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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)
• Prefixes
– sarco- “flesh”
• sarcoplasm = cytoplasm of a muscle cell (fiber)
– my- “muscle”
• myocyte = muscle fiber
Characteristics of Muscle Tissue
• Irritability
– the ability to receive and respond to stimuli
• Conductivity
– the ability to conduct an electrical impulse called an
action potential along the cell membrane
• an action potential (AP) is caused by the diffusion
of ions (typically Na+ and K+) across the cell
membrane through opened gated ion channels
• Contractility
– the ability to shorten forcibly
• Extensibility
– the ability to be stretched or extended
• Elasticity
– the ability to recoil after being stretched
Microscopic Anatomy of a Skeletal Muscle Fiber
• Each fiber is long (up to 30 cm) and cylindrical with
multiple nuclei just beneath the cell membrane
– the cell membrane contains ion channels capable of
generating an action potential
– portions of the cell membrane called transverse (t) tubules fold inward toward the center of the fiber
• Occupying most of the space within the cell are long
filamentous contractile proteins that are organized into
bundles called myofibrils
– each myofibril is composed of 2 types of proteins
(myofilaments) that overlap and slide past one
another during contraction and relaxation
• “thin”
• “thick”
Microscopic Anatomy of a Skeletal Muscle Fiber
Striations of Skeletal Muscle
• When viewed longitudinally, the overlapping
arrangement of myofilaments creates a repeating
pattern of dark and light striations (stripes) called
sarcomeres
– the contractile unit of skeletal (and cardiac) muscle
Sarcomeres
Segments of a Sarcomere
• Z disc
– a protein that creates a thin, dark vertical line in the
middle of a light vertical band
– the distance between successive Z discs represents
the length of a single sarcomere
– anchors the thin filaments during contraction
• 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
Motor Unit: The Nerve-Muscle Functional Unit
• In order to contract, a skeletal muscle must be
stimulated by a motor neuron
• The location where the end of a motor neuron and a
skeletal muscle fiber meet is called the neuromuscular
junction (NMJ)
• A single motor neuron is capable of stimulating
multiple skeletal muscle fibers to contract
simultaneously
– one neuron branches allowing it to stimulate
multiple muscle fibers simultaneously
– the anatomical relationship between a single motor
neuron and all skeletal fibers that it controls 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, precise 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 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
The Neuromuscular Junction
• Between the motor neuron and the skeletal muscle
fiber is a small space called a symaptic cleft
• A motor neuron stimulates the contraction of a skeletal
muscle fiber by exocytosing a chemical messenger
called a neurotransmitter into the synaptic cleft
• The specific neurotransmitter released onto skeletal
muscle fibers is called acetylcholine (ACh)
• Acetylcholine diffuses through the ECF within the cleft
and binds to integral membrane proteins of the
skeletal muscle fiber called ACh receptors
• The binding of ACh to ACh receptors creates an
action potential in the cell membrane of the skeletal
muscle fiber which will ultimately cause the cell to elicit
a twitch
• Linking the action potential to the contraction of a
muscle fiber is called excitation-contraction coupling
NMJ Function
Muscle Fiber Relaxation
• After ACh creates an action potential in the fiber it is
rapidly hydrolyzed into acetate and choline by the
enzyme Acetylcholine esterase located in the synaptic
cleft of the NMJ
– prevents prolonged stimulation (and contraction) of
a skeletal muscle fiber allowing it to relax
• Skeletal muscle fibers contain an elaborate, smooth
sarcoplasmic (endoplasmic) reticulum (SR) which is
the storage site of intracellular calcium (Ca+2)
• Action potentials travel along the sarcolemma into the
t-tubules which open Ca2+ channels in the SR to open
resulting in the diffusion of Ca2+ out of the SR into the
sarcoplasm
Sliding Filament Model of Contraction
• An increase in the
amount of Ca2+ in the
sarcoplasm, allows the
thick filaments to pull
the thin filaments
toward the center of
the sarcomere causing
the sarcomere to
shorten
• As all of the
sarcomeres in a
muscle shortens, the
entire muscle shortens
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
• attach to and pull on the protein actin of thin
filaments causing the sarcomere to shorten
Structure of Thick Filaments
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
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 promoting the interaction
between myosin and actin (CONTRACTION)
• Linking the action potential to the contraction of a
muscle fiber is called excitation-contraction coupling
Molecular Events of Contraction
•
Myosin pulls on actin in a repetitive (cyclic) fashion
progressively moving the thin filaments toward the
center of the sarcomere
• Each cycle consists of 4 steps
1. 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
2. Cross bridge formation
• myosin cross bridge attaches to actin filament
3. Power stroke
• myosin head pivots and pulls thin filament
4. Cross bridge detachment
• the binding of a molecule of ATP to the myosin
head causes it to detach from actin
(Cross Bridge Cycling)
Muscle Fiber Relaxation
• Within the membrane of the SR is a primary active
transporting pump called the Ca2+-ATPase
• The Ca2+-ATPase constantly pumps Ca2+ out of the
sarcoplasm into the SR
• During an action potential, Ca2+ diffuses into the
sarcoplasm faster than the Ca2+-ATPase can remove
it. However, when the action potential is over the
Ca2+-ATPase pumps the Ca2+ back into the SR ending
contraction
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
• Isometric contraction = “same length”
– muscle contracts and produces tension, but the
muscle but does not shorten or lengthen
Isotonic Contractions
• Isotonic contraction = “same tension”
– muscle contracts and produces tension
– shortens as it contracts, but maintains a constant
tension as it shortens
Types of Skeletal Muscle Fibers
• There are 3 different types skeletal muscle fibers
– slow oxidative fibers
– fast oxidative fibers
– fast glycolytic fibers
• Slow fibers have a slow twitch speed (use ATP slowly)
• Fast fibers have a fast twitch speed (use ATP quickly)
• Oxidative fibers contain an iron complexed protein
called myoglobin (provides a darker color to fibers)
which binds oxygen to maintain a high concentration
of oxygen within the fiber to facilitate aerobic
respiration and contain greater amounts of
mitochondria compared to glycolytic fibers
• Glycolytic fibers lack myoglobin (resulting in a light
color to fibers) and contains few mitochondria and
therefore use glycolysis as the main method to make
ATP
Characteristics of Skeletal Muscle Fiber Types
• Slow oxidative fibers:
– muscle fibers used to maintain posture
– high resistance to fatigue since they can make lots
of ATP and use it somewhat slowly
• Fast oxidative fibers:
– muscle fibers used for non-exertive movement
– moderate resistance to fatigue since they can make
lots of ATP but use it somewhat quickly
• Fast glycolytic fibers:
– muscle fibers used for powerful movements
– low resistance to fatigue since they make little ATP
and use it very quickly
• Skeletal muscles of your body contain a combination
of all three fiber types, but their ratio determines the
overall function of that muscle
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
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
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
actively stimulating muscle fibers
– altering the frequency of muscle stimulation
• determined by how often the motor neuron
releases ACh onto the muscle fiber
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
– recruitment
• 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 and individual twitches blend