Chapter 7: The Muscular System

Types of Muscle

The Muscular System Skeletal Muscle Has Three Primary Functions: Skeletal movement Posture maintenance Heat generation

The Muscular System Muscle Structure Fascicles Bundles of muscle cells (fibers) that make up a whole muscle Connective tissue Hold fascicles and whole muscle together Three layers Endomysium Perimysium Epimysium Tendons

The Muscular System Muscle Cells in Action Motor unit: A single neuron and all the muscle fibers it stimulates Small motor units used for fine movements Large motor units used for broad movements

The Muscular System Muscle Cells in Action Neuromuscular junction (NMJ): The point at which a nerve fiber contacts a muscle fiber A type of synapse NMJ anatomy Motor neuron Neurotransmitter (acetylcholine; ACh) Motor end plate (on muscle fiber) Contains acetylcholine receptors

The Muscular System Muscle Cells in Action NMJ allows motor neuron to stimulate muscle fiber to become electrically excited (action potential) Action potential stimulates muscle contraction Events at the NMJ Ach is released from motor neuron into synaptic cleft Ach diffuses across synaptic cleft towards motor end plate Ach binds to receptors on motor end plate and stimulates action potential

The Muscular System Muscle Cells in Action The sarcomere is the functional unit of contraction in the skeletal muscle fiber Sarcomere anatomy: Thick filaments (myosin) Thin filaments (actin) Regulatory proteins Troponin Tropomyosin

The Muscular System Muscle Cells in Action Sarcomeres contract via the sliding filament mechanism: Myosin heads bind to actin, forming cross-bridges Using stored energy, myosin heads pull actin filaments together within the sarcomeres and the cell shortens New ATP is used to detach myosin heads and move them back into position for another “power stroke”

Figure 7-4 Sliding filament mechanism of skeletal muscle contraction. 10

The Muscular System Muscle Cells in Action Calcium regulates sarcomeric contraction within the muscle cell: Action potential from NMJ travels to sarcoplasmic reticulum (SR) SR releases calcium into cytoplasm Calcium shifts troponin and tropomyosin off of thin filament soit blocks the sites on actin filaments where cross-bridges form Muscle relaxes when stimulation ends and calcium is pumped back into SR

The Muscular System Summary of Events in a Muscle Contraction ACh is released from neuron ending into synaptic cleft at NMJ ACh binds to motor end plate and produces action potential Action potential travels to sarcoplasmic reticulum (SR) SR releases calcium into cytoplasm Calcium shifts troponin and tropomyosin so that binding sites on actin are exposed

The Muscular System Summary of Events in a Muscle Contraction (continued) Myosin heads bind to actin, forming cross-bridges Using stored energy, myosin heads pull actin filaments together within sarcomeres and cell shortens New ATP is used to detach myosin heads and move them back to position for another “power stroke” Muscle relaxes when stimulation ends and calcium is pumped back into SR

The Muscular System Energy Sources Muscle contraction requires ATP Skeletal muscle prefers to produces ATP via aerobic metabolism, which requires Oxygen Glucose Storage compounds ensure an adequate supply of oxygen and glucose for aerobic ATP metabolism Myoglobin Stores additional oxygen, located in muscle cells Glycogen

The Muscular System Energy Sources During strenuous activity, muscle cells use anaerobic ATP metabolism which causes an oxygen debt and does not require ATP Breakdown of creatine phosphate Anaerobic glycolysis Lactic acid accumulation and oxygen debt Excess postexercise oxygen consumption After strenuous exercise, person takes in extra oxygen (via rapid breathing) to remove lactic acid and replenish energy stores

The Muscular System Effects of Exercise Improved balance, joint flexibility Increased muscle size (hypertrophy) Improved muscle tissue Vasodilation Strengthened heart muscle Improved breathing and respiratory efficiency Weight control Stronger bones

The Muscular System Types of Muscle Contractions Partial (muscle tone or tonus) Partially contracted state that is normal Isotonic No change in tension Muscle length shortens Movement Isometric Great increase in tension Muscle length unchanged No movement

The Mechanics of Muscle Movement Tendons attach muscles to bones. Origin: Attached to more fixed part of skeleton Insertion: Attached to more movable part of skeleton

Figure 7-6 Muscle attachments to bones. 19

The Mechanics of Muscle Movement Muscles Work Together Many muscles function in pairs Prime movers Movement by a muscle Antagonists Produces opposite movment Synergists helpers

The Mechanics of Muscle Movement Levers and Body Mechanics Musculoskeletal system as a lever system Lever—bone Fulcrum—joint Force—applied by muscle Three classes of levers First class Second class Third class—most body systems

