Muscles Anatomy Physiology

• Attached to bones and skin
• Striated
• Voluntary (i.e., conscious control)
• Powerful
• Primary topic of this chapter
• Each muscle is served by one artery, one nerve, and one or more veins

• Only in the heart
• Striated
• Involuntary
• More details in Chapter 18

• In the walls of hollow organs, e.g., stomach, urinary bladder, and airways
• Not striated
• Involuntary
• More details later in this chapter

• Excitability (responsiveness or irritability): ability to receive and respond to stimuli
• Contractility: ability to shorten when stimulated
• Extensibility: ability to be stretched
• Elasticity: ability to recoil to resting length

• Movement of bones or fluids (e.g., blood)
• Maintaining posture and body position
• Stabilizing joints
• Heat generation (especially skeletal muscle)

• Epimysium: dense regular connective tissue surrounding entire muscle
• Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers)
• Endomysium: fine areolar connective tissue surrounding each muscle fiber

Directly—epimysium of muscle is fused to the periosteum of bone or perichondrium of cartilage

Indirectly—connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis

*Cylindrical cell 10 to 100 micrometers in diameter, up to 30 cm long

*Multiple peripheral nuclei

*Many mitochondria

*Glycosomes for glycogen storage, myoglobin for O2 storage

*Also contain myofibrils, sarcoplasmic reticulum, and T tubules

• *Densely packed, rodlike elements
• *~80% of cell volume
• *Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands

• *Smallest contractile unit (functional unit) of a muscle CELL
• *The region of a myofibril between two successive Z discs
• *Composed of thick and thin myofilaments made of contractile proteins

• *Thin filaments: run the length of the I band and partway into the A band
• *Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
• *Thick filaments: run the entire length of an A band
• *H zone: lighter midregion where filaments do not overlap
• *M line: line of protein myomesin that holds adjacent thick filaments together

• Composed of the protein myosin
• *Myosin tails contain: 2 interwoven, heavy polypeptide chains
• *Myosin heads contain:
• - 2 smaller, light polypeptide chains that act as cross bridges during contraction
• -Binding sites for actin of thin filaments
• -Binding sites for ATP
• -ATPase enzymes

• *Twisted double strand of fibrous protein F actin
• *F actin consists of G (globular) actin subunits
• *G actin bears active sites for myosin head attachment during contraction
• *Tropomyosin and troponin: regulatory proteins bound to actin

• *Network of smooth endoplasmic reticulum surrounding each myofibril
• *Pairs of terminal cisternae form perpendicular cross channels
• *Functions in the regulation of intracellular Ca2+ levels

• *Continuous with the sarcolemma
• *Penetrate the cell’s interior at each A band–I band junction
• *Associate with the paired terminal cisternae to form triads that encircle each sarcomere

• *T tubules conduct impulses deep into muscle fiber
• *Integral proteins protrude into the intermembrane space from T tubule and SR cisternae membranes
• *T tubule proteins: voltage sensors
• *SR foot proteins: gated channels that regulate Ca2+ release from the SR cisternae

• *The generation of force
• *Does not necessarily cause shortening of the fiber
• *Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening

*In the relaxed state, thin and thick filaments overlap only slightly

*During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line

*As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

1. Activation: neural stimulation at aneuromuscular junction

• 2. Excitation-contraction coupling:
• -Generation and propagation of an action potential along the sarcolemma

-Final trigger: a brief rise in intracellular Ca2+ levels

• *Skeletal muscles are stimulated by somatic motor neurons
• *Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles
• *Each axon forms several branches as it enters a muscle
• *Each axon ending forms a neuromuscular junction with a single muscle fiber

• 
• 1. Actionpotential arrives at the axon terminal of a motor neauron
• 2.Voltage Gated Calcium (Ca2) chammels open and the Ca2 enters th axon terminal
• 3.Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.
• 4.Acetylcholine, aneurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.
• 5.ACh binding opens ionchannels that allow simultaneous passage of Na+ into the musclefiber and K+ out of the muscle fiber.
• 6.ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.

*ACh effects are quickly terminated by the enzyme acetylcholinesterase

*Prevents continued muscle fiber contraction in the absence of additional stimulation

• 1.Local depolarization (end plate potential):
• -ACh binding opens chemically (ligand) gated ion channels
• -Simultaneous diffusion of Na+ (inward) and K+ (outward)
• -More Na+ diffuses, so the interior of the sarcolemma becomes less negative
• -Local depolarization – end plate potential
• 2.Generation and propagation of an action potential:
• End plate potential spreads to adjacent membrane areas
• Voltage-gated Na+ channels open
• Na+ influx decreases the membrane voltage toward a critical threshold
• If threshold is reached, an action potential is generated
• Local depolarization wave continues to spread, changing the permeability of the sarcolemma
• Voltage-regulated Na+ channels open in the adjacent patch, causing it to depolarize to threshold
• 3.Repolarization:
• Na+ channels close and voltage-gated K+ channels open
• K+ efflux rapidly restores the resting polarity
• Fiber cannot be stimulated and is in a refractory period until repolarization is complete
• Ionic conditions of the resting state are restored by the Na+-K+ pump

• At higher intracellular Ca2+ concentrations:
• Ca2+ binds to troponin Troponin
• changes shape and moves tropomyosin away from active sites
• Events of the cross bridge cycle occur
• When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends

• Continues as long as the Ca2+ signal and adequate ATP are present
• Cross bridge formation—high-energy myosin head attaches to thin filament
• Working (power) stroke—myosin head pivots and pulls thin filament toward M line
• Cross bridge detachment—ATP attaches to myosin head and the cross bridge detaches
• “Cocking” of the myosin head—energy from hydrolysis of ATP cocks the myosin head into the high-energy state

• 1.Contraction happens in single cells and in whole muscles
• 2.Contraction produces tension (force)
• 3.Contraction does not always shorten a muscle
• 4.Force and duration of contraction vary in response to stimuli of different frequencies and intensities

ATP is regenerated by:

• Direct phosphorylation of ADP by creatine phosphate (CP)
• Anaerobic pathway (glycolysis)
• Aerobic respiration

• White = Fast glycolytic cells = short fast activity
• Red = Slow oxidative cells= long slow activity
• (Pink= Fast oxidative cells)= long and fast!