Chapter 10: Muscle tissue

Excitability

The fundamental ability of a muscle cell to receive and respond to chemical or electrical stimuli (e.g., neurotransmitters, stretch) by generating an electrical impulse.

Contractility

The unique ability of muscle cells to shorten forcibly when adequately stimulated, primarily due to the sliding interaction of their myofilaments.

Extensibility

The capacity of a muscle cell to be stretched or extended beyond its resting length when relaxed, without tearing.

Elasticity

The ability of a muscle fiber to recoil and return to its original resting length after being stretched, owing to the elastic properties of connective tissue and specialized proteins.

Maintain Posture

One of the key functions of muscles, achieved by making continuous, tiny adjustments (e.g., standing upright, sitting) that resist the force of gravity.

Stabilize Joints

Muscles help keep joints stable by exerting tension across them, reinforcing the articulating bones (e.g., rotator cuff muscles stabilizing the shoulder joint).

Generate Heat

A significant byproduct of muscle contraction is the production of heat, which is vital for maintaining normal body temperature, especially skeletal muscle activity (e.g., shivering in cold conditions).

Insertion

The attachment point of a muscle to the movable bone or structure. During contraction, the insertion typically moves toward the origin.

Origin

The less movable or fixed attachment point of a muscle, often located closer to the body's midline or a stable part of the skeleton.

Direct Attachment

A type of muscle attachment where the epimysium (outermost connective tissue wrapping) of the muscle directly fuses with the periosteum of a bone or the perichondrium of a cartilage.

Indirect Attachment

A more common type of muscle attachment where the connective tissue wrappings (epimysium, perimysium, endomysium) extend beyond the muscle belly to form a ropelike tendon or a sheet-like aponeurosis, which then attaches to bone, cartilage, or fascia.

Myo, Mys, and Sarco

Common prefixes used in muscle terminology; 'Myo' and 'Mys' refer to muscle, while 'Sarco' refers to 'flesh' or muscle tissue components (e.g., sarcoplasm, sarcomere).

Skeletal Muscle Tissue and Fibers

This tissue is packaged into individual skeletal muscles, which are typically attached to bones or, in some cases, directly to skin (e.g., facial muscles). It is composed of the longest of all muscle fibers. Skeletal muscle fibers exhibit visible striations (stripes under a microscope) and are under voluntary control, meaning their contractions can be consciously commanded.

Cardiac Muscle Tissue

Specialized muscle tissue found exclusively in the heart walls. It is characterized by striations (similar to skeletal muscle) but is involuntary, meaning its contractions are not consciously controlled. Cardiac muscle cells are branched and interconnected by intercalated discs.

Smooth Muscle Tissue

Muscle tissue found in the walls of hollow internal organs (e.g., stomach, intestines, bladder, blood vessels). Unlike skeletal and cardiac muscle, it lacks striations and is involuntary, responsible for processes like peristalsis and regulating blood vessel diameter.

Sarcoplasm

The cytoplasm of a muscle fiber (muscle cell). It contains typical cellular organelles, but also specialized structures like abundant glycosomes (granules of stored glycogen for energy) and myoglobin (a red pigment that stores oxygen).

Myofibrils

Long, rod-like organelles that are densely packed within the sarcoplasm, accounting for approximately ~80\% of the muscle cell volume. They are the contractile elements of skeletal muscle cells, composed of repeating units called sarcomeres.

Sarcomere

The smallest contractile unit of a muscle fiber, representing the functional unit of skeletal muscle. Each sarcomere extends from one Z disc to the next and contains an orderly arrangement of actin (thin) and myosin (thick) myofilaments.

A Bands

The dark regions within a sarcomere, corresponding to the full length of the thick (myosin) filaments. They also include the overlapping portions of the thin (actin) filaments, contributing to the characteristic striated appearance of skeletal muscle.

H Zone

A lighter, central region within the dark A band of a sarcomere. This zone appears lighter because in a relaxed muscle, it contains only thick (myosin) filaments and no overlapping thin (actin) filaments.

M Line

A line of protein (myomesin) that bisects the H zone vertically within the A band. This line serves to anchor the thick (myosin) filaments in place within the sarcomere.

I Bands

The lighter regions within a sarcomere, which contain only thin (actin) filaments and are not overlapped by thick (myosin) filaments. Each I band is bisected by a Z disc.

Z Disc

A coin-shaped, disc-like sheet of proteins that forms the midline of each light I band. It serves as the anchor for the thin (actin) filaments and connects adjacent myofibrils, marking the boundaries of a sarcomere.

Myofilaments

The highly organized arrangement of contractile proteins, primarily actin and myosin, within each sarcomere. These filaments slide past each other during muscle contraction.

Actin Myofilaments

Also known as thin filaments, these are composed primarily of the protein actin, along with regulatory proteins tropomyosin and troponin. They are anchored to the Z discs at either end of the sarcomere.

Myosin Myofilaments

Also known as thick filaments, these are composed primarily of the protein myosin. They possess 'heads' that can bind to actin during the cross-bridge cycle and are connected at the M line in the center of the sarcomere.

Cross Bridge Cycle

The cyclical series of events during muscle contraction where myosin heads bind to actin (formation), pull the actin filament toward the M line (working stroke), detach from actin (detachment), and then re-cock into a high-energy position (cocking of myosin head) in preparation for another cycle, all powered by ATP hydrolysis.

Isometric Contraction

A type of muscle contraction where the muscle tension increases, but the muscle length does not change. The force generated is less than or equal to the load, resulting in no visible movement (e.g., holding a heavy object stationary).

