Chapter 10
What promotes a power stroke?
A power stroke occurs when ADP + Pi are released from the myosin head.
The main immediate energy is stored as _______ in the muscle fiber and is rapidly consumed during muscle contraction.
ATP
diffuses across the synaptic cleft where it can bind to ligand- gated channels found in the motor end plate of the muscle fiber plasma membrane.
Acetylcholine
the neurotransmitter released from a motor neuron that stimulates a muscle fiber.
Acetylcholine
a contractile protein that has active sites that bind with the myosin heads of the thick filament
Actin
When glucose levels are exhausted, what molecules can be catabolized to generate ATP?
Amino acids and fatty acids.
What is an end-plate potential?
An end-plate potential is a small region of the plasma membrane that has been depolarized.
What do skeletal muscle cells and cardiac muscle cells have in common?
Both skeletal and cardiac muscle cells have striations, giving both cell types a striped appearance
are structurally similar to skeletal fibers with some noticeable major differences.
Cardiac muscle cells
are autorhythmic. Pacemaker cells are found in specific regions of the heart where they spontaneously generate action potentials.
Cardiac pacemaker cell
Compare the number of mitochondria, myoglobin and the blood supply in fast-twitch fibers to slow-twitch fibers.
Compared to slow-twitch fibers, fast-twitch fibers have fewer mitochondria and lower levels of myoglobin and smaller blood supply, giving them their characteristic "white meat" appearance.
Define contractility.
Contractility is the ability to contract where proteins in the cell draw closer together.
of smooth muscle involves influx of extracellular calcium ions that bind to calmodulin, which activates myosin light chain kinase. This in turn activates myosin ATPase.
Contraction
occur when the muscle length remains unchanged because
Isometric contractions
How is an end-plate potential accomplished?
Ligand-gated channels open when they bind acetylcholine which allows Na+ ions to enter the muscle fiber generating an end-plate potential.
a dark line in the middle of the A band made up of structural proteins.
M line
due to an unequal distribution of ions near the plasma membrane resulting in a polarized resting state.
Membrane potentials
found in the uterus, eye, and skin. These are individual cells that contract independently to allow for precision.
Multi-unit smooth muscl
chemicals that trigger changes in a target tissue when released, allowing for cell-to-cell communication (Core Principle).
Neurotransmitters
What does the cocked myosin head bind?
Once cocked the myosin head is able to bind to the active site of actin.
What are some other functions of muscle tissue?
Other functions of muscle tissue are to create movement, maintain posture, stabilize joints, generate heat, and regulate the flow of materials through hollow organs.
occurs when Ca++ is removed from the cytosol, MLCK is deactivated, and the myosin ATPase is deactivated.
Relaxation
Summarize how repolarization occurs
Repolarization occurs as voltage- gated K+ channels open allowing K+ ions to exit the cell returning the cell back to its resting membrane potential.
What makes smooth muscle different from skeletal and cardiac muscle tissues?
Smooth muscle cells do not have striations unlike skeletal and cardiac muscle tissue.
What other features do smooth muscle cells lack?
Smooth muscle cells lack motor end plates, the SR is much less extensive, and there are no T-tubules.
________ and _________ ions move through the sarcolemma using protein channels and carriers.
Sodium and potassium
enlarged sections of SR found flanking each T-tubule.
Terminal cisternae
Describe what happens during the relaxation period
The relaxation period begins as calcium ion levels are reduced in the cytosol by SR pumps and tension diminishes.
What is the function of the pacemaker cells?
These cells coordinate electrical activity and trigger the contraction of surrounding cells.
are deep inward extensions of sarcolemma that are filled with extracellular fluid.
Transverse tubules (T-tubules)
results when fibers are stimulated about 50 times per second and the fiber partially relaxes between stimuli. Tension pulsates and increases to a maximum of 80% of the maximum
Unfused tetanus
How does this change in position return the muscle to a relaxed state?
With troponin and tropomyosin back to their original blocking positions, the active sites on the actin filaments are no longer exposed for the crossbridge cycle to occur.
The ________ is found in the middle of the I band and is composed of structural proteins.
Z disc
The sarcoplasm is the ___________ of the myocyte.
cytoplasm
Myocytes
muscle cells
A _______ is the smallest unit of contraction.
muscle twitch
The______________ is the space between axon terminal and muscle fiber.
synaptic cleft
Define distensibility.
the ability of a cell that allows it to be stretched without being ruptured.
Define elasticity
the ability of a cell that allows it to return to its original length after it has been stretched.
What causes EPOC?
EPOC is caused by changes in body temperature, by imbalances of intracellular and extracellular ion concentrations, and by blood pH imbalances.
are composed of a single massive, spring-like structural protein called titin that stabilizes the myofibril structure and resists excessive stretching force.
Elastic filaments
How is a functional muscle contraction produced?
Multiple end-plate potentials must be generated to produce a functional muscle contraction.
the electrical potential across the sarcolemma of a resting muscle fiber and measures -85 mV.
Resting membrane potential
is found in all hollow organ walls where they are linked electrically by gap junctions.
Single unit smooth muscle
____________fibers are thin cylinders but can be quite long and thick. Skeletal muscle fibers are formed by the fusion of many embryonic myoblasts giving each fiber multiple nuclei.
Skeletal muscle
What is the latch state?
