Muscle Structure & Function

terminology

• Muscle terminology: Myo, mys, and sarco are prefixes for muscle

  • Example: sarcoplasm refers to muscle cell cytoplasm

types

• Three types of muscle tissue: Skeletal, Cardiac, Smooth

• Only skeletal and smooth muscle cells are elongated and referred to as muscle fibers

skeletal muscle:

• Skeletal muscle:

  • Organs that are attached to bones and skin

  • Fibers are the longest of all muscles and have striations

  • Voluntary control

  • Contract rapidly, tire easily, and are powerful

cardiac muscle:

• Cardiac muscle:

  • Found only in the heart

  • Makes up the bulk of heart walls

  • Striated

  • Involuntary control

  • Contracts at a steady rate due to the heart's own pacemaker, but the nervous system can increase the rate

smooth muscle:

• Smooth muscle:

  • Found in the walls of hollow organs such as the stomach, urinary bladder, and airways

  • Not striated

  • Involuntary control

characteristic of muscles:

• Characteristics of muscles:

  • Excitability (responsiveness): ability to receive and respond to stimuli

  • Contractility: ability to shorten forcibly when stimulated

  • Extensibility: ability to be stretched

  • Elasticity: ability to recoil to resting length

important muscle functions:

• Important muscle functions:

  • Produce movement: responsible for all locomotion and manipulation

  • Maintain posture and body position

  • Stabilize joints

  • Generate heat as they contract

connective tissue sheaths:

• Connective tissue sheaths:

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

  • Support cells and reinforce the whole muscle

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

connective tissue sheaths:

• Connective tissue sheaths:

  • Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers)

  • Endomysium: fine areolar connective tissue surrounding each muscle fiber

attachments:

• Attachments:

  • Muscles attach to bone in at least two places

  • Insertion: attachment to movable bone

  • Origin: attachment to immovable or less movable bone

attachments:

• Attachments:

  • Attachments can be direct or indirect

  • Direct (fleshy): epimysium fused to bone or cartilage

  • Indirect: connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis

muscle cell anatomy:

• Muscle cell anatomy:

  • Skeletal muscle fibers are long, cylindrical cells that contain multiple nuclei

  • Sarcolemma: plasma membrane

  • Sarcoplasm: cytoplasm

  • Contains glycosomes for glycogen storage and myoglobin for O2 storage

  • Modified organelles: Myofibrils, Sarcoplasmic reticulum, T tubules

myofibris:

• Myofibrils:

  • Striations: stripes formed from a repeating series of dark and light bands

  • A bands: dark regions

  • H zone: region in the middle of the A band that anchors myosin

  • M line: area where actin is not present

  • I bands: lighter regions

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

sarcomere:

• Sarcomere:

  • Smallest contractile unit of a muscle fiber

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

  • Consists of the area between Z discs

  • Individual sarcomeres align end to end

myofilaments:

• Myofilaments:

  • Proteins that make up the sarcomere

  • Actin myofilaments: thin filaments that extend across the I band and partway into the A band, anchored to Z discs

  • Myosin myofilaments: thick filaments that extend the length of the A band

myosin:

• Myosin:

  • Two parts: Heavy chains form the tail, Light chains form the globular head

  • During contraction, myosin heads grab thin filaments, forming cross bridges

  • Myosins are offset, resulting in a staggered array of heads at different points

actin: thin filament:

• Actin: Thin filament

  • Contains active sites for myosin head attachment during contraction

  • Tropomyosin and troponin: regulatory proteins bound to actin

other proteins:

• Other proteins help form the structure of the myofibril:

  • Elastic filament: Holds thick filaments in place, helps recoil after stretch, and resists excessive stretching

  • Dystrophin: Links thin filaments to proteins of the sarcolemma

    • Duchenne muscular dystrophy (DMD) is the most common and serious form of muscular dystrophies

sarcoplasmic reticulum:

• Sarcoplasmic reticulum:

  • Network of smooth endoplasmic reticulum tubules surrounding each myofibril

  • Most run longitudinally

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

  • Stores and releases Ca2+

t tubules:

• • Tube formed by the protrusion of the sarcolemma deep into the cell interior

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

  • When an electrical impulse passes by, T tubules release calcium into the cytoplasm, initiating contraction

sliding filament:

• • When relaxed, thin and thick filaments overlap only slightly at the ends of the A band

  • When the nervous system stimulates a muscle fiber, myosin heads bind to actin, forming cross bridges, and contraction begins

  • Neither thick nor thin filaments change length, they just overlap more

contraction:

• Contraction:

  • Cross bridge attachments form and break several times, each time pulling thin filaments a little closer toward the center of the sarcomere

