lecture 28. Action potential 2

Question 1

2 types of axons

Answer 1

1) Unmyelinated axons: small diameter (~1 um); transmission of APs slow,
continuous
2) Myelinated axons: larger diameter (5-10 um) ; transmission of APs fast,
‘saltatory’ (in large steps)

Question 2

2 stages of action potential transmission

Answer 2

1) Passive spread

2) Generation of Action potentials

Question 3

passive spread of current

Answer 3

1. (Subthreshold) depolarisation at one region of the membrane. Can be induced
2. Passive current flow ( inside and outside the axon)
3. Initial depolarisation causes depolarisation of adjacent parts of membrane

dissipates quickly

Question 4

Why can passive current not be used for long distances?

Answer 4

it dissipates quickly as it moves away from the initial depolarization
(~1 mm away) as the charges leak out of the “leak channels”

Question 5

transmission in an axon always involves:

Answer 5

both passive transmission and action potential

Question 6

Action potential in unmyelinated axons

Answer 6

1. Action potential
2. AP induces ‘Passive’ current flow
3. Depolarization of adjacent parts of membrane to threshold
4. If threshold is reached- Voltage-gated Na+ channels in adjacent parts of membrane open.
5. New full size AP generated in adjacent parts of membrane

*multiple APs generated along the length of the axon

Question 7

unmyelinated vs myelinated

Answer 7

unmyelinated- slow ~ 1 m/sec
conduction velocity is slow as AP need to be generated multiple times

myelinated- fast ~20-100 m/sec

Question 8

Myelination

Answer 8

-glia cells:

• Myelin sheath formed:

• by oligodendrocytes in the CNS
• by Schwann cells in the PNS

• Myelination is discontinuous; interrupted
at nodes of Ranvier( without gaps would not generate APs)

Question 9

How does myelination increase passive spread of current?

Answer 9

• Due to the insulating properties of myelin, there is less current dissipation
as it flows along the axon.

Current can only flow out at the node of ranvier

Myelination increases speed of AP conduction by increasing the efficiency
of passive spread, and the fact that APs do not need to be regenerated at every
part of the cell membrane.
• APs are generated only at nodes of Ranvier (current flows passively between nodes)
• This process is called “saltatory conduction”

Question 10

saltatory conduction

Answer 10

In myelinated axon:

Saltatory conduction describes the way an electrical impulse skips from node to node down the full length of an axon, speeding the arrival of the impulse at the nerve terminal in comparison with the slower continuous progression of depolarization spreading down an unmyelinated axon.

Question 11

saltatory conduction

Answer 11

In myelinated axon:

Saltatory conduction describes the way an electrical impulse skips from node to node down the full length of an axon, speeding the arrival of the impulse at the nerve terminal in comparison with the slower continuous progression of depolarization spreading down an unmyelinated axon.

Question 12

How is myelination more energy efficient?

Answer 12

-AP requires energy from the Na+/K+ ATPase to recreate the conc. gradient
less AP generated-> more ATP saved

Question 13

Why are not all axons myelinated?

Answer 13

Myelination takes up a lot of space and energy and nutrients to produce-> not all axons need to be myelinated

Question 14

Why does AP conduct in only one direction under physiological conditions?

Answer 14

While the axon is in refractory period where the current was generated, the current can only go forward

By the time the absolute refractory period is over, the AP has already moved far away

Question 15

PNS contains:

Answer 15

• axons and cell bodies of sensory neurons( input)
• axons of motoneurones( output)
• neurons forming the ‘autonomic nervous system’- under involuntary control

Question 16

Sensory neurons features:

Answer 16

no dendrites

long axons

Question 17

Muscle spindle

Answer 17

a special muscle cell,
when that muscle cells stretches, mechanically gated ion channels will open
tells your brain how stretched your muscle is

Question 18

How are APs generated in sensory neurons

Answer 18

• When a stimulus acts on receptors in sensory neurons (e.g. a mechanical stimulus acting on muscle spindles–> mechanically gated ion channels open), it does not immediately evoke APs
• First it evokes a graded depolarisation known as ‘the receptor potential’
• The receptor potential spreads passively to more distally located ‘trigger zone’ where APs are generated. ( if reaches the threshold-AP, if not-nothing)
• APs then spread along the axon (myelinated or unmyelinated) towards the CNS.
• Information about the strength of the stimulus is coded in the amplitude of the receptor potential
and the frequency of APs (analog-to-digital converter!)

Question 19

graded depolarisation

Answer 19

dependent on the stimulus strength( not Voltage gated)

Question 20

how is the strength/amplitude of the receptor potential is converted in AP

Answer 20

through the frequency