Skeletal Muscle Flashcards
skeletal muscle is responsible for
maintaining posture and improving movement
the muscle fibers of skeletal muscles are
- multi-nucleated
- surrounded by a cell membrane called the sarcolemma
what are the three layers of connective tissue that surround muscle fibers
- endomysium
- perimysium
- epimysium
what contains the calcium needed to initiate the contractile process
sarcoplasmic reticulum separating the myofibrils
sarcomeres are composed of
think and thin filaments bound together by a band of proteins called the Z line
thin filaments contain what three proteins
- actin
- tropomyosin
- troponin
think filaments are composed of
200 - 300 single protein molecules called myosin protected by globular cross bridges
thick filaments are held together by
organized proteins called myomesin that form the M line
what connects the M and Z line and what is its function
- elastic protein called titin
- creates elasticity that allows muscles to spring back into original shape after contraction
skeletal muscle fibers are activated by
single α-motoneurons
the α-motoneuron releases
enough acetylcholine to depolarize all the skeletal muscle fibers innervated by that single motoneuron
excitation-contraction coupling (EC)
initiation of muscle contraction
what are the four steps of excitation-contraction coupling
- generation of AP and depolarization of the T-tubule
- release of calcium from the sarcoplasmic reticulum
- calcium binds to troponin initiating cross bridge
- reaccumulation of calcium
T-tubule membrane contains a voltage sensor called
dihydropyridine
the sarcoplasmic reticulum contains a calcium channel called
ryanodine receptor
the coupling of the dihydropyridine receptor to the ryanodine receptor by a protein causes what
protein blocks the release of Ca2+ when the membrane is polarized
in the resting state, tropomysoin
covers the myosin binding sites on actin
troponin is composed of 3 proteins
- troponin C: binding sites for calcium
- troponin T: binds troponin complex to tropomyosin and actin
- troponin I: keeps troponin bound to actin and inhibits interactions between myosin cross bridges and actin
sliding filament theory
contraction of a skeletal muscle occurs when myosin cross bridges attach to thin filaments
how is calcium removed from the sarcoplasmic reticulum
ATP dependent calcium pumps called SERCA
calsequestrin
storage protein for calcium inside the sarcoplasmic reticulum
what are the three major phases of a twitch contraction
- latent period
- contraction phase
- relaxation phase
latent period
period of time from the action potential to the onset of contraction. The time delay is due to the excitation- contraction coupling.
contraction phase
the time that tension is developing due to the cross-bridge cycling.
relaxation phase
time that the tension is decreasing (i.e., relaxing) and is longer than the contraction phase. This is due to the amount of time it takes to get all the Ca2+ sequestered.
isometric contraction
total length of the muscle does not change
preload
initial length of muscle prior to contraction
the force produced during an isometric contraction depends on
- the preload (overlap of thick and thin filaments)
- number of muscle fibers activated
- frequency of action potentials generated by the muscle
isotonic contraction
muscle develops force and shortens
an isotonic contraction consists of three phases
- contracts isometrically until the force developed is equal to the afterload
- shortens isotonically at a constant velocity until the end of its capability
- contracts isometrically until it relaxes
afterload
load lifted by the muscle when it shortens
the velocity and amount of shortening during an isotonic contraction is
inversely proportional to the afterload
the force produced during an isometric contraction depends on
preload
the force produced during an isometric contraction depends on the
number of fibers activated by an alpha motoneuron
what is recruitment in an isometric contraction
central nervous system varies the amount of force produced by a muscle by varying the number of alpha motoneurons that are active
what does rotation refer to in an isometric contraction
During a sustained tension (less than maximum tension), units cycle through periods of activity and inactivity: allows motor units to rest
twitch contraction
contractile force produced by a single action potential
tetanus
occurs when the frequency of action potentials is high enough to prevent any relaxation of contraction between action potentials and the force
The increased force produced by summation and tetanus results from
stretching out a series elastic element
series elastic component (SEC)
what connects contractile proteins to bone
the greater the stretch of the SEC the
greater the force transferred to the bone
stimulating the muscle fiber before the muscle relaxes produces
a greater stretch of the SEC and a greater force of contraction
increasing the afterload during an isotonic contraction
- increases the the latency between activation of the muscle and shortening.
- Increases the force produced by the muscle during shortening.
- Decreases the velocity of shortening.
- Decreases the amount of shortening.
with heavier loads during isotonic twitching
- the latent period is longer
- the velocity of shortening is slower
- the duration of the twitch is shorter
- the distance shortened is less
