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BIOLOGY 251
Human Anatomy & Physiology
Chapter 10
Muscle Tissue
Lecture Notes
Chapter 10
Muscular
Tissue
Lecture slides prepared by Curtis DeFriez, Weber State University
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Properties of Muscle Tissue
• Extensibility
– ability to be stretched without damaging the tissue
• Elasticity
– ability to return to original shape after being stretched
• Excitability
– respond to chemicals released from nerve cells
• Conductivity
– ability to propagate electrical signals over membrane
• Contractility
– ability to shorten and generate force
Functions of Muscular Tissue
Like nervous tissue, muscles are excitable or "irritable”
they have the ability to respond to a stimulus
Unlike nerves, however, muscles are also:
Contractible (they can shorten
in length)
Extensible (they can extend or
stretch)
Elastic (they can return to their
original shape)
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Three Types of Muscular Tissue
(a) Skeletal muscle
(b) Cardiac muscle
(c) Visceral smooth muscle
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Three Types of Muscular Tissue
Location
Skeletal
skeleton
Cardiac
heart
Visceral
(smooth muscle)
G.I. tract,
uterus, eye,
blood vessels
Function
movement,
heat, posture
Appearance
Control
striated, multinucleated, fibers
voluntary
parallel
pump blood
striated, one central
continuously
nucleus
involuntary
Peristalsis,
blood pressure,
no striations, one
pupil size,
central nucleus
involuntary
erects hairs
Copyright © John Wiley and Sons, Inc. All rights reserved.
Skeletal Muscle
Location
Skeletal
skeleton
Cardiac
heart
Visceral
(smooth muscle)
G.I. tract,
uterus, eye,
blood vessels
Function
movement,
heat, posture
Appearance
Control
striated, multinucleated, fibers
voluntary
parallel
pump blood
striated, one central
continuously
nucleus
involuntary
Peristalsis,
blood pressure,
no striations, one
pupil size,
central nucleus
involuntary
erects hairs
Copyright © John Wiley and Sons, Inc. All rights reserved.
Skeletal Muscle
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Skeletal Muscle
Skeletal muscle fibers are very long “cells” - next to
neurons (which can be over a meter long),
perhaps the longest in the body
The Sartorious muscle contains
single fibers that are at least
30 cm long
A single skeletal muscle fiber
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Organization of Muscle Tissue
The epimysium, perimysium, and
endomysium all are continuous with
the connective tissues that form
tendons and ligaments (attach
skeletal muscle to bone) and muscle
fascia (connect muscles to other
muscles to form groups of muscles)
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Organization of Muscle Tissue
Organization of a fasciculus
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Organization of Muscle Tissue
Organization of a muscle fiber
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Organization of Muscle Tissue
In groups of muscles the
epimysium continues to
become thicker, forming
fascia which covers many
muscles
This graphic shows the
fascia lata enveloping the
entire group of quadriceps
and hamstring muscles in
the thing
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Organization
of Muscle
Tissue
Many large muscle
groups are encased in
both a superficial
and a deep fascia
Real Anatomy, John Wiley and Sons
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Organization of Muscle Tissue
Veins, arteries, and
nerves are located in the
deep fascia between
muscles of the thigh.
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The Skeletal Muscle Fiber
Beneath the connective tissue endomysium is found
the plasma membrane (called the sarcolemma) of an
individual skeletal muscle fiber
The cytoplasm (sarcoplasm) of skeletal muscle fibers
is chocked full of
contractile proteins
arranged in myofibrils
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The Skeletal Muscle Fiber
You should learn the names of the internal structures of
the muscle fiber
Sarcolemma
Sarcoplasm
Myofibril
T-tubules
Triad (with
terminal cisterns
Sarcoplasmic reticulum
Sarcomere
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The Skeletal Muscle Fiber
Increasing the level of magnification, the myofibrils are
seen to be composed
of filaments
Thick filaments
Thin filaments
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The Skeletal Muscle Fiber
The basic functional unit of skeletal muscle fibers is the
sarcomere: An arrangement of thick and thin filaments
sandwiched between two Z discs
A scanning electron micrograph of a sarcomere
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The Skeletal Muscle Fiber
Muscle contraction occurs in the sarcomeres
The “Z line” is really a Z disc when considered in 3
dimensions. A sarcomere extends from Z disc to Z disc.
