Journal #1: List and Sketch
the 3 types of muscle tissue

Objective:


describe the
organization of
muscle at the
tissue level
identify the
structural
components of
a sarcomere
Chapter 10:
Muscle Tissue
(Interactive pgs.284-292)
Muscle Overview
3 types of muscle tissue:
 skeletal, cardiac, and smooth
 These types differ in structure,
location, function, and activation

Muscle Similarities


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Skeletal and smooth muscle cells are
elongated and are called muscle fibers
Muscle contraction depends on two kinds of
myofilaments – actin and myosin
Muscle terminology:



Sarcolemma – muscle plasma membrane
Sarcoplasm – cytoplasm of a muscle cell
Prefixes – myo, mys, and sarco all refer to
muscle
SKELETAL MUSCLE TISSUE
Functions:
1. Produces skeletal movement
2. Maintain body position
3. Support soft tissues
4. Guard body openings
5. Maintain body temperature
Organization of
Connective Tissues

Connective tissue wrappings



epimysium-surrounds the
‘whole’ muscle
perimysium- surrounds the
‘fascicle’
endomysium- surrounds each
muscle fiber
all of these sheaths are
continuous w/ each other as
well as tendons &
aponeuroses that connect to
bone

transmits force of contraction to
the bone to be moved
Skeletal Muscle: Nerve and
Blood Supply



Each skeletal muscle fiber is
supplied with a nerve ending that
controls contraction
Contracting fibers require
continuous delivery of oxygen and
nutrients via arteries
Wastes must be removed via
veins
Microscopic anatomy of skeletal
muscle

Muscle fiber = muscle cell


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Fibers are 10 to 100 m in diameter, and up to
hundreds of centimeters long
Plasma membrane = sarcolemma


Long, cylindrical, and multinucleated
lemma = husk
Cytoplasm = sarcoplasm

contain large amts of glycosomes (stored glycogen)
& myoglobin (store O2 w/in mm cell)
Myofibrils
•
•
•
•
The contractile elements of skeletal muscle cells
account for 80% of cell volume
100’s to 1000’s are in a single muscle fiber (cell)
Made up of bundles of protein filaments (myofilaments)
Striations
• Result from darker A
Bands & lighter I
Bands
• a sarcomere is the
region of a myofibril
between 2 successive
Z-Discs
– the sarcomere is the
smallest contractile
unit of a muscle fiber
(cell)
A band



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Dark
contain thick filaments
(myofilaments)
has a central H zone
visible only when the
muscle is in a relaxed
state (thin filaments do not
overlap the thick ones in
this region)
has a slightly darker M line
in middle of H zone
because of protein strands
(desmin) that hold
adjacent thick filaments
together
I band



Light
interrupted in the middle by a
darker line called the Z disc
(AKA Z line)
 composed of proteins
(connectins)
 anchors thin filaments &
connects each myofibril to
the next throughout the
width of the muscle cell
Titin - Strands of protein that
reach from tips of thick
filaments to the Z line to
stabilize the filaments
Transverse Tubules (T tubules)

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Transmit action potential through cell
Allow entire muscle fiber to contract
simulataneously
Have same properties as sarcolemma
Sarcoplasmic Reticulum

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A membranous structure surrounding each
myofibril similar in structure to smooth e.r.
Helps transmit action potential to myofibril
Forms chambers (terminal cisternae) attached
to T tubules
Function to regulate intracellular levels of ionic
calcium

stores calcium & releases it on demand when
the muscle fiber is stimulated to contract
Skeletal Muscle Contraction

In order to contract, a skeletal muscle must:
 Be stimulated by a nerve ending
 Propagate an electrical current, or action
potential, along its sarcolemma
 Have a rise in intracellular Ca2+ levels, the
final trigger for contraction
Types of Myofilaments

Actin (thin filaments) AKA F actin

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subunit called G actin has sites to bind to myosin
contains 2 types of proteins (troponin & tropomyosin)
tropomyosin stiffens the actin & blocks active sites in a
relaxed muscle so myosin cannot bind
troponin helps to position tropomyosin on the actin &
helps to bind calcium ions
Myosin (thick filaments)

each molecule has a tail & 2 heads (cross bridges)


heads contain ATP binding sites & ATPase enzymes to split ATP
to generate E for contraction
~200 myosin molecules per each thick filament within a
sarcomere
4 Thin Filament Proteins
F actin: is 2 twisted rows of globular G actin
1.

the active sites on G actin strands bind to myosin
Nebulin: holds F actin strands together
Tropomyosin: is a double strand
2.
3.

prevents actin–myosin interaction
Troponin: binds tropomyosin to G actin
4.

controlled by Ca2+
The Myosin Molecule


Tail: binds to other myosin molecules
Head: made of 2 globular protein subunits


reaches the nearest thin filament
During contraction, myosin heads interact with
actin filaments, forming cross-bridges

pivot, producing motion
Skeletal Muscle Contraction

Sliding filament theory:




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Contractions that produce
a shortening of the muscle
cell
thin filaments of sarcomere
slide between thick
filaments toward M line
A bands move closer
together but do not change
in length
Z lines move closer
together
I bands are shortened
Excitation–Contraction
Coupling

