Muscle Physiology (skeletal) Flashcards
Muscle is classified as:
- Skeletal muscle
- Smooth muscle
- Cardiac muscle
- It is multinucleated
- It contains many mitochondria
- It has special structures called Transverse tubules (T tubules)
- It has myofibrils and sarcomeres
- It has specific terms for some of the intracellular
Skeletal Muscle Fiber
Skeletal Muscle Fiber
equivalent of Plasma membrane
Sarcolemma
Skeletal Muscle Fiber
equivalent of Cytoplasm
Sarcoplasm
Skeletal Muscle Fiber
equivalent of Smooth ER
Sarcoplasmic reticulum
are the structures that give skeletal and cardiac muscle their characteristic striated appearance
alternaring light and dark bands
Myofibrils
The term contraction does not necessarily mean shortening
true
Myofibrils are orderly arrangements of thick and thin filaments called
Actin (thin)
Myosin (thick)
It simply refers to activation of the force-generating sites within muscle fibers—the cross-bridges.
contraction
a type of muscle contraction where the muscle generates force without changing its length or causing movement of a joint
Isometric contraction
a type of muscle contraction where the muscle shortens while generating force, overcoming resistance, and causing movement, like lifting a weight.
concentric contraction
occurs when a muscle actively resists lengthening under a load, meaning the muscle contracts while simultaneously lengthening
eccentric contraction
what Regulatory protein binds to Ca2+ reversibly and once bound changes conformation to pull tropomyosin away from the myosin interaction sites
Troponin
Thin Filaments and Associated Proteins
Actin
Tropomyosin
Troponin
The ability of a muscle fiber to generate force and movement depends on the interaction of the contractile proteins actin and myosin.
produces shortening of a skeletal muscle fiber, the overlapping thick and thin filaments in each sarcomere move past each other, propelled by movements of the cross-bridges.
Sliding Filament Mechanism
- Contractile protein
- has a binding site for myosin.
- Think of pearls strung together on a string and then the strands of pearls are twisted together.
Actin
- Regulatory protein
- Forms a complex with the other proteins of the thin filament (actin and tropomyosin).
Troponin
- Regulatory protein
- Overlaps binding sites on actin for myosin and inhibits interaction when in the relaxed state.
Tropomyosin
regulates skeletal muscle contraction because it moves the tropomyosin away and allows myosin to interact with the actin.
Ca2+ binding to troponin
low amount og calcium in the muscles = relaxed state
true
defined as the motor neuron and the skeletal muscle fibers it innervates
Within a whole muscle there are many
Motor Unit
One motor neuron innervates many ______
muscle fibers
nerve cells whose axons innervate skeletal muscle fibers are known as ____________
motor neurons (or somatic efferent neurons),
muscle fiber is innervated by only one
motor neuron
____________ is the only mechanism by which action potentials are initiated in skeletal muscle
Stimulation of the nerve fibers to a skeletal muscle
axons of ________ are myelinated and are the largest-diameter axons in the body for saltatory contractions.
motor neurons
motor neurons (or somatic efferent neurons) cell bodies are located in either the
brainstem or the spinal cord.
in the Neuromuscular Junction, vesicles contain the neurotransmitter
acetylcholine (ACh)
The region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon is known as the
motor end plate
axon terminals of a motor neuron contain vesicles similar to the vesicles found at synaptic junctions between two neurons.
true
why do motor neurons have the largest-diameter axons in the body
They propagate action potentials at high velocities, allowing signals from the central nervous system to travel to skeletal muscle fibers with minimal delay
The junction of an axon terminal with the motor end plate is known as a
neuromuscular junction
All neuromuscular junctions are ________.
excitatory
In addition to receptors for ACh, the synaptic junction contains the enzyme ______________, which breaks down ACh, just as it does at ACh mediated synapses in the nervous system.
acetylcholinesterase
a deadly arrowhead poison used by indiginous peoples of South America. It binds strongly to nicotinic ACh receptors. It does not open their ion channels, however, and acetylcholinesterase does not destroy it.
Curare
poisin where ACh cannot bind to the receptor. Therefore, although the motor nerves still conduct normal action potentials and release ACh, there is no resulting EPP in the motor end plate and no contraction.
