Week 4 ELM 8: Transmission within Neurons

Transmission: An Introduction

Dendrites: Current is attenuated (signal decreases over distance).
- When current is injected into a dendrite, the recorded current decreases over time and distance.
Axon: Current is not attenuated (signal remains constant over distance).
- When current is injected into an axon, the recorded current remains relatively constant over time and distance.

Victorian Era: Transatlantic telegraph cables experienced current attenuation.
- Current was lost as it flowed from the source (USA) to the receiving station (UK).
- I{in} > I{out}, indicating leaking current.
- The voltage decreases exponentially with distance: V = V_0 e^{-x/\lambda}, where:
  - V is the voltage at distance x.
  - V_0 is the original voltage at the source.
  - x is the distance from the source.
  - \lambda is the length constant.
Length Constant (\lambda): The distance over which the voltage drops to 37% of its original value.

To achieve efficient transmission, cables need a large length constant (\lambda).
The length constant depends on:
- R_m: Membrane resistance (insulation).
- R_i: Internal resistance (conducting core).
- d: Diameter of the cable.
Cable engineers can improve transmission by:
- Increasing R_m: Better insulation to reduce leakiness.
- Decreasing R_i: Better conducting cores to improve conductivity.
- Increasing d: Fatter cables to reduce resistance.

Attenuation affects broadband speed, which drops off with distance from the telephone exchange.
- This is less of a problem in dendrites due to short distances and multiple inputs.
- Dendrites can generate action potentials, but dendritic transmission is generally passive (does not involve a wave of action potentials).

If axons behaved like dendrites, long-distance transmission would be impossible.
An axon capable of 1-meter passive transmission would need a diameter of 1 cm, which is impractical.

Axons have a much higher density of sodium channels compared to dendrites.
- Example: 200 Na+ channels/mm^2 in axons vs. 1 Na+ channel/mm^2 in dendrites.
This high density of sodium channels is key to non-attenuated transmission via action potential (AP) waves.

A) Increase Axonal Diameter:
- Strategy used by primitive animals like squid for rapid escape.
- Drawback: Not suitable for complex nervous systems; leads to impractically large heads.
B) Decrease Leak of Current:
- Achieved through myelination.

Myelination reduces current leak, similar to taping a leaky garden hose.
Structure:
- Myelin sheath consists of internodes (approx. 1mm long) spaced at regular intervals.
- Nodes of Ranvier are gaps between internodes.