Download The Muscular System

Muscle Tissue
Muscle Structure and Function
Types of Muscle Tissue
• Skeletal Muscle Tissue – moves the body by pulling
on bones of the skeleton
▫ Allows us to walk, move, pick up and throw objects
 Voluntary – we can control
• Cardiac Muscle Tissue – pumps blood through the
circulatory system
▫ Involuntary – we can’t control
• Smooth Muscle Tissue – pushes material through
the digestive tract and controls the diameter of small
arteries.
▫ Involuntary – we can’t control
Functions of Skeletal Muscle
1.
2.
3.
4.
5.
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Produce skeletal movement
Maintain posture and body positioning
Support of soft tissues
Guard entrances and exits
Maintain body temperature
Store nutrient reserves
Organization of Muscle Tissue
• From smallest structure to largest structure:
▫ Muscle fiber (cell)
Muscle fascicle (bundle of
cells)
Skeletal Muscle (organ)
Anatomy of Skeletal Muscle
• 3 layers of connective tissue
▫ Epimysium
 Surrounds entire muscle
▫ Perimysium
 Divides skeletal muscle into compartments called
fascicles
 Contains blood vessels and nerve fibers
▫ Endomysium
 Surrounds each individual muscle fiber
 Contains capillaries that supply blood to fiber, satellite
cells (stem cells that repair muscle cells), nerve fibers that
control the muscle.
Anatomy of Skeletal Muscle
• The fibers of the epimysium, endomysium and
perimysium are interwoven to form either a
bundle (tendon) or a broad sheet (aponeurosis)
Skeletal Muscle Fibers (cells)
• Different than typical cells
▫ Very large
 Can run the length of your thigh (30cm)
▫ Multi-nucleated
 Contain hundreds of nuclei
 Control production of enzymes and proteins necessary
for muscle function
Skeletal Muscle Fibers (cells)
• Formed during development from the fusing of
multiple embryonic cells (myoblasts)
• Some myoblasts don’t fuse
▫ Called satellite cells in adult muscles
 Repair damaged muscle
Skeletal Muscle Fibers (cells)
• Parts of a muscle fiber
▫ Sarcolemma – cell membrane of muscle cells
▫ Sarcoplasm – cytoplasm of muscle cells
▫ Transverse tubules (T-tubules) – narrow tubes
that carry the electric signal for contraction deeper
into the cell
Skeletal Muscle Fibers (cells)
• Parts of a muscle fiber
▫ Myofibrils
 Consist of bundles of protein filaments
 Thin filaments – composed of actin
 Thick filaments - composed of myosin
 Actively shortening component of muscle
 Responsible for muscle contractions
Skeletal Muscle Fibers (cells)
• Sarcoplasmic reticulum (SR)
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Similar to endoplasmic reticulum in regular cells
Tightly bound to the T-tubules
Forms a network around each myofibril
Stores Ca++ ions for muscle contractions
 Up to 40,000 time the amount found in the
sarcoplasm
 A contraction begins when Ca++ ions are released
into the sarcoplasm
• Video Clip
Skeletal Muscle Fibers (cells)
• Sarcomeres
▫ Functional unit of skeletal muscle
 Actual contracting unit
▫ About 10,000 sarcomeres, end-to-end, make up a
myofibril
Skeletal Muscle Fibers (cells)
• Each sarcomere has dark bands (A bands) and
light bands (I)
• The A band
▫ Made up of thick filaments (myosin)
▫ 3 subdivisions
 M line – dark, central line where the thick filaments
are connected to their neighbors
 H zone – light area around the M line. Has thick
filaments but no thin filaments
 Zone of overlap – thin and thick filaments overlap
Skeletal Muscle Fibers (cells)
• The I band
▫ Contains just the thin (actin) filaments
▫ Extends from the A band from one sarcomere to
the A band of the next
▫ Z line – marks the boundary between adjacent
sarcomeres
• The A and I bands are visible with a light
microscope and are called striations, thus
skeletal muscle is also known as striated muscle
Skeletal Muscle Fibers (cells)
• Thin Filaments
▫ Contains strands of proteins (actin)
▫ Has active sites that are used during muscle
contraction
Skeletal Muscle Fibers (cells)
• Thick Filaments
▫ Contains roughly 300 myosin molecules
 The myosin tail is long and is bound to other myosin
molecules in the thick filament
 The free head projects out toward the nearest thin
filament
 When the head interacts with the thin filament during a
contraction, it is called a cross-bridge
Skeletal Muscle Fibers (cells)
• Thick Filaments
▫ Myosin molecules are arranged with their tails
towards the M line
▫ The heads are arranged in a spiral
▫ H zone contains no myosin heads
Sliding Filament Theory
• When a muscle contracts:
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H zones and I bands get smaller
Zones of overlap get larger
Z lines move closer together
Width of the A band remains constant
• This only makes sense if the thin filaments slide
alongside the thick filaments toward the center
of the sarcomere (M line)
▫ This is known as the sliding filament theory
Muscle Tissue
Muscle Contraction
Muscle Contraction
• When muscle fibers contract, they actively pull
on the tendon fiber the way people pull on a
rope.