Curare
Because the skeletal muscles responsible for breathing, like all skeletal muscles, depend upon neuromuscular transmission to initiate their contraction, curare poisoning can cause death by ___________
asphyxiation
are the main ingredients in certain pesticides and “nerve gases” (the latter developed for chemical warfare), inhibit acetylcholinesterase. This results in skeletal muscle paralysis and death from asphyxiation.
organophosphates
in muscle is homologous to the endoplasmic reticulum found in most cells. Ca2+ is stored and is released following membrane excitation
sarcoplasmic reticulum (SR)
T-tubules and SR are connected with junctions. These junctions involve two integral membrane proteins, which are
one in the T-tubule membrane, and the other in the membrane of the sarcoplasmic reticulum
In Sarcoplasmic Reticulum
T-tubule protein is a modified voltage-sensitive Ca2+ channel known as the __________ receptor, which acts as a voltage sensor
T-tubule
The protein embedded in the SR membrane is known as the ___________ receptor, which forms a Ca2+channel.
ryanodine
Neuromuscular transmission can also be blocked by inhibiting
acetylcholinesterase
Thus, the antidote for organophosphate and nerve gas exposure includes both _______________, the muscarinic receptor antagonist
atropine
also cause ACh to build up at muscarinic synapses, where parasympathetic neurons inhibit cardiac pacemaker cells
Nerve gases
Drugs that block neuromuscular transmission are sometimes used in small amounts to prevent muscular contractions during certain types of surgical procedures
One example is _______, which actually acts as an agonist to the ACh receptors and produces a depolarizing/desensitizing block similar to acetylcholinesterase inhibitors.
succinylcholine
Patients must be artificially ventilated, however, to maintain respiration until the drug has been removed from the system
true
Thus, the antidote for organophosphate and nerve gas exposure includes both _______________, which reactivates acetlycholinesterase,
pralidoxime
Nondepolarizing neuromuscular junction blocking drugs that act more like curare and last longer are also used, such as
rocuronium and vecuronium
The use of such paralytic agents in surgery reduces the required dose of general anesthetic, allowing patients to recover faster and with fewer complications.
true
The toxin produced by the bacterium _____________, blocks the release of acetylcholine from nerve terminals.
Clostridium botulinum
A muscle fiber generates force called tension in order to oppose a force called the
which is exerted on the muscle by an object
load
is an enzyme that breaks down proteins of the SNARE complex that are required for the binding and fusion of ACh vesicles with the plasma membrane of the axon terminal.
Botulinum toxin
Botulinum toxin roduces the food poisoning called _________, is one of the most potent poisons known because of the very small amount necessary to produce an effect.
botulism
The increase in muscle tension from successive action potentials occurring during the phase of mechanical activity is known as
summation
is increasingly being used for clinical and cosmetic procedures, including the inhibition of overactive extraocular muscles, prevention of excessive sweat gland activity, treatment of migraine headaches, and reduction of aging-related skin wrinkles
botulinum toxin
A muscle fiber generates force called
in order to oppose a force called the load
tension
The mechanical response of a muscle fiber to a single action potential is known as a
twitch
3 major phases to a twitch contraction
Latent Period
Contraction Phase
Relaxation Phase
twitches that do generate tension but do not shorten the muscle (load is greater than the force generated by the muscle…i.e., postural muscles)
Isometric twitches
what happens to the muscle shortening when more load is placed on it
slower shortening
3 major phases to a twitch contraction
This is the period of time from the action potential to the onset of contraction. The time delay is due to the excitation-contraction coupling.
Latent Period
twitches that do generate tension and do shorten the muscle
Isotonic twitches
3 major phases to a twitch contraction
This is the time that the tension is decreasing (i.e., relaxing) and is longer than the contraction phase. This is due to the amount of time it takes to get all the Ca2+ sequestered
Relaxation Phase
3 major phases to a twitch contraction
This is the time that tension is developing due to the cross-bridge cycling.
Contraction Phase
what happens to the muscle shortening when less load is placed on it
faster shortening
A maintained contraction in response to repetitive stimulation is known as a
tetanus (tetanic contraction)
When a stimulus is applied before a fiber has completely relaxed from a twitch, it induces a contractile response with a peak tension _________ than that produced in a single twitch (S3 and S4).
greater
it is possible for a second action potential to be initiated during the period of mechanical activity
true
The spring-like characteristic of the protein _______ is responsible for most of the passive elastic properties of relaxed muscle fibers.
titin
With increased stretch, the passive tension in a relaxed fiber increases, not from active cross-bridge movements but from elongation of the
titin filaments.
If the stretched fiber is released, it will return to an equilibrium length, much like what occurs when releasing a stretched rubber band.
true
There are three ways a muscle fiber can form ATP
- Phosphorylation of ADP by creatine phosphate
- Oxidative phosphorylation of ADP in the mitochondria
- Phosphorylation of ADP by the glycolytic pathway in the cytosol
the amount of ___________ a muscle fiber develops during contraction can also be altered by changing the length of the fiber. If you stretch a muscle fiber to various lengths and tetanically stimulate it at each length, the magnitude of the _______ will vary with length,
active tension
happens when a skeletal muscle fiber is repeatedly stimulated, the tension the fiber develops eventually decreases even though the stimulation continues.
decreased shortening velocity and a slower rate of relaxation
Muscle Fatigue
decline in muscle tension as a result of previous contractile activity is known as
muscle fatigue
The onset of fatigue and its rate of development depend on the type of skeletal muscle fiber that is active, the intensity and duration of contractile activity, and the degree of an individual’s fitness.
true
Muscle Fatigue Causes
The muscle action potential can fail to be conducted into the fiber along the T-tubules, which halts the release of Ca2+ from the sarcoplasmic reticulum.
