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Chapter 11
Lecture Outline
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Muscle Tissue







Types and characteristics of muscular tissue
Microscopic anatomy of skeletal muscle
Nerve-Muscle relationship
Behavior of skeletal muscle fibers
Behavior of whole muscles
Muscle metabolism
Cardiac and smooth muscle
2
Introduction to Muscle
 Movement
is a fundamental characteristic of
all living things
 Cells capable of shortening and converting
the chemical energy of ATP into mechanical
energy
 Types of muscle

skeletal, cardiac and smooth
 Physiology

of skeletal muscle
basis of warm-up, strength, endurance and
fatigue
3
Characteristics of Muscle
 Responsiveness

(excitability)
to chemical signals, stretch and electrical
changes across the plasma membrane
 Conductivity

local electrical change triggers a wave of
excitation that travels along the muscle fiber
 Contractility
-- shortens when stimulated
 Extensibility -- capable of being stretched
 Elasticity -- returns to its original resting length
after being stretched
4
Skeletal Muscle
 Voluntary
striated muscle attached to bones
 Muscle fibers (myofibers) as long as 30 cm
 Exhibits alternating light and dark transverse
bands or striations

reflects overlapping arrangement of
internal contractile proteins
 Under
conscious
control (voluntary)
5
Connective Tissue Elements
 Attachments

endomysium, perimysium, epimysium, fascia,
tendon
 Collagen

between muscle and bone
is extensible and elastic
stretches slightly under tension and recoils when
released
• protects muscle from injury
• returns muscle to its resting length
 Elastic


components
parallel components parallel muscle cells
series components joined to ends of muscle
6
The Muscle Fiber
7
Muscle Fibers
 Multiple
flattened nuclei inside cell
membrane


fusion of multiple myoblasts during development
unfused satellite cells nearby can multiply to
produce a small number of new myofibers
 Sarcolemma
has tunnel-like infoldings or
transverse (T) tubules that penetrate the cell

carry electric current to cell interior
8
Muscle Fibers 2
 Sarcoplasm


is filled with
myofibrils (bundles of myofilaments)
glycogen for stored energy and myoglobin for
binding oxygen
 Sarcoplasmic



reticulum = smooth ER
network around each myofibril
dilated end-sacs (terminal cisternea) store
calcium
triad = T tubule and 2 terminal cisternea
9
Thick Filaments
 Made

of 200 to 500 myosin molecules
2 entwined polypeptides (golf clubs)
 Arranged
in a bundle with heads directed
outward in a spiral array around the bundled
tails

central area is a bare zone with no heads
10
Thin Filaments
 Two

intertwined strands fibrous (F) actin
globular (G) actin with an active site
 Groove

holds tropomyosin molecules
each blocking 6 or 7 active sites of G actins
 One
small, calcium-binding troponin
molecule on each tropomyosin molecule
11
Elastic Filaments
 Springy
proteins called titin
 Anchor each thick filament to Z disc
 Prevents overstretching of sarcomere
12
Regulatory and Contractile Proteins


Myosin and actin are contractile proteins
Tropomyosin and troponin = regulatory proteins



switch that starts and stops shortening of muscle cell
contraction activated by release of calcium into sarcoplasm and
its binding to troponin,
troponin moves tropomyosin off the actin active sites
13
Overlap of Thick and Thin Filaments
14
Striations = Organization of Filaments

Dark A bands (regions) alternating with lighter I bands (regions)


A band is thick filament region


anisotrophic (A) and isotropic (I) stand for the way these regions affect polarized
light
lighter, central H band area
contains no thin filaments
I band is thin filament region


bisected by Z disc protein called
connectin, anchoring elastic and thin
filaments
from one Z disc (Z line) to the next is a sarcomere
15
Striations and Sarcomeres
16
Relaxed and Contracted Sarcomeres

