CHAPTER 6 The Muscular System. Overview of Muscle Tissue Table 6.1, page 179.

CHAPTER 6

The Muscular System

Overview of Muscle Tissue

• Table 6.1, page 179

• Skeletal muscle– Cylindrical, multinucleated, striated, voluntary– Contracts rapidly and with great force, but

tires easily– Cells are surrounded and bundled by

connective tissue

• A muscle is made up of bundles of fascicles. Fascicles are made up of bundles of muscle fibers (muscle cells)

• Muscles are surrounded by connective tissue called epimysium. Fasicles are surrounded by connective tissue called perimysium. Muscle fibers are surrounded by connective tissue called endomysium.

Connective Tissue Wrappings of Skeletal Muscle

• Epimysium – covers the entire skeletal muscle

• Fascia – on the outside of the epimysium

Figure 6.1

Connective Tissue Wrappings of Skeletal Muscle

• Endomysium – around single muscle fiber

• Perimysium – around a fascicle (bundle) of fibers

Figure 6.1

Connective tissue coverings of muscles

• Fascia – fibrous connective tissue– Separates individual muscles from adjacent

muscles– Surrounds each muscle– Projects beyond the end of the muscle to form

tendons and aponeuroses which intertwine with bone periosteum and attach muscles to bones

• Epimysium blends into a connective tissue attachment– Tendon – cord-like structure– Aponeuroses – sheet-like structure

• Beneath the fascia – other layers of connective tissue– Epimysium – just beneath the fascia,

surrounding the muscle– Perimysium – extends inward from epimysium

to separate the muscle tissue into compartments that contain muscle fiber bundles called fascicles, surrounds fascicles

– Endomysium – thin covering of each muscle fiber

• A muscle is made up of bundles of fascicles. Fascicles are made up of bundles of muscle fibers (muscle cells)

• Muscles are surrounded by connective tissue called epimysium. Fasicles are surrounded by connective tissue called perimysium. Muscle fibers are surrounded by connective tissue called endomysium.

Skeletal Muscle Attachments

• Sites of muscle attachment– Bones– Cartilages– Connective tissue coverings

Smooth Muscle Characteristics

• Has no striations• Spindle-shaped

cells• Single nucleus• Involuntary – no

conscious control• Found mainly in the

walls of hollow organs

Figure 6.2a

• Smooth muscle – Arranged in sheets or layers, one running

circularly and one running longitudinally– The two layers alternately contract and relax,

changing the shape of the organ– Contraction is slow and sustained

Cardiac Muscle Characteristics

• Has striations• Usually has a single

nucleus• Joined to another

muscle cell at an intercalated disc

• Involuntary• Found only in the

heartFigure 6.2b

• Cardiac muscle– Branching cells joined by special junctions

called intercalated discs– This structure allows for the special kind of

contraction in cardiac muscle– Contracts at steady rate

Function of Muscles

• Produce movement– Mobility of body, facial expressions, circulate

blood, move fluids

• Maintain posture

• Stabilize joints

• Generate heat– By-product of muscle activity, maintains

normal body temp

Microscopic Anatomy of Skeletal Muscle

• Cells are multinucleate

• Nuclei are just beneath the sarcolemma

Figure 6.3a


• Sarcolemma – specialized plasma membrane• Sarcoplasmic reticulum – specialized smooth

endoplasmic reticulum

Figure 6.3a

Skeletal Muscle Fibers

• A muscle fiber is a single cell

• Thin, elongated cylinder which may be the full length of the muscle

• Cell membrane is called sarcolemma

• Cytoplasm is called sarcoplasm and contains many nuclei, many mitochondria, many myofibrils, sarcoplasmic reticulum (endoplasmic reticulum)

• Myofibrils are the organelles within the muscle cell that produce contraction– Contain chains of sarcomeres which are

chains of tiny contractile units aligned end to end along the length of a myofibril

– Smaller structures called myofilaments are within the sarcomeres and produce the banding pattern of the muscle cell

Important terms

• Muscle fiber = muscle cell

• Sarcolemma

• Sarcoplasm

• Sarcoplasmic reticulum

• Sarcomere

• Myofibril

• Myofilament

Figure 6.3b


• Myofibril– Bundles of myofilaments– Myofibrils are aligned to give distinct bands

• I band =

light band• A band =

dark band


• Sarcomere– Contractile unit of a muscle fiber

Figure 6.3b

Myofibrils

• Function in muscle contraction

• Contain 2 kinds of protein filaments: thick filaments of myosin, and thin filaments of actin

