lecture 18

Question 1

what are chemically gated ion channels

Answer 1

Chemically gated ion channels or ligand gated ion channels, open when they bind to specific chemicals. The receptors for ACh at the neuromuscular junction are chemically gated ion channels.

Question 2

describe voltage-gated sodium channels in the axon

Answer 2

these have two gates. One that opens on stimulation, activation gate, and one that closes to stop ion entry, inactivation gate. For these to open another stimuli must cause the resting membrane potential to change to -60mV, at which point the activation gate opens.

Question 3

what are mechanically gated ion channels

Answer 3

these open in response to mechanical stimuli that physically distort the neuron membrane surface.

Question 4

what is a local or graded potential

Answer 4

this is a neuron and neurotransmitter interaction, the specific part of the neuron is the dendrites. This interaction results in a localised change of membrane potential. The magnitude of these is regulated by the strength of the stimulus.

Question 5

what causes a local potential

Answer 5

due to the result of Neurotransmitters opening Na+ chemical gated channels and causing depolarisation. However the signals can also open K+ channels to lower the depolarisation, perhaps even hyperpolarise.

Question 6

where are chemically gated ion channels common

Answer 6

in the dendrites of nerve cells

Question 7

where is the action potential initally propogated

Answer 7

the initial segment of the axon hillock

Question 8

what needs to happen to cause an action potential

Answer 8

local potentials must summate well enough to cause depolarisation to -60mV, ie threshold must be met in the cell, in the initial segment of the axon hillock

Question 9

action potential can be briefly defined as

Answer 9

a short increase in permeability to Na+, once threshold is reached, followed by termination of said permeability and then an increase in K+ permeability, which results in a quick hyperpolarisation.

Question 10

local potentials can be what or what

Answer 10

inhibitory or excitiory

Question 11

whats special about the inital segment

Answer 11

it has a high density of sodium gated ion channels, it is the site where an AP is propogated

Question 12

step one of propagation of an AP in a neuron

Answer 12

The stimulus that initiates an AP is a graded depolarisation that is large enough to open voltage gated sodium channels. The opening of these gated channels occurs at a membrane potential called the threshold.

Question 13

step 2 of propagation of an AP in a given area of an axon

Answer 13

Activation of sodium ion channels and rapid depolarisation
When a Na+ ion activation gate opens, the plasma membrane becomes more permeable to Na+. Sodium ions then rush into the cytosol, rapidly depolarising the cell.

Question 14

step 3 of propagation of an action potential in a given area of axon

Answer 14

Inactivation of sodium ion channels and activation of potassium channels:
As the membrane potential reached 30mV, the inactivation gates of the voltage-gated sodium channels close. This is called sodium channel inactivation, this occurs as the voltage-gated K+ channels open. Positively charged K+ moves out of cytosol, lowering the membrane potential, thus repolarisation begins.

Question 15

step 4 of propagation of an AP in a given axon segment

Answer 15

The voltage gated sodium channels remain inactivated until near threshold voltage is reached. At this time the voltage gated sodium channels move back to normal status of closed but ready to open. The voltage gated K+ channels begin closing as the cell membrane potential repolarises to near -70mV. Until all potassium channels close, the K+ keeps leaving so some small hyper polarisation momentarily occurs.

Question 16

what is absolute refractory

Answer 16

At the +30mV point is absolute refractory as no more AP will be conducted as the Na+ ion channels aren’t available for use. As the inactivation gate is closed. This occurs till about -40mV,

Question 17

what is relative refractory

Answer 17

-40mV, at this point some inactivation gates are open and some activation close, ie the voltage can can now allow Na+ through. This is Relative Refractory as it would take a large stimulated an AP but an AP can generate. The AP can generate as the activation gates can open to allow the movement of Na+ into the cell.

Question 18

whats a local current

Answer 18

the movement of ions down the axon bringing membrane down the axon to threshold. moving down the axon as its down a concentration gradient.

Question 19

step one of AP propogation

Answer 19

Propagation is progressive depolarisation. At this point, an AP has generated in the axon hillock so lots of Na+ is in the cell. the initial segment depolarises to 30mV

Question 20

step two of AP propagation

Answer 20

The electrochemical gradient for Na+ is now down the axon within the cell. Thus Na+ will move down the axon in a graded potential. The movement makes further segments of the axon also reach threshold,

Question 21

step three of AP propagation

Answer 21

as the second site is at threshold more Na+ enters the cell and then another local current moves down the axon. Making another region hit threshold and so on. and the initial segment begins refractory

Question 22

step four of AP propagation

Answer 22

the graded potential in the second segment moves down to the 3rd segment causing threshold in that segment and the process repeats.

Question 23

why is AP conduction in myelinated axons faster

Answer 23

between the myelin sheath is a node of Ranvier is like its own axon hillock. Myelin prevents the loss of Na+ ions through the membrane as travelling. The myelin means only small areas must be made to threshold not the whole axon. So its like the AP instantly “jumps” between nodes. Thus this process is fast at 20-100 meters per second.

Question 24

what is repolarisation caused by

Answer 24

the exit of k+ from the inside of the cell membrane

Question 25

Explain how the speed of conduction of the action potential can be enhanced by myelin

Answer 25

Myelin prevents short range local currents across the membrane, so current spreads further along the inside of
the axon to depolarize membrane to threshold at next Node of Ranvier. Increase CV up to 20-100m/sec. also prevents sodium ion leaks