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Skeletal Muscle
Physiology
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)
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
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
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.
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
Muscle Tissue Types
Skeletal Muscle
Long cylindrical
cells
Many nuclei per
cell
Striated
Voluntary
Rapid contractions
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 attachment
Insertion – more
movable attachment
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
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
Muscle Attachments
Antagonistic Muscles
Microanatomy of Skeletal Muscle
Parts of a Muscle
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
Motor Unit: The NerveMuscle 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)
Motor Unit
All the muscle cells controlled by one
nerve cell
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
Mosby—Year Book, Inc., 1993.
Three Potential Actions During Muscle Contraction:
• shortening
Biceps muscle shortens
during contraction
Isometric-muscle
does not change its
length
• isometric
• lengthening
Biceps muscle lengthens
during contraction
Most likely to cause
muscle injury
Recall The Motor Unit:
motor neuron and the muscle fibers it innervates
Spinal
cord
• 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
To increase force:
1. Recruit more M.U.s
2. Increase freq.
(force –frequency)
unit contract simultaneously,
pressures for gene expression
(e.g. frequency of stimulation,
load) are identical in all fibers
of a motor unit.
Increased use: strength training
Early gains in strength appear to be predominantly due to neural
factors…optimizing recruitment patterns.
Long term gains almost solely the result of hypertrophy i.e.
increased size.
Performance (% of peak)
Performance Declines with Aging
--despite maintenance of physical activity
100
80
60
40
Shotput/Discus
Marathon
Basketball (rebounds/game)
20
0
10
20
30
40
50
60
Age (years)
D.H. Moore (1975) Nature 253:264-265.
NBA Register, 1992-1993 Edition
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 36:74-182.
Motor unit remodeling with aging
Central
nervous
system
AGING
Motor
neuron
loss
Muscle
Fewer motor units
• More fibers/motor unit
•
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!!
Maximum Isometric Force (mN)
225
200
Adult
Old
175
150
125
100
75
50
25
0
FF
FI
FR
Motor Unit Classification
S
Kadhiresan et al., (1996)
J Physiol 493:543-552.
Muscle injury may play a role in the development of
atrophy with aging.
• Muscles in old animals are more susceptible to contractioninduced 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.
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
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
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
Aerobic Respiration
 Needs oxygen for respiration
 Glucose + Oxygen  Carbon Dioxide + Water + Energy
 Energy=ATP
ATP
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
Muscle Fatigue
 Lack of oxygen causes ATP deficit
 Lactic acid builds up from anaerobic respiration
Muscle Fatigue
Muscle Atrophy
 Weakening and shrinking of a muscle
 May be caused
 Immobilization
 Loss of neural stimulation
Muscle Hypertrophy
Enlargement of a muscle
More capillaries
More mitochondria
Caused by
Strenuous exercise
Steroid hormones
Steroid Hormones
 Stimulate muscle growth and hypertrophy
Muscle Tonus
 Tightness of a muscle
 Some fibers always contracted
Tetany
 Sustained contraction of a muscle
 Result of a rapid succession of nerve impulses
Tetanus
Refractory Period
 Brief period of time in which muscle cells will not
respond to a stimulus
Refractory
Refractory Periods
Skeletal Muscle
Cardiac Muscle
Isometric Contraction
 Produces no movement
 Used in
 Standing
 Sitting
 Posture
Isotonic Contraction
 Produces movement
 Used in
 Walking
 Moving any part of the body
Muscle Spindle
Muscle Spindle Responses
Alpha / Gamma Coactivation
Golgi Tendon Organs
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
 Women’s skeletal muscle makes up 36% of their
body mass
 Men’s skeletal muscle makes up 42% of their
body mass
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
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