Functional Human Physiology for the Exercise and Sport Sciences Muscle Physiology

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Functional Human Physiologyfor the Exercise and
Sport Sciences Muscle Physiology

• Jennifer L. Doherty, MS, ATC
• Department of Health, Physical Education, and
  Recreation
• Florida International University

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Types of Muscle Tissue - Classified by location,
appearance, and by the type of nervous system
control or innervation.

• Skeletal muscle
• Located throughout the body connected to bones
  and joints
• Striated in appearance
• Under voluntary nervous control.
• Smooth or visceral muscle
• Located in the walls of organs
• No striations
• Under involuntary or unconscious nervous control.
• Cardiac muscle
• Located only in the heart
• Striated in appearance
• Under involuntary or unconscious nervous control.

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Skeletal Muscle

• Most skeletal muscles are connected to at least
  two bones
• Muscles attach directly to bone
• Or muscles attach indirectly to bone through
  tendons
• Muscles produce movement by producing tension
  between its ends
• Skeletal Muscle Structure
• Cellular Level
• Molecular Level

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Skeletal Muscle Structure Cellular Level

• A Skeletal muscle fiber is an individual muscle
  cell
• Muscle fibers are long and narrow in shape
• Sarcolemma
• The plasma membrane of the muscle cell
• Surrounds the sarcoplasm
• Many nuclei (multi-nucleated)
• Located in the periphery of the muscle cell just
  beneath the sarcolemma

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Skeletal Muscle Structure Cellular Level

• Each muscle fiber contains various organelles
  specifically designed to meet the needs of the
  contractile skeletal muscle fiber
• Abundant mitochondria
• High demand for energy (ATP) required for muscle
  contraction
• Myoglobin
• Protein with a high affinity for oxygen
• Transfers oxygen from the blood to the
  mitochondria of the muscle cell

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Skeletal Muscle Structure Cellular Level

• Each muscle fiber contains
• Myofibrils a cylindrical bundle of contractile
  proteins, which are called Myofilaments, within a
  muscle fiber
• Located in the sarcoplasm of the muscle cell
• Myofilaments the contractile protein filaments
  that make up the Myofibrils
• Actin thin filament
• Myosin thick filament

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Skeletal Muscle Structure Cellular Level

• Sarcoplasmic reticulum (SR)
• Saclike membranous network of tubules
• Elaborate form of smooth endoplasmic reticulum
• Surrounds each myofibril
• Contains terminal cisternae
• Located where the SR ends, which is near the area
  where actin and myosin overlap
• The SR tubules and terminal cisternae store high
  concentrations of calcium, which is important in
  the process of skeletal muscle contraction

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Skeletal Muscle Structure Cellular Level

• Transverse tubules (T-tubules)
• Closely associated with SR
• Connected to the sarcolemma
• Penetrate the sarcolemma into the interior of the
  muscle cell (invaginations)
• Bring extracellular materials into close
  proximity of the deeper parts of the muscle fiber
• SR and T-tubules Function
• Activate skeletal muscle contraction when the
  muscle cell is stimulated by a nerve impulse
• Transmit nerve impulses from the sarcolemma to
  the myofibirls

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Skeletal Muscle Structure Molecular Level

• Sarcomere
• Smallest contractile unit of the muscle fiber
• Arrangement of Myofilaments
• Alternating bands of light and dark areas
• Due to the organization of the actin and myosin
• Striated appearance

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Sarcomere Components

• Z-lines borders of the sarcomere
• Perpendicular to long axis of the muscle fiber
• Anchor thin myofilaments (actin)
• M-lines
• Perpendicular to long axis of the muscle fiber
• Anchor thick myofilaments (myosin)

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Sarcomere Components

• A-Bands
• Dark area where actin and myosin overlap
• Equal to the length of the thick myofilaments
  (myosin)
• Contains the H-Zone
• Lighter area within the A-Band that contains only
  myosin
• The M-Line is located with the H-zone
• I-Bands
• Light area composed of actin only
• Contains the Z line, which is the boarder of the
  sarcomere
• Actin is directly attached the Z-Line
• Appears as a darker line through the I-Band.

