Nervous Tissue

NERVOUS TISSUE

Nervous tissue or nerve tissue:

Is a complex network of cells that makes up the brain,spinal cord and nerves which are responsible for controlling and coordinating many of body’s activities.

• It creates two parts of the nervous system,which are;
• 1.The brain and spinal cord of the Central nervous system (CNS) and

2.The peripheral nerves of the Peripheral nervous system (PNS) Nervous tissue is distributed throughout the body as an integrated communications network.

Components of Nervous Tissue

• Structurally, nerve tissue consists of two classes of cell types.

1.Neurons,

Nervous tissue is made up of billions of cells called neurons that are specialized to respond to stimuli and to transmit a signal to activate other cells.

2.Glial cells or neuroglia (non-excitable supporting cells), They participate in neural activity, neural nutritional and defence processes of the central nervous system.

A typical Neuron consist of:

Cell body, which has many radiating processes called dendrites, which are specialized to receive signals from other neurons.

Axon, a single long process, which is capable of generating a nerve impulse and conducting it over a long distance to stimulate other neurons in the CNS, or muscular or secretory cells elsewhere in the body.

Neurons usually receive information through dendrites; the information is integrated in the cell body and transmitted onwards via axons.

Types of Neurons

1.Unipolar neurons,

These neurons have single processes, the axon arising from the cell body.Unipolar neurons have no dendrites.

• 2.Bipolar neurons,

Have two processes (both axons) emerging directly and oppositely from the cell body.

Bipolar neurons are found in the cochlear and vestibular ganglia as well as retina and olfactory mucosa.

• 3.Multipolar neurons,
• Has one axon and many dozens of dendrites.Most neurons in the CNS are multipolar.

4.Pseudo unipolar neurons,

Has a single short process (an axon) with a T-shaped branching, one branch extending towards the CNS, and the other extending to a periphery ending.In typical pseudo unipolar neurons, both branches are axons on both structural and electrophysiological grounds.

Classification of Neurons according to their Functional Roles

Motor neurons,

They are involved in stimulating muscles or glands (effector organs) in the periphery.

Sensory neurons,

Sensory neurons receive sensory stimuli from the environment or from tissues and organs of the body.

Inter-neurons,

They maintain connection between neurons in the CNS, forming complex functional chains or circuits

Neuroglia (Glial Cells)

These are supporting cells found in the central nervous system and peripheral nervous system.There are six types of supporting cells whereas four are found in the CNS and two are found in the PNS.

• Neuroglia found in central nervous system are;

Astrocytes

Star-shaped, abundant, and versatile cells which guide the migration of developing neurons, they are found in large numbers adjacent to blood vessels.

They form a blood-brain barrier, i.e. the blood is separated from neurones by capillary wall and a layer of astrocytes foot process.

Microglia

Specialized immune cells that act as the macrophages of the CNS and they derived from monocytes.

Ependymal Cells

These cells form the epithelial lining of the ventricles of the brain and the central canal of the spinal cord.

Oligodendrocytes

These are found in clusters around cell bodies.They form and maintain the myelin, having the same functions as schwann cells in peripheral nerves.

These cells have clinical importance as most tumours of the brain are arising from them.

Glia Cells Found in the Peripheral Nervous System

Satellite cells, surround clusters of neuronal cell bodies in the PNS, they are of unknown function Schwann cells, form myelin sheaths around the larger nerve fibers in the PNS, they are for neuronal regeneration