Nervous Tissue

Neuron
This is a colored scanning electron micrograph (SEM) of a neuron (nerve cell). The cell body is the central structure with neurites (long and thin structures) radiating outwards from it. A neurite is a general term used for processes connecting nerve cells together to form a network of nervous tissue.

STEVE GSCHMEISSNER/Getty Images

Nervous tissue is the primary tissue that composes the central nervous system and the peripheral nervous system. Neurons are the basic unit of nervous tissue. They are responsible for sensing stimuli and transmitting signals to and from different parts of an organism. In addition to neurons, specialized cells known as glial cells serve to support nerve cells. As structure and function are very much intertwined within biology, the structure of a neuron is uniquely suited to its function within nervous tissue.

Neurons

A neuron consists of three major parts:

  • Cell Body: The central cell body contains the neuron's nucleus, associated cytoplasm, and other organelles.
  • Axons: This part of the neuron transmits information and extends away from the soma or cell body. It typically carries signals away from the cell body, but occasionally receives impulses from axoaxonic connections.
  • Dendrites: Dendrites are similar to axons, but tend to be multibranched extensions that typically carry signals toward the cell body. They generally receive neurochemical impulses from the axons of other cells.

Neurons usually have one axon (can be branched, however). Axons usually terminate at a synapse through which the signal is sent to the next cell, most often through a dendrite. This is known as an axodendritic connection. However, axons can also terminate on the cell body, an axosomatic connection, or on the length of another axon, known as an axoaxonic connection. Unlike axons, dendrites are usually more numerous, shorter and more branched. As with other structures in organisms, there are exceptions. There are three types of neurons: sensory, motor, and interneurons. Sensory neurons transmit impulses from sensory organs (eyes, skin, etc.) to the central nervous system. These neurons are responsible for your five senses. Motor neurons transmit impulses from the brain or spinal cord toward muscles or glands. Interneurons relay impulses within the central nervous system and act as a link between sensory and motor neurons. Bundles of fibers composed of neurons form nerves. Nerves are sensory if they consist of dendrites only, motor if they consist of axons only, and mixed if they consist of both.

Glial Cells

Glial cells, sometimes called neuroglia, do not conduct nerve impulses but perform a number of support functions for nervous tissue. Some glial cells, known as astrocytes, are found in the brain and spinal cord and form the blood-brain barrier. Oligodendrocytes found in the central nervous system and Schwann cells of the peripheral nervous system wrap around some neuronal axons to form an insulating coat known as the myelin sheath. The myelin sheath aids in the faster conduction of nerve impulses. Other functions of glial cells include nervous system repair and protection against microorganisms.