Lab Exam 3

Histology: Muscle Tissues

Histological characteristics of muscle tissues. Source: musmus culus / Getty Images

• Skeletal Muscle

  • Microscopic Features: Long, cylindrical, unbranched fibers; highly visible cross-striations; multinucleated with nuclei located along the periphery.

  • Function: Voluntary movement, locomotion, facial expression, and posture maintenance.

  • Location: Attached to bones by tendons, tongue, pharynx, and external sphincters.

• Cardiac Muscle

  • Microscopic Features: Short, branching fibers; visible striations; single central nucleus per cell; joined by distinct dark cross-bands called intercalated discs (containing gap junctions and desmosomes).

  • Function: Involuntary rhythmic contraction to propel blood into circulation.

  • Location: Myocardium (walls of the heart).

• Smooth Muscle

  • Microscopic Features: Spindle-shaped (fusiform) fibers with pointed ends; no striations; single central, cigar-shaped nucleus; fibers arranged in closely packed sheets.

  • Function: Involuntary propulsion of substances along internal passageways (peristalsis) and regulation of vessel diameter.

  • Location: Walls of hollow organs (stomach, intestines, blood vessels, bladder, uterus, respiratory passages).

Muscular Anatomy & Actions

Major head and neck facial muscles. Source: TefiM / Getty Images

• Head & Neck

  • Extraocular Muscles: Move the eyeball and upper eyelid.

    • Levator palpebrae superioris: Elevates upper eyelid.

    • Superior/Inferior/Medial/Lateral Recti: Turn eye upward, downward, medially, and laterally.

    • Superior Oblique: Depresses eye and rotates it medially (moves eye down and out via trochlea).

    • Inferior Oblique: Elevates eye and rotates it laterally (moves eye up and out).

  • Masseter: Elevates mandible (closes jaw for chewing).

  • Temporalis: Elevates and retracts mandible.

  • Occipitofrontalis: Raises eyebrows and wrinkles forehead (frontalis belly); pulls scalp posteriorly (occipitalis belly).

  • Orbicularis Oris: Closes and protrudes lips ("puckering/kissing muscle").

  • Buccinator: Compresses cheeks inward (whistling, blowing, holding food against teeth).

  • Zygomaticus Minor & Major: Elevate lateral corners of mouth upward and laterally (smiling).

  • Orbicularis Oculi: Closes eye (winking, blinking, squinting).

  • Platysma: Tenses neck skin; depresses mandible.

  • Sternocleidomastoid: Bilateral contraction flexes neck; unilateral contraction rotates head to opposite side.

  • Splenius Capitis & Cervicis: Extend and laterally flex head and neck.

  • Scalenes (Anterior, Middle, Posterior): Elevate ribs 1–2 during forced inhalation; flex and rotate neck.

• Chest & Abdomen

  • Pectoralis Major: Flexes, adducts, and medially rotates humerus.

  • Pectoralis Minor: Draws scapula anteriorly and inferiorly; depresses shoulder.

  • Serratus Anterior: Protracts scapula and holds it flat against ribcage ("boxer’s muscle").

  • Rectus Abdominis: Flexes lumbar vertebral column; compresses abdominal contents.

  • External Abdominal Oblique: Flexes vertebral column; compresses abdomen; aids lateral rotation.

  • Internal Abdominal Oblique: Flexes vertebral column; aids trunk rotation and lateral flexion.

  • Transverse Abdominis: Compresses abdominal contents (innermost abdominal wall layer).

  • Linea Alba: Tendinous midline seam extending from xiphoid process to pubic symphysis; anchors abdominal muscles.

  • Rectus Sheath: Fibrous envelope formed by aponeuroses enclosing the rectus abdominis.

• Back & Shoulder

  • Trapezius: Elevates, depresses, retracts, and rotates scapula.

  • Latissimus Dorsi: Extends, adducts, and medially rotates humerus ("swimmer's muscle").

  • Deltoid: Prime mover of arm abduction (anterior fibers flex/medially rotate; posterior fibers extend/laterally rotate).

  • Levator Scapulae: Elevates scapula and tilts glenoid cavity downward.

  • Rhomboid Major & Minor: Retract (adduct) and stabilize scapula.

  • Serratus Posterior (Superior/Inferior): Elevates/depresses ribs during respiration.

  • Rotator Cuff Group (SITS):

    • Supraspinatus: Initiates arm abduction.

    • Infraspinatus: Laterally rotates humerus.

    • Teres Minor: Laterally rotates humerus.

    • Subscapularis: Medially rotates humerus.

  • Teres Major: Extends, adducts, and medially rotates humerus (not a rotator cuff muscle).

Upper arm and forearm muscular organization. Source: leonello / Getty Images

• Hip & Thigh

  • Gluteus Maximus: Major extender and lateral rotator of thigh.

  • Gluteus Medius: Abducts and medially rotates thigh; stabilizes pelvis during walking.

  • Tensor Fasciae Latae: Tenses iliotibial tract; abducts and flexes thigh.

  • Iliotibial Tract (IT Band): Thick lateral fascia band extending from iliac crest to lateral tibial condyle.

  • Anterior Thigh:

    • Pectineus: Adducts and flexes thigh.

    • Adductor Longus: Adducts, flexes, and medially rotates thigh.

    • Sartorius: Flexes, abducts, and laterally rotates thigh ("tailor’s muscle" - crosses legs).

    • Quadriceps Femoris (Vastus Lateralis, Vastus Medialis, Rectus Femoris, Vastus Intermedius): Prime movers for knee extension (Rectus femoris also flexes thigh).

    • Gracilis: Adducts thigh and flexes knee.

  • Posterior Thigh (Hamstrings):

    • Biceps Femoris, Semitendinosus, Semimembranosus: Extend thigh and flex knee.

    • Adductor Magnus: Adducts thigh; anterior part flexes, posterior part extends thigh.

• Lower Leg

  • Tibialis Anterior: Dorsiflexes and inverts foot.

  • Extensor Digitorum Longus: Dorsiflexes foot; extends toes II–V.

  • Flexor Digitorum Longus: Plantarflexes and inverts foot; flexes toes II–V.

  • Fibularis (Peroneus) Longus & Brevis: Plantarflex and evert foot.

  • Gastrocnemius & Soleus: Plantarflex foot via calcaneal tendon (gastrocnemius also flexes knee).

• Upper & Lower Arm

  • Upper Arm:

    • Biceps Brachii: Flexes elbow and supinates forearm.

    • Brachialis: Prime mover of elbow flexion.

    • Coracobrachialis: Flexes and adducts humerus.

    • Brachioradialis: Synergist in elbow flexion (semi-pronated position).

    • Triceps Brachii: Prime mover of elbow extension.

  • Lower Arm (Superficial Posterior - Extensors):

    • Extensor Carpi Radialis (Longus/Brevis): Extends and abducts wrist.

    • Extensor Digitorum: Extends wrist and fingers.

    • Extensor Digiti Minimi: Extends little finger.

    • Extensor Carpi Ulnaris: Extends and adducts wrist.

    • Anconeus: Assists triceps in elbow extension.

  • Lower Arm (Superficial Anterior - Flexors/Pronators):

    • Flexor Carpi Ulnaris: Flexes and adducts wrist.

    • Flexor Carpi Radialis: Flexes and abducts wrist.

    • Pronator Teres: Pronates forearm.

    • Palmaris Longus: Tenses palmar aponeurosis; weak wrist flexor.

Muscular Origins & Insertions