I. The Structural and Functional Organization of Muscles

A. The Functions of Muscles

1. Muscle functions include: movement, stability, communication (facilitate speech, writing), control of body openings and passages, and heat production.

B. Connective Tissues of a Muscle (fig. 11.1)

1. Skeletal muscle cells, called muscle fibers, are each surrounded by a thin layer of areolar tissue called the endomysium.

2. Muscle fibers are grouped into bundles called fascicles, each of which is surrounded by a connective tissue sheath, the perimysium.

3. Each whole muscle is covered by yet another layer of connective tissue, the epimysium. The epimysium grades into fascia.

4. Muscles can attach to bone directly through collagen fibers between the epimysium and periosteum, or indirectly through extensions of deep fascia called tendons.

C. General Anatomy of Skeletal Muscles (figs. 11.3, 11.4)

1. A muscle attachment at the stationary end is the origin; the relatively movable end is the insertion. The thicker middle region is the belly of the muscle.

2. Fusiform muscles are thick in the middle and tapered at each end.

3. Parallel muscles are long, straplike muscles of uniform width and parallel fascicles.

4. Convergent muscles are fan-shaped.

5. Pennate muscles are feather-shaped. Pennate muscles can be unipennate, bipennate, or multipennate.

6. Circular muscles (sphincters) surround body openings.

D. Coordinated Action of Muscle Groups

1. The movement of a muscle is its action; muscles seldom act alone.

2. The prime mover (agonist) is the muscle producing the most force.

3. A synergist is a muscle that aids the prime mover.

4. The antagonist is a muscle that opposes the prime mover. An antagonistic pair of muscles act on opposite sides of a joint.

5. A fixator is a muscle that prevents the movement of a bone.

E. Intrinsic and Extrinsic Muscles

1. An intrinsic muscle is entirely contained within a particular region.

2. An extrinsic muscle acts upon a designated region but has its origin elsewhere.

F. Muscle Innervation

1. Muscles of the head and neck are supplied by cranial nerves.

2. Muscles elsewhere are innervated by spinal nerves.

G. How Muscles Are Named (table 11.1)

1. In 1895, Latin was chosen as the official language for muscle nomenclature. Terms used in this text are English modifications of the Latin names.

2. Muscle nomenclature provides clues to location, appearance, origin, insertion, or action.

H. A Learning Strategy

1. Suggestions for studying muscles include: use visual models or images, palpate muscles on yourself, locate origins and insertions on a skeleton, study the derivation of each muscle name, and say the names aloud.

II. Muscles of the Head and Neck

A. Muscles of Facial Expression (figs. 11.5–11.7, table 11.2)

1. Humans have an amazing variety of facial expressions when compared with other animals.

2. The frontalis of the forehead and the occipitalis at the back of the head collectively form the occipitofrontalis, attached across the top of the skull by the galea aponeurotica.

3. The orbicularis oculi closes the eyelid, while the levator palpebrae superioris opens it. Other muscles of the eye and nose areas are the corrugator supercilii, procerus, and nasalis.

4. A sphincter, the orbicularis oris, surrounds the mouth. Five muscles approach the orbicularis oris from all directions. These are the levator labii superioris, zygomaticus minor, zygomaticus major, levator anguli oris, and risorius.

5. Muscles that draw the lower lip downward are the depressor anguli oris, the depressor labii inferioris, and a pair of mentalis muscles.

6. The buccinator muscle of the cheek helps in blowing, sucking, and chewing (pushing food against the teeth).

7. The platysma under the chin depresses the mandible, widens the mouth, and tenses the skin of the neck.

B. Muscles of Chewing and Swallowing (figs. 11.8–11.10, table 11.3)

1. Extrinsic muscles that connect the tongue to other structures in the head and neck are the genioglossus, hyoglossus, styloglossus, and palatoglossus.

2. The four paired muscles of mastication are the temporalis, masseter, and medial and lateral pterygoid muscles.

3. Eight pairs of hyoid muscles aid in chewing and swallowing. Four are superior to the hyoid: the digastric, geniohyoid, mylohyoid, and stylohyoid. The inferior group includes the omohyoid, sternohyoid, sternothyroid, and thyrohyoid.

4. When food enters the pharynx, the superior, middle, and inferior pharyngeal constrictors contract in that order to force the bolus downward.

C. Muscles Acting on the Head (figs. 11.10–11.12, table 11.4)

1. The principal flexors of the neck are the sternocleidomastoid and three scalenes.

2. The extensors of the head are located in the back of the neck. Three primary extensors are the trapezius, the splenius capitis, and the semispinalis capitis.

III. Muscles of the Trunk

A. Muscles of Respiration (fig. 11.13, table 11.5)

1. The lungs are ventilated primarily by the diaphragm, which forms the floor of the thoracic cavity; 11 pairs of external intercostal muscles between the ribs; and 11 pairs of internal intercostal muscles between the ribs deep to the external intercostals.

B. Muscles of the Abdomen (figs. 11.14- 11.16, table 11.6)

1. The anterior and lateral walls of the abdomen have four pairs of sheetlike muscles that support the viscera, stabilize the vertebral column, and aid in respiration, urination, defecation, vomiting, and childbirth. They are the rectus abdominis, the external and internal obliques, and the transversus abdominis.

