Nervous System: Action Potential Generation and Transmission part 2

Question 1

Unmyelinated axon diameter

Answer 1

1μm

Question 2

Description of Unmyelinated axon AP transmission

Answer 2

Slow transmission of action potentials, continuous

Question 3

Mylinated axon diameter

Answer 3

5-10μm

Question 4

Description of Myelinated axon AP transmission

Answer 4

Fast transmission of action potentials, “saltatory” (in large steps)

Question 5

The two stages of action potential transmission in both types of axons

Answer 5

1. Passive spread of current

2. Generation of action potentials

Question 6

Stage 1 of the passive spread of current

Answer 6

there is a depolarisation att one segment of the cell membrane. This depolarisation has a higher potential leading to current to flow from + to -

Question 7

Stage 2 of the passive spread of current

Answer 7

Current flows passively (inside/outside axon). It goes in both directions and flows across the membrane through leak channels and then flows back through ‘circuit’

Question 8

Stage 3 of the passive spread of current

Answer 8

The passive flow of current causes the depolarization of adjacent parts of the membrane (rule #2) causing electric signals to spread across the axon

Question 9

How far can passive current spread and why

Answer 9

Only short distances (less than 1mm) because current goes through the path of least resistance so will dissipate over time

Question 10

What occurs under a anode

Answer 10

the membrane potential becomes hyperpolarized

Question 11

What occurs under a cathode

Answer 11

the membrane potential becomes depolarised

Question 12

Process of Action potential transmission in unmyelinated axons

Answer 12

1. an action potential is generated under a cathode, causing the passive spread of current in both directions
2. current flows passively in both directions
3. adjacent parts of the cell membrane depolarise to threshold
4. Voltage gated Na+ channels in adjacent parts of the membrane open
5. New full size AP is generated in adjacent parts of the membrane

Question 13

Speed of AP transmission in unmyelinated axons

Answer 13

~1m/sec AP (slow)

Question 14

Why is the passive spread of current in unmylinated axons slow?

Answer 14

Because AP must be regenerated at every point in the membrane

Question 15

What cells form myelin sheaths in the CNS

Answer 15

oligodendrocytes

Question 16

What cells form myeline sheaths in the PNS

Answer 16

Schwann cells

Question 17

Gaps in myelination

Answer 17

Nodes of Ranvier

Question 18

Length of Nodes of Ranvier

Answer 18

1mm

Question 19

Spead of AP transmission in myelinated axons

Answer 19

20-100m/sec

Question 20

How does myelination increase speed of AP transmission?

Answer 20

It acts as an insulator so it reduces current dissipation at it flows along the axon and AP ONLY need to be regeenrated at nodes of ranvier (not every other point)

Question 21

Where are AP generated in myelinated axons and why

Answer 21

Nodes of Ranvier - this is the point of least resistance in myelinated axons

Question 22

Name of current flow in myelinated axons

Answer 22

Saltatory Conduction

Question 23

Why are unmyelinated axons used in the brain

Answer 23

Despite taking longer, they have a small diameter so more can pack into nervous tissue

Question 24

Why do AP’s only flow in one direction in physiological conditions

Answer 24

The abolsute refractory period (1-2ms) means AP’s cannot flow backwards as it won’t be able to reactivate voltage gated Na channels

Question 25

What is a muscle spindle?

Answer 25

the sensory part of a muscle

Question 26

How does information travel from the muscle spindle to the CNS

Answer 26

sensory unipolar neurons

Question 27

Structural features of muscle spindle and its function

Answer 27

It contains stretch sensistive ion channels that sense the displacement of the cell membrane and causes channels to open, causing the depolarisation of the most distal part of the axon (AP).

Question 28

Where is the synaptic terminal located?

Answer 28

in the CNS

Question 29

What is a receptor potential

Answer 29

the graded depolarisation of the muscle spindle (small depolarisations due to small stimuli do not cause an AP, the bigger the stimuli = bigger receptor potential)

Question 30

What codes for the strength of the stimulus in the muscle spindle

Answer 30

the amplitude of the receptor potential and the frequency of the action potential