Study Notes: The Muscular System (Anatomy & Physiology Lab I, Lesson 8)

Chapter 13: The Muscular System

Overview of Muscle Classification

The muscular system consists of specialized tissues responsible for movement, posture, and heat production. Muscles are classified by their function in movement:

• Agonists (Prime Movers): Muscles primarily responsible for producing a particular movement.

• Antagonists: Muscles that oppose, restrict, or reverse a movement.

• Synergists: Muscles that aid the action of agonists, providing additional force or reducing unnecessary movement.

• Fixators: Specialized synergists that immobilize the origin of the agonist muscle, ensuring all tension is exerted at the insertion.

Naming of Muscles

Muscles are named according to several criteria, which help identify their location, function, and structure:

• Direction of Muscle Fiber: Rectus (straight), transverse (right angles).

• Relative Size: Maximus (largest), longus (long).

• Location: Frontalis (at frontal bone).

• Number of Origins: Biceps (2 origins).

• Location of Origin/Insertion: Sternocleidomastoid (originates at sternum and clavicle, inserts at mastoid process).

• Shape: Deltoid (triangular).

• Action: Adductor (adduction action).

Patterns of Fascicle Arrangement in Muscles

Muscle fascicles are arranged in specific patterns that influence muscle function:

Pattern	Example
Circular	Orbicularis oris
Convergent	Pectoralis major
Parallel	Sartorius
Unipennate	Extensor digitorum longus
Multipennate	Deltoid
Fusiform	Biceps brachii
Bipennate	Rectus femoris

Muscles of the Head and Neck

Facial Expression Muscles

• Epicranius frontalis (frontal belly): Raises eyebrows. Origin: Epicranial aponeurosis Insertion: Skin of eyebrows & root of nose

• Epicranius occipitalis (occipital belly): Pulls scalp posteriorly. Origin: Occipital and temporal bones Insertion: Epicranial aponeurosis

• Orbicularis oculi: Closes eye, pulls eyebrow down.

• Orbicularis oris: Closes and protrudes lips.

Muscles of Mastication

• Masseter: Closes jaw. Origin: Zygomatic arch & maxilla Insertion: Angle & ramus of mandible

• Temporalis: Closes jaw.

Neck Muscles

• Platysma: Opens jaw, pulls lower lip down. Origin: Over pectoral muscles Insertion: Lower margin of mandible, skin, muscle at mouth corner

• Sternocleidomastoid: Neck flexion, head rotation toward shoulder. Origin: Manubrium of sternum & medial portion of clavicle Insertion: Mastoid process of temporal bone & superior nuchal line of occipital bone

• Digastric (anterior and posterior belly): Opens jaw, elevates hyoid bone. Origin: Lower margin of mandible & mastoid process Insertion: Connective tissue loop to hyoid bone

• Mylohyoid: Elevates hyoid bone & base of tongue.

• Sternohyoid: Depresses larynx & hyoid bone.

Muscles of the Anterior Thorax & Shoulder

• Deltoid: Abducts arm; flexes, extends, and rotates humerus.

• Pectoralis major: Adducts and medially rotates arm; pulls chest upward (inhalation). Origin: Clavicle, sternum, cartilage of ribs 1-6, aponeurosis of external obliques Insertion: Short tendons into intertubercular sulcus of humerus

• Pectoralis minor: Moves scapula anterior/inferior; raises ribs (inhalation).

• Serratus anterior: Fixes scapula for arm abduction; pulls scapula anterior/inferior.

Muscles of the Abdominal Wall

• Rectus abdominis: Flexes vertebral column; depresses ribs; compresses abdomen (exhalation). Origin: Pubic crest & symphysis Insertion: Xiphoid process & costal cartilage of ribs 5-7

• External oblique: Flexes vertebral column; depresses ribs; rotates trunk; compresses abdomen (exhalation).

• Internal oblique: Flexes vertebral column; depresses ribs; rotates trunk; compresses abdomen (exhalation).

• Transversus abdominis: Compresses abdomen (exhalation).

Muscles of the Posterior Trunk

• Trapezius: Extends head; adducts scapula & lateral clavicle.

• Latissimus dorsi: Extends, adducts, and medially rotates arm.

Muscles of the Upper Arm

Posterior Compartment

• Triceps brachii: Extends forearm. Heads: Lateral (#16), Long (#17), Medial (#18)

Anterior Compartment

• Biceps brachii (#11-13): Flexes forearm. Origin: Short head – coracoid process; Long head – intertubercular sulcus & within capsule of shoulder joint Insertion: Radial tuberosity

• Brachioradialis (#19): Flexes forearm.

• Brachialis (#14): Flexes forearm.

Muscles of the Forearm

Anterior Compartment

• Pronator teres (#22): Pronates & weakly flexes arm.

• Flexor carpi radialis (#23): Flexes & abducts hand at wrist.

