Muscle Tissue
Excitability (responsiveness)—to chemical signals, stretch, and electrical changes across the 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 rest length after being stretched
Skeletal muscle—voluntary, striated muscle usually attached to bones
Striations—alternating light and dark transverse bands
Voluntary—usually subject to conscious control Muscle cell is a muscle fiber (myofiber)—as long as 30 cm
Endomysium: connective tissue around muscle cell
Perimysium: connective tissue around muscle fascicle
Epimysium: connective tissue surrounding entire muscle
Tendons- are attachments between muscle and bone matrix
Collagen- is somewhat extensible and elastic
Sarcolemma—plasma membrane of a muscle fiber
Sarcoplasm—cytoplasm of a muscle fiber
Myofibrils: long protein cords occupying most of sarcoplasm
Glycogen: carbohydrate stored to provide energy for exercise
Myoglobin: red pigment; provides some oxygen needed for muscle activity
Myoblasts: stem cells that fused to form each muscle fiber early in development
Satellite cells: unspecialized myoblasts remaining between the muscle fiber and endomysium
Sarcoplasmic reticulum (SR)—smooth ER that forms a network around each myofibril:
Terminal cisterns—dilated end-sacs of SR which cross the muscle fiber from one side to the other
T tubules—tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side
Triad—a T tubule and two terminal cisterns associated with it
Thick filaments—made of several hundred myosin molecules
Two chains intertwined to form a shaft-like tail
• Double globular head
• Heads directed outward in a helical array around the bundle
• Heads on one half of the thick filament angle to the left, while heads on other half angle to the right
• Bare zone with no heads in the middle
Thin filaments- composed primarily of two intertwined strands of a protein called fibrous actin
Globular actin- subunits of fibrous actin
Tropomyosin molecules- each blocking six or seven active sites
Troponin molecule: small, calcium-binding protein on each tropomyosin molecule
Elastic filaments- made of huge springy protein called titin
- Help stabilize and position the thick filament
- Prevent over stretching
Contractile proteins—myosin and actin do the work of contraction
Regulatory proteins—tropomyosin and troponin
- Determines when fiber can and cannot contract
Dystrophin—clinically important protein
- Links actin in outermost myofilaments to membrane proteins that link to endomysium
- Transfers forces of muscle contraction to connective tissue ultimately leading to tendon
- Genetic defects in dystrophin produce disabling disease muscular dystrophy
A band: dark; “A” stands for anisotropic
• Darkest part is where thick filaments overlap a hexagonal array of
thin filaments
• H band: not as dark; middle of A band; thick filaments only
• M line: middle of H band
• I band: light; “I” stands for isotropic
• The way the bands reflect polarized light
• Z disc: provides anchorage for thin filaments and elastic filaments
Sarcomere—segment from Z disc to Z disc
- Z disc (Z lines) are pulled closer together as thick and thin filaments slide past each other
Denervation atrophy: shrinkage of paralyzed muscle when nerve remains disconnected
Somatic motor neurons
• Nerve cells whose cell bodies are in the brainstem and spinal cord that serve skeletal muscles
• Somatic motor fibers—their axons that lead to the skeletal muscle
• Each nerve fiber branches out to a number of muscle fibers
• Each muscle fiber is supplied by only one motor neuron
Motor unit—one nerve fiber and all the muscle fibers innervated by it
Muscle fibers of one motor unit
• Dispersed throughout muscle
• Contract in unison
• Produce weak contraction over wide area
• Provide ability to sustain long-term contraction as motor units take turns contracting
• Effective contraction usually requires contraction of several motor units at once
Small motor units—fine degree of control
• Three to six muscle fibers per neuron
• Eye and hand muscles
Large motor units—more strength than control
• Powerful contractions supplied by large motor units with hundreds of fibers
• Quadriceps femoris and gastrocnemius have 1,000 muscle fibers per neuron
Synapse—point where a nerve fiber meets its target cell
Neuromuscular junction (NMJ)—when target cell is a muscle fiber
• Axon terminal—swollen end of nerve fiber
• Contains synaptic vesicles with acetylcholine (ACh)
• Synaptic cleft—gap between axon terminal and sarcolemma
Basal lamina—thin layer of collagen and glycoprotein separating Schwann cell and muscle cell from surrounding tissues
Neuromuscular Junction: Nerve impulse causes synaptic vesicles to undergo exocytosis releasing ACh into synaptic cleft, Muscle cell has millions of Ach receptors—proteins incorporated into its membrane
Electrophysiology—the study of the electrical activity of cells
Voltage (electrical potential)—a difference in electrical charge from one point to another
Resting membrane potential—about −90 mV in skeletal muscle cells
• Maintained by sodium–potassium pump
In an unstimulated (resting) cell:
• There are more anions (negatively charged particles) on the inside of the membrane than on the outside
• These anions make the inside of the plasma membrane negatively charged by comparison to its outer surface
• The plasma membrane is electrically polarized (charged) with a negative resting membrane potential (RMP)
• There are excess sodium ions (Na+) in the extracellular fluid (ECF)
• There are excess potassium ions (K+) in the intracellular fluid (ICF)
In a stimulated (active) muscle fiber or nerve cell:
• Na+ ion gates open in the plasma membrane
• Na+ flows into cell down its electrochemical gradient
• These cations override the negative charges in the ICF
• Depolarization: inside of plasma membrane becomes positive
• Immediately, Na+ gates close and K+ gates open
• K+ rushes out of cell partly repelled by positive sodium charge and partly because of its concentration gradient
• Loss of positive potassium ions turns the membrane negative again (repolarization)
• This quick up-and-down voltage shift (depolarization and repolarization) is called an action potential
