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basic components of skeletal muscle
Tendons
Fasciculus
Muscle fiber
Myofibril
Actin
Myosin
role of connective tissue
Tendons- Bands of tough, fibrous CT that connect to bone
Fasciculus-Small bundle of muscle fibers
Muscle Fiber- long multinucleated cell that generates force when stimulated
Myofibril- Portion of muscle composed of thin & thick myofilaments
Actin & myosin- Contractile proteins
sarcomere action during contraction
Actin Filaments
slide over myosin
H zone disappear as actin filaments slide into it
I band shorten as actin & myosin slide over each other
Z line approach ends of myosin filaments
sarcomere action during relaxation
It returns to original length
H zone & I bands return to original size & appearance
Less overlap between actin & myosin
Titin (non contractile protein)
Connects z line to m line
stabilize myosin in longitudinal axis
Limits ROM of sarcomere contributes to passive stiffness
Nebulin
Extends from Z line & is localized to I band
Stabilizes actin by binding with actin monomers
actin filament
Thin(2 intertwined helices of actin molecules
Active site: where heads of myosin crossbridges bind actin.
Wrapped by tropomyosin and troponin
Subunits of troponin
Troponin I: holds to actin
Troponin T: holds to tropomyosin
Troponin C: can bind calcium
Myosin filament
Thick(has globular head, hinge pivot point and fibrous tail
Heads: made up of enzyme myosin ATPase
Tails: Intertwine to form myosin filament
Crossbridge
consist of 2 myosin molecules, with 2 heads
interacts with actin
Develops force to pull actin over myosin
Features different isoforms of ATPase
Sliding filament theory
Electrical impulse is generated at neuromuscular junction
Impulse spreads across sarcolemma into T-tubules
impulse reaches sarcoplasmic reticulum release Ca++ into cytosol of muscle fiber
Ca++ binds to troponin C subunit - causes conformational change in troponin/tropomyosin
Tropomyosin uncovers active sites of actin
Myosin crossbridge heads bind actin, form actomyosin complex
Heads pulls actin toward center of sarcomere (power stroke)
force is produced
Cross-bridge cycling
Active site covered. ADP and P are connected on myosin
The release of calcium binds to troponin changing its shape to have the active sites exposed for myosin filaments
Pi leaves then myosin head causing a power stroke and moving filament
Release ADP from the head causes myosin head to tightly bind to the active site.
Then ATP binds to the myosin head then is broken down to ADP + Pi going back through the same process. steps 1-5
Concentric-(force-velocity curve)
decrease in force partly due to inability to form actomyosin complexes at high velocity
Eccentric-(force-velocity curve)
increase force partly due to increased elasticity
strength curve
amount of force produce over ROM which is explained through length tension relationship
length-tension relationship
highest force occurs at a length where there is optimal overlap between actin and myosin
- optimal sarcomere length = 2.0 – 2.25 mm
> 2.25 decreased interaction because of stretch
< 2.0 decreased interaction because of overlap
muscle adaptation
Hypertrophy(size)
Hyperplasia(#)
neural adaptation
Initial strength gain primarily due to neural adaptations
improve neural recruitment pattern
increase CNS activation
Hypertrophy
Increase in size of muscle fibers
Results from addition of contractile protein & new myofibrils to existing fibers, making them larger
increase sarcomeres —> increase myofibrils (#) —> increase muscle fiber diameter
Hyperplasia
Increase in number of muscle fibers
by incorporating satellite cells into new fibers
Occurrence is controversial - intensive resistance training may increase fiber #
• Generally considered that most strength gains due to increase in fiber size not an increase in fiber #
oxygen delivery
Increase I # of capillaries per muscle fiber
increase in capillary density
increase in myoglobin concentration, which increases the rate of oxygen transport from capillaries to mitochondria, causing mitochondria to increase.
Length tension relationship
Optimal over lap of actin and myosin leading to optimal force produce.
Detraining
The opposite of strength gains
Rapid loss due mostly to neural component
Slower strength loss due to atrophy
Most strength gains can be maintained with decrease frequency (1-2 times per week)
________is an enzyme found in the head of myosin which breaks down ATP and staining of this enzyme can be used to determine muscle ___________
ATPase
Fiber type
Muscle contraction is regulated by _______, which once bound to __________ induces conformational change and pulls tropomyosin from active site on actin.
Calcium
troponin C
muscle cell is also known as
myocyte