Nervous Tissues: Structure and Function of Nervous Tissue

Nervous tissue is a major part of the nervous system, which is essential for responding to stimuli and transmitting impulses from one body organ to another. In the human body, nervous system is divided as - the central nervous system (CNS) and the peripheral nervous system (PNS). The former encompasses nervous tissue of the brain and spinal cord, whereas the latter consists of all the remaining nervous tissues excluding those of the brain and spinal cord such as cranial nerves, spinal nerves and autonomic nervous system (ANS).

Nervous Tissue: Structure and Function

Speaking about the anatomical structure of the nervous tissue, it is made up of neurons and support cells called glial cells or neuroglia. In the human brain, there are more than 100 billion neurons. Based on the functions, neurons are categorized into three types, namely, sensory, motor and interneurons. Sensory neurons, also referred to as afferent neurons, transmit impulses from the sensory organs to the CNS, whereas motor neurons conduct impulses from the CNS to the target organs. Interneurons, on the other hand, colligate the sensory and motor neurons.

Neurons
Neurons are the basic structural and functional units of nerve tissue. They perform the major function of transmitting nerves impulses. Anatomically, a nerve tissue comprises of many neurons that are joined by the connective tissues. Though neurons differ in shape and size, the basic structure is same for all the three types. A neuron consists of the following structural parts:

Cell body - The cell body (soma or perikaryon) is enclosed by a plasma membrane. It is made up of nucleus and cytoplasm. Inside the cytoplasm, granules (Nissl bodies), mitochondria, golgi complex, lysosomes and other cell organelles are present. The cell body is responsible for controlling the metabolic activities of the neuron.

Axon - An axon is an elongated structure (few millimeters to one meter) covered by a protective, insulating medullary sheath that helps in stimulating the nerve transmission. Medullary sheath is further composed of two layers; an inner fatty layer called the myelin sheath and an outside cellular layer called the neurilemma or sheath of Schwann cells. Nodes of Ranvier are present at specific intervals in the medullary sheath. In general, a nerve tissue consists of only one axon, which conducts nerve impulses from the cell body to muscles, other nerves and/or glands. Thus, an axon performs the function of transmitting output signals from the neuron.

Dendrites - Dendrites are short and non myelinated structures that conduct impulses from various nerves or organs towards the cell body. In simpler terms they transmit input signals to the neuron.

Neuroglia
Neuroglia or glial cells are protective and supportive structures of nervous tissue. They are found in bunches surrounding the neurons and have the ability to regenerate in case of injury. Neuroglia provide nutrition and immune protection to the neurons. In addition, they are responsible for the formation of myelin sheath and maintaining homeostasis inside the neurons. Some of the forms of neuroglia are astrocytes (provide metabolic support to nervous tissue) and oligodendrocytes (support axons), microglia (repair the damage of neurons).

The nerve tissues of the peripheral nervous system are responsible for collecting the signals from the organs and transmitting them to the central nervous system. Overall, the nervous system regulates and controls various functions of the body such as memory, emotion, reasoning and muscle contraction.