Skeletal Muscle Physiology

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Skeletal Muscle Physiology
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Muscular System Functions

• Body movement (Locomotion)
• Maintenance of posture
• Respiration
• Diaphragm and intercostal contractions
• Communication (Verbal and Facial)
• Constriction of organs and vessels
• Peristalsis of intestinal tract
• Vasoconstriction of b.v. and other structures
  (pupils)
• Heart beat
• Production of body heat (Thermogenesis)

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Properties of Muscle

• Excitability capacity of muscle to respond to a
  stimulus
• Contractility ability of a muscle to shorten and
  generate pulling force
• Extensibility muscle can be stretched back to
  its original length
• Elasticity ability of muscle to recoil to
  original resting length after stretched

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Types of Muscle

• Skeletal
• Attached to bones
• Makes up 40 of body weight
• Responsible for locomotion, facial expressions,
  posture, respiratory movements, other types of
  body movement
• Voluntary in action controlled by somatic motor
  neurons
• Smooth
• In the walls of hollow organs, blood vessels,
  eye, glands, uterus, skin
• Some functions propel urine, mix food in
  digestive tract, dilating/constricting pupils,
  regulating blood flow,
• In some locations, autorhythmic
• Controlled involuntarily by endocrine and
  autonomic nervous systems
• Cardiac
• Heart major source of movement of blood
• Autorhythmic
• Controlled involuntarily by endocrine and
  autonomic nervous systems

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Connective Tissue Sheaths

• Connective Tissue of a Muscle
• Epimysium. Dense regular c.t. surrounding entire
  muscle
• Separates muscle from surrounding tissues and
  organs
• Perimysium. Collagen and elastic fibers
  surrounding a group of muscle fibers called a
  fascicle
• Endomysium. Loose connective tissue that
  surrounds individual muscle fibers
• Collagen fibers of all 3 layers come together at
  each end of muscle to form a tendon or
  aponeurosis.

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Nerve and Blood Vessel Supply

• Motor neurons
• stimulate muscle fibers to contract
• Capillary beds surround muscle fibers
• Muscles require large amts of energy
• Extensive vascular network delivers necessary
  oxygen and nutrients and carries away metabolic
  waste produced by muscle fibers

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Muscle Tissue Types
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Skeletal Muscle

• Long cylindrical cells
• Many nuclei per cell
• Striated
• Voluntary
• Rapid contractions

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Basic Features of a Skeletal Muscle

• Muscle attachments
• Most skeletal muscles run from one bone to
  another
• One bone will move other bone remains fixed
• Origin less movable attach- ment
• Insertion more movable attach- ment

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Basic Features of a Skeletal Muscle

• Muscle attachments (continued)
• Muscles attach to origins and insertions by
  connective tissue
• Fleshy attachments connective tissue fibers are
  short
• Indirect attachments connective tissue forms a
  tendon or aponeurosis
• Bone markings present where tendons meet bones
• Tubercles, trochanters, and crests

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Skeletal Muscle Structure

• Composed of muscle cells (fibers), connective
  tissue, blood vessels, nerves
• Fibers are long, cylindrical, and multinucleated
• Tend to be smaller diameter in small muscles and
  larger in large muscles. 1 mm- 4 cm in length
• Develop from myoblasts numbers remain constant
• Striated appearance
• Nuclei are peripherally located

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Muscle Attachments
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Antagonistic Muscles
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Microanatomy of Skeletal Muscle
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Parts of a Muscle
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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
• Large weight-bearing muscles (thighs, hips) have
  large motor units

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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 (bundle of skeletal
  muscle fibers surrounded by perimysium)
• Therefore, contraction of a single motor unit
  causes weak contraction of the entire muscle
• Stronger and stronger contractions of a muscle
  require more and more motor units being
  stimulated (recruited)

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Motor UnitAll the muscle cells controlled by one
nerve cell
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Power Output The Most Physiologically Relevant
Marker of Performance
Power work / time force x distance / time
force x velocity
Peak power obtained at intermediate loads and
intermediate velocities.
Figure from Berne and Levy, Physiology MosbyYear
Book, Inc., 1993.
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Three Potential Actions During Muscle Contraction
Biceps muscle shortens during contraction

• shortening

Isometric-muscle does not change its length

• isometric

• lengthening

Biceps muscle lengthens during contraction
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Recall The Motor Unit motor neuron and the
muscle fibers it innervates

• The smallest amount of
• muscle that can be activated
• voluntarily.
• Gradation of force in skeletal
• muscle is coordinated largely
• by the nervous system.
• Recruitment of motor units
• is the most important means
• of controlling muscle tension.