Muscle cells shorten because their individual
sarcomeres shorten


pulling Z discs closer together
pulls on sarcolemma

Notice neither thick nor thin filaments change
length during shortening
 Their overlap changes as sarcomeres shorten
17
Nerve-Muscle Relationships
 Skeletal
muscle must be stimulated by a
nerve or it will not contract
 Cell bodies of somatic motor neurons in
brainstem or spinal cord
 Axons of somatic motor neurons = somatic
motor fibers


terminal branches supply one muscle fiber
Each motor neuron and all the muscle
fibers it innervates = motor unit
18
Motor Units

A motor neuron and the muscle fibers it
innervates




Fine control



dispersed throughout the muscle
when contract together causes weak
contraction over wide area
provides ability to sustain long-term
contraction as motor units take turns resting
(postural control)
small motor units contain as few as
20 muscle fibers per nerve fiber
eye muscles
Strength control

gastrocnemius muscle has 1000
fibers per nerve fiber
19
Neuromuscular Junctions (Synapse)

Functional connection between
nerve fiber and muscle cell
 Neurotransmitter (acetylcholine/ACh) released from
nerve fiber stimulates muscle cell
 Components of synapse (NMJ)


synaptic knob is swollen end of nerve fiber (contains ACh)
junctional folds region of sarcolemma
• increases surface area for ACh receptors
• contains acetylcholinesterase that breaks down ACh and causes
relaxation


synaptic cleft = tiny gap between nerve and muscle cells
Basal lamina = thin layer of collagen and glycoprotein
over all of muscle fiber
20
The Neuromuscular Junction
21
Neuromuscular Toxins
 Pesticides


(cholinesterase inhibitors)
bind to acetylcholinesterase and prevent it from
degrading ACh
spastic paralysis and possible suffocation
 Tetanus
or lockjaw is spastic paralysis
caused by toxin of Clostridium bacteria

blocks glycine release in the spinal cord and
causes overstimulation of the muscles
 Flaccid
paralysis (limp muscles) due to
curare that competes with ACh

respiratory arrest
22
Electrically Excitable Cells
 Plasma


membrane is polarized or charged
resting membrane potential due to Na+ outside of
cell and K+ and other anions inside of cell
difference in charge across the membrane =
resting membrane potential (-90 mV cell)
 Stimulation

opens ion gates in membrane
ion gates open (Na+ rushes into cell and K+
rushes out of cell)
• quick up-and-down voltage shift = action potential

spreads over cell surface as nerve signal
23
Muscle Contraction and Relaxation
 Four




actions involved in this process
excitation = nerve action potentials lead to
action potentials in muscle fiber
excitation-contraction coupling = action
potentials on the sarcolemma activate
myofilaments
contraction = shortening of muscle fiber
relaxation = return to resting length
 Images
will be used to demonstrate the
steps of each of these actions
24
Excitation of a Muscle Fiber
25
Excitation (steps 1 and 2)

Nerve signal opens voltage-gated calcium channels. Calcium
stimulates exocytosis of synaptic vesicles containing ACh =
ACh release into synaptic cleft.
26
Excitation (steps 3 and 4)
Binding of ACh to receptor proteins opens Na+ and K+
channels resulting in jump in RMP from -90mV to +75mV
27
forming an end-plate potential (EPP).
Excitation (step 5)
Voltage change in end-plate region (EPP) opens nearby
voltage-gated channels producing an action potential
28
Excitation-Contraction Coupling
29
Excitation-Contraction Coupling (steps 6 and 7)
Action potential spreading over sarcolemma enters T
tubules -- voltage-gated channels open in T tubules
causing calcium gates to open in SR
30
Excitation-Contraction Coupling (steps 8 and 9)

Calcium released by SR binds to troponin
 Troponin-tropomyosin complex changes shape
and exposes active sites on actin
31
Contraction (steps 10 and 11)

Myosin ATPase in myosin head hydrolyzes an ATP
molecule, activating the head and “cocking” it in an
extended position
 It binds to actin active site forming a cross-bridge 32
Contraction (steps 12 and 13)

