2.2 Muscular system and muscle fiber types

The muscular system is a complex network of tissues that enable movement and maintain bodily functions. This section focuses on the major muscle groups, their locations, and naming conventions. It also explores the three types of muscle tissue: skeletal, smooth, and cardiac.

Skeletal muscle fibers are the building blocks of movement, with a unique structure of myofibrils and sarcomeres. Understanding the differences between Type I (slow-twitch) and Type II (fast-twitch) fibers is crucial for optimizing training and performance in various physical activities.

Major Muscle Groups

Location and Naming of Major Muscle Groups

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• Major muscle groups include the chest (pectorals), back (latissimus dorsi, trapezius, rhomboids), shoulders (deltoids), arms (biceps, triceps), abdominals (rectus abdominis, obliques), gluteals, quadriceps, hamstrings, and calves (gastrocnemius, soleus)
• Muscles are named based on their location (e.g., pectorals in the chest), shape (e.g., deltoids resembling the Greek letter delta), size (e.g., gluteus maximus being the largest gluteal muscle), depth (e.g., rectus abdominis as the superficial abdominal muscle), origin and insertion points (e.g., sternocleidomastoid originating from the sternum and clavicle and inserting on the mastoid process), number of origins (e.g., biceps brachii having two origins), or action (e.g., flexors and extensors)

Skeletal Muscle Attachment and Function

• Skeletal muscles are attached to bones via tendons, which are dense connective tissue structures that transmit the force generated by the muscle to the bone
• Muscles work together to produce movement at joints, with the origin of the muscle typically remaining stationary while the insertion point moves as the muscle contracts
• Muscles often work in antagonistic pairs, with one muscle contracting (agonist) while the other relaxes (antagonist) to produce smooth, coordinated movement
  • For example, the biceps brachii and triceps brachii work as an antagonistic pair to flex and extend the elbow joint, respectively

Muscle Tissue Types

Skeletal Muscle

• Skeletal muscle is striated, meaning it has a banded appearance due to the arrangement of contractile proteins (actin and myosin) within the muscle fibers
• It is voluntarily controlled by the somatic nervous system, allowing for conscious control of movement
• Skeletal muscle is attached to bones and is responsible for producing movement and maintaining posture
  • Examples include the muscles responsible for locomotion (e.g., quadriceps, hamstrings) and postural support (e.g., erector spinae)

Smooth Muscle

• Smooth muscle is non-striated, lacking the banded appearance of skeletal muscle due to the different arrangement of contractile proteins
• It is involuntarily controlled by the autonomic nervous system, meaning it functions without conscious control
• Smooth muscle is found in the walls of hollow organs and plays a role in functions such as digestion, blood flow regulation, and urination
  • Examples include the muscles in the walls of the stomach, intestines, blood vessels, and bladder

Cardiac Muscle

• Cardiac muscle is striated, similar to skeletal muscle, but is involuntarily controlled by the autonomic nervous system
• It is found only in the heart and is responsible for pumping blood throughout the body
• Cardiac muscle fibers are branched and connected by intercalated discs, which allow for the rapid transmission of electrical impulses and coordinated contraction of the heart

Structural Differences

• Skeletal muscle fibers are long, cylindrical, and multinucleated (having multiple nuclei), while smooth muscle fibers are shorter, spindle-shaped, and uninucleated (having a single nucleus)
• Cardiac muscle fibers are branched, uninucleated, and connected by intercalated discs, which are specialized junctions that allow for rapid communication between cells

Skeletal Muscle Fiber Structure

Myofibrils and Sarcomeres

• Each skeletal muscle fiber contains numerous myofibrils, which are long, cylindrical structures composed of repeating units called sarcomeres
• Sarcomeres are the basic functional units of muscle contraction and consist of thick filaments (myosin) and thin filaments (actin) that slide past each other during contraction
• The arrangement of thick and thin filaments gives skeletal muscle its striated appearance under a microscope, with dark A-bands (containing thick filaments) and light I-bands (containing thin filaments) alternating along the length of the myofibril

Sarcoplasmic Reticulum and T-Tubules

• The sarcoplasmic reticulum is a specialized form of smooth endoplasmic reticulum that surrounds each myofibril and plays a crucial role in muscle contraction by storing and releasing calcium ions (Ca2+)
• Transverse tubules (T-tubules) are invaginations of the sarcolemma (cell membrane) that run perpendicular to the myofibrils and are closely associated with the sarcoplasmic reticulum
• T-tubules conduct action potentials from the sarcolemma into the interior of the muscle fiber, triggering the release of Ca2+ from the sarcoplasmic reticulum, which initiates muscle contraction

Muscle Contraction Mechanism

• During muscle contraction, thick and thin filaments slide past each other, shortening the sarcomere and generating force
• This sliding filament mechanism is driven by the cyclic attachment and detachment of myosin heads (cross-bridges) to binding sites on actin filaments
• The release of Ca2+ from the sarcoplasmic reticulum exposes the binding sites on actin, allowing myosin heads to attach and pull the thin filaments towards the center of the sarcomere
• The contraction of multiple sarcomeres along the length of a myofibril, and the coordinated contraction of multiple myofibrils within a muscle fiber, results in the overall shortening and force production of the muscle

Type I vs Type II Muscle Fibers

Type I (Slow-Twitch) Fibers

• Type I fibers are fatigue-resistant and capable of sustaining prolonged, low-intensity activities such as endurance exercise (e.g., long-distance running, cycling)
• They have a high concentration of mitochondria, which are the powerhouses of the cell and generate ATP through aerobic metabolism
• Type I fibers also contain high levels of myoglobin, an oxygen-binding protein that gives them a reddish appearance and enhances their oxygen storage capacity
• The high mitochondrial and myoglobin content allows Type I fibers to rely primarily on aerobic metabolism, which is more efficient for sustained energy production

Type II (Fast-Twitch) Fibers

• Type II fibers are less fatigue-resistant and are better suited for short-duration, high-intensity activities such as sprinting or weightlifting
• They have fewer mitochondria and lower myoglobin content compared to Type I fibers, giving them a paler appearance
• Type II fibers rely more on anaerobic metabolism, which provides rapid energy production but results in the accumulation of metabolic byproducts (e.g., lactate) that contribute to fatigue
  • Type IIa fibers have intermediate characteristics between Type I and Type IIx fibers, with a moderate resistance to fatigue and a balance between aerobic and anaerobic metabolism
  • Type IIx fibers have the highest force production and speed of contraction but fatigue rapidly due to their heavy reliance on anaerobic metabolism

Fiber Type Proportion and Adaptability

• The proportion of Type I and Type II fibers in a muscle varies depending on its primary function and can be influenced by genetics and training
• Muscles responsible for postural support and endurance activities (e.g., soleus in the calf) have a higher proportion of Type I fibers, while muscles involved in powerful, explosive movements (e.g., gastrocnemius in the calf) have a higher proportion of Type II fibers
• Fiber type composition can be modified to a certain extent through specific training methods
  • Endurance training can increase the proportion of Type I fibers and enhance their oxidative capacity, while resistance training can cause a shift towards Type II fibers and increase their size (hypertrophy) and force-generating capacity