• Special organization of these filaments produces light and dark striations of skeletal muscle fibers

• Striations:• Light band – I band – thin actin filaments• Dark band – A bands – thick myosin

filaments that overlap actin filaments• H zone – only thick filaments• M line – an extra thick section of myosin• Z line – center of thin filamnet with no

myosin overlap• Sarcomere – segment that extends from Z

to Z


• Organization of the sarcomere– Thick filaments = myosin filaments

• Composed of the protein myosin• Has ATPase enzymes

Figure 6.3c


• Organization of the sarcomere– Thin filaments = actin filaments

• Composed of the protein actin

Figure 6.3c


• Myosin filaments have heads (extensions, or cross bridges)

• Myosin and actin overlap somewhat

Figure 6.3d

Properties of Skeletal Muscle Activity

• Muscle cells have 2 important properties that enable them to function: irritability and contractility

• Irritability – ability to receive and respond to a stimulus

• Contractility – ability to shorten when an adequate stimulus is received

Irritability

• Muscle cells must be stimulated by nerve impulses

• One neuron (nerve cell) can stimulate a few muscle cells or hundreds of muscle cells

• One neuron and the muscles cells it stimulates are called a motor unit

• The axon of the nerve cell forms a junction with the sarcolemma of each muscle cell. This junction is called the neuromusclular junction.

• There is a gap between the nerve cell and the muscle cells called a synaptic cleft.

Nerve Stimulus to Muscles

• Skeletal muscles must be stimulated by a nerve to contract

• Motor unit– One neuron– Muscle cells

stimulated by that neuron

Figure 6.4a


• Neuromuscular junctions – association site of nerve and muscle

Figure 6.5b


• Synaptic cleft – gap between nerve and muscle– Nerve and muscle

do not make contact

– Area between nerve and muscle is filled with interstitial fluid

Figure 6.5b

Transmission of StimulusFigure 6.5

• Nerve impulse reaches axon terminal

• Chemical known as neurotransmitter is released – acetylcholine, ACh

• ACh diffuses across synaptic cleft

• ACh attaches to membrane (sacrolemma) receptor

• Membrane becomes permeable to Na+

• Na+ diffuse into cell and K+ diffuse out

• More Na+ enters the cell than K+ leaves

• Cell interior has an excess of positive ions which generates an electric current called and action potential.

• Action potential travels over the entire sarcolemma.

• Action potential travels over the entire sarcolemma. The impulse spreads from one end of cell to the other.

• The impulse causes the contraction of the muscle cell.

• ACh is broken down while action potential occurs. Only one impulse is produced, only one contraction occurrs.

• Muscle cell returns to resting state. – K+ diffuse out of cell– Active transport pump moves sodium and

potassium ions back to initial positions

Skeletal Muscle Contraction

• Complex interaction between cell parts and biochemicals

• Filaments of actin and myosin within myofibrils slide past each other, shortening muscle fiber (pulls on attached bones)

The Sliding Filament Theory of Muscle Contraction

• Activation by nerve causes myosin heads (crossbridges) to attach to binding sites on the thin filament

• Myosin heads then bind to the next site of the thin filament

Figure 6.7

The Sliding Filament Theory of Muscle Contraction

• This continued action causes a sliding of the myosin along the actin

• The result is that the muscle is shortened (contracted)

Figure 6.7

Sliding Filament Theory

• Head of myosin cross-bridge attaches to actin binding site and bends slightly

• When cross-bridge bends, it pulls the actin filament

• Head of cross-bridge releases actin binding site and attaches to another site along actin filament

• Bending and pulling process continues

The Sliding Filament Theory

Figure 6.8

Figure 6.8

• (a) – In relaxed state, proteins on the actin filament prevent myosin from binding to actin.

• After the nerve stimulates the muscle, the action potential that is produced stimulates the sarcoplasmic reticulum to release calcium ions.

Figure 6.8

• (b) – Ca+2 binds to regulatory proteins and causes them to be altered so that the myosin binding sites on the actin filament are available.

• (c) – The myosin heads attach to the actin. The myosin heads pull toward the center of the sarcomere and pull the actin filament along toward the center also

• ATP provides energy to release the myosin from the actin. Myosin then binds to another actin binding site and the pulling continues

• The myofilaments slide past each other and the muscle shortens. Figure 6.7

• When the action potential ends, the calcium ions are returned to the sarcoplasmic reticulum, the regulatory proteins go back to normal conditions blocking the binding sites. The myosin can no longer attach to the actin. The muscle relaxes – goes back to normal state.