Power stroke =
myosin head releases
ADP and phosphate as
it flexes pulling the thin
filament past the thick
 With the binding of more
ATP, the myosin head
extends to attach to a
new active site


half of the heads are bound to a thin filament at
one time preventing slippage
thin and thick filaments do not become shorter,
just slide past each other (sliding filament
theory)
33
Relaxation (steps 14 and 15)
Nerve stimulation ceases and acetylcholinesterase
removes ACh from receptors. Stimulation of the muscle
cell ceases.
34
Relaxation (step 16)

Active transport needed to pump calcium back
into SR to bind to calsequestrin
 ATP is needed for muscle relaxation as well as
muscle contraction
35
Relaxation (steps 17 and 18)

Loss of calcium from sarcoplasm moves troponintropomyosin complex over active sites


stops the production or maintenance of tension
Muscle fiber returns to its resting length due to recoil
of series-elastic components and contraction of
antagonistic muscles
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Rigor Mortis





Stiffening of the body beginning 3 to 4 hours after
death
Deteriorating sarcoplasmic reticulum releases
calcium
Calcium activates myosin-actin cross-bridging and
muscle contracts, but can not relax.
Muscle relaxation requires ATP and ATP production
is no longer produced after death
Fibers remain contracted until myofilaments decay
37
Length-Tension Relationship

Amount of tension generated depends on length of
muscle before it was stimulated


Overly contracted (weak contraction results)


thick filaments too close to Z discs and can’t slide
Too stretched (weak contraction results)


length-tension relationship (see graph next slide)
little overlap of thin and thick does not allow for very many
cross bridges too form
Optimum resting length produces greatest force
when muscle contracts

central nervous system maintains optimal length
producing muscle tone or partial contraction
38
Length-Tension Curve
39
Muscle Twitch in Frog

Threshold = voltage producing an
action potential


a single brief stimulus at that voltage
produces a quick cycle of contraction
and relaxation called a twitch (lasting
less than 1/10 second)
A single twitch contraction is not
strong enough to do any useful
work
40
Muscle Twitch in Frog 2

Phases of a twitch contraction

latent period (2 msec delay)
• only internal tension is generated
• no visible contraction occurs since
only elastic components are being
stretched

contraction phase
• external tension develops as muscle
shortens

relaxation phase
• loss of tension and return
to resting length as calcium returns to SR
41
Contraction Strength of Twitches

Threshold stimuli produces twitches
 Twitches unchanged despite increased voltage
 “Muscle fiber obeys an all-or-none law”
contracting to its maximum or not at all

not a true statement since twitches vary in strength
• depending upon, Ca2+ concentration, previous stretch of the
muscle, temperature, pH and hydration

Closer stimuli produce stronger twitches
42
Recruitment and Stimulus Intensity

Stimulating the whole nerve with higher and higher
voltage produces stronger contractions
 More motor units are being recruited


called multiple motor unit summation
lift a glass of milk versus a whole gallon of milk
43
Twitch and Treppe Contractions

Muscle stimulation at variable frequencies

low frequency (up to 10 stimuli/sec)
• each stimulus produces an identical twitch response

moderate frequency (between 10-20 stimuli/sec)
• each twitch has time to recover but develops more tension
than the one before (treppe phenomenon)


calcium was not completely put back into SR
heat of tissue increases myosin ATPase efficiency
44
Incomplete and Complete Tetanus

Higher frequency stimulation (20-40 stimuli/second)
generates gradually more strength of contraction

each stimuli arrives before last one recovers
• temporal summation or wave summation


incomplete tetanus = sustained fluttering contractions
Maximum frequency stimulation (40-50 stimuli/second)



muscle has no time to relax at all
twitches fuse into smooth, prolonged contraction called complete
tetanus
rarely occurs in the body
45
Isometric and Isotonic Contractions

