Conduction within cells

How do neurons process information?

1. Electrical signals - Conducted within a neuron
2. Chemical signals - Transmitted between neurons

 

Early research on electrical stimulation:

• Luigi Galvani (18th century) – electrical stimulation of dissected nerve induces muscle twitch.
• Gustav Theodore Fritsch and Eduard Hitzig (mid 19th century) – stimulation of (motor) neocortex in animals causes limb movement.
•  Roberts Bartholow (1874) – First report of human brain stimulation

Slow flow - more frog experiments

• Helmholtz (19th century)
• Flow of information in the nervous system is too slow to be a flow of electricity
• Nerve conduction: 30–40 metres/second
• Electricity: 3 × 108 metres/second
• Instead, information travels along the axon as a wave of charge (Bernstein, 1886)

Squid experiments

• Hodgkin and Huxley (1930s and 1940s)
• Experiments on the giant axon of squid
• Neurons have a natural electric charge - Resting potential
• Stimulation with electricity results in the conduction of an electric impulse along the axon - Action potential

The resting potential 

• Difference in electrical charge across the cell membrane
• A store of negative energy on the intracellular side relative to the extracellular side
• Resting potential is approx. -70 mV

Graded potential

• An increase of positive charge inside the membrane causes a depolarisation.
• An increase of negative charge inside of the membrane causes a hyperpolarisation.
• Graded potentials typically occur in the cell body (soma)

Summation of inputs

- Inputs from presynaptic neurons are either excitatory or inhibitory 

• Excitory inputs depolarise the membrane
• Inhibitory inputs hyperpolarise the membrane
• Firing of a nerve impulse determined by the summation of excitatory and inhibitory inputs

The Action Potential

• An electrical impulse
• Large, brief reversal in polarity of an axon
• Lasts approximately 1 millisecond

Threshold potential

• Voltage on a neural membrane at which an action potential is triggered
• Approximately -50mV relative to extracellular surround

3 phases of Action Potential

1. Depolarisation phase
2. Repolarisation phase
3. Hyperpolarisation phase (Refractory period)
4. file:///home/chronos/u-e5c2a397a3d77f3868f00ca2c09225dc13341547/MyFiles/Downloads/Screenshot%202021-05-08%207.56.23%20PM.png

 

Saltatory conduction an myeline sheaths

Myelin - A fatty salty sheath that wraps around the axon

Node of Ranvier - Part of an axon that is not covered by myelin - tiny gaps in the myelin sheath

Saltatory conduction: Fast propagation of an action potential at successive nodes of Ranvier

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• Instead, information travels along the axon as a wave of charge (Bernstein, 1886)