Muscle Anatomy and Function

Muscles

Types of Muscle Action

  • Agonist (Prime Mover): A muscle that is primarily responsible for producing a particular movement.

  • Antagonist: A muscle that opposes the movement of the agonist.

  • Synergist: A muscle that assists the agonist in performing a movement.

Naming Skeletal Muscles

Skeletal muscles are named based on several criteria:

  • Location: Indicates where the muscle is situated in the body.

  • Shape: The physical shape of the muscle.

  • Relative Size: Comparison of muscle size, such as maximus (largest), minimus (smallest), or longus (long).

  • Direction of Muscle Fibers: The orientation of muscle fibers, for example, rectus (straight), oblique (angled).

  • Number of Origins: Refers to the number of muscle origins (heads) the muscle has, e.g., biceps (two heads).

  • Location of Attachments: Location at which muscles attach, using terms like origin and insertion.

  • Action: The function or movement the muscle produces.

Fascicle Arrangement

Muscle fascicles can be arranged in several patterns:

  • Circular: Fascicles are arranged in concentric rings (e.g., orbicularis oris).

  • Convergent: Fascicles converge toward a single insertion point (e.g., pectoralis major).

  • Parallel: Fascicles run parallel to the long axis of the muscle (e.g., sartorius).

  • Fusiform: A modified parallel arrangement with an expanded belly (e.g., biceps brachii).

  • Pennate: Fascicles attach obliquely to a central tendon.

    • Unipennate: Fascicles insert on one side of the tendon (e.g., extensor digitorum longus).

    • Bipennate: Fascicles insert on both sides of the tendon (e.g., rectus femoris).

    • Multipennate: Fascicles attach at multiple points to multiple tendons (e.g., deltoid).

Lever Systems

A lever system in the human body consists of:

  • Lever: A rigid bar that moves around a fixed point.

  • Fulcrum: The fixed pivot point of the lever.

  • Effort: The force applied to the lever.

  • Load: The resistance the lever must overcome.

  • Mechanical Advantage: Achieved when the load is close to the fulcrum, allowing a smaller effort to move a larger load over a greater distance.

  • Mechanical Disadvantage: Occurs when the load is far from the fulcrum, requiring a greater effort to move a smaller load over a lesser distance.

Types of Lever Systems in the Body

  1. First-Class Lever: Arrangement is load-fulcrum-effort. Example: Raising head off the chest.

  2. Second-Class Lever: Arrangement is fulcrum-load-effort. Example: Standing on tip-toe with the weight of the body as load.

  3. Third-Class Lever: Arrangement is load-effort-fulcrum. Example: Flexing the forearm by the biceps brachii muscle.

Muscles of the Head

  • Frontalis: Muscle of the forehead that raises eyebrows.

  • Occipitalis: Located at the back of the head.

  • Epicranial Aponeurosis: Connective tissue structure connecting the frontalis and occipitalis.

Muscles of the Face

  • Corrugator Supercilii: Draws eyebrows together, creating vertical wrinkles.

  • Orbicularis Oculi: Closes the eyelids.

  • Zygomaticus: Elevates the corners of the mouth (smiling muscle).

  • Risorius: Draws corners of the mouth laterally.

  • Levator Labii Superioris: Elevates the upper lip.

  • Depressor Labii Inferioris: Draws lower lip inferiorly.

  • Depressor Anguli Oris: Depresses the corner of the mouth.

  • Orbicularis Oris: Closes and protrudes the lips.

  • Mentalis: Raises and protrudes the lower lip.

  • Buccinator: Compresses cheek, holds food in place.

  • Platysma: Tenses the skin of the neck.

Muscles of Mastication

  • Masseter: Elevates and protracts the mandible.

  • Temporalis: Elevates and retracts the mandible.

  • Medial Pterygoid: Elevates mandible and assists in side-to-side movement.

  • Lateral Pterygoid: Protracts mandible and helps with side-to-side movement.

  • Buccinator: Also plays a role in mastication by compressing the cheeks.

Muscles of Tongue Movement

  • Genioglossus: Protrudes the tongue.

  • Hyoglossus: Depresses the tongue.

  • Styloglossus: Retracts the tongue.

Hyoid Muscles

  • Suprahyoid Muscles: Raises the hyoid bone.

    • Digastric: Two muscle bellies; elevates and steadies hyoid bone.

    • Stylohyoid: Elevates hyoid bone.

