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Chapter 12
Lecture
Outline
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Chapter 12 Outline
Skeletal
Muscles
Mechanisms of Contraction
Contractions of Skeletal Muscle
Energy Requirements of Skeletal Muscle
Neural Control of Skeletal Muscles
Cardiac and Smooth Muscle
12-2
Skeletal Muscles
12-3
Skeletal Muscles
 Are
attached to bone on each end by tendons
 Insertion is the more movable attachment; is pulled toward
origin-the less moveable attachment
 Contracting muscles cause tension on tendons which move
bones at a joint
 Flexors decrease angle of joint
 Extensors increase angle of joint
 Prime mover of any skeletal movement is agonist muscle
 Antagonistic muscles are muscles (flexors and extensors) that
act on the same joint to produce opposite actions
12-4
Skeletal Muscle Structure
 Fibrous
connective
tissue from tendons
forms sheaths
(epimysium) that
extend around and
into skeletal muscle
 Inside the muscle
this connective
tissue divides
muscle into
columns called
fascicles
 Connective tissue
around fascicles is
called perimysium
12-5
Skeletal Muscle Structure continued
 Muscle
fibers are muscle cells
 Ensheathed by thin connective tissue layer called
endomysium
 Plasma membrane is called sarcolemma
 Muscle fibers are similar to other cells except are multinucleate
and striated
12-6
Skeletal Muscle Structure continued
Most
distinctive feature of skeletal muscle is its
striations
12-7
Neuromuscular Junction
12-8
Neuromuscular Junction (NMJ)
 Includes
the single synaptic ending of the motor neuron
innervating each muscle fiber and underlying specializations of
sarcolemma
12-9
Neuromuscular Junction (NMJ) continued
Place
on sarcolemma where NMJ occurs is the motor
end plate
12-10
Motor Unit
12-11
Motor Unit
 Each
motor
neuron branches
to innervate a
variable # of
muscle fibers
 A motor unit
includes each
motor neuron
and all fibers it
innervates
12-12
Motor Unit continued
When
a motor neuron is activated, all muscle fibers in
its motor unit contract
Number of muscle fibers in motor unit varies according
to degree of fine control capability of the muscle
Innervation ratio is # motor neurons : muscle fibers
Vary from 1:100 to 1:2000
Fine control occurs when motor units are small, i.e.
1 motor neuron innervates small # of fibers
12-13
Motor Unit continued
Since
individual motor units fire "all-or-none," how do
skeletal muscles perform smooth movements?
Recruitment is used:
Brain estimates number of motor units required
and stimulates them to contract
It keeps recruiting more units until desired
movement is accomplished in smooth fashion
More and larger motor units are activated to
produce greater strength
12-14
Mechanisms of Contraction
12-15
Structure of Muscle Fiber
 Each
fiber is packed with myofibrils
 Myofibrils are 1 in diameter and extend length of fiber
 Packed with myofilaments
 Myofilaments are composed of thick and thin filaments
that give rise to bands which underlie striations
12-16
Structure of Myofibril
A
band is dark, contains
thick filaments (mostly
myosin)
 Light area at center of
A band is H band
 = area where actin
and myosin don’t
overlap
 I band is light, contains
thin filaments (mostly
actin)
 At center of I band
is Z line/disc where
actins attach
12-17
12-18
Sarcomeres
 Are
contractile units of skeletal muscle consisting of
components between 2 Z discs
 M lines are structural proteins that anchor myosin during
contraction
 Titin is elastic protein attaching myosin to Z disc that contributes
to elastic recoil of muscle
12-19
How Fiber Contracts
12-20
Sliding Filament Theory of Contraction
Muscle
contracts because myofibrils get shorter
Occurs because thin filaments slide over and
between thick filaments towards center
Shortening distance from Z disc to Z disc
12-21
Sliding Filament Theory of Contraction continued
 During
contraction:
 A bands (containing actin)
move closer together, do
not shorten
 I bands shorten because
they define distance
between A bands of
successive sarcomeres
 H bands (containing