• Since all fibers in the motor
• unit contract simultaneously,
• pressures for gene expression
• (e.g. frequency of stimulation,
• load) are identical in all fibers
• of a motor unit.

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Increased use strength training
Early gains in strength appear to be
predominantly due to neural factorsoptimizing
recruitment patterns. Long term gains almost
solely the result of hypertrophy i.e. increased
size.
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Performance Declines with Aging --despite
maintenance of physical activity
100
80
60
Performance ( of peak)
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Shotput/Discus
Marathon
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Basketball (rebounds/game)
0
20
30
40
50
60
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Age (years)
D.H. Moore (1975) Nature 253264-265.

NBA Register, 1992-1993 Edition
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Number of motor units declines during aging
AGE-ASSOCIATED ATROPHY DUE TO BOTH Individual
fiber atrophy (which may be at least partially
preventable and reversible through exercise).
Loss of fibers (which as yet appears
irreversible).
Campbell et al., (1973) J Neurol Neurosurg Psych
3674-182.
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Motor unit remodeling with aging
Central nervous system
Muscle
Motor neuron loss
AGING

• Fewer motor units
• More fibers/motor unit

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• Mean Motor Unit Forces
• FF motor units get smaller in old age and
  decrease in number
• S motor units get bigger with no change in
  number
• Decreased rate of force generation and POWER!!

225
200
Adult
175
Old
150
125
Maximum Isometric Force (mN)
100
75
50
25
0
FF
FI
FR
S
Kadhiresan et al., (1996) J Physiol 493543-552.
Motor Unit Classification
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Muscle injury may play a role in the development
of atrophy with aging.

• Muscles in old animals are more susceptible to
  contraction-
• induced injury than those in young or adult
  animals.

• Muscles in old animals show delayed and
  impaired recovery
• following contraction-induced injury.

• Following severe injury, muscles in old animals
  display
• prolonged, possibly irreversible, structural
  and functional
• deficits.

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Disorders of Muscle Tissue

• Muscle tissues experience few disorders
• Heart muscle is the exception
• 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
• Duchenne muscular dystrophy
• Myotonic 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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Aerobic Respiration

• Needs oxygen for respiration
• Glucose Oxygen ? Carbon Dioxide Water
  Energy
• EnergyATP

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ATP
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Anaerobic Respiration

• Without oxygen for respiration
• Glucose ? Lactic Acid Energy
• Incomplete breakdown of glucose
• 5 of energy released by aerobic
• respiration
• Lactic Acid-produces an oxygen
• debt because oxygen is needed to
• oxidize lactic acid (liver)
• REST

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Muscle Fatigue

• Lack of oxygen causes ATP deficit
• Lactic acid builds up from anaerobic respiration

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Muscle Fatigue
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Muscle Atrophy

• Weakening and shrinking of a muscle
• May be caused
• Immobilization
• Loss of neural stimulation

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Muscle Hypertrophy

• Enlargement of a muscle
• More capillaries
• More mitochondria
• Caused by
• Strenuous exercise
• Steroid hormones

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Steroid Hormones

• Stimulate muscle growth and hypertrophy

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Muscle Tonus

• Tightness of a muscle
• Some fibers always contracted

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Tetany

• Sustained contraction of a muscle
• Result of a rapid succession of nerve impulses

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Tetanus
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Refractory Period

• Brief period of time in which muscle cells will
  not respond to a stimulus

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Refractory
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Refractory Periods
Skeletal Muscle
Cardiac Muscle
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Isometric Contraction

• Produces no movement
• Used in
• Standing
• Sitting
• Posture

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Isotonic Contraction

• Produces movement
• Used in
• Walking
• Moving any part of the body

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Muscle Spindle
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Muscle Spindle Responses
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Alpha / Gamma Coactivation
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Golgi Tendon Organs
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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 AspectsMale 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