  • Causes shortening of the muscle fiber

  • Z discs are pulled toward the M line

  • I bands shorten

  • Z discs become closer

  • H zones disappear

  • A bands move closer to each other

initiation of contraction:

• • Contraction is activated by the brain

  • Signal is transmitted down the spinal cord, to motor neurons, to muscle fibers

  • Neurons and muscle cells are excitable cells

  • These cells can change resting membrane potential

  • Difference of ions inside vs. outside cell membrane

nuromuscular juction:

• What is it?:

  • Axons travel from the central nervous system to skeletal muscle

  • Divides into many branches as it enters the 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

• Synaptic Cleft

  • fluid-filled space between axon terminal and sarcolemma of muscle

  • Neurotransmitter continues the message to the muscle cell

• Acetylhcoline (ACh)

  • is stored in membrane-bound synaptic vesicles

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

action potential:

• Action Potential:

  • Resting sarcolemma is polarized, meaning a voltage exists across the membrane

  • Inside of the cell is negative compared to the outside

  • Action potential is caused by changes in electrical charges

end plate potential:

• End plate potential:

  • ACh released from the motor neuron binds to ACh receptors on the sarcolemma

  • Causes chemically gated ion channels on the sarcolemma to open

  • Na+ diffuses into the muscle fiber, resulting in local depolarization called end plate potential

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

  • Results in local depolarization called end plate potential

depolarization:

• Depolarization:

  • Generation and propagation of an action potential (AP)

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

  • Large influx of Na+ through channels into the cell triggers an AP that leads to muscle fiber contraction

repolarization:

• Repolarization:

  • Restoration of resting conditions

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

  • K+ efflux rapidly brings the cell back to the initial resting membrane voltage

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

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

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

• Action Potential Leads to Ca2+ Release:

  • AP is propagated along the sarcolemma and down into T tubules where voltage-sensitive proteins in tubules stimulate Ca2+ release from SR

  • Ca2+ release leads to contraction,

  • AP is brief and ends before contraction is seen

excitation-contraction sequence

• Action potential along the sarcolemma leads to contraction

• Steps in E-C Coupling:

  • Sarcolemma Voltage-sensitive t tubule

  • Action potential (AP) tubule protein propagates along the sarcolemma and down the tubules

  • Ca2+ Calcium release Transmission channe T tubules proteins to Terminal change cistern in the of SR sarcoplasmic reticulum (SR)

  • Ca2+ flows into the cytosol

  • Actin I Troponin Tropomyosin blocking active sites

  • Myosin Myosin-binding sites exposed and ready for myosin binding

  • Myosin cross bridge

• Aftermath:

  • Muscle AP ceases, voltage-sensitive tubule proteins return to original shape

  • Ca2+ release channels of the SR close

  • Ca2+ levels in the sarcoplasm fall as Ca2+ is pumped back into the SR

  • Blocking action of tropomyosin is restored, myosin-actin interaction is inhibited, and relaxation occurs

• Each time an AP arrives at the neuromuscular junction, the sequence of E-C coupling is repeated

muscle contraction

• At low intracellular Ca2+ concentration:

  • Tropomyosin blocks active sites on actin

  • Myosin heads cannot attach

  • Muscle fiber remains relaxed

• Action potential leads to channels in the SR to release Ca2+ to cytosol

cross bridge cycling

• At high intracellular Ca2+ concentrations, it binds to troponin

• Troponin changes shape and moves tropomyosin away from myosin-binding sites

• Myosin heads are then allowed to bind to actin, forming cross bridge

• Cycling is initiated, causing sarcomere shortening and muscle contraction

• When nervous stimulation ceases, Ca2+ is pumped back into SR, and contraction ends

four steps of the cross bridge cycle

1. Cross bridge formation: high-energy myosin head attaches to actin thin filament active site

2. Working (power) stroke: myosin head pivots and pulls thin filament toward M line

3. Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach

4. Cocking of myosin head: energy from hydrolysis of ATP "cocks" myosin head into high-energy state

   • This energy will be used for power stroke in the next cross bridge cycle

rigor mortis

• 3–4 hours after death, muscles begin to stiffen

• Peak rigidity occurs about 12 hours postmortem

• Intracellular calcium levels increase, ATP is no longer synthesized, calcium cannot be pumped back into SR

• Results in cross bridge formation

• ATP is also needed for cross bridge detachment

• Results in myosin head staying bound to actin, causing a constant state of contraction

• Muscles stay contracted until muscle proteins break down, causing myosin to release

myasthenia gravis

• Many toxins, drugs, and diseases interfere with events at the neuromuscular junction

• Example: myasthenia gravis: disease characterized by drooping upper eyelids, difficulty swallowing and talking, and generalized muscle weakness

• Involves a shortage of Ach receptors because a person's ACh receptors are attacked by own antibodies (autoimmune disease)