Contraction of a Skeletal Muscle

• Muscle fiber contraction is “all or none”– Muscle cell will contract to fullest extent

• Within a skeletal muscle, not all fibers may be stimulated during the same interval

• Different combinations of muscle fiber contractions may give differing responses

• Graded responses – different degrees of skeletal muscle shortening– Produced by: changing the frequency of

stimulation, and changing the number of muscle cells being stimulated

Types of Graded Responses

• Twitch– Single stimulus is delivered and muscle

contracts & relaxes, brief contraction– Not a normal muscle function

Figure 6.9a–b

Response to increase in frequency

• Nerve impulses delivered at a rapid rate

• Muscle cannot relax completely between stimuli

• Successive contractions are summed together

• Contractions get stronger & smoother


• Tetanus (summing of contractions)– One contraction is immediately followed by

another– The muscle does

not completely return to a resting state

– The effects are added

Figure 6.9a–b

• When contractions are summed together, they can be unfused or fused tetanus– Tetanus is the tense, contracted state of a

muscle

• Unfused tetanus occurs as stimuli are delivered at a faster rate

• Fused tetanus results from a very rapid stimulation and produces a smooth, continuous contraction with no relaxation


• Unfused (incomplete) tetanus– Some relaxation occurs between contractions– The results are summed

Figure 6.9c–d


• Fused (complete) tetanus– No evidence of relaxation before the following

contractions– The result is a sustained muscle contraction

Figure 6.9c–d

Muscle Response to Strong Stimuli - # of cells stimulated

• Muscle force depends upon the number of fibers stimulated

• More fibers contracting results in greater muscle tension

• Muscles can continue to contract unless they run out of energy

• Amount of contraction depends on stimulus and energy

Energy for Muscle Contraction

• Initially, muscles used stored ATP for energy– Bonds of ATP are broken to release energy– Only 4-6 seconds worth of ATP is stored by

muscles

• After this initial time, other pathways must be utilized to produce ATP

• Working muscles use 3 pathways to generate ATP for energy needed for muscle contraction:– Direct phosphorylation of ADP by creatine

phosphate– Aerobic respiration– Anaerobic glycolysis and lactic acid

formation


• Direct phosphorylation– Muscle cells contain creatine

phosphate (CP)• CP is a high-energy molecule

– After ATP is depleted, ADP is left

– CP transfers energy to ADP, to regenerate ATP

– CP supplies are exhausted in about 20 seconds

Figure 6.10a


• Aerobic Respiration– Series of metabolic

pathways that occur in the mitochondria

– Glucose is broken down to carbon dioxide and water, releasing energy, 36 ATP molecules

– This is a slower reaction that requires continuous oxygen & food source

Figure 6.10b


• Anaerobic glycolysis– Reaction that breaks

down glucose without oxygen

– Glucose is broken down to pyruvic acid to produce 2 ATP

Figure 6.10c

• If enough oxygen is present, pyruvic acid continues into the mitocondria to aerobic respiration

• If muscle activity is intense, or oxygen and glucose are inadequate, aerobic respiration cannot occur

• Pyruvic acid would then be converted to lactic acid through anaerobic glycolysis, producing only 5% as much ATP as aerobic respiration


• Anaerobic glycolysis (continued)– This reaction is not as

efficient, but is fast• Huge amounts of glucose

are needed• Lactic acid produces

muscle fatigue

Figure 6.10c

Muscle Fatigue

• A muscle is fatigued when it is unable to contract even though it is still being stimulated

• Muscle fatigue results when a person is not able to take in oxygen fast enough to provide for aerobic respiration– This oxygen debt occurs during prolonged

muscle activity– How long a muscle can work depends on how

good the blood supply is

• Lack of oxygen causes lactic acid to accumulate in muscles, and ATP to run low

• Increasing acidity and low ATP cause muscles to contract less effeciently and finally to stop contracting

• True muscle fatigue causes muscle to stop contracting entirely - marathon runners

• Oxygen debt is replaced even when fatigue does not occur – breathing rapidly and deeply

Terms relating to muscle contraction and effect of exercise

• Isotonic contractions

• Isometric contractions

• Muscle tone

• Aerobic, endurance exercise

• Resistance, isometric exercise

Terms relating to body movements

• Skeletal muscles attach to bones at 2 points– Origin – attached to the immovable bone– Insertion – attached to the movable bone

