The Nerve Tissue & The Nervous System
The nervous system is the most complex system in the human body & is formed by a network of more than 100 million nerve cells (neurons) assisted by many more glial cells.
Nerve tissue is distributed throughout the body as integrated communications network. Anatomically, the nervous system is divided into the central nervous system consisting of the brain & spinal cord; & the peripheral nervous system, composed of nerve fibers & small aggregates of nerve cells called nerve ganglia.
Structurally, the nerve system consists of two cell type:- nerve cells (neurons), which usually show long processes; & several types of glial cells, which has short processes, support & protect neurons, & defense processes of the central nervous system, while the function of neurons which respond to environmental changes (stimuli) by altering electrical potential that exist between inner & outer surface of their membrane.
The neurons are independent anatomic & functional units with complex morphologic characteristics. They are responsible for reception, transmission & processing of stimuli. Most neurons consist of three parts; cell body (perikaryon), dendrites & axon.
The neurons divided according to the size & shape of their processes; multipolar, bipolar & unipolar & pseudounipolar, which have single process that is close to the perikaryon & divides into two branches.
Neurons can also be classified according to their functional roles, motor (efferent) neurons control effector organs such as a muscle fibers & exocrine, endocrine glands. Sensory (afferent) neurons are involved in the reception of sensory stimuli from the environment & from the body.
In the central nervous system, nerve cell bodies are present only in the grey matter, white matter contains neuronal processes but no nerve cell bodies.
In the peripheral nervous system, cell bodies are found in ganglia & in some sensor region
(e.g. olfactory mucosa).
· Glial Cells:
Although neurons are the principle cells of nerve tissue, glial cells play an important supporting role, they surround both cell bodies & their axonal, dendrite processes that occupy the interneuronal spaces.
Nervous tissue has only a very small amount of extracellular matrix & glial cells, & they are classified according to the function & site which are present on it into three types:
1. Macroglia which are divided into:
a. Astrocytes
b. Oligodendrocytes
2. Ependam
3. Microglia
1. Macroglia:
A- Astrocytes: are star-shaped cells, because have multiple processes, astrocytes bind neurons to capillaries & to the pia mater (a thin connective tissue that covers the central nervous system). Astrocytes with few long processes are called (fibrous astrocytes) & are located in the white matter; & astrocytes with many short-branched processes, are found in the grey matter called (protoplasmic astrocytes). The astrocytes have processes with expanded end-feet (perivascular feet) that are linked to the endothelial cells by junctional complexes and transfer molecules & ions from the blood to the neurons.
B- Oligodendrocytes: produce the myelin sheath that provides the electrical insulation of neurons in the central nervous system.
2. Ependymal cells:
Are low columnar epithelial cells lining the ventricles of the brain & central canal of spinal cord, in some location ependymal cells are ciliated, which facilitates the movement of cerebrospinal fluid.
3. Microglia:
Are small elongated cells with short irregular processes. They are phagocytic cells that represent the mononuclear phagocytic system in the nerve tissue & not have the perivascular feet.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .