Muscular System: Histology and Physiology

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Muscular SystemHistology and Physiology

• Chapter 8.2

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Sliding Filament Model of Contraction

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• In the relaxed state, thin and thick filaments
  overlap only slightly
• Upon stimulation, myosin heads bind to actin and
  sliding begins

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Sliding Filament Model of Contraction

• Each myosin head binds and detaches several times
  during contraction, acting like a ratchet to
  generate tension and propel the thin filaments to
  the center of the sarcomere
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Neuromuscular Junction

• Region where the motor neuron stimulates the
  muscle fiber
• The neuromuscular junction is formed by
• 1. End of motor neuron axon (axon terminal)
• Terminals have small membranous sacs (synaptic
  vesicles) that contain the neurotransmitter
  acetylcholine (ACh)
• 2. The motor end plate of a muscle
• A specific part of the sarcolemma that contains
  ACh receptors
• Though exceedingly close, axonal ends and muscle
  fibers are always separated by a space called the
  synaptic cleft

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Neuromuscular Junction
Figure 9.7 (a-c)
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Motor Unit The Nerve-Muscle Functional Unit

• A motor unit is a motor neuron and all the muscle
  fibers it supplies
• The number of muscle fibers per motor unit can
  vary from a few (4-6) to hundreds (1200-1500)
• Muscles that control fine movements (fingers,
  eyes) have small motor units
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Motor Unit The Nerve-Muscle Functional Unit
Figure 9.12 (a)
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Motor Unit The Nerve-Muscle Functional Unit

• Muscle fibers from a motor unit are spread
  throughout the muscle
• Not confined to one fascicle
• Therefore, contraction of a single motor unit
  causes weak contraction of the entire muscle
• -

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Smooth Muscle

• Cells are not striated
• Fibers smaller than those in skeletal muscle
• Spindle-shaped single, central nucleus
• More actin than myosin
• No sarcomeres
• Not arranged as symmetrically as in skeletal
  muscle, thus NO striations.
• Caveolae indentations in sarcolemma
• May act like T tubules
• Dense bodies instead of Z disks
• Have noncontractile intermediate filaments

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Smooth Muscle

• Grouped into sheets in walls of hollow organs
• Longitudinal layer muscle fibers run parallel
  to organs long axis
• Circular layer muscle fibers run around
  circumference of the organ
• Both layers participate in peristalsis

Figure 9.24
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Smooth Muscle

• Is innervated by autonomic nervous system (ANS)
• Visceral or unitary smooth muscle
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• -
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• Multiunit
• Cells or groups of cells act as independent units
• Arrector pili of skin and iris of eye

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Cardiac Muscle

• Found only in heart where it forms a thick layer
  called the myocardium
• Striated fibers that branch
• Each cell usually has one centrally-located
  nucleus
• Fibers joined by intercalated disks
• IDs are composites of desmosomes and gap
  junctions
• Allow excitation in one fiber to spread quickly
  to adjoining fibers
• Under control of the ANS (involuntary) and
  endocrine system (hormones)
• Some cells are autorhythmic
• -

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Cardiac Muscle Tissue
Figure 10.10a
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Disorders of Muscle Tissue

• Muscle tissues experience few disorders
• -
• Skeletal muscle remarkably resistant to
  infection
• Smooth muscle problems stem from external
  irritants

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Disorders of Muscle Tissue

• Muscular dystrophy a group of inherited muscle
  destroying disease
• Affected muscles enlarge with fat and connective
  tissue
• Muscles degenerate
• Types of muscular dystrophy
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Disorders of Muscle Tissue

• Myofascial pain syndrome pain is caused by
  tightened bands of muscle fibers
• Fibromyalgia a mysterious chronic-pain syndrome
• Affects mostly women
• Symptoms fatigue, sleep abnormalities, severe
  musculoskeletal pain, and headache

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Developmental Aspects Regeneration

• Cardiac and skeletal muscle become amitotic, but
  can lengthen and thicken
• Myoblast-like satellite cells show very limited
  regenerative ability
• Cardiac cells lack satellite cells
• Smooth muscle has good regenerative ability
• There is a biological basis for greater strength
  in men than in women
• Womens skeletal muscle makes up 36 of their
  body mass
• Mens skeletal muscle makes up 42 of their body
  mass

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Developmental Aspects Male and Female

• These differences are due primarily to the male
  sex hormone testosterone
• With more muscle mass, men are generally stronger
  than women
• Body strength per unit muscle mass, however, is
  the same in both sexes

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Developmental Aspects Age Related

• With age, connective tissue increases and muscle
  fibers decrease
• Muscles become stringier and more sinewy
• By age 80, 50 of muscle mass is lost
  (sarcopenia)
• Decreased density of capillaries in muscle
• Reduced stamina
• Increased recovery time
• Regular exercise reverses sarcopenia