Muscular System Chapter 6

MUSCULAR SYSTEM

• Total muscles in human body: 650• Muscles required to smile: 17• Muscles required to frown: 33• Muscles required to walk: at least 200• Longest muscle: Gluteus maximus ( butt muscle)• Shortest muscle: ear muscle

Characteristics of Muscles

• Muscle cells are elongated (muscle cell = muscle fiber)

• Contraction of muscles is due to the movement of microfilaments (actin and myosin)

• All muscles share some terminology– Prefix myo refers to muscle– Prefix mys refers to muscle– Prefix sarco refers to flesh

Functions of Muscular system• Contract:• Skeletal Muscles: body movement• Smooth muscles: movement of contents through blood

vessels, digestive organs• Cardiac Muscles: Pumping of heart

• Generate heat : generate heat, by product of ATP needed for muscle contraction

• Stabilize joints: Skeleton muscles pull on bones for movement, also stabilize joints

• Maintaining Posture: Keeps the body straight against gravity

• Muscles are responsible for all types of body movement

• Three basic muscle types are found in the body– Skeletal muscle– Cardiac muscle– Smooth muscle

The Muscular System

Smooth Muscle

• Smooth muscles are non striated, uni-nucleated and spindle shaped

• Involuntary

• Found in the walls of visceral organs, eg. Stomach, urinary bladder, respiratory passages

Smooth Muscles• Arranged in two layers:• circular layer• longitudinal layer

• These two layers alternately contract and relax

• And move food through digestive tract, emptying the bowels & bladder

• Maintain housekeeping activities

• Slow and steady

Cardiac Muscle

• Responsible for heart pumping

• Cardiac muscles are striated, cylindrical, uninucleated & branched

• The muscle is under involuntary control and contract at regular intervals

Cardiac Muscle• Cardiac muscle cells are joined to another cell by intercalated discs

• Discs are important in transport of impulses

• Present in heart only

• Cardiac fibers are arranged in spiral or eight-shaped bundles

Skeletal Muscle

• Under voluntary or CNS control

• Respond to impulses

• Muscles are striated (actin and myosin are alternatively arranged)

• Muscle cells are cylindrical or cigar shaped

• multinucleated

Skeletal Muscle

• Attached to bones with the help of tendons

• A person has same number of muscle fiber from his birth to adult hood

• However, their size increases or decreases depending on the usage

Structure of Skeletal Muscle

• Each muscle fiber is enclosed in a connective tissue called endomysium

• Each muscle fiber is supplied with capillary and nerve cell extensions

• Several muscle fibers (cells) called fascicle are enclosed in a connective tissue called perimysium

Structure of Skeletal Muscle

• Several fascicles are surrounded by third or heavy layer of connective tissue layer called epimysium

• Thus each skeletal muscle is surrounded by epimysium

• In order to form a long muscle, muscle fibers are arranged end to end

• Epimysium blends into a connective tissue attachment– Tendons — cord-like

structures • Mostly collagen fibers

– Aponeuroses — sheet-like structures

• Attach muscles indirectly to bones, cartilages, or connective tissue coverings

Skeletal Muscle Attachments

Microscopic Anatomy of Skeletal Muscle

• Skeletal cells are multinucleate

• Outer boundary of the cell is made of plasma membrane – sarcolemma

• Cytoplasm of muscle cell - sarcoplasm

• Endoplasmic reticulum – Sarcoplasmic reticulum

• Sarcoplasm is packed with myofibrils

• Other organelles, such as mitochondria, glycogen granules are found between myofibrils

• Myofibrils: are thread like organelles

– Composed of protein threads called myofilaments:

– thin (actin) – thick (myosin)

– Sarcomeres: repeating units of myofilaments

• Interaction between actin and myosin filaments leads muscle to shorten or contract

• The tropomyosin/troponin complex regulates the interaction between actin and myosin


• Repeating units of myofibrils - Sarcomere

• The distance between two Z discs is the sarcomere

• The point where actin originates is called Z disk

• Each sarcomere has alternating actin and myosin filaments

Structure of Sarcomere

• The arrangement of myosin (dark in color and is called anisotropic band or A band)

