The nervous system is a complex collection of nerves and specialized cells known as neurons that transmit signals between different parts of the body. Vertebrates — animals with backbones and spinal columns — have central and peripheral nervous systems.
The central nervous system is made up of the brain, spinal cord and retina. The peripheral nervous system consists of sensory neurons, ganglia (clusters of neurons) and nerves that connect to one another and to the central nervous system.
Description of the nervous system
The nervous system is essentially the body’s electrical wiring. It is composed of nerves, which are cylindrical bundles of fibers that start at the brain and central cord and branch out to every other part of the body.
Neurons send signals to other cells through thin fibers called axons, which cause chemicals known as neurotransmitters to be released at junctions called synapses. A synapse gives a command to the cell and the entire communication process typically takes only a fraction of a millisecond.
Sensory neurons react to physical stimuli such as light, sound and touch and send feedback to the central nervous system about the body’s surrounding environment. Motor neurons, located in the central nervous system or in peripheral ganglia, transmit signals to activate the muscles or glands.
Glial cells, derived from the Greek word for “glue,” support the neurons and hold them in place. Glial cells also feed nutrients to neurons, destroy pathogens, remove dead neurons and act as traffic cops by directing the axons of neurons to their targets. Specific types of glial cells (oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system) generate layers of a fatty substance called myelin that wraps around axons and provides electrical insulation to enable them to rapidly and efficiently transmit signals.
Diseases of the nervous system
There are a number of tests and procedures to diagnose conditions involving the nervous system. Aside from MRIs and CT scans, an electroencephalogram (EEG) is often used to record the brain’s continuous electrical activity by attaching electrodes to the scalp. Positron emission tomography (PET) is a procedure that measures the metabolic activity of cells.
A spinal tap places a needle into the spinal canal to drain a small amount of cerebral spinal fluid that is tested for infection or other abnormalities.
A number of nerve disorders can affect the nervous system, including vascular disorders such as stroke, transient ischemic attack (TIA), subarachnoid hemorrhage, subdural hemorrhage and hematoma, and extradural hemorrhage.
The nervous system can also experience functional difficulties, which result in conditions such as epilepsy, Parkinson’s disease, multiple sclerosis, amyotrophic lateral sclerosis (ALS), Huntington’s chorea, and Alzheimer’s disease.
Infections such as meningitis, encephalitis, polio, and epidural abscess can also affect the nervous system.
Structural disorders such as brain or spinal cord injury, Bell’s palsy, cervical spondylosis, carpal tunnel syndrome, brain or spinal cord tumors, peripheral neuropathy, and Guillain-Barré syndrome also strike the nervous system.
Study of the nervous system
The branch of medicine that studies and treats the nervous system is called neurology and doctors who practice in this field of medicine are called neurologists. Once they have completed medical training, neurologists complete additional training for their specialty and are certified by the American Board of Psychiatry and Neurology.
Neurosurgeons perform surgeries involving the nervous system and are certified by the American Association of Neurological Surgeons.
There are also physiatrists, who are physicians who work to rehabilitate patients who have experienced disease or injury to their nervous systems that impact their ability to function.
Some milestones in the study of the nervous system include:
- Ancient Egypt: Documents describe the meninges, membranes that envelop the central nervous system; the external surface of the brain; the cerebrospinal fluid; and the intracranial pulsations.
- Ancient Greece: Medical practitioners dissect the nervous system. Aristotle distinguishes between the cerebrum and the cerebellum.
- 1543: Andreas Vesalius published his “De humani corporis fabrica.” It includes detailed images depicting the ventricles, cranial nerves, pituitary gland, meninges, structures of the eye, the vascular supply to the brain and spinal cord, and an image of the peripheral nerves.
- 1664: Thomas Willis publishes his “Anatomy of the Brain,” followed by “Cerebral Pathology” in 1676. He removed a brain from the cranium, and was able to describe it more clearly, setting forth the circle of Willis — the circle of vessels that enables arterial supply of the brain. He describes epilepsy, apoplexy and paralysis.
- 1837: J.E. Purkinje (1787–1869) provides the first description of neurons, a very early description of cells of any kind.
- 1878: William McEwen (1848–1924) removes a meningioma — a brain tumor — and the patient survived for many years.
- 1886: Victor Horsley (1857–1916) develops a surgical procedure for medically intractable epilepsy.
- 1906: Sir Charles Scott Sherrington publishes “The Integrative Action of the Nervous System,” which describes the synapse and motor cortex.
- 1909: American surgeon Harvey Cushing (1869–1939) successfully removes a pituitary adenoma. Treating endocrine hyperfunction by neurosurgery was a major neurological landmark.
- 1960: Oleh Hornykiewicz shows that brain dopamine is lower than normal in Parkinson’s disease patients.
- 1974: M. E. Phelps, E. J. Hoffman and M. M. Ter Pogossian develop the first PET scanner.
- 1986: Stanley Cohen and Rita Levi-Montalcini share the Nobel Prize for Physiology and Medicine for their work on the control of nerve cell growth.
—Kim Ann Zimmermann