Lecture 8 - Neurophysiology

Question 1

What is a gated ion channel?

Answer 1

An ion channel that can open or close in response to a stimulus

Question 2

What is a leak channel?

Answer 2

A channel that is always open

Question 3

What is a voltage gated ion channel?

Answer 3

A channel whose open or close state depends on the value (polarity) of membrane potential

Question 4

What is a ligand gated ion channel?

Answer 4

a. A channel in which a ligand (chemical) binding to a receptor opens the channel
b. Can also be called a receptor mediated ion channel

Question 5

What is a mechano-gated ion channel?

Answer 5

a. Mechanical stimulus opens or closes the ion channel
b. E.g. stretch receptors, touch receptors, baroreceptors

Question 6

What are the three key types of V-gated channels?

Answer 6

a. Voltage gated Na+ channels
b. Voltage gated K+ channels
c. Voltage gated Ca+ channels

Question 7

Voltage gated Na+ channels

Answer 7

a. Has three possible conductance states
i. Open, closed, inactive
b. Vna transition
i. Must close before they can repoen
ii. Refractory
c. Notable structural features of Vna
i. Voltage sensory
ii. Inactivation gate
d. Relative fast opening

Question 8

Voltage gated K+ channels

Answer 8

a. Two possible conductance states
i. Open and closed
b. Dynamics
i. Slow opening, slow closing

Question 9

Threshold potential positive-feedback cycle

Answer 9

a. Triggering event
b. Depolarization (decreased membrane potential)
c. Opening of some voltage-gated Na+ channels
d. Influx of Na+ (which further decreases membrane potential)

Question 10

The cell membrane permeability at rest

Answer 10

75 x more permeable to K+ relative to Na+

Question 11

Cell membrane during depolarization

Answer 11

a. Vna is open
b. The membrane is permeable to Na+ sp Na+ enter the cell
c. Na+ entering the cell bring positive charge into the cell
d. Vk remains closed

Question 12

Cell membrane during repolarization

Answer 12

a. Vna become inactive and gate closes so Na+ can no longer enter the cell
b. Vk opens the membrane becomes very permeable to K+
c. K+ leaves the cell to positive charge leaves the cell and the membrane potential falls

Question 13

Cell membrane during hyperpolarization

Answer 13

a. Vk remains open while Vna remains closed
b. Membrane even more permeable to K+ than it would be at rest
c. Fall in membrane potential “overshoots” and the membrane potential becomes even more negative than RMP

Question 14

What are refractory periods?

Answer 14

During and immediately after an action potential their periods when it is hard to generate new action potential

Question 15

Properties of action potentials

Answer 15

a. All or none (i.e. no half action potentials)
b. Self-regenerating
c. Travel down the axon
d. Initiated in one segment
e. Driven by voltage-gated ion channels
f. Travel the entire length of an axon without diminishment
g. High fidelity
h. No summation

Question 16

What are graded potentials?

Answer 16

a. Any change in the membrane potential below threshold
b. They occur because gated ion channels are open or closed causing influx or efflux of ions

Question 17

What is myasthenia gravis?

Answer 17

a. An autoimmune disease leading to destruction of nicotinic acetyl-choline receptors at the neuromuscular junction
i. Leads to muscle weakness
ii. Nerve function intact

Question 18

What is EPSP

Answer 18

a. Excitory post synaptic potential
b. Movement of ions across the cell membrane at the synapse depolarizes the membrane
c. Membrane potential moves closer to the threshold

Question 19

What is IPSP

Answer 19

a. Inhibitory post synaptic potential
i. Movement of ions across the cell membrane at the synapse hyperpolarizes the membrane
ii. Membrane potential moves further away from threshold

Question 20

Temporal and spatial summation

Answer 20

a. Temporal summation occurs in time
b. Spatial summation occurs when PSPs at two different synapses summate
c. Summation occurs at the trigger zone

Question 21

Describe the events that occur at the synapse

Answer 21

a. Action potential invades axon terminal
b. Depolarization of the axon potential leads to opening of Vca
c. Diffusion from high to low concentration triggers the movement of neurotransmitters
d. Neurotransmitters bind to receptors
e. Neurotransmitter function is terminated

Question 22

The membrane potential can be calculated using the Goldman-Hodgkin-Katz equation. What would be altered by the opening of a gated cation channel?

Answer 22

a. The permeability value for potassium
b. The permeability value for sodium

Question 23

Why is the relative permeability of the cell membrane to potassium 75 time greater than that of sodium?

Answer 23

due to the presence of K+ leak channels on the cell membrane

Question 24

Summary of absolute refractory periods

Answer 24

After an action potential, the absolute refractory period of a cell occurs due to the voltage-gated sodium channels being in the inactive state. This prevents the initiation of further action potentials. As the cell repolarises, sodium channels re-enter the closed state. However, the initiation of further action potentials is rendered difficult due to the cells continued permeability to potassium which will tend to hyperpolarise the cell

Question 25

What are two consequences of opening a ligand gated cation channel on a post-synaptic neuron?

Answer 25

• Na+ influx
• generation of an EPSP