Neurons

Although the relationships between neurons—the cells that make up the nervous system—are extremely complex, all these cells share a common basic structure: they consist of a cell body containing a nucleus and processes—one axon and one or more dendrites. Neurons vary greatly in the shape of the cell body and in the length, number and degree of branching of axons and dendrites.

Neurons are divided into sensitive (sensory), motor and intercalary. In sensory neurons, dendrites are connected to receptors, and axons are connected to other neurons; in motor neurons, dendrites are connected to other neurons, and axons are connected to some effector; In interneurons, both dendrites and axons connect to other neurons.

The simplest path along which a nerve impulse can travel consists of three neurons: one sensory, one intercalary and one motor. Nerve trunks, or nerves, consist of a large number of axons and dendrites united in a common connective tissue sheath.

The cell bodies of neurons and their processes are not scattered randomly, but form clusters called ganglia if they are located outside the brain and spinal cord, and nerve centers if they are located in the brain or spinal cord.

The axon or dendrite is covered, in addition to the cell membrane, by one or two more sheaths: the outer neurilemma and the inner myelin sheath. The neurilemma consists of cells. The myelin sheath is made of a noncellular, fat-like material that makes the nerves it covers appear white.

The myelin sheath appears to increase the speed of nerve impulses. In humans, a nerve impulse travels along the myelin fiber at a speed of about 100 m/sec, and along nerves with a thin myelin sheath at a speed of 7-15 m/sec.

The neurilemma appears to play some role in the regeneration of severed nerve fibers. In case of fiber rupture, if the ends of the nerve trunk are connected, the nerve fibers can grow into the neurilemmal tubes of the degenerated fibers and restore the lost functions.