Conduction Failure
- Number of active fibers
a. Number of fibers per motor unit
b. Number of active motor units
Muscle Fatigue Causes
– The buildup of ADP and Pi within muscle fibers during intense activity may directly inhibit cross-bridge.
– Slowing the rate of this step delays cross-bridge detachment from actin, and thus slows the overall rate of cross-bridge cycling.
– These changes contribute to the reduced shortening velocity and impaired relaxation observed in muscle fatigue resulting from high-intensity exercise
Inhibition of Cross-Bridge Cycling
Muscle Fatigue Causes
results from the buildup of potassium ions in the small volume of the T-tubule during the repolarization of repetitive action potentials. Elevated external potassium ion concentration leads to a persistent depolarization of the membrane potential, and eventually causes a failure to produce action potentials in the T-tubular membrane
Conduction Failure
Muscle Fatigue Causes
– Elevated hydrogen ion concentration alters protein conformation and activity.
– Thus, the acidification of muscle by lactic acid may alter a number of muscle proteins, including the proteins involved in Ca2+ release.
– The function of the Ca2+-ATPase pumps of the sarcoplasmic reticulum is also affected, which may in part explain the impaired relaxation of fatigued muscle.
Lactic Acid Buildup
- Tension developed by each fiber
a. Action potential frequency (frequency-tension relation)
b. Fiber length (length-tension relation)
c. Fiber diameter
d. Fatigue
Another type of fatigue quite different from muscle fatigue occurs when the appropriate regions of the cerebral cortex fail to send excitatory signals to the motor neurons.
This may cause a person to stop exercising even though the muscles are not fatigued.
Central Command Fatigue
Fibers are classified on the basis of:
- Their maximal velocities of shortening (fast or slow)
- The major pathway they use to form ATP—oxidative or glycolytic
The total tension a muscle can develop depends upon two factors:
By controlling these two factors, the nervous
system controls whole-muscle tension as well
as shortening velocity.
- The amount of tension developed by each fiber
- The number of fibers contracting at any time
Factors Determining Muscle Tension
- Tension developed by each fiber
- Number of active fibers
what happens during cramping?
action potentials fire at abnormally high rates, a much greater rate than occurs during maximal voluntary contraction
electrolyte imbalances in the extracellular fluid surrounding both the muscle and nerve fibers
Muscle Adaptation to Exercise
An increase in the amount of contratile activity increases the size of muscle fibers and increases their capacity for ATP production
There are 2 types of atrophy
– Disuse atrophy (like an arm in a cast)
– Denervation atrophy (nerve damage = loss of function)
explanation for Use it or lose it
Muscles that are not used will atrophy.
Involuntary tetanic contraction of skeletal muscles produces
muscle cramps
is a viral disease that destroys motor neurons, leading to the paralysis of skeletal muscle, and may result in death due to respiratory failure
poliomyelitis
may arise from overexercise or persistent dehydration, and they can directly induce action potentials in motor neurons and muscle fibers.
electrolyte imbalances
_________ within the muscle stimulate sensory receptors in the muscle, and the motor neurons to the area are activated by reflex when those signals reach the spinal cord
chemical imbalances
the involuntary tetanic contraction of skeletal muscles that occurs when the extracellular Ca2+ concentration falls to about 40 percent of its normal value.
normally, we use sarcoplasmic reticulum Ca2+, not extracellular Ca2+
increases the opening of Na+ channels in excitable membranes, leading to membrane depolarization and the spontaneous firing of action potentials.
Hypocalcemic Tetany
are caused by the absence or defect of one or more
proteins that make up the costameres in striated muscle
Muscular dystrophy
disease is one of the most frequently encountered genetic diseases
associated with the progressive degeneration of skeletal and cardiac muscle fibers, weakening the muscles and leading ultimately to death from respiratory or cardiac failure.
Muscular dystrophy
(costa = “rib”) are clusters of structural and regulatory proteins that link the Z-disks of the outermost myofibrils to the sarcolemma and extracellular matrix
Costameres
is a sex-linked recessive disorder caused by a defect in a gene on the X chromosome that codes for the protein, dystrophin
Duchenne muscular dystrophy
was the first costamere protein discovered to be related to a muscular dystrophy, which is how it earned its name
Dystrophin
is a treatment that involves replacing the liquid fraction of blood (plasma), which contains the offending antibodies for Myasthenia Gravis
Plasmapheresis
The most common cause is the destruction of nicotinic ACh receptor proteins of the motor end plate, mediated by antibodies of a person’s own immune system.
Myasthenia Gravis
Myasthenia Gravis
A number of approaches are currently used to treat the disease. One is to administer ___________. This can partially compensate for the reduction in available ACh receptors by prolonging the time that acetylcholine is available at the synapse.
acetylcholinesterase inhibitors
