Chapter 10 The Muscular System*

 

I. Interactions of Skeletal Muscles in the Body

    

     A.  Origin and Insertion

1.       The immovable end of a muscle is the origin;

2.     The movable end is the insertion;

3.     Contraction pulls the insertion toward the origin.           

     B.  Interaction of Skeletal Muscles

1.       Skeletal muscles are arranged in opposing groups across body joints so that one group can reverse or modify the action of the other.

 

C.     Classification of muscles

1.       Of a group of muscles, the one doing the majority of the work is the prime mover or agonist.

2.     Opposing muscles are called antagonists (they reverse, or oppose, the action of another muscle);

3.     Helper muscles are called synergists (they aid the prime mover by effecting the same action, stabilizing joints, or preventing undesirable movements); and

4.     Fixators function to immobilize a bone or a muscle's origin.

 

II.            Naming Skeletal Muscles

 

A.    Muscles are named according to any of the following criteria: relative size, shape, location, action, number of origins, attachment sites (origin/insertion), or direction of its fibers (fascicles).

 

III.       Muscle Mechanics:  Importance of Leverage and Fascicle Arrangement

 

A.    A lever is a bar that moves on a fulcrum.

1.       When an effort is applied to the lever, a load is moved.

2.     In the body, bones are the levers, joints are the fulcrums, and the effort is exerted by skeletal muscles at their insertions.

 

B.     Types of Movement

1.       When the effort is farther from the fulcrum than is the load, the lever operates at a mechanical advantage (is slow and strong);

2.     When the effort is exerted closer to the fulcrum than is the load, the lever operates at a mechanical disadvantage (is fast and promotes a large degree or movement).

 

*(Human Anatomy & Physiology, 6th Ed.; Marieb, 2004)

 

C.     Classes of Levers

1.       1st Class Levers (effort-fulcrum-load) may operate at a mechanical advantage or disadvantage.

2.     2nd Class Levers (fulcrum-load-effort) all operate at a mechanical advantage.

3.     3rd Class Levers (fulcrum-effort-load) always operate at a mechanical disadvantage.

 

D.    Common Patterns of Fascicle Arrangement

1.       Parallel

a.      The long axes of the fascicles run parallel to the long axis of the muscle;              (ie. sartorius)

b.     Shorten the most. 

2.     Fusiform

a. Spindle-shaped muscles with an expanded belly (ie. biceps brachii)

3.     Pennate

a.      The fascicles are short and attach obliquely to a central tendon that runs the length of the muscle.

¨      Unipennate:  Fascicles insert into only one side of the tendon (ie. extensor digitorum muscle of the leg);

¨      Bipennate:  Fascicles insert into the tendon from opposite sides, resembling a feather.  (ie. rectus femoris)

¨      Multipennate:  Fascicles insert into the tendon from multiple sides; resembling feathers situated side by side. (ie. deltoid)

b.     Stocky pennate muscles shorten little but are the most powerful muscles.

4.     Convergent

a.      The muscle has a broad origin, and its fascicles converge toward a single tendon of insertion.

b.     Muscle is triangular or fan shaped (ie. pectoralis major)

5.     Circular

a.      The fascicles are arranged in concentric rings; called sphincters

b.     Surround external body openings (ie. orbicularis oris and oculi)

         

 

IV.           Major Skeletal Muscles of the Body

 

A. Muscles of Facial Expression

1.       Muscles of the head that produce facial expression tend to be small.

2.     They insert into soft tissue (skin and other muscles) rather than into bone.

3.     These muscles open and close the eyes and mouth, compress the cheeks, allow smiling and other types of facial language.

4.   Muscles aiding in facial expression include: epicranius (frontalis, galea aporneurotica and occipitalis), orbicularis oculi, orbicularis oris, buccinator, zygomatigus, and platysma.

 

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     B.  Muscles of Mastication

1.       Chewing movements include up and down as well as side-to-side grinding motions of muscles attached to the skull and mandible.

2.      Muscles of the head involved in mastication include: masseter and temporalis that elevate the mandible and two deep muscle pairs (medial pterygoid, and lateral pterygoid) that promote grinding and sliding jaw movements.

3.     Extrinsic muscles of the tongue anchor the tongue and control its movements.

 

C.  Muscles of the Anterior Neck and Throat Permitting Swallowing    

1.       Deep muscles of the anterior neck promote swallowing movements, including elevation/depression of the hyoid bone, closure of the respiratory passages, and peristalsis of the pharynx.

2.     Muscles involved in swallowing include: Suprahyoid muscles (Digastric, Stylohyoid, Mylohyoid and Geniohyoid); Infrahyoid Muscles (Sternohyoid, Sternothyroid, Omohyoid, and Thyrohyoid); Pharyngeal Constrictor Muscles (Superior, Middle and Inferior)

 

D.   Muscles that Move the Head and Vertebral Column

1.       Neck muscles and deep muscles of the vertebral column promote head and trunk movements.

2.     The deep muscles of the posterior trunk can extend large regions of the vertebral column and head simultaneously.

3.     Paired muscles in the neck and back flex, extend, and turn the head as well as hold the torso in an upright position.

