Anatomy and Physiology 1 - Muscular System

Classification of Muscles Muscles are classified into three main types: Skeletal, Smooth, and Cardiac. Types of Muscular Tissues The three types of muscular tissues are: Skeletal: attached to bones, striated, and voluntary Smooth: found in hollow organs, non-striated, and involuntary Cardiac: found in the heart, striated, and involuntary The following table summarizes the characteristics of each type of muscular tissue: Type Location Structure Function Skeletal Bones Striated Voluntary Smooth Hollow organs Non-striated Involuntary Cardiac Heart Striated Involuntary Functions of Skeletal Muscles Skeletal muscles have several important functions: Produce movement Maintain posture Stabilize joints Generate heat “The ability of skeletal muscles to convert chemical energy (ATP) to mechanical energy (and heat) is the basis for all these functions.” Muscle Contraction Muscle contraction occurs when myosin filaments bind to and move actin filaments, shortening the muscle. This process is regulated by the availability of calcium ions. The following steps explain the contraction process: In the relaxed state, myosin binding sites on actin molecules are covered by tropomyosin. Excitation rises calcium level. Calcium binds to troponin C. Calcium-troponin C complex interacts with tropomyosin, uncovering the myosin binding sites on actin molecules. Myosin head binds to actin. Muscle Organization Skeletal muscle is organized into: Whole muscle: composed of bundles of muscle fibers Fascicles: bundles of muscle fibers surrounded by connective tissue Muscle fiber: a single cell with multiple nuclei Myofibrils: rod-like structures within muscle fibers Sarcomere: the functional unit of the muscle, composed of thick and thin filaments The following table summarizes the structure of a sarcomere: Component Description Z disc protein structure that flanks the sarcomere A band region of the sarcomere containing thick filaments I band region of the sarcomere containing only thin filaments H zone region of the sarcomere containing only thick filaments M line structure that holds thick filaments together Sliding Filament Mechanism The sliding filament mechanism explains how muscles contract: “The myosin filaments bind to and move the actin filaments, shortening the muscle. This process is regulated by the availability of calcium ions.” The following diagram illustrates the sliding filament mechanism: Myosin filaments bind to actin filaments Myosin heads flex, sliding actin filaments along myosin filaments Sarcomere shortens, causing muscle contraction## Muscle Contraction The process of muscle contraction involves the troponin complex, tropomyosin, and G-actin. During contraction, calcium ions (Ca²⁺) bind to troponin, moving tropomyosin out of the way and uncovering binding sites for myosin on the actin. “The cross-bridge cycle is the process by which myosin heads bind to actin and produce muscle contraction.” The concentration of free, ionized calcium (Ca²⁺) in the cytosol surrounding the thick and thin filaments is very low, only about 10⁻⁷ mol/L. At this low calcium concentration, very few of the calcium-binding sites on troponin are occupied, and thus cross-bridge activity is blocked by tropomyosin. Role of ATP ATP provides the chemical energy which is stored in the myosin head. This chemical energy is transformed into mechanical energy during the power stroke of the cross-bridge. The energy from ATP can only be used if there is Ca²⁺. Source of Ca²⁺ The source of Ca²⁺ that binds to troponin in skeletal muscle is entirely from the sarcoplasmic reticulum (SR). No extracellular calcium is involved. The cell membrane of skeletal muscle does not have voltage-gated calcium channels. Excitation-Contraction Coupling The process of excitation-contraction coupling involves the following steps: An action potential is introduced at the neuromuscular junction The action potential is propagated along the T-tubules The action potential releases calcium from the sarcoplasmic reticulum The calcium binds to troponin, moving tropomyosin out of the way and uncovering binding sites for myosin on actin Effects of Exercise on Skeletal Muscle Regular exercise can improve muscle tone and posture, result in more efficient heart and lung functioning, and reduce fatigue. The effects of exercise on skeletal muscle can be summarized in the following table: Type of Exercise Effect on Muscle Strength training Increases muscle size and myofilaments Endurance training Increases blood flow and oxygen delivery to the muscle Prolonged inactivity Causes disuse atrophy Movements Produced by Skeletal