There are two types of cells in nervous tissue: neurons, which conduct nerve impulses, and neuroglial cells, which support, protect, and nourish the neurons.
Neurons
Neurons vary in size and shape, but they all have three parts: the dendrite(s), the cell body, and the axon. The dendrites receive information from other neurons and conduct signals typically toward the cell body. The axon, on the other hand, conducts nerve impulses typically away from the cell body. The cell body contains the nucleus and other organelles typically found in cells.
There are two types of neurons. Motor neurons, which have short dendrites and a long axon, conduct nerve impulses from the central nervous system to effector organs-for example, either muscles or glands. Muscle tissue is located in the walls of internal organs and in the skeletal muscles. Glands are located throughout the body. Because motor neurons cause muscle fibers to contract and glands to secrete, they are said to innervate these structures.
Sensory neurons conduct nerve impulses from peripheral body parts to the CNS. Sensory neurons begin at specialized endings called receptors, which are sensitive to either external and/or internal stimuli. When a receptor is stimulated, a nerve impulse begins that travels along the sensory neuron to the CNS.
Sometimes, a sensory neuron is referred to as an afferent neuron, and a motor neuron is called an efferent neuron. These words, which are derived from Latin, mean "running to" and "running away from," respectively. Obviously, they refer to the relationship of these neurons to the CNS.
A third type of neuron, called an interneuron, or association neuron, is only found within the CNS. An interneuron conducts nerve impulses between various parts of the central nervous system, such as from one side of the brain or spinal cord to the other, or from the brain to the cord, and vice versa. An interneuron has short dendrites and either a long axon or a short axon.
The dendrites and axons of neurons are sometimes called processes or neuron fibers. Outside the eNS, long fibers are covered by a white myelin sheath formed by Schwann cells, which wrap themselves tightly around these fibers. The portions of the Schwann cells that contain cytoplasm and nuclei form a neurilemmal sheath. The neurilemmal sheath promotes regeneration of a nerve fiber if it is injured. Gaps between Schwann cells form the nodes of Ranvier and are important in nerve cell conduction.
Multiple sclerosis (MS) is a disease of the myelin sheath. Lesions develop that soon become hardened scleroses, or scars, that interfere with normal conduction of nerve impulses. The effects are widespread because of the number of fibers covered by a myelin sheath. The disease is chronic and tends to worsen with time.
Neuroglial Cells
Schwann cells are a type of neuroglial cell found outside the CNS. Microglial cells, astrocytes, and oligodendroglial cells and ependymal cells are among the neuroglia found inside the CNS. Microglial cells are small, phagocytic cells that migrate to the site of an injury, where they engulf microbes and clean away debris. Astrocytes have cytoplasmic processes that provide structural support by forming bridges between neurons and capillaries. They may have a nutritive function involving the transport of molecules between capillaries and neurons. Oligodendroglial cells produce the myelin sheath that envelops neurons in the CNS. Ependymal cells line cavities in the CNS and assist in the production and circulation of the cerebrospinal fluid that fills the cavities of the CNS.
Nervous tissue contains neurons and neuroglial cells. Each type of neuron has three parts (dendrite[s], cell body, and axon) but is specific as to function. Neuroglial cells support. protect, and nourish the neurons.