▫ This pull is called tension
▫ It is an active force, so it requires energy
• For movement to occur, the tension must
overcome the resistance of the object.
▫ Resistance depends on the weight, shape, friction,
and other factors.
• MUSCLES PULL…THEY DO NOT PUSH!!!!
Overview of Skeletal Muscle
Contraction
• Skeletal muscle fibers are activated by neurons
(nerve cells)
▫ Activated by stimulation of the sarcolemma
• Excitation-contraction coupling occurs next
▫ Calcium ions are released from sarcoplasmic reticulum
• Calcium ions trigger interactions between thick and
thin filaments, resulting in fiber contraction and the
consumption of energy in the form of ATP
• Tension is produced.
Control of Skeletal Muscle
• Skeletal muscle only contracts under control of
the nervous system
▫ Communication that occurs between muscle and
nerve takes place at what is known as a
neuromuscular junction (NMJ)
• Each muscle fiber is controlled by a neuron at a
single NMJ midway along its length.
Control of Skeletal Muscle
• The neuron branches when it reaches the muscle
▫ At the end of each branch, there is a synaptic
terminal
 Contains the neurotransmitter Acetylcholine (Ach)
• The synaptic cleft is the narrow space between the
synaptic terminal and the sarcolemma.
• The sarcolemma surface of the synaptic cleft is
known as the motor end plate
• The synaptic terminal and the sarcolemma contain
acetylcholinesterase (AChE)
▫ Breaks down ACh
Control of Skeletal Muscle
• Stimulation of the muscle occurs through 5 steps.
1. Arrival of action potential
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Electrical impulse arrives at synaptic terminal
2. Release of ACh
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The action potential triggers the release of ACh into the
synaptic cleft
3. ACh binds at the Motor End Plate
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ACh molecules diffuse across cleft and bind to
receptors on Motor End Plate
Increases the sarcolemma’s permeability of sodium
ions, and sodium ions rush into the sarcolemma
Control of Skeletal Muscle
4. Appearance of action potential in the
sarcolemma
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The rush of sodium ions causes an action potential
in the sarcolemma
Travels inward via the T-tubules
5. Return to initial state
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ACh is broken down by AChE.
ACh is recycled
Excitation-Contraction Coupling
• The step between the generation of the action
potential in the sarcolemma and the start of a
muscle contraction is called excitationcontraction coupling.
• The action potential in sarcolemma triggers the
release of calcium ions(Ca++) from the
sarcoplasmic reticulum.
Excitation-Contraction Coupling
• Remember that the thin filament has active sites
on it.
▫ At rest, these are covered
• After the Ca++ is released from the sarcoplasmic
reticulum, the active site is uncovered, allowing
the myosin head to bind.
• Muscle contraction now begins
The Contraction Cycle
• The myosin head is already energized, ready to act.
• Step 1: Exposure of Active Sites
 Ca++ binds to troponin, exposing the active sites
• Step 2: Formation of Cross-Bridges
 The myosin heads bind to the exposed active sites
• Step 3: Pivoting of myosin heads
 When at rest, the myosin head points away from the M
line. Myosin head is “cocked”
 After cross-bridge formation, the head pivots toward the
M line as energy is released (called the power stroke)
The Contraction Cycle
• Step 4: Detachment of Cross-Bridges
 When another ATP binds to the myosin head, it
detaches from the active site
 Active site can now form another cross-bridge
• Step 5: Reactivation of Myosin
 Occurs when myosin head splits the ATP
 This energy is used to re-cock the myosin head
• This cycle can be repeated several time each
second
The Contraction Cycle
• Each power stroke shortens the sarcomere by 1
percent
▫ Because all sarcomeres contract together, the
entire muscle shortens at the same rate.
• To better understand how tension is produced in
a muscle fiber, think of a tug-of-war.
Relaxation
• Duration of contraction depends on:
▫ Duration of stimulation at NMJ
▫ Presence of free Ca++ in the sarcoplasm
▫ Availability of ATP
Relaxation
• If one action potential arrives at the NMJ, Ca++
levels in the sarcoplasm will quickly return to
normal.
• Two mechanisms are involved in this process:
▫ Active transport of Ca++ across the cell membrane
into the extracellular fluid
▫ Active transport of Ca++ into the SR
 This one is much more important
Relaxation
• As the Ca++ levels in the sarcoplasm fall,
▫ Active sites are re-covered.
• The contraction ends
Return to Resting Length
• Since muscle can’t actively lengthen, outside
forces must lengthen the muscle
▫ Opposing muscle contractions
▫ Gravity
Rigor Mortis
• Within a few hours after death, muscle fibers
run out of ATP
▫ The SR can not pump Ca++ out of the sarcoplasm,
triggering a sustained contraction
▫ Myosin heads don’t detach from active sites.
