Week 1 Lecture 3

Question 1

What happens when a patch of excitable membrane generates an action potential?

Answer 1

An influx of Na+ and reverses the potential difference across the membrane.

Question 2

Are most cells excitable?

Answer 2

No, most cells are not excitable however they conduct passive current, but not generate action potentials.

Question 3

Why are most cells not excitable?

Answer 3

They lack voltage-gated Na+ channels (and axons)

Question 4

What cells generate action potentials?

Question 5

Is biological tissue conductive?

Answer 5

No, when voltage is measured, 1/2 of the amplitude is loss indicating some signal is lost.

Question 6

length constant (lambda)

Answer 6

how far a signal can be carried before the signal is lost, larger potential difference = longer length constant = signal will be carried out w/o being lost

Question 7

How is the length constant increased?

Answer 7

Increasing the diameter of the axon results in less internal resistance and therefore less voltage is lost as the currents travel down the membrane

Increasing membrane resistance results in less current being leaked out while traveling across membrane

Question 8

How is the length constant defined?

Answer 8

it is defined with internal resistance, extracellular fluid resistance, and membrane resistance however since extracellular fluid resistance is not adjustable and relatively low, only internal resistance and membrane resistance is considered

Question 9

What is the most efficient way of increasing conduction velocity?

Answer 9

Increasing membrane resistance i.e. myelination

Question 10

What cells are involved in myelination?

Answer 10

Schwann cells (in the PNS) and oligodendrocytes (in the CNS) which are specialized glial cells.

Question 11

What are glial cells?

Answer 11

Cells that assist the nervous system that are required for nutrition and increased membrane resistance

Question 12

How does myelination occur?

Answer 12

Layers (50-100) of myelin are wrapped around sections of the axon by Schwann and oligodendrocytes in nervous system to create myelin sheath

Question 13

Are all axons myelinated?

Answer 13

Only some axons are myelinated since they add bulk and take up space

Question 14

Is an entire axon myelinated?

Answer 14

No, small gaps are left between adjacent portions of the myelin sheath by glial cells

Question 15

What are the small gaps left between adjacent glial cells?

Answer 15

Nodes of Ranvier

Question 16

Why are Nodes of Ranvier important?

Answer 16

They are where action potentials are generated because voltage-gated Na+ channels are present in the axon

Question 17

What is multiple sclerosis (MS)?

Answer 17

Loss of myelination, message/signals are slowed down or blocked

Question 18

What is saltatory conduction?

Answer 18

The ‘jumping’ mode of conduction that occurs with myelinated axons in which only Nodes of Ranvier, unmyelinated regions with voltage-gated Na+ channels, are excitable