Muscle Contractions Skeletal muscle contractions can produce various types of movements, including: Angular movements: flexion, extension, abduction, and adduction Circular movements: rotation and circumduction Special movements: dorsiflexion, plantar flexion, inversion, and eversion Skeletal Muscle Groups The skeletal muscles can be grouped into the following categories: Muscles of the head and neck: facial muscles, masseter, temporal, and sternocleidomastoid Muscles of the trunk: abdominal muscles, respiratory muscles, and diaphragm Muscles of the upper extremities: pectoralis major, latissimus dorsi, deltoid, and biceps brachii Muscles of the lower extremities: iliopsoas, gluteus maximus, adductor muscles, and hamstring muscles The following table summarizes the main skeletal muscle groups: Muscle Group Muscles Head and neck orbicularis oculi, orbicularis oris, zygomaticus Trunk rectus abdominis, external oblique, internal oblique, transversus abdominis Upper extremities pectoralis major, latissimus dorsi, deltoid, biceps brachii Lower extremities iliopsoas, gluteus maximus, adductor muscles, hamstring muscles Muscle Structure and Function The human body uses muscles every day to perform various activities, such as breathing, circulating blood, and moving limbs. There are three types of muscle tissues: cardiac, smooth, and skeletal. Cardiac and smooth muscle tissues are involuntary, meaning they are not consciously controlled. On the other hand, skeletal muscles are voluntary, allowing for conscious control over their actions. Muscle Composition Skeletal muscles are composed of bundles of muscle fibers. These muscle fibers are long, cylindrical cells containing several nuclei. Each muscle fiber is composed of many myofibrils, which contain contractile units called sarcomeres. “A sarcomere is the functional unit of a muscle fiber, responsible for contraction and relaxation.” Muscle Contraction Mechanism The muscle contraction mechanism involves the sliding filament theory, where thick filaments (composed of myosin) and thin filaments (composed of actin) slide past each other, causing the sarcomere to shorten. The thick filaments are anchored at the center of the sarcomere, called the M-line, while the thin filaments are anchored to the Z-lines on the outer edges of the sarcomere. The following table summarizes the key components involved in muscle contraction: Component Description Myosin Thick filament protein Actin Thin filament protein M-line Center of the sarcomere, where myosin filaments are anchored Z-line Outer edge of the sarcomere, where actin filaments are anchored Cross-bridge Region where myosin heads attach to actin filaments Muscle Contraction Process The muscle contraction process involves the following steps: A bound ATP is hydrolyzed to ADP and inorganic phosphate, causing the myosin head to extend and attach to a binding site on actin, forming a cross-bridge. The power stroke is triggered, allowing myosin to pull the actin filament toward the M-line, thereby shortening the sarcomere. ADP and inorganic phosphate are released during the power stroke. The myosin remains attached to actin until a new molecule of ATP binds, freeing the myosin to either go through another cycle of binding and contraction or remain unattached to allow the muscle to relax. Regulation of Muscle Contraction Muscle contraction is controlled by the actions of calcium ions. The thin actin filaments are associated with regulatory proteins called tropinin and tropomyosin. When a muscle is relaxed, tropomyosin blocks the cross-bridge binding sites on actin. However, when calcium ion levels are high enough and ATP is present, calcium ions bind to tropinin, displacing tropomyosin and exposing the myosin binding sites on actin, allowing myosin to attach and form a cross-bridge. The following bullet points summarize the key points about calcium regulation: Calcium ions are stored in the sarcoplasmic reticulum. Calcium ions are released in response to signals from the nervous system. Calcium ions bind to tropinin, displacing tropomyosin and exposing myosin binding sites on actin. Calcium ions trigger muscle contraction by allowing myosin to attach to actin and form a cross-bridge. Introduction to Movement Terms The study of movement terms is crucial in understanding how the body moves in different ways. To describe these movements, we use unique terms that apply to specific joints. Planes and Axes of Motion The body exists and moves in three dimensions, and its movements can be described in terms of planes and axes of motion. Movement Terms The following are some key movement terms: Abduction: movement