• And actin (light in color and is called isotropic or I band) alternatively

• gives the muscle a striated appearance

• H zone (bare zone) - lacks actin filament

Structure of Sarcomere


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

• When contraction occurs, actin and myosin overlap

Properties of Skeletal Muscle Activity

• Irritability – ability to receive and respond to a stimulus

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

Nerve Stimulus and Action Potential

• Skeletal muscles must be stimulated by a nerve to contract

• One motor neuron can stimulate many muscle cells – Motor unit

• Axon of the neuron branches into many axon terminals at Neuromuscular junctions

• Neuromuscular junctions – association site of nerve and muscle


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

not make contact– Area between nerve

and muscle is filled with interstitial fluid


• When nerve impulse reaches the axon terminal, it releases the chemical known as

Neurotransmitter

• The neurotransmitter for skeletal muscle is acetylcholine

• Neurotransmitter attaches to receptors on the sarcolemma

• Sarcolemma becomes permeable to sodium (Na+)

• Sodium rushing into the cell generates an action potential

– Once started, muscle contraction cannot be stopped

• Stimulates release of Ca+2 from the sarcoplasmic reticulum

• • Ca +2 binds to the troponin, troponin

changes shape and removes the blocking action of tropomyosin

• Actin active sites exposed

• Actin is now ready to bind to the head of the myosin


The Sliding Filament Theory of Muscle Contraction

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

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

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

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

The Sliding Filament Theory

Contraction of a Skeletal Muscle as a whole

• Muscle cell contraction is “all or none”

• But in whole 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

• They are of 4 types:– Twitch– Tetanus– Unfused– fused

• Twitch– Single, brief contraction– Not a normal muscle

function• Following stages

– Stimulus : receives information from nerve

– Lag phase: muscle cell gets ready physiologically

– Contraction phase: actin and myosin slide one over the other

– Relaxation phase: back to non contracted stage

• 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

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

• Fused (complete) tetanus– No evidence of relaxation before the following contractions– The result is a smooth and sustained muscle contraction

Muscle Response to Stronger Stimuli

• 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

Energy for Muscle Contraction• Initially, muscles use 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 for ATP production

• Direct phosphorylation of ADP by creatine phosphate

– Muscle cells contain creatine phosphate (CP)

• CP is a high-energy molecule

– ADP is left, after ATP is depleted,

– CP transfers energy to ADP, to regenerate ATP

– CP supplies are exhausted in about 15 seconds

• Aerobic Respiration

– Series of metabolic pathways occur in the mitochondria, require oxygen

– Known as oxidative phosphorylation

– Glucose is broken down to carbon dioxide and water, & release energy in the form of ATP

– This is a slower reaction that requires continuous oxygen and nutrient fuel

– 36 ATP/ glucose

• Anaerobic glycolysis– Reaction that breaks

down glucose without oxygen

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

– Pyruvic acid is converted to lactic acid

• Anaerobic glycolysis

• This reaction is not as efficient, but is fast

• 2ATP/glucose

• Huge amounts of glucose are needed

• Lactic acid produces muscle fatigue

Muscle Fatigue and Oxygen Debt

• When a muscle is fatigued, it is unable to contract

• The common reason for muscle fatigue is oxygen debt

– Oxygen must be “repaid” to tissue to remove oxygen debt

– Oxygen is required to get rid of accumulated lactic acid

• Increasing acidity (from lactic acid) and lack of ATP causes the muscle to get tired (contract less)

Muscle Tone

• Some fibers are contracted even in a relaxed muscle

• Contraction is not visible but muscles remain firm, healthy

• This state of continuous partial contraction is Muscle tone

Effects of Exercise on Muscle• Exercise increases muscle size, strength, and

endurance

– Aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue

• Makes body metabolism more efficient• Improves digestion, coordination

– Resistance (isometric) exercise (weight lifting) increases muscle size and strength

Muscles and Body Movements

• Movement is attained due to a muscle moving an attached bone

• Muscles are attached to at least two points– Origin –

• attachment to an immovable bone

– Insertion –• attachment to a moveable

bone

Flexion: movement of a body part anterior to the coronal plane

Extension: movement of a body part posterior to the coronal plane

Abduction: movement away from the median plane

Adduction: movement toward the median plane

Pronation/Supination:

