The Muscular System

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

Functions and Types of Muscles 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

Functions and Types of Muscles o

Skeletal Muscle • • •

Fibers are tubular, multinucleated, and striated Make up muscles attached to the skeleton Contraction is voluntary

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

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

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 Filament Model 





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

Contraction of Skeletal Muscle 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

Contraction of Skeletal Muscle 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

Contraction of Skeletal Muscle 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

Contraction of Skeletal Muscle 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

Contraction of Skeletal Muscle •

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

Contraction of Skeletal Muscle •

Fermentation   

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

Contraction of Skeletal Muscle 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 Smooth muscle fibers contain thick and thin filaments

o • •

o o o

Filaments are not arranged into myofibrils that create striations Thin filaments are anchored to the sarcolemma or dense bodies

When contracted, the elongated cells become shorter and wider Contraction occurs very slowly 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 A whole muscle shows degrees of contraction

o •

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

Muscular Responses Threshold Stimulus • minimal strength required to cause contraction Recording a Muscle Contraction • twitch • latent period • period of contraction • period of relaxation • refractory period • all-or-none response

Summation • process by which individual twitches combine • produces sustained contractions • can lead to tetanic contractions

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 • •

o

A nerve fiber together with all of the muscle fibers it innervates Obeys the all-or-none law

Recruitment • •

o

As the intensity of nervous stimulation increases, more motor units are activated Results in stronger muscle contractions

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

Skeletal Muscles of the Body o

Naming Muscles • • • • • • •

Size Shape Direction of fibers Location Attachment Number of attachments Action

Skeletal Muscles of the Body

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

Skeletal Muscles of the Body 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

Skeletal Muscles of the Body 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

Skeletal Muscles of the Body Muscles of the Shoulder (Table 7.4)

o

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

Skeletal Muscles of the Body o

Muscles of the Arm • • •

o

Biceps brachii Brachialis Triceps brachii

Muscles of the Forearm • •

Flexor carpi and extensor carpi Flexor digitorum and extensor digitorum

Tab. 7.4

Skeletal Muscles of the Body o

Muscles of the Hip and Lower Limb (Table 7.5) •

Muscles that move the thigh Iliopsoas Gluteus maximus Gluteus medius Adductor group muscles

   

   

Pectineus Adductor longus Adductor magnus Gracilis

Skeletal Muscles of the Body •

Muscles that move the leg Quadriceps femoris group



   

Rectus Vastus Vastus Vastus

femoris lateralis medialis intermedius

Sartorius Hamstring group

 

  

Biceps femoris Semimembranosus Semitendinosus

Skeletal Muscles of the Body •

Muscles that move the ankle and foot     

Gastrocnemius Tibialis anterior Fibularis longus Fibularis brevis Flexor and extensor digitorum longus

Tab. 7.5

Effects of Aging o o o

Muscle mass and strength tend to decrease Endurance decreases Exercise at any age can stimulate muscle buildup

Homeostasis o o o o o o o

Cardiac muscle contraction forces blood into the arteries and arterioles Smooth muscle in arteries and arterioles help maintain blood pressure Smooth muscle contraction moves food along the digestive tract and assists in the voiding of urine Skeletal muscles protect internal organs and stabilizes joints Skeletal muscles are active during breathing Heat produced by skeletal muscle contraction helps maintain normal body temperature Skeletal muscle contraction allows us to relocate our bodies