Muscle Tissue & Skeletal Muscle Fiber

Muscle Tissue Types

• Skeletal muscle tissue is voluntary, striated, has large cells, and is multinucleated.
• Smooth muscle tissue is involuntary, has tapered cells, and is single nucleated.
• Cardiac muscle tissue is involuntary, striated, and single nucleated.

Skeletal Muscle Fiber

• A single skeletal muscle cell is known as a muscle fiber.
• Myofibrils are abundant, parallel, threadlike structures within muscle fibers.
• Myofibrils play a fundamental role in muscle contraction.
• Myofibrils consist of thick and thin protein filaments.
• Thick filaments are composed of myosin.
• Thin filaments are composed of actin.
• The organization of actin and myosin filaments creates the striated pattern seen in skeletal and cardiac muscle.

Sarcomere Structure

• Sarcomeres are units of the myofibril that contain actin and myosin filaments.
• Sarcomeres are arranged end to end to produce a myofibril; myofibrils are packaged to form a muscle fiber.
• The sarcomere is the functional unit of a muscle.
• Troponin and tropomyosin are two proteins associated with actin filaments within the sarcomere.
• At rest, the troponin-tropomyosin complex covers myosin binding sites on the actin molecule.

Skeletal Muscle Contraction

• Skeletal muscle contracts in response to a signal from the nervous system.
• The neuromuscular junction is where a neuron contacts a muscle fiber.
• Acetylcholine (ACh) is released from the neuron into the synaptic cleft.
• ACh instructs the muscle fiber to contract.
• Sarcoplasmic reticulum is an internal membrane system that releases calcium ions into the cytosol of the muscle fiber.
• Calcium ions bind to troponin, causing the troponin-tropomyosin complex to expose myosin binding sites on actin.
• A myosin cross-bridge forms once the myosin binding sites of the actin molecule are exposed.
• The myosin head, attached to the actin molecule, breaks down ATP.
• The energy from ATP is used to pull the actin filament toward the center of the sarcomere.
• The width of the sarcomeres shortens, leading to muscle contraction.

Neuromuscular Junction Pathology

• Myasthenia gravis is an autoimmune disorder where antibodies block ACh receptors, preventing muscle contraction.
• Botulism toxin poisoning is caused by bacterial toxins that block the release of ACh, preventing muscle contraction.
• Tetanus toxin poisoning is caused by bacterial toxins that prevent inhibitory neurons from instructing muscles to relax, causing continuous muscle spasm.

The Muscular System

• The muscular system consists of skeletal muscles only, which are usually subject to conscious control.
• Skeletal muscles function primarily to produce movement and stability.
• Muscles attach to bones either directly or indirectly.
• Direct attachment: Little separation between muscle and bone, muscle tissue emerges directly from the bone.
• Indirect attachment: Muscle ends short of its destination, bridged by a fibrous band called a tendon.
• The origin is the bony site of attachment at the relatively stationary end of the muscle.
• The insertion is the attachment site at the more mobile end of the muscle.

Anatomy of Skeletal Muscle

• Skeletal muscle fibers are organized and packaged by connective tissue sheaths.
• Endomysium: a sleeve surrounding each muscle fiber individually.
• Perimysium: a sheath wrapping muscle fibers together into bundles called fascicles.
• Epimysium: a fibrous sheath surrounding the entire muscle.
• Fascia: dense connective tissue on the muscle surface separating muscles from one another, forming the tendon, a cordlike projection attaching muscle to bone.

Muscles Acting on the Shoulder - Anterior Group

• Pectoralis Minor draws the scapula laterally and depresses the apex of the shoulder.
• Serratus Anterior draws the scapula laterally and protracts it, it is the prime mover in all forward reaching and pushing actions.

Muscles Acting on the Shoulder - Posterior Group

• Trapezius elevates the apex of the shoulder and retracts the scapula.
• Levator Scapulae elevates the scapula and depresses the apex of the shoulder.
• Rhomboideus Minor retracts the scapula.
• Rhomboideus Major retracts the scapula.

Muscles Acting on the Arm - Axial Muscles

• Pectoralis Major flexes, adducts, and medially rotates the humerus.
• Latissimus Dorsi extends, adducts, and medially rotates the humerus.

Muscles Acting on the Arm - Scapular Muscles

• Deltoid flexes with anterior fibers, extends with posterior fibers, and abducts with medial fibers the humerus.
• Teres Major extends and medially rotates the humerus.
• Coracobrachialis flexes and medially rotates the arm.

Muscles Acting on the Arm - Rotator Cuff

• Supraspinatus abducts the humerus and resists downward sliding of the humeral head.
• Infraspinatus rotates the humerus laterally and resists upward sliding of the humeral head.
• Teres Minor rotates the humerus laterally and resists upward sliding of the humeral head.
• Subscapularis rotates the humerus medially and resists upward sliding of the humeral head.

Muscles Acting on the Forearm

• Brachialis, located in the arm, is the prime elbow flexor.
• Biceps Brachii, located in the arm, supinates the forearm and assists in flexion.
• Triceps Brachii, located in the arm, extends the elbow.
• Brachioradialis, located in the forearm, flexes the elbow.
• Pronator Quadratus/Teres, located in the forearm, pronates the forearm.
• Supinator, located in the forearm, supinates the forearm.

Muscles Acting on the Wrist and Hand- Anterior (Flexor) Compartment

• Flexor Carpi Radialis/Ulnaris flexes the wrist.
• Flexor Digitorum Superficialis/Profundus flexes digits.
• Flexor Pollicis Longus flexes the thumb.
• Palmaris Longus flexes the wrist.