Unique rotation of the forearm

Pronation: palm faces posteriorly

Supination: palm faces anteriorly

Circumduction The circular or conical

movement of a body part

Consists of a combination of flexion, extension, adduction, and abduction

Occurs at freely movable joints

Eg. Windmilling the arms or rotating the hand from the wrist

– Plantar flexion: standing on the toes

– Dorsiflexion: foot lifted toward the shin,

such as walking on the heels

• Elevation: moves a structure superior• Depression: moves a structure inferior• Examples: shrugging the shoulders, opening and

closing the mouth

• Protraction: • Movement of a bone

anteriorly• Eg. Thrusting the jaw

forward, shoulder forward

• Retraction: • Moves structure back to

anatomic position or even further posteriorly

• Lateral: moving mandible to the right or left of midline• Such as in grinding the teeth or chewing the food• Medial: return the mandible to the midline

• Opposition: movement of thumb and little finger toward each other

• Reposition: return to anatomical position

• Inversion: • Inversion is a

movement in which the soles are turned medially

• Eversion: • Eversion is a turning

of the soles to face laterally

Interaction of Skeletal Muscles in the Body

• Prime mover – muscle with the major responsibility for a certain movement– Biceps is the prime mover for

flexion– Triceps is the prime mover of

extension

• Antagonist – muscle that opposes or reverses a prime mover– Biceps and triceps work against

one another

• Synergist – muscle that aids a prime mover in a movement and reduce undesirable movement– Cross one or more

joints that connect two or more bones

– Brachialis and brachioradialis help bicep brachii

Naming of Skeletal Muscles

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

– Rectus abdomonius


• Relative size of the muscle– Example: maximus

(largest)– Gluteus muscles– Maximus– Medius– Minimus


• Location of the muscle– Example:

many muscles are named for bones (e.g., temporalis)


• Location of the muscle’s origin and insertion– Example: sterno (on

the sternum)

• Number of origins– Example: triceps

(three heads)


• Shape of the muscle– Example: deltoid

(triangular)


• Action of the muscle– Example: flexor and extensor (flexes or extends a bone)

– Hamstring( flexor)– Extensor( quadriceps)

Head and Neck Muscles• Facial muscles

– Frontalis— raises eyebrows– Orbicularis oculi— closes

eyes, squints, blinks, winks– Orbicularis oris— closes

mouth and protrudes the lips– Buccinator— flattens the

cheek, chews– Zygomaticus— raises corners

of the mouth• Chewing muscles

– Masseter— closes the jaw and elevates mandible

– Temporalis— synergist of the masseter, closes jaw

Head and Neck Muscles

• Neck muscles

– Platysma—pulls the corners of the mouth inferiorly

– Sternocleidomastoid—flexes the neck, rotates the head

• Anterior muscles– Pectoralis major—

adducts and flexes the humerus

– Intercostal muscles

• External intercostals—raise rib cage during inhalation

• Internal intercostals—depress the rib cage to move air out of the lungs when you exhale forcibly

Muscles of Trunk

Muscles of Trunk

• Muscles of the abdominal girdle– Rectus abdominis—– flexes vertebral column and

compresses abdominal contents (defecation, childbirth, forced breathing)

– External and internal obliques— flex vertebral column; rotate trunk and bend it laterally

– Transversus abdominis— compresses abdominal contents

Muscles of Trunk• Posterior muscles

– Trapezius— elevates, depresses, adducts, and stabilizes the scapula

– Latissimus dorsi— extends and adducts the humerus

– Deltoid— arm abduction

• Biceps brachii— supinates forearm, flexes elbow

• Brachialis— elbow flexion

• Brachioradialis— weak muscle

• Triceps brachii— elbow extension (antagonist to biceps brachii)

Muscles of the Upper Limb

Muscles of Lower Limb

• Gluteus maximus— hip extension

• Gluteus medius— hip abduction, steadies pelvis when walking

• Iliopsoas— hip flexion, keeps the upper body from falling backward when standing erect

• Adductor muscles—adduct the thighs

Muscles of the Lower Limb

• Muscles causing movement at the knee joint– Hamstring group—

thigh extension and knee flexion

• Biceps femoris• Semimembranosus• Semitendinosus


• Muscles causing movement at the knee joint– Sartorius— flexes the

thigh– Quadriceps group—

extends the knee• Rectus femoris• Vastus muscles (three)


• Muscles causing movement at ankle and foot– Tibialis anterior—

dorsiflexion and foot inversion

– Extensor digitorum longus— toe extension and dorsiflexion of the foot

– Fibularis muscles— plantar flexion, everts the foot

– Soleus— plantar flexion

Muscular System Chapter 6

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