PT15: Skeletal Muscles [pages 1 - 29]

three types of muscle tissue

smooth, cardiac, skeletal

skeletal and smooth muscle

elongated and are called muscle fibers

actin and myosin

contractile proteins

Sarcolemma

muscle plasma membrane

Sarcoplasm

cytoplasm of a muscle cell

Sarcoplasmic reticulum

- modified, highly specialized endoplasmic reticulum
- interconnecting tubules & sacs of the SR runs longitudinally & surround each myofibril
- interconnecting tubules & cisternae (sacs) form perpendicular cross channels
- stores calcium & release on demand when muscle fiber is stimulated to contract

characteristics of skeletal muscle tissue

striated, skeletal, & voluntary

location of skeletal muscle tissue

attached to skeletons

shape of skeletal muscle tissue

elongated, cylindrical

muscle fibers of skeletal muscle tissue

longest; cigar-shaped

size of skeletal muscle tissue

largest muscle

nuclei of skeletal muscle tissue

multinucleated; peripheral

type of contraction of skeletal muscle tissue

rapidly but tires easily

characteristics of cardiac muscle tissue

cardiac, striated, & involuntary

location of cardiac muscle tissue

heart; bulk of heart walls & roots of large blood vessels

shape of cardiac muscle tissue

elongated, cylindrical that branch

nuclei of cardiac muscle tissue

uninucleate & central

type of contraction of cardiac muscle tissue

fairly at a steady rate set by heart's pacemaker

characteristics of smooth muscle tissue

viscera, non-striated, & involuntary

location of smooth muscle tissue

lining walls of hollow visceral organs

shape of smooth muscle tissue

elongated, spindle-shaped

nuclei of of smooth muscle tissue

uninucleate & central

layers of smooth muscle tissue

inner circular & outer longitudinal

type of contraction of smooth muscle tissue

slow, sustained contraction

muscle functions

- movement
- maintains poster & body position
- stabilize joints
- generates heat

movement of Skeletal muscles

responsible for all locomotion, manipulation, respond quickly to external environment & facial expressions

movement of Cardiac muscle

responsible for coursing the blood through the body

movement of Smooth muscle

helps maintain blood pressure; & squeezes or propels substances (i.e., food, feces) through internal body channels

Maintains Posture & Body Position

maintains erect, upright position despite downward pull of gravity

Stabilize joints

as the skeletal muscle pulls on bones to cause movement, joints of the skeleton are stabilized; tendons reinforce & helps stabilize joints with poorly articulating surfaces

Generates heat

as a by-product of muscle activity from ATP used to power muscle contraction, nearly ¾ of its energy escapes as heat; vital in maintaining normal body temperature

functional characteristics of muscle tissue

excitability/irritability, contractility, extensibility, elasticity

excitability or irritability

ability to receive and response to stimulus; electrical changes on sarcolemma

contractility

stimulus dictates muscle to shorten or contract

extensibility

ability to be extended again; recoil and resume normal resting length

elasticity

important for muscles not to slacken again when resting

40%

percent of body mass made up of skeletal muscles

epimysium

covers entire muscle; fused with periosteum of the bone

perimysium

covers fascicle

endomysium

covers individual muscle fiber (synonymous with sarcolemma)

fascicle

bundle of individual muscle fibers enclosed in a perimysium

direct attachment of skeletal muscles

epimysium of the muscle is fused to the periosteum of a bone

indirect attachment of skeletal muscles

connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis

structural hierarchy of a skeletal muscle

Muscle → muscle bundle → fascicle → muscle fiber (muscle cell) → myofibril → sarcomere (myofilaments)

Muscle

contractile organ composed of bundles (fascicles) of tightly packed long, parallel muscle cells (muscle fibers)

Muscle fiber

a single muscle cell enclosed in the sarcolemma that contains densely packed bundles of (myofibrils) contractile proteins; enclosed in an endomysium

Myofibril

a bundle of protein myofilaments filling most of the sarcoplasm; each are surrounded by sarcoplasmic reticulum (SR) & mitochondria

Sarcomere

a segment of a myofibril from one Z-disc to the other Z-disc; "smallest functional contractile unit" of the muscle fiber

Myofilaments

fibrous contractile proteins that carry out the contractile process.

myoglobin

red protein pigment that stores oxygen within the muscle cells

transverse tubules (T Tubules)

- branching network of transverse tubules closely associated with the terminal cisternae of the sarcoplasmic reticulum
- conduct nerve impulses to release calcium

troponin and tropomyosin

regulatory proteins

titin & nebulin

accessory proteins

arrangement of myofibrils

arranged in a perfectly aligned repeating series of dark A bands and light I bands giving the striated appearance/banding pattern

Z-disc

- darker area w/c is in the middle of the I-band
- "between" in German
- coin-shaped proteins which are usually labeled as connectings
- zigzag structure which serve as attachment of thin filaments
- anchor thin filaments in the sarcomere

number of Z-discs per sarcomere unit

2 Z-discs

I-band

- light-colored bands in the sarcomere
- only composed of actin filaments
- Z-disc runs in the middle of the I-band so that each half of I-band belongs to two sarcomeres
- end of myosin with Z-disc in between

isotopic

reflects light uniformly under a polarized microscope

A-band

- dark-colored band; darkest area in sarcomere filled by entire length of myosin
- outer edges is actin & myosin overlap
- center is occupied only by myosin

anisotopic

does not reflect light evenly

H-zone

- central region of A-band
- sometimes referred as the bare region
- lighter than outer edges of A-band
- only occupied by myosin filaments

M-line

- middle of sarcomere/ H-zone
- exactly opposite Z-disc; attachment of myosin
- divides A-band into halves

titin

- anchors myosin filament to Z-disc
- huge, elastic
- stabilize thick filaments during muscle contraction
- framework holding and lining up myosin and actin filaments
- serves as a 'mold' where myosin filaments are positioned during muscle contraction

nebulin

- inelastic giant protein
- lies alongside actin filament & attaches to the Z-disc
- helps align actin filaments of the sarcomere

functions of titin molecules

- stabilize the position of the contractile filament
- act as framework that holds & lines up the myosin & the actin filaments to make the contractile machinery of the sarcomere work
- elasticity returns stretched muscle to their resting length, prevents overstretching / recoils like a spring when a muscle is stretched
- template for initial formation of portions of contractile filaments of the sarcomere, especially myosin filaments

dystrophin

- clinically important accessory protein
- enormous protein located between the sarcolemma & the outermost myofilaments
- links actin filaments to a peripheral protein on the inner surface of the sarcolemma and through a series of links, leads to the fibrous endomysium surrounding the muscle fiber

muscular dystrophy

- disabling disease caused by genetic defects in dystrophin

banding pattern

reveals the working structure of the myofibrils

thick myofilaments

made-up of bundled protein molecules of Myosin and extends the entire length of the dark A band

thin myofilaments

- composed of contractile proteins Actin & extends across the I-band & partway into the A-band
- contains regulatory proteins

desmin

protein that causes M-line to appear darker

ultrastructure of myosin

head (business end) has a cross-bridge that binds to actin molecule; also contains ATPase enzyme

myosin heads

two smaller, light polypeptide chains

myosin tails

two interwoven, heavy polypeptide chains

ultrastructure of actin

- contains troponin and tropomyosin
- has the active binding site
- anchored to Z-discs

G-actin

helical polymer of globular subunits found in each actin molecule

tropomyosin

- rod-shaped protein w/ two strands that are wrapped around actin helix
- cover or block the active binding site on the actin

troponin

- complex of 3 subunits
- makes sure tropomyosin is anchored firmly to actin filament provided you have favorable conditions that remove the inhibitory effect of the troponin complex away from the binding site that allow binding of myosin and actin

troponin I (TnI)

inhibitory subunit with strong affinity to actin

troponin T (TnT)

strong affinity to tropomyosin & helps position it on actin

troponin C (TnC)

strong affinity to calcium ions

triads

successive groupings of the three membranous structures: terminal cisternae, T tubules, & terminal cisternae

triad relationships

- T tubules & SR provide tightly linked signals for muscle contraction
- A double zipper of integral membrane proteins protrudes into the inter-membrane space
- T tubule proteins act as voltage sensors
- SR foot proteins are receptors that regulate Ca2+ release from the SR cisternae

requirements for skeletal muscle to contract

- must be stimulated by a nerve ending at the neuromuscular junction
- must generate & propagate an electrical current or an action potential, along its sarcolemma
- must have a rise in intracellular Ca2+ levels - the final trigger for contraction

denervation atrophy

a condition wherein paralyzed skeletal muscles becomes "wasted" or "wastes" & shrinks away if the damage, severed or poisoned nerve connections are not restored

somatic (voluntary) motor neurons

nerve cells that serve skeletal muscles

somatic motor fibers

large, myelinated nerve fiber axons that innervate the muscles

axon

long threadlike extension of the neuron/nerve cell

neuromuscular junction

a junction between the axon terminals and the muscle membrane

synapse

when an ending of a nerve fiber meets a target cell

motor unit

consists of one, single motor neuron & all the muscle fibers innervated & stimulate.

muscles that control fine movements

have smaller motor units (nerve fibers > muscle fibers)

large weight-bearing muscles

have larger motor units; large cell bodies and less sensitive neurons

major steps of skeletal muscle contract

excitation, excitation-contraction coupling, contraction, relaxation

Excitation

events at the neuromuscular junction where chemical signal from somatic neuron is converted into electrical signals in the muscle fiber; AP in nerve fiber becomes AP in muscle fiber

Excitation-contraction coupling

refers to a series of events that link AP (electrical signal) on the sarcolemma to the activation of the myofilaments, preparing them to contract → muscle contraction; initiates calcium signals

Contraction

the step in which the muscle fiber develops tension & may shorten

Relaxation

when muscle fiber returns to its resting length after the nerve fiber stops stimulating it

events in skeletal muscle contraction

- Nerve impulse & an action potential travels along a motor nerve, reaching the axon terminal / nerve ending on muscle fibers
- Calcium-gated channels open on the membrane of the axon terminals & Ca2+ enters the axon terminal stimulating the synaptic vesicles to release & secretes a small amount of neurotransmitter - acetylcholine (ACh).
- ACh acts on a local area of the sarcolemma to open multiple acetylcholine-gated protein channels
- Opening ACh-gated channels allows Na+ to flow into the interior of the muscle fiber membrane initiating an action potential
- AP travels along the length of the sarcolemma.
- AP depolarizes the muscle membrane & deep within the muscle fiber through the T tubules, stimulating the release Ca2+ from the sarcoplasmic reticulum.
- Muscle contraction occurs
- Ca+ return to SR after contraction stops

motor end plate

a complex formed by branching ends of large, myelinated nerve fiber terminals (axon terminals) of the somatic motor neurons that invaginate (junctional folds) into the surface of the muscle fiber just outside the sarcolemma