myosin) shorten
12-22
12-23
Cross Bridges
 Are
formed by heads of myosin molecules that extend toward
and interact with actin
 Sliding of filaments is produced by actions of cross bridges
 Each myosin head contains an ATP-binding site which
functions as an ATPase
12-24
Cross Bridges continued
 Myosin
can’t bind to actin unless it is “cocked” by ATP
 After binding, myosin undergoes conformational change
(power stroke) which exerts force on actin
 After power stroke myosin detaches
12-25
12-26
Control of Contraction
 Control
of cross bridge attachment to actin is via troponintropomyosin system
 Serves as a switch for muscle contraction and relaxation
 The filament tropomyosin lies in grove between double row
of G-actins (that make up actin thin filament)
 Troponin complex is attached to tropomyosin at intervals of
every 7 actins
12-27
Control of Contraction continued
 In
relaxed muscle,
tropomyosin blocks
binding sites on actin so
crossbridges can’t occur
 This occurs when Ca++
levels are low
 Contraction can occur
only when binding sites
are exposed
12-28
Role of Ca++ in Muscle Contraction
Ca++ levels rise, Ca++
binds to troponin causing
conformational change which
moves tropomyosin and
exposes binding sites
 Allowing crossbridges and
contraction to occur
 Crossbridge cycles stop
when Ca++ levels decrease
 When
12-29
Role of Ca++ in Muscle Contraction
 Ca++
levels decrease
because it is
continually pumped
back into the
sarcoplasmic
reticulum (SR - a
calcium reservoir in
muscle)
 Most Ca++ in SR is in
terminal cisternae
 Running along
terminal cisternae are
T tubules
12-30
Excitation-Contraction Coupling
 Skeletal
muscle
sarcolemma is
excitable
 Conducts Action
Potentials
 Release of ACh at
NMJ causes large
depolarizing end-plate
potentials and APs in
muscle
 APs race over
sarcolemma and
down into muscle via
T tubules
12-31
Excitation-Contraction Coupling continued
T
tubules are extensions
of sarcolemma
 Ca++ channels in SR are
mechanically linked to
channels in T tubules
 APs in T tubules cause
release of Ca++ from
cisternae via V-gated
and Ca++ release
channels
 Called
electromechanical
release
 channels are 10X
larger than V-gated
channels
12-32
Excitation-Contraction Coupling continued
12-33
Muscle Relaxation
Ca++
from SR diffuses to troponin to initiate
crossbridge cycling and contraction
When APs cease, muscle relaxes
Because Ca++ channels close and Ca++ is pumped
back into Sarcolplamic Reticulum by Ca++-ATPase
pumps.
Therefore, ATP is needed for relaxation as well as
contraction.
12-34
Contractions of Skeletal Muscles
12-35
Twitch, Summation, and Tetanus
A
single rapid contraction and relaxation of muscle fibers is a
twitch
 If 2nd stimulus occurs before muscle relaxes from 1st, the 2nd
twitch will be greater (summation)
 Contractions of varying strength (graded contractions) are
obtained by stimulation of varying numbers of fibers
12-36
Twitch, Summation, and Tetanus continued
 If
muscle is stimulated by an increasing frequency of electrical
shocks, its tension will increase to a maximum (incomplete
tetanus)
 If frequency is so fast that no relaxation occurs, a smooth
sustained contraction results called complete tetanus or tetany
12-37
Twitch, Summation, and Tetanus continued
If
muscle is repeatedly stimulated with maximum
voltage to produce individual twitches, successive
twitches get larger
This is Treppe or staircase effect
Caused by accumulation of intracellular Ca++
12-38
Velocity of Contraction
For
muscle to shorten it must generate force greater
than the load
The lighter the load the faster the contraction and vice
versa
12-39
Isotonic, Isometric, Eccentric, and
Concentric Contractions
During
isotonic contraction, force remains constant
throughout shortening process, length changes
During isometric contraction, exerted force does not
cause load to move and length of fibers remains
constant
During eccentric contraction, load is greater than
exerted force and fibers lengthen despite its
contraction
During concentric contraction, muscle tension is
