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Physio CH 12

Terms in this set (184)

A skeletal muscle is composed of a bundle of ________, each composed of many muscle fibers wrapped by connective tissue.

A) muscle cells
B) sarcomeres
C) myofilaments
D) myofibrils
E) fascicles
E) fascicles
What structure, composed of connective tissue, transmits force from contracting skeletal muscle to bone?

A) fascicle
B) tendon
C) aponeurosis
D) ligament
E) myofibril
B) tendon
The fundamental repeating unit of the myofibril, called a ________, gives skeletal muscle its striated appearance and is bordered by ________.

A) crossbridge : A bands
B) crossbridge : Z lines
C) sarcomere : I bands
D) sarcomere : Z lines
E) sarcomere : M lines
D) sarcomere : Z lines
What is the region of the striated muscle's banding patterns that contains only the connections between the tails of myosin molecules and is the mid-point of the sarcomere?

A) M line
B) H zone
C) Z line
D) I band
E) A band
A) M line
The contractile portion of the thin filament is composed of what protein?

A) myosin
B) actin
C) tropomyosin
D) titin
E) troponin
B) actin
What is the regulatory protein component of the thin filament that binds to calcium, thereby initiating skeletal muscle contraction?

A) titin
B) myosin
C) tropomyosin
D) troponin
E) actin
D) troponin
Which of the following is a structural protein that extends along each thick filament from M line to Z line?

A) tropomyosin
B) myosin
C) actin
D) troponin
E) titin
E) titin
The shortening of a skeletal muscle fiber during contraction involves which of the following?

A) the sarcomeres shortening

B) the thin filaments shortening

C) the Z lines not changing their position

D) the A bands shortening

E) the thick filaments shortening
A) the sarcomeres shortening
During skeletal muscle contraction, as the muscle shortens, the thick and thin filaments

A) lengthen.
B) slide past one another.
C) condense.
D) shorten.
E) do not interact.
B) slide past one another.
When a skeletal muscle is passively stretched, that cell has a tendency to spring back once the force that was stretching the muscle is removed due to

A) the active interaction between actin and myosin (energy required).

B) titin acting as a spring using the energy stored by the stretching.

C) actin and myosin acting as a spring using the energy stored by the stretching.

D) the passive interaction between actin and myosin (no energy required).

E) the elastic connective tissue that surrounds the muscle cells and fascicles
B) titin acting as a spring using the energy stored by the stretching.
The repeated, oscillating interaction between actin and myosin which results in the generation of force by a skeletal muscle cell is called what?

A) crossbridge cycling
B) Z line interaction
C) the sliding-filament model
D) titin cycling
E) sarcomeric facilitation
A) crossbridge cycling
What converts the myosin head into the high-energy state?

A) binding to titin
B) binding to ATP only
C) the condensation of ATP
D) the hydrolysis of ATP
E) binding to actin
D) the hydrolysis of ATP
The release of an inorganic phosphate from the myosin molecule directly results in which of the following?

A) development of rigor
B) power stroke
C) cocking of the myosin head
D) breaking of the actin myosin complex
E) binding of actin to myosin
B) power stroke
In order for crossbridge cycling to occur, the actin-myosin complex must be broken by which of the following?

A) binding of ATP to actin

B) binding of the troponin complex to actin

C) conformational change that occurs as the myosin head changes from the high to low energy
state

D) binding of ATP to myosin

E) binding of tropomyosin to myosin
D) binding of ATP to myosin
The sequence of events that links the muscle's action potential to changes in skeletal muscle force development is called what?

A) excitation-contraction coupling

B) oxidative phosphorylation

C) crossbridge cycling

D) myoaction coupling

E) the sliding-filament model
A) excitation-contraction coupling
During skeletal muscle contraction, multiple crossbridge cycles are occurring between the same thick and thin filament. Which of the following best describes this process?

A) Tropomyosin only exposes one binding site on actin at a time allowing only one crossbridge to form with actin at a given time.

B) Crossbridge cycling is asynchronous between a certain thick and thin filament.

C) No more than one myosin head detaches from the thin filament at the same time.

D) No more than one myosin head links to the thin filament at the same time.

E) Crossbridge cycling is highly synchronized between a certain thick and thin filament.
B) Crossbridge cycling is asynchronous between a certain thick and thin filament
Contraction of skeletal muscle fibers is stimulated by what type of neuron?

A) sympathetic
B) afferent
C) autonomic
D) parasympathetic
E) motor
E) motor
Which of the following is NOT a specialization observed at the neuromuscular junction?

A) Each muscle fiber is innervated by multiple motor neurons.