The latch state is an alternative to relaxation where the cell remains contracted in an energy-efficient mode.
Describe what happens during the latent period.
The latent period is the time it takes the action potential to propagate across the sarcolemma.
How is the concentration gradient maintained?
The concentration gradient is maintained by the Na+/K+ pump.
Describe what happens during the contraction period
The contraction period begins as repeated crossbridge cycles generate tension.
Summarize how the contraction phase begins.
The contraction phase begins when Ca++ bind troponin, which pulls tropomyosin away from actin's active site.
What is required for the crossbridge cycle to repeat?
The cycle may be repeated as long as the stimulus to contract continues and ATP is available.
Summarize how the excitation phase begins.
The excitation phase begins when an action potential of a motor neuron signals the release of acetylcholine from the axon terminal into the synaptic cleft.
The ____________ of the neuron contains synaptic vesicles filled with acetylcholine.
axon terminal
Describe repolarization.
begins after Na+ channels have closed and voltage-gated K+ channels have opened, allowing K+ to diffuse out of the cell. K+ channels close once the cell returns to its resting membrane potential.
Describe depolarization.
begins when voltage-gated Na+ channels open, allowing Na+ to flow inward. The membrane quickly reaches 0 mV and peaks at approximately +30 mV.
What do voltage-gated channels open and close in response to?
changes in the membrane potential of the plasma membrane.
The _____________ is a specialized region of the muscle fiber plasma membrane that has ligand-gated Na+ channels.
motor end plate
A single ___________ and all the muscle fibers that it innervates define a motor unit.
motor neuron
The sarcolemma is the __________ of the myocyte.
plasma membrane
The ___________ is modified endoplasmic reticulum that forms a web-like network surrounding the myofibrils. It stores and releases calcium ions.
sarcoplasmic reticulum (SR)
Cardiac muscle cells, found only in the heart, are
short and highly branched.
The three types of cells in muscle tissue are
skeletal, cardiac, and smooth muscle
Define conductivity.
the ability of a cell to conduct electrical changes across the entire plasma membrane.
Define excitability.
the ability of a cell to respond to a stimulus or signal.
What do ligand-gated channels (chemically-gated) open in response to?
the presence of a chemical or ligand
The pump moves _________ Na+ ions out of the cell and _______ K+ ions into the cell.
3 and 2
The __________ ("a" in dark, mnemonic) contains the zone of overlap, the region where we find thick and thin filaments and where tension is generated during contraction.
A band
____________ fibers have high myoglobin
Fast-twitch
occurs when the fiber is stimulated at a rate of 80-100 stimuli per second and the fiber does not relax between stimuli. Tension stays constant at nearly 100% of the maximum
Fused tetanus
What function do all three muscle tissue types share in common?
Generating a force called muscle tension is a basic function common to each muscle tissue type.
is a series of reactions that occurs in all cells' cytosol. Glycolysis breaks glucose down into pyruvate and provides energy (ATP) for muscle contraction once immediate sources of energy are depleted.
Glycolysis
In the middle of the A band where only thick filaments exist is the
H zone
What happens to pyruvate if oxygen is abundant
If oxygen is abundant pyruvate formed by glucose catabolism enters the mitochondria for oxidative catabolism.
What happens to pyruvate if oxygen is not abundant?
If oxygen is not abundant the pyruvate is converted to lactic acid. Lactic acid can either be converted back into glucose by the liver or taken up in the mitochondria for oxidative catabolism.
How do the positions of troponin and tropomyosin change in the absence of calcium ions?
In the absence of calcium ions, troponin and tropomyosin shift back to their original blocking positions
link cells together both electrically by gap junctions and physically by desmosomes, permitting the heart to contract as a coordinated unit.
Intercalated discs
maintain constant tension while the muscle shortens.
Isotonic concentric contractions
maintain constant tension, but the muscle lengthens
Isotonic eccentric contractions
___________is a baseline level of involuntary activation of motor units.
Muscle Tone
______________ consists of many fibers and their surrounding endomysium.
Muscle tissue
cylindrical organelles found in each of the three muscle cell types. Myofibrils are made up of bundles of specialized proteins that allow for contraction.
Myofibrils
How is another crossbridge cycle accomplished?
Myosin pulls the actin towards the M line of the sarcomere
An end-plate potential leads to the opening of voltage-gated ______ channels in the sarcolemma surrounding the motor end plate, which triggers an action potential.
Na+
How are the two main types of skeletal muscle fibers classified?
Skeletal muscle fibers are classified mainly on myosin ATPase activity that determines how fast or how slowly a power stroke can occur.
__________ fibers have low myosin ATPase activity.
Slow-twitch
are composed of bundles of myosin.
Thick filaments
are composed of the proteins actin, tropomyosin, and troponin
Thin filaments
Two terminal cisternae and their corresponding T-tubule form a
Triad
a long rope-like regulatory protein that twists around actin, covering up its active sites.
Tropomyosin
a small globular regulatory protein that holds tropomyosin in place and assists with turning contractions on and off.
Troponin
Skeletal muscle cells are known as
fibers due to their length and appearance.
Define fatigue
is the inability to maintain a given level of intensity during activity.
The ______________ states that the optimal length of a sarcomere is about 100-120% of the natural length of the sarcomere.
length-tension relationship