greater than the load and muscle shortens
12-40
Series-Elastic Component
Tendons
and connective tissue are elastic and absorb
tension as muscle contracts
They recoil as muscle relaxes and spring back to
resting length
12-41
Length-Tension Relationship
Strength
of muscle contraction influenced by:
Frequency of stimulation
Thickness of each muscle fiber
Initial length of muscle fiber
Ideal resting length is that which can generate
maximum force
12-42
Length-Tension Relationship
 Too
little overlap
yields less tension
because fewer
cross bridges can
form
 With no overlap
force cannot be
generated
because cross
bridges cannot
form
12-43
Energy Requirements of Skeletal
Muscles
12-44
Metabolism of Skeletal Muscles
Skeletal
muscles respire anaerobically 1st 45-90 sec
of moderate-to-heavy exercise
Cardiopulmonary system requires this time to
increase O2 supply to exercising muscles
If exercise is moderate, aerobic respiration
contributes majority of muscle requirements after 1st
2 min
12-45
Maximum Oxygen Uptake
 Maximum
oxygen uptake (aerobic capacity) is maximum rate of
oxygen consumption (V02 max)
 Determined by age, gender, and size
 Lactate (anaerobic) threshold is % of max O2 uptake at which
there is significant rise in blood lactate levels
 In healthy individuals this is at 50–70% V02 max
12-46
Metabolism of Skeletal Muscles
During
light exercise, most energy is derived from
aerobic respiration of fatty acids
During moderate exercise, energy derived equally
from fatty acids and glucose
During heavy exercise, glucose supplies 2/3 of energy
Liver increases glycogenolysis
GLUT-4 carrier is moved to muscle cell’s plasma
membrane
12-47
12-48
Oxygen Debt
When
exercise stops, rate of oxygen uptake does not
immediately return to pre-exercise levels
Because of oxygen debt accumulated during exercise
When oxygen is withdrawn from hemoglobin and
myoglobin
And because of O2 needed for metabolism of lactic
acid produced by anaerobic respiration
12-49
Phosphocreatinine
 During
exercise ATP can be used faster than can be generated
by respiration
 Phosphocreatine (creatine phosphate) is source of high energy
phosphate to regenerate ATP from ADP
 So efficient that muscle ATP concent. dec. only slightly from
rest to heavy exercise
12-50
Types of Skeletal Muscle
12-51
Slow- and Fast-Twitch Fibers
Skeletal
muscle fibers can be divided on basis of
contraction speed and resistance to fatigue:
Slow-twitch, slow fatigue (Type I fibers)
Fast-twitch, fast fatigue (Type IIA and IIX fibers)
a=fast twitch extraocular, b=gastrocnemius muscle,
and c=slow-twitch soleus
12-52
Type I Fibers
Also
called red slow oxidative
Adapted to contract slowly without fatigue
Uses mostly aerobic respiration
Has rich capillary supply, many mitochondria, and
aerobic enzymes
Has lots of myoglobin (O2 storage molecule)
Gives fibers red color
Have small motor neurons with small motor units
12-53
Type II Fibers
Type
IIX fibers also called white fast glycolytic
Adapted to contract fast using anaerobic metabolism
Has large stores of glycogen, few capillaries and
mitochondria, little myoglobin
Type II A fibers also called white fast oxidative
Adapted to contract fast using aerobic metabolism
Intermediate to Type I and Type IIX
Have large motor neurons with large motor units
12-54
12-55
Muscle Fatigue
Is
exercise-induced reduction in ability of muscle to
generate force
Sustained muscle contraction fatigue is due to
accumulation of extracellular K+
From K+ efflux during AP
Occurs in moderate exercise as slow-twitch fibers
deplete glycogen stores
Fast twitch fibers are then recruited, converting
glucose to lactic acid which interferes with Ca2+
transport
Central fatigue caused by changes in CNS rather than
by fatigue in muscles themselves
12-57
Adaptations of Muscles to Exercise Training
Endurance
training improves aerobic capacity (by
20%) and lactate threshold (by 30%)
Resistance training increases muscle size by
increasing # of myofibrils/fiber (hypertrophy)
Once a myofibril has attained a certain thickness, it
may split into two myofibrils.
