Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Structure of the human body
Tissues, organs, organ systems, and apparatuses
Nervous tissue

Nervous Tissue is the primary structural element of the Organs of The Nervous system. It consists of Nerve Cells (neurocytes, or Neurons) and neuroglial cells, which are anatomically and functionally associated with them.

Neurons are capable of perceiving stimuli, entering a state of excitation, and generating and transmitting nerve impulses. They also participate in Processing, storing, and retrieving information from memory.

Neuroglial cells perform boundary, supportive, protective, and trophic Functions.

Each nerve Cell has a cell body, processes, and nerve endings (Fig. 13, see color insert). A nerve cell is surrounded by a Plasma Membrane, which can perceive external influences, conduct excitation, and ensure Metabolic exchange between The Cell and its environment. The cell body contains The Nucleus, as well as membrane-bound Organelles (Endoplasmic reticulum, Ribosomes, Mitochondria, Golgi apparatus, Lysosomes) and non-membrane-bound organelles (microtubules, neurofilaments, and microfilaments). Neurons are characterized by the presence of specialized structures: chromophilic substance (Nissl substance) and neurofibrils. Under stained histological preparations, the chromophilic substance appears as basophilic clumps (aggregates of granular endoplasmic reticulum), the presence of which indicates a high level of Protein Synthesis. Neurofibrils are bundles of microtubules and neurofilaments that participate in The transport of various substances.

Mature neurons have Two Types of processes. One long process is the neurite, or axon, which conducts nerve impulses away from the nerve cell body toward the effector organ. Depending on the velocity of Nerve Impulse propagation, two types of axonal transport are distinguished: slow transport, proceeding at a rate of 1—3 mm per day, and fast transport, proceeding at 5—10 mm per hour. The other processes of nerve cells are short and are called dendrites. In most cases, dendrites branch extensively, which gives rise to their name. Dendrites conduct nerve impulses toward the nerve cell body at a rate of 3 mm per hour (dendritic Transport of substances). Based on the number of processes, neurons are classified as unipolar (having a single process), bipolar (cells with two processes), and multipolar (having three or more processes). Pseudounipolar neurons are a variation of bipolar cells. A single common process emerges from their cell body, which then T-branches into an axon and a dendrite. Both dendrites and neurites terminate in nerve endings—sensory endings for dendrites and effector endings for neurites.

According to their functional significance, nerve cells are divided into receptor (sensory) neurons, effector neurons, and associative neurons. Sensory (afferent) neurons perceive external stimuli and conduct them toward the Spinal Cord or Brain. Effector (efferent) nerve cells transmit nerve impulses to working organs (Muscles, glands). Associative (intercalary, Relay) neurons transmit nerve impulses from the afferent neuron to the efferent one. There are also neurons whose function is to produce neurosecretion; these are secretory neurons.

In addition to neurons, nervous tissue contains neuroglial cells, which perform boundary, supportive, protective, and trophic functions. Neuroglia are subdivided into macroglial cells (gliocytes) and microglial cells (glial macrophages).

Macroglia, which develop from the embryonic elements of the neural tube, include ependymocytes (lining the spinal canal and the cavities of all brain ventricles), as well as astrocytes and oligodendrocytes. Ependymocytes, which cover the choroid plexus in the brain ventricles, are cuboidal in shape and participate in The formation of CEREBROSPINAL FLUID. Astrocytes are small cells with numerous branched outgrowths that form the supporting framework of the BRAIN AND SPINAL cord. Astrocytes also perform boundary and trophic functions, participating in metabolic processes. Oligodendrocytes surround the cell bodies and processes of neurons and form their sheaths.

Microglial cells are small cells originating from the mesenchyme that function as glial macrophages due to their mobility.

Nerve cell processes covered with sheaths are called nerve fibers. Structurally, nerve fibers are divided into thin unmyelinated (non-myelinated, amyelinated) and thick myelinated fibers. Each fiber consists of a nerve cell process (axon or dendrite) located in the center, known as the axis cylinder, and a surrounding sheath. In both unmyelinated and Cytology/practical/65.html">Myelinated nerve fibers, the sheath is formed by neuroglial cells (oligodendrocytes) termed neurolemmocytes (Schwann cells). In an unmyelinated nerve fiber, the axis cylinder is surrounded by a thin sheath (neurolemma) that may enclose not just one, but several (up to 10—20) axis cylinders belonging to different nerve cells.

Myelinated nerve fibers are thicker than unmyelinated ones. In myelinated fibers, the axis cylinder is surrounded by a sheath containing myelin (Lipids) in its inner layers. Externally, the myelinated fiber is covered by the outer sheath of neurolemmocytes, to which the Cytoplasm and nuclei of these cells are adjacent.

All nerve fibers terminate in terminal apparatuses—nerve endings. Functionally, three groups of endings are distinguished: receptor (sensory receptors), effector (effectors), and interneuronal endings, which mediate communication between neurons.

Receptor (sensory) nerve endings are the terminal apparatuses of the dendrites of sensory neurons. According to their Structure, they are divided into free and non-free nerve endings. Free nerve endings consist solely of terminal dendritic branches. Non-free nerve endings consist of the nerve fiber termination surrounded by a sheath (capsule). When a Connective Tissue capsule is present, the endings are called encapsulated. If there is no connective tissue capsule and only glial elements are present, the endings are referred to as unencapsulated.

Effector nerve endings are the terminal apparatuses of neurites in organs and tissues through which nerve impulses are transmitted to the tissues of effector organs (e.g., neuromuscular junctions) and glands (secretory endings).

Interneuronal nerve endings (synapses) are specialized terminal structures of the nervous system. Interneuronal synapses consist of a presynaptic membrane on the nerve ending and a postsynaptic membrane on another nerve cell. Between these membranes lies the synaptic cleft, into which BIOLOGICALLY ACTIVE SUBSTANCES (Neurotransmitters) are released from the presynaptic vesicles of the presynaptic component during the transmission of a nerve impulse. Depending on their Location, synapses are classified as axosomatic (axon ending on the cell body of another neuron), axodendritic (axon ending contacting the dendrite of another cell), and axoaxonic (axon of one cell contacting the axon of another nerve cell).

Within nervous tissue, nerve cells contact one another to form neuronal chains. The neurite of one cell establishes contact with the dendrites or cell bodies of other cells, which in turn form connections with subsequent nerve cells. At these contact sites, the membranes of two adjacent cells are separated by a cleft up to 20 nm wide. This close membrane apposition facilitates the transmission of nerve impulses from one nerve cell to neighboring ones. By connecting with other cells via synapses, nerve cells mediate all organismal responses to stimuli. The ensemble of neurons through which nerve impulses are transmitted forms a reflex arc.