Muscular Tissue

Overview of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and various bodily functions. There are three main types of muscle tissue in the human body, each with distinct structural and functional characteristics.

• Muscle tissue types share some properties but differ in histology, location, and control mechanisms.

• All muscle tissues are controlled by the nervous system, but the specific control (voluntary or involuntary) varies.

Types of Muscle Tissue

Skeletal Muscle

Skeletal muscle is responsible for voluntary movements and is attached to bones, skin, or fascia. It is characterized by its striated appearance and multinucleated, cylindrical cells.

• Location: Attached to bones, skin, or fascia.

• Appearance: Striated (alternating light and dark bands under the microscope), unbranched, cylindrical fibers.

• Nuclei: Multiple, peripherally located nuclei.

• Control: Voluntary (conscious) control of contraction and relaxation.

Cardiac Muscle

Cardiac muscle is found only in the heart wall and is responsible for pumping blood throughout the body. It is striated like skeletal muscle but has unique features for its function.

• Location: Heart wall.

• Appearance: Striated, branched fibers with a single, centrally located nucleus.

• Special Features: Cells are connected by intercalated discs containing gap junctions, allowing rapid electrical communication.

• Control: Involuntary; has autorhythmicity (self-initiated contraction) and is modulated by hormones and the autonomic nervous system.

Smooth Muscle

Smooth muscle is found in the walls of hollow organs and blood vessels. It lacks striations and is involved in involuntary movements such as peristalsis and vasoconstriction.

• Location: Walls of hollow organs (e.g., blood vessels, GI tract, bronchi), arrector pili muscles in skin.

• Appearance: Non-striated (smooth), spindle-shaped cells with a single, centrally located nucleus.

• Control: Involuntary; some cells have autorhythmicity and are controlled by the autonomic nervous system.

Functions and Properties of Muscle Tissue

Functions

• Producing body movements: Locomotion and manipulation of the environment.

• Stabilizing body positions: Maintaining posture and joint stability.

• Storing and moving substances: Movement of blood, food, urine, and air within the body.

• Generating heat: Muscle activity produces heat, contributing to body temperature regulation.

Properties

• Electrical excitability: Ability to respond to stimuli by producing electrical signals (action potentials).

• Contractility: Ability to shorten and generate force.

• Extensibility: Ability to be stretched without being damaged.

• Elasticity: Ability to return to original shape after being stretched.

Structure of Skeletal Muscle

Organization

Skeletal muscle is organized into bundles of muscle fibers (cells) surrounded by connective tissue layers. This organization allows for efficient force generation and transmission.

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds bundles of 10-100 muscle fibers (fascicles).

• Endomysium: Surrounds individual muscle fibers.

• Connective tissue layers extend beyond the muscle to form tendons.

Muscle Fiber Anatomy

• Muscle fiber: Long, cylindrical, multinucleated cell.

• Sarcolemma: Muscle cell membrane.

• Sarcoplasm: Cytoplasm of muscle cell, containing myoglobin (oxygen-binding protein) and glycogen (energy storage).

• Myofibrils: Rod-like structures containing contractile proteins (actin and myosin).

• Myofilaments: Thin (actin) and thick (myosin) protein filaments arranged in repeating units called sarcomeres.

• Sarcomere: Basic contractile unit of muscle fiber, defined by Z discs.

Specialized Structures

• Transverse (T) tubules: Invaginations of the sarcolemma that conduct action potentials into the muscle fiber.

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions () for muscle contraction.

Mechanism of Muscle Contraction

Sliding Filament Theory

Muscle contraction occurs when myosin heads bind to actin filaments and pull them toward the center of the sarcomere, shortening the muscle fiber.

• Calcium release: is released from the SR into the cytosol.

• ATP hydrolysis: Provides energy for myosin head movement.

• Cross-bridge cycling: Myosin heads attach to actin, pivot, detach, and reattach, pulling actin filaments inward.

• Result: Sarcomeres shorten, leading to muscle contraction. Myofilaments themselves do not change length.

Rigor Mortis

• After death, leaks out of the SR, allowing myosin heads to bind to actin.

• ATP synthesis ceases, so cross-bridges cannot detach, resulting in muscle rigidity (rigor mortis) until proteins degrade.

Neuromuscular Junction (NMJ)

Structure and Function

The neuromuscular junction is the synapse between a motor neuron and a skeletal muscle fiber, enabling nerve impulses to trigger muscle contraction.

• Axon terminals: Contain synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).

• Motor end plate: Region of the muscle fiber's sarcolemma with ACh receptors.

• Synaptic cleft: Gap between the neuron and muscle fiber.

• Process: ACh is released, binds to receptors, and initiates an action potential in the muscle fiber.

Clinical Relevance

• Paralysis: Certain toxins and drugs can block ACh receptors or release, causing muscle paralysis (e.g., botulinum toxin/Botox).

• Botox: Used medically and cosmetically to relax muscles by inhibiting ACh release.

Development, Growth, and Regeneration of Muscle

Development

• Myoblasts: Embryonic cells that fuse to form multinucleated skeletal muscle fibers.

• Satellite cells: Myoblast-like cells that aid in muscle repair and regeneration.

Growth

• Hypertrophy: Increase in muscle fiber size due to increased myofibrils, SR, and mitochondria from forceful, repetitive activity.

• Atrophy: Decrease in muscle size due to disuse or loss of nerve supply.

Regeneration

• Skeletal muscle has limited regenerative capacity; satellite cells can repair minor damage, but extensive damage leads to fibrosis (scar tissue formation).

• Cardiac muscle has very limited regeneration; smooth muscle (e.g., uterus) can divide and regenerate more readily.

Classification of Skeletal Muscle Fibers

Types of Skeletal Muscle Fibers

Skeletal muscle fibers are classified based on their contraction speed, metabolic pathway, and resistance to fatigue.

Effects of Anabolic Steroids

• Anabolic steroids: Synthetic derivatives of testosterone used to increase muscle size, strength, and endurance.

• Side effects: Liver cancer, kidney damage, heart disease, mood swings, stunted growth, masculinization in females, testicular atrophy, and baldness in males.

Additional info:

• Muscle contraction is regulated by the interaction of actin, myosin, troponin, and tropomyosin, with binding to troponin to initiate contraction.

• ATP is required both for contraction and for detachment of myosin heads during relaxation.