Nerve signals
Information travels as brief electrical pulses, action potentials, whose rate carries the message; at synapses it becomes chemical.
An action potential is a wave of sodium ions rushing into the axon followed by potassium flowing out, restoring the resting state within a millisecond. Because each pulse is identical, a stronger stimulus is signalled by more pulses per second, not bigger ones.
Sensory signals travel from receptors through the spinal cord and thalamus to the cortex; motor commands travel down the corticospinal tract, cross to the opposite side in the medulla, and leave through spinal nerves. This crossing is why a stroke in the left hemisphere weakens the right side of the body.
Local anaesthetics block sodium channels so pain signals never start; multiple sclerosis strips myelin so signals slow or fail; nerve compression, as in carpal tunnel syndrome, produces tingling and numbness along the nerve's territory.
Key facts
| Fact | Value |
|---|---|
| Action potential duration | ≈1 ms |
| Synaptic delay | ≈0.5 ms |
| Neurotransmitters known | >100 |