12-58
12-59
Neural Control of Skeletal Muscles
12-60
Neural Control of Skeletal Muscles
Motor
neuron cell bodies are in ventral horn of spinal
cord; axons leave in ventral root
Called lower motor neurons and final common
pathway
Activity influenced by sensory feedback from
muscles and tendons
And facilitory and inhibitory activity from upper
motor neurons
12-61
Sensory Feedback
To
control skeletal muscle movements, NS must
receive continuous sensory feedback
Including information on tension from Golgi tendon
organs
And on length of muscle from muscle spindle
apparatus
12-62
Muscle Spindle Apparatus
Consists
of modified thin
muscle cells called
intrafusal fibers
Regular muscle fibers
are extrafusal fibers
Spindles are arranged in
parallel with extrafusal
fibers
Insert into tendons at
each end of muscle
12-63
Muscle Spindle Apparatus
 Intrafusal
fibers have
nuclei in central region
instead of contractile
filaments
 Nuclear bag fibers
have nuclei arranged in
loose aggregate
 Nuclear chain fibers
have nuclei arranged in
rows
12-64
Muscle Spindle Apparatus continued
 Nuclear
bag and chain
fibers are innervated
by primary,
annulospiral sensory
endings
 Which wrap around
central regions
 Respond most at
onset of stretch
12-65
Muscle Spindle Apparatus continued
 Nuclear
chain fibers
additionally have
secondary, flower-spray
endings located at ends
 Respond to
sustained stretch
12-66
Muscle Spindle Apparatus continued
Both
nuclear bag and chain fibers respond strongly to
sudden, rapid stretching
Their activation causes a reflex contraction of
muscle
12-67
Alpha and Gamma Motor Neurons
 Fast
conducting alpha
motor neurons innervate
extrafusal fibers and
cause muscle
contraction
 Slower conducting
gamma motor neurons
innervate and induce
tension in intrafusal
fibers (=active stretch)
 Increases sensitivity
of muscle to passive
stretch
12-68
Coactivation of Alpha and Gamma Motor
Neurons
Upper
motor neurons usually stimulate alpha and
gamma motor neurons simultaneously (coactivation)
Stimulation of alpha motor neurons results in
muscle contraction and shortening
Stimulation of gamma motor neurons causes
intrafusal fibers to take up slack
Activity of gamma motor neurons maintains normal
muscle tone
12-69
Monosynaptic-Stretch Reflex
 Consists
of only 1
synapse within CNS
 Striking patellar ligament
passively stretches
spindles activating
annulospiral sensory
neurons
 Which synapse on
alphas causing them to
stimulate extrafusals
 Produces knee-jerk
reflex
12-70
Golgi Tendon Organ Reflex
 Involves
2 synapses
in the CNS
(=disynaptic reflex)
 Sensory axons from
Golgi tendon organ
synapse on
interneurons
 Which make
inhibitory synapses
on motor neurons
 Prevents excessive
muscle contraction or
passive muscle
stretching
12-71
Reciprocal Innervation
 Occurs
in stretch
reflexes because
sensory neurons
stimulate motor neuron
and interneuron
 Interneuron inhibits
motor neurons of
antagonistic muscles
 When limb is flexed,
antagonistic extensors
are inhibited from doing
stretch reflex
12-72
Crossed-Extensor Reflex
 Involves
double reciprocal innervation
 Affecting muscles on contralateral side of cord
 e.g. if step on tack, foot is withdrawn by contraction of flexors
and relaxation of extensors
 And contralateral leg extends to support body (crossed extensor
reflex)
12-73
Upper Motor Neuron Control of Skeletal
Muscles
Influence
lower motor neurons
Axons of neurons in precentral gyrus form pyramidal
tracts
Extrapyramidal tracts arise from neurons in other
areas of brain
12-74
Upper Motor Neuron Control of Skeletal
Muscles continued
 Cerebellum
receives sensory input from spindles, Golgi tendon
organs, and areas of cortex devoted to vision, hearing, and
equilibrium
 No descending tracts arise from cerebellum
 Influences motor activity indirectly
 All output from cerebellum is inhibitory
 Aids motor coordination
 Cerebral ganglia exert inhibitory effects on activity of lower
motor neurons
12-75
Cardiac and Smooth Muscles
12-76
Cardiac Muscle (Myocardium)
 Contractile
apparatus similar to skeletal
 Striated like skeletal but involuntary like smooth
 Branched; adjacent myocardial cells joined by intercalated disks
(gap junctions)
 Allow Action Potentials to spread throughout cardiac muscle
12-77
Smooth Muscle
 Has
no sarcomeres
 Has gap junctions
 Contains 16X more actin
than myosin
 Allows greater
stretching and
contracting
 Actin filaments are
anchored to dense
bodies
12-78
Smooth Muscle Contraction
by Ca++ but different from striated
Has little SR and no troponin/tropomyosin
Ca++ enters thru voltage gated channels in plasma
membrane
Binds with calmodulin
Ca++-calmodulin complex activates myosin light
chain kinase (MLCK)
Which phosphorylates and activates myosin
Myosin forms crossbridges with actin
Controlled
12-79
Smooth Muscle Contraction
occurs when Ca++ concentration decreases
Myosin is dephosphorylated by myosin phosphatase
Myosin can no longer form crossbridges
Smooth muscle has slower contractions than
striated
Can form a state of prolonged binding of myosin
to actin (latch state)
 Maintains force using little energy
Relaxation
12-80
12-81
Single and Multiunit Smooth Muscle
Single
unit is spontaneously active (myogenic)
Some cells are pacemakers
Has gap junctions to spread electrical activity
Multiunit requires nerve stimulation by ANS
NT released along a series of synapses called
varicosities
Called synapses en passant (in passing)
12-82
12-83