Isotonic Contraction

A type of muscle contraction where the muscle shortens and moves the load because the muscle tension successfully exceeds the resistance. This can be concentric (muscle shortens as it contracts, e.g., lifting a weight) or eccentric (muscle lengthens despite contracting, e.g., lowering a weight slowly).

Muscle Twitch

The simplest, briefest contraction of a muscle fiber in response to a single action potential. It consists of three distinct phases: the Latent Period (excitation-contraction coupling occurs, but no measurable tension is seen), the Period of Contraction (cross-bridges form, and muscle tension rapidly increases), and the Period of Relaxation ( \text{Ca}^{2+} re-enters the sarcoplasmic reticulum, and tension declines to zero).

Recruitment

Also known as multiple motor unit summation, this is the process of increasing the number of active motor units within a muscle to increase the overall force and precision of muscle contraction. As more motor units are recruited, the contraction becomes stronger, faster, and more prolonged.

Muscle Tone

A constant, slightly contracted state of all muscles, even when they appear to be at rest. This baseline tension is maintained by spinal reflexes that alternately activate small groups of motor units, keeping the muscles firm, healthy, and ready for action without causing movement.

EPOC

Excess Postexercise Oxygen Consumption, commonly referred to as 'oxygen debt'. It refers to the additional amount of oxygen the body consumes after strenuous exercise to replenish oxygen reserves, convert lactic acid back to glucose, and restore ATP and creatine phosphate levels in muscle tissue.

Fatigue

The physiological inability of a muscle to contract or sustain contraction despite continued stimulation. It often results from factors such as ionic imbalances (e.g., \text{K}^{+} accumulation), depletion of ATP and glycogen reserves, or accumulation of metabolic byproducts like lactic acid.

Relative Size of Fibers

One of the key factors influencing muscle force: The greater the cross-sectional area (bulk) of a muscle or muscle fiber, the more myofibrils and sarcomeres are arranged in parallel, enabling the muscle to generate more tension and force.

Hypertrophy

An increase in the size (diameter) of individual muscle fibers, primarily due to an increase in the number of myofibrils within the cells. This occurs in response to resistance exercise and leads to greater muscle strength.

Frequency of Stimulation

The rate at which a muscle is stimulated. When stimuli are delivered at a high frequency, successive contractions combine (summate) because the muscle does not have enough time to relax completely between stimuli, leading to stronger, more sustained contractions (tetanus).

Degree of Muscle Stretch

Also known as the length-tension relationship, this principle states that a muscle generates its maximum force when its sarcomeres are at an optimal length, typically 80-120% of their normal resting length. At this range, there is optimal overlap between actin and myosin filaments, allowing for the greatest number of cross-bridges to form.

Muscle Fiber Type Classification

Skeletal muscle fibers are broadly classified based on two main characteristics: their speed of contraction (how quickly myosin ATPases split ATP) and the primary metabolic pathways they use to synthesize ATP (aerobic vs. anaerobic).

Speed of Contraction

Refers to how quickly a muscle fiber can contract and relax. Fibers are categorized as 'slow' or 'fast' based on the speed at which their myosin ATPases hydrolyze ATP and the pattern of electrical activity of their motor neurons.

Oxidative Fibers

Muscle fibers that primarily generate ATP through aerobic respiration, which requires oxygen. These fibers are characterized by high endurance and fatigue resistance due to their efficient energy production and abundant mitochondria and capillaries.

Glycolytic Fibers

Muscle fibers that primarily generate ATP through anaerobic glycolysis, which does not require oxygen. These fibers are well-suited for short bursts of powerful activity but have limited endurance and fatigue more quickly due to the rapid accumulation of lactic acid.

Skeletal Muscle Fiber Types

Based on their speed of contraction and metabolic pathways, skeletal muscle fibers are primarily classified into three types: Slow Oxidative (SO) fibers, Fast Oxidative (FO) fibers, and Fast Glycolytic (FG) fibers, each optimized for different types of activity.

Genetics and Muscle Fibers

An individual's genetic predisposition largely determines the specific percentage and distribution of each muscle fiber type (slow oxidative, fast oxidative, fast glycolytic) in their muscles. This plays a significant role in influencing an individual's natural athletic potential for different activities.

Slow Oxidative Fibers

These are red, fatigue-resistant muscle fibers that contract slowly and primarily use aerobic pathways for ATP synthesis. They are rich in mitochondria, capillaries, and myoglobin, making them ideal for sustained, low-intensity activities like marathon running, maintaining posture, or prolonged walking.

Fast Oxidative Fibers

These are red-to-pink, moderately fatigue-resistant fibers that contract quickly and utilize both aerobic and anaerobic pathways for ATP synthesis. They are suitable for activities requiring moderate intensity and quick, repetitive movements, such as sprinting, walking, or moderate-duration sports.

Fast Glycolytic Fibers

These are white, easily fatigued muscle fibers that contract very rapidly and rely heavily on anaerobic glycolysis for ATP synthesis. They are characterized by a high power output but low endurance, making them optimal for short-term, intense, or powerful movements like weightlifting, jumping, or hitting a baseball.

Load on Muscles

The velocity and duration of muscle contraction are inversely related to the load placed upon the muscle. Muscles contract fastest and for a longer duration when there is little or no load. Conversely, increasing the load significantly reduces both the velocity and the duration of the contraction.

Aerobic Exercise Benefits

Regular aerobic exercise leads to significant physiological adaptations in muscles, including an increase in muscle capillaries (improving oxygen and nutrient delivery), a greater number of mitochondria (enhancing efficient aerobic ATP production), and increased myoglobin synthesis (improving oxygen storage). These changes collectively result in greater muscle endurance, improved cardiovascular health, and enhanced resistance to fatigue.