    • Mylohyoid: Forms floor of mouth; elevates hyoid bone.

    • Geniohyoid: Pulls hyoid bone forward.

  • Infrahyoid Muscles: Lowers the hyoid bone.

    • Sternohyoid: Depresses hyoid.

    • Sternothyroid: Depresses the thyroid cartilage.

    • Omohyoid: Depresses the hyoid bone.

    • Thyrohyoid: Elevates thyroid cartilage and depresses hyoid.

    • Pharyngeal Constrictors: Aid in swallowing.

Anterolateral Neck Muscles

  • Sternocleidomastoid: Rotates and flexes the head.

  • Scalenes: Elevate the first two ribs during deep inspiration.

Posterior Neck Muscles

  • Splenius Capitus: Extends and rotates the head.

  • Splenius Cervicis: Similar function as splenius capitus but acts on cervical spine.

Erector Spinae Muscles

  • Iliocostalis: Lateral column, extends and laterally bends the spine.

  • Longissimus: Intermediate column, extensors of the back.

  • Spinalis: Medial column, extends the spine.

Other Back Muscles

  • Semispinalis: Rotates and extends the spine.

  • Quadratus Lumborum: Laterally flexes the lumbar spine.

Muscles of Breathing

  • External Intercostals: Elevate ribs during inhalation.

  • Internal Intercostals: Aid in forced expiration by depressing the ribs.

  • Diaphragm: Primary muscle of respiration; flattens during inhalation to increase thoracic cavity volume.

Muscles of the Abdomen

  • Rectus Abdominis: Flexes lumbar vertebrae; aids in posture.

  • External Oblique: Compresses abdomen, rotates trunk.

  • Internal Oblique: Similar to external but fibers run in the opposite direction.

  • Transversus Abdominis: Provides compressive support to abdominal contents.

Muscles of the Pelvis

  • Levator Ani: Supports pelvic organs; can be divided into pubococcygeus and iliococcygeus.

  • Coccygeus: Assists in pelvic floor support.

  • Deep Transverse Perineal Muscle: Supports the pelvis.

  • External Urethral Sphincter: Controls urethral opening.

  • Ischiocavernosus: Maintains erection by compressing the deep arteries.

  • Bulbospongiosus: Assists in erection and expulsion of urine.

  • Superficial Transverse Perineus: Supports perineum.

Muscles of the Thorax

  • Anterior Muscles:

    • Pectoralis Major: Adducts and medially rotates the arm.

    • Pectoralis Minor: Stabilizes the scapula.

    • Serratus Anterior: Protracts and rotates scapula, keeps it against thoracic wall.

    • Subclavius: Stabilizes and depresses the clavicle.

  • Posterior Muscles:

    • Trapezius: Stabilizes and moves the scapula.

    • Levator Scapulae: Elevates scapula.

    • Rhomboids (Major and Minor): Retracts the scapula.

    • Latissimus Dorsi: Extends, adducts, and medially rotates the arm.

Shoulder Muscles

  • Deltoid: Abducts the arm.

  • Subscapularis: Medial rotation of the arm.

  • Supraspinatus: Initiates abduction of the arm.

  • Infraspinatus: Lateral rotation of the arm.

  • Teres Major and Minor: Assist in arm movement.

  • Coracobrachialis: Flexes and adducts the arm.

Muscles of the Elbow

  • Triceps Brachii: Extends the forearm.

  • Anconeus: Assists in elbow extension.

  • Biceps Brachii: Flexes and supinates the forearm.

  • Brachialis: Primary flexor of the elbow.

  • Brachioradialis: Flexes forearm at elbow.

Anterior Forearm Muscles

Superficial Muscles:

  • Pronator Teres: Pronates the forearm.

  • Flexor Carpi Radialis: Flexes and abducts the wrist.

  • Palmaris Longus: Flexes the wrist.

  • Flexor Carpi Ulnaris: Flexes and adducts the wrist.

  • Flexor Digitorum Superficialis: Flexes fingers.

Deep Muscles:

  • Flexor Pollicis Longus: Flexes the thumb.

  • Flexor Digitorum Profundus: Flexes the distal phalanges.

  • Pronator Quadratus: Pronates the forearm.

Posterior Forearm Muscles

Superficial Muscles:

  • Extensor Carpi Radialis Longus and Brevis: Extend and abduct wrist.

  • Extensor Digitorum: Extends fingers.

  • Extensor Carpi Ulnaris: Extends and adducts wrist.

