Muscle Structure and Fascicle Arrangement

Patterns of Fascicle Arrangement

The arrangement of muscle fascicles determines the shape and function of each muscle. Fascicles are bundles of muscle fibers, and their organization affects the muscle's range of motion and force production.

• Circular: Fascicles are arranged in concentric rings, commonly found in sphincter muscles such as the orbicularis oris.

• Convergent: Fascicles converge toward a single tendon, as seen in the pectoralis major.

• Parallel: Fascicles run parallel to the long axis of the muscle, typical in muscles like the sartorius.

• Unipennate: Fascicles insert into only one side of the tendon, as in the extensor digitorum longus.

• Bipennate: Fascicles insert into the tendon from both sides, as in the rectus femoris.

• Fusiform: Muscle is spindle-shaped with a central belly, as in the biceps brachii.

• Multipennate: Fascicles attach obliquely from many directions to several tendons, as in the deltoid.

Example: The orbicularis oris muscle forms the lips and controls mouth opening and closing.

Superficial Muscles of the Body

Anterior View

The anterior view of the human body reveals the major superficial muscles responsible for movement and posture. These muscles are grouped by region and function.

• Head and Neck: Includes muscles such as the temporalis, masseter, sternocleidomastoid, and platysma.

• Thorax: Muscles like the pectoralis major, pectoralis minor, and serratus anterior are involved in movements of the shoulder and chest.

• Abdomen: The rectus abdominis, external oblique, internal oblique, and transversus abdominis support trunk movement and abdominal compression.

• Upper Limb: Muscles such as the biceps brachii, triceps brachii, brachialis, and brachioradialis control arm and forearm movement.

• Lower Limb: Includes iliopsoas, rectus femoris, vastus lateralis, vastus medialis, sartorius, adductor longus, gracilis, gastrocnemius, and soleus.

Posterior View

The posterior view highlights muscles involved in extension, stabilization, and movement of the back, shoulders, and legs.

• Neck and Back: Trapezius, latissimus dorsi, rhomboid major, infraspinatus, and deltoid are key muscles.

• Shoulder and Arm: Triceps brachii, brachialis, brachioradialis, extensor carpi ulnaris, and extensor digitorum.

• Hip and Thigh: Gluteus maximus, gluteus medius, biceps femoris, adductor magnus, semimembranosus, and semitendinosus.

• Leg: Gastrocnemius, soleus, fibularis longus, and calcaneal (Achilles) tendon.

Muscles of the Scalp, Face, and Neck

Facial Muscles

Muscles of the face are responsible for expressions, mastication, and movement of the scalp.

• Epicranius: Composed of frontal and occipital bellies connected by the galea aponeurotica.

• Orbicularis oculi: Closes the eyelids.

• Zygomaticus: Elevates the corners of the mouth.

• Buccinator: Compresses the cheek.

• Masseter: Elevates the mandible for chewing.

• Platysma: Tenses the skin of the neck.

Neck Muscles

The neck muscles facilitate head movement and support the cervical spine.

• Sternocleidomastoid: Flexes and rotates the head.

• Scalene muscles: Assist in lateral flexion and elevation of the first two ribs during inspiration.

• Platysma: Superficial muscle that tenses the skin of the neck.

Muscles of the Abdominal Wall

Abdominal Muscles

The abdominal wall is composed of several layers of muscles that protect internal organs and aid in trunk movement.

• Rectus abdominis: Flexes the vertebral column and compresses the abdomen.

• External oblique: Rotates and laterally flexes the trunk.

• Internal oblique: Functions similarly to the external oblique but lies deeper.

• Transversus abdominis: Compresses abdominal contents.

• Linea alba: Midline tendinous structure separating left and right rectus abdominis.

• Inguinal ligament: Formed by the inferior border of the external oblique aponeurosis.

Muscles of the Thorax and Shoulder

Superficial Muscles Acting on the Scapula and Arm

Muscles of the thorax and shoulder are essential for movements of the upper limb and stabilization of the scapula.

• Pectoralis major: Adducts and medially rotates the arm.

• Pectoralis minor: Stabilizes the scapula.

• Serratus anterior: Protracts the scapula.

• Deltoid: Abducts the arm.

• Subclavius: Stabilizes the clavicle.

Muscles of the Forearm

Anterior Compartment

Muscles in the anterior compartment of the forearm are primarily responsible for flexion of the wrist and fingers.

• Biceps brachii: Flexes the elbow and supinates the forearm.

• 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 the fingers.

Posterior Compartment

Muscles in the posterior compartment extend the wrist and fingers.

• Extensor carpi radialis longus and brevis: Extend and abduct the wrist.

• Extensor digitorum: Extends the fingers.

• Extensor carpi ulnaris: Extends and adducts the wrist.

• Extensor pollicis longus and brevis: Extend the thumb.

• Abductor pollicis longus: Abducts the thumb.

Muscles of the Thigh and Leg

Anterior and Medial Compartment

Muscles in the anterior and medial compartments of the thigh promote movements such as flexion, extension, and adduction.

• Iliopsoas: Flexes the thigh.

• Quadriceps femoris: Extends the knee; includes rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius.

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

• Adductor longus, magnus, and gracilis: Adduct the thigh.

Posterior Compartment

The posterior compartment contains muscles responsible for extension of the hip and flexion of the knee.

• Gluteus maximus and medius: Extend and abduct the thigh.

• Hamstrings: Include biceps femoris, semitendinosus, and semimembranosus; flex the knee and extend the hip.

• Adductor magnus: Adducts and extends the thigh.

Muscles of the Leg

Anterior Compartment

Muscles in the anterior compartment of the leg are involved in dorsiflexion and extension of the toes.

• Tibialis anterior: Dorsiflexes and inverts the foot.

• Extensor digitorum longus: Extends the toes.

• Extensor hallucis longus: Extends the big toe.

• Fibularis longus and tertius: Evert and dorsiflex the foot.

Posterior Compartment

The posterior compartment contains muscles responsible for plantar flexion and flexion of the toes.

• Gastrocnemius: Plantar flexes the foot and flexes the knee.

• Soleus: Plantar flexes the foot.

• Flexor digitorum longus: Flexes the toes.

• Flexor hallucis longus: Flexes the big toe.

• Plantaris: Assists in plantar flexion.

• Tibialis posterior: Inverts and plantar flexes the foot.

Key Terms and Concepts

• Origin: The fixed attachment point of a muscle.

• Insertion: The movable attachment point of a muscle.

• Aponeurosis: A broad, flat tendon.

• Retinaculum: A band that holds tendons in place.

Additional info:

• Muscle actions are often described in terms of their effect on joints and bones, such as flexion, extension, abduction, adduction, and rotation.

• Muscles work in groups to produce coordinated movements, with agonists, antagonists, and synergists playing distinct roles.