Copyright © John Wiley and Sons, Inc. All rights reserved.
Muscle Proteins
Myofibrils are built from three groups of proteins
Contractile proteins generate force during contraction
Regulatory proteins help switch the contraction process
on and off
Structural proteins keep the thick and thin filaments in
proper alignment and link the myofibrils to the
sarcolemma and extracellular matrix
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Muscle Proteins
The thin filaments are comprised mostly of the
contractile protein actin, and the thick filaments are
comprised mostly of the contractile protein myosin
However, in both types of filaments, there are also other
structural and regulatory proteins
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Muscle Proteins
In the thin filaments actin proteins are strung together
like a bead of pearls
In the thick filaments myosin proteins look like golf clubs
bound together
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Muscle Proteins
In this first graphic, the myosin binding sites on the actin
proteins are readily visible.
The regulatory proteins troponin and tropomyosin have
been added to the bottom graphic: The myosin binding
sites have been
covered
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Muscle Proteins
In this graphic the troponin-tropomyosin complex has
slid down into the “gutters” of the actin molecule
unblocking the myosin binding site
Myosin binding site exposed
The troponin-tropomyosin complex can slide back and
forth depending on the presence of Ca2+
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Muscle Proteins
Ca2+ binds to troponin which changes the shape of
the troponin-tropomyosin complex and uncovers the
myosin binding sites on actin
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Muscle Proteins
• Besides contractile and regulatory proteins, muscle
contains about a dozen structural proteins which
contribute to the alignment, stability, elasticity, and
extensibility of myofibrils
• Titan is the third most plentiful protein in muscle,
after actin and myosin - it extends from the Z disc and
accounts for much of the elasticity of myofibrils
• Dystrophin is discussed later as it relates to the disease
of muscular dystrophy
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Muscle Proteins
Structural proteins cont’d.
• Myomesin is the protein that forms the M line. The
M line proteins bind to Titan and connect the
adjacent thick filaments to one another and hold they
in alignment at the M line.
• Nebulin is a long non-elastic protein wrapped around
the entire length of each thin filament. It helps
anchor the thin filaments to the Z disc.
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The Sliding-Filament Mechanism
With exposure of the myosin binding sites on actin (the
thin filaments)—in the presence of Ca2+ and ATP—the
thick and thin filaments “slide” on one another and the
sarcomere is shortened
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The Sliding-Filament Mechanism
The “sliding” of actin on myosin (thick filaments on thin
filaments) can be broken down into a 4 step process
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The Sliding-Filament Mechanism
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Step 1: ATP hydrolysis
Step 2: Attachment
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Step 3: Power Stroke
Step 4: Detachment
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The Sliding-Filament Mechanism
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Length-Tension Relationship
Sarcomere shortening produces tension within a muscle
Compressed
thick
filaments
Limited
contact
between actin
and myosin
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Sources of Muscle Energy
Stored ATP
3 seconds
Energy transferred from stored creatine phosphate
12 seconds
Anaerobic glucose use
30-40 seconds
Aerobic ATP production
Minutes - hours
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Sources of Muscle Energy
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Sources of Muscle Energy
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Sources of Muscle Energy
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Neuromuscular Junction
Excitation-Contraction coupling (EC coupling) involves
events at the junction between a motor neuron and a
skeletal muscle fiber
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Neuromuscular Junction
An enlarged view of the neuromuscular junction
The presynaptic membrane is on the neuron while the
postsynaptic membrane is the motor end plate on the
muscle cell. The two membranes are
separated by a space,
or “cleft”
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Neuromuscular Junction
Conscious thought (to move a muscle) results in activation
of a motor neuron, and release of the neurotransmitter
acetylcholine (ACh) at the NM junction
The enzyme
acetylcholinesterase
breaks down ACh
after a short period
