Muscle Tissue Structure

Microscopic Anatomy of Skeletal Muscle

Skeletal muscle tissue is composed of long, cylindrical cells called myocytes or muscle fibers. These fibers are multinucleated and display a characteristic striated appearance due to the arrangement of contractile proteins.

• Capillaries: Small blood vessels that supply oxygen and nutrients to muscle fibers.

• Myocyte nuclei: Multiple nuclei located at the periphery of each muscle fiber.

• I-bands (light): Regions containing only thin (actin) filaments; appear lighter under the microscope.

• A-bands (dark): Regions containing thick (myosin) filaments, including areas of overlap with actin; appear darker.

Example: The striated pattern seen in skeletal muscle is due to the regular arrangement of A-bands and I-bands.

Muscle Mass Regulation

Muscle Remodeling

Muscle mass is constantly remodeled through the balance of protein synthesis and protein degradation. This process is influenced by mechanical stress, nutrient availability, growth factors, and age.

• Protein synthesis > Protein degradation: Leads to increased muscle mass.

• Protein degradation > Protein synthesis: Leads to muscle atrophy.

Increasing Muscle Mass

• Hypertrophy: Increase in muscle fiber diameter due to increased synthesis of contractile proteins (actin and myosin).

• Hyperplasia: Increase in the number of muscle fibers (rare in humans, more common in some animal species).

• Myosatellite cells: Stem cells involved in muscle repair; can fuse with existing fibers or form new fibers during development.

Example: Resistance training stimulates hypertrophy by increasing the rate of protein synthesis in muscle fibers.

Additional info: Muscle fiber splitting and hyperplasia are not significant contributors to muscle growth in adult humans.

Muscle Hypertrophy

Hypertrophy occurs when skeletal muscle is subjected to overload, such as during resistance exercise.

• Increased number of sarcomeres (contractile units) in parallel, leading to greater muscle fiber diameter.

• Increased myofibrils and sarcoplasmic storage (e.g., glycogen).

• Greater hypertrophy observed in type II (fast-twitch) fibers compared to type I (slow-twitch) fibers.

Example: Bodybuilders exhibit pronounced muscle hypertrophy due to chronic overload and adaptation.

Muscle Atrophy

Atrophy is the loss of muscle mass due to decreased protein synthesis or increased protein degradation.

• Disuse atrophy: Caused by immobilization, bed rest, or unloading (e.g., spaceflight).

• Food deprivation: Leads to muscle protein breakdown for energy.

• Disease-related atrophy: Chronic diseases (e.g., ALS, muscular dystrophy) can cause muscle wasting and weakness.

Example: Astronauts can lose up to 20% of muscle mass during spaceflights lasting 5–11 days due to lack of gravitational loading.

Skeletal Muscle Reflexes

Overview of Reflexes

Skeletal muscle reflexes are automatic responses that help maintain posture and protect the body from injury. They involve sensory receptors, integration in the central nervous system (CNS), and motor responses.

• Sensory receptors: Detect changes in joint movement, muscle tension, and length.

• Integrating center: CNS processes sensory input and determines response.

• Efferent neurons: Carry motor commands to effectors (muscles).

• Effectors: Skeletal muscles that contract or relax in response.

Proprioceptors

Proprioceptors provide information about body position, movement, and muscle force.

• Muscle spindles: Detect changes in muscle length and rate of stretch.

• Golgi tendon organs: Detect muscle tension and help prevent excessive force.

• Joint receptors: Located in joint capsules and ligaments; respond to mechanical distortion.

Muscle Spindles

Muscle spindles are stretch receptors embedded within skeletal muscles. They consist of intrafusal fibers innervated by sensory and gamma motor neurons.

• Provide information to the CNS about muscle length and changes in length.

• Spindles are tonically active, firing even when the muscle is relaxed (contributing to muscle tone).

• When a muscle is stretched, spindle firing rate increases; when the muscle contracts, firing rate decreases.

Example: The knee-jerk (patellar tendon) reflex is mediated by muscle spindles.

Golgi Tendon Organs

Golgi tendon organs are located at the junction of muscle and tendon. They monitor muscle tension and can trigger inhibitory reflexes to prevent muscle damage from excessive force.

• Consist of sensory nerve endings interwoven among collagen fibers inside a connective tissue capsule.

• Activation leads to inhibition of the contracting muscle and activation of its antagonist.

Reflex Pathways

• Monosynaptic reflex: Involves a single synapse between sensory and motor neuron (e.g., stretch reflex).

• Polysynaptic reflex: Involves one or more interneurons (e.g., withdrawal reflex).

• Reciprocal inhibition: Activation of one muscle group is accompanied by inhibition of its antagonist.

Patellar Tendon (Knee Jerk) Reflex

• Tap on the patellar tendon stretches the quadriceps muscle.

• Muscle spindle afferents send signals to the spinal cord.

• Motor neurons stimulate quadriceps contraction and inhibit hamstrings (antagonist).

Withdrawal and Crossed Extensor Reflexes

• Withdrawal reflex: Pulls limb away from painful stimulus.

• Crossed extensor reflex: Supports body by extending the opposite limb during withdrawal.

Cardiac Muscle

Structure and Function

Cardiac muscle is found only in the heart and is responsible for pumping blood throughout the body. It shares some features with skeletal muscle but also has unique characteristics.

• Composed of cardiac myocytes (myocardial cells): short, branched, usually with a single nucleus.

• Cells are interconnected by intercalated disks containing desmosomes (mechanical linkage) and gap junctions (electrical coupling).

• Striated appearance due to sarcomeres with thick and thin filaments.

• Less abundant but larger T-tubules compared to skeletal muscle.

• Smaller amounts of sarcoplasmic reticulum; relies on extracellular Ca2+ for contraction.

• Abundant mitochondria (about 1/3 of cell volume) for oxidative metabolism and ATP production.

Example: Cardiac muscle cells can contract nearly 3 billion times in an average human lifespan.

Summary Table: Comparison of Skeletal and Cardiac Muscle