetate and choline by AChE (enzyme)

Question 27

Synaptic transmission:

step 5

Answer 27

The axon terminal reabsorbs choline from the synaptic cleft and uses it to synthesize new molecules of ACh

Question 28

Electrical Synapse

Answer 28

Occur when pre- and postsynaptic cell membranes sit very close together and are joined by gap junctions. An electrical event in the presynaptic cell leads to an electrical event in the postsynaptic cell without the involvement of chemical signals. Cannot produce an AP in the postsynaptic cell.

Question 29

EPSP

Answer 29

Excitatory post synaptic potential

Question 30

IPSP

Answer 30

Inhibitory post synaptic potential

Question 31

Postsynaptic potential summation:

Temporal

Answer 31

Summation of potentials over time.
An AP arrives, causes a postsynaptic response that doesn’t reach threshold but before it decays away another signal arrives and brings the cell to threshold and the cell fires an AP in the postsynaptic cell.

Question 32

Postsynaptic potential summation:

Spatial

Answer 32

Summation of potentials in a certain space.
When two AP’s arrive at the same time anad are close enough, their voltage adds together and is therefore closer to threshold, increasing the likelihood of another AP being fired in the postsynaptic cell. If the combined voltage reaches threshold, an AP will be fired.

Question 33

Nerve-nerve junctions (4 facts)

Answer 33

• Synapses are tiny, each synapse may be one of thousands on the post-synaptic cell
• AP in an individual neuron will rarely bring the post-synaptic cell to threshold
• Inputs may be excitatory or inhibitory (EPSP’s & IPSP’s), many transmitters used
• AP may result at axon hillock if depolarisation is strong enough at that point

Question 34

Nerve-muscular junctions (4 facts)

Answer 34

• Synapses are huge, each muscle fibre receives input from only one neuron at one site
• AP in a neuron is very likely to bring muscle fibre to threshold
• Only excitatory inputs (no inhibition or IPSP’s), only ACh used
• AP initiated at nerve-muscle junction