M&R - Nerve Impulse

Question 1

What does a low membrane capacitance result in?

Answer 1

Increased conduction velocity as less work is required to depolarise adjacent regions of membrane to threshold for an action potential to be generated

Question 2

How is an electrical signal propagated directionally down an axon?

Answer 2

After an action potential has fired in a region of nerve membrane, that part of the membrane becomes refractory to the firing of a second action potential because of the inactivation of the Na+ channels.

Therefore, a nerve impulse cannot re-enter a region of nerve axon that has just fired and so it is propagated directionally down a nerve axon

Question 3

What are the two types of glia cell?

Answer 3

Oligodendrocytes - CNS

Schwaan cells - PNS

Question 4

What are the unmyelinated regions between the internodes called?

Answer 4

Noes of Ranvier

Question 5

What effect does myelination have on the electrical capacitance of the internodal length?

What happens?

Answer 5

Reduces it substantially

Electrical activity only occurs at nodes of Ranvier

Question 6

Where are the Na+ channels located in a myelinated nerve?

Answer 6

Exclusively at the nodes of Ranvier

Question 7

What happens to the electrical current in demyelination?

Answer 7

Excitation of successive nodes becomes progressively slowed because of the increased membrane capacitance.

This causes local current dissipation and conduction of the action potential ultimately fails.

Question 8

What can restore some excitability after complete demyelination?

Answer 8

Insertion of new Na+ channels in internodal regions

Conduction velocity is slowed

Question 9

Where does electrical stimulation occur?

What will happen to the excitability under an anode?

What is this used to stimulate?

Answer 9

Under a cathode (negatively charged)

Reduced - as anode is positively charged

An axon/group of axons to threshold, thus initiating an action potential

Question 10

How is conduction velocity calculated?

Answer 10

Measuring the distance between the stimulating electrode and the recording electrode and the time gap between stimulus and action potential being registered by the recording electrode.

Conduction velocity = distance/time

Question 11

How is action potential conducted along an axon?

Answer 11

Change in membrane potential in one part can spread to adjacent areas of the axon due to the local current spread.

Local current spread causes depolarisation of part of the axon to threshold then an action potential is initiated

Question 12

What properties of the axon lead to a high conduction velocity?

Answer 12

High membrane resistance

Low membrane capacitance

Large axon diameter –> low cytoplasmic resistance

Question 13

What does the membrane resistance depend on?

Answer 13

Number of ion channels open

Question 14

What happens in low membrane resistance?

Answer 14

More ion channels are open

More loss of local current occurs across the membrane

Limiting the spread of local current effect

Question 15

What is the effect of myelin?

Answer 15

Reduce the capacitance

Increase the resistance of the axonal membrane

Question 16

Name a disease which causes demyelination of axons?

Answer 16

Multiple sclerosis - autoimmune attack of the myeline sheath

Can have dramatic effects on the ability of previously myelinated axons to conduct action potentials properly.

This leads to decreased conduction velocity, complete block or cases where only some action potentials are transmitted

Question 17

What is electrical capacity (capacitance)?

Answer 17

Ability to store electrical charge

Question 18

When during development does myelination begin to occur?

Answer 18

After 4 months of fetal development

Question 19

Where in an myelinated nerve does electrical activity occur?

Answer 19

Node of Ranvier

Question 20

How does the myeline sheath reduced the capcitance?

Answer 20

Reduces the dissipation of local current within the nerve fibre, thereby, permitting more distant regions of the fibre membrane to be brought to threshold for firing an action potential

Question 21

Where are the Na+ and K+ channels located in a myelinated nerve fibre?

Answer 21

Na+ channels at the node of Ranvier

K+ channels more widely distributed

Question 22

What may happen to the conduction of nerve impulses during nerve fibre demyelination?

Answer 22

Conduction may stop due to the dissipation of local currents because of the increase capacitance of the membrane

After a period of time, a demyelinated nerve may begin to conduct nerve impulses again due to the incorporation of ion channels in the internodal regions

Treatment of demyelinated nerves with K+ channel blockers can restore nerve impulse conduction to otherwise defective nerve.