Chapter 10: Muscle Tissue – Structure, Function, and Physiology

Introduction to Muscle Tissue

Overview of Muscle Tissue

• Muscle cells are one of the four basic tissue types in the human body.

• The skeletal muscles collectively make up about 40% of total body weight in males and about 32% in females.

• Muscle tissues serve a variety of functions, including movement, posture, and heat production.

Properties of Muscle

Key Functional Properties

• Contractility: Ability of a muscle to shorten with force.

• Excitability: Capacity of muscle to respond to a stimulus.

• Extensibility: Muscle can be stretched to its normal resting length and beyond to a limited degree.

• Elasticity: Ability of muscle to resume its original resting length after being stretched.

Classification of Muscle Tissue

Types and Control

• Striated muscle: Includes skeletal and cardiac muscle.

• Unstriated muscle: Smooth muscle.

• Voluntary muscle: Skeletal muscle, controlled by the somatic nervous system.

• Involuntary muscle: Cardiac and smooth muscle, controlled by the autonomic nervous system.

Structural and Functional Comparison

• Skeletal muscle: Long, cylindrical, striated, multinucleate; voluntary control.

• Cardiac muscle: Short, branched, striated, usually single nucleus; interconnected by intercalated discs; involuntary control.

• Smooth muscle: Short, spindle-shaped, non-striated, single central nucleus; involuntary control.

Table: Comparison of Skeletal, Cardiac, and Smooth Muscle Tissues

Property	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Striations	Yes	Yes	No
Nuclei	Multiple, peripheral	Single, central	Single, central
Control	Voluntary	Involuntary	Involuntary
Location	Attached to bones	Heart	Walls of hollow organs
Special Features	None	Intercalated discs	Gap junctions

Skeletal Muscle Structure

Organization and Connective Tissue Layers

• Epimysium: Fibrous connective tissue surrounding the entire muscle.

• Fascicles: Bundles of muscle fibers within the muscle.

• Perimysium: Connective tissue around fascicles.

• Endomysium: Thin connective tissue layer surrounding each muscle fiber.

• Each muscle fiber is supplied by blood vessels and a branch of a somatic motor neuron.

Skeletal Muscle Fibers

• Muscle fibers are long, cylindrical, multinucleate, and striated.

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm of muscle fiber.

• Sarcoplasmic reticulum (SR): Stores calcium ions, essential for muscle contraction.

The Sarcomere

Structural Unit of Contraction

• The sarcomere is the repeating unit between two Z-lines in a myofibril.

• Striations are due to the arrangement of actin (thin) and myosin (thick) filaments.

• A band: Dark, thick filaments (myosin).

• I band: Light, thin filaments (actin).

• H zone: Center of A band, where actin and myosin do not overlap.

• M line: Structural proteins anchoring myosin.

• Titin: Elastic protein attaching myosin to Z disc, contributing to recoil.

Mechanisms of Contraction

Sliding Filament Theory

• During contraction, thin filaments slide over thick filaments, shortening the sarcomere.

• A bands remain the same length, but I bands and H zones shorten.

• Muscle contraction results from the shortening of all sarcomeres in a muscle fiber.

Cross-Bridge Cycle

1. Contraction cycle begins: Ca2+ binds to troponin, exposing active sites on actin.

2. Cross-bridge formation: Myosin heads bind to actin, forming cross-bridges.

3. Power stroke: Myosin head pivots, pulling actin filament.

4. Cross-bridge detachment: ATP binds to myosin, causing detachment from actin.

5. Myosin reactivation: ATP hydrolysis re-cocks the myosin head.

Excitation-Contraction Coupling

Neuromuscular Junction (NMJ)

• NMJ is the site where a motor neuron stimulates a muscle fiber.

• Acetylcholine (ACh) is released from the neuron, binds to receptors on the sarcolemma, and initiates an action potential.

• Action potentials travel along the sarcolemma and down T tubules, triggering Ca2+ release from the SR.