2. The right and left rectus muscles are separated by a vertical fibrous strip called the linea alba.

C. Muscles of the Back (figs. 11.17–11.19, table 11.7)

1. Muscles acting on the spine include a superficial group (the erector spinae), divided into three columns: the iliocostalis, longissimus, and spinalis.

2. The major deep muscle is the semispinalis, which is divided into three parts: the semispinalis capitis, the semispinalis cervicis, and the semispinalis thoracis.

3. The major deep lumbar muscle is the quadratus lumborum.

D. Muscles of the Pelvic Floor (fig. 11.20, table 11.8)

1. The floor of the pelvic cavity is formed by three layers of muscles and fasciae that span the pelvic outlet and support the viscera.

2. The superficial perineal space contains the ischiocavernosus and bulbospongiosus that function mostly during sexual intercourse.

3. The middle layer of the pelvic floor is spanned by the urogenital diaphragm, composed of a fibrous membrane and two muscles, the deep transverse perineus and the external urethral sphincter.

4. The deepest layer of the pelvic floor (pelvic diaphragm) consists of the levator ani and coccygeus in both sexes.

IV. Muscles Acting on the Shoulder and Upper Limb

A. Muscles Acting on the Scapula (figs. 11.15, 11.17, 11.21, table 11.9)

1. The muscles that act on the pectoral girdle are divided into anterior and posterior groups.

2. Anterior group muscles include the pectoralis minor and serratus anterior.

3. The posterior group consists of a superficial muscle, the trapezius, and three deep muscles: the levator scapulae, rhomboideus major, and rhomboideus minor.

B. Muscles Acting on the Humerus (figs. 11.22–11.24, tables 11.10, 11.11)

1. Nine shoulder muscles cross the shoulder joint and insert on the humerus.

2. Two axial muscles are the pectoralis major (flexor) and latissimus dorsi (extensor). These are the prime movers of the shoulder joint.

3. Three of the seven scapular muscles are the deltoid (prime mover that flexes, extends, and abducts the shoulder), the teres major (extensor), and the coracobrachialis (flexor and adductor). The remaining four scapular muscles form the rotator cuff: the infraspinatus, supraspinatus, subscapularis, and teres minor.

C. Muscles Acting on the Forearm (figs. 11.22, 11.25, tables 11.12, 11.13)

1. The prime flexors are the biceps brachii and brachialis. The brachioradialis is a synergist in elbow flexion.

2. The prime extensor is the triceps brachii, with a weak synergistic anconeus.

3. Pronation is achieved by the pronator teres and the pronator quadratus.

4. Supination is achieved by the biceps brachii and the supinator.

D. Muscles Acting on the Wrist and Hand (figs. 11.26–11.28, tables 11.14, 11.15)

1. The hand is acted upon both by extrinsic muscles in the forearm and intrinsic muscles in the hand itself.

2. The intrinsic muscles are divided into three groups and are contained entirely within the hand: thenar muscles, hypothenar muscles, and midpalmar muscles (fig. 11.29, table 11.16).

V. Muscles Acting on the Hip and Lower Limb

A. Muscles Acting on the Hip and Femur (figs. 11.30–11.34, table 11.17)

1. Most of the muscles acting on the femur originate on the os coxae.

2. The two principal anterior flexors are the iliacus and the psoas major.

3. On the lateral and posterior sides of the hip are the tensor fasciae latae and three gluteal muscles: the gluteus maximus, gluteus medius, and gluteus minimus.

4. Five medial muscles act on the hip joint: the adductor longus, adductor brevis, adductor magnus, gracilis, and pectineus.

B. Muscles Acting on the Knee (figs. 11.32, 11.34, table 11.18)

1. Several muscles form most of the mass of the thigh and act on the knee joint.

2. The anterior extensors are the large quadriceps femoris and its four heads: the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. All four converge on the patellar tendon.

3. Crossing the quadriceps from the lateral side of the hip to the medial side of the knee is the sartorius (flexor).

4. Muscles of the posterior compartment are the biceps femoris, semimembranosus, and semitendinosus.

C. Muscles Acting on the Foot (figs. 11.36, 11.37, table 11.19)

1. The anterior dorsiflexors are the extensor digitorum longus, extensor hallucis longus, peroneus tertius, and tibialis anterior.

2. The posterior compartment has three superficial plantar flexors: the gastrocnemius, soleus, and plantaris.

3. Deep flexors are the flexor digitorum longus, flexor hallucis longus, tibialis posterior, and popliteus.

4. The lateral group consists of the peroneus brevis and peroneus longus that plantar flex and evert the foot.

5. The intrinsic muscles of the foot support the arches and act on the toes to aid locomotion (fig. 11.39, table 11.20). The superficial muscles are the extensor digitorum brevis, flexor digitorum brevis, quadratus plantae, and four lumbrical muscles.

6. Deeper muscles include the adductor hallucis, flexor digiti minimi brevis, and flexor hallucis brevis.

7. The deepest layer consists of four dorsal and three plantar interosseous muscles.