Histology of Muscle Tissue

Overview of Muscle Tissue Types

Muscle tissue is specialized for contraction and is essential for movement, posture, and many physiological functions. There are three main types of muscle tissue, each with distinct structural and functional characteristics.

• Skeletal Muscle: - Attached to bones; responsible for voluntary movement of the skeleton. - Multinucleated cells. - Highly organized with visible striations. - Fibers are long and cylindrical. - Voluntary control.

• Smooth Muscle: - Located in the walls of internal organs (e.g., digestive tract, blood vessels). - Also called visceral muscle. - Involuntary control. - Single nucleus per cell. - Lacks striations; fibers are spindle-shaped and tapered at the ends.

• Cardiac Muscle: - Found only in the heart wall. - Involuntary control. - Single nucleus per cell. - Has visible striations. - Fibers are branched and interconnected. - Contains intercalated discs (unique to cardiac muscle), which facilitate synchronized contraction.

Gross Anatomy of the Muscular System

Muscle Cell Structure

Muscle cells (also called muscle fibers) have specialized structures for contraction and force generation.

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions, essential for muscle contraction.

• Myofibrils: Cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.

• Sarcomere: The functional unit of muscle contraction, defined by the area between two Z-lines. Contains thick (myosin) and thin (actin) filaments.

Muscle Organization

Muscle tissue is organized into bundles surrounded by connective tissue layers:

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Surrounds bundles of muscle fibers called fascicles.

• Epimysium: Surrounds the entire muscle.

• Connective tissue layers merge at the ends of muscles to form tendons (cord-like) or aponeuroses (sheet-like), which attach muscle to bone.

Neuromuscular Junction

Mechanism of Muscle Activation

The neuromuscular junction is the site where a motor neuron communicates with a skeletal muscle fiber to initiate contraction.

• A nerve impulse (action potential) travels down the motor neuron to the axon terminal.

• The axon terminal contains synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).

• ACh is released into the synaptic cleft via exocytosis.

• ACh binds to receptors on the motor end plate of the muscle fiber, increasing membrane permeability to sodium ions.

• Sodium influx causes depolarization of the sarcolemma, leading to muscle contraction.

Muscles of the Head and Neck

Identification, Origin, Insertion, and Action

Muscles are identified by their location, size, shape, direction of fibers, number of origins, and action. Understanding the origin, insertion, and action is essential for describing muscle function.

• Origin: The fixed attachment point of the muscle (usually proximal).

• Insertion: The movable attachment point (usually distal).

• Action: The movement produced when the muscle contracts.

• Prime mover (agonist): Muscle responsible for a specific movement.

• Antagonist: Muscle that opposes the action of the agonist.

Examples of Naming Conventions:

• Location: frontalis (forehead), femoris (thigh)

• Size: maximus (large), minimus (small), longus (long), brevis (short)

• Shape: deltoid (triangle), orbicularis (circular)

• Direction of fibers: rectus (straight), transversus (transverse), oblique (angled)

• Number of origins: biceps (two heads), triceps (three heads)

• Action: flexor (flexes), extensor (extends), adductor (adducts)

Review of Muscle Actions

Muscle actions are described by the movement they produce:

• Abduction: Moves a limb away from the midline.

• Adduction: Moves a limb toward the midline.

• Medial rotation: Rotates a limb toward the midline.

• Lateral rotation: Rotates a limb away from the midline.

Selected Muscles of the Face and Neck

Key muscles and their functions:

• Zygomaticus major/minor: Originates from the zygomatic bone; elevates the upper lip (smiling).

• Levator scapulae: Originates from transverse processes of cervical vertebrae; inserts on the medial border of the scapula; elevates and rotates the scapula.

• Thyrohyoid: Originates from thyroid cartilage; inserts on the hyoid bone; depresses the hyoid and elevates the larynx.

• Masseter: Originates from the zygomatic arch; inserts on the mandibular ramus; elevates and protrudes the mandible (chewing).

Table: Example Muscle Origins, Insertions, and Actions

Additional info:

• Muscle contraction is regulated by the sliding filament theory, where actin and myosin filaments slide past each other to shorten the sarcomere.

• Calcium ions released from the sarcoplasmic reticulum are essential for initiating contraction.

• Muscle names often reflect their anatomical position, function, or structure.