Isometric muscle contraction



develops tension without changing length
important in postural muscle function and antagonistic
muscle joint stabilization
Isotonic muscle contraction


tension while shortening = concentric
tension while lengthening = eccentric
46
Muscle Contraction Phases

Isometric and isotonic phases of lifting



tension builds though the box is not moving
muscle begins to shorten
tension maintained
47
ATP Sources


All muscle contraction depends on ATP
Pathways of ATP synthesis

anaerobic fermentation (ATP production limited)
• without oxygen, produces toxic lactic acid

aerobic respiration (more ATP produced)
• requires continuous oxygen supply, produces H2O and CO2
48
Immediate Energy Needs

Short, intense exercise (100 m
dash)


Phosphagen system



oxygen need is supplied by
myoglobin
myokinase transfers Pi groups
from one ADP to another forming
ATP
creatine kinase transfers Pi
groups from creatine phosphate to
make ATP
Result is power enough for 1
minute brisk walk or 6 seconds
of sprinting
49
Short-Term Energy Needs
 Glycogen-lactic

acid system takes over
produces ATP for 30-40 seconds of maximum
activity
• playing basketball or running around baseball
diamonds

muscles obtain glucose from blood and stored
glycogen
50
Long-Term Energy Needs

Aerobic respiration needed for prolonged
exercise


After 40 seconds of exercise, respiratory and
cardiovascular systems must deliver enough
oxygen for aerobic respiration


Produces 36 ATPs/glucose molecule
oxygen consumption rate increases for first 3-4
minutes and then levels off to a steady state
Limits are set by depletion of glycogen and
blood glucose, loss of fluid and electrolytes
51
Fatigue
 Progressive





weakness from use
ATP synthesis declines as glycogen is
consumed
sodium-potassium pumps fail to maintain
membrane potential and excitability
lactic acid inhibits enzyme function
accumulation of extracellular K+ hyperpolarizes
the cell
motor nerve fibers use up their acetylcholine
52
Endurance
 Ability
to maintain high-intensity exercise
for >5 minutes

determined by maximum oxygen uptake
• VO2 max is proportional to body size, peaks at age
20, is larger in trained athlete and males

nutrient availability
• carbohydrate loading used by some athletes


packs glycogen into muscle cells
adds water at same time (2.7 g water with each
gram/glycogen)
• side effects include “heaviness” feeling
53
Oxygen Debt

Heavy breathing after strenuous exercise



known as excess postexercise oxygen consumption
(EPOC)
typically about 11 liters extra is consumed
Purposes for extra oxygen




replace oxygen reserves (myoglobin, blood
hemoglobin, in air in the lungs and dissolved in
plasma)
replenishing the phosphagen system
reconverting lactic acid to glucose in kidneys and liver
serving the elevated metabolic rate that occurs as
long as the body temperature remains elevated by
exercise
54
Slow- and Fast-Twitch Fibers
 Slow



oxidative, slow-twitch fibers
more mitochondria, myoglobin and
capillaries
adapted for aerobic respiration and
resistant to fatigue
soleus and postural muscles of the back
(100msec/twitch)
55
Slow and Fast-Twitch Fibers
 Fast



glycolytic, fast-twitch fibers
rich in enzymes for phosphagen and
glycogen-lactic acid systems
sarcoplasmic reticulum releases calcium
quickly so contractions are quicker (7.5
msec/twitch)
extraocular eye muscles, gastrocnemius and
biceps brachii
 Proportions
genetically determined
56
Strength and Conditioning

Strength of contraction

muscle size and fascicle arrangement
• 3 or 4 kg / cm2 of cross-sectional area



Resistance training (weight lifting)


size of motor units and motor unit recruitment
length of muscle at start of contraction
stimulates cell enlargement due to synthesis of more
myofilaments
Endurance training (aerobic exercise)

produces an increase in mitochondria, glycogen and
density of capillaries
57
Cardiac Muscle 1

