BIO2305
Muscle Physiology
Muscle accounts for nearly half of the body’s mass
- Muscles have the ability to change chemical energy (ATP) into mechanical energy
Types of Muscle Tissure
3 types:
1. Skeletal - tissue is attached to bones, it is striated, voluntary control, multinucleated
2. Smooth - located in the walls of hollow internal structures, nonstriated, involuntary
3. Cardiac - forms most of the heart, it is striated, involuntary, autorhythmic
Histology Review
1. Sarcolemma - a muscle fiber plasma (cell) membrane
2. Sarcoplasm - muscle fiber cytoplasm, almost completely filled with contractile filaments called
myofilaments. Sarcoplasm contains glycosomes (granules of glycogen) and the oxygen-binding protein
called myoglobin
3. Sarcoplasmic reticulum - (endoplasmic reticulum) a network of tubes surrounding myofibrils,
functions to reabsorb calcium ion during relaxation, release them to cause contraction.
4. Transverse tubules - tubules formed by invaginations of the sarcolemma and flanked by the
sarcoplasmic reticulum, they carry action potentials deep into the muscle fiber. T tubules and SR provide
tightly linked signals for muscle contraction. T tubules at each A band/I band junction - continuous with
the sarcolemma. Conduct electrical impulses to the throughout cell (every sarcomere) - signals for the
release of Ca2+ from adjacent terminal cisternae
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T tubule proteins (Dihydropyridine) act as voltage sensors
SR foot proteins are (ryanodine) receptors that regulate Ca2+ release from the SR cisternae
5. Myofibril - bundle of thread like contractile elements consisting of myofilaments, 80% of the muscle
volume, contain the contractile elements of skeletal muscle cells
6.
Myofilaments - extremely fine thread like proteins, there are three types;
1. thick filaments (16nm) called myosin. Each myosin molecule (two interwoven
polypeptide chains) has a rodlike tail and two globular heads
2. thin filaments (8nm) called actin;
3. elastic filaments.
During muscle contraction, the myosin Heads link the thick and thin filaments together, forming cross
bridges. Actin provides active sites where myosin heads attach during contraction. Tropomyosin and
Troponin are regulatory subunits bound to actin.
7. Sarcomere – smallest contractile unit of a muscle fiber, a compartment of myofibrils
The sarcomere is characterized by alternating light and dark bands or zones produced by the myofilaments
Z disc - a line that separates one sarcomere from another
M line - central line of the sarcomere where myosin filaments are anchored
H zone - the area where only myosin filaments are present
Z
M
Z
I band - the area where only actin filaments are present
A band - includes overlapping myosin and actin filaments
I
A
H
A
I
For contraction to occur, 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 stimulus for contraction
Molecular Basis of Skeletal Muscle Contraction (Sliding Filament Model)
Excitation-Contraction Coupling is the sequence of events linking the transmission of an action potential
along the sarcolemma to muscle contraction (the sliding of myofilaments)
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The sarcolemma, like other plasma membranes is polarized. There is a potential difference
(voltage) across the membrane
When Ach binds to its receptors on the motor end plate, chemically (ligand) gated ion channels in
the receptors open and allow Na+ and K+ to move across the membrane, resulting in a transient
change in membrane potential - Depolarization
End plate potential - a local depolarization that creates and spreads an action potential across the
sarcolemma
The AP lasts only 1-2 ms and ends before contraction occurs. The period between action potential initiation
and the beginning of contraction is called the latent period. E-C coupling occurs within the latent period.
1. The action potential is propagated along (across) the sarcolemma and travels through the T tubules
At the triads, the action potential causes voltage sensitive T tubule Dihdropyridine receptors to be activated.
This in turn, causes the SR foot proteins of the terminal cisternae (lateral sacs) to open Ca2+ release
channels and Ca2+ is released into the sarcoplasm (where the myofilaments are)
2. The released calcium combines with troponin, troponin then pulls the tropomyosin changing its
position so that the actin binding sites are exposed. Myosin heads can now alternately attach and detach,
pulling the actin filaments toward the center of the sarcomere (ATP hydrolysis is necessary)
3. The ATP attached to the myosin head is split by ATPase causing the myosin heads to be activated.
4. The activated myosin head attaches to the actin binding site, then swivels, producing a power stroke
which results in the sliding of the filaments. The ADP and P are released. Contraction refers to the
activation of myosin’s cross bridges – the sites that generate the force
5. Once the power stroke is complete, ATP again attaches to the myosin head causing the head to detach
from the actin site and return to its original position.
