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Muscle Overview
• 3 different types of muscle tissue provide movement:
– Skeletal
• attached to the bones of the skeleton
• controlled consciously (voluntary)
– Cardiac
• heart
• controlled unconsciously (involuntary)
– Smooth
• airways of the lungs, blood vessels, the digestive,
urinary, and reproductive tracts
• controlled unconsciously (involuntary)
Characteristics of Muscle Tissue
• Excitability, or irritability
– the ability to receive and respond to stimuli
• Conductivity
– the ability to create and conduct an action
potential along the cell membrane
• Contractility
– the ability to shorten forcibly through the hydrolysis
of ATP by contractile proteins
• Extensibility
– the ability to be stretched or extended
• Elasticity
– the ability to recoil after being stretched
Muscle Terminology
• Prefixes
– sarco- “flesh”
• sarcolemma = muscle plasma membrane
• sarcoplasm = cytoplasm of a muscle fiber (cell)
– my- “muscle”
• myocyte = muscle fiber
• epimysium = the sheath of connective tissue that
surrounds a skeletal muscle
Motor Unit: The Nerve-Muscle Functional Unit
• A skeletal fiber will contract only after it is excited
• A skeletal fiber is excited by the exocytosis of the
neurotransmitter acetylcholine from a motor neuron
at a synapse called the neuromuscular junction (NMJ)
– generates a graded potential which can lead to an
action potential in the fiber to trigger contraction
• A single motor neuron is capable of stimulating
multiple skeletal muscle fibers to contract
simultaneously
– one axon branches creating multiple axon termini
– the anatomical relationship between a motor neuron
and all skeletal fibers that it causes to contract is
called a motor unit
Motor Unit: The Nerve-Muscle Functional Unit
Motor Unit: The Nerve-Muscle Functional Unit
• The number of muscle fibers per motor unit can range:
– few (small motor unit)
• control fine movements (fingers, eyes)
– several hundred (large motor unit)
• control gross movements (arms, legs)
• large weight-bearing muscles (back)
Muscle Twitch
• The contraction followed by the relaxation of a muscle
fiber to a single, brief threshold stimulus by a motor
neuron is called a twitch
• There are three phases of a muscle twitch
– Latent (lag) period
• time between the stimulation by a motor neuron
and the beginning of contraction (few
milliseconds)
– Contractile period
• contractile proteins within the fiber hydrolyze ATP
causing the fiber to shorten resulting in an
increase in tension (force)
– Relaxation period
• fiber lengthens causing tension to decrease
Muscle Twitch
Contraction of Skeletal Muscle
• The two types of muscle contractions are:
– Isometric contraction = “same length”
• muscle contracts and produces tension, but does
not shorten
• trying to lift a car
– Isotonic contraction = “same tension”
• muscle contracts and produces tension
• shortens as it contracts
• lifting a pencil
Isometric Contractions
• Tension increases to the muscle’s capacity, but the
muscle neither shortens nor lengthens
• Occurs if the load is greater than the tension the
muscle is able to develop
Isotonic Contractions
• In isotonic contractions, the muscle changes in length
and moves the load
Types of Skeletal Muscle Fibers
• There are 3 different types skeletal muscle fibers
based on the duration of a twitch and the method of
ATP production
– slow oxidative fibers
– fast oxidative fibers
– fast glycolytic fibers
• Skeletal muscles of your body contain a combination
of all three fiber types, but their ratio determines the
overall function of that muscle
Oxidative vs. Glycolytic fibers
• Oxidative fibers contain greater amounts of
mitochondria compared to glycolytic fibers
• Oxidative fibers contain an oxygen-binding protein
called myoglobin to maintain a high concentration of
oxygen within the fiber for aerobic respiration
– similar in structure to the blood protein hemoglobin
– provides a red color to oxidative fibers
– a lack of myoglobin in glycolytic fibers results in a
white color
Characteristics of Skeletal Muscle Fiber Types
• Slow oxidative fibers:
– have a slow twitch (use ATP slowly)
– fatigue resistant
– muscle fibers used to maintain posture
• Fast oxidative fibers:
– have a fast twitch (use ATP quickly)
– moderate resistance to fatigue
– muscle fibers used for non-exertive movement
(walking)
• Fast glycolytic fibers:
– have a fast twitch (use ATP quickly)
– easily fatigued
– muscle fibers used for powerful movements
(jumping and sprinting)
Fatigue
• Weakening of contracting muscle caused by:
– the rate of ATP hydrolysis exceeds the rate of
synthesis
– lactic acid accumulation (↓ pH) inhibits muscle
protein function
– motor neurons run out of acetylcholine
Resistance to Fatigue
• Fibers that use ATP slowly
and have a high capacity to
synthesize ATP are resistant
to fatigue
• Fibers that use ATP quickly
and have a high capacity to
synthesize ATP have
moderate resistance to
fatigue
• Fibers that use ATP quickly
and have a low capacity to
synthesize ATP have no
resistance to fatigue
Variety of Muscle Responses
• Variations in the force of muscle contraction is
