The Muscular System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The Muscular System

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Functions and Types of Muscles

o Smooth Muscle• Located in the walls of hollow organs

and blood vessels• Involuntary contraction• Moves materials through organs and

regulates blood flow• Cylindrical cells with pointed ends• Each cell is uninucleate


o Cardiac Muscle• Forms the heart wall• Fibers are uninucleated, striated,

tubular, and branched• Fibers interlock at intercalated disks,

which permit contractions to spread quickly throughout the heart

• Contraction does not require outside nervous stimulation

• Nerves do affect heart rate and strength of contraction


o Skeletal Muscle• Fibers are tubular, multinucleated,

and striated• Make up muscles attached to the

skeleton• Contraction is voluntary


o Connective Tissue Coverings of Skeletal Muscle

• Endomysium Thin layer of areolar connective tissue Surrounds each skeletal muscle fiber

• Perimysium – surrounds bundles of muscle fibers (fascicles)

• Epimysium Layer that surrounds the entire muscle Becomes part of the fascia (separates muscles

from each other) Collagen fibers extend from epimysium to form

tendons that attach muscles to bone


o Functions of Skeletal Muscles• Support the body• Make bones and other body parts

move• Help maintain a constant body

temperature• Assists movement in cardiovascular

and lymphatic vessels• Help protect bones and internal

organs, and stabilize joints

Microscopic Anatomy

o Muscle fiber components• Sarcolemma – plasma membrane• Sarcoplasm – cytoplasm

Contains glycogen that provides energy for muscle contraction

Contains myoglobin which binds oxygen until needed

• Sarcoplasmic reticulum – endoplasmic reticulum

• T (transverse) tubules Formed by the sarcolemma penetrating into the

cell Come into contact with expanded portions of the

sarcoplasmic reticulum

Microscopic Anatomy

o Myofibrils and Sarcomeres• Myofibrils run the length of the muscle fiber• Composed of numerous sarcomeres

Extends between two vertical Z lines Contains two types of protein myofilaments

Thick filaments – made up of myosin Thin filaments – made up of actin, tropomyosin, and

troponin I band contains only thin filaments A band in the center of the sarcomere contains

thick and thin filaments H zone in the center of the A band has only

myosin filaments

Microscopic Anatomy

o Myofilaments• Thick filaments

Composed of several hundred of molecules of myosin

Myosin molecules end in a cross-bridge • Thin filaments

Two strands of actin Double strands of tropomyosin coil of

each actin strand Troponin occurs at intervals on the

tropomyosin strand

Microscopic Anatomy

• Sliding filaments Occurs when sarcomeres shorten (during

muscle contraction) Actin filaments slide past the myosin

filaments Thick and thin filaments remain the same

length

Contraction of Skeletal Muscle

o Neuromuscular junction• Axon terminals

Come into close proximity to the sarcolemma

Have vesicles that contain acetylcholine (Ach)

• Synaptic cleft – a small gap that separates the axon from the sarcolemma

Fig 7.4


o Steps involved in skeletal muscle contraction

• Nerve signal arrives at the axon terminal• The synaptic vesicles release Ach• Ach binds to receptors on the sarcolemma• The sarcolemma generates a signal that

travels down the T tubules to the SR• The SR releases calcium• Calcium from the SR causes the filaments to

slide past one another


o The Role of Actin and Myosin• Myosin binding sites on actin

molecules Covered by tropomyosin when muscle is

relaxed Released calcium combines with troponin

and myosin binding sites are exposed• Cross-bridges of myosin have two

binding sites One site binds to ATP Second binding site binds to actin


o Energy for Muscle Contraction• ATP present before strenuous exercise

only lasts a few seconds• Muscles acquire new ATP in three

ways Creatine phosphate breakdown Cellular respiration Fermentation


• Creatine Phosphate Breakdown Does not require oxygen (anaerobic) Regenerates ATP by transferring its phosphate to

ADP Fastest way to make ATP available to muscles ATP produced only lasts about 8 seconds

