Chapter #10 Muscle Tissue

Skeletal Muscle

Skeletal Muscle

This tissue is packaged into skeletal muscles, organs that are attached to bones and skin.

Skeletal muscle fibers are the longest of all muscles and have striations (stripes)
Also called voluntary muscle: can be consciously controlled
Contract rapidly; tire easily; powerful
Keywords for skeletal muscle: skeletal, striated, and voluntary

Cardiac Muscle

This tissue is found only in heart

Makes up bulk of heart walls

Striated

Involuntary: cannot be controlled consciously

Contracts at steady rate due to heart’s own pacemaker, but nervous system can increase rate

Key words for cardiac muscle: cardiac, striated, and involuntary

Smooth Muscle

This tissue is found in walls of hollow organs

Examples: stomach, urinary bladder, and airways

Not striated

Involuntary: cannot be controlled consciously

Key words for smooth muscle: visceral, nonstriated and involuntary

Muscle Tissue Comparisons

Skeletal Muscle ( Explained)

Skeletal muscle is an organ made up of different tissues with three features: nerve and blood supply, connective tissue sheaths, and attachments

Each muscle receives a nerve, artery, and veins

Consciously controlled skeletal muscle has nerves supplying every fiber to control activity

Contracting muscle fibers require huge amounts of oxygen and nutrients

Also need waste products removed quickly

Each skeletal muscle, as well as each muscle fiber, is covered in connective tissue

Support cells and reinforce whole muscle

Sheaths from external to internal:

Epimysium »Perimysium»Endomysium

Epimysium

dense irregular connective tissue surrounding the entire muscle; may blend with fascia

Perimysium

fibrous connective tissue surrounding fascicles (groups of muscle fibers)

Endomysium

fine areolar connective tissue surrounding each muscle fiber

Sarcolemma

muscle fiber plasma membrane

Sarcoplasm

: muscle fiber cytoplasm

Myofibrils

Myofibrils are densely packed, rodlike elements
Single muscle fiber can contain 1000s
Accounts for ~80% of muscle cell volume

Myofibril features Striations, Sarcomeres, Myofilaments, Molecular composition of myofilaments

Sarcomere

Smallest contractile unit (functional unit) of muscle fiber

A bands: dark regions

H zone: lighter region in middle of dark A band

M line: line of protein (myomesin) that bisects H zone vertically

I bands: lighter regions

Z disc (line): coin-shaped sheet of proteins on midline of light I band

Contains A band with half of an I band at each end

Consists of area between Z discs

Striations

: stripes formed from repeating series of dark and light bands along length of each myofibril

Actin myofilaments

: thin filaments Extend across I band and partway in A band and Anchored to Z discs

Myosin myofilaments

: thick filaments Extend length of A band Connected at M line

Thick filaments

: composed of protein myosin that contains two heavy and four light polypeptide chains
Heavy chains intertwine to form myosin tail
Light chains form myosin globular head

Myosins are offset from each other, resulting in staggered array of heads at different points along thick filament

Thin filaments

: composed of fibrous protein actin
Actin is polypeptide made up of kidney-shaped G actin (globular) subunits
G actin subunits bears active sites for myosin head attachment during contraction
G actin subunits link together to form long, fibrous F actin (filamentous)
Two F actin strands twist together to form a thin filament
Tropomyosin and troponin: regulatory proteins bound to actin

Sarcoplasmic reticulum

: network of smooth endoplasmic reticulum tubules surrounding each myofibril and Most run longitudinally

Sacroplasmic Reticulum (Explained)

Terminal cisterns form perpendicular cross channels at the A–I band junction

SR functions in regulation of intracellular Ca2+ levels and Stores and releases Ca2+

SR cistern membranes also have integral membrane proteins that protrude into intermembrane space

SR integral proteins control opening of calcium channels in SR cisterns

When an electrical impulse passes by, T tubule proteins change shape, causing SR proteins to change shape, causing release of calcium into cytoplasm

Triad

: area formed from terminal cistern of one sarcomere, T tubule, and terminal cistern of neighboring sarcomere

Triad relationships T tubule contains integral membrane proteins that protrude into intermembrane space (space between tubule and muscle fiber sarcolemma)

T tubules

Tube formed by protrusion of sarcolemma deep into cell interior

Increase muscle fiber’s surface area greatly

Lumen continuous with extracellular space

Allow electrical nerve transmissions to reach deep into interior of each muscle fiber

Tubules penetrate cell’s interior at each A–I band junction between terminal cisterns

Tubule proteins act as voltage sensors that change shape in response to an electrical current

Excitable cells

are capable of changing resting membrane potential voltages

Neurons and muscle cells are excitable cells capable of action potentials

Contraction

: the activation of cross bridges to generate force

Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening

Contraction ends when cross bridges become inactive

Extension/Relaxed State of Muscle

In the relaxed state, thin and thick filaments overlap only slightly at ends of A band

Sliding filament model of contraction states that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more

Neither thick nor thin filaments change length, just overlap more

Muscle Fiber Stimulation

When nervous system stimulates muscle fiber, myosin heads are allowed to bind to actin, forming cross bridges, which cause sliding (contraction) process to begin

Cross bridge attachments form and break several times, each time pulling thin filaments a little closer toward center of sarcome in a ratcheting action

Causes shortening of muscle fiber

Z discs are pulled toward M line I bands shorten Z discs become closer H zones disappear and A bands move closer to each other

Ion Channels

Play a major role in changing of membrane potentials

Two classes of ion channels:

Chemically gated ion channels – opened by chemical messengers such as neurotransmitters
Example: ACh receptors on muscle cells

Voltage-gated ion channels – open or close in response to voltage changes in membrane potential

Chemically gated ion channels

– opened by chemical messengers such as neurotransmitters
Example: ACh receptors on muscle cells

Voltage-gated ion channels

– open or close in response to voltage changes in membrane potential

Anatomy of Motor Neurons and Neuromuscular Junction

Skeletal muscles are stimulated by somatic motor neurons

Axons (long, threadlike extensions of motor neurons) travel from central nervous system to skeletal muscle

Each axon divides into many branches as it enters muscle

Axon branches end on muscle fiber, forming neuromuscular junction or motor end plate

Each muscle fiber has one neuromuscular junction with one motor neuron

Axon Terminal

Axon terminal (end of axon) and muscle fiber are separated by gel-filled space called synaptic cleft

Stored within axon terminals are membrane-bound synaptic vesicles

Synaptic vesicles contain neurotransmitter acetylcholine (ACh)

Infoldings of sarcolemma, called junctional folds, contain millions of ACh receptors

NMJ consists of axon terminals, synaptic cleft, and junctional folds

Action potential

is caused by changes in electrical charges

Resting sarcolemma is polarized, meaning a voltage exists across membrane

Inside of cell is negative compared to outside

Neuromuscular Junction of Skeletal Muscle Fiber

Neuromuscular Junction of Skeletal Muscle Fiber ( Explained)

Occurs in three steps

End plate potential

ACh released from motor neuron binds to ACh receptors on sarcolemma

Causes chemically gated ion channels (ligands) on sarcolemma to open

Na+ diffuses into muscle fiber Some K+ diffuses outward, but not much

Because Na+ diffuses in, interior of sarcolemma becomes less negative (more positive)

Results in local depolarization called end plate potential

Depolarization: generation and propagation of an action potential (AP)

If end plate potential causes enough change in membrane voltage to reach critical level called threshold, voltage-gated Na+ channels in membrane will open

Large influx of Na+ through channels into cell triggers AP that is unstoppable and will lead to muscle fiber contraction

AP spreads across sarcolemma from one voltage-gated Na+ channel to next one in adjacent areas, causing that area to depolarize

Repolarization: restoration of resting conditions

Na+ voltage-gated channels close, and voltage-gated K+ channels open

K+ efflux out of cell rapidly brings cell back to initial resting membrane voltage

Refractory period: muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete

Ionic conditions of resting state are restored by Na+-K+ pump

Na+ that came into cell is pumped back out, and K+ that flowed outside is pumped back into cell

Refractory period: muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete

Depolarization: generation and propagation of an action potential (AP)