OVERVIEW OF MUSCLE TISSUE
• Skeletal muscle tissue
– attached to bones
– striated and voluntary
• Cardiac muscle tissue
– forms wall of heart
– striated and involuntary
• Smooth (visceral) muscle tissue
– walls of hollow organs, blood vessels
– nonstriated in appearance
– involuntary
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Functions of Muscle Tissue
1) Produce body movements
2) Stabilize body positions
3) Storage/movement of substances within the body
– bands of smooth muscle called sphincters
– blood, lymph, urine, air, food and fluids, sperm
4) Heat production
– involuntary contractions of skeletal muscle (shivering)
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Properties of Muscle Tissue
• Excitability = ability to generate electrical signals (a.p.’s)
– in response to autorhythmic electrical signal in heart
– in response to chemicals released from nerve cells (NTs)
• Contractility
– ability to contract forcefully when stimulated by act. pot.
• Extensibility
– ability to be stretched
– can forcefully contract even when stretched
• Elasticity = ability to return to original shape after being
stretched
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Skeletal Muscle: CT components
• Superficial fascia= loose C.T. underlying the sub-Q layer of skin
• Deep fascia = dense irregular C.T. around muscle
• C.T. components of the muscle include
– epimysium = surrounds whole muscle
– perimysium = surrounds bundles (fascicles) of 10-100
muscle cells
– endomysium = separates individual muscle cells
• All these connective tissue layers extend beyond the muscle
belly to form the tendon
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Nerve and Blood Supply
• Well-vascularized & highly innervated
• Somatic motor neurons
– axon extends from brain to group of fibers
– axon branches @ muscle, each branch supplies diff. fiber
• Each muscle cell supplied by at least one capillary
– bring in oxygen/nutrients
– remove wastes
• Nerve fibers & capillaries are found in the endomysium
between individual cells
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Microscopic Anatomy of Skeletal Muscle
• Muscle fibers = muscle cells
• Sarcolemma = plasma membrane of muscle cell
• T tubules = tiny in-folds of the sarcolemma
– tunnel from surface of cell to center
– open to outside of fiber  filled with interstitial fluid
– allows muscle action potential to propagate to all
parts of muscle fiber simultaneously
• Sarcoplasm = muscle cell cytoplasm
– contains a large amount of glycogen for energy production
– myoglobin binds oxygen for storage
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Microscopic Anatomy of Skeletal Muscle
• Sarcoplasmic reticulum
– fluid-filled system of membranous sacs
– similar to endoplasmic reticulum
– encircles each myofibril
– terminal cisternae store Ca+2 ions in relaxed muscle
– release of Ca+2 ions triggers muscle contraction
– intersection w/ T-tubule forms triad
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Microscopic Anatomy of Skeletal Muscle
• Myofibrils = contractile organelles of muscle fibers
– extend entire length of muscle fiber
– dark & light bands give striated appearance
– sarcomere = functional unit of myofibril
• contain myofilaments which do not extend entire
length of cell
• defined as the region btwn two Z discs
– thin filaments
• make up I band of sarcomere
• actin protein
• anchored together by Z disc in middle of I band
• slide relative to myosin during muscle contraction
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Microscopic Anatomy of Skeletal Muscle
• Myofibrils, ctd.
– thick filaments
• myosin protein
• “A” band of sarcomere  dark-staining
– “zone of overlap” = ends of A bands where
thick/thin filaments overlap
• H zone of sarcomere = region of only thick filaments
– middle of A band where thin filaments do not reach
– M line in center anchors myosin vertically
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Filaments and the Sarcomere
• Myofilaments do not extend entire length of muscle fiber 
arranged into sarcomeres
• Sarcomere = functional unit of myofibril
• Thick and thin filaments overlap each other in a pattern that
creates striations (light I bands and dark A bands)
• I band region contains only thin filaments
• Separated by Z discs
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Muscle Proteins
• Contractile proteins generate force during muscle contraction
• myosin = motor protein
– push/pull cell structures to produce movement
• actin = main component of thin filaments
– myosin binding site on each molecule
• Regulatory proteins
– tropomyosin
• covers myosin binding sites in relaxed muscle
• prevents myosin binding in absence of Ca+2 ions
– troponin holds tropomyosin in place
• binds calcium
• moves tropomyosin & exposes myosin binding sites
• Structural proteins contribute to proper alignment, elasticity and
extensibility
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Sliding Filament Mechanism Of Contraction
• Myosin cross bridges pull on thin filaments
• Thin filaments slide toward M-line
• Z Discs come toward each other
• ***H & I bands narrow***
• Sarcomeres shorten at same time muscle fiber shortens
muscle shortens
 Notice: Thick/thin filaments do not change in
length!!
