ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

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

1. Features of Animal Tissues

1.3. Muscle Tissue

Muscle Tissues are characterized by their ability to contract in Response to nerve impulses, as the Cytoplasm of muscle fiber Cells contains specialized contractile filaments — myofibrils (from Greek myos — muscle, New Latin fibrilla — fiber, thread). This enables the movement of the body and its individual parts. Muscle tissues also possess the capacity for regeneration.

The following types of Muscle tissue are distinguished: striated (skeletal and cardiac) and non-striated.

✵ Cytology/practical/58.html">Striated Skeletal Muscle tissue (Fig. 9) consists of cylindrical fibers (both long and short). Each muscle fiber is covered by an excitable Cell/33.html">Plasma Membrane — the sarcolemma, which encloses the sarcoplasm containing numerous nuclei. The sarcoplasm of each muscle fiber contains A large number of contractile threads — myofibrils, ranging in diameter from 0.5 to several μm. The bulk of a myofibril is composed of protein filaments — myofilaments, or protofibrils.

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Fig. 9. Striated skeletal muscle tissue (individual fibers)

Two Types of myofilaments are distinguished:

✵ thin Actin filaments (1000-1100 nm long, 5-8 nm in diameter);

✵ thick Myosin filaments (1500 nm long, 10-15 nm in diameter).

These filaments give the Muscles their striated appearance, which can be observed under a Microscope.

Although these Proteins are called contractile, they are not actually capable of contracting on their own. The Mechanism of muscle fiber shortening lies in the contraction of the actin-myosin complex formed by these two proteins.

Actin and myosin molecules interact with each other, causing the myosin filaments to slide past the actin filaments, which shortens the myofibril.

Since the work of a muscle fiber is highly intensive and requires a lot of energy and oxygen, it contains numerous Mitochondria. This tissue is part of the skeletal and facial muscles, as well as the Muscles of the Mouth, Tongue, Pharynx, Larynx, upper Esophagus, and Diaphragm.

Groups of muscle fibers surrounded by a Connective Tissue sheath form muscle fascicles. Groups of muscle fascicles surrounded by a connective tissue sheath form a muscle.

Striated skeletal muscles are capable of rapid contraction. They contract both voluntarily and involuntarily.

✵ Striated cardiac muscle tissue, like skeletal muscle, has striated myofibrils. Unlike skeletal muscle, cardiac muscle cells (cardiomyocytes) do not have connective tissue sheaths; instead, they form specialized junctions with each other — Gap Junctions (nexuses, where outer membranes meet), linking The cardiac muscle into a single functional unit. Unlike skeletal muscle, cardiac muscle contracts involuntarily. Its activity occurs automatically and is regulated by impulses originating within The Heart itself, as well as those arriving from the Central Nervous system via specific nerves.

Cardiac muscle cells do not divide, so a connective tissue scar forms at the site of injury. Such a scar can block the conduction of contractile impulses and disrupt heart function.

✵ Non-striated (smooth) muscle tissue (Fig. 10) consists of mononuclear, spindle-shaped cells that lack striations. The cytoplasm of these cells contains actin and myosin contractile proteins, but they do not form an ordered Actomyosin complex. Therefore, contractions of these muscles occur much more slowly than those of striated muscles, but they can be sustained for a long time without significant Energy Expenditure.

Fig. 10. Non-striated (smooth) muscle tissue.

Non-striated (smooth) muscle tissue is part of the walls of most Internal Organs — The Stomach, intestines, Blood Vessels, Spleen, and Urinary Bladder. Contraction of this tissue occurs involuntarily (without conscious control). It is capable of remaining in a partially contracted state for a long time, which is crucial for maintaining the lumen of tubular structures (vessels, intestines).