nervous system
Meaning of nervous system: Most of what it does never reaches awareness, since heartbeat, digestion and blood vessel tone are governed with no conscious involvement at all.
Definition of nervous system
The nervous system is the body's signaling network, built from nerve cells that carry electrical impulses and pass chemical messages to one another. It divides into the central nervous system, meaning the brain and spinal cord, and the peripheral nervous system, the nerves reaching every other part of the body. It senses, decides and directs.
How is it divided?
| Division | What it covers |
|---|---|
| Central | Brain and spinal cord; integrates information and decides; shielded by skull, vertebrae and membranes |
| Peripheral somatic | Nerves carrying sensation inward and voluntary movement commands outward |
| Sympathetic | Prepares for exertion: faster heart, wider airways, blood diverted to muscle |
| Parasympathetic | Supports rest and digestion: slower heart, active gut, conserved energy |
| Enteric | A semi-independent network in the gut wall able to run digestion on its own |
The last three form the autonomic division. A finding traced to the spinal cord is investigated quite differently from one traced to a peripheral nerve, and the balance between the autonomic nervous system branches, particularly the parasympathetic nervous system, sets the resting state of the heart, gut and blood vessels.
What are the cells that do the work?
Nerve cells receive input through branching extensions, generate an electrical impulse along a single long fiber, and hand the signal on at a junction where a chemical messenger crosses a narrow gap to the next cell. Supporting cells outnumber them and are anything but passive: some wrap fibers in a fatty insulating sheath that multiplies conduction speed, some maintain the chemical environment and clear used messengers, and some act as the immune cells of nervous tissue. Damage to the insulating sheath alone slows or blocks conduction even when the fiber inside is intact, which is how demyelinating diseases produce their effects.
Why does damage reach beyond the site of injury?
Because this is a network of long connections rather than a collection of independent parts. Injury at one point disrupts everything served downstream of it, so a small lesion in a spinal tract affects every region that tract supplies below that level. A stroke in a language area leaves the muscles of speech working perfectly while the ability to assemble words is lost. Repair capacity differs sharply across the divide: fibers in peripheral nerves can regrow slowly along their old path, while those in the brain and spinal cord largely cannot, which is why an injury of the same size carries such different consequences depending on which side of that boundary it falls.
Used in a sentence
Imaging was obtained to evaluate the central nervous system after the patient's neurological symptoms developed.
Good to know
A finding traced to the central nervous system, meaning the brain or spinal cord, is assessed differently from one traced to the peripheral nervous system, since the two divisions differ in how they are reached, imaged and treated.
Word origin
Where “nervous system” comes from
Formed from nervous, from Latin nervus, meaning sinew or nerve, combined with system, from Greek systema, meaning an organized whole.