Thick cells shaped like a log with uneven, notched
ends
 Linked to each other at intercalated discs


electrical gap junctions allow cells to stimulate their
neighbors
mechanical junctions keep the cells from pulling apart

Sarcoplasmic reticulum less developed but large T
tubules admit Ca+2 from extracellular fluid
 Damaged cells repaired by fibrosis, not mitosis
58
Cardiac Muscle 2
 Autorhythmic
due to pacemaker cells
 Uses aerobic respiration almost
exclusively


large mitochondria make it resistant to fatigue
very vulnerable to interruptions in oxygen
supply
59
Smooth Muscle
 Fusiform




cells with one nucleus
30 to 200 microns long and 5 to 10 microns wide
no striations, sarcomeres or Z discs
thin filaments attach to dense bodies scattered
throughout sarcoplasm and on sarcolemma
SR is scanty and has no T tubules
• calcium for contraction comes from extracellular fluid
 If

present, nerve supply is autonomic
releases either ACh or norepinephrine
60
Types of Smooth Muscle
 Multiunit



smooth muscle
largest arteries, iris, pulmonary air passages,
arrector pili muscles
terminal nerve branches synapse on
myocytes
independent contraction
61
Types of Smooth Muscle
 Single-unit



smooth muscle
most blood vessels and viscera as circular
and longitudinal muscle layers
electrically coupled by gap junctions
large number of cells contract as a unit
62
Stimulation of Smooth Muscle
63
Stimulation of Smooth Muscle
 Involuntary
and contracts without nerve
stimulation


hormones, CO2, low pH, stretch, O2 deficiency
pacemaker cells in GI tract are autorhythmic
 Autonomic
nerve fibers have beadlike
swellings called varicosities containing
synaptic vesicles

stimulates multiple myocytes at diffuse junctions
64
Features of Contraction and Relaxation

Calcium triggering contraction is extracellular

calcium channels triggered to open by voltage,
hormones, neurotransmitters or cell stretching
• calcium ions bind to calmodulin
• activates light-chain myokinase which activates myosin ATPase
• power stroke occurs when ATP hydrolyzed

Thin filaments pull on intermediate filaments
attached to dense bodies on the plasma
membrane

shortens the entire cell in a twisting fashion
65
Features of Contraction and Relaxation
 Contraction
and relaxation very slow in
comparison

slow myosin ATPase enzyme and slow pumps
that remove Ca+2
 Uses
10-300 times less ATP to maintain
the same tension

latch-bridge mechanism maintains tetanus
(muscle tone)
• keeps arteries in state of partial contraction
(vasomotor tone)
66
Contraction of Smooth Muscle
67
Responses to Stretch 1
 Stretch
opens mechanically-gated calcium
channels causing muscle response

food entering the esophagus brings on peristalsis
 Stress-relaxation
response necessary for
hollow organs that gradually fill (urinary
bladder)

when stretched, tissue briefly contracts then
relaxes
68
Responses to Stretch 2
 Must
contract forcefully when greatly
stretched


thick filaments have heads along their entire
length
no orderly filament arrangement -- no Z discs
 Plasticity
is ability to adjust tension to
degree of stretch such as empty bladder is
not flabby
69
Muscular Dystrophy
 Hereditary
diseases - skeletal muscles
degenerate and are replaced with adipose
 Disease of males


appears as child begins to walk
rarely live past 20 years of age
 Dystrophin
links actin filaments to cell
membrane

leads to torn cell membranes and necrosis
 Fascioscapulohumeral
shoulder muscle only
MD -- facial and
70
Myasthenia Gravis
 Autoimmune
disease - antibodies attack
NMJ and bind ACh receptors in clusters


receptors removed
less and less sensitive to ACh
• drooping eyelids and double vision, difficulty
swallowing, weakness of the limbs, respiratory failure
 Disease
of women between 20 and 40
 Treated with cholinesterase inhibitors,
thymus removal or immunosuppressive
agents
71
Myasthenia Gravis
Drooping eyelids and weakness of muscles of eye movement
72