Chapter 11

Universal Characteristics of Muscle

Excitability (responsiveness): to chemical signals, stretch, & electrical changes across plasma membrane

Conductivity: local electrical excitation sets off a wave of excitation that travels along the muscle fiber

Contractility: shortens when stimulated

Extensibility: capable of being stretched between contractions

Elasticity: returns to its original resting length after being stretched

Skeletal

Has striations, usually subject to conscious control

Tendons

Attachments between muscle & bone

Collagen

Extensible and elastic, stretches under tension and recoils when released

Sarcolemma

plasma membrane of a muscle fiber

Sarcoplasm

Cytoplasm of a muscle fibers

Myofibrils

bundles of protein filaments occupy most of sarcoplasm

Mitochondria

found in spaces between myofibrils

Glycogen

storage form of glucose, energy for exercise

Myoglobin

iron & O2 binding protein; skeletal & heart muscle

The Muscle Fiber

Multiple nuclei—flat, inside of sarcolemma

The Muscle Fiber

Sarcoplasmic reticulum: regulates excitation-contraction-coupling (ECC) & intracellular Ca++  for contraction & relaxation

Terminal cisterns: calcium reservoir, releases Ca++ through channels, activates contraction

T tubules: tubular in folding of sarcolemma, penetrate through cell, emerge on other side

Triad: T tubule & two terminal cisterns associated with it

Myosin (Thick) filament

Composed of myosin molecules, 2 chains intertwined,

The Muscle Fiber

Multiple nuclei—flat, inside of sarcolemma,

Myoblasts

stem cells, divide to create multinucleated muscle fibers, activated to repair damaged muscles

Satellite cells

unspecialized myoblasts found between muscle fiber & endomysium, regeneration of damaged skeletal muscle

Actin (thin) filaments

regulatory proteins, determine when fiber contracts; contraction activated by release of Ca2+ into SR & binding to troponin, changes shape, moving tropomyosin off actin active sites

Tropomyosin

globular (G) actin subunits, active site, binds to myosin head

Troponin

Ca2+ binding protein on each tropomyosin molecule

Titin/Elastic Filament

largest of all known proteins, springy

Sarcomere

functional contractile unit, Z disc to Z disc, Muscle cells shorten because individual sarcomeres shorten

•Z discs pulled closer, thick & thin filaments slide past each other

•Actin & myosin do not change length, shorten/contraction is overlap

•During shortening, dystrophin & linking proteins pull on EC proteins

A (Anisotropic) band

dark, thick filaments overlap thin H band: middle of A band; thick filaments only M line: middle of H band

I (Isotropic) band

bands reflect polarized light, Z disc: anchor for thin & elastic filaments

Dystrophin

important protein complex (dystrophin-glycoprotein complex,

crucial for muscle functions

Links actin to membrane proteins

anchors muscle fiber to EC matrix

Transfers force of muscle to tendon

The Nerve—Muscle Relationship

Skeletal muscle cannot contract unless stimulated by a nerve

Somatic motor neuron

Skeletal muscle nerve cells with cells, with cell bodies in brainstem and spinal cord

Somatic motor fibers

Lead to skeletal muscle, each nerve branches it to muscle fibers, each muscle fiber supplied by only one motor neuron

Motor unit

one nerve fiber (axon) & all muscle fibers it innervates, dispersed though out muscle, sustain long term contraction

Action of a muscle

The effect the muscle has on a part of the body

Levator Scapulae

Elevates scapula, abducts and rotates neck

Scalenes

Flexes and rotates neck

Sternocleidomastoid

Abducts rotates and flexes head

Sternohyoid

Depressed hyoid bone

Sternothyroid

Depressed thyroid cartilage

Omohyoid

Depresses hyoid bone

Platysma

Depresses inferior lip, opens jaw

Digastric

Elevates hyoid bone and mandible

Mylohyoid

Elevates hyoid bone and tongue

Frontal belly occipitofrontalis

Raises eyebrows, draws scalp anteriorly

Occipital belly of occipitofrontalis

Retracts scalp

Temporalis

Elevates mandible

Masseter

Elevates mandible

Pterygoids

Medial excursion of mandible

Orbicularis oculi

Closes eyes

Corrugator supercilii

Adducts eyebrows

Mentalis

Protrudes lower lip

Buccinator

Compresses check

Zygomaticus Major and minor

Elevates corners of mouth

Depressor labii inferioris

Depresses inferior lip

Levator Labii Superioris

Elevates superior lip, flares nostril

4 Major phases of contraction and relaxation

Excitation

Excitation-contraction coupling

Contraction

Relaxation

Phase 1

Ca2+ enters

Ca2+ stimulates release of ACh

ACh diffuses, binds to receptor

Na+ flows in while K+ flows out

Polarity changes, AP created

Phase 2

AP propagate down T-tubules

Ca2+ diffuses out of SR

Ca2+ binds to troponin

Troponin-tropomyosin changes shape

Phase 3

Hydrolysis of ATP to ADP and Pi

Formation of myosin-actin

Actin slides over myosin

New ATP binds to myosin

Somatic motor neuron

Skeletal muscle nerve cells with cell bodies in brainstem and spinal cord

Average motor unit contains ___ muscle fibers

200