Chapter 9: Muscles – Structure and Function in Anatomy & Physiology

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Muscle tissue is specialized for contraction and is classified into three main types, each with distinct structural and functional characteristics.

Skeletal Muscle: - Voluntary control (consciously controlled by the nervous system) - Multinucleated (multiple nuclei per cell) - Striated (exhibits a banded appearance under the microscope)
Cardiac Muscle: - Involuntary control (not under conscious control) - Uninucleated (one nucleus per cell) - Striated
Smooth Muscle: - Involuntary control - Uninucleated - Non-striated (lacks visible banding)

Muscle tissue possesses several unique properties that enable its function:

Excitability (Responsiveness): Ability to receive and respond to stimuli, typically from nerve impulses.
Contractility: Ability to shorten forcibly when adequately stimulated.
Extensibility: Ability to be stretched or extended without being damaged.
Elasticity: Ability to recoil and resume its resting length after being stretched.

Muscles perform several essential functions in the body:

Movement: Responsible for movement of bones (skeletal muscle) and fluids such as blood (cardiac and smooth muscle).
Maintaining Posture and Body Position: Continuous adjustments by skeletal muscles maintain posture.
Stabilizing Joints: Muscles reinforce and stabilize joints.
Heat Generation: Especially by skeletal muscle, which produces heat as a byproduct of contraction, helping maintain body temperature.

Skeletal muscle fibers are organized and protected by three layers of connective tissue:

Epimysium: Dense connective tissue that surrounds the entire muscle.
Perimysium: Connective tissue that surrounds groups of muscle fibers, forming bundles called fascicles.
Endomysium: Fine connective tissue that surrounds each individual muscle fiber within a fascicle.

Example: In a biceps muscle, the epimysium covers the whole muscle, perimysium wraps each fascicle, and endomysium surrounds each muscle cell.

The structure of skeletal muscle can be visualized as follows:

Muscle → Fascicle (bundle of fibers) → Muscle Fiber (cell) → Myofibril (contractile element)
Connective tissue layers: Epimysium (outermost), Perimysium (around fascicles), Endomysium (around fibers)

Muscles attach to bones or other structures in two main ways:

Direct Attachment: The epimysium of the muscle fuses directly to the periosteum of a bone or perichondrium of cartilage. Example: Tibialis anterior muscle.
Indirect Attachment: Connective tissue wrappings extend beyond the muscle as a tendon (rope-like) or aponeurosis (sheet-like) to attach to bone, cartilage, or fascia of another muscle. Example: Biceps brachii muscle.

Skeletal muscle fibers are highly specialized cells with the following features:

Multiple Peripheral Nuclei: Each muscle fiber contains many nuclei located at the periphery of the cell.
Many Mitochondria: To meet high energy demands.
Glycosomes: Specialized organelles for glycogen storage, providing a readily available energy source.
Myoglobin: Oxygen-binding protein that stores O2 within the muscle fiber.
Also contains myofibrils (contractile elements), sarcoplasmic reticulum (Ca2+ storage), and T tubules (transmit action potentials).

Additional info: Myofibrils are composed of repeating units called sarcomeres, which are the basic contractile units of muscle.