of time
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Neuromuscular Junction
The plasma membrane on the “far side” of the NMJ belongs
to the muscle cell and is called the motor end plate
The motor end plate is rich in chemical (ligand) - gated
sodium channels that respond to ACh. Another way to say
this: The receptors for ACh are on the ligand-gated sodium
channels on the motor end plate
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Neuromuscular Junction
The chemical events at the NMJ transmit the electrical
events of a neuronal action potential into the electrical
events of a muscle action potential
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Muscle Action Potential
The muscle AP is propagated over the surface of the
muscle cell membrane (sarcolemma) via voltage
(electrical)-gated Na+ and K+ channels
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Muscle Action Potential
By placing a micropipette inside a muscle cell, and then
measuring the electrical potential across the cell
membrane, the phases of an
action potential
(AP) can be
graphed (as in this
figure)
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Muscle Action Potential
The behavior of the Na+
and K+ channels, at various
points in the AP, are seen
in this graphic
Na+ gates open during the
depolarization phase
K+ gates open during the
repolarization phase
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Excitation-Contraction Coupling
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Excitation-Contraction Coupling
EC coupling involves putting it all together
The thought process going on in the brain
The AP arriving at the neuromuscular junction
The regeneration of an AP on the muscle membrane
Release of Ca2+ from the sarcoplasmic reticulum
Sliding of thick on thin filaments in sarcomeres
Generation of muscle tension (work)
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Excitation-Contraction Coupling
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Excitation-Contraction Coupling
Role Players in E-C coupling
The brain
Regenerate AP
The motor neuron
The T-tubules
Acetylcholine (ACh)
The SR
Acetylcholinesterase enzyme
Ca2+ release
Ach receptors on the
Troponin/Tropomyosin
motor endplate
ATP
Na+-K+ channels on the
Myosin binding
sarcolemma
Filaments slide
Na+ flow
Muscles contract
K+ flow
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The Motor Unit
Motor Unit is composed of a motor neuron plus all of the
muscle cells it innervates
 High
precision
• Fewer muscle fibers per neuron
• Laryngeal and extraocular muscles (2-20)
 Low
precision
• Many muscle fibers per neuron
• Thigh muscles (2,000-3,000)
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The Motor Unit
Florescent dye is used to show the terminal processes of a
single neuron which terminate on a few muscle fibers
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The Motor Unit
Activities requiring extreme precision (like the subtle and rapid
movements of the eye) involve muscles with very small motor
units (1-4 muscle fibers/neuron)
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The Motor Unit
All-or-none principle of muscle contraction
When an individual muscle fiber is stimulated to
depolarization, and an action potential is propagated
along its sarcolemma, it must contract to it’s full force—
it can’t partially contract
Also, when a single motor unit is recruited to contract,
all the muscle fibers in that motor unit must all contract
at the same time
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Tension in a Muscle
There is a brief delay called the latent period as the AP
sweeps over the sarcolemma and Ca2+ ions are released
from the sarcoplasmic reticulum (SR)
During the next phase the fiber is actively contracting
This is followed by relaxation as the Ca2+ ions are re-
sequestered into the SR and myosin binding sites are
covered by tropomyosin
Temporary loss of excitability is call the refractory period –
All muscle fibers in a motor unit will not respond to a
stimulus during this short time
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Tension in a Muscle
A twitch is recorded when a stimulus that results in
contraction (force) of a single muscle fiber is measured
over a very brief millisecond time frame
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Tension in a Muscle
Applying increased numbers of action potentials to a
muscle fiber (or a fascicle, a muscle, or a muscle group)
results in fusion of contractions (tetanus) and the
performance of useful work
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Tension in a Muscle
Two motor units, one in green, the other in purple,
demonstrate the concept of progressive activation of a
muscle known as recruitment
Recruitment allows a muscle to accomplish increasing
gradations of contractile strength
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Figure 2.4
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