The Muscular System

Introduction to the Muscular System

The muscular system consists exclusively of skeletal muscles and is responsible for producing movement, maintaining posture, and generating heat. The organization of muscles, including the arrangement of muscle fibers (fascicles), significantly influences the power, range, and speed of movement.

Fascicle Arrangement in Skeletal Muscles

Types of Fascicle Arrangements

Skeletal muscle fibers are grouped into bundles called fascicles. The arrangement of these fascicles determines the muscle's shape and functional properties. The main types of fascicle arrangements are:

• Parallel muscles: Fascicles run parallel to the long axis of the muscle. These muscles can be flat or cylindrical with a central belly. Example: biceps brachii. Tension depends on the number of myofibrils.

• Convergent muscles: Muscle fibers spread out like a fan and converge on a single attachment point (tendon, aponeurosis, or raphe). Example: pectoralis major. Fibers can pull in different directions.

• Pennate muscles: Fascicles are arranged at an angle to the tendon. These muscles contain more myofibrils and develop more tension but do not move tendons as far as parallel muscles. Types include unipennate (e.g., extensor digitorum), bipennate (e.g., rectus femoris), and multipennate (e.g., deltoid).

• Circular muscles (sphincters): Fascicles are arranged in concentric rings around an opening. Contraction closes the opening. Example: orbicularis oris.

Levers and Muscle Efficiency

Classes of Levers

Muscles attach to bones and act as levers to produce movement. A lever is a rigid structure that moves around a fixed point called the fulcrum. Levers can change the direction, distance, speed, and strength of applied force. There are three classes of levers:

• First-class lever: The fulcrum is between the applied force and the load. Example: extension of the neck (lifting the head).

• Second-class lever: The load is between the applied force and the fulcrum. Example: ankle extension (plantar flexion) by calf muscles.

• Third-class lever: The applied force is between the load and the fulcrum. This is the most common lever in the body and maximizes speed and distance at the expense of force. Example: flexion at the elbow by the biceps brachii.

Origins, Insertions, and Muscle Actions

Muscle Attachments and Actions

The origin of a muscle is its fixed attachment, usually proximal, while the insertion is the movable attachment, usually distal. Muscle actions are described based on their effects on bones or joints (e.g., flexion, extension, abduction).

• Agonist (prime mover): Main muscle responsible for a movement.

• Antagonist: Opposes the action of the agonist.

• Synergist: Assists the agonist in performing its action.

• Fixator: A synergist that stabilizes the origin of the agonist.

Agonists and antagonists work in pairs; when one contracts, the other relaxes (e.g., flexors and extensors).

Naming Skeletal Muscles

Muscle Naming Conventions

Muscle names provide information about their location, appearance, or function. Common naming criteria include:

• Region: e.g., temporalis (temple), brachialis (arm)

• Position/direction: e.g., externus (superficial), internus (deep), rectus (straight), oblique (slanted)

• Structural characteristics: e.g., number of tendons (biceps, triceps), shape (deltoid = triangle, rhomboid = parallelogram), size (maximus, minimus, longus, brevis)

• Action: e.g., flexor, extensor, abductor, adductor, pronator, supinator

Some names reflect occupation or habits (e.g., buccinator means "trumpeter").

Axial and Appendicular Muscles

Divisions of the Muscular System

The muscular system is divided into axial muscles (60% of skeletal muscles, position head, vertebral column, and rib cage, and form the pelvic floor) and appendicular muscles (move and support the pectoral and pelvic girdles and limbs).

Axial Muscles

Muscles of the Head and Neck

• Muscles of facial expression: Originate on the skull; e.g., orbicularis oris (mouth), buccinator (cheek).

• Muscles of mastication: Move the mandible; e.g., masseter, temporalis, pterygoid muscles.

• Muscles of the tongue: Names end in -glossus; e.g., palatoglossus, styloglossus, genioglossus, hyoglossus.

• Muscles of the pharynx: Involved in swallowing; e.g., pharyngeal constrictors, palatal muscles, laryngeal elevators.

• Extrinsic eye muscles: Control eye position; e.g., inferior rectus, medial rectus, superior rectus, lateral rectus, inferior oblique, superior oblique.

• Muscles of the anterior neck: Control larynx position, depress mandible, support tongue and pharynx; e.g., digastric, mylohyoid, geniohyoid, stylohyoid, sternocleidomastoid, omohyoid.

Muscles of the Vertebral Column

• Erector spinae (superficial and deep layers): Includes spinalis, longissimus, and iliocostalis groups (superficial); semispinalis, multifidus, interspinales, intertransversarii, and rotatores (deep).

• Spinal flexors: Longus capitis and longus colli (neck); quadratus lumborum (lumbar region).

Oblique and Rectus Muscles

• Oblique muscles: Compress underlying structures and rotate the vertebral column. Regions: cervical (scalene muscles), thoracic (intercostals, transversus thoracis), abdominopelvic (external oblique, internal oblique, transversus abdominis).

• Rectus muscles: Flex the vertebral column and oppose erector spinae. Example: rectus abdominis.

• Diaphragm: Major muscle of respiration, separates thoracic and abdominopelvic cavities.

Muscles of the Pelvic Floor

• Support pelvic organs, flex sacrum and coccyx, and control movement through urethra and anus.

• Divided into the anterior urogenital triangle and posterior anal triangle; the pelvic diaphragm forms the muscular foundation of the anal triangle.

Appendicular Muscles

Muscles of the Shoulders and Upper Limbs

• Muscles that position the pectoral girdle: Trapezius, serratus anterior, subclavius, pectoralis minor, rhomboid major/minor, levator scapulae.

• Muscles that move the arm: Deltoid, supraspinatus, subscapularis, teres major/minor, infraspinatus, coracobrachialis, pectoralis major, latissimus dorsi.

• Rotator cuff: Supraspinatus, infraspinatus, teres minor, subscapularis.

• Muscles that move the forearm and hand: Biceps brachii, triceps brachii, brachialis, brachioradialis, pronator teres, supinator, pronator quadratus, flexor and extensor carpi muscles.

• Muscles that move the fingers: Extrinsic (forearm) and intrinsic (hand) muscles provide gross and fine movements, respectively.

Muscles of the Pelvis and Lower Limbs

• Muscles that move the thigh: Gluteal muscles (gluteus maximus, medius, minimus), tensor fasciae latae, lateral rotators, adductors, iliopsoas (psoas major, iliacus).

• Muscles that move the leg: Flexors (hamstrings: biceps femoris, semitendinosus, semimembranosus, sartorius, popliteus), extensors (quadriceps femoris: vastus muscles, rectus femoris).

• Muscles that move the foot and toes: Gastrocnemius, soleus, fibularis muscles, tibialis anterior/posterior, extensor/flexor digitorum longus, extensor/flexor hallucis longus.

• Intrinsic muscles of the foot: Originate on tarsal/metatarsal bones, move toes, and maintain the foot's arch.

Functional Relationships and Effects of Exercise

Integration with Other Body Systems

The muscular system works closely with other body systems:

• Cardiovascular system: Delivers oxygen and nutrients, removes carbon dioxide.

• Respiratory system: Meets the oxygen demand of muscles.

• Integumentary system: Disperses heat generated by muscle activity.

• Nervous and endocrine systems: Direct and coordinate muscle activity and responses.

Exercise enhances muscle performance and induces beneficial adaptations in other body systems, such as increased cardiovascular efficiency and improved respiratory capacity.

*Additional info: This summary integrates and expands upon the provided lecture content, ensuring a comprehensive and academically robust overview suitable for college-level Anatomy & Physiology students.*