Muscle Relaxation

• Action potentials cease, Ca2+-ATPase pumps move Ca2+ back into the SR.

• No more Ca2+ is available to bind to troponin, so cross-bridges are no longer formed.

Motor Unit

Definition and Function

• A motor unit consists of a single motor neuron and all the muscle fibers it innervates.

• When a motor neuron is activated, all fibers in its unit contract.

• The number of muscle fibers per motor unit varies by muscle and function (fine control vs. strength).

• Control and strength are trade-offs: more motor units = finer control; larger units = greater strength.

Muscle Energy Requirements

ATP and Muscle Contraction

• ATP is required for muscle contraction and relaxation.

• Muscles store limited ATP and creatine phosphate (CP) for rapid energy needs.

• ATP is generated by:

  • Aerobic metabolism: Fatty acids + O2 → 34 ATP

  • Anaerobic metabolism: Glycogen to glucose → lactic acid + 2 ATP

Energy Sources During Exercise

• At rest/mild exercise: fatty acids

• Moderate exercise: fatty acids and glycogen

• Heavy exercise: glycogen and blood glucose

Maximal Oxygen Uptake (VO2 max)

• VO2 max is the maximum rate of oxygen consumption during exercise.

• Determined by age, sex, size, and training.

Lactate Threshold

• The point at which blood lactate levels rise rapidly during exercise (50–70% VO2 max).

• Indicates the shift to anaerobic metabolism.

Types of Skeletal Muscle Fibers

Classification by Contraction Speed and Metabolism

• Type I (slow oxidative): Contract slowly, resist fatigue, use aerobic metabolism, rich in myoglobin and mitochondria.

• Type IIa (fast oxidative): Contract quickly, intermediate fatigue resistance, use aerobic and anaerobic metabolism.

• Type IIx (fast glycolytic): Contract rapidly, fatigue quickly, use anaerobic metabolism, low myoglobin.

Muscle Fatigue and Adaptation

Causes of Muscle Fatigue

• Accumulation of extracellular K+, depletion of glycogen, reduced Ca2+ release, lactic acid buildup, increased ADP, and central fatigue.

Adaptation to Training

• Endurance training: Increases mitochondria, capillaries, and oxidative enzymes.

• Strength training: Causes hypertrophy (increased fiber size), especially in Type II fibers.

Muscle Repair and Aging

• Satellite cells repair damaged muscle fibers.

• Aging leads to reduced muscle mass, fewer satellite cells, and decreased capillary supply.

Neural Control of Skeletal Muscles

Motor Neurons and Sensory Feedback

• Motor neuron cell bodies are in the spinal cord; axons innervate muscle fibers.

• Sensory feedback from muscle spindles and Golgi tendon organs regulates muscle contraction and prevents overstretching.

Bone Structure and Remodeling

Types of Bone Tissue

• Compact bone: Dense, solid, organized into osteons (Haversian systems).

• Spongy bone: Less dense, contains trabeculae, no osteons, nutrients diffuse from compact bone.

Bone Remodeling and Homeostasis

• Bone is continuously remodeled by osteocytes, osteoblasts, and osteoclasts.

• Remodeling is influenced by exercise, hormones, and nutrition.

• Key minerals: calcium, phosphorus, magnesium, fluoride, iron, manganese.

• Vitamins: C, A, K, B12; hormones: growth hormone, thyroxine, sex hormones, calcitonin, parathyroid hormone.

Aging and Bone Health

• Osteopenia: Inadequate ossification with age, leading to weaker bones.

• Osteoporosis: Severe bone loss, increased fracture risk.

Clinical Applications

• Understanding muscle and bone physiology is essential for diagnosing and treating musculoskeletal disorders, managing exercise programs, and addressing age-related decline.

Additional info: Some explanations and terminology have been expanded for clarity and completeness, including the mechanisms of contraction, energy metabolism, and clinical relevance.