4.     Head flexion and rotation are effected by the anteriorly located sternocleidomastoid and scalene muscles.

5.     Muscles of the neck and column permitting head and trunk movements include:  sternocleidomastoid, scalenes (anterior, middle and posterior), splenius (capitis and cervicis), erector spinae group (iliocostalis, longissimus, spinalis), semispinalis (thoracis, cervicis, and capitis), and quadratus lumborum.

 

E.    Muscles of the Thorax Permitting Breathing

1.       Movements of quiet breathing are promoted by the diaghragm and the external intercostal muscles of the thorax.

2.     Downward movement of the diaphragm increases intra-abdominal pressure.

3.     The sternocleidomastoid and scalene muscles of the neck help lift the ribs during forced inspiration.

4.     Muscles of the thorax permitting normal breathing include: external intercostals, internal intercostals, and diaphragm.

             

 

 

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F.  Muscles of the Abdominal Wall

1.       This group of muscles connects the rib cage and vertebral column to the pelvic girdle.

2.     The four muscle pairs forming the abdominal wall are layered like plywood to form a natural muscular girdle that protects, supports, and compresses abdominal contents.

3.     These muscles also flex and laterally rotate the trunk.

4.  A band of tough connective tissue, the linea alba, extending from the xiphoid process to the symphysis pubis, serves as an attachment for certain abdominal wall muscles.

5.     The muscles of the abdominal wall permitting trunk movement and compression the the abdominal viscera include: rectus abdominis, external oblique, internal oblique, and transverse abdominis.

 

G.   Muscles of the Pelvic Floor and Perineum 

1.       Muscles of the pelvic floor and perineum support the pelvic viscera, resist increases in intraabdominal pressure, inhibit urination and defecation, and aid erection.

2.     The muscles of the pelvic floor and perineum that support the abdominopelvic organs include:

(a)  Muscles of the pelvic diaphragm (levator ani and coccygeus); the pelvic diaphragm forms the floor of the pelvic cavity

(b) Muscles of the urogenital diaphragm (deep transverse perineus and spincter urethrae), and

(c)  Muscles of the superficial space (ischiocavernosus, bulbospongiosus, and superficial transverse perineus).  The superficial urogenital diaphragm fills the space within the pubic arch, and the deeper pelvic.

 

H.   Superficial Muscles of the Anterior and Posterior Thorax: Movements of the Scapula

1.       Except for the pectoralis major and the latissimus dorsi, the superficial muscles of the thorax act to fix or promote movements of the scapula.

2.     Scapular movements are effected primarily by posterior thoracic muscles.

3.  Major muscles moving the scapula include: pectoralis minor, serratus anterior, subclavius, trapezius, levator scapulae, and rhomboids (major and minor).

 

I.    Muscles Crossing the Shoulder Joint Permitting Movement of the Arm                        

1.       Nine muscles cross the shoulder joint to effect movements of the humerus.

2.     Of these nine muscles, seven originate on the scapula and two arise from the axial skeleton.

3.     Four muscles contribute to the "rotator cuff" helping to stabilize the multiaxial shoulder joint.                                            

4.     Generally speaking, muscles located anterior to the shoulder joint flex the arm.

5.     Muscles located posterior to the shoulder joint extend the arm.

6.     The deltoid muscle of the shoulder is the prime mover of shoulder abduction.

 

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7.  Muscles connect the humerus to the pectoral girdle, ribs, and vertebral column, making the arm freely movable.

8.     Types of muscle movements:

          a.  Flexor muscles include: the coracobrachialis and pectoralis major.

                   b.  Extensor muscles include: the teres major and latissimus dorsi.

              c.  Abductor muscles include: the supraspinatus and the deltoid.

                   d.  Rotator muscles include: the subscapularis, infraspinatus, and teres minor.  

 

J.  Muscles Crossing the Elbow Joint Permitting Flexion and Extension of the Forearm  

1.       Muscles causing movements of the forearm form the flesh of the arm.

2.     These muscles arise from the humerus or pectoral girdle and connect to the ulna and radius.

3.     Anterior arm muscles are forearm flexors.

4.     Posterior muscles are forearm extensors.

5.     Muscles that move the forearm and their types of movements

a.      Flexors include: the biceps brachii, brachialis, and brachioradialis.

b.  An extensor is the triceps brachii which is assisted a little by the anconeus muscle.

                   c.  Rotators include: the supinator, pronator teres, and pronator quadratus.

