Skeletal Muscles

Functions of Skeletal Muscles

force production for locomotion and breathing

force generation for postural support

heat production

considered and endocrine organ and assist in regulating a variety of body systems

Epimysium

plasma membrane that surrounds the entire muscle

Perimysium

Surrounds bundles of muscle fibers

Endomysium

surrounds individual muscle fiber

Sarcolemma

Muscle cell membrane

Breakdown of a Muscle

muscle - the whole structure

fascicle - bundle of muscle fibers

muscle fibers - single muscle cell (myofiber)

myofibril - long cylindrical structure within the muscle fiber, consisting of repeating units of sarcomeres

sarcomere - smallest functional unit of a muscle fiber

Neuromuscular Junction (NMJ)

junction between motor neuron and muscle fiber

Motor End Plate

the “pocket” formed around motor neuron by sarcolemma

Neuromuscular Cleft

the “short gap” between neuron and muscle fiber

Sarcoplasmic Reticulum

Storage sites for calcium

terminal cisternae: 2 enlarged portion of the sarcoplasmic reticulum

Transverse Tubules

extend from sarcolemma to sarcoplasmic reticulum

Satellite Cells

play role in muscle growth and repair

Muscle Shortening

occurs because of the movement of the actin filament over the myosin filament which forms cross bridges between actin and myosin filaments

Sarcomeres Shortening During Muscle Contraction

the heads of the myosin filaments latch on to myosin binding sites on the actin filament

as the heads cock to one side they pull the actin along contracting the muscle

E-C Coupling - Muscle Excitation

nerve signals arrive at the synaptic knob

Acetylcholine is released into synaptic cleft and binds to receptors on motor end plate, opening ion channels and allowing sodium to enter the muscle fiber

E-C Coupling - Contraction

depolarization of t-tubules causes release of Ca++ from sarcoplasmic reticulum

Ca++ binds to troponin, causing shift in tropomyosin to uncover myosin binding sites on actin

myosin binds to actin to form cross-bridge

Pi released from myosin and cross-bridge movement occurs

new ATP attaches to myosin, breaking the cross bridge; ATP is then broken down to ADP + Pi, which energizes myosin

E-C Coupling - Muscle Relaxation

motor neuron stimulation ends acetylcholine is no longer released and muscle fiber repolarizes

Ca++ is pumped back into SR and tropomyosin returns to original position covering myosin binding sites on actin, and muscle relaxation occurs

Biochemical Properties of Muscles

1) oxidative properties — how many capillaries, mitochondria, and how much myoglobin?

2) type of myosin ATPase activity — determines the speed of ATP utilization

3) abundance of contractile protein within the fiber — larger amounts of actin and myosin generate more force than fibers with low levels of contractile proteins

Contractile Properties of Muscles

1) maximal force production — force per unit of cross sectional area (specific tension)

2) speed of contraction — Vmax is determined by the rate of cross-bridge cycling, the key biochemical here is Myosin ATPase activity

3) maximal power output — power = force x shortening velocity

4) muscle fiber efficiency — more efficient fiber would require less energy to perform certain amount of work produced (amount of force production / ATP used)

Types of Skeletal Muscle

type I

slow twitch fibers

slow oxidative fibers

type IIa

intermediate fibers

fast-oxidative glycolytic fibers

type IIx

fast twitch fibers

fast-glycolytic fibers