The nervous system is your body’s control and communication system, made up of nerves and the brain. Nerves are your body’s hot-lines, carrying instant messages from the brain to every organ and muscle – and sending back an endless stream of data to the brain about what is going on both inside and outside your body.
The central nervous system (CNS) is the brain and spinal cord.
The peripheral nervous system (PNS) is made up of the nerves that branch out from the CNS to the rest of the body.
The main branches of the PNS are the 12 cranial nerves located in the head, and the 31 pairs of spinal nerves that branch off the spinal cord.
The nerves of the PNS are made up of long bundles of nerve fibres, which in turn are made from the long axons (tails) of nerve cells, bound together like the wires in a telephone cable.
In many places, sensory nerves (which carry sense – signals from the body to the brain) run alongside motor nerves (which carry the brain’s commands telling muscles to move).
Some PNS nerves are as wide as your thumb. The longest is the sciatic, which runs from the base of the spine to the knee.
The autonomic nervous system (ANS) is the body’s third nervous system. It controls all internal body processes such as breathing automatically, without you even being aware of it.
The ANS is split into two complementary (balancing) parts – the sympathetic and the parasympathetic. The sympathetic system speeds up body processes when they need to be more active, such as when the body is exercising or under stress. The parasympathetic slows them down.
Nerves are made of very specialized cells called neurons.
Neurons are shaped like a spider, with a nucleus at the centre, lots of branching threads called dendrites, and a winding tail called an axon which can be up to 1 m long. 336
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, arc 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 metres per second.
Ordinary nerves send signals at about 1 to 2 metres 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.