Download Muscle and Muscle Tissue

Muscle and Muscle Tissue
Make up about half of total body mass
Exerts force by converting chemical energy, ATP, to mechanical energy
Muscle tissue is classified based on
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Shape
Number and position of nuclei
Presence of striations
Voluntary or involuntary control
Functional Characteristics of Muscle Tissue
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Excitability
o Ability to receive and respond to stimuli
Contractility
o Ability to shorten after adequate stimulation
Extensibility
o Ability to stretch
Elasticity
o Ability to return to its original length after contraction
Functions of Muscle
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Movement
o Skeletal
o Cardiac
o Smooth
Maintain posture
Stabilize joints
Generate heat
Types of Muscle Tissue
Skeletal muscle
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Cells are elongated
Multinucleated
Nuclei are peripherally placed
Striated
Voluntary
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They attach to and cover the bony skeleton
Cardiac
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Cells branch
Single centrally placed nucleus
Striated
Involuntary
Located in the heart
Smooth
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Spindle shaped fibers
Centrally placed nucleus
Non-striated
Involuntary
Located in the walls of hollow visceral organs
Types of Smooth Muscle
Smooth muscles in different organs differ in:
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Fiber arrangement and organization
Responsiveness to stimuli
Innervation
Categorized into two main categories both of which are innervated by the ANS and respond to hormonal
controls
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Single-unit smooth muscle
Multiunit smooth muscle
Single-Unit Smooth Muscle
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Also called visceral muscle
Cell contract rhythmically and as a single unit
Cells are connected to each other via gap junctions and are arranged in opposing sheets
Exhibit spontaneous action potentials (stress-relaxation response)
Multi-unit Smooth Muscle
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Muscles fibers are independent of each other (Gap junctions rare)
Richly innervated and each nerve forms a motor unit with a number of muscle fibers
(spontaneous depolarizations are infrequent)
Responds to neural stimulation with graded contractions
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Examples: Arrector pilli muscles, eye muscles that adjust pupil size, muscles in large airways and
large arteries
Gross Anatomy of Skeletal Muscle
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Each muscle has a nerve and blood supply that allows neural control and ensures adequate
nutrient delivery and waste removal.
Connective tissue sheaths are found at various structural levels of each muscle: endomysium
surrounds each muscle fiber, perimysium surrounds groups of muscle fibers, and epimysium
surrounds whole muscles. (Fig 9.2)
Attachments span joints and cause movement to occur from the movable bone (the muscle’s
insertion) toward the less movable bone (the muscle’s origin)
Muscle attachments may be direct or indirect.
Microscopic Anatomy of Skeletal Muscle
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Skeletal muscle fibers are long cylindrical cells with multiple nuclei beneath the sarcolemma.
Myofibrils account for roughly 80% of cellular volume, and contain the contractile elements of
the muscle cell. (Tab 9.1)
Microscopic Anatomy of Skeletal Muscle
Striations are due to a repeating series of dark A bands and light I bands.
Myofilaments make up the myofibrils, and consist of thick and thin filaments.
Ultrastructure and Molecular Composition of the Myofilaments
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There are two types of myofilaments in muscle cells: thick filaments composed of bundles of
myosin, and thin filaments composed of strands of actin.
Tropomyosin and troponin are regulatory proteins present in thin filaments.
Ultrastructure and Molecular Composition of the Myofilaments
The sarcoplasmic reticulum is a smooth endoplasmic reticulum surrounding each myofibril.
T tubules are infoldings of the sarcolemma that conduct electrical impulses from the surface of
the cell to the terminal cisternae. (Fig 9.5)
Sliding Filament Theory
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The sliding filament model of muscle contraction states that during contraction, the thin
filaments slide past the thick filaments. Overlap between the myofilaments increases and the
sarcomere shortens
Physiology of a Skeletal Muscle Fiber
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The neuromuscular junction is a connection between an axon terminal and a muscle fiber that is
the route of electrical stimulation of the muscle cell.
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A nerve impulse causes the release of acetylcholine to the synaptic cleft, which binds to
receptors on the motor end plate, triggering a series of electrical events on the sarcolemma.
