Nerves are made of very specialized cells called neurons.
Neurons are shaped like a spider, with a nucleus at the center, lots of branching threads called dendrites, and a winding tail called an axon which can be up to 1 m long.
Axon terminals on the axons of one neuron link to the dendrites or body cell of another neuron.
Neurons link up like beads on a string to make your nervous system.
Most cells are short-lived and are constantly being replaced by new ones. Neurons, however, are very long-lived – some are never actually replaced after you are born.
Nerve signals travel as electrical pulses, each pulse lasting about 0.001 seconds.
When nerves are resting there are extra sodium ions with a positive electrical charge on the outside of the nerve cell, and extra negative ions inside.
When a nerve fires, gates open in the cell wall all along the nerve, and positive ions rush in to join the negative ions. This makes an electrical pulse.
Long-distance nerves are insulated (covered) by a sheath of a fatty substance, myelin, to keep the signal strong.
Myelinated (myelin-sheathed) nerves shoot signals through very fast – at more than 100 meters per second.
Ordinary nerves send signals at about 1 to 2 meters per second.
Motor nerves are connected to your muscles and tell your muscles to move.
Each major muscle has many motor nerve-endings that instruct it to contract (tighten).
Motor nerves cross over from one side of your body to the other at the top of your spinal cord. This means that signals from the right side of your brain go to the left side of your body, and vice versa.
Each motor nerve is paired to a proprioceptor on the muscle and its tendons (see co-ordination). This sends signals to the brain to say whether the muscle is tensed or relaxed.
If the strain on a tendon increases, the proprioceptor sends a signal to the brain. The brain adjusts the motor signals to the muscle so it contracts more or less.
Motor nerve signals originate in a part of the brain called the motor cortex (see the cortex).
All the motor nerves (apart from those in the head) branch out from the spinal cord.
The gut has no motor nerve-endings but plenty of sense endings, so you can feel it but cannot move it consciously.
The throat has motor nerve-endings but few sense endings, so you move it but not feel it.
Motor neuron disease attacks motor nerves within the central nervous system.