Section 1, Chapter 9
Muscular System
Muscle is derived from Musculus, for “Mouse”
Imagine a mouse running beneath the skin.
Functions of Muscles:
1. Body movement
2. Maintain posture
3. Produces heat
4. Propel substances
through body
5. Heartbeat
Types of muscles include:
1. Smooth muscle
2. Cardiac muscle
3. Skeletal muscle
Smooth Muscle
Characteristics of smooth muscles
• Involuntary control
• Tapered cells with a single, central nucleus
• Lack striations
Smooth Muscle
There are two types of smooth muscles
• Multi-unit Smooth Muscle
• unorganized cells that contract
as individual cells
•Located within the iris of eye
and the walls of blood vessels
• Visceral (single-unit) Smooth Muscle
• Form sheets of muscle
• Cells are connected by gap junctions
• Muscle fibers contract as a group
• Rhythmic contractions
• Within walls of most hollow organs
(viscera)
Cardiac Muscle
•Located only in the heart
•Striated cells
•Intercalated discs
• Muscle fibers branch
•Muscle fibers contract
as a unit
• Self-exciting and rhythmic
Skeletal Muscle
• Usually attached to bone
• Voluntary control
• Striated (light & dark bands)
• Muscle fibers form bundles
• Several peripheral nuclei
Coverings of Skeletal Muscle
Fascia
• Dense connective tissue surrounding skeletal muscles
Tendons
• Dense connective tissue that attaches muscle to bones
• Continuation of muscle fascia and bone periosteum
Aponeurosis
• Broad sheet of connective tissue attaching muscles to
bone, or to other muscles.
Coverings of Skeletal Muscle
Epimysium
• Connective tissue that covers the
entire muscle
• Lies deep to fascia
Perimysium
• Surrounds organized bundles of
muscle fibers, called fascicles
Endomysium
• Connective tissue that covers
individual muscle fibers (cells)
Figure 9.3 Scanning electron micrograph of a fascicle
surrounded by its perimysium. Muscle fibers within the
fascicle are surrounded by endomysium.
Organization of Skeletal Muscle
Fascicle
Organized bundle of muscle fibers
Muscle Fiber
Single muscle cell
Collection of myofibrils
Myofibrils
Collection of myofilaments
Myofilaments
Actin filament
Myosin filament
Figure 9.2
Skeletal muscle
organization
Skeletal Muscle Fibers
Sarcolemma
• Cell membrane of muscle fibers
Sarcoplasm
• Cytoplasm of muscle fibers
Sarcoplasmic Reticulum
• Modified Endoplasmic Reticulum
• Stores large deposits of Calcium
sarcolemma
Skeletal Muscle Fibers
(Transverse)T-tubules:
• invaginations of sarcolemma,
extending into the sarcoplasm.
Cisternae:
• enlarged region of sarcoplasmic
reticulum, adjacent to the t-tubules
Triad
• T-tubule + adjacent cisternae
Openings into t-tubules
Myofibrils
Myofibrils are bundles of
actin and myosin filaments.
• Actin – thin filament
• Myosin – thick filament
Striations appear from the
organization of actin and
myosin filaments
Figure 9.4 Organization of actin and myosin filaments
Sarcomere
A sarcomere is the functional unit of
skeletal muscle
• A sarcomere is the area between
adjacent Z-lines.
•During a muscle contraction the z-lines
move together and the sarcomere shortens.
Sarcomere
Striations appear from alternate light and dark
banding patterns.
Z Line is the attachment site of actin
filaments (center of I bands)
I Bands (light band): consists of only
actin filaments
A Bands (dark band) : consists of
myosin filaments and the overlapping
portion of actin filaments
Figure 9.5 thin and thick filaments in a
sarcomere.
filaments
Thin filaments
composed of actin proteins
Thin filaments are
associated with troponin
and tropomyosin proteins
Thick filaments
composed of myosin proteins
During muscle contraction the
heads on myosin filaments
bind to actin filaments forming
a Cross-bridge
Cross-Bridges
When a muscle is at rest, myosin heads are extended in the “cocked” position.
During a contraction, myosin heads bind to actin, forming a cross-bridge and the
myosin head pivot forward (Power Stroke) and back (Recovery stroke)
Troponin-Tropomyosin Complex
The troponin-tropomyosin complex prevents cross-bridge
formation when the muscle is at rest.