B) There is a high density of acetylcholine receptors in the motor end plate.

C) Every action potential that reaches the axon terminal of the motor neuron will generate an
action potential in the healthy muscle fiber.

D) The motor end plate is relatively large compared with other synapses.

E) The terminal bouton fans out over a wide area of the sarcolemma.
A) Each muscle fiber is innervated by multiple motor neurons.
Increases in the amount of cytoplasmic calcium required to initiate a muscle contraction are mediated by the coupling between a ________ on the T tubule and a ________ on the membrane of
the sarcoplasmic reticulum.

A) dihydropyridine receptor : ryanodine receptor

B) ryanodine receptor : calcium pump

C) dihydropyridine receptor : calcium pump

D) calcium-induced calcium release channel : dihydropyridine receptor

E) calcium pump : ryanodine receptor
A) dihydropyridine receptor : ryanodine receptor
Which of the following is the voltage sensor that initiates an increase in cytoplasmic calcium in response to an action potential?

A) calcium-induced calcium release channel

B) dihydropyridine receptor

C) sodium channel

D) ryanodine receptor

E) calcium pump
B) dihydropyridine receptor
The binding of calcium to troponin will directly allow which of the following?

A) the further release of calcium into the cytoplasm

B) the movement of tropomyosin, thereby exposing the myosin-binding site on the actin molecule

C) the binding of ATP to myosin

D) the movement of tropomyosin, thereby exposing the actin-binding site on the myosin molecule

E) the hydrolysis of ATP
B) the movement of tropomyosin, thereby exposing the myosin-binding site on the actin molecule
What area of a sarcomere shows up as light striations when viewed under a microscope?

A) Z line
B) I band
C) A band
D) M line
E) H zone
B) I band
Which of the following best describes the interaction between transverse (T) tubules and the sarcoplasmic reticulum in skeletal muscle contraction?

A) Action potentials in T tubules cause a depolarization of the sarcoplasmic reticulum membrane, thereby opening calcium channels to trigger calcium release.

B) Action potentials in T tubules trigger the release of norepinephrine, which binds to receptors on the sarcoplasmic reticulum and triggers calcium release.

C) Action potentials in T tubules trigger the release of acetylcholine, which binds to receptors on the sarcoplasmic reticulum and triggers calcium release.

D) Action potentials in T tubules are detected by DHP receptors, which are coupled to ryanodine receptors in the sarcoplasmic reticulum and open channels for calcium.

E) Action potentials in T tubules are detected by ryanodine receptors, which are coupled to DHP receptors in the sarcoplasmic reticulum and open channels for calcium.
D) Action potentials in T tubules are detected by DHP receptors, which are coupled to ryanodine receptors in the sarcoplasmic reticulum and open channels for calcium.
During the cross-bridge cycle, ATP binding to myosin causes which of the following?

A) the myosin head to interact with calcium channels, triggering calcium release from the sarcoplasmic reticulum

B) the myosin head to detach from actin

C) the myosin head to attach to actin

D) the myosin head to swing forward pulling actin toward the M line

E) the myosin head to be in its high-energy form
B) the myosin head to detach from actin
What is the function of T tubules?

A) They provide elasticity to the muscle.

B) Actin and myosin are synthesized here.

C) They store calcium.

D) They hold the thick filaments to the Z line.

E) They conduct action potentials from the sarcolemma to the interior of the muscle cell.
E) They conduct action potentials from the sarcolemma to the interior of the muscle cell.
What is a motor unit?

A) all the muscle fibers in a fascicle

B) all the myofibrils in a muscle fiber

C) a motor neuron and all the muscle fibers it innervates

D) a spinal nerve and all the muscle fibers it innervates

E) a muscle fiber and all the motor neurons that innervate it
C) a motor neuron and all the muscle fibers it innervates
In order to allow discrete contractile events to occur, calcium is rapidly removed from the cytoplasm via

A) ryanodine channels.

B) calcium-binding proteins on the sarcoplasmic reticulum.

C) calcium pumps (active transport of calcium).

D) dihydropyridine receptors.

E) calcium-sequestering proteins in the cytoplasm.
B) calcium-binding proteins on the sarcoplasmic reticulum.
Which of the following is not part of the process whereby skeletal muscles relax?

A) The amount of calcium bound to troponin decreases.

B) The binding of calcium to a low-affinity site closes sarcoplasmic reticulum calcium channels.

C) Sarcolemmal calcium channels open to allow the efflux of calcium.

D) A calcium pump actively removes calcium.

E) A myosin-binding site on the actin molecule is blocked by tropomyosin.
A) The amount of calcium bound to troponin decreases.
What enzyme catalyzes the reaction that creates creatine phosphate?