TA 7.1


• Cellular Respiration Usually provides most of a muscle’s ATP Uses glucose from stored glycogen and fatty acids from

stored fats Required oxygen Myoglobin can make oxygen available to muscle

mitochondria Carbon dioxide and water are end products Heat is a by-product


• Fermentation Anaerobic process Produces ATP for short bursts of exercise Glucose is broken down to lactate (lactic acid)


o Oxygen Debt• Occurs when muscles use

fermentation to supply ATP• Requires replenishing creatine

phosphate supplies and disposing of lactic acid

Contraction of Smooth Muscle

o Smooth muscle fibers contain thick and thin filaments

• Filaments are not arranged into myofibrils that create striations

• Thin filaments are anchored to the sarcolemma or dense bodies

o When contracted, the elongated cells become shorter and wider

o Contraction occurs very slowlyo Contractions can last for long

periods of time without fatigue

Muscle Responses in the Laboratory

o All-or-none law – a muscle fiber contracts completely or not at all

o A whole muscle shows degrees of contraction

• Muscle twitch – a single contraction that lasts only a fraction of a second

Latent period Contraction period Relaxation period

• Summation – increased muscle contraction• Tetanic contraction – maximal sustained

contraction

Muscle Responses in the Laboratory

o Fatigue• Muscle relaxes even though

stimulation continues• Reasons for fatigue

ATP is depleted Accumulation of lactic acid in the

sarcoplasm inhibits muscle function ACh may become depleted

Muscle Responses in the Body

o Motor unit• A nerve fiber together with all of the muscle

fibers it innervates• Obeys the all-or-none law

o Recruitment• As the intensity of nervous stimulation

increases, more motor units are activated• Results in stronger muscle contractions

o Tone• Some muscle fibers are always contracting• Important in maintaining posture

Muscle Responses in the Body

o Athletics and muscle contraction• Size of muscles

Atrophy – a decrease in muscle size Hypertrophy – an increase in muscle size

• Slow-twitch fibers (Type I fibers) Tend to be aerobic Have more endurance Have many mitochondria Dark in color because they contain

myoglobin Highly resistant to fatigue

Muscle Responses In the Body

• Fast-twitch fibers (Type II fibers) Tend to be anaerobic Designed for strength Light in color Have fewer mitochondria, little or no

myoglobin, and fewer blood vessels than fast-twitch fibers

Vulnerable to accumulation of lactic acid and can fatigue easily

Skeletal Muscles of the Body

o Basic Principles• Origin – attachment of a muscle to the

immovable bone• Insertion – attachment of a muscle to

the bone that moves• Prime mover – muscle that does most

of the work in a movement• Synergist – muscles that assist the

prime mover• Antagonists – muscles that work

opposite one another to bring about movement in opposite directions


o Naming Muscles• Size• Shape• Direction of fibers• Location• Attachment• Number of attachments• Action


o Muscles of the Head• Muscles of Facial

Expression Frontalis Orbicularis oculi Orbicularis oris Buccinator Zygomaticus

• Muscles of Mastication Masseter muscles Temporalis muscles

Fig 7.13


o Muscles of the Neck• Swallowing

Tongue and buccinators Suprahyoid and infrahyoid muscles Palatini muscles Pharyngeal constrictor muscles

• Muscles that move the head Sternocleidomastoid Trapezius muscles


o Muscles of the Trunk• Muscles of the thoracic wall

External intercostal muscles Diaphragm Internal intercostal muscles

• Muscles of the abdominal wall External and internal obliques Transversus abdominis Rectus abdominis


o Muscles of the Shoulder• Muscles that move the scapula

Trapezius Serratus anterior

• Muscles that move the arm Deltoid Pectoralis major Latissimus dorsi Rotator cuff muscles

Supraspinatus Infraspinatus Teres minor Subscapularis

Fig 7.15


o Muscles of the Arm• Biceps brachii• Brachialis• Triceps brachii

o Muscles of the Forearm• Flexor carpi and extensor carpi• Flexor digitorum and extensor

digitorum


o Muscles of the Hip and Lower Limb• Muscles that move the thigh

Iliopsoas Gluteus maximus Gluteus medius Adductor group muscles

Pectineus Adductor longus Adductor magnus Gracilis


• Muscles that move the leg Quadriceps femoris group

Rectus femoris Vastus lateralis Vastus medialis Vastus intermedius

Sartorius Hamstring group

Biceps femoris Semimembranosus Semitendinosus


• Muscles that move the ankle and foot Gastrocnemius Tibialis anterior Fibularis longus Fibularis brevis Flexor and extensor digitorum longus

Effects of Aging

o Muscle mass and strength tend to decrease

o Endurance decreaseso Exercise at any age can stimulate

muscle buildup

Homeostasis

o Cardiac muscle contraction forces blood into the arteries and arterioles

o Smooth muscle in arteries and arterioles help maintain blood pressure

o Smooth muscle contraction moves food along the digestive tract and assists in the voiding of urine

o Skeletal muscles protect internal organs and stabilizes joints

o Skeletal muscles are active during breathingo Heat produced by skeletal muscle contraction

helps maintain normal body temperatureo Skeletal muscle contraction allows us to

relocate our bodies

Documents

The Muscular System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display