• Palmaris longus (#24): Flexes hand at wrist. Origin: Medial epicondyle of humerus Insertion: Palmar aponeurosis, skin & fascia of palm

• Flexor carpi ulnaris (#25): Flexes & adducts hand at wrist.

Posterior Compartment

• Extensor carpi radialis (longus #20 & brevis #21): Extends & abducts hand at wrist.

• Extensor digitorum (#30): Extends fingers & hand at wrist. Origin: Lateral epicondyle of humerus Insertion: By 4 tendons into distal phalanges of fingers 2-5

• Extensor carpi ulnaris (#32): Extends & adducts hand at wrist.

Muscles of the Anterior & Medial Thigh

• Iliopsoas (#25): Flexes trunk, thigh at hip.

• Sartorius (#24): Flexes, abducts, & laterally rotates thigh. Origin: Anterior superior iliac spine Insertion: By aponeurosis into medial aspect of proximal tibia

• Adductor group (Adductor magnus #28, Adductor longus #26, Adductor brevis): Adducts, flexes, & medially rotates thigh.

• Pectineus: Adducts, flexes, and medially rotates thigh.

• Gracilis (#27): Adducts, flexes, & medially rotates thigh.

Anterior Compartment

• Quadriceps femoris: Extends lower leg at knee & flexes thigh.

  • Rectus femoris (#20): Origin: Anterior inferior iliac spine & superior margin of acetabulum Insertion: Tibial tuberosity and patella

  • Vastus lateralis (#21)

  • Vastus intermedius (#22)

  • Vastus medialis (#23)

• Tensor fasciae latae (#18): Flexes, abducts, & medially rotates thigh.

Muscles of the Hip & Posterior Thigh

Posterior Pelvis

• Gluteus maximus (#11): Laterally rotates, abducts, & extends thigh. Origin: Dorsal ilium, sacrum, & coccyx Insertion: Gluteal tuberosity of femur and iliotibial tract

• Gluteus medius (#12): Abducts & medially rotates thigh.

Posterior Compartment: "Hamstrings"

• Biceps femoris (#31 long head, 32 short head): Extends thigh; laterally rotates leg; flexes knee.

• Semitendinosus (#30): Extends thigh, medially rotates leg; flexes knee.

• Semimembranosus (#29): Extends thigh, medially rotates leg; flexes knee.

Muscles of the Lower Leg

Lateral Compartment

• Fibularis longus (Peroneus longus) (#35): Plantar flexes & everts foot. Origin: Head and upper portion of fibula Insertion: By long tendon under foot to metatarsal 1 & medial cuneiform

Anterior Compartment

• Tibialis anterior (#33): Dorsiflexes & inverts foot.

• Extensor digitorum longus (#34): Dorsiflexes foot & extends toes.

Posterior Compartment

• Triceps surae: Plantar flexes foot; composed of:

  • Gastrocnemius (#37 lateral head, 38 medial head): Plantar flexes & flexes lower leg. Origin: By two heads from medial and lateral condyles of femur Insertion: Calcaneus via calcaneal tendon

  • Soleus (#39): Plantar flexes foot.

• Flexor digitorum longus (#42): Flexes toes, plantar flexes & inverts foot.

Summary Table: Major Muscle Groups and Actions

Region	Major Muscles	Primary Actions
Head/Neck	Epicranius, Orbicularis oculi/oris, Masseter, Temporalis, Platysma, Sternocleidomastoid, Digastric, Mylohyoid, Sternohyoid	Facial expression, mastication, neck movement, swallowing
Thorax/Shoulder	Deltoid, Pectoralis major/minor, Serratus anterior	Arm abduction, adduction, rotation, scapular movement
Abdominal Wall	Rectus abdominis, External/Internal oblique, Transversus abdominis	Trunk flexion, rotation, abdominal compression
Posterior Trunk	Trapezius, Latissimus dorsi	Head extension, scapular adduction, arm movement
Upper Arm	Biceps brachii, Triceps brachii, Brachialis, Brachioradialis	Forearm flexion/extension
Forearm	Pronator teres, Flexor/extensor carpi radialis/ulnaris, Palmaris longus, Extensor digitorum	Hand/wrist movement, finger extension
Thigh	Iliopsoas, Sartorius, Adductors, Quadriceps femoris, Tensor fasciae latae, Gluteus maximus/medius, Hamstrings	Thigh flexion, extension, abduction, rotation, knee movement
Lower Leg	Fibularis longus, Tibialis anterior, Extensor digitorum longus, Gastrocnemius, Soleus, Flexor digitorum longus	Foot dorsiflexion, plantar flexion, inversion, eversion, toe movement

Additional Info

• Muscle origin refers to the fixed attachment, while insertion refers to the movable attachment.

• Muscle actions are determined by the direction of fibers and the joints they cross.

• Understanding muscle groups and their actions is essential for clinical assessment and rehabilitation.