Human Anatomy and Physiology - N. I. Fedyukovich 2003

Cells and Tissues
Tissues
Nervous Tissue

Nervous Tissue is the primary component of The Nervous system, enabling the conduction of signals (impulses) to the Brain, their transmission and integration, establishing The connection between the body and the external environment, participating in the coordination of internal bodily Functions, and ensuring its integrity. Nervous tissue consists of Nerve Cells—Neurons (neurocytes), which have a specialized Structure and function, and neuroglia, which perform trophic, supportive, protective, and other functions. Nervous tissue forms the Central Nervous System (BRAIN AND SPINAL cord) and the Peripheral Nervous System—nerves (plexuses, ganglia).

Neurons are the functional units of the nervous system, forming numerous connections. They are sensitive to stimuli and capable of transmitting electrical impulses from peripheral receptors to effector Organs (Fig. 7). Nerve cells differ in shape, size, and the branching of their processes. Neurons with a single process are called unipolar, those with two are bipolar, and those with three or more are multipolar (Fig. 8).

Two Types of processes are distinguished: dendrites and axons. Dendrites conduct excitation toward The Cell body of the nerve cell. They are short and branch into fine ramifications. Along the axon, the Nerve Impulse travels away from the cell body to an effector organ (a gland or Muscle) or to another nerve cell. Neuroglial cells line the ventricles of the brain and the central canal of the Spinal Cord, form the supportive framework of the central nervous system, and surround the cell bodies and processes of neurons.

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Fig. 7. Structure of a neuron (diagram):

I — sensory neuron: 1 — nerve endings; 2 — axon; 3 — Nucleus; 4 — cell body; 5 — dendrite; 6 — myelin sheath; 7 — receptor; 8 — organ; 9 — neurilemma; II — motor neuron: 1 — dendrites; 2 — axon; 3 — synaptic terminal; 4 — node of Ranvier; 5 — nucleus of Schwann cell; 6 — Schwann cell; III — interneuron: 1 — axon; 2 — dendrites; 3 — nucleus; 4 — cell body; 5 — dendron

Fig. 8. Types of neurons:

A — unipolar; B — bipolar; C — multipolar

Axons are thinner than dendrites, and their length can reach up to 1.5 m. The distal region of the axon branches into numerous terminals with synaptic vesicles at their ends, connecting via junctions (synapses) with other neurons or organs. At synapses, excitation is transmitted from one cell to another or to an organ by Neurotransmitters (acetylcholine, norepinephrine, serotonin, dopamine, etc.). Bundled together, these processes form nerve fascicles. Nerve fibers can be myelinated (medullated) or unmyelinated (non-medullated). In the former, the nerve fiber is covered by a myelin sheath resembling a sleeve. The myelin sheath is interrupted at regular intervals, forming the nodes of Ranvier. Externally, the myelin sheath is surrounded by a non-elastic membrane—the neurilemma.

Cytology/practical/64.html">Unmyelinated nerve fibers lack a myelin sheath and are predominantly found in Internal Organs.

Bundles of nerve fibers form nerves, which are covered by a Connective Tissue sheath called the epineurium. Inward extensions of the epineurium form the perineurium, which divides the nerve fibers into smaller bundles and surrounds them.

Nerve fibers terminate in specialized structures called nerve endings. Depending on their function, they are classified into sensory (receptors) and motor (effectors) endings. Sensory nerve endings perceive stimuli from the external and internal environments, convert them into nerve impulses, and transmit them to other cells and organs. Receptors that detect stimuli from the external environment are called exteroceptors, while those from the internal environment are interoceptors. Proprioceptors detect stimuli within body tissues, located in Muscles, ligaments, tendons, bones, etc. Depending on The Nature of the stimulus, they are classified into thermoreceptors (detecting Temperature changes), mechanoreceptors (detecting Touch and pressure on the Skin), and nociceptors (detecting painful stimuli).

Motor nerve endings transmit nerve impulses (excitation) from nerve cells to the effector organ. Effectors that transmit impulses to the smooth muscles of internal organs, Blood Vessels, and glands are structured as follows: the terminal branches of motor neurons approach the cells and make contact with them.

Motor nerve endings in skeletal muscles have a complex structure and are called motor endplates.

Nerves that transmit impulses to the central nervous system are called afferent (sensory), while those carrying impulses from the center are efferent (motor). Afferent and efferent neurons are connected via interneurons. Mixed nerves transmit impulses in both directions. The transmission of a nerve impulse from one neuron to another occurs through junctions called synapses.