chapter 3 Skeletal muscle system

basic components of skeletal muscle

Tendons

Fasciculus

Muscle fiber

Myofibril

Actin

Myosin

role of connective tissue

1. Tendons- Bands of tough, fibrous CT that connect to bone 

2. Fasciculus-Small bundle of muscle fibers

3. Muscle Fiber- long multinucleated cell that generates force when stimulated

4. Myofibril- Portion of muscle composed of thin & thick myofilaments 

5. 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

1. Electrical impulse is generated at neuromuscular junction

2. Impulse spreads across sarcolemma into T-tubules

3. impulse reaches sarcoplasmic reticulum release Ca⁺⁺ into cytosol of muscle fiber

4. Ca⁺⁺ binds to troponin C subunit - causes conformational change in troponin/tropomyosin

5. Tropomyosin uncovers active sites of actin

6. Myosin crossbridge heads bind actin, form actomyosin complex 

7. Heads pulls actin toward center of sarcomere (power stroke) 

8. force is produced  

Cross-bridge cycling

1. Active site covered. ADP and P are connected on myosin 

2. The release of calcium binds to troponin changing its shape to have the active sites exposed for myosin filaments

3. P_i leaves then myosin head causing a power stroke and moving filament

4. Release ADP from the head causes myosin head to tightly bind to the active site. 

5. Then ATP binds to the myosin head then is broken down to ADP + P_i  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