Anatomy and Physiology of the Skeletal and Muscular Systems

The Muscular System: Mechanics and Physiology

The muscular system is responsible for movement, maintaining posture, and generating body heat. To understand how muscles work, we must distinguish between the three types of muscle tissue:

1. Skeletal Muscle: Striated, voluntary muscle attached to bones.
2. Smooth Muscle: Non-striated, involuntary muscle found in the walls of internal organs.
3. Cardiac Muscle: Striated, involuntary muscle found exclusively in the heart.

Muscle Anatomy and Contraction

Skeletal muscles are organized into layers of connective tissue: the epimysium (surrounding the whole muscle), the perimysium (surrounding bundles called fascicles), and the endomysium (surrounding individual muscle fibers).

At the microscopic level, the functional unit of contraction is the sarcomere. Contraction occurs when thick (myosin) and thin (actin) filaments slide past one another. This process is heavily dependent on calcium ions and ATP.

The signal for contraction begins at the neuromuscular junction (NMJ), where a motor neuron meets a muscle fiber. The release of neurotransmitters triggers an action potential, involving the movement of sodium, potassium, and calcium ions. A single motor neuron and all the muscle fibers it innervates is called a motor unit. To increase the force of a contraction, the body uses recruitment, which is the activation of additional motor units.

Muscle Movement and Levers

Muscles move bones through specific actions, such as flexion, extension, abduction, and adduction. Muscles are attached to bones at specific points, often categorized as origin (stationary) and insertion (moving).

Movement is often described in terms of levers. A lever consists of three essential parts:

1. The Fulcrum (the pivot point).
2. The Effort (the force applied by the muscle).
3. The Load (the weight being moved).

Metabolism and Nutrition

Muscle function is energy-intensive, requiring constant metabolic support.

Energy Production

• Aerobic Respiration: Uses oxygen to produce a large amount of ATP efficiently.
• Anaerobic Respiration: Occurs when oxygen is scarce, producing ATP quickly but resulting in the buildup of metabolic byproducts.

When the demand for energy exceeds the supply, or when metabolic byproducts accumulate, muscle fatigue occurs.

Protein and Amino Acids

Muscle tissue is built from proteins, which are composed of amino acids.

• Essential Amino Acids: Must be obtained from the diet because the body cannot synthesize them.
• Nonessential Amino Acids: Can be produced by the body.
• Complete Proteins: Contain all essential amino acids.
• Incomplete Proteins: Lack one or more essential amino acids.

Muscular Disorders and Aging

Like the skeletal system, the muscular system is subject to various pathologies:

• Atrophy: The wasting away or decrease in size of muscle tissue, often due to disuse.
• Tendinitis: Inflammation of a tendon.
• Carpal Tunnel Syndrome: Compression of the median nerve in the wrist, affecting muscle function in the hand.
• Compartment Syndrome: Increased pressure within a muscle compartment that can restrict blood flow and damage tissue.
• Fibromyalgia: A condition characterized by widespread musculoskeletal pain.

Exam Tip: When studying muscle contraction, remember the critical roles of Calcium and ATP. Calcium allows the binding sites to be exposed, while ATP provides the energy for the "power stroke" and the detachment of the myosin head.