M6 L3: Action potentials II

Question 1

What are the two types of axons, diameter and style of transmission

Answer 1

Unmyelinated axons: small 1 um diameter with slow continuous transmission
Myelinated axons: large 5-10um diameter. with fast saltatory (in steps) transmission

Question 2

What are the two stages of action potential transmission in both types of axons

Answer 2

1. Passive spread

2. Generation of action potentials

Question 3

How does current passively spread along the axolemma

Answer 3

1. At one region of hte membrane there is a subthreshold local depolarisation of axoplasm. 2. Depolarisation causes a passive flow of current (starting at the depolarisation) from plus to minus in both directions inside the cell and outside the cell to make 4 circuits (leak channels connect inside and outside). And this causes depolarisation of adjacent parts of the membrane

Question 4

What is the relationship of amplitude of depolarisation over distance from local depolarisation

Answer 4

It is an exponential decay. The current quickly dissapates as it flows along the axon, escaping through leak channels. Therefore not effective for long axons

Question 5

Describe the 4 steps of Action potential transmission in unmyelinated axon

Answer 5

1. Firstly an outside stimulus sends in current which creates a local hyperpolarisation and depolarisation which makes an action potential.
2. Action potential depolarisation makes a passive current flow which is sufficiently strong enough to depolarise adjacent parts of the membrane to threshold.
3. Voltage gated NA+ channels in adjacent parts of the membrane open
4. New full size AP generated in adjacent parts of the membrane

Question 6

Compare the speed of AP transmission between unmyelinated axons and myelinated axons

Answer 6

Unmyelinated axons speed is 1 m/s, Passive current flow between two adjacent points is fast, but AP must be regenerated at everypoint on the membrane so conduction velocity is slow.
Myelinated axons speed is 20-100m/s. Passive spread is more eficient and APs do not have to be regenerated at every part of axonal membrane, only nodes of Ranvier where current passively flows.

Question 7

What forms the myelin sheath in CNS and PNS

Answer 7

CNS: oligodendrocytes
PNS: Schwann cells

Question 8

What is the structure of myelin

Answer 8

Bilipid layer of cell membrane of glia cells that are layered on top of each other

Question 9

Why does myelination increase passive spread of current

Answer 9

Myelin is insulated so there is less current dissipation as it flows along the axon. The current takes path of least resistance to dissipate at leak channels at the Nodes of Ranvier. As a result,it has a higher amplitude of depolarisation for longer (not as steep)

Question 10

In what direction does passive flow of current go vs the action potential

Answer 10

PC goes in both directions but Action potential in body goes only in one.

Question 11

Describe how myelination increases action potential conduction velocity

Answer 11

Myelin insulation means that passive current spread generates action potentials (through depolarisation and opening of voltage gated… at further points along the membrane nodes of Ranvier. With large enough depolarisation, the AP can jump 2 nodes of ranvier

Question 12

Why does AP conduct in only one direction in physiological conditions (cell body to axon terminals)

Answer 12

The AP does trigger passive conduction going both ways but the passive current going back to the previous node of ranvier will not be able to reactivate voltage gated Na+ channels because membrance is in absolute refractory period which lasts 1-2 ms. When this is over, AP has already moved down the axon by 2 more nodes so too great a distance to send bacj enough current to depolarise it once ARP is over.

Question 13

How are action potentials generated in sensory neurons of the PNS (3 steps)

Answer 13

When a stimulus acts one on receptors in the sensory neuron it doesn’t immediately evoke AP.

1. stimulus evokes graded depolarisation in sensory ending called receptor potential
2. The receptor potential spreads passively to the more distally located trigger zone where APs are generated.
3. The APs then spread along the axon (m or unm) towards the CNS.

Question 14

What is a sensory neuron look like compared to CNS neurons

Answer 14

They are unipolar, has no dendrites so doesn’t receive any synaptic input. From the left of the cell body is the distal axon (to the stimulus) and to the right is the proximal axon (to synaptic terminal with the spinal cord where neurotransmitters are released) Sensory neurons are in the skin, internal organs or muscle.

Question 15

What is the muscle spindle structure and function

Answer 15

structure that contains ion channels that are stretch sensitive so mechanical displacement (change in shape or length) triggers opening of channels leading to small graded depolarisation known as receptor potential

Question 16

Why are neurons called an analog to digital converter

Answer 16

The information about the strength of the stimulus is first recorded in the amplitude of the receptor potential (analog signal) and this is converted at the trigger zone to APs which are digital - 0,1 - the frequency of APs