Nervous System

The functional unit of the nervous system is the neuron, a cell specialized to receive and transmit impulses. The types and functions of neurons found in organisms seem to be directed by several regulatory genes and by certain cues that occur during development. Once neurons mature, they lose the ability to divide. An exception is the olfactory neurons which are replaced every 60 days from stem cells resting beneath them, which ensures a supply as they wear out. Even though there are a variety of neurons, the essential structures are the same in each: a cell body containing the nucleus and two kinds of processes extending from it, the axon and dendrite. Axons transmit impulses away from the cell body to the dendrites of adjoining neurons. Some axons may be over 3 ft (1 m) in length, such as the sciatic nerve which runs from the spinal cord to the lower leg. The axons of the peripheral nerves are enclosed in a fatty (myelin) sheath formed from specialized cells called Schwann cells. The myelin sheath acts to insulate the axon, which helps to accelerate the transmission of a nerve impulse. Gaps along the sheath expose the axon fiber and are important in allowing nerve impulses to jump from one section of the axon to another. The speed at which nerve impulses travel depends on the diameter of the axon and the presence of the myelin sheath with some impulses from the large motor nerves to the leg muscles traveling as fast as 394 ft (120 m) per second. Damage to the sheath in multiple sclerosis patients causes impaired muscle control and other symptoms, an indication of the importance of the myelin sheath in the transmission of nerve impulses. Axons are bundled together and enclosed by connective tissue to form nerves. Dendrites are usually highly branched extensions of the cell body which receive impulses from axons. In some cases they may be very small, as seen in some of the neurons of the brain, or long, as is the sensory dendrite which runs from the foot to the spinal cord.