• Rigor mortis lasts until the Z-lines are broken
down 15-25 hours after rigor mortis sets in.
Muscle Tissue
Tension Production
Tension Production
• Tension depends on the amount of pivoting
cross-bridges.
• There is no mechanism to regulate the amount
of tension by changing the number of
contracting sarcomeres
▫ When Ca++ is released, it is released from all SR
in the muscle fiber
▫ Muscle fiber is either “on” or “off”
Tension Production
• Tension at the muscle fiber level does vary. It
depends on:
▫ The fiber’s resting length at the time of
stimulation
▫ The frequency of stimulation
Length-Tension Relationship
• The amount of tension depends on the number
of cross-bridges along the length of the fiber.
▫ Depends on the degree of overlap between thick
and thin fibers.
▫ When a fiber is stimulated to contract, only the
myosin heads in the zone of overlap can bind to
the active sites.
 The more myosin heads in the zone, the more
tension…To a point
Length-Tension Relationship
• A sarcomere works most efficiently within an
optimal range
• If the sarcomere is stretched, there will be less
myosin heads in the zone of overlap.
• If the sarcomere is compressed/shortened, it has
less room to shorten because the actin filaments
are already close to the M line.
Length-Tension Relationship
Frequency of Stimulation
• Twitch
▫ A single stimulation producing a single
contraction
▫ Lasts 7-100 milliseconds depending on the muscle
▫ Can be divided into 3 phases
 Latent period – begins at stimulation, lasts 2 msec
 Action potential sweeps across sarcolemma, and Ca++
is released from SR
 NO TENSION IS PRODUCED YET
Frequency of Stimulation
 Contraction phase – tension rises to a peak
 Cross-bridges are forming
 Ends about 15msec after stimulation
 Relaxation phase – lasts about 25msec
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Ca++ levels fall
Active sites are being covered
Decrease in cross-bridges
Tension falls back to resting levels
Frequency of Stimulation
Frequency of Stimulation
• Treppe
▫ When the muscle is stimulated immediately after
the relaxation phase has ended.
 The resulting contraction will produce slightly more
tension than the first
 This will continue for the first 30-50 contractions,
then tension levels off
 Tension rises because there is extra Ca++ left over
from previous stimulus. SR doesn’t have enough
time to pump all Ca++ back in.
Frequency of Stimulation
• Wave Summation
▫ If a second stimulus arrives before the relaxation
phase has ended, a second, more powerful
contraction occurs.
 A stimulus of greater of 50 per second will produce
wave summation
• Incomplete Tetanus
▫ If stimulation continues and the muscle is never
allowed to relax completely, tension will rise until
it reaches a peak 4X that of treppe.
▫ This is incomplete tetanus
Frequency of Stimulation
• Complete Tetanus
▫ Occurs when a higher stimulation frequency
eliminates the relaxation phase completely.
 SR doesn’t have time to reclaim the Ca++ at all.
Frequency of
Stimulation
Motor Units
• Amount of tension a muscle produces as a whole is
the sum of the tensions generated by the individual
muscle fibers
▫ You control the amount of tension in skeletal muscle
by controlling the number of stimulated muscle fibers
• Motor Unit - All the muscle fibers controlled by a
single motor neuron
▫ The smaller the unit, the finer the control you have
over the muscle.
 Eye muscles have motor units of 4-6 muscle fibers
 Leg muscles may have motor units consisting of 10002000 muscle fibers.
Motor Units
• When a movement is performed, the smallest
motor units in the muscle are activated
▫ These muscle fibers contract slowly
• As the movement continues, larger, faster, more
powerful motor units are activated.
▫ Tension production rises steeply
• Recruitment – smooth steady increase in
tension by increasing the number of activated
motor units
Muscle Tone
• Muscle tone – the resting tension in skeletal muscle
▫ Activated muscle units are changed so that they may
relax and recover
• Resting muscle tone:
▫ Stabilizes positions of bones
 Ex. – balance and posture
▫ Prevents sudden, uncontrolled changes in the
positions of bones and joints
▫ Higher resting tone accelerates the recruitment
process because some of the motor units are already
activated
Types of Muscle Contraction
• Isotonic Contraction
▫ Tension rises and the muscle changes length
 Muscle can shorten AND lengthen while activated
▫ Concentric Contraction – tension EXCEEDS
resistance and muscle shortens
 Ex: flexion of elbow
▫ Eccentric Contraction – tension is LESS than the
load and the muscle elongates
 Due to pull of another muscle, gravity, etc.
 Ex: extension of elbow
Types of Muscle Contraction
• Isometric Contraction
▫ Muscle length does not change
 Tension produced never exceeds resistance
 Ex: holding a bag of groceries, standing upright