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Nervous tissue is the main component of the two parts of the nervous system; the brain and spinal cord of the central nervous system (CNS), and the branching peripheral nerves of the peripheral nervous system (PNS), which regulates and controls bodily functions and activity. It is composed of neurons, or nerve cells, which receive and transmit impulses, and neuroglia, also known as glial cells or more commonly as just glia,(from the Greek, meaning glue) which assist the propagation of the nerve impulse as well as providing nutrients to the neuron.
All living cells have the ability to react to stimuli. Nervous tissue is specialized to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus. Neurons are easily stimulated and transmit impulses very rapidly. A nerve is made up of many nerve cell fibers bound together by connective tissue. A sheath of dense connective tissue, the epineurium surrounds the nerve. This sheath penetrates the nerve to form the perineurium which surrounds bundles of nerve fibers. Blood vessels of various sizes can be seen in the epineurium. The endoneurium, which consists of a thin layer of loose connective tissue, surrounds the individual nerve fibers.
The cell body is enclosed by a cell (plasma) membrane and has a central nucleus. Granules called Nissl bodies are found in the cytoplasm of the cell body. Within the cell body, extremely fine neurofibrils extend from the dendrites into the axon. The axon is surrounded by the myelin sheath, which forms a whitish, non-cellular, fatty layer around the axon. Outside the myelin sheath is a cellular layer called the neurilemma or sheath of Schwann cells. The myelin sheath together with the neurilemma is also known as the medullary sheath. This medullary sheath is interrupted at intervals by the nodes of Ranvier.
Nerve cells are functionally made to meet each other at a junction known as a synapse, where the terminal branches of an axon and the dendrites of another neuron lie close to each other but normally without direct contact. Information is transmitted across the gap by chemical secretions called neurotransmitters. It causes activation in the post-synaptic cell. All cells possess the ability to respond to stimuli. The messages carried by the nervous system are electrical signals called impulses.
Classification of neurons
Neurons are classified both structurally and functionally.
Neurons are grouped structurally according to the number of processes extending from their soma (cell body). Three major neuron groups make up this classification:
- Multipolar neurons (3+ processes)
- These are the most common neuron type in humans (more than 99% of neurons belong to this class) and the major neuron type in the central nervous system (CNS).
- Bipolar neurons
- Bipolar neurons are spindle-shaped, with a dendrite at one end and an axon at the other. An example can be found in the light-sensitive retina of the eye. They also rapidly grow.
- Unipolar neurons
- Sensory neurons have only a single process or fibre which branches close to the cell body into an axon and a dendrite.
Neoplasms (tumors) in nervous tissue include:
- Gliomatosis cerebri, Oligoastrocytoma, Choroid plexus papilloma, Ependymoma, Astrocytoma (Pilocytic astrocytoma, Glioblastoma multiforme), Dysembryoplastic neuroepithelial tumour, Oligodendroglioma, Medulloblastoma, Primitive neuroectodermal tumor
- Neuroepitheliomatous tumors
- Ganglioneuroma, Neuroblastoma, Atypical teratoid rhabdoid tumor, Retinoblastoma, Esthesioneuroblastoma
- Neurofibroma (Neurofibrosarcoma, Neurofibromatosis), Schwannoma, Neurinoma, Acoustic neuroma, Neuroma
- Nerve fascicle
- Connective tissue in the peripheral nervous system