of a joint away from the midline of the body Adduction: movement of a joint towards the midline of the body The midline of the body is an imaginary line that runs down the center of the body, and it is used as a reference point for describing movement. To remember the difference between abduction and adduction, consider the following: Abduction can be thought of as an "alien abduction" taking a limb away from the body Adduction can be thought of as "adding" to the size of the body Flexion and Extension Flexion: a decrease in the angle between two joints Extension: an increase in the angle between two joints Flexion and extension can be thought of in terms of the fetal position, where the hips and knees are flexed, and the shoulders, elbows, and wrists are also flexed. To remember the difference between flexion and extension, consider the following: Flexion is like the fetal position, where the joints are bent Extension is the opposite of flexion, where the joints are straightened Rotation Medial rotation: rotation of a joint towards the midline of the body Lateral rotation: rotation of a joint away from the midline of the body The axis of rotation is an imaginary line that runs through the long bone of a joint, and it is used as a reference point for describing rotation. Movement Terms Table Movement Term Definition Example Abduction movement away from the midline lifting arm away from body Adduction movement towards the midline bringing arm back to body Flexion decrease in angle between joints bending elbow Extension increase in angle between joints straightening elbow Medial rotation rotation towards the midline rotating shoulder towards body Lateral rotation rotation away from the midline rotating shoulder away from body Specialized Movement Terms Some joints have specialized movement terms, such as: Supination: rotation of the forearm so that the palm faces upwards Pronation: rotation of the forearm so that the palm faces downwards Dorsiflexion: movement of the foot upwards towards the shin Plantarflexion: movement of the foot downwards away from the shin Inversion: movement of the foot inwards towards the midline Eversion: movement of the foot outwards away from the midline These movement terms can be remembered using the following tricks: Supination is like asking for soup Pronation is like a group of people putting their hands in Dorsiflexion is like pointing the toes towards the dorsal aspect of the foot Plantarflexion is like planting the toes on the ground Inversion is like tripping over the feet Eversion is like standing comfortably with the feet apart Scapular Movement The scapula can move in the following ways: Elevation: movement of the scapula upwards Depression: movement of the scapula downwards Protraction: movement of the scapula forwards Retraction: movement of the scapula backwards These movements can be remembered using the following tricks: Elevation is like lifting the scapula up Depression is like lowering the scapula down Protraction is like moving the scapula forwards Retraction is like moving the scapula backwards## Movement Types The major movements of the human body include circumduction, which is a circular motion that occurs at joints such as the neck, shoulders, and hips. Movement Terminology As a clinician, it is essential to understand movement terms to document range of motion and functional movements. There are three types of movements: Active motion: the patient moves the joint through their own effort Passive motion: the motion is performed by an external force, such as a clinician's hands or a machine Resisted motion: a type of motion where resistance is applied “A key distinction between these movements is that they can help with assessment and diagnosis. For example, if a patient experiences painful shoulder flexion during active motion but not during passive motion, this information can be used to inform treatment.” Movement Assessment Understanding the differences between active, passive, and resisted motion is crucial for accurate assessment. The following table summarizes the key characteristics of each type of movement: Movement Type Description Example Active Patient-generated motion A patient lifting their arm Passive Clinician-generated or machine-generated motion A clinician moving a patient's arm Resisted Motion with applied resistance A patient trying to lift their arm against resistance Additional Resources For students interested in learning more about movement and musculoskeletal biomechanics, a recommended resource is the musculoskeletal biomechanics and kinesiology playlist, which includes a video on origins and insertions.