Muscles and Skeleton

Muscles work by contracting (active movement) and

lengthening (passive). Three types in mammals:

1. Skeletal muscle – used to move the skeleton, most abundant form of muscle in body• also called striated muscle (has “striped”

appearance)• most is under voluntary control• contracts as a simple twitch (single, quick

contraction)• most movement occurs as a result of skeletal

muscle receiving a series of separate stimuli timed very close together – these produce a single sustained contraction called tetanus

Two types of skeletal muscle fibers (musclecells):

• white muscle fibers – specialized for rapid response, generate a lot of power but fatigue quickly – obtain most of their energy from glycolysis - have few mitochondria

• red (dark) muscle fibers – specialized for slower, endurance activities

– require a steady flow of oxygen, have many mitochondria, rich blood supply

– rich in myoglobin (red pigment similar to hemoglobin) which stores oxygen and enhances rapid diffusion of oxygen during strenuous exercise

Skeletal Muscle

2. Cardiac muscle – located only in the heart

– spontaneously active – initiates its own contractions

– influenced by nerves and hormones– striated in appearance

3. Smooth muscle – lacks striations – lines the walls of digestive tract and large blood

vessels– produces slow, sustained contractions– involuntary

Muscles are made up of muscle cells called

muscle fibers– each muscle fiber runs the entire length of the

muscle– each muscle fiber is made up of myofibrils –

individual contractile subunits that extend from one end of the fiber to the other

• each myofibril is surrounded by the sarcoplasmic reticulum

– series membranes forming hollow tubes that store high concentrations of calcium

•plasma membrane (sarcolemma) has multiple inward extensions that form transverse tubules (T tubules)

•T tubules pass very close to sarcoplasmic reticulum and are crucial to muscle control

• each myofibril is made up of two types of myofilaments:

– myosin filaments – thick filaments– actin filaments – thin filaments, also

contain tropomyosin and the troponin complex (regulate actin’s interactions with myosin)

– myosin and actin filaments slide past each other during contraction

• myofilaments are arranged in repeating subunits called sarcomeres – basic unit of contraction – aligned end to end the length of the myofibril

Muscle contraction – Sliding Filament Model

• muscle contraction occurs when the sarcomeres contract

• myosin “heads” bond to exposed receptor sites on actin forming cross bridges

• at rest, receptor sites are blocked by tropomyosin and troponin complex

• when stimulated, complex pulls away from actin exposing receptor sites to form cross bridges

• myosin heads bend in toward the center of the sarcomere then cross bridges break

• cross bridges reform at the next actin receptor site and bend some more

• process repeats causing thin filaments to slide over thick pulling the sarcomere inward (shortens between Z lines)

• ATP is used to break cross bridges and allow myosin heads to bond to the next receptor site after powerstroke

• A band length stays the same

• I band length shortens

• H zone length shortens (may disappear)

• Z lines are drawn closer to each other

Role of calcium in muscle contraction• muscle fiber membrane is polarized – action

potential is generated in similar manner as neurons – action potential activates contraction indirectly by causing the release of calcium

• action potential is generated by the release of acetylcholine (a neurotransmitter) at the neuromuscular junction – acetylcholine binds to the sarcolemma and causes the membrane to depolarize

• action potential depolarizes membrane of T tubules which are closely associated with sarcoplasmic reticulum – reticular membranes become permeable to and release large amounts of calcium

•troponin complex and tropomyosin block the receptor sites on actin when muscle is at rest•calcium binds to troponin complex and tropomyosin and pulls them out of the way to expose receptor sites to myosin heads

Skeletal Structure

• Arthropod and vertebrate skeletal and muscular systems have many functional similarities

• each have hard jointed skeleton• most skeletal muscle is arranged w/one

end attached to one section of skeleton and the other attached to a different section

• Arthropods – exoskeleton – hard body covering w/muscles and organs located inside – requires molting for growth

Vertebrates – endoskeleton – framework w/muscles attached embedded within the organism – composed of two types of bone:

1. Cancellous – “spongy”– very porous and light weight– filled w/red bone marrow (produce RBCs)– found in flat bones of ribs, skull, and ends of long bonds

2. Compact Bone – composed of structural units called Haversian Systems

– concentric layers of hard inorganic matrix surrounding a central Haversian Canal

– has radiating canaliculi that allow for exchange of mat’l b/w bone cells and blood vessels

Cartilage – firm, flexible tissue• primary component of embryonic

skeletons• replaced w/bone as fetus develops –

retained in some areas where firmness and flexibility is needed: ends of ribs, external ear, tip of nose, where some bones meet

• Chondricthyes and Agnathans – retain cartilagenous skeleton throughout life

• Vertebrate skeleton divided into 2 major parts:– Axial skeleton – skull, vertebral column and

rib cage– Appendicular skeleton – pectoral and pelvic

girdles w/paired appendages

• Ligaments – tough “strings” of tissue holding bone to bone

• Tendons – attach muscle to bones• Origin – end of musc. attached to an essentially

stationary bone• Insertion – end of musc. attached to bone that

moves• a single muscle may have multiple

origins/insertions• Movement resulting from a musc. contraction

depends on position of origin/insertion and type of joint b/w bones (read about diff’t types of joints in text)