Human Anatomy and Physiology (with Age-Specific Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Structure of the Human Body
Tissues
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 their associated neuroglial cells.

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

Each nerve Cell has a cell body, processes, and nerve endings. The nerve cell is surrounded by a Plasma Membrane capable of conducting excitation and facilitating 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: chromatophilic substance (Nissl bodies) and neurofibrils. Chromatophilic substance appears as basophilic clumps (aggregations of rough endoplasmic reticulum), the presence of which indicates a high level of Protein Synthesis. Neurofibrils are bundles of microtubules and neurofilaments involved in The transport of various substances.

Mature neurons have Two Types of processes. One is a long process called a neurite, or axon, which conducts nerve impulses away from the nerve cell body. Depending on the speed of Nerve Impulse movement, two types of axonal transport are distinguished: slow, proceeding at a rate of 1—3 mm per day, and fast, proceeding at a rate of 5—10 mm per hour. Other processes of nerve cells are short and are called dendrites. In most cases, they branch profusely, which gives them 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 one 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 extends from their cell body and then branches in a T-shape into an axon and a dendrite.

Both dendrites and neurites terminate in nerve endings. In dendrites, these are sensory endings, while in neurites, they are effector endings.

Functionally, nerve cells are divided into receptor neurons (sensory, afferent), which bring impulses to the Brain; effector neurons (causing action, efferent), which carry impulses away from the brain; and associative (interneurons) neurons. Sensory (afferent) neurons perceive external stimuli and conduct them toward the Spinal Cord or brain. Effector (efferent) nerve cells transmit nerve impulses to effector organs (Muscles, glands). Associative (interneurons, projection) neurons transmit nerve impulses from afferent to efferent neurons. There are also neurons whose function is to produce secretions; these are neurosecretory neurons.

Neuroglia. In addition to neurons, nervous tissue contains neuroglial cells, which perform barrier, supportive, protective, and trophic Functions. These are classified into macroglia (gliocytes), which develop from neural tube elements, and microglia (glial macrophages), which develop from the mesenchyme.

Macroglia include ependymocytes, which line the cavities of the brain ventricles, as well as astrocytes and oligodendrocytes.

Astrocytes serve as support for nerve cells, isolate and bundle nerve fibers, and participate in metabolic processes.

Oligodendrocytes surround the cell bodies and processes of neurons, forming their sheaths.

Microglial cells are small cells that function as glial macrophages and are capable of amoeboid movement.

The processes of nerve cells, covered by sheaths, are called nerve fibers. Based on their Structure, nerve fibers are divided into thin unmyelinated (non-myelinated, amyelinic) and thick myelinated fibers. Each nerve fiber consists of a nerve cell process, which lies at the center of the fiber and is called the axon (axial cylinder), and its surrounding sheath. Unmyelinated and Cytology/practical/65.html">Myelinated nerve fibers have different sheaths.

An unmyelinated nerve fiber has a thin sheath around the axon called the neurolemma. A myelinated nerve fiber has a myelin layer consisting of Lipids around the axon, with the neurolemma located externally to it.

All nerve fibers terminate in end-organs known as nerve endings. Based on their functional significance, three groups of endings are distinguished: receptor (sensory receptors), effector (effectors), and interneuronal, which establish connections between neurons.

Receptor (sensory) nerve endings are the terminal apparatuses of the dendrites of sensory neurons. They are classified into free nerve endings (consisting only of dendritic branchings), encapsulated endings (consisting of dendritic branchings, glial cells, and a Connective Tissue capsule), and non-encapsulated endings (lacking a connective tissue capsule).

Effector nerve endings are the terminal apparatuses of axons in organs and tissues, through which nerve impulses are transmitted to the tissues of effector organs.

Interneuronal nerve endings (synapses) are located on adjacent nerve cells. Synapses enable nerve impulses to be transmitted from one nerve cell to another.

In nervous tissue, nerve cells contact each other, forming chains of neurons. The axon of one cell makes contact with the dendrites or cell bodies of other cells, which, in turn, form connections with subsequent nerve cells. At these contact sites, known as synapses, the membranes of two adjacent cells are separated by a cleft up to 20 nm wide. This close proximity of membranes facilitates the transmission of nerve impulses from one nerve cell to neighboring ones. By connecting to other cells via synapses, nerve cells mediate all of the body's responses to stimuli.

The pathway of neurons through which nerve impulses are transmitted forms a reflex arc. A reflex arc is a chain of neurons connected by synapses, including the first (sensory) neuron and the last (effector) neuron, along which a nerve impulse travels from its point of origin (at a sensory nerve ending) to an effector organ (Muscle, gland). The simplest reflex arc consists of two neurons: a sensory neuron and a motor neuron. In the vast majority of cases, one or more interneurons (associative neurons) are interposed between the sensory and motor neurons (Fig. 10).