Generation of an Action Potential Across the Sarcolemma
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Like plasma and nerve cell membranes the sarcolemma is polarized
The potential difference between the extracellular space and the intracellular space is called the
Resting Potential
Resting Potential
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Potential difference is the result of an unequal distribution of ions between the inside and the
outside of the cell
Typical resting potential (Nerve) is -70mV meaning the inside of the cell is more negative than
the outside
Difference is due to selective ionic permeability of the cell membrane
It is maintained by Na+/K+ pump
K+ ion concentration is higher inside than outside
Negatively charged proteins are trapped inside the cell
Resting membrane potential is created because the membrane is selectively permeable K+ ions
K+ ions diffuse down its concentration gradient
Na+ is not allowed to enter the cell thus the cell remains polarized
Transmission of action potentials lead to disruption of the ionic gradients which must then be
restored by the Na+/K+ pump that uses ATP to transport 3 Na+ out for every 2 K+ it transports in
Action Potential
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When a muscle cell receives excitatory impulse of sufficient strength, depolarization occurs
The inside of the cell becomes progressively less negative and an action potential is generated
Voltage changes on the membrane result in the opening of voltage-gated ion channels
An action potential begins when voltage-gated Na+ channels open in response to depolarization
Na+ ions rush down its electrochemical gradient into the cell
The segment of the cell where this occurs is depolarized
The Na+ channels then close
Voltage gated K+ ion channels then open
K+ ions rush out down its electrochemical gradient
The cell is repolarized (returns to a more negative potential)
Ionic concentrations of the resting state are restored by Na+/K+ ATPase
An action potential is therefore a transient reversal of the resting membrane potential
The inside of the cell may become more negative than normal after repolarization
(hyperpolarization)
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Immediately after an action potential, it may become very difficult or impossible to initiate
another action potential
This period is referred to as the refractory period (relative and absolute refractory periods)
An action potential is propagated along the entire sarcolemma
All-or-none response
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An action potential with consistent size and duration is produced only when a threshold
membrane potential is reached
Physiology of a Skeletal Muscle Fiber
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Generation of an action potential across the sarcolemma occurs in response to acetylcholine
binding with receptors on the motor end plate. It involves the influx of sodium ions, which
makes the membrane potential slightly less negative.
Excitation-contraction coupling is the sequence of events by which an action potential on the
sarcolemma results in the sliding of the myofilaments.
Ionic calcium in muscle contraction is kept at almost undetectable levels within the cell through
the regulatory action of intracellular proteins.
Muscle fiber contraction follows exposure of the myosin binding sites, and follows a series of
events
Contraction of a Skeletal Muscle
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A motor unit consists of a motor neuron and all the muscle fibers it innervates. It is smaller in
muscles that exhibit fine control.
The muscle twitch is the response of a muscle to a single action potential on its motor neuron.
Contraction of a Skeletal Muscle
There are three kinds of graded muscle responses: wave summation, multiple motor unit
summation (recruitment), and treppe.
Muscle Tone
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A state of partial contraction exhibited by relaxed muscles
Results from spinal reflexes that activate a group of motor units in response to stretch receptor
activation in muscles and tendons
Does not produce movements
Keeps muscles healthy and firm so they can respond when stimulated
Helps stabilize joints and maintain posture
Isotonic Contraction
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Same tension
Muscle contracts and shortens to move a load
Concentric contractions occur when muscle contracts as it shortens (picking a book)
Eccentric contractions (more forceful) occurs when the muscle contracts as it lengthens
Isometric Contractions
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Same length
Occurs when a muscle tries to move a load that is greater than the force the muscle is able to
generate
Tension builds up in the muscle but it does not shorten
Muscle Metabolism
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Muscles contain very little stored ATP, and consumed ATP is replenished rapidly through
phosphorylation by creatine phosphate, glycolysis and anaerobic respiration, and aerobic
respiration.
Muscles will function aerobically as long as there is adequate oxygen, but when exercise
demands exceed the ability of muscle metabolism to keep up with ATP demand, metabolism
converts to anaerobic glycolysis. (Fig 9.20)
Muscle fatigue is the physiological inability to contract due to the shortage of available ATP.
Oxygen debt is the extra oxygen needed to replenish oxygen reserves, glycogen stores, ATP and
creatine phosphate reserves, as well as conversion of lactic acid to pyruvic acid, and then to
glucose after vigorous muscle activity
Heat production during muscle activity is considerable. It requires release of excess heat
through homeostatic mechanisms such as sweating and radiation from the skin.
Effect of Exercise on Muscles
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Aerobic, or endurance, exercise promotes an increase in capillary penetration, the number of
mitochondria, and increased synthesis of myoglobin, leading to more efficient metabolism, but
no hypertrophy.
Resistance exercise, such as weight lifting or isometric exercise, promotes an increase in the
number of mitochondria, myofilaments and myofibrils, and glycogen storage, leading to
hypertrophied cells.
Force of Muscle Contraction
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Number of muscle fibers stimulated
Size of the muscle fiber stimulated
Frequency of stimulation
Degree of Muscle stretch
Length-tension relationship
Velocity and Duration of Contraction
Velocity and duration of contraction is influenced by:
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Fiber type
o Fast and slow fibers exist
o Difference in speed is dependent on the rate at which myosin ATPase splits ATP
Load
Recruitment