• Flexion – decreases angle of the joint, brings 2 bones closer

• Extension – increases angle between bones

• Rotation – movement around a longitudinal axis, shaking head no

• Abduction – moving limb away from midline

• Adduction – moving limb toward midline

• Circumduction – proximal end of limb is stationary, distal end of limb moves in circle

Interactions of Skeletal muscles

• Movements are the result of the activity of 2 or more muscles acting together or against each other

• Prime mover

• Antagonist

• Synergist

Names of muscles (p 197-198)

• Direction of fibers

• Size of muscle

• Location of muscle

• Number of origins

• Location of origin and insertion

• Shape pf muscle

• Action of muscle

Naming of Skeletal Muscles

• Direction of muscle fibers– Example: rectus (straight)

• Relative size of the muscle– Example: maximus (largest)

Naming of Skeletal Muscles

• Location of the muscle– Example: many muscles are named for bones

(e.g., temporalis)

• Number of origins– Example: triceps (three heads)

• Location of origin and insertion– Sternocleidomastoid

• Shape of muscle– Deltoid

• Action of the muscle– adductor

Gross Anatomy of Skeletal Muscles

• Head and neck– Facial, chewing, neck

• Trunk– Anterior, posterior

• Upper limb

• Lower limb– Movement at the hip joint, movement at the

knee joint, movement at the ankle & foot

Head & Neck musclesFigure 6.15

• Facial muscles are inserted into soft tissue– Frontalis – covers front of frontal bone from

cranial aponeurosis to eyebrow (insertion)• Raise eyebrows, wrinkle forehead

– Occipitalis – covers posterior skull– Orbicularis oculi – circular fibers around the

eye• Close eyes, squint, blink, wink

– Orbicularis oris – circular muscle of the lips• Close mouth, kiss

– Buccinator – runs horizontally across the cheek originates in maxilla & mandible, inserting into orbicularis oris

• Flattens cheek, whistling, sucking

– Zygomaticus – from cheekbone (origin) to corner of the mouth (insertion), raises corners of mouth to smile

• Raises corner of mouth, smile

• Chewing muscles– Masseter – from zygomatic process of

temporal bone(origin) to mandible(insertion)• Closes the jaw

– Temporalis – fan-shaped muscle over the temporal bone, inserts into mandible

• Synergist of (helps action of) masseter, closes jaw

• Neck muscles – move head and shoulder girdle– Platysma – sheetlike muscle on front and side

of neck, originates in chest muscle, inserts into mouth area

• Pulls corners of mouth downward

– Sternocleidomastoid – two muscles, one on each side of neck, originate in sternum and clavicle (2 heads), insert into temporal bone

• Flex neck, bowing head

Trunk Muscles

• Three groups– Anteriors – move ribs, head, arms– Muscles of the abdominal wall – move

vertebral column, form abdominal body wall– Posterior – move vertebral column

Trunk MusclesFigure 6.16

• Anterior muscles– Pectoralis major – fan-shaped muscle

covering upper chest, originates in sternum, shoulder girdle, upper ribs, inserts into humerus

• Adduct and flex arm

– Intercostal muscles – deep muscles between ribs

• Raise and depress rib cage

• Muscles of the Abdominal Girdle - Rectus abdominis, external and internal obliques, transversus abdominis– Rectus abdominis – superficial, enclosed in

aponeurosis, origin at pubis insertion at rib cage

• Flex vertebral column, compress abdominal contents

– External oblique – 2 muscles on lateral walls of abdomen, fibers run downward and medially

• Origin – lower ribs insertion – iliac crest• Flex vertebral column, rotate trunk, bend laterally

– Internal oblique – 2 muscles deep to external obliques, fibers run at right angles to externals

• Origin iliac crest insertion – lower ribs• Same action as externals

– Transverse abdominis – deepest of abdominal muscles, fibers run horizontally across abdomen

• Origin – lower ribs and iliac crest• Insertion - pubis• Compresses abdominal contents

• Posterior muscles– Trapezius – 2 muscles run from occipital bone

down the vertebral column to the thoracic vertebrae, flare laterally and insert into scapula and clavicle

• Elevate, adduct scapula, extend head

– Latissimus dorsi – flat muscle covering lower back, origin in lower spine and ilium, inserts into proximal humeus

• Extend and adduct humerus

– Erector spinae – deep muscles of the back, 3 muscle columns that run the length of vertebral column

