The neuron (nerve cell) is the basic functional unit of the nervous system. It is highly specialized to transmit electrochemical impulses from one neuron to another, forming a complex network of billions of nerve cells, thus constituting the parenchyma of the nervous system. It has two main anatomical characteristics; these are two types of protoplasmic processes, or outgrowths, that extend out from the neuron body, making it different in appearance and function from other cells in the body. Therefore, it has a short branching process, called 'dendrite', and a long thread-like extension, which is called 'axon'. The nerve cell receives nerve impulses from another neuron or sensory organ (eye, ear) through its dendrite, and then it relays this electrochemical impulse to another neuron through the axon.
Neuron dendrites are short, branched outgrowths which are specialized to receive information; these structures have a receptive membrane and are sensitive to specific physiological stimuli. The excitatory and inhibitory processes that are localized in the receptive membrane accumulate and act on the stimulus region, the most excitable area of the surface membrane of the neuron. This serves as the origin for the spreading bioelectric potentials. The longest outgrowth, the axon (or axis cylinder), is covered by a layer of phospholipid (fat), which is called myelin. It is an electrically excitable conducting membrane that serves to transmit the potentials. Having reached the terminal sections of the axon, the nerve impulse excites the secretory membrane; as a result of this, a physiologically active substance is secreted from the nerve endings. This active substance is called neurotransmitter.
The junction across which the nerve impulse jumps from a neuron axon terminal to another neuron dendritic branch is called synapse. There are also synapse in which the nerve impulse jumps from an axon to specialized sensitive fibers in skeletal muscles. This type of nerve impulse is called 'efferent' impulse, and this particular synapse is called 'neuromuscular junction' and the axon terminal endings 'motor end plate'.
Aside from its branching processes, a neuron has a nucleus and the same type of organelles contained in the cytoplasm as most Eukaryotic cells; These include lysosomes, endoplasmic reticulum, Golgi apparatus, and, above all, mitochondria. To produce enough energy in the form of ATP, a neuron needs a large amount of mitochondria, between 2,000 and 3,500 mitochondria per cell. The nerve cell requires a lot of energy to be able to generate the nerve impulse that makes the nervous system work. The life and function of a nerve cell depends on two types of support cells and a microphage; these form what is called the neuroglia, which consists of astrocytes, oligodendrocytes, and microglia.
Types of Neurons
The structure, dimensions, and shape of neurons vary widely. Neurons of the cerebral cortex, cerebellum, and some other areas of the central nervous system are small and have a pyramidal shape. The nerve cell in the motor cortex area (frontal lobe) is called Betz cell; this cell has the longest axon in the central nervous system. This axon arises from a Betz cell in the frontal lobe and then runs all the way down to the brainstem and spinal cord. On the anterior side of the medulla, billions of Betz cell myelinated axons cross from the left side over to the right, and those coming down on the right side cross over to the left. This is called 'decussation of the pyramids'.
Nerve cells having more than one dendrite is called multipolar neurons, which are characteristic of the brain of vertebrates. In such neurons, several dendrites and one axon emerge from the cell-body; the initial section of the axon serves as the excitatory region. Numerous nerve endings from the outgrowths of other neurons converge on the cell body and dendrites of a multipolar neuron. Neurons with two outgrowths are called bipolar; they occur most often as peripheral sensory neurons having one axon and one dendrite, which impinges on the cell surface.
Neurons are also classified according to their position in a reflex arc; these are: 1) afferent (sensory) neurons, which receive information from the external environment or from receptor cells; 2) interneurons, or internuncial neurons, are relay nerve cell connecting one neuron with another; 3) efferent neurons, or motoneurons, which transmit impulses to the organs of response, such as muscle or glands.
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| Above, two types of neurons: A) multipolar, which is common in the cerebral cortex; and B) bipolar, which is a sensory nerve cell for the transmission of sensory information from eye retina and from inner ear. |
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| A schematic picture of neuron synapse. |