Action potential reaches a triad:


releasing Ca2+ and triggering contraction
Requires myosin heads to be in “cocked”
position:

loaded by ATP energy
Exposure of binding sites

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Cross bridge attachment to actin requires calcium
ions
Nerve impulses increase calcium levels in the cell
Low levels of Ca causes muscle relaxation &
tropomyosin blocks binding sites on actin
Ca binds to sites on troponin causing it to change
shape & the tropomyosin moves away from the
myosin binding sites
5 Steps of the Contraction
Cycle
1.
2.
3.
4.
5.
Exposure of active sites
Formation of cross-bridges
Pivoting of myosin heads
Detachment of cross-bridges
Reactivation “cocking” of myosin
The Contraction Cycle
http://www.youtube.com/watch?
v=Ct8AbZn_A8A

A single working stroke of all the cross
bridges in a muscle results in a shortening
of only about 1 %


routinely muscles contract between 30-35% of
their total resting length
probably only one-half of the myosin
heads are actively exerting a pulling force
at the same time

the other half are actively seeking their next
binding site
The Process of Contraction

A skeletal muscle fiber must be stimulated by
a nerve ending and must propagate an
electrical current (action potential) along its
sarcolemma in order to contract.


causes excitation–contraction coupling
Cisternae of SR release Ca2+ which triggers
interaction of thick and thin filaments

consuming ATP and producing tension
Chapter 10: Part II
Neuromuscular Junction
Muscle Tissue cont…
(Interactive pgs.293-300)
Voluntary nervous system

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Motor neurons of the somatic (voluntary)
nervous system stimulate skeletal muscle to
contract
cell bodies reside in the brain/spinal cord
axons (efferent) travel to the muscle cells they
serve

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axons divide many times to form neuromuscular
junctions w/ individual muscle fibers
each muscle fiber has only one neuromuscular
junction located at the approximate middle of the
fiber
Neuromuscular Junction
• The neuromuscular junction is formed from:
– Axonal endings, which have small membranous sacs (synaptic
vesicles) that contain the neurotransmitter acetylcholine
(ACh)
– The motor end plate of a muscle, which is a specific part of
the sarcolemma that contains ACh receptors and helps form
the neuromuscular junction
• Axonal ends and muscle fibers are separated by a
space called the synaptic cleft
The Neurotransmitter
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Acetylcholine or ACh:
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travels across the synaptic cleft & binds to
receptors on sarcolemma (motor end plate)
Causes Na+ to rush into sarcoplasm causing
interior of cell to become less negative (more
positive)
This event is called depolarization
Is quickly broken down by enzyme
(acetylcholinesterase or AChE)

This destruction prevents continued muscle fiber
contraction in the absence of additional stimuli
Action Potential
Generated by increase in sodium ions in
sarcolemma
 Travels along the T tubules
 Leads to excitation–contraction coupling
http://www.youtube.com/watch?v=y7X7IZ_ubg4
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Rigor Mortis
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A fixed muscular contraction after death
Caused when:
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ion pumps cease to function
calcium builds up in the sarcoplasm
Tension of a Single Muscle
Fiber
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All or None: muscle contracts or is relaxed
Depends on:
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The number of pivoting cross-bridges
The fiber’s resting length at the time of stimulation
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Normal resting sarcomere length is 75% to 130% of
optimal length
The frequency of stimulation
3 Phases of Twitch
Latent period before
contraction:
1.


the action potential
moves through
sarcolemma
causing Ca2+ release
Contraction phase:
2.

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calcium ions bind
tension builds to peak
Relaxation phase:
3.
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Ca2+ levels fall
active sites are covered
tension falls to resting
levels
Myogram
Complete Tetanus

If stimulation
frequency is high
enough, muscle
never begins to
relax, and is in
continuous
contraction
Motor Units in a Skeletal
Muscle
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Contain hundreds of muscle fibers that contract at the same
time
Controlled by a single motor neuron
Recruitment – multiple motor unit stimulation
Muscle Tone

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
The normal tension and firmness of a muscle
at rest
Muscle units actively maintain body position,
without motion
Increasing muscle tone increases metabolic
energy used, even at rest
2 Types of Skeletal Muscle
Tension
Isotonic contraction - skeletal muscle
changes length resulting in motion
1.

If muscle tension > resistance:


If muscle tension < resistance:

2.
muscle shortens (concentric contraction)
muscle lengthens (eccentric contraction)
Isometric contraction - skeletal muscle
develops tension, but is prevented from
changing length
Isotonic Contraction
Isometric Contraction
Resistance and Speed of
Contraction
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
Are inversely related
The heavier the
resistance on a muscle:


the longer it takes for
shortening to begin
and the less the muscle
will shorten
Muscle Relaxation

After contraction, a muscle fiber returns to
resting length by:

elastic forces: the pull of elastic elements (tendons
and ligaments)

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Expands the sarcomeres to resting length
opposing muscle contractions : antagonists
gravity
Muscle metabolism

ATP is the only E source for contraction

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mm only store ~2-4 seconds worth of ATP
3 main pathways for the regeneration of ATP