 

K.    Muscles that Move the Wrist, Hand and Fingers

1.       Movements of the wrist, hand and fingers are effected mainly by muscles originating on the forearm.

2.     Except for the two pronator muscles, the anterior forearm muscles are wrist and/or finger flexors.

3.     The posterior forearm muscles are wrist and /or finger extensors.

4.     Movements of the hand are caused by muscles originating from the distal humerus, and the radius and ulna.

5.     Muscles and their types of movements:

a.    Flexors include:   the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, flexor pollicic longus, and flexor digitorum profundus.

                   b.  Extensors include: the extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor carpi ulnaris, extensor pollicis longus, extensor pollicis brevis, and extensor indicis.

¨        An extensor retinaculum forms sheaths for extensor muscles.

                                                                  

 

 

 

 

 

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L.    Intrinsic Muscles of the Hand Permitting Fine Movements of the Fingers

1.       Intrinsic muscles of the hand aid in precise movements of the fingers and in opposition, which helps us grip things in our palm.

2.      These small muscles are divided into thenar, hypthenar, and mid-palmar groups.

3.      The thenar muscles in the ball of the thimb include:  abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, and adductor pollicis muscles.

4.      The hypthenal muscles of the little finger include:  abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi muscles.

5.      The midpalmar muscles include:  lumbricals, palmar interossei, and dosal interossei.

         

M.  Muscles Crossing the Hip and Knee Joints Permitting Movements of the Thigh and Leg

1.       Muscles crossing the hip and knee joints effect thigh and leg movements.

2.     The muscles that move the thigh are attached to the femur and to the pelvic girdle.

3.     Anteromedial muscles include thigh flexors and/or adductors and knee extensors.

4.     Muscles of the posterior gluteal region extend and rotate the thigh.

5.     Posterior thigh muscles extend the hip and flex the knee.

          6.  Types of muscles in the Thigh and Leg:

a.  Anterior and medial group of muscles with origin on the pelvis includes:       ilipsoas (iliacus and psoas major) and sartorius muscles.

b.     Medial group of thigh adductors includes: the pectineus, adductor longus, adductor magnus, adductor brevis, and gracilis muscles.

c.      Anterior group of muscles of the thigh includes: quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius) and tensor fasciae latae.

d.      Posterior group of muscles with origin on the pelvis includes: gluteus maximus, gluteus medius, gluteus minimus, and lateral rotators (piriformis, obturator internus, gemellus, and quadratus femoris).

e.      Posterior group of muscles of the thigh includes:  hamstrings (biceps femoris, semitendinosus, and semimembranosus.

          7.   Muscles that move the leg connect the tibia or fibula to the femur or pelvic girdle.

              a.  Muscles that Move the Leg include:

¨      Flexors include: the biceps femoris, semitendinosus, semimembranosus, and sartorius.

¨      An extensor is the quadriceps femoris group made up of four parts: rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius.

 

 

 

 

 

 

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     N.  Muscles that Move the Foot, Ankle and Toes

1.       Muscles in the leg act on the ankle and toes.

2.     Muscles that move the foot are attached to the femur, fibula, or tibia, and move the foot upward, downward, or in a turning motion.

3.     Anterior compartment muscles are largely ankle dorsiflexors.

4.     Lateral compartment muscles are plantar flexors and foot evertors.

5.     Muscles of the posterior leg are plantar flexors.

6.     Intrinsic foot muscles support foot arches and help effect toe movements.

7.     Types of muscle movements:

a.      Dorsal flexors include: the tibialis anterior, fibularis (peroneus) tertius, and extensor digitorum longus, and extensor hallucis longus.

b.  Plantar flexors include:  the fibularis (peroneus) longus, fibularis (peroneus) brevis, gastrocnemius, soleus, plantaris, flexor digitorum longus, and flexor hallucis longus, and tibialis posterior.

c.  Invertors include: tibialis anterior, flexor digitorum longus, and tibialis posterior.

d.  Evertors  include: the fibularis (peroneus) longus, fibularis (peroneus) brevis, and fibularis tertius.

8.  Retinacula form sheaths for tendons beneath the foot and on the lateral ankle.

 

O.  Intrinsic Muscles of the Foot: Toe Movement and Arch Support 

1.       The intrinsic muscles support the foot arches and help move the toes.

2.     Most occur in the sole, arranged in four layers.

3.     They resemble the small muscles in the palm of the hand.

4.     Muscles of the foot:

a.      Muscle on the dorsum of the foot:  extensor digitorum brevis extends toes at the metatarsophalangeal joints.

b.     Muscle layers of the sole of the foot:

¨      1st layer (most superficial) contains:  flexor digitorum brevis, abductor hallucis, and abductor digiti minimi;

¨      2nd layer contains:  flexor accessorius (quadratus plantae) and lumbricals;

¨      3rd layer contains:  flexor hallucis brevis, adductor hallucis, and flexor digiti minimi brevis;

¨      4th layer contains:  3 plantar interossei and 4 dorsal interossei.

    

 

 

 

 

 

 

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