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Nervous tissue
Nervous tissue is distributed throughout the body as an integrated communication network. Two fundamental functions are:
- To detect, analyze, integrate and transmit all information generated by sensory stimuli and by mechanical and chemical changes that take place in the internal and external environment
- To organize and coordinate directly or indirectly most functions of the body especially the motor, visceral, endocrine, and mental activities
Division of the nervous system
A. Central nervous system(CNS) - compromising the brain and spinal cord. CNS contains nerve cells and supporting tissue neuroglia.
B. Peripheral nervous system (PNS) - all nervous tissue outside the CNS. PNS contains nerve fibers, ganglia and the supporting tissue loose areolar connective tissue.
In addition the nervous system is further divided into
A. Somatic nervous system - involved in voluntary functions B. Autonomic nervous system - involved in involuntary functions
Unit structure of nervous tissue is the nerve cell/ neuron or neurocytes. The system contains at least 10 billion neurons. Structurally NT consists of two cell type classifications: ● Nerve cells - shows numerous processes ● several types of glial cells or neuroglial cells which support and protect neurons and participate in neutral activity, neural nutrient and defense processes of the central nervous tissue
Meninges - connective tissue covering of the brain and spinal cord
cells of the nervous tissue
● neurons
Consists of the cell body(perikaryon) together with all of its processes up to their peripheral terminations. Found in gray matter of CNS. These cells are large and complex in shape. Size is about 4 to 400 diameters. The shape of a neuron depends on the number of cell processes.
Parts of the neuron (multipolar nerve cells)
- The nucleus is large, spherical or slightly ovoid, pale and centrally located in the cell body with a single conspicuous nucleolus with fine chromatin particles. It appears empty and pale or vesicular due to uniform dispersions of the chromatin so the nucleus is described as “owl’s eyes” or “fish eye” in appearance.
- Perikaryon/cyton/soma is the nerve cell body, large and surrounded by a thin plasma membrane.
- Neuroplasm is the cytoplasm filled with organelles and inclusions arranged concentrically around nucleus, which includes neurofibrils, chromophilic substance or Nissan bodies, mitochondria, centra some and various inclusions a. Nissl bodies appear as deeply basophilic masses or blocks wherein the principal constituent is the ribonucleoprotein. These structures vary in form, size and distribution. It has a nutrition function to the cell and its processes
b. Neurofibrils appear as slender, interlacing threads
c. Golgi apparatus as network of irregular wavy strands that are coarser than neurofibrils
d. Mitochondria are elongated and smaller
Processes of neurons
- Dendrites are broad processes tapering gradually along its length toward the ends. It is relatively short and divided into primary, secondary, tertiary and higher order branches. The surface is covered with numerous minute thorny spines or dendritic spines known as Gemmules, which often serve as sites of synaptic content. This process carries impulses toward the nerve cell body.
- Axon arises from a small conical elevation on the perikaryon devoid of Nissl bodies known as Axon Hillock or Implantation One. This is usually thinner, longer than dendrites and does not contain Nissl bodies. There is only one axon to each neuron.
Myelin sheath is a prominent sheath covering the axon and appears black in tissue.
GANGLIA
Ganglia are a collection of nerve cells located outside the CNS.
NEUROGLIA
Neuroglia is the supporting tissue of the CNS. The cells seen in this tissue are known as glial cells or neuroglial cells. There are several varieties of neuroglial cells:
1. Astrocytes This is the largest glial cell possessing numerous long processes and stellate in shape. The nucleus is large, ovoid and centrally located while the cytoplasm is granular. There are two types of astrocytes:
a. Protoplasmic astrocytes/mossy cell/short-rayed astrocytes. This is provided with numerous processes that are short, wavy giving off several short branches. Cytoplasm is filled with granules producing several spheroidal swellings called Gliosomes. One or more processes may have terminal expansions which contact blood vessels called perivascular foot or sucker foot plug the pores of the brain capillaries serving as a blood-brain barrier.
b. Fibrous astrocytes/spider cell/long-rayed astrocytes. This possesses fewer processes that appear longer and more or less straight with fewer branches. Within the cytoplasm are fibrillar structures. Gliosomes and perivascular foot are present and more numerous in the white matter.
2. Oligodendrocytes/Oligodendroglia These are smaller glial cells with a dark staining nucleus and scanty cytoplasm with very few short processes. 3. Microglia/mesoglia These are small cells with small but deeply stained nucleus surrounded by scanty cytoplasm, with few extensions are short and twisted in various ways covered with numerous tiny pointed twigs or spines. These cells are scattered everywhere throughout the brain and spinal cord. These cells are capable of ameboid movement and phagocytosis so this is considered as the macrophage of the brain.