6. Cycle can then be repeated over and over again as long as calcium and ATP are present.
7. Relaxation is caused by the breaking down of ACh by the enzyme acetylcholinesterase and the
reabsorption of calcium back into the SR
Muscle Tension Physiology
• Motor unit - One motor neuron and the muscle fibers it innervates
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Number of muscle fibers varies among different motor units
Number of muscle fibers per motor unit and number of motor units per muscle vary widely
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Muscles that produce precise, delicate movements contain fewer fibrs per motor unit
Muscles performing powerful, coarsely controlled movement have larger number of fibers per
motor unit
Individual muscle fibers contract to their fullest extent; they do not partially contract, this follows the all or
none principle.
A twitch contraction is a brief contraction of all the muscle fibers in a motor unit in response to a single
action potential. It involves three phases which can be recorded on a myogram
1. Latent period - time elapsed from the application of a stimulus to the beginning of the contraction
(when Ca is being released)
2. Contraction period - cross bridges are active and the muscle shortens if the tension is great enough to
overcome the load
3. Relaxation period - active transport of Ca++ back into the sarcoplasmic reticulum and degradation of
ACh.
Graded muscle responses are variations in the degree or strength of muscle contraction in response to
demand, required for proper control of skeletal movement
Muscle contraction can be graded (varied) in two ways:
1. Frequency of the stimulation
2. Number of motor units stimulated
Motor unit recruitment - The process of increasing the number of active motor units in a muscle for
stronger contractions
A single stimulus results in a single contractile response – a muscle twitch (contracts and relaxes)
More frequent stimuli increases contractile force – wave summation - muscle is already partially
contracted when next stimulus arrives and contractions are summed
A sustained contraction that lacks even partial relaxation is known as tetanus.
Asynchronous recruitment of motor units is used to prevent fatigue. While some motor units are active
others are inactive. This pattern of firing of motor neurons prevents fatigue while maintaining contraction
by allowing a brief rest for the inactive units.
Recruitment also helps provide smooth muscle action rather than jerky movements.
Muscle tone
• The constant, slightly contracted state of all muscles
• Does not produce active movements
• Keeps the muscles firm and ready to respond to stimulus
• Helps stabilize joints and maintain posture
• Due to spinal reflex activation of motor units in response to stretch receptors in muscles and
tendons
Strength of muscle tension depends on
Number of motor units recruited, recruitment also helps provide smooth muscle action rather than
jerky movements
• The relative size of the muscle fibers – the bulkier the muscle fiber (greater cross-sectional area),
the greater its strength
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Asynchronous recruitment of motor units -while some motor units are active others are inactive this pattern of firing provides a brief rest for the inactive units preventing fatigue
Degree of muscle stretch
Types of Contractions
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The force exerted on an object by a contracting muscle is called muscle tension, the opposing
force or weight of the object to be moved is called the load.
Isometric - the muscle does not or cannot shorten, but the tension on the muscle increases
Isotonic - moving a constant load through the range of muscle motion, there are two types of isotonic
contractions
1. Concentric - the muscle shortens and pulls that produces a movement to reduce the angle at a joint
2. Eccentric - the overall muscle lengthens during a contraction
Metabolism
A. several pathways supply ATP to muscle cells:
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ATP is the only energy source that is used directly for contractile activity
As soon as available ATP is hydrolyzed (4-6 seconds), it is regenerated by three pathways:
1.
Transfer of high-energy phosphate from creatine phosphate to ADP, first energy
storehouse tapped at onset of contractile activity. Transfer of energy as a phosphate
group is moved from CP to ADP – the reaction is catalyzed by the enzyme creatine
kinase
• Creatine phosphate + ADP → creatine + ATP
Stored ATP and CP provide energy for maximum muscle power for 10-15 seconds
2.