Tetanus (lockjaw) is a form of spastic paralysis caused by toxin Clostridium tetani
Glycine in the spinal cord normally stops motor neurons from producing unwanted muscle contractions
Spastic paralysis: a state of continual contraction of the muscles; possible suffocation
Flaccid paralysis—a state in which the muscles are limp and cannot contract
Curare: competes with ACh for receptor sites, but does not stimulate the muscles
Botulism—type of food poisoning caused by a neuromuscular toxin secreted by the bacterium Clostridium botulinum
Excitation: a process in which nerve action potentials lead to muscle action potentials
Excitation–contraction coupling: events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract
Contraction: the step in which the muscle fiber develops tension and may shorten
Relaxation: when stimulation ends, a muscle fiber relaxes and returns to its resting length
Length–tension relationship—the amount of tension generated by a muscle depends on how stretched or shortened it was before it was stimulated
Rigor mortis—hardening of muscles and stiffening of body beginning 3–4 hr after death
Myogram—a chart of the timing and strength of a muscle’s contraction
Threshold—minimum voltage necessary to generate an action potential in the muscle fiber and produce a contraction
Twitch—a quick cycle of contraction and relaxation when stimulus is at threshold or higher
Latent period—very brief delay between stimulus and contraction
Contraction phase—time when muscle generates external tension
Relaxation phase—time when tension declines to baseline
With subthreshold stimuli—no contraction at all
At threshold intensity and above—twitch produced
Even if the same voltage is delivered, different stimuli cause twitches varying in strength, because:
• The muscle’s starting length influences tension generation
• Muscles fatigue after continual use
• Warmer muscles’ enzymes work more quickly
• Muscle cell’s hydration level influences cross-bridge formation
• Increasing the frequency of stimulus delivery increases tension output
Stimulating the nerve with higher voltages produces stronger contractions
• Higher voltages excite more nerve fibers which stimulate more motor units to contract
• Recruitment or multiple motor unit (MMU) summation—the process of bringing more motor units into play with stronger stimuli
• Occurs according to the size principle: weak stimuli (low voltage) recruit small units, while strong stimuli recruit small and large units for powerful movements
Low frequency stimuli produce identical twitches
Higher frequency stimuli (e.g., 20 stimuli/s) produce temporal (wave) summation
Isometric muscle contraction
• Muscle produces internal tension but external resistance causes it to stay the same length
• Can be a prelude to movement when tension is absorbed by elastic component of muscle
• Important in postural muscle function and antagonistic muscle joint stabilization
Isotonic muscle contraction
• Muscle changes in length with no change in tension
• Concentric contraction: muscle shortens as it maintains tension (example: lifting weight)
• Eccentric contraction: muscle lengthens as it maintains tension (example: slowly lowering weight)
At the beginning of contraction—isometric phase
Muscle begins to shorten and move the load—isotonic phase
Anaerobic fermentation—enables cells to produce ATP in the absence of oxygen; yields little ATP and lactate, which needs to be disposed of by the liver
Aerobic respiration—produces far more ATP; does not generate lactate; requires a continual supply of oxygen
Short, intense exercise (100 m dash)
• Oxygen is briefly supplied by myoglobin but is rapidly depleted
• Muscles meet most ATP demand by borrowing phosphate groups (Pi) from other molecules and transferring them to ADP
• Two enzyme systems control these phosphate transfers
• Myokinase: transfers Pi from one ADP to another, converting the latter to ATP
• Creatine kinase: obtains Pi from a phosphate-storage molecule creatine phosphate (CP) and gives it to ADP
• Phosphagen system—the combination of ATP and CP which provides nearly all energy for short bursts of activity
• Enough energy for 6 s of sprinting
As the phosphagen system is exhausted, muscles shift to anaerobic fermentation
• Muscles obtain glucose from blood and their own stored glycogen
• In the absence of oxygen, glycolysis can generate a net gain of 2 ATP for every glucose molecule consumed
• Converts glucose to lactate
Anaerobic threshold (lactate threshold)—point at which lactate becomes detectable in the blood
Glycogen–lactate system—the pathway from glycogen to lactate
Produces enough ATP for 30–40 s of maximum activity
After about 40 s, the respiratory and cardiovascular systems start to deliver oxygen fast enough for aerobic respiration to meet most of muscle’s ATP demand
Aerobic respiration produces more ATP per glucose than glycolysis does (another 30 ATP per glucose)
• Efficient means of meeting the ATP demands of prolonged exercise
• After 3–4 min, the rate of oxygen consumption levels off to a steady state where aerobic ATP production keeps pace with demand
• For 30 min energy comes equally from glucose and fatty acids
• Beyond 30 min, depletion of glucose causes fatty acids to become the more significant fuel
Muscle fatigue—progressive weakness from prolonged use of muscles
Fatigue in high-intensity exercise is thought to result from:
• Potassium accumulation in the T tubules reduces excitability
• Excess ADP and slow cross-bridge movements, inhibit calcium release and decrease force production in myofibrils
Fatigue in low-intensity (long duration) exercise is thought to result from:
• Fuel depletion as glycogen and glucose levels decline
• Electrolyte loss through sweat can decrease muscle excitability
• Central fatigue when less motor signals are issued from brain
• Brain cells inhibited by exercising muscles’ release of ammonia
• Psychological will to persevere—not well understood
VO2 max: the point at which the rate of oxygen consumption plateaus and does not increase further with added workload
EPOC meets a metabolic demand also known as oxygen debt
It is the difference between the elevated rate of oxygen consumption following exercise and the usual resting rate