The Muscular System

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The Muscular System

• ANS 215
• Anatomy Physiology of Domesticated Animals

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Primary Functions

• Movement
• Blood
• Ingesta/excreta
• Gametes/conceptus
• Support
• Generation of body heat

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Calssifications

• Smooth
• No striations
• Individual cells are spindle shaped with
  centrally located nuclei
• Regulated by the autonomic nervous system
• Located in visceral structures
• Aggregates of myofilaments are composed of actin
  and myosin
• Filaments are not arranged in order

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Smooth Muscle
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Classifications

• Cardiac
• Found only in the heart
• Regulated by the autonomic nervous system
• Shows striations characterized by alternating
  white and dark bands
• Composed of elongated, branching cells with
  irregular contours at junctions with other cells

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Classifications

• Cardiac
• Boundary area where cells meet is called
  intercalated disk
• This specialized structure facilitates
  transmission of nerve impulses
• Each cell has a nucleus (sometimes 2) that is
  centrally located

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Cardiac Muscle
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Classifications

• Skeletal
• Long bundles of cells striated
• Multinucleated with nuclei at periphery of cells
• Innervated by spinal or cranial nerves
• Three types
• red or dark (highest myoglobin and mitochondria
  content)
• white or pale (lowest myoglobin and mitochondria
  content)
• intermediate (characteristics between white and
  red fibers)

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Classifications

• Skeletal
• Skeletal muscle comprises majority of body muscle
  mass
• Skeletal muscle fiber can run the length of the
  muscle with which it is a part
• Skeletal muscle fibers do not branch or
  anastomose

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Classifications

• Skeletal
• Often described according to type of movement
  performed
• Flexors if they are located on the side of the
  limb towards which the joint decreases the joint
  angle
• Extensors if they are located on the side of
  the limb towards which the joint increases the
  joint angle
• Adductors if they pull a limb toward the median
  plane
• Abductors if they pull a limb away from the
  median plane
• Sphincters are arranged circularly to constrict
  body openings

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Skeletal Muscle
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Arrangement

• Function of muscles is to contract or shorten and
  thereby move an object
• Primary consideration for accomplishing this goal
  is arrangement
• Examples include
• Sheets
• Sheets rolled into tubes
• Bundles
• Rings
• Cones
• Discrete cells or clusters of cells

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Arrangement

• The effects of skeletal muscles (apart from
  sphincters) are noted some distance from their
  location
• This means the contraction must be transmitted
• One end must be anchored and the other attached
  directly to a tendon or moveable part
• Accordingly, anatomic description of a skeletal
  muscle sometimes refers to its origin and
  insertion

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Arrangement
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Skeletal Muscle Harnessing

• Harness for skeletal muscle fibers is composed of
  connective tissue elements
• Epimysium
• Perimysium
• Endomysium
• Continuous from the individual muscle fibers to
  the connective tissue structure to which it
  attaches and on which it exerts its contraction

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Skeletal Muscle Harnessing

• Connective tissue elements of skeletal muscle
  are
• Muscle fibers comprise a muscle bundle which are
  attached by their cell covering (sarcolema) to
  connective tissue division, endomysium
• The endomysium is continuous with the connective
  tissue that envelopes muscle bundles, the
  perimysium

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Skeletal Muscle Harnessing

• The perimysium is continuous with the connective
  tissue that envelops the muscle (collection of
  bundles), the epimysium
• Then epimysium is continuous with the tendon or
  aponeurosis, which can travel some distance from
  its attachment

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Microstructure of Skeletal Muscles

• Depending on diameter of the muscle fiber, there
  might be several hundred to several thousand
  myofibrils within one muscle
• Each myofibril has striations or banding
• Myofibrils are further broken into repetitive
  units (sarcomeres)

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Microstructure of Skeletal Muscles

• Sarcomeres contain the protein myofilaments actin
  and myosin
• Give rise to striations by their arrangement
• Sarcomeres of a myofibril are in alignment with
  the sarcomeres of all the other myofibrils of the
  muscle fiber
• The Z line is located at each end of a sarcomere
  and is common to both sarcomeres that it
  separates

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Microstructure of Skeletal Muscles

• Actin filaments project from the Z line into the
  sarcomeres that it separates
• Actin of two sarcomeres common to the same Z line
  comprise an I band

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Microstructure of Skeletal Muscles

• The myosin filaments are centrally located within
  a sarcomere, coupled with the overlap of actin
  filaments provide for the dark banding (A band)
• There is a regular 21 ratio of actin to myosin

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Sarcotubular System

• Skeletal muscle fibers contain a network of
  tubules known as the sarcotubular system
• Tubules are located within the muscle fiber, but
  outside the myofibrils

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Sarcotubular System

• Two tubule sets
• Arranged transversely (right angles) to
  myofibrils
• Open to the outside of fibers
• contain extracellular fluid
• Found at boundary of A and I bands
• each sarcomere has two

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Neuromuscular Junction

• Each skeletal muscle fiber is innervated by a
  motor neuron
• Axon terminal end bulb forms synapse with muscle
  fiber
• Release of Acetylcholine (Ach) causes
  depolarization
• One motor neuron innervates many muscle fibers
• Motor unit
• Motor unit ration of 1150 means one nerve fiber
  supplies 150 muscle fibers
• Smaller ration increases precision

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Scanning Electron Micrograph of Neuromuscular
Junction
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Skeletal Muscle Contraction

• Depolarization of muscle fibers
• Acetylcholine released into synapse
• Ca facilitates release
• Parturient paresis occurs if not enough Ca
• Opens ligand-coated Na channels
• Causes a propagated action potential
• sarcolema
• T-tubule
• sarcoplasmic reticulum
• Cholinesterase destroys Ach

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Skeletal Muscle Contraction

• Depolarization of muscle fibers
• Depolarization of sarcoplasmic reticulum causes
  Ca release
• Ca diffuses to myofibrils
• Ca initiates contraction
• Ca pumped back into sarcoplasmic reticulum
• active transport
• Ca ATPase Pump

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