Tropomyosin
Blocks binding sites on
actin when a muscle is
at rest
Troponin
Ca2+ binds to troponin
during a muscle contraction.
Troponin moves repositions the
tropomyosin filaments, so the
myosin and actin filaments can
interact.
End of section 1, chapter 9
Chapter 9, Section 2
Muscle Contractions
Synapse
Synapse: Functional (not physical) junction between an axon
of a neuron and another cell
The two cells are separated by a physical
space, called the synaptic cleft.
Neurotransmitters are stored within
synaptic vesicles of the presynaptic cell
and they’re released into the synapse.
Neuromuscular Junction
Neuromuscular Junction (NMJ) refers to the synapse
between an axon and a muscle fiber.
Motor End Plate is a highly folded region of muscle fiber at
NMJ that contain abundant mitochondria
Figure 9.8a.
General NMJ
Motor Unit
Motor neurons innervate
effectors (muscles or glands)
A motor unit includes a
motor neuron and all of the
muscle fibers it controls
1 motor unit may control between
1 and 1000 muscle fibers
Figure 9.9 two motor units. The muscle fibers
of a motor unit are innervated (controlled) by a
single motor neuron.
Stimulus for Contraction
Acetylcholine (ACh) is the only neurotransmitter
that initiates skeletal muscle contraction
Sequence of Actions
1. A nerve impulse (Action Potential)
reaches axon terminal
2. The impulse opens calcium channels at
the axon terminal
• Calcium diffuse into axon
3. The calcium triggers the release of ACh
from vesicles into synaptic cleft.
Stimulus for Contraction
Sequence of Actions…Continued
4. ACh diffuses across synaptic cleft &
binds to receptors on motor endplate.
5. ACh opens Na+ channels on muscle
6. Na+ floods into the muscle, initiating a
muscle impulse.
7. A muscle impulse (action potential) is
propagated across the entire muscle.
Stimulus for a muscle
impulse. Corresponds to
steps 1-7 in the previous
slides.
The muscle impulse causes the release of calcium from the SR. Calcium binds
to troponin and tropomyosin is repositioned exposing the actin filaments.
Stimulus for contraction continued…
8. The muscle impulse diffuses across
sarcolemma and down the t-tubules into
the cisternae of sarcoplasmic reticula.
9. The sarcoplasmic reticula release
their calcium supplies into the
sarcoplasm.
10. Calcium binds to troponin and the
troponin repositions the tropomyosin,
so the myosin can bind to actin.
11. Cross-bridge cycling causes the
muscle to contract.
Excitation-Contraction Coupling
Calcium released from sarcoplasmic reticulum binds to troponin.
Troponin moves tropomyosin, exposing actin filaments to myosin
cross-bridges.
myosin heads bind to actin, forming a cross bridge and cross-bridge
cycling causes the muscle to contract.
End of section 2, chapter 9
ivyanatomy.com
section 3, chapter 9
Sliding Filament Theory of Contraction
The Sliding Filament Model of Muscle Contraction
During a muscle contraction
Thick (myosin) filaments and thin (actin)
filaments slide across one another
The filaments do not change lengths
Z-bands move closer together causing the
sarcomere to shorten.
I bands appear narrow
Figure 9.11a. Individual sarcomeres shorten as thick and
thin filaments slide past one another.
Cross Bridge Cycling
1.
When a muscle is relaxed, tropmyosin covers the
binding sites on actin.
A molecule of ADP and Phosphate remains
attached to myosin from the previous
contraction.
Cross Bridge Cycling
2.
During a contraction, Calcium binds to troponin.
Tropomyosin is repositioned, exposing the myosin
binding sites on actin filaments
Cross Bridge Cycling
3. Myosin heads bind to actin filaments.
The phosphate is released.
Cross Bridge Cycling
4. Myosin heads spring forward “Power Stroke” pulling the actin
filaments.
ADP is released from Myosin
Cross Bridge Cycling
5. Myosin is released from actin.
A new molecule of ATP binds to myosin, causing it to be released from the actin
filament.
• ATP is not yet broken down, but it is essential to release the cross-bridges.
Cross Bridge Cycling
6. ATP is broken down, providing the energy to
“cock” the myosin filaments (recovery stroke).
7. Steps 1-6 are repeated several times.
Figure 9.10. The cross-bridge cycle. The
cycle continues as long as ATP is present,
and nerve impulses release Acetylcholoine.
Watch the You-Tube video
“Sliding Filament” to view
cross-bridge cycling in action.
Relaxation
When a nerve impulse ceases, two events relax muscle fibers.
1.
Acetylcholinesterase breaks down Ach in the synapse.
• Prevents continuous stimulation of a muscle fiber.
2.
Calcium Pumps (Ca2+ATPase) remove Ca2+ from the sarcoplasm and
returns it to the SR.
• Without calcium, tropomyosin covers the binding sites on actin
filaments.
Relaxation
Rigor Mortis is a partial contraction of skeletal muscles that occurs a few
hours after death.
• After death calcium leaks into sarcoplasm, triggering the muscle
contractions.
• But ATP supplies are diminished after death, so ATP is not available to
remove the cross-bridge linkages between actin and myosin.
• muscles do not relax*.
• Contraction is sustained until muscles begin to decompose.
* Notice that ATP is required for muscle relaxation!
End of Chapter 9, Section 3
ivyanatomy.com
section 4, chapter 9
Energy Sources for Contraction
Energy Sources for Contraction
ATP provides the energy to power the interaction between
actin & myosin filaments.
• However, ATP is quickly spent and must be replenished
New ATP molecules are synthesized by
1. Hydrolysis of Creatine Phosphate
2. Glycolysis (anaerobic respiration)
3. Aerobic Respiration
Creatine Phosphate
Creatine Phosphate can be hydrolyzed into Creatine, releasing
energy that is used to make new ATP.
The energy from creatine phosphate hydrolysis cannot be used
to directly power muscles.
Instead, it’s used to produce new ATP.
Creatine Phosphate…continued
When cellular ATP is abundant, creatine phosphate can
be replenished by phosphorylating creatine.
Creatine Phosphate provides energy for only about 10
seconds of a high intensity muscle contraction.
Glycolysis
Anaerobic respiration (glycolysis) occurs in the cytosol of the cell
and does not require oxygen.
Glucose molecules are partially broken down producing just 2 ATP
for each glucose.
If there isn’t sufficient oxygen available, glycolysis produces lactic
acid as a byproduct.
Oxygen debt of glycolysis
Exercise and strenuous activity depends on
anaerobic respiration for ATP supplies.
During exercise anaerobic respiration causes
lactic acid to accumulate in the cells.
After exercise, when oxygen is available
the O2 is used to convert lactic acid back to
glucose in the liver.
Oxygen debt is the amount of oxygen
needed by liver cells to convert
accumulated lactic acid back to glucose.
Oxygen debt
Aerobic Respiration
Aerobic respiration (uses oxygen) occurs in the mitochondria and it
includes the citric acid cycle & electron transport chain.
Aerobic respiration is a slower reaction than glycolysis, but it produces
the most ATP.
Myoglobin
Oxygen binding protein (similar to hemoglobin) within muscles
-Provides additional oxygen supply to muscles
Aerobic Respiration
Aerobic respiration is used primarily at rest or during
light exercise.
Muscles that rely on aerobic respiration have plenty of
mitochondria and a good blood supply.
Energy Sources for Contraction
Figure 9.13. The oxygen required for aerobic respiration is
carried in the blood and stored in myoglobin. In the absence of
oxygen, anaerobic respiration uses pyruvic acid to produce lactic
acid.
Muscle Fatigue
• Muscle Fatigue = Inability for the muscle to contract
• Several factors can cause muscle fatigue:
• Decreased blood flow
• Ion imbalances across the sarcolemma
• Lactic acid accumulation – (greatest cause of fatigue)
• Cramp:
• A cramp is a sustained, involuntary, and painful muscle
contraction
• It’s due to electrolyte imbalance surrounding muscle
Heat Production
• Heat is produced as a by-product of cellular respiration
• Muscle cells are major source of body heat
• Blood transports heat throughout body core
End of Chapter 9, Section 4
ivyanatomy.com
section 5, chapter 9
muscular responses
Muscle Response
A muscle contraction can be observed by removing a single skeletal
muscle and connecting it to a device (myograph) that senses and
records changes in the overall length of the muscle fiber.