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Skeletal Muscle Groups Characteristics for Naming Muscles Characteristic Example Location Temporalis; near the skull’s temple Size Gluteus maximus; the largest Shape Serratus anterior; serrated edge Direction External oblique; runs obliquely Number of heads Biceps brachii; two heads Action Flexor digitorum; flexes toes

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Skeletal Muscle Groups Muscles of the Head and Neck Name Location Function Orbicularis oculi Encircles eyelid Closes eye Levator palpebrae superioris Posterior orbit to upper eyelid Opens eye Orbicularis oris Encircles mouth Closes lips* Buccinator Fleshy part of cheek Flattens cheek; helps in eating, whistling, and blowing wind instruments Temporalis Above and near ear Closes jaw Masseter At angle of jaw Closes jaw-used for chewing* Sternocleidomastoid Along lateral neck, to mastoid process Flexes head; rotates head toward opposite side from muscle, moves head*

Figure 7-10 Muscles of the head and neck. 27

Skeletal Muscle Groups Muscles That Move the Shoulder and Arm Name Location Function Trapezius Posterior neck and upper back to clavicle and scapula Raises shoulder and pulls it back; superior portion extends and turns head Latissimus dorsi Middle and lower back, to humerus Extends and adducts arm Pectoralis major Superior, anterior chest, to humerus Flexes and adducts arm Serratus anterior Inferior to axilla on lateral chest Moves shoulder forward; aids in raising arms, punching, or reaching forward Deltoid Covers shoulder joint, to lateral humerus Abducts arm; flexes and extends arm

Skeletal Muscle Groups Muscles That Move the Forearm and Hand Name Location Function Biceps brachii Anterior arm along humerus, to radius Flexes forearm; supinates forearm and hand Brachialis Deep to biceps brachii; inserts at anterior elbow joint Forceful flexor of forearm Brachioradialis Lateral forearm from distal end of humerus Flexes forearm Triceps brachii Posterior arm, to ulna Extends forearm

Skeletal Muscle Groups Muscles That Move the Forearm and Hand (continued) Name Location Function Flexor carpi group Anterior forearm, to hand Flexes hand Extensor carpi group Posterior forearm, to hand Extends hand Flexor digitorum group Anterior forearm to fingers Flexes fingers Extensor digitorum group Posterior forearm to fingers Extends fingers

Figure 7-11 Muscles that move the forearm and hand. 31

Skeletal Muscle Groups Muscles of the Trunk Name Location Function Diaphragm Dome-shaped partition between thoracic and abdominal cavities Dome descends to enlarge thoracic cavity from top to bottom Intercostals Between ribs Elevate ribs and enlarge thoracic cavity Muscles of abdominal wall: External oblique Internal oblique Transversus abdominus Rectus abdominis Anterolateral abdominal wall Compress abdominal cavity and expel substances from the body; flex spinal column Levator ani Pelvic floor Aids defecation Erector spinae Group of deep vertical muscles between sacrum and skull Extends vertebral column to produce erect posture

Figure 7-12 Muscles of respiration. 33

Figure 7-13 Muscles of the abdominal wall. 34

Skeletal Muscle Groups Muscles That Move the Leg and Thigh Name Location Function Gluteus maximus Superficial buttock, to femur Extends thigh Gluteus medius Deep buttock, to femur Abducts thigh Iliopsoas Crosses anterior hip joint, to femur Flexes thigh Adductor group Medial thigh, to femur Adducts thigh Sartorius Crosses anterior thigh from ilium to medial tibia Flexes thigh and leg (to sit cross-legged) Gracilis Pubic bone to medial surface of tibia Adducts thigh at hip; flexes

Skeletal Muscle Groups Muscles That Move the Leg and Thigh (continued) Name Location Function Quadriceps femoris Rectus femoris Vastus medialis Vastus lateralis Vastus intermedius Anterior thigh, to tibia Extends leg Hamstring group Biceps femori Semimembranosus Semitendinosus Posterior thigh; ischium and femur to tibia and fibula Flexes leg at knee; extends and rotates thigh at hip

Figure 7-15 Muscles of the thigh. How many muscles make up the quadriceps femoris? 37

Skeletal Muscle Groups Muscles That Move the Foot Name Location Function Gastrocnemius Posterior leg, to calcaneus, inserting by the Achilles tendon Plantar flexes foot Soleus Posterior leg deep to gastrocnemius Tibialis anterior Anterior and lateral leg, to foot Dorsiflexes and inverts foot Fibularis longus Lateral leg, to foot Everts foot Flexor digitorum group Posterior leg and foot to inferior surface of phalanges Flexes toes Extensor digitorum group Anterior surface of leg bones to superior surface of phalanges Extends toes

Figure 7-16 Muscles that move the foot. 39

Effects of Aging on Muscles Beginning at about age 40 Gradual loss of muscle cells Loss of power Tendency to flex hips and knees Decrease in height