ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 5. THE TISSUE LEVEL OF ORGANIZATION OF LIVING MATTER. ANIMAL TISSUES

1. Features of Animal Tissues

1.4. Nervous Tissue

Nervous Tissue forms The basis of the Organs of The Nervous system. Its Functions include receiving, storing, and Processing information, as well as regulating and coordinating the activities of various body systems.

Nervous tissue consists of Nerve Cells (Neurons) and neuroglia.

✵ A neuron (from Greek neuron — sinew, thread, nerve) is the Structural and functional unit of nervous tissue. Each neuron is a mononuclear Cell consisting of a cell body and Two Types of processes: dendrites and axons (Fig. 11).

A dendrite (from Greek dendron — tree) is a short, highly branched process of a neuron that conducts excitation to The Cell body from receptors or other nerve cells. The collective cell bodies and dendrites form the Gray matter of the Brain AND SPINAL cord.

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Fig. 11. Neuron:

A — multipolar; B — unipolar; C — bipolar; 1 — axon; 2 — dendrite.

An axon (from Greek axon — axis) is an elongated process of a neuron that transmits impulses away from the cell body to other neurons or organs. The collective axons form the White matter OF the brain and Spinal Cord.

The SHAPES AND SIZES of neuron cell bodies vary significantly, as do the type, number, and length of their processes:

- a unipolar neuron has a single process that divides into two branches of a single axon;

- a bipolar neuron has two processes;

- a multipolar neuron has multiple processes and is localized in the Cerebral Cortex.

A single cell can have anywhere from one to 1000 dendrites. The length of an axon can range from a few centimeters to 1-1,5 meters. An axon can make contact with hundreds of cells. The long processes of cells form nerve fibers, which consist of an axis cylinder made of Cytoplasm with its neurofibrils, and two sheaths. The inner, thicker sheath, composed of a fat-like substance, is called the myelin sheath. The myelin sheath is interrupted at regular intervals, leaving sections of the axis cylinder exposed. These are the nodes of the nerve fiber — the nodes of Ranvier. The outer sheath, consisting of flat cells, is called the Schwann sheath. Most nerves that innervate the body (Muscles, ligaments, tendons, periosteum) are myelinated. Unmyelinated fibers are thin, isolated from one another only by a thin, structureless endothelial sheath, and are found primarily in the NERVES OF THE Autonomic nervous system.

The MOST IMPORTANT PROPERTIES of nerve cells are excitability and conductivity. Excitability is The ability to respond to a stimulus with a specific type of activity. In a neuron, stimulation triggers an increase in vital processes, known as excitation. Excitation that originates in one area spreads throughout the entire neuron and is then transmitted to neighboring neurons. This ability of excitation to propagate is called conductivity.

Neurons are unable to divide and reproduce.

✵ Neuroglia consists of cells (gliocytes) that surround neurons and makes up the majority of cells in the nervous system. Neuroglial cells are 3-4 times smaller than neurons and have few processes. They do not conduct impulses themselves, but neurons cannot survive without them. Neuroglia performs various functions: supportive, trophic, protective, and secretory.

Unlike neurons, neuroglia does not lose the ability to divide. Thus, if a nerve fiber is damaged or severed, its severed part dies off. In this case, neuroglial cells multiply, aligning themselves into bands. The processes of neurons then grow along these bands, thereby restoring the damaged fibers.