EXSC 329 Muscle and Fiber Types

What are the differences between each muscle type?

cardiac: involuntary with striations

skeletal: voluntary with striations

smooth: involuntary with NO striations

What is the order of structures surrounding a muscle cell?

epimysium → fascicle surrounded by perimysium → muscle fiber surrounded by endomysium → myofibril

What are the structures within a muscle fiber?

plasmalemma

t-tubules

myofibril

sarcoplasm

sarcoplasmic reticulum

What is the I-band and H-zone?

I-band: the area with no myosin that shortens during muscle contraction

H-zone: the area with no actin that shortens during muscle contraction

What is the A-band?

the dark region of a sarcomere that spans the length of the myosin filaments

Where are troponin and tropomyosin located and what is their purpose?

within the actin filaments and regulate muscle contraction by covering or exposing myosin binding sites on actin filaments

What is the purpose of titin?

to stabilize myosin, prevent overstretching, and generate spring-like force during eccentric contractions as a result of calcium binding

What is a motor unit?

an alpha motor neuron and all the fibers it innervates

What are the steps of excitation contraction coupling?

• motor neuron excitation

• release of Ach from synaptic terminal

• depolarization of muscle cell at NMJ

• AP propagates into t-tubules

• depolarization triggers Ca2+ release from SR

• Ca2+ binds to troponin

• tropomyosin reveals myosin binding sites on actin

• myosin binds to actin

• ADP and Pi release from myosin, causing a power stroke

• ATP binds to myosin head, breaking actomyosin complex

• ATP hydrolysis puts myosin into “ready” position

What is the role of ATP during muscle contraction?

removal of Pi during hydrolysis of ATP causes myosin heads to shift and produce a power stroke

What is the role of ATP during muscle relaxation?

ATP is used to pump Ca2+ back into SR during relaxation

Slow-oxidative fiber types show what characteristics regarding ATPase activity, SR, motor units, and aerobic endurance?

ATPase: slower speed of contraction isoform

SR: less developed

MU: smaller cell body, innervates less fibers, and have smaller fiber CSA

Endurance: greater aerobic endurance and most often used for low intensity activity

Fast-oxidative-glycolytic and Fast-glycolytic fiber types show what characteristics regarding ATPase activity, SR, motor units, and aerobic endurance?

ATPase: fast speed of contraction isoform

SR: greater developed

MU: larger cell body, innervates more fibers, and greater fiber CSA

Endurance: poor aerobic endurance and are better for shorter, high intensity activities

What is progressive recruitment and the size principle?

motor units are recruits from smaller to larger and in a progressive order (SO → FOG → FG)