Oxidative phosphorylation (citric acid cycle and electron transport system - takes place
within muscle mitochondria if sufficient O2 is present
-Glucose is broken down into pyruvic acide to yield 2 ATP
-When oxygen demand cannot be met, pyruvic acid is converted into lactic acid
-Lactic acid diffuses into the bloodstream – can be used as energy source by the liver,
kidneys, and heart
-Can be converted back into pyruvic acid, glucose, or glycogen by the liver
3.Glycolysis - supports anaerobic or high-intensity exercise,
Aerobic respiration occurs in mitochondria - requires O2
series of reactions breaks down glucose for high yield of ATP
• Glucose + O2 → CO2 + H2O + ATP
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Muscle fatigue – the muscle is physiologically not able to contract
Occurs when oxygen is limited and ATP production fails to keep pace with ATP use
Lactic acid accumulation and ionic imbalances may also contribute to muscle fatigue
Depletion of energy stores – glycogen
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When no ATP is available, contractures (continuous contraction) may result because cross bridges
are unable to detach
- Insufficient O2, build up of lactic acid
- Depletion of energy reserves - glycogen
Ionic imbalance, neural fatigue
Central Fatigue – psychological, it hurts
For a muscle to return to its pre-exercise state:
- Oxygen reserves must be replenished
- (Lactic acid must be converted to pyruvic acid?)
- Glycogen stores must be replaced
- ATP and CP reserves must be resynthesized
Oxygen debt – the extra amount of O2 needed for the above restorative processes
Muscle Fiber Types
Type I slow oxidative - fatigue resistant fibers containing large amounts of myoglobin, many
mitochondria, a many capillaries, high capacity for generating ATP, slow contraction rate. Abundant in
postural muscle groups
Type IIA fast oxidative - same as Type I except fast contraction rate. Abundant is muscle groups requiring
speed (sprinter)
Type IIB fast glycolitic - easily fatigueable fibers, low myoglobin, few mitochondria, few capillaries, large
amounts of glycogen , large diameter fibers used in muscles requiring strong and rapid, but brief
contractions (arms).
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Speed of contraction – determined by how fast their myosin ATPases split ATP
• Oxidative fibers – use aerobic pathways
• Glycolytic fibers – use anaerobic glycolysis
Smooth Muscle
• Occurs within most organs
• Walls of hollow visceral organs, such as the stomach
• Urinary bladder
• Respiratory passages
• Arteries and veins
• Helps substances move through internal body channels via peristalsis
• No striations
• Filaments do not form myofibrils
• Not arranged in sarcomere pattern found in skeletal muscle
• Is Involuntary
• Single Nucleus
• Composed of spindle-shaped fibers with a diameter of 2-10 m and lengths of several hundred m
Smooth Muscle contraction
• Cells usually arranged in sheets within muscle
• Organized into two layers (longitudinal and circular) of closely apposed fibers
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Have essentially the same contractile mechanisms as skeletal muscle
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Cell has three types of filaments
• Thick myosin filaments
• Longer than those in skeletal muscle
• Thin actin filaments
• Contain tropomyosin but lack troponin
• Filaments of intermediate size
• Do not directly participate in contraction
• Form part of cytoskeletal framework that supports cell shape
Have dense bodies containing same protein found in Z lines
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Whole sheets of smooth muscle exhibit slow, synchronized contraction
Smooth muscle lacks neuromuscular junctions
Action potentials are transmitted from cell to cell
Some smooth muscle cells:
• Act as pacemakers and set the contractile pace for whole sheets of muscle
• Are self-excitatory and depolarize without external stimuli
Muscle fiber stimulated
Ca2+ released into the cytoplasm from ECF
Ca2+ binds with calmodulin
Ca2+/Calmodulin activates mysoin kinase
Myosin kinase phosphorylates myosin
Myosin can now bind with actin
Ca2+ for smooth muscle contraction comes largely from outside the cell.
Cardiac Muscle
• Occurs only in the heart
• Is striated like skeletal muscle but but has a branching pattern with intercalated Discs
• Usually one nucleus, but may have more
• Is not voluntary
• Contracts at a fairly steady rate set by the heart’s pacemaker
• Neural controls allow the heart to respond to changes in bodily needs