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Skeletal Muscle Structure Molecular Level

• Actin
• G-actin (globular actin) the basic component of
  each actin myofilament
• Contains myosin binding sites
• The actin myofilament consists of two strands of
  G-actin molecules
• The two strands of G-action molecules are twisted
  together with two regulatory proteins
• tropomyosin
• troponin

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Skeletal Muscle Structure Molecular Level

• Tropomyosin
• Rod-shaped protein that occupies the groove
  between the twisted strand of actin molecules
• Blocks the myosin binding sites on the G-actin
  molecules
• Troponin
• A complex of three globular proteins.
• One is attached to the actin molecule
• One is attached to tropomyosin
• One contains a binding site for calcium

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Skeletal Muscle Structure Molecular Level

• Myosin
• Crossbridges
• Composed of a rod-like tail and two globular
  heads
• The tails form the central portion of the myosin
  myofilament
• The two globular headsface outward and in
  opposite directions
• Interact with actin during contraction.
• Contain binding sites for both actin and ATP
• The enzyme ATP-ase is located at the ATP binding
  site for hydrolysis of ATP

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Skeletal Muscle Structure Molecular Level

• Titin
• Connects myosin to the Z-lines in the sarcomere
• It is very elastic
• Able to stretch up to 3 times its resting length
• Important molecule because it is responsible for
  muscle flexibility

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Skeletal Muscle Contraction

• The chemical components and reactions that occur
  when a muscle is stimulated by a motor nerve
  result in the sliding of the myofibrils past one
  another.
• The sliding of each myofibril within a muscle
  fiber cause the muscle fiber to shorten.
• When many muscle fibers shorten, the result is
  contraction of the skeletal muscle.

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Skeletal Muscle Contraction

• Role of Actin and Myosin
• These myofilaments are responsible for muscle
  contractility
• Arrangement of actin and myosin
• Cross bridges are oriented around the myosin
  myofilament in rows so that they may interact
  with actin molecules
• The purpose of this complex structure is the
  production of tension (pulling force) within the
  muscle causing the muscle to shorten, thus
  causing movement

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Skeletal Muscle Contraction Force Generation

• Chemical or heat energy in the body is converted
  to mechanical work or movement.
• A nerve impulse arrives at the neuromuscular
  junction (NMJ) and stimulates the beginning of
  the contraction process
• NMJ synapse between a motor neuron and a
  skeletal muscle cell
• Stimulation of the skeletal muscle cell triggers
  the release of calcium ions from the terminal
  cisternae of the sarcoplasmic reticulum
• Calcium catalyzes the contraction process

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Skeletal Muscle Contraction Force Generation

• Calcium ions bind to troponin causing a
  conformational change
• Troponin then pushes tropomyosin away thus
  exposing the active site that it is covering on
  actin
• Myosin crossbridges have a strong affinity for
  the exposed active site on the actin molecule
• Myosin binds to the exposed active site
• Myosin crossbridges pull on the actin myofilament
  pulling it toward the center of the sarcomere
• This motion physically shortens the sarcomere,
  the myofibril, and the muscle fiber.

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Skeletal Muscle Contraction Force Generation

• After the sarcomere is shortened, the calcium
  ions are pumped back into the sarcoplasmic
  reticulum
• Calcium ions are stored until another nerve
  stimulus arrives at the NMJ
• Tropomyosin moves back to its original position
  of covering the active site
• This causes the myosin crossbridges to release
  their hold on the actin myofilament
• The actin myofilaments slide back to their
  original position

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The Sliding-Filament Model

• A muscle contracts because the myosin and actin
  myofilaments slide past each other
• Myosin cross bridges attach and pull, release,
  reattach and pull, sliding the actin toward the
  center of the sarcomere
• Results in shortening of the I-band and the
  H-zone
• Neither actin nor myosin actually change length
  even though the sarcomere is shortened in the
  contraction process
• The A-band remains the same length (length of
  myosin)
• A single attachment of the cross bridge results
  in about a 1 shortening of the total muscle
• Muscles normally shorten 35 to 50 of their total
  resting length

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The Sliding-Filament Model

• Each myosin cross bridge must attach and reattach
  many times during a single contraction
• Called crossbridge cycling
• Power Stroke - Attachment of the myosin cross
  bridge to actin requires energy
• Breakdown of ATP into ADP and P provides the
  energy required for pulling on the actin
  myofilament
• ATP-ase catalyzes the breakdown of ATP
• Rigor low-energy, strong bond between myosin
  and actin
• ADP and P are released from the myosin head thus
  breaking the bond between the myosin crossbridge
  and actin
• Now the muscle is in a state of relaxation
• Cocking - Upon completion of the pulling
  mechanism, another ATP attaches to the myosin
  crossbridge
• Preparation for another crossbridge cycle