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How Does Contraction Begin?
1. Nerve impulse reaches axon terminal & synaptic vesicles
release acetylcholine (ACh)
2. ACh diffuses across synaptic cleft & binds to receptors on
the sarcolemma; Na+ channels open& Na+ rushes into cell
3. A muscle action potential spreads over sarcolemma and
down into the transverse tubules
4. SR releases Ca+2 into the sarcoplasm
5. Ca+2 binds to troponin & causes troponin-tropomyosin
complex to move & reveal myosin binding sites on actin
contraction cycle begins
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Contraction Cycle
• Once Ca+2 is released from terminal cisternae & has bound
troponin, myosin binding sites on actin are available for
crossbridge formation
• 4 steps to contraction cycle
– ATP hydrolysis  puts myosin in hi-energy position
– attachment of myosin to actin to form crossbridges
– Release of ADP & Pi causes conformational change in
myosin which pulls thin filaments toward center of
sarcomere  this is the power stroke
– detachment of myosin from actin when new ATP binds 
mysoin returns to hi-energy conformation (this is position of
myosin in resting muscle)
• Cycle repeats as long as ATP is available & [Ca+2] is high
near the filaments
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Steps in the Contraction Cycle
• Refer to figure 10.7
– Notice how myosin head attaches & pulls on thin filament
with energy released from ATP
Excitation - Contraction Coupling
• All the steps that occur from the muscle action potential
reaching the T tubule to contraction of the muscle fiber
• Refer to figure 10.8
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Relaxation
• Acetylcholinesterase (AChE) breaks down ACh @ synaptic cleft
• Muscle action potentials cease
• Ca+2-release channels close
• Ca+2 actively resequestered by SR/ Ca+2-ATPase pump
– ↓ in sarcoplasmic Ca+2 returns troponin-tropomyosin to
original conformation
– myosin binding sites unavailable
• Contraction stops but myosin heads remain in high-energy
conformation, ready for contraction cycle to begin again
***Remember: ATP is required to detach crossbridges, so
resting muscle is in energized state***
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Length-Tension Relationship
• Fiber length @ onset of contration influenes contraction
strength
• Maximal tension develops when fiber contracts @ + 30% of
its optimal length
– fibers shorter/longer than optimal length result in decreased
force of contraction
– too long: no overlap between myosin/actin  no Xbridge
formation
– too short: too much overlap, thick filaments compressed by
Z lines  fewer interactions btwn myson/actin
• Figure 10.9 illustrates this relationship
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Structures of NMJ Region
• NMJ = synapse btwn somatic motor neuron (SMN) & skeletal
muscle fiber
– synapse = area of communication btwn neuron & another
cell (in this case, a muscle fiber)
– synaptic cleft = gap separating neuron & muscle cell
– neurotransmitter (NT) = chem released by neuron into cleft
which activates the muscle fiber
– axon terminal = end of motor neuron
• synaptic end bulbs are swellings axon terminal
• contain synaptic vesicles filled w/ ACh
– motor end plate = region of sarcolemma opposite axon
terminal
• contains ACh receptors/cation channels
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Generation of Muscle Action Potential
1. Arrival of nerve impulse at nerve terminal causes release of
ACh from synaptic vesicles
2. ACh binds to receptors on motor end plate & opens gated ion
channels  Na+ rushes into muscle cell
3. Inside of muscle cell becomes more positive, triggering a
muscle action potential that travels over the cell and down the
T tubules
4. The release of Ca+2 from the SR is triggered and the muscle
cell will shorten & generate force
5. Acetylcholinesterase breaks down the ACh attached to the
receptors on the motor end plate so the muscle action
potential will cease and the muscle cell will relax.
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NMJ Antagonists
• Botulinum toxin (C. botulinim)
– blocks exocytosis of ACh  muscle cannot contract
– causes flaccid paralysis
– death occurs from paralysis of the diaphragm
• Curare (plant poison from poison arrows)
– binds ACh receptors but does not activate them
– causes flaccid paralysis
– used to relax skeletal muscle during surgery because does
not affect heart muscle
• Black widow spider venom
– causes massive ACh release
– spastic paralysis results  exaggerated reflexes, ↑ tone
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Muscle Metabolism
Production of ATP in Muscle Fibers
• Active muscle has high demand for ATP
– required for contraction cycle
– required for calcium uptake by SR
– required for other metabolic reactions
• Sarcoplasmic ATP only lasts for few seconds
• 3 sources of ATP production within muscle
– creatine phosphate
– anaerobic cellular respiration
– aerobic cellular respiration
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Creatine Phosphate
• Hi-energy compound found only in muscle
• [Creatine phosphate] 3-6 times greater than [ATP] in muscle
• Excess ATP within resting muscle used to form creatine
phosphate
– ATP + Cr  ADP + Cr—P (at rest)
• As ATP is used during exercise, [ADP] ↑  energy transferred
from Cr—P to ADP
– ADP + Cr—P  ATP + Cr (in active muscle)
• Sustains short, powerful bursts of maximal contraction
– lasts about 15 sec
– as in 100 meter dash
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Anaerobic Cellular Respiration
• Generates ATP w/o requiring O2
• When Cr—P used up, glucose broken down to form ATP
– glycolysis yields 2 net ATP
– pyruvate converted to lactic acid instead of entering aerobic
metabolic cycles
• Can provide ATP for 30 to 40 sec of maximal activity (200
meter race)
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Aerobic Cellular Respiration
• Series of reactions requiring O2 to generate ATP
• If sufficient O2 is available, glc completely oxidized to yield 36 or
38 ATP
– pyruvate enters mitochondria rather than being converted to
lactate
• 2 sources of oxygen
– myoglobin
– diffusion from blood
• Provides ATP for prolonged activity if O2 & nutrients are
available
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Muscle Fatigue
• Inability to contract after prolonged activity
• Factors that contribute to fatigue:
– central (psychological) fatigue = feeling of tiredness &
desire to stop physical activity
– neuromuscular fatigue = insufficient release of ACh from
motor neurons
– muscular fatigue:
• depletion of creatine phosphate, glycogen
• inadequate release of Ca+2 from SR
• insufficient oxygen
• lactate accumulation
• Oxygen debt = amount of O2 required to replenish ATP/Cr—P
used during exercise
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The Motor Unit
• Motor unit = one somatic motor neuron & all skeletal muscle
cells (fibers) it stimulates
• Muscle fibers of a unit normally scattered throughout belly of
muscle (not all in one location)
• All fibers within one unit contract in unison
• Total strength of a contraction depends on # motor units
activated & size of motor units
– precise movements require large # of small motor units
(few fibers/unit)
– large/powerful movements involve large motor units (many
fibers/unit)
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Twitch Contraction
• Brief contraction of all fibers in a motor unit in response to
single action potential in its motor neuron
• Latent period = delay btwn stimulus & onset of contraction
– Muscle a.p. moving along sarcolemma & Ca+2 is released
from SR
• Contraction period = period where crossbridges form and peak
muscle tension develops
• Relaxation period = time during which Ca+2 is actively taken up
by SR pump
• Refractory period = period when muscle cannot contract even if
it receives an additional stimulus
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Frequency of Stimulation
• Wave summation = repeated stimuli arriving @ different times,
(but before muscle has completely relaxed from previous
stimulus) produces an additive effect
– results in stronger contraction
• Incomplete (unfused) tetanus = sustained muscle contraction
that permits partial relaxation between stimuli
– stimulation rate ~20-30x/sec
• Complete (fused) tetanus = sustained contraction that lacks
partial relaxation between stimuli
– stimulation rate ~ 80-100x/sec
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Motor Unit Recruitment
• Process in which # of active motor units is increased
• Generally, motor units in a whole muscle fire asynchronously
– some fibers are active & others are relaxed
• Asynchronous contraction delays muscle fatigue  allows
for sustained contraction
• Produces smooth muscular contraction
– not series of jerky movements
• Weak motor units are recruited first
– then progressively larger units recruited as needed
– size of unit recruited dependent upon amt of force needed
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TYPES OF SKELETAL MUSCLE FIBERS
• Slow oxidative fibers
– Least powerful of three fiber types
– Large amounts of myoglobin  dark in color
– Generate ATP primarily by aerobic cellular respiration
• have many large mitochondria for this function
– Highest resistance to fatigue of all fiber types  adapted
for endurance activities
• Fast oxidative-glycolytic fibers
– Lots of myoglobin, but less than slow oxidative fibers
– Generate ATP via aerobic respiration
– High glycogen content  can also generate fair amt of
ATP via anaerobic means
– Fairly fatigue resistant
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TYPES OF SKELETAL MUSCLE FIBERS
• Fast glycolytic fibers
– Generate quickest, most powerful contractions of all fiber
types
– Low myoglobin content  appear white
– Lots of glycogen  primarily generate ATP via anaerobic
means
– Adapted for short, powerful movements  weight lifting,
throwing a ball, sprinting, etc.
– Fatigue quickly
• Relative ratios of fiber types are genetically determined
• All muscles contain the three fiber types, but distribution of
fibers dependent upon particular function/need of the muscle
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CARDIAC MUSCLE TISSUE
• Found only in the heart wall
– fibers arranged similarly to skeletal muscle fibers but are
branched rather than parallel
– fibers connect to adjacent fibers by intercalated discs
• desmosomes prevent cell separation during contraction
• gap junctions allow a.p. to spread quickly
– contractions last longer than skeletal muscle twitch due to
prolonged delivery of calcium ions from sarcoplasmic
reticulum and the extracellular fluid.
• Cardiac muscle fibers contract when stimulated by their own
autorhythmic fibers.
– This continuous, rhythmic activity is a major physiological
difference between cardiac and skeletal muscle tissue.
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SMOOTH MUSCLE
• Nonstriated, involuntary & autorhythmic
• Two types
– Visceral (single unit) smooth muscle
• found in walls of hollow viscera & small blood vessels;
• fibers work as single unit, an a.p. stimulates all fibers to
contract at once
– Multiunit smooth muscle
• found in large blood vessels, large airways, arrector pili
muscles, eye muscles
• each fiber has own neuron
• fibers operate singly rather than as one unit
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Microscopic Anatomy of Smooth Muscle
• One centrally located nucleus
• Thick and thin filaments are not organized into sarcomeres
• Contains intermediate filaments attached to dense bodies
• Small SR, but no T-tubules
• Caveolae = structures containing XC Ca+2 for use by sm. musc.
• Lengthwise shortening of fiber results from dense bodies pulling
on filament
– Twists into helix during contraction
– Reverse motion during relaxation
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Physiology of Smooth Muscle
• Contraction starts slowly & lasts longer
– no transverse tubules & very little SR
– Ca+2 must flow in from outside
• Can contract/stretch to greater extent than skeletal muscle
• Regulatory protein = calmodulin
– binds Ca in cytosol  facilitates myosin-actin binding
• Smooth muscle tone = state of c’td partial contraction
– result of prolonged presence of calcium ions
• Stress-relaxation response: smooth muscle can change in
length without generating tension
– retains contractile ability while stretched
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