• Extensors of back

– Deltoid – triangular muscles forming the shoulder, origin at scapula and clavicle, inserts at proximal humerus

• Abductors of the arm

Muscles of the Upper Limb

• Three groups– Muscles that arise from shoulder girdle and

cross the shoulder joint to insert into the humerus – pectoralis major, latissimus dorsi, deltoid – move the arm

– Muscles that cause movement at the elbow– Muscles of the forearm – insert into hand

bones – move the hand

• Anterior Muscles that cause movement at the elbow – flexion– Biceps brachii – 2 heads, originate in shoulder

girdle insert on radius - bulges when elbow is flexed

• Flexes the forearm, rotates forearm outward (supination)

– Brachialis – deep to the biceps brachii• Elbow flexion

Posterior Muscle that causes extension at the elbow– Triceps brachii – posterior muscle of arm – 3

heads originate in shoulder girdle and proximal humerus and insert in ulna

• Elbow extension

• Muscles of the forearm– Brachioradialis – originates on humerus

inserts on the distal forearm – weak muscle– Anterior – flexor muscles – flexor carpi

radialis, flexor carpi ulnaris, flexor digitorum• Flex wrist and fingers

– Posterior – extensor muscles – extensor carpi radialis, extensor digitorum

• Extend wrist and fingers

Muscles of the Lower Limb

• Three groups of muscles cause movement at the hip, knee, and foot joints

• Hip joint – gluteus maximus, gluteus medius, iliopsoas, adductors

• Knee joint – hamstring group, sartorius, quadriceps group

• Foot joints – tibialis anterior, extensor digitorum longus, fibularis muscles, gastrocnemius, soleus

• Largest and strongest muscles in the body

• Movement at hip joint

• Posterior muscles include:– Gluteus maximus – forms most of the flesh of

the buttock – originates in sacrum and illium and inserts into femur

• Powerful hip extensor

– Gluteus medius – beneath gluteus maximus – originates in illium and inserts into femur

• Hip abductor

• Anterior muscles include:– Iliopsoas – originates in the iliac bone and

lower vertebrae, is deep in pelvis (anterior), inserts into femur

• Hip flexion

– Adductor muscles – muscle mass at medial side of each thigh – originate on pelvis and insert on proximal side of femur

• Adduct thighs, press thighs together

• Movement at knee joint – hamstrings, sartorius, quadriceps

• Hamstring group – posterior thigh muscles– Biceps femoris, semimembranous,

semitendonosus – originate on the ischial tuberosity (ischial part of the coxal bone) and insert into proximal tibia, biceps inserts into head of fibula

• Flex knee

• Sartorius – most superficial anterior muscle of the thigh– Anterior illium to medial side of tibia, runs

obliquely across thigh• Weak thigh flexor

• Quadriceps group – anterior thigh muscles– Rectus femoris – originates in the pelvis and

inserts into tibia via patellar ligamnet– Vastus lateralis, vastus medialis – originate in

femur and insert into tibia via patellar ligament• Powerful knee extendors• Rectus femoris can also flex the hip

• Movement at ankle and foot– Tibialis anterior – superficial muscle on

anterior of leg – originates at upper tibia and inserts into tarsals

• Flexes and inverts the foot

– Extensor digitorum longus – originates in lateral tibia and inserts into phalanges

• dorsiflexes foot, extends toe

– Fibularis muscles – fibularis longus, fibularis brevis, fibularis tertius – originate on fibula and insert in the metatarsals

• Flex the foot

– Gastrocnemius – posterior leg, forms the curve of the calf – originate on each side of the distal femur and inserts into heel bone via the calcaneal tendon (achilles tendon)

• Plantar flexion – pointing toes

– Soleus – deep to the gastrocnemius – arises from tibia and fibula and inserts into calcaneus

• Plantar flexion of foot

Development of Muscular System

• In the embryo– Development of muscles and control by

nervous system occurs early in pregnancy

• Childhood– Nervous system must mature before baby can

control muscles– Through childhood control becomes more

precise– Midadolescence brings peak control

• As we age, amount of muscle tissue decreases– Amount of connective tissue increases– Muscle strength decreases– Mass and strength can be rebuilt with regular

exercise

• Homeostatic imbalance– Muscular dystrophy– Myasthenia gravis

CHAPTER 6 The Muscular System. Overview of Muscle Tissue Table 6.1, page 179.

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Transcript of CHAPTER 6 The Muscular System. Overview of Muscle Tissue Table 6.1, page 179.