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1. Creatine phosphate (CP)
2. Anaerobic glycolysis - breaks down glucose
from glycogen stored in skeletal muscles
3. Aerobic respiration - of fatty acids in the
mitochondria
Muscle fatigue

Definition- the physiological inability to contract
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Absence of ATP leads to contractures
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Different than psychological fatigue
i.e. writer’s cramp
 No ATP available detach the cross-bridges
Intense exercise produces rapid muscle fatigue (with
rapid recovery)
Na+-K+ pumps cannot restore ionic balances quickly
enough
Low pH (lactic acid)
SR is damaged and Ca2+ regulation is disrupted
Oxygen debt


Definition- the extra amount of oxygen to be taken in
by the body to restore reserves of glycogen, ATP, and
CP
Example

To run the 100 yard dash in 12 seconds your body would
need ~ 6 L of oxygen for totally aerobic respiration. VO2 max
(amt of oxygen delivered to & used by your mm) is ~ 1.2 L
during that interval. The oxygen debt is then 4.8L which gets
repaid by heavy breathing after exercise triggered by
increased lactic acid in the blood.
3 Types of Skeletal Muscle
Fibers
Fast fibers (white)- Have large diameter,
large glycogen reserves, few mitochondria
1.

Have strong contractions, fatigue quickly
Slow fibers (red)- Have small diameter, more
mitochondria, contain myoglobin (red pigment,
binds oxygen)
2.

Have high oxygen supply
Intermediate fibers (pink)- Have more
capillaries than fast fiber, slower to fatigue
3.

Have low myoglobin
Fast versus Slow Fibers
Figure 10–21
Muscle Hypertrophy
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Muscle growth from heavy training:


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increases diameter of muscle fibers
increases number of myofibrils
increases mitochondria, glycogen reserves
Muscle Atrophy
• Lack of muscle activity:
– reduces muscle size, tone, and power
Structure of Cardiac Tissue
1. Automaticity:
–
–
contraction without neural
stimulation
controlled by pacemaker
cells
2. Variable contraction
tension:
–
controlled by nervous
system
3. Extended contraction time
4. Prevention of wave
summation and tetanic
contractions by cell
membranes
Characteristics of Cardiocytes

Unlike skeletal muscle, cardiac muscle cells:

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are small, branched & uninucleate
have short, wide T tubules
have SR with no terminal cisternae
are aerobic (high in myoglobin, mitochondria)
have intercalated discs
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Are specialized contact points between cardiocytes
Join cell membranes of adjacent cardiocytes (gap
junctions, desmosomes)
Because intercalated discs link heart cells mechanically,
chemically, and electrically, the heart functions like a
single, fused mass of cells
Smooth Muscle in Body Systems
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Found lining hollow organs
In blood vessels:
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In reproductive and glandular systems:
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produces movements
In digestive and urinary systems:
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regulates blood pressure and flow
forms sphincters
produces contractions
In integumentary system:

arrector pili muscles cause goose bumps
Characteristics of Smooth Muscle Cells
1.
2.
3.
4.
5.
6.
7.
8.
Long, slender, and spindle shaped
Have a single, central nucleus
Have no T tubules, myofibrils, or sarcomeres
Have no tendons or aponeuroses
Have scattered myosin fibers
Myosin fibers have more heads per thick filament
Have thin filaments attached to dense bodies
Dense bodies transmit contractions from cell to
cell
Excitation–Contraction
Coupling



Actin & myosin are
scattered in
sarcoplasm
Free Ca2+ in
cytoplasm triggers
contraction
Ca2+ binds with
calmodulin in the
sarcoplasm

activates myosin light
chain kinase
Regulation of contraction



30x longer to contract than skeletal mm
Can maintain contraction at 1% the E cost of
skeletal muscle
ATP efficiency is very important to homeostasis


i.e. maintaining smooth muscle tone in arteries
Smooth muscle tone - Maintains normal
levels of activity

Modified by neural, hormonal, or chemical factors
Regulation of contraction,
cont.

Neural regulation

The effect of a neurotransmitter on a smooth mm
cell depends on the types of receptors on the
sarcolemma ( + or - )


Ach in sk mm is always excitatory
Examples of neurotransmitters include Ach,
epi/norepi
Regulation of contraction,
cont.

Local regulation


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Some smooth mm cells have no nerve supply at
all
Some respond to both neural & chemical stimuli
Examples include…hormones, lack of oxygen,
rise in carbon dioxide, low pH

All either enhance or inhibit calcium ion entry into the
sarcoplasm
Response to Stretch

Smooth muscle exhibits a phenomenon called
stress-relaxation response in which:




Smooth muscle responds to stretch only briefly, and
then adapts to its new length
The new length, however, retains its ability to
contract
This enables organs such as the stomach and
bladder to temporarily store contents
Skeletal & cardiac muscle respond to stretch with a
more forceful contraction
Length & tension changes


Smooth mm can function normally from
~30% shorter to 30% longer than its resting
length
It can also contract from twice its normal
length to half of its normal (resting) length

This is a change of 150%!