Deep Muscles:

  • Supinator: Supinates the forearm.

  • Abductor Pollicis Longus: Abducts the thumb.

  • Extensor Pollicis Longus and Brevis: Extends the thumb.

  • Extensor Indicis: Extends the index finger.

Thenar and Hypothenar Muscles

  • Thenar Muscles: Involved in thumb movement.

    • Abductor Pollicis Brevis: Abducts the thumb.

    • Flexor Pollicis Brevis: Flexes the thumb.

    • Opponens Pollicis: Allows opposition of the thumb.

    • Adductor Pollicis: Adducts the thumb.

  • Hypothenar Muscles: Involved in little finger movement.

    • Abductor Digiti Minimi: Abducts the little finger.

    • Flexor Digiti Minimi: Flexes the little finger.

    • Opponens Digiti Minimi: Allows opposition of the little finger.

Midpalmar Muscles

  • Lumbricals: Flex fingers at the MCP joint and extend at the PIP and DIP joints.

  • Palmar Interossei: Adduct fingers.

  • Dorsal Interossei: Abduct fingers.

Hip Muscles

Anterior Muscles:

  • Iliopsoas: Major hip flexor consisting of two parts:

    1. Iliacus: Flexes thigh at the hip.

    2. Psoas Major: Flexes thigh at the hip.

  • Sartorius: Flexes, abducts, and laterally rotates thigh.

  • Adductor Group: Includes Medius, Longus, and Brevis which adduct the thigh.

  • Pectineus: Flexes and adducts the thigh.

  • Gracilis: Adducts thigh and flexes the leg.

  • Tensor Fasciae Latae: Flexes and abducts thigh.

  • Quadriceps Femoris: Major extensor of the knee, comprised of:

    • Rectus Femoris

    • Vastus Lateralis

    • Vastus Medialis

    • Vastus Intermedius

Posterior Muscles:

  • Gluteus Maximus: Extends and laterally rotates thigh.

  • Gluteus Medius: Abducts and medially rotates thigh.

  • Gluteus Minimus: Similar function as gluteus medius.

  • Piriformis: Laterally rotates and abducts thigh.

  • Obturator Externus & Internus: Lateral rotators.

  • Gemelli (Superior and Inferior): Assist in lateral rotation.

  • Quadratus Femoris: Lateral rotator of the hip.

  • Hamstrings: Flexors of the knee and extensors of the hip, consisting of

    1. Biceps Femoris

    2. Semitendinosus

    3. Semimembranosus

Leg Muscles

Anterior Compartment:

  • Tibialis Anterior: Dorsiflexes and inverts foot.

  • Extensor Digitorum Longus: Extends toes and dorsiflexes foot.

  • Fibularis (Peroneus) Tertius: Dorsiflexes and everts foot.

  • Extensor Hallucis Longus: Extends the big toe and dorsiflexes foot.

Lateral Compartment:

  • Fibularis (Peroneus) Longus: Evert and plantar flex the foot.

  • Fibularis (Peroneus) Brevis: Evert the foot.

Superficial Posterior Compartment:

  • Gastrocnemius: Plantar flexes foot and flexes knee.

  • Soleus: Plantar flexes the foot.

  • Plantaris: Weakly assists in knee flexion and plantar flexion.

Deep Posterior Compartment:

  • Popliteus: Flexes and rotates leg medially.

  • Flexor Digitorum Longus: Flexes toes and plantar flexes foot.

  • Flexor Hallucis Longus: Flexes the big toe and plantar flexes foot.

  • Tibialis Posterior: Plantar flexes and inverts foot.

Foot Muscles

Dorsum:

  • Extensor Digitorum Brevis: Extends toes.

Sole of the Foot (4 Layers):

  • 1st Layer:

    • Flexor Digitorum Brevis: Flexes toes.

    • Abductor Hallucis: Abducts and flexes big toe.

    • Abductor Digiti Minimi: Abducts and flexes little toe.

  • 2nd Layer:

    • Flexor Accessorius: Aids in flexing toes.

    • Lumbricals: Flex toes at MTP joints.

  • 3rd Layer:

    • Flexor Hallucis Brevis: Flexes big toe.

    • Adductor Hallucis: Adducts big toe.

    • Flexor Digiti Minimi Brevis: Flexes little toe.

  • 4th Layer:

    • Plantar Interossei: Adducts toes.

    • Dorsal Interossei: Abducts toes.