Histology and Physiology of the Muscular System

Muscle Tissue Types

Skeletal muscle is primarily attached to bones and functions under voluntary control to facilitate body movement and posture. Histologically, it consists of long, cylindrical, striated fibers with multiple peripheral nuclei. Cardiac muscle is located in the heart wall and is involuntary and striated. Its cells are shorter and branching, connected by intercalated discs that contain gap junctions for coordinated rhythmic contraction. Smooth muscle is found in the walls of hollow organs and blood vessels. It is involuntary, non-striated, and spindle-shaped with a single central nucleus, specializing in moving substances through tubular organs and regulating passageway diameters.

Functional Characteristics and Muscle Roles

Muscle tissue is defined by four major properties: excitability, contractility, extensibility, and elasticity. Excitability is the responsiveness to stimuli such as nerve impulses or hormones. Contractility is the ability to generate tension and force. Extensibility allows muscle to stretch without damage, while elasticity is the ability to return to resting length. In functional groups, an agonist or prime mover is the muscle primarily responsible for a movement, while an antagonist produces the opposite action. Synergists assist the prime mover, and fixators specifically stabilize the origin of the prime mover.

Physiological Functions and Thermoregulation

The system produces movement by pulling on bones and maintains posture through muscle tone, which is a continuous, partial contraction of motor units. It is essential for thermoregulation; muscle contraction involves metabolic pathways that are not $100\%$ efficient, converting energy into heat. Shivering involves rapid, involuntary skeletal muscle contractions to increase $ATP$ consumption and heat production. The system also facilitates breathing via the diaphragm and external intercostals, protects internal soft tissues, and supports functions requiring internal pressure, such as defecation, urination, and childbirth.

Mechanisms of Substance Movement and Circulation

Smooth muscle facilitates peristalsis, a coordinated series of contractions that propels contents through the esophagus, stomach, and intestines. It also regulates blood flow via vasoconstriction and vasodilation and air flow through bronchoconstriction and bronchodilation. In the cardiovascular system, cardiac muscle contraction maintains pulmonary and systemic circulation by acting as a pump. Specialized circular muscles called sphincters, such as the pyloric or anal sphincters, control the opening and closing of body passageways to regulate the movement of materials.

Cellular Mechanism of Contraction

Skeletal muscle movement begins with nervous system stimulation through a motor neuron. The neuron releases acetylcholine (ACh) at the neuromuscular junction, triggering an action potential across the muscle fiber. This electrical signal causes the sarcoplasmic reticulum to release $Ca^{2+}$. These calcium ions allow myosin (thick filaments) and actin (thin filaments) to interact via the cross-bridge cycle. As actin is pulled toward the center of the sarcomere, the sarcomere shortens, generating tension that is transferred to bone through tendons. This process relies on $ATP$ sourced from stored $ATP$, phosphocreatine, anaerobic glycolysis, and aerobic cellular respiration.

Specialized Functions and Metabolic Homeostasis

Skeletal muscles are vital for non-verbal communication and speech. Facial expression is primarily controlled by cranial nerve VII (facial nerve). Mastication involves the Masseter, Temporalis, Medial pterygoid, and Lateral pterygoid muscles. Metabolically, skeletal muscle acts as a major site for glucose utilization and is a reservoir for glycogen, which is the storage form of glucose used to maintain metabolic homeostasis. Contractions are categorized as isometric, where tension develops without length change, or isotonic. Isotonic contractions include concentric (muscle shortens) and eccentric (muscle lengthens) movements.