Nervous system:
Cells
The nervous system consists of two main categories or types of cells: neurons and glial cells.
Neurons
The nervous system is defined by the presence of a special type of cell, a neuron (sometimes called a "neurone" or "nerve cell"). Neurons can be separated from other cells in many ways, but their most important property is that they communicate with other cells through synapses, a compound that contains molecular mechanisms that allow for the rapid transfer of signals, either electrical or chemical. Many types of neurons have axons, protrasmic protrusion that can extend to distant parts of the body and form thousands of synaptic contacts. Axons often travel through the body through bundles called nerves (in the PNS) or tissues (in the CNS).
Even in the same nervous system as humans, there are hundreds of different types of neurons, with a wide variety of morphologies and functions. These include sensory nerves that transmit stimuli such as light and sound into neural signals, as well as motor neurons that transmit neural signals to activate muscles or glands. In most species, however, most neurons receive all their input from other neurons and transmit their effect to other neurons.
Body cells
Glial cells (named after the Greek word for “glue”) are non-neuronal cells that provide support and nourishment, maintain homeostasis, form myelin, and participate in signal transduction in the nervous system (Allen, 2009). In the human brain, it is currently estimated that the total number of glia is approximately equal to the number of neurons, although size varies in different parts of the brain (Azevedo et al., 2009). Among the most important functions of glial cells are supporting neurons and holding them in place; nutrient supply to neurons; to protect neurons electronically; destroying viruses and removing dead neurons; and to provide directional guidance that directs the axons of neurons to their targets. The most important set of glial cells (oligodendrocyte in the spinal CNS, and Schwann cells in the PNS) produce layers of fatty material called myelin that wrap axons and provide electrical implants that allow them to transmit signals faster and more efficiently.
Anatomy of the vertebrate
Figure 1: Major segregation of the spinal nervous system.
The nervous system of vertebrates is divided into two parts, the central nervous system (CNS) and the peripheral nervous system (PNS).
The CNS is the largest organ, and it includes the brain and spinal cord. The CNS is closed and protected by the meninges, a system consisting of three strands separated by a membrane, including a hard outer layer of skin called the dura mater. The brain is also protected by a skull, and a spinal cord with vertebral bones. The blood vessels that enter the CNS are surrounded by cells that form a strong chemical marker called the blood-brain barrier, which prevents many types of chemicals present in the body from entering the CNS.
The peripheral nervous system (PNS) is the collective name for the structures of the nervous system that are not within the CNS. Most axons called nerve endings are considered PNS, even if the cells of the hygienic cells reside within the brain or spinal cord. PNS is divided into "somatic" and "visceral" components. The somatic component contains nerves that do not allow skin, joints, and tissues. The cells of the somatic sensory nerve cells lie at the roots of the spinal cord. The visceral part, also known as the autonomic nervous system, contains neurons that protect the internal organs, blood vessels, and glands. The autonomic nervous system itself consists of two parts: the sensory nervous system and the parasympathetic nervous system. Some authors also include nerve endings with cell bodies lying at the edges (of nerve-like sensors) as part of the PNS; others, however, leave them (Hubbard, 1974, p. vii).
The spinal cord system can also be divided into so-called gray areas ("gray matter" in British spelling) and white matter. Gray (which is gray only in stored tissue, and better defined as pink or light brown in living tissues) contains a high number of nerve cell carcasses. The white matter is composed mainly of axes wrapped in myelin, and takes its color from myelin. The white matter encompasses all the senses of the body, and much of the interior of the brain and spinal cord. Graying is found in groups of neurons in the brain and spinal cord, as well as in the cortical organs that replace them. There is a body assembly where a group of neurons in the brain is called a "nucleus", and a group of peripheral neurons is called a "ganglion". There are, however, a few exceptions to this rule, especially the part of the brain called the basal ganglia.
Comments
Post a Comment