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Excitation-Contraction Coupling

• Sequence of events that links the nerve impulse
  and skeletal muscle contraction
• Motor Neurons stimulates skeletal muscles
• Excitatory effect
• When a skeletal muscle cell receives input from a
  motor neuron, it depolarizes
• Depolarization causes the muscle cell to fire an
  action potential

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Excitation-Contraction Coupling

• Action Potentials
• Large changes in cell membrane potential (charge)
• Inside of the cell becomes more positive relative
  to the outside of the cell
• Function to transmit information over long
  distances

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Excitation-Contraction Coupling

• Neuromuscular Junction (NMJ)
• The synapse between the motor neuron and the
  muscle cell
• Synaptic Cleft
• The extra-cellular space between the motor neuron
  and the muscle cell

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Excitation-Contraction Coupling

• The NMJ releases a neurotransmitter from the
  motor neuron into the synaptic cleft
• The neurotransmitter is acetylcholine (ACh)
• This neurotransmitter is synthesized by the nerve
  cell and stored in synaptic vesicles
• When an nerve impulse reaches the NMJ, the
  synaptic vesicles release acetylcholine into the
  synaptic cleft.

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Excitation-Contraction Coupling

• 4) Acetylcholine rapidly diffuses across the
  synaptic cleft to combine with receptors on
  muscle cell membrane (sarcolemma)
• The muscle cell is also called the motor end
  plate membrane
• 5) ACh causes depolarization of the muscle cell
  membrane
• Generates an action potential
• 6) Acetylcholine bound to the receptor is
  rapidly decomposed by acetylcholinesterase
  (enzyme) preventing continuous stimulation of the
  muscle fiber.

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Excitation-Contraction Coupling

• Stimulation of Contraction
• Action potential propogates along the sarcolemma
  and down the T-tubules to reach the sarcoplasmic
  reticulum
• Sarcoplasmic reticulum releases calcium
• Calcium is actively pumped into and stored in the
  SR leaving a small concentration of calcium ions
  in the sarcoplasm
• The action potential causes the calcium ions to
  be released from the SR into the sarcoplasm

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Excitation-Contraction Coupling

• When released from the SR, calcium travels toward
  the myofilaments
• Calcium binds with troponin on the actin
  myofilament causing a conformational change,
  which results in moving tropomyosin off the
  active site
• Myosin heads are then able to bind to the G-actin
  on the active sites
• This begins the contraction process of
  crossbridge cycling

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Excitation-Contraction Coupling

• Crossbridge cycling continues as long as there is
  an adequate supply of ATP and if there is
  stimulation from a motor neuron
• Crossbridge cycling stops if there is an
  inadequate supply of ATP or if the motor neuron
  impulse stops
• When the motor neuron impulse stops, calcium ions
  are rapidly pumped back into the sarcoplasmic
  reticulum for storage
• The calcium ion concentration in the sarcoplasm
  decreases
• Tropomyosin returns to its original position
  blocking the myosin binding site on actin
• The muscle cell relaxes

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Muscle Cell Metabolism

• How Muscle Cells Provide ATP to Drive the
  Crossbridge Cycle
• The sources of ATP
• Available ATP in the sarcoplasm
• Creatine phosphate
• Glucose

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Muscle Cell Metabolism

• Available ATP
• There is a limited supply of readily available
  ATP
• A small amount of ATP is stored in the myosin
  crossbridges immediately available when the
  muscle begins to contract.
• Contraction uses up this source of ATP in about 6
  seconds making it necessary to have other sources
  of ATP available

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Muscle Cell Metabolism

• Creatine Phosphate (CP)
• When the ATP stores in the myosin crossbridges
  are exhausted, ADP and CP are used to regenerate
  ATP.
• CP ADP ATP Creatine.
• The energy available from stored ATP and from the
  reaction of joining ADP with CP provides only
  about 20 seconds worth of energy
• The muscles could contract only long enough to
  run a 100 m dash on the energy from these sources

