Lecture 7 – SALTATORY CONDUCTION

Question 1

Transmission can be broken down into two components

Answer 1

1. Electrical (within neurons) (THIS LECTURE)
2. Chemical (communication between neurons)
   - Transmission is the ability to relay signals within or between neurons

Question 2

Why does dendritic current attenuate?

Answer 2

• Leaky hose analogy (diminished as it goes down the dendrite)
• Attenuation in the Victorian era (rubber for insulation, copper as a conducting core)
• Telegraph current attenuation (below) where current is lost as if flows from source (USA) to the receiving station (UK) – too much attenuation then it will not reach the other side
• I(in) > I(out)

Question 3

Cable Theory:

Answer 3

• distance which the voltage drops by a certain factor
• starting point at 0%, at an increased distance the current drops off
• bigger the current value, the less it is going to be attenuated
• attenuation is the reduction of the force, reduction of the amplitude of a signal

Question 4

The Transatlantic Solution:

Answer 4

1. To get efficient transmission we want cables with big length constants
2. Length constant depends on:
   - R(m) which is leakiness
   - R(i) which is conductivity
   - Diameter
3. Cable engineer’s options to get good transmission you would need to increase Rm for better insulation (thick rubber), Decrease Ri for better conducting cores (more copper) and increase the diameter for fatter cables

Question 5

Attenuation in dendrites:

Answer 5

1. Not a big problem because:
   - Short distances involved
   - Many inputs due to a big starting signal
2. Dendrites can generate action potentials
3. Dendritic transmission is passive but does not involve a wave of action potentials

Question 6

What about Axons?

Answer 6

• An axon cable of 1 metre passive transmission need to have a diameter of 1 cm which would mean a BIG HEAD
• To get fast transmission, we need axons with big length constants
• Increase Rm for better insulation – increase Schwann cells

Question 7

Properties of axons:

Answer 7

• Axons have a far higher density of sodium channels
• AP wave is key to non-attenuated transmission
• The axonal solution is to get rid of Ri and still have increased Rm for better insulation (Schwann cells) and increase the axonal diameter which is a strategy used in primitive animals (squid) and to decrease the leak of current
• Fatter diameter, bigger distance (longer)
• Primitive animals do not need to take up that much space with their neurons

Question 8

How to decrease the current leak:

Answer 8

• Myelinated neurons
• Decrease capacitance of myelination of axons
• Effectively the current jumps ahead creating a salutatory conduction
• Internodes are about 1mm long and spaced at regular intervals

Question 9

The Node of Ranvier:

Answer 9

• In axon under myelin – intermodal region where Na+ density is very low (100/ um2)
• In unmyelinated axons Na+ density intermediate (1000/um2)
• High density of channels in node decreases rise time of AP
• Passive transmission to the next Node of Ranvier, the insulated bit has a much faster transmission
• More energy efficient
• Depolarisation is only seen at the nodes

Question 10

Sensory Nerve Fibres:

Answer 10

Advantages of myelination are:

• Speed
• Compactness of neurones
• Energy efficiency

Question 11

Diseases involving altered myelination:

Answer 11

1. Multiple sclerosis
   - Immune mediated
   - Demyelinating disease
   - Central nervous system
   - Guillain-Barre syndrome which is an autoimmune attack on myelin in the Peripheral NS and attacks the Schwann cells
   - Signals become slower (attenuated) as you go along a de-myelinated axon which leads to the person being less sensitive
   - Sodium channels would spread out in a normal un-myelinated axon
   - Paresis is weakness
   - Paraesthesia is pins and needles

Question 12

conduction in MS

Answer 12

• damage to myelin sheath
• decreased conduction in axons
• not same as reverting to non-myelinated