Muscles Acting on the Wrist and Hand- Posterior (Extensor) Compartment

• Extensor Carpi Radialis Longus/Brevis extend and abduct the wrist.
• Extensor Carpi Ulnaris extends and adducts the wrist.
• Extensor Digitorum extends the digits.
• Extensor Digiti Minimi extends the little finger.
• Extensor Pollicis Longus/Brevis extends the thumb.
• Extensor Indicis extends the index finger.
• Abductor Pollicis Longus abducts the thumb.

Muscles Acting on the Hip and Lower Limb

• Lower limbs are adapted for strength rather than precision.
• Muscles will be discussed based on anatomical region and action.

Muscles Acting on the Hip and Thigh - Anterior Muscles of the Hip

• Iliacus flexes the thigh at the hip when the trunk is fixed and flexes the trunk at the hip when the thigh is fixed.
• Psoas also flexes the thigh at the hip when the trunk is fixed and flexes the trunk at the hip when the thigh is fixed.

Muscles Acting on the Hip and Thigh - Lateral and Posterior Muscles of the Hip

• Tensor Fasciae Latae (TFL) extends the knee and laterally rotates the tibia.
• Gluteus Maximus extends the thigh at the hip and abducts the thigh.
• Gluteus Medius and Gluteus Minimus abduct and medially rotate the thigh.

Muscles Acting on the Hip and Thigh - Lateral Rotators

• Piriformis laterally rotates and abducts the thigh.
• Gemellus Superior/Inferior, Obturator Externus/Internus, Quadratus Femoris all assist in lateral rotation and abduction of the thigh.

Muscles Acting on the Hip and Thigh - Medial (Adductor) Compartment of the Thigh

• Adductor Brevis/Longus/Magnus adduct and medially rotate the thigh.
• Gracilis flexes and medially rotates the tibia at the knee. Pectineus adducts and flexes the thigh.

Muscles Acting on the Knee and Leg - Anterior (Extensor) Compartment of the Thigh

• Quadriceps Femoris:
  • Rectus Femoris extends the knee and flexes the thigh at the hip.
  • Vastus Lateralis extends the knee and applies lateral tension on the patella.
  • Vastus Medialis extends the knee and applies medial tension on the patella.
  • Vastus Intermedius extends the knee.
• Sartorius knee and hip flexion, abduction and lateral rotation of the thigh.

Muscles Acting on the Knee and Leg - Posterior (Flexor) Compartment of the Thigh

• Biceps Femoris flexes the knee, extends the hip, and laterally rotates the tibia.
• Semitendinosus flexes the knee and medially rotates the tibia.
• Semimembranosus flexes the knee and medially rotates the tibia.
• Popliteus rotates the tibia medially and unlocks the knee to allow flexion.

Muscles Acting on the Foot - Anterior (Extensor) Compartment of the Leg

• Extensor Digitorum Longus extends toes II-V and dorsiflexes the foot.
• Extensor Hallucis Longus extends the great toe and dorsiflexes the foot.
• Tibialis Anterior dorsiflexes and inverts the foot.

Muscles Acting on the Foot - Posterior (Flexor) Compartment of the Leg

• Gastrocnemius plantar flexes the foot and flexes the knee.
• Soleus plantar flexes the foot.
• Flexor Digitorum Longus flexes digits II-V.
• Flexor Hallucis Longus flexes the great toe.
• Tibialis Posterior inverts the foot.

Muscles Acting on the Foot - Lateral Compartment of the Leg

• Fibularis (Peroneus) Longus/Brevis/Tertius everts the foot.

Muscles of Facial Expression

• Frontalis elevates eyebrows, draws scalp forward, and wrinkles forehead skin.
• Orbicularis Oculi closes the eye in blinking.
• Orbicularis Oris encircles the mouth, closes and protrudes lips.
• Buccinator compresses cheek against teeth and gums.

Muscles of Mastication (Chewing)

• Temporalis elevates and retracts the mandible.
• Masseter elevates the mandible.
• Medial Pterygoid elevates and protracts the mandible.
• Lateral Pterygoid depresses and protracts the mandible.

Muscles Acting on the Head -Flexors of the Neck

• Sternocleidomastoid: lateral flexion and rotation of the head toward the contralateral side.
• Scalenes (Anterior, Middle, Posterior): unilateral contraction causes ipsilateral flexion, bilateral contraction causes forward flexion of the head.

Muscles Acting on the Head - Extensors of the Neck

• Upper Trapezius extends and laterally flexes the neck.
• Splenius Capitis/Semispinalis Capitis extends the head.

Muscles of Respiration

• Diaphragm acts as the prime mover of inspiration.
• External Intercostals assist in expanding the thoracic cavity.
• Internal Intercostals assist in retracting the thoracic cavity.

Muscles of Anterior Abdomen

• External Abdominal Oblique increases intraabdominal pressure, unilateral contraction causes contralateral waist rotation.
• Internal Abdominal Oblique increases intraabdominal pressure, unilateral contraction causes ipsilateral waist rotation.
• Transverse Abdominal increases pressure.
• Rectus Abdominis produces forward bending at the waist.

Muscles of the Back

• Erector Spinae (Iliocostalis, Longissimus, Spinalis) extends and laterally flexes the vertebral column.
• Quadratus Lumborum: unilateral contraction causes ipsilateral flexion, bilateral contraction aids in extension of the lumbar spine.
• Multifidus maintains posture and stabilizes adjacent vertebrae.