A threshold stimulus is the minimum
stimulus that elicits a muscle fiber contraction
all-or-none response
A threshold stimulus will cause the muscle
fiber to contract fully and completely.
A stronger stimulus does not produce a
stronger contraction!
muscle
subthreshold stimulus
The muscle fiber will not contract at all if
the stimulus is less than threshold.
Myograph
Recording of a Muscle Contraction
A twitch is a single contractile response to a stimulus
A twitch can be divided into three periods.
1. Latent period
brief delay between the stimulus
and the muscle contraction
The latent period is less than 2
milliseconds in humans
2. Period of contraction
3. Period of relaxation
Summation
If the muscle is allowed to relax
completely before each stimulus than the
muscle will contract with the same force.
If the muscle is stimulated again before
it has completely relaxed, then the force
of the next contraction increases.
i.e. stimulating the muscle at a rapid
frequency increases the force of
contraction. This is called summation
Figure 9.17a
series of twitches
Figure 9.17b
summation
Summation
Tetanic Contraction (c)
If the muscle is stimulated at a high frequency the contractions fuse
together and cannot be distinguished.
A tetanic contraction results in a maximal sustained contraction without
relaxation
Figure 9.17c
Recruitment of Motor Units
all-or-none response
A muscle that is stimulated with threshold potential contracts
completely and fully.
A stronger stimulus does not produce a stronger contraction!
Instead, the strength of a muscle is increased by recruitment of
additional motor units.
Recruitment of Motor Units
Recruitment – progressive activation of motor units to
increase the force of a muscle contraction.
Recall that a motor unit is a motor neuron plus all of the fibers it controls.
• Muscles are composed of many motor units.
• As a general rule, motor units are recruited in order of their size
• Small motor units are stimulated with light activities, but additional
motor units are recruited with higher intensity activity.
As the intensity of stimulation increases,
recruitment of motor units continues until
all motor units are activated.
Sustained Contractions
The central nervous system can increase the
strength of contractions in 2 ways:
1.
Recruitment
•
Smaller motor units are recruited first, followed by larger motor units.
•
The result is a sustained contraction of increasing strength.
2.
Increased firing rate
•
A high frequency of action potentials results in summation of the muscle
contractions.
•
If the frequency is too high, however, it may produce tetanic contractions, in which
case the muscle does not relax.
Muscle tone is produced because some muscles are in a continuous
state of partial contraction in response to repeated nerve impulses from
the spinal cord.
Types of Contractions
Isotonic – muscle contracts and changes length
Concentric – shortening of muscle (a)
Eccentric – lengthening of muscle (b)
Isometric – muscle contracts but does not change length (c)
Isometric contractions stabilizes posture and holds the
body upright
Figure 9.18. muscle contractions
Fast twitch and slow twitch muscle fibers
Fast & Slow twitch refers to the contraction speed, and to whether muscle
fibers produce ATP oxidatively (by aerobic respiration) or glycolytically
(by glycolysis)
Slow-twitch fibers (Type I)
• Always oxidative and resistant to fatigue
• Contain myoglobin for oxygen storage “red fibers”
• Also have many mitochondria for aerobic respiration
• Good blood supply
Slow-twitch fibers (Type I)
Slow-twitch fibers are best suited for endurance
exercise over a long period with little force.
Slow-twitch fibers rely on aerobic respiration
for energy (ATP) and are resistant to fatigue.
Slow-Twitch fibers contain abundant myoglobin
for oxygen storage “red fibers” and mitochondria
to carry out aerobic respiration.
Because of their oxygen demands, slow-twitch
fibers have a good blood supply.
Fast twitch muscle fibers – two types
Fast-twitch glycolytic fibers contract rapidly
and with great force, but they fatigue quickly.
They are best suited for rapid
contractions over a short duration.
Fast-twitch glycolytic fibers (type IIa) contain
very little mitochondria and myoglobin and are
“white fibers”
Fast twitch muscle fibers – two types
Fast-twitch intermediate or fast oxidative fibers
contain intermediate amounts of myoglobin.
They contract rapidly but also have the capacity
to generate energy through aerobic respiration.
Fast twitch and slow twitch muscle fibers
Migrating birds have abundant slowtwitch fibers for flying long distances,
which is why their flesh is dark.
Chickens that can only flap around the
barnyard have abundant fast-twitch
muscles and mostly white flesh.
End of Chapter 9