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Unit 2 Day 10
Today’s Topics:
Ch 7 Muscular System
Muscle Structure
Muscle Physiology

Muscular System
Muscles utilize chemical energy
from the breakdown and
metabolism of food to perform
useful work. There are three kinds
of muscle cells: skeletal,
smooth, and cardiac. This
lecture will review only skeletal
muscle and will examine the
mechanism of muscle contraction
as it applies to movement. The
body contains over 600 different
skeletal muscles. These muscles
perform three principal functions:
(1) movement, (2) heat
production, and (3) body
support and posture.
Skeletal Muscle Anatomy
Skeletal Muscle Anatomy
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Myofiber or muscle fiber = muscle cell
Endomysium = conn. tissue sheet around
muscle cell
Fascicle = bundle of muscles cells
Perimysium = conn. tissue sheet around
fascicle
Skeletal Muscle Anatomy

Epimysium (fascia) = conn. tissue sheet
around entire muscle
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Fascia can surround muscles and even
muscle groups
Skeletal Muscle Cell or Fiber
Structure of Skeletal
Muscle Cell
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Myofiber = muscle cell
Sarcolemma(muscle cell membrane)
transmits the action potential (AP).
 Sarcoplasmic reticulum (SR)
the system of tubes controlling the
release of Ca++, which is essential for
contraction.
lacework of smooth E.R., stores
calcium ions

Myofiber:

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Sarcoplasmcytoplasm (very little)
Myofibrils contain the contractile
proteins of the muscle (thin and thick
myofilaments) & are surrounded by a
mesh-like network of tubes containing
calcium ions (Ca++)
Myofiber:
- myofibrils  cylindrical organelles,
contains bundles of myosin (thick) &
actin (thin) myofilaments
Thick
myofilament
Thin
myofilament
MYOFIBRIL
Myofiber:

transverse (T) tubulesnarrow tube-like
projections beginning at the cell membrane
and extending into the cell
carries muscle AP deep into the interior
where the contractile proteins are located
the S.R. becomes more permeable to
Ca++ and diffuses into the sarcoplasm
Myofiber:
 terminal cisternae or cisternae
a membranous enlargement of the SR, close to
the T tubule where the action potential travels.
expanded endings of S.R., concentrated area of
calcium ions (Ca2+)
 mitochondriaproduces ATP for muscle contraction
Myofiber:
 -myoglobinred pigment protein molecule in
cytoplasm, similar to hemoglobin, stores O2
stores O2 temporarily
provides O2 to muscle in times of inadequate
blood flow
contributing factor to darker appearance of
chicken leg or duck breast
associated with slow-twitch muscle fibers,
resists fatigue and contract slowly
Skeletal Muscle Anatomy
Whole muscles  bundles of fasciculi.
Each fascicle  groups of muscle
cells or fibers.
Each muscle cell  many bundles of
myofibrils.
Each myofibril  thin and thick
myofilaments.
Thin myofilaments  mostly the
protein actin along with troponin and
tropomyosin.
Thick myofilaments  protein
myosin.
It is this interaction of thin and
thick myofilaments that result in
muscle contraction!!!!
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Characteristics of
muscle/myofibers
1.
2.
3.
4.
Contractility
Excitability
Extensibility
Elasticity
Muscle Physiology
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Includes muscle contraction & muscle
relaxation
Requires:
Nerve impulse = stimulus

Neurotransmitter = acetylcholine (ACh),
released by neuron and binds to muscle cell
membrane (sarcolemma)
 ATP = energy  needed for contraction & relaxation

Calcium ions
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Acetylcholinesterase (AChE) = enzyme breaks down
ACh

Stimulating a Myofiber
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Neurons stimulate myofibers (ie nerve
impulse (AP) stimulate muscle cells)
 Neuromuscular Junction = site where
a neuron meets a myofiber, also called
synapses
Neuromuscular junction (NMJ)

NMJ= space between the muscle cell &
neuron
Synaptic cleft or
synapsegeneral term refers
to a cell-to-cell
junction (ie neuron to
neuron or neuron to
effector cell)
Neuromuscular junction