A) creatine hydrolase

B) creatine kinase

C) creatine hydrogenase

D) ATP hydrolase

E) creatine phosphatase
B) creatine kinase
Since the creatine phosphorylation reaction is substrate driven, an increase in the concentration of creatine within a skeletal muscle fiber will directly have what effect?

A) increase ATP generation

B) decrease creatine phosphate concentration

C) increase glucose metabolism

D) increase creatine phosphate concentration

E) decrease creatine kinase activity
D) increase creatine phosphate concentration
During intense (heavy) exercise, the initial ability of oxidative phosphorylation to provide enough ATP for the sudden demand of increased force generation by the skeletal muscle is ________, thereby causing the muscle fiber to quickly and massively boost its energy production from ________ until oxygen delivery and other mechanisms come up to speed.

A) enhanced : glucose

B) enhanced : substrate-level phosphorylation

C) enhanced : fatty acids

D) inadequate : fatty acids

E) inadequate : substrate-level phosphorylation
E) inadequate : substrate-level phosphorylation
As the intensity of exercise increases (and the potential gap between energy demand and creation threatens to widen), the muscles quickly underwrite their escalating energy demands by augmenting oxidative with substrate-level phosphorylation, resulting in the increased generation of which of the following?

A) lactic acid
B) glucose
C) ATP
D) fatty acid
E) water from the electron transport chain
A) lactic acid
When an action potential is generated within a motor neuron,

A) every muscle cell of the motor unit is stimulated to contract.

B) the muscle cells of the motor unit will occasionally contract.

C) all of the muscle cells within the motor unit are stimulated to relax.

D) only select muscle cells within the motor unit are stimulated to contract.

E) the muscle cells from a neighboring motor unit will contract.
A) every muscle cell of the motor unit is stimulated to contract.
Which is the longest phase of a twitch?

A) latent
B) relaxation
C) initial
D) contraction
E) plateau
B) relaxation
Which of the following is NOT part of the explanation for the all-or-nothing property of an isometric twitch contraction of skeletal muscle?

A) equivalent calcium reuptake by the sarcoplasmic reticulum

B) equivalent calcium released by each action potential

C) equivalent activation of calmodulin stimulating myosin light-chain kinase activity

D) all-or-nothing property of the action potential generated by a skeletal muscle

E) equal numbers of crossbridges activated by the calcium released
C) equivalent activation of calmodulin stimulating myosin light-chain kinase activity
Which of the following is a property of isometric skeletal muscle contraction?

A) Load is greater than the force generated by the muscle.

B) Load is less than the force generated by the muscle.

C) rapid lengthening of the muscle

D) rapid shortening of the muscle

E) Load is equal to the force generated by the muscle.
A) Load is greater than the force generated by the muscle.
Which of the following is a property of isotonic skeletal muscle contraction?

A) There is a rapid increase in force with no change in muscle length.

B) Load is greater than the force generated by the muscle.

C) Load is less than the force generated by the muscle.

D) Muscle length will be increased by contraction.

E) There is a slow increase in force with no change in muscle length.
C) Load is less than the force generated by the muscle.
Which of the following statements about excitation-contraction coupling in skeletal muscle is FALSE?

A) An action potential in the sarcolemma stimulates the opening of voltage-gated calcium channels in the plasma membrane and the sarcoplasmic reticulum.

B) Calcium binds to troponin, causing a shift in tropomyosin, thereby exposing the binding sites on actin for myosin.

C) Calcium ATPases in the sarcoplasmic reticulum are necessary for relaxation of the muscle.

D) DHP receptors on the T tubules detect changes in voltage and communicate to ryanodine
receptors on the sarcoplasmic reticulum to open calcium channels.

E) An end-plate potential triggers an action potential on the sarcolemma.
A) An action potential in the sarcolemma stimulates the opening of voltage-gated calcium channels in the plasma membrane and the sarcoplasmic reticulum.
Which of the following statements about end-plate potentials is FALSE?

A) They are graded potentials.

B) They are almost always of sufficient magnitude to generate an action potential in the sarcolemma.

C) They are a result of acetylcholine binding to muscarinic cholinergic receptors.

D) They are always depolarizations.

E) They are terminated by removal of acetylcholine from the synaptic cleft.
C) They are a result of acetylcholine binding to muscarinic cholinergic receptors.
Which of the following best describes an isotonic contraction?

A) When the contractile elements shorten, they lengthen the elastic elements but do not move the load.

B) When the contractile elements lengthen, they lengthen the elastic elements but do not move the load.

C) When the contractile elements lengthen, they shorten the elastic elements and move the load.