required for proper control of skeletal movement
– moving a pencil vs. a textbook with your hand uses
the same muscles, but requires a different amount
of force
• Skeletal muscle contractions are varied by:
– altering the number of muscle fibers that contract
• determined by the number of motor units that are
propagating action potentials to a muscle and
which muscle fiber types are contracting to
perform a particular task
– altering the frequency of muscle stimulation
• determined by the frequency of action potentials
traveling down a motor neuron arriving at a fiber
Muscle Response: Motor Unit Recruitment
• The first observable muscle contraction occurs
following a threshold stimulus
– activates one motor unit
• As stimulus strength is increased more motor units are
activated
– recruitment
• The maximum force that a muscle is capable of
generating is reached when all motor units are
activated
– an increase in stimulus intensity results in no further
increase in force generated
Stimulus
Intensity and
Muscle
Tension
Motor Unit
Recruitment
• Slow oxidative fibers are
first stimulated to
contract
– provide basal muscle
tension (tone)
• If additional muscle
tension is required, fast
oxidative fibers are
stimulated to contract
• Finally, the fast glycolytic
fibers are stimulated to
bring muscle tension to
maximum
Muscle Response: Stimulation Frequency
• Rapidly delivered stimuli result in the summation of
muscle twitches creating an incomplete (unfused)
tetanus (constant submaximal contractile force where
each twitch is visibly distinct)
– muscle tension does not return to baseline
• If stimuli are given quickly enough, complete (fused)
tetanus is observed where the contractile force
reaches a maximum, but individual twitches blended
together
ATP Sources During Muscle Contraction
• Resting muscle fibers synthesizes and stores enough
ATP (by cellular respiration) for 5 seconds of maximal
sustained contraction. After that the muscle must
make ATP in order to continue contraction
• During resting periods, skeletal muscle uses ATP that
it synthesizes to energize the amino acid derivative
creatine into creatine phosphate which can be stored
– during contraction creatine phosphate is converted
back into creatine as ADP is converted to ATP
• Glucose delivered to the muscle as well as stored
glycogen (once hydrolyzed) is used by the muscle for
additional ATP synthesis via glycolysis and oxidative
phosphorylation
3 Sources of ATP Formation in Skeletal Muscle
Monitoring of Muscle Length and Tension
• Within skeletal muscle are 2 sensory receptors that
monitor muscle length and tension
• Muscle spindles are modified muscle fibers called
intrafusal muscle fibers that are wrapped around by a
neuron which sends information to the brain/spinal
cord about the length of a muscle and the speed at
which the length changes during contraction or
stretching
– extrafusal fibers are those that contract to produce
tension and movement
• Golgi tendon organs are neurons that are wrapped
around the collagen fibers of a tendon near the
attachment to muscle which sends information about
the tension that a muscle produces during contraction
Muscle Spindles and
Golgi Tendon Organs
• Neurons associated with the
spindle will generate
additional or fewer APs
which propagate to the
brain/spinal cord when the
length of the muscle
(spindle) increases or
decreases, respectively
• Tension within a tendon (by
either contraction or passive
stretching) generates APs in
the neuron which propagate
to the brain/spinal cord
Muscle
Spindles
• A lengthened spindle generates more APs, a
shortened spindle generates fewer APs
• The brain/spinal cord interprets the change in the AP
frequency from the spindle as a change in length
Myotatic Reflex
• Reflex that causes the
contraction of a muscle
following an increase in
that muscles length
• APs from the lengthened
spindle synapse with
neurons in the spinal cord
causing:
– contraction of the
extensors (pathway A
and C)
– relaxation of the
opposing flexors
(pathway B
– sensory (pathway D) for
perception by the brain
Golgi Tendon Organ
• Tension within a
tendon generates
APs in the neuron
which propagate
to the brain/spinal
• The greater the
tension the higher
the frequency of
APs are generated
so the brain/spinal
cord can monitor
the amount of
stress in the
tendon
Golgi Tendon Reflex
• Protective reflex that
prevents over contraction
of a muscle resulting in
damage to the muscle,
tendon or bone
• Contraction of the extensor
muscle on the thigh
stretches the Golgi tendon
organ and generates APs
causing:
– inhibition of the motor
neurons that innervate
the extensor (A)
– excitation in the
opposing flexor’s motor
neurons (B)
Microscopic Anatomy of a Skeletal Muscle Fiber
• Each fiber is long (up to 30 cm) and cylindrical with
multiple nuclei just beneath the sarcolemma
– the sarcolemma contains both voltage-gated Na+
and K+ capable of generating an action potential
– portions of the sarcolemma called transverse (t) tubules fold inward toward the center of the fiber
• propagate APs to the center of the muscle cell
• Muscle fibers contain an elaborate, smooth
sarcoplasmic (endoplasmic) reticulum (SR)
– physically associated with the t-tubules
• storage site of intracellular calcium (Ca+2)
• An action potential in the t-tubules causes the release
of from the SR into the sarcoplasm which increases
the cytoplasmic level of Ca+2
• triggers the contraction of a muscle fiber
Microscopic Anatomy of a Skeletal Muscle Fiber
Contractile Proteins
• Occupying most of the space within the cell, long
filamentous contractile proteins are arranged in long
bundles called myofibrils
– composed of 2 types of contractile proteins
(myofilaments) that overlap and slide past one
another during contraction and relaxation
• “thin”
• “thick”
Structure of Thin Filaments
• Thin filaments are composed of 3 proteins
– F (fibrous) Actin is a helical polymer of G
(globular) actin protein subunits
• each subunit contains a binding site for the
protein myosin of the thick filaments
– Tropomyosin blocks the interaction between actin
and myosin
• prevents an unstimulated muscle from
contracting
– Troponin C is attached to tropomyosin
• binds to Ca2+ in the sarcoplasm during
contraction
Structure of Thin Filaments
Structure of Thick Filaments
• Thick filaments are composed of many molecules of
the protein myosin
• Each myosin protein has a rodlike tail and two heads
– Myosin heads:
• hydrolyze a molecule of ATP
–uses the chemical energy to contract
• Temporarily bind to actin
• pull on actin causing the shortening sarcomere
Structure of Thick Filaments
Arrangement of the Filaments in a Sarcomere
Striations of Skeletal Muscle
• The overlapping arrangement of myofilaments creates
a repeating pattern of striations (stripes) called
sarcomeres when viewed longitudinally
Segments of a Sarcomere
• Z disc
– constitutes the end of a sarcomere
– anchors the thin filaments
• A band
– the length of the thick filaments
• I band
– the length of thin filaments within a sarcomere that
is not overlapping with the thick filaments
• H (bare) zone
– the length of thick filaments within in a sarcomere
that is not overlapping with the thin filaments
• During contraction, the thin and thick filaments slide
past one another as the sarcomere shortens
Sarcomeres
Sliding Filament Model of Contraction
• In the relaxed state,
thin and thick filaments
overlap only slightly
• Upon stimulation, the
thick filaments pull the
thin filaments toward
the center of the
sarcomere
– filaments overlap to
a greater degree
– shortening the
sarcomere
• As all of the
sarcomeres in a
muscle shortens, the
entire muscle shortens
Skeletal Muscle Contraction
• In order to contract, a skeletal muscle must be
stimulated by a motor neuron
– generates an action potential in the muscle fiber
• causes an increase in the amount of
cytoplasmic Ca2+
–causes the muscle fiber to contract
• Linking the action potential to the contraction of a
muscle fiber is called excitation-contraction coupling
Neuromuscular Junction
• The axon termini have synaptic vesicles that contain
the neurotransmitter acetylcholine (ACh)
• ACh receptors (ligand-gated Na+ channels) are
localized to a portion of the sarcolemma called the
motor end plate
Neuromuscular Junction
Neuromuscular Junction
NMJ Function
Excitation-Contraction Coupling
• Binding of ACh to its receptors opens the channel and
allows both Na+ and K+ to diffuse
– diffusion of more Na+ than K+ causes the membrane
potential to depolarize (endplate potential)
Excitation-Contraction Coupling
• The endplate potential brings the membrane potential
to threshold
– opens voltage-gated Na+ and K+ channels to
generate an action potential in the sarcolemma
Excitation-Contraction Coupling
• Action potentials propagate along the sarcolemma into
the t-tubules
– action potentials in the t-tubules cause the release
of Ca2+ from the SR into the sarcoplasm
Excitation-Contraction Coupling
• Ca2+ in the sarcoplasm binds to troponin C
– changes the position of troponin C
• moves tropomyosin away from the myosin
binding site on actin
Events of Contraction (Cross bridge cycling)
• Muscle fiber shortening occurs as myosin pulls on
actin in a repetitive ratcheting fashion
• Thin filaments move toward the center of the
sarcomere
– Activation of the myosin head
• a molecule of ATP is hydrolyzed and the energy
is used by the myosin head to change the shape
of myosin into the high-energy state
– Cross bridge formation
• myosin cross bridge attaches to actin filament
– Power stroke
• myosin head pivots and pulls thin filament over
thick filament
– Cross bridge detachment
• The binding of a molecule of ATP to the myosin
head causes it to detach from actin
Events of Contraction
(Cross Bridge Cycling)
Muscle Fiber Relaxation
• The motor neuron stops the exocytosis of ACh
• The remaining ACh is hydrolyzed into acetate and
choline by the enzyme Acetylcholine esterase located
in the synaptic cleft of the NMJ
– ACh receptors close
• membrane potential returns to resting value
Muscle Fiber Relaxation
• Ca2+ is pumped back into the SR by Ca2+-ATPase in
the SR membrane
– decreases Ca2+ in the sarcoplasm
• troponin C moves back to resting position
–Tropomyosin recovers the binding site for
myosin on G actin