Motor end plate = highly folded area of
muscle cell membrane (sarcolemma).
Increases stimulation of muscle cell
Excitation-Contraction
Coupling
Muscle Mechanics
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Consists of 4 steps
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Excitation = nerve impulse,
neurotransmitter & excitation of
myofiber
Coupling = ATP & calcium ions
Contraction = muscle cells shorten
Relaxation = muscle cell resume
normal resting length (ATP)
Neuromuscular junction
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Neuron releases a chemical called Acetylcholine
(ACh). The ACh diffuses across the synaptic cleft
and then binds to sarcolemma (muscle cell
membrane), stimulating it.
ACh binds to receptor sites on sodium channels in
the sarcolemma
The combination or binding event leads to an
increase in the permeability of the sarcolemma to
sodium (Na+)
Movement of Na+ into the muscle cell initiates an
AP
Excitation Stage
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Where?
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At the neuromuscular junction, where
a neuron meets (innervates) a
myofiber (muscle cell)
Neuromuscular Junction Structure
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Neuron = nerve cell conducts impulse
Synaptic knob = neuron axon swollen
ending
Synaptic gap or cleft = small space
between myofiber & neuron
Excitation Stage
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Neuromuscular Junction Structure
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Synaptic vesicles = small
membranous sacs which contain
neurotransmitter (acetylcholine)
ACh receptors = location on
sarcolemma where ACh binds when
stimulating myofiber
Muscle Excitation
Step 1: Nerve impulse travels down axon
Step 2: As impulse reaches the synaptic
knob, neurotransmitter, ACh is released
Step 3: Neurotransmitter diffuses across
synaptic cleft (gap) & binds to
sarcolemma, stimulating it.
Step 4: A muscle impulse is produced in
sarcolemma
Muscle Mechanics
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Consists of muscle contraction and
relaxation
Requires:
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ATP and calcium (why?)
Myofibrils
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Myofibrils shorten  muscle cell
contraction
Thick & thin myofilaments slide
across each other, causing myofibrils
to shorten  muscle contraction
(shorten)
Muscle Contraction
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Is due to the arrangement of the
myofilaments (actin & myosin) in
the myofibrils
Let’s take a closer look at the
myofilaments
Arrangement of
myofilaments
Sarcomere –
smallest structural
and functional unit
of muscle. Many
sarcomeres joined
end to end form
myofibrils
Mechanism of muscle
contraction
What Happened?
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Did the myofibril become
shorter?
Did the thin & thick
myofilaments become
shorter?
Any chemical bonding
between thick & thin
myofilaments?
Let’s Take A closer Look!

Myofilaments:
 thick myofilaments = protein called
myosin, resembles golf club (stick
& head)
Myosin
Myosin
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Golf Club Head =>
 myosin globular heads
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Contain ATP binding site, which
binds ATP and breaks it down
(hydrolysis) to ADP+ Pi. ATP
energizes the globular head
Golf Club Stick =>tail
MYOSIN
Tail
Globular Head
ATP
ATP
binding site
ATP energizes myosin to change shape
Let’s Take A Closer Look!

Myofilaments:
 thin myofilaments = protein
called actin, resembles golf
balls
Myofilaments
Actin

Golf Balls =>
 strung together like beads

Each contains myosin binding site,
which binds myosin globular heads
and forms a CROSS BRIDGE
ACTIN
Myosin binding site
Myosin binding site on actin is
not always exposed; otherwise myosin
heads would bind constantly and our
muscles would stay in a state of
contraction (cramp!!!).
ACTIN
Ca2+
Ca2+
Ca2+
Ca2+ Ca2+
Myosin binding site
Muscle contraction is controlled in
many ways. One is by exposing this
myosin binding site on actin only when
calcium ions leave S.R. and bind to
the thin myofilament.
Ca2+
Ca2+
Ca2+
ATP
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
video
Sarcomere Shortening
Sarcomere Shortening
Relaxation
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Action potentials cease
Calcium no longer released from SR
Calcium pumped back in to SR
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Up concentration gradient and requires ATP
In absence of calcium, tropomyosin covers
myosin binding sites again.
Myosin becomes unable to bind actin
Power stroke will not occur-> muscle
relaxes
Changing the Force of
Contraction
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Example:

You are lifting weights at the gym. You pick
up dumbells that differ in weight. In the left
hand, you have the lighter dumbell and in
the right hand the heavier dumbell. What is
happening in each biceps muscle that allows
you to pick up objects of different weight
and control them in the same manner (ie
when doing biceps curls)?
Altering the force of
contraction…..
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Answer:
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You are able to alter the force of
contraction to adjust to the weight
being lifted through 2 mechanisms.
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1. recruit motor units
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2. summation of twitch contractions
video
Motor Unit
-motor unit is a motor neuron
and all of the muscle
cells/fibers it causes to
contract.
-usually one motor neuron will
contact (or innervate) several
muscle cells, but each muscle
cell is innervated by only one
motor neuron.
-large motor unit has a motor
nerve in contact with a large
number of muscle cells (up to
roughly 200).
-small motor unit is one in
which the motor neuron
contacts only a few muscle
cells.
Recruitment

Motor unit recruitment – the progressive activation of
motor units resulting in a more forceful contraction
Summation of Twitch
Contractions