D) When the contractile elements shorten, they create enough force to move the load.

E) The contractile elements stay the same length as the elastic elements shorten and move the load.
D) When the contractile elements shorten, they create enough force to move the load.
The speed with which the skeletal muscle generates force is determined primarily by

A) the amount of energy available.

B) actin.

C) troponin.

D) the type of myosin and its ATPase present.

E) tropomyosin.
D) the type of myosin and its ATPase present.
Which of the following is the property of skeletal muscle whereby an increase in the frequency of action potentials enhances the force developed by the muscle cell?

A) length-tension relationship
B) external tension
C) internal tension
D) summation
E) force-velocity relationship
D) summation
The tension produced during skeletal muscle contraction can be increased by all of the following EXCEPT

A) increasing the frequency of action potentials in the motor neuron innervating the muscle.

B) initiating the contraction when the muscle is at its optimum length.

C) increasing the load on the muscle.

D) recruiting motor units.

E) increasing the frequency of action potentials in the muscle fiber.
C) increasing the load on the muscle
Which of the following statements about summation and tetanus is FALSE?

A) Summation and tetanus occur at high frequencies of skeletal muscle stimulation.

B) Summation and tetanus result in greater tension in the muscle.

C) The maximum tension developed during summation and tetanus is treppe.

D) Summation and tetanus result from increases in cytosolic calcium levels.

E) Summation and tetanus do not occur in cardiac muscle.
C) The maximum tension developed during summation and tetanus is treppe.
Once a skeletal muscle cell begins to shorten during an isotonic twitch contraction,

A) the muscle will continue to shorten until the force generated by the muscle stops increasing.

B) the muscle will stop shortening once the force is greater than the load.

C) the amount of force generated by the muscle will start to decrease.

D) the amount of force generated by the muscle will remain constant.

E) the amount of force generated by the muscle will continue to increase.
D) the amount of force generated by the muscle will remain constant.
Which of the following conditions will result in a skeletal muscle that produces a greater amount of force?

A) fewer myofibrils per muscle cell
B) fewer sarcomeres in parallel
C) more sarcomeres in series
D) more sarcomeres in parallel
E) fewer sarcomeres in series
D) more sarcomeres in parallel
Which of the following is NOT an accurate description of specific muscle fiber types?

A) Slow oxidative fibers are the smallest diameter fibers.

B) Fast glycolytic fibers produce their ATP by glycolysis.

C) Fast oxidative fibers have a high mitochondrial density.

D) Slow oxidative fibers are quick to fatigue.

E) Fast glycolytic fibers are the largest diameter fibers.
D) Slow oxidative fibers are quick to fatigue.
As skeletal muscle is further stretched beyond the length where optimum force is developed,

A) the thin filaments overlap one another, thereby reducing their ability to interact with myosin.

B) the amount of calcium released by the sarcoplasmic reticulum is reduced as length increases.

C) the thin filaments are pulled away from the thick filaments, thereby reducing actin's ability to interact with myosin.

D) the thick filaments overlap one another, thereby reducing their ability to interact with actin.

E) the thick filaments are pulled away from one another, thereby reducing their ability to
interact with actin.
C) the thin filaments are pulled away from the thick filaments, thereby reducing actin's ability to interact with myosin.
As the sarcomere length of skeletal muscle is reduced beyond the length where optimum force is developed,

A) the thin filaments are pulled away from one another, thereby reducing their ability to interact with myosin.

B) the thin filaments overlap one another with myosin bumping into the Z-line, thereby causing the force generated by crossbridges to be exerted on the sarcomere itself rather than transmitted to the ends of the muscle fiber.

C) the amount of calcium released by the sarcoplasmic reticulum is reduced as length increases.

D) the thick filaments overlap one another, thereby reducing their ability to interact with actin.

E) the thick filaments are pulled away from one another, thereby reducing their ability to interact with actin.
B) the thin filaments overlap one another with myosin bumping into the Z-line, thereby causing the force generated by crossbridges to be exerted on the sarcomere itself rather than transmitted to the ends of the muscle fiber.
In the body, the sarcomere length of skeletal muscle cells is usually

A) much greater than optimum length.

B) much less than optimum length.

C) 3 microns.

D) quite variable from one muscle to the next.

E) near optimal.
E) near optimal.
Which of the following does NOT influence the force generated by an individual muscle fiber?

A) frequency of stimulation
B) length at onset of contraction
C) recruitment
D) fiber diameter
E) summation
C) recruitment
What is an increase in the number of active motor units that would increase the force developed by a skeletal muscle called?

A) summation
B) treppe
C) length-tension relationship
D) tetanus
E) recruitment
E) recruitment
Contraction of motor units in a(n) ________ manner allows a muscle to maintain a smooth level of force for a duration of time without fatiguing.

A) synchronous
B) random
C) asymmetrical
D) symmetrical
E) asynchronous
E) asynchronous
Which of the following BEST describes the relationship between motor neuron size, motor unit size, and order of recruitment?

A) large motor neurons : small motor unit : last to be recruited

B) large motor neurons : small motor unit : first to be recruited

C) large motor neurons : large motor unit : first to be recruited

D) large motor neurons : large motor unit : last to be recruited

E) There is no relationship
D) large motor neurons : large motor unit : last to be recruited
A muscle is stimulated at a frequency that allows the muscle to relax completely between contractions. However, the amount of tension increases with each contraction. What is this called?

A) tetanus
B) recruitment
C) treppe
D) summation
E) twitch
C) treppe
The mechanism responsible for the size principle involves

A) smaller motor neurons that innervate the least number of muscle cells and reach threshold easier than larger neurons.

B) smaller sympathetic neurons that contain the least number of muscle cells and reach threshold easier than larger neurons.

C) smaller motor units that contain the greatest number of muscle cells and reach threshold easier than larger neurons.

D) larger sympathetic neurons that contain the greatest number of muscle cells and reach threshold easier than smaller neurons.

E) larger motor neurons that innervate the greatest number of muscle cells and reach threshold easier than smaller neurons.
A) smaller motor neurons that innervate the least number of muscle cells and reach threshold easier than larger neurons.
Which of the following is NOT a property of glycolytic fibers as compared to oxidative fibers?

A) absence of myoglobin

B) readily fatigable

C) high capacity for lactic acid production

D) fewer capillaries

E) poor ability to produce ATP in the absence of oxygen
E) poor ability to produce ATP in the absence of oxygen
What molecule, present primarily within oxidative skeletal muscle cells, acts as an oxygen buffer due to its ability to bind oxygen?

A) hemoglobin
B) myosin light chain
C) myosin
D) lactic acid
E) myoglobin
E) myoglobin
What causes some muscle fibers to appear red and dark?

A) presence of lots of mitochondria

B) presence of myoglobin and lots of mitochondria

C) presence of myoglobin

D) high concentration of actin and myosin

E) greater blood supply
C) presence of myoglobin
What is the site where a muscle attaches to a stationary bone called?

A) adductor
B) abductor
C) origin
D) insertion
E) flexor
C) origin
What connects muscles to bone?

A) capillaries
B) tendons
C) T tubules
D) ligaments
E) cartilage
B) tendons
Relative to the triceps muscle, the biceps is its

A) antagonist.
B) abductor.
C) insertion.
D) agonist.
E) origin.
A) antagonist.
What type of receptor detects muscle length?

A) DHP
B) ryanodine
C) Golgi tendon organ
D) nociceptor
E) muscle spindle
E) muscle spindle
What is an annulospiral ending?

A) sensory endings in the muscle spindle that detect muscle length

B) sensory endings in the Golgi tendon organ that detect muscle tension

C) a structure in the semicircular canals that contains hair cells

D) a structure in the utricle that contains hair cells

E) a structure in the cochlea that contains hair cells
A) sensory endings in the muscle spindle that detect muscle length
What type of efferent neuron innervates intrafusal muscle fibers?

A) type Ia
B) type Ib
C) type II
D) alpha motor neurons
E) gamma motor neurons
E) gamma motor neurons
Which of the following statements is TRUE regarding activation of alpha and gamma motor neurons?

A) Gamma motor neurons are activated before alpha motor neurons.

B) Alpha and gamma motor neurons are activated at the same time.

C) Alpha motor neurons are activated before gamma motor neurons.

D) During light activity, alpha motor neurons are activated first; during heavy activity, gamma motor neurons are activated first.

E) During light activity, gamma motor neurons are activated first; during heavy activity, alpha motor neurons are activated first.
B) Alpha and gamma motor neurons are activated at the same time.
Which of the following is the correct order of muscle fiber recruitment, from first to last?

A) fast glycolytic : slow oxidative : fast oxidative

B) slow oxidative : fast oxidative : fast glycolytic

C) fast glycolytic : fast oxidative : slow oxidative

D) fast oxidative : fast glycolytic : slow oxidative

E) slow oxidative : fast glycolytic : fast oxidative
B) slow oxidative : fast oxidative : fast glycolytic
Which of the following muscle types has the slowest contraction time?

A) smooth muscle
B) fast oxidative skeletal muscle
C) slow oxidative skeletal muscle
D) fast glycolytic skeