Ch 12 Skeletal Muscle Physiology

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types of muscle

1. skeletal
2. smooth
2. cardiac

striated muscle

-describing skeletal muscle's transverse striations


the point of attachment by skeletal muscle closest to the spine


-point of attachment on the distal region (on the far side of the joint)


-is where the points of attachment occur at the end of the muscle
-connective tissue


-contraction of the flexor muscle on one side results in decrease in the angle of the elbow joint (bringing the forearm closer to the shoulder)


-contraction of the extensor muscle results in reverse motion (extending the arm)

muscle fibers

-muscle cells
-muscle is made of up of numerous muscle fibers


-connective tissue layer that surrounds each of the muscle fibers


-groups of individual muscle fibers
-fasicles themselves are joined together to create muscle


connective tissue that surrounds the fasicles
-contain the blood vessles and nerves that supply the individual muscle fibers


-the connective tissue sheath that surrounds the muscle


-at the end of the muscles the connective tissue layers come together to form a tendon, which attaches the muscle to the skeleton

connective tissue layers

-composed mainly of elastin and collagen fibers and serve to transmit movement of the actin and myosin molecules t the skeleton to effect movement

relaxation of skeletal muscle

-occurs as intracellular ca is resequestered by the SR
-uptake of ca into the SR is due to Ca pump (ie ca ATPase)=SERCA


-sarcoplasmic endoplasmic reticulum ATPase
-most abundant protein in the SR of skeletal muscle
-distributed along the longitudinal tubules and terminal cisternae
-transports two molecules of ca into its lumen for each molecule of ATP hydrolyzed
-its transports 2 ca for 2 h ions (h pumped out of the cell)


-low affinity ca binding protein that is present throughout the longitudinal tubules of the SR and nonjunctional regions of the terminal cisternae
-thought to be involved in the transfer of ca from sites of ca uptake in the longitudinal tubules to sites of ca release in the terminal cisternae

actin myosin interaction- crossbridge

-ca released from the SR binds to tropnin C
-once bound with ca, troponin c facilitates movement of the associated tropomyosin molecule toward the cleft of the actin filament
-this movement exposes myosin binding site on the actin filament and allows cross bridge to form, thereby generating tension
-binding of myosin to actin filaments appears to cause further shift in tropomyosin

-although, it is hypothesized that the strong binding of myosin to actin results in a movement of an adjacent tropomyosin molecule, perhaps exposing as many as 14 actin molecules

troponin C

-has 4 ca binding sites
-two of these have a high affinity for ca, but also bind Mg at rest
-these sites seem to be involved in controlling and enhancing the interaction between troponin 1 and troponin T subunits

-the other sites have lower affinity and bind ca as its concentration rises after release from the SR

troponin I

-facilitates the inhibition of myosin binding to actin by tropomyosin

troponin T

-binds to tropomyosin


-located at end of thin filament
-may participate in setting the length of the thin filament

alpha actinin and capZ

-proteins that serve to anchor the thin filament to the z line


-elongated cytoskeletal protein that extends along the length of the thin filament and may participate in regulation of the length of the thin filament


-dimers of the protein extend over the entire actin filament and cover myosin binding sites on actin molecules
-each tropomyosin dimer extends over seven actin molecules, with sequential tropomyosin dimers arranged in a head to tail configuration

-although, it is hypothesized that the strong binding of myosin to actin results in a movement of an adjacent tropomyosin molecule, perhaps exposing as many as 14 actin molecules

steps in cross bridge cycle

a. in resting state myosin is thought to have partially hydrolyzed ATP.
- When ca is released from the terminal cisternae of the SR, it binds to troponin C, which promotes movement on the actin filament such that myosin binding sites on actin are exposed.

b.-This then allows the "energized" myosin head to bind to the underlying actin

c.-myosin next undergoes a conformational change termed "rachet action" that pulls the actin filament toward the center of the sarcomere
-myosin releases ADP and P during the transition to state

d.-binding of ATP to myosin decreases the affinity of myosin for actin, thereby resulting in the release of myosin from the actin filament
-myosin then partially hydrolyzes the ATP is used to recock the head and return to the resting state
-if intracellular ca is still elevated, myosin will undergo another cross-bridge cycle and produce further contraction of the muscle

-cycle continues until SERCA pumps ca back into the SR
-as ca levels fall, ca dissociates from troponin c and troponin-tropomyosin complex moves and blocks the myosin binding sites on the actin filaments

rigor mortis

-when the supply of ATP is depleted (as in death)
-cycle stops in state c, with the formation of permanent actin-myosin complexes
-state if muscle is ridgid and conditio

sliding filament theory

-the cross bridge cycling mechansisim

fast twitch

-activity of glycolytic fibers is high and activity of oxidative enzymes is low
-have fewer mitochondria than slow twitch
-have more extensive SR than slow twitch
-fatigue rapidly due to dependence of glycolytic metabolically
-type 11A or B
-reuptake of ca into sr happens more quicky, so has faster relaxation time
-develop tension at a higher ca concentration than slow fibers do (troponin has two low afffinity ca binding site)

slow twitch

-meet metabolic demands by oxidative phosphorlyation
-have low glycolytic fibers and high activity of oxidative enzymes
-high more mitochondria than fast twitch
-fatigue slower
-type 1
-develop tension at a lower ca concentration than fast fibers do (troponin has only a single low affinity ca binding site)

fast IIA fibers

-found in mammals but uncommon in humans
-have both high glycolytic and high oxidative capacity


oxygen binding protein
-is red
-sometime called 'red fibers"


increasing force of contraction of a muscle= recuit more muscle fibers (motor units)

slow twitch motor units

-innervated by motor neuron that is easily excited
-tend to be recruited before fasttwitch motor units

fast twich motor units

-innervated by motor neurons that are less easily excited

spatial summation

process of recuiting additional motor units


at high level of stimulation, intracellular ca increases and is maintained throughout the period of stimulation and the amount of force developed greatly exceeds that seen during a twtich
-seem to be due to a series of elastic components in the muscle
-slow fibers can be tetanized at lower frequencies than is the case with fast fibers because fast fibers develop a larger maximal force, they are larger in diameter and have more fibers in fast motor units than slow motor units

muscle spindles (intrafusal fibers)

-sense amount of stretch of the muscle as well as speed of contraction
-procide feedback to the muscle in terms of its length

stretch reflex

-in a stretch reflex, rapid stretching lengthens the muscle and results in an increased frequency of action potentials in the afferent sensory neurons of the spindle
-afferent fibers in turn excite the alpha- motor neurons in the spinal cord that innerbate the stretch muscle
-result is the reflex arc i s a stretched induced contraction that does not require higher input from the brain

glogi tendon organs

-located in the tendons of muscles and provide feedback regarding contraction of the muscle
-main component is an elongated fasicle of collagen bundles that is in series with the muscle fibers and can respond to contractions of individual muscle fibers
-may attach to several fast twitch or slow twitch fibers (or both) and sends impulses through Ib afferent nerve fibers to the spinal chord in response of muscle contraction.

skeletal muscle tone

-firmness or tone (even in a relaxed state) is caused by low levels of contractile activity in some motor units and is driven by reflex arcs from the muscle spindles

creatine phosphate

used to convert ADP TO ATP and thus replensih the ATP store during muscle contraction
-enzyme creatine phosphokinase (CPK) catalyzes this reaction
-most CPK is present in the myoplasm but a small amount is located in the thick filament (near the M line)


-associated with creatine phosphate store (but does not cause it)
-in fast units: depletion of glycogen, creatine phosphate stores and accumulation of lactic acid
-during intense exercise, accumulation of p and lactic acid
-lacticacid changes ph which alters myosin/ actin interaction by affecting ca binding to troponon c

oxygen debt

-when energy demands are not met by oxidative phosphorylation
-after exercise, respiration remains above the resting level to repay debt (restore metabolite levels- creatine, phosphate, atp) and to metabolize the lactate generated by glycolysis
-oxygen debt is much greater with strenuous exercise, when fast glycolytic motor units are used


-if a motor neuron is cut, fasciculation occurs
-small, irregular contractions cause by release of acetylcholine from the terminals of the degenerating distal portion of the axon


characterized by spontaneous, repetitive contraction

isometric contraction

muscle length is held constant and force is generated during the contraction is then measured

isotonic contraction

force or tone is held constant and the change in length of the muscle is then measure

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