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Muscle Cell Metabolism

• Glucose
• Cellular respiration of glucose is an energy
  source utilized to generate ATP
• Muscle contractions that are longer than 15 - 20
  seconds depend on cellular respiration of glucose
  as a source of ATP

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Muscle Cell Metabolism

• Recall
• Cells store glucose in the sarcoplasm in the form
  of glycogen
• The cell must break apart the glycogen molecules
  to release the individual glucose molecules
  this is called glycogenolysis
• The breakdown of glucose, called glycolysis,
  occurs in the sarcoplasm of the muscle cell and
  does not require oxygen, it is anaerobic
• Glycolysis produces pyruvic acid, and a small
  amount of ATP.
• The majority of the ATP used by muscles is formed
  by aerobic processes in the mitochondria.
• At low intensities, the muscle cell depends on
  aerobic glycolysis during which oxidative
  phosphorylation becomes more important

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Muscle Cell Metabolism Changes with Exercise
Intensity

• Anerobic Metabolism
• Oxygen is not readily available
• During intense exercise, when the supply of
  oxygen cannot keep up with metabolic demand of
  the cells, pyruvic acid produced during
  glycolysis is converted to lactic acid.
• Lactic acid accumulates in the muscle resulting
  in the burning sensation during short duration,
  high intensity muscular exercise such as lifting
  weights
• Lactic acid is quickly removed from the muscle
  and taken to the liver where it is converted to
  glucose

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Muscle Cell Metabolism Changes with Exercise
Intensity

• Aerobic Metabolism
• Oxygen is readily available
• During prolonged, low-intensity exercise, the
  muscles are supplied with adequate oxygen by the
  protein myoglobin
• Myoglobin
• Similar to hemoglobin (oxygen binding protein in
  the blood)
• Myoglobin has a high affinity for oxygen and
  binds to it loosely inside muscle cells
• Myoglobin brings oxygen into the muscle cell and
  stores it temporarily
• This provides a continuous supply of oxygen even
  when blood flow to the muscle is reduced

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Muscle Cell Metabolism Changes with Exercise
Intensity

• When exercise stops, the body's need for oxygen
  continues for a period of time
• The body responds to this need by continuing to
  breathing heavily until all the sources of ATP
  have been replenished
• Oxygen Debt
• The amount of oxygen necessary to restore the
  resting metabolic state of the body
• A better, and more currently accepted, term to
  describe the events following exercise is
  recovery oxygen consumption

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Muscle Cell Metabolism Changes with Exercise
Intensity

• Recovery oxygen consumption
• Includes the oxygen needed to
• Restore muscles to their resting metabolic
  condition
• Convert lactic acid to pyruvic acid in the liver
• Replenish cellular stores of glycogen, creatine
  phosphate, and ATP
• Return resting body temperature to normal
• Return the heart muscle and the muscles of
  respiration to normal, which need repair from the
  minor tissue damage that occurs due to exercise
• The amount of oxygen needed to meet recovery
  oxygen consumption demands depends on an
  individual's physical condition and the duration
  and intensity of the exercise session.

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Types of Skeletal Muscle Fibers

• Not all muscle fibers are the same
  physiologically
• Muscles vary depending on
• The predominant pathway utilized to synthesize
  ATP
• Oxidative fibers - predominantly aerobic pathways
• Oxidative phosphorylation in the mitochondria
• Fatigue-resistant fibers
• Glycolytic fibers predominantly anaerobic
  pathways
• Glycolysis in the sarcoplasm
• Fatigable fibers
• The amount of myoglobin
• Red fibers - high amounts of myoglobin
• White fibers - small amounts of myoglobin
• Efficiency of ATPase
• Fast twitch fibers - decompose ATP rapidly
• Slow twitch fibers - decompose ATP slowly

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Types of Skeletal Muscle Fibers

• Slow-twitch fatigue-resistant fibers
• Slow oxidative fibers, or red muscle fibers.
• Contain abundant myoglobin giving them their red
  color.
• Slow acting ATPase enzymes
• Abundant mitochondria
• Depend upon aerobic pathways for production of
  ATP
• Endurance type muscles
• Able to deliver strong, prolonged contractions.
• Examples
• Postural muscles - spinal extensors
• Anti-gravity muscles - calf muscle

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Types of Skeletal Muscle Fibers

• Fast-twitch fatigable fibers
• Fast glycolytic fibers, or white muscle fibers.
• Contain small amounts of myoglobin
• Fast acting ATPase enzymes
• Allows the muscle fiber to contract rapidly
• Few mitochondria
• Contract for limited periods of time because
  fatigue rapidly
• Plenty of glycogen
• Depends on anaerobic metabolism
• Extensive sarcoplasmic reticulum
• Rapidly releases and stores calcium ions
  contributing to rapid contractions
• Best suited for short duration, high intensity
  contractions

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Types of Skeletal Muscle Fibers

• Intermediate Fibers
• Fast-twitch fatigue-resistant fibers
• Fast glycolytic fibers
• Pale muscle fibers
• Characteristics lie between the red and white
  fibers

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Types of Skeletal Muscle Fibers

• Most of the body's muscles contain a mixture of
  fiber types.
• It is the motor nerve that innervates the muscle
  cell that determines its type
• Therefore, all of the muscle cells in a single
  motor unit are of the same type
• Motor Unit a motor neuron and all of the muscle
  fibers it innervates
• Examples
• Running the motor nerve stimulates the motor
  units containing fast-twitch fibers.
• Posture the motor nerve stimulates the motor
  units containing slow-twitch fibers.

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Types of Skeletal Muscle Fibers

• Slow twitch fibers are recruited first
• This is because they are found in small motor
  units
• Fast twitch fibers are recruited last
• This is because they are found in large motor
  units

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Types of Skeletal Muscle Fibers

• People are genetically predisposed to have
  relatively more of one fiber type than another
• People who excel at marathon running have higher
  percentages of slow twitch fatigue resistant
  muscle fibers
• People who excel at sprinting have higher
  percentages of fast twitch fatigable fibers

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Other Muscle Types Smooth Muscle

• In comparison to skeletal muscle fibers
• Smooth muscle fibers are shorter and thinner
• They have a single, centrally located nucleus
• Lack striations
• Although smooth muscle fibers do contain actin
  and myosin, the filaments are thin and randomly
  arranged so that it lacks striations
• No T-tubules
• A poorly developed sarcoplasmic reticulum

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Other Muscle Types Smooth Muscle

• Smooth muscle fibers contract in a similar manner
  to skeletal muscles with a few important
  functional similarities and differences.
• Similarities
• Both contractile mechanisms depend on the action
  of actin and myosin
• Both are triggered by membrane impulses and the
  release of calcium ions and
• Both require ATP.

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Other Muscle Types Smooth Muscle

• Differences in smooth muscle include
• Actin has no troponin, the protein that binds to
  myosin in skeletal muscle. Rather smooth muscle
  has a calcium binding protein called calmodulin.
  This protein activities the actin and myosin
  crossbridge formation.
• Most of the calcium required for contraction
  comes into the cell by diffusion from the
  extracellular fluid.
• Smooth muscle is more resistant to fatigue and
  produces a slower, longer lasting contraction
  than skeletal muscle.
• It is more energy efficient than skeletal muscle
  in that it can maintain a more forceful
  contraction for a longer period of time with the
  same amount of ATP.

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Other Muscle Types Smooth Muscle

• Autonomic nervous system control
• Unconscious control of smooth muscle contraction
• Nuerotransmitters
• Acetylcholine (as in skeletal muscle)
• Norepinephrine.
• Neurotransmitters for smooth muscle can be either
  excitatory (cause muscle contraction), or
  inhibitory (prevent muscle contraction) depending
  on the receptor on the smooth muscle cell
  membrane. Whereas, the neurotransmitter for
  skeletal muscle is always excitatory.
• Smooth muscle is also stimulated by certain
  hormones such as oxytocin, which stimulates
  smooth muscle contraction in the walls of the
  uterus during childbirth.

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Other Muscle Types Smooth Muscle

• Multiunit smooth muscle
• Fibers are not very well organized
• Occur as separate fibers scattered throughout the
  sarcoplasm rather than in sheets.
• Requires stimulation by a motor nerve impulse
  from the autonomic nervous system.
• This type of smooth muscle is found in the irises
  of the eyes, arrector pili muscles, blood
  vessels, and large airways of the lungs

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Other Muscle Types Smooth Muscle

• Single Unit Smooth Muscle
• Also called Visceral Smooth Muscle because it is
  found in the walls of the hollow visceral organs
  such as the stomach, intestines, urinary bladder
  and uterus.
• More common of the two types of smooth muscle.
• The muscle fibers are organized into sheets of
  cells held in close contact by gap junctions.
• Organized into two layers
• Longitudinal layer
• Outer layer directed longitudinally along the
  length of the structure.
• Contraction of this layer causes the structure to
  dilate and shorten
• Circular layer
• Inner layer arranged circularly around the
  structure.
• Contraction of this layer causes the structure to
  constrict and elongate.

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Other Muscle Types Smooth Muscle

• Intrinsic Control of Smooth Muscle Contraction
• Myogenic Response
• Smooth muscle is stimulated to contract when it
  is stretched
• Smooth muscle is able to distend, or stretch,
  without great increases in tension or tightness
• Allows hollow organs to be filled
• When the smooth muscle reaches is stretching
  capacity, it will contract and force the contents
  out
• Such as occurs in the intestines or urinary
  bladder.

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Other Muscle Types Cardiac Muscle

• Found only in the heart
• Composed of interconnecting, branching fibers
  that are striated
• Each cell has a single nucleus similar to
  skeletal muscle
• Contains actin and myosin similar to smooth
  muscle.
• Abundant mitochondria
• Depends on aerobic metabolism
• It cannot sustain an oxygen debt and still
  function efficiently
• No motor units
• Not every cardiac muscle cell is innervated by a
  nerve in order to stimulate contraction

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Other Muscle Types Cardiac Muscle

• Extensive system of T-tubules
• Release large quantities of calcium ions
• Well developed sarcoplasmic reticulum
• Terminal cisternae contain less calcium than in
  skeletal muscle
• Strength of the cardiac muscle contraction
  depends largely on the influx of calcium from the
  extracellular space in addition to that released
  from the T-tubules and sarcoplasmic reticulum
• Contains intercalated disks
• Membrane junctions that hold adjacent cells
  together and transmit the contraction force to
  each cell
• Gap Juntions
• Most important intercellular junction that allow
  interchange and communication between the
  sarcoplasm of connected cardiac muscle cells

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Other Muscle Types Cardiac Muscle

• Communication between Cardiac Muscle Cells is
  important to allow the nerve impulse to rapidly
  travel from cell to cell to stimulate contraction
• Stimulation of part of the cardiac muscle cell
  results in impulses sent across the entire area
  of the heart muscle tissue
• All-or-none Response
• The entire heart muscle contracts as a unit, or
  in syncytium

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Other Muscle Types Cardiac Muscle

• Two syncytium are in heart
• The atrial syncytium and the ventricular
  syncytium
• They are almost completely separated from each
  other by fibrous tissue
• The all-or-none response applies to the entire
  syncytium
• Either both atria contact, or both do not
  contract at all
• Either both ventricles contact, or both do not
  contract at all

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Other Muscle Types Cardiac Muscle

• Cardiac Muscle Contraction
• Plateau Phase
• The prolonged depolarization in cardiac muscle
  due to Calcium influx from the extra-cellular
  fluid
• The prolonged plateau phase prevents tetany, or
  prolonged contractions, that would interfere with
  the pumping ability of the heart
• Refractory Period
• Due to the calcium influx in cardiac muscle,
  there is a prolonged absolute refractory period
  of cardiac muscle lasting about 250 msec.
• Much longer than skeletal muscle which lasts
  about 1-2 msec.
• Repolarization
• Calcium is pumped back into sarcoplasmic
  reticulum and out of cell to the extracellular
  space.

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Other Muscle Types Cardiac Muscle

• Cardiac muscle is self-exciting
• It is able to stimulate itself to contract
• Cardiac muscle is autorhythmic
• It contracts in a periodic manner
• Autorhythmicity causes the automatic contraction
  and relaxation of the heart
• Known as the heartbeat.

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Other Muscle Types Cardiac Muscle

• Autorhythmicity
• Ability of cardiac muscle to repeatedly and
  rhythmically contract without external
  stimulation
• Due to the presence of Pacemaker Cells in the
  heart
• Specialized smooth muscle cells that depolarize
  spontaneously at regular intervals causing
  excitation of the muscle cells without nervous
  system stimulation
• The spontaneous impulses travel into the
  surrounding muscle tissue through gap junctions
  that connect the cell membranes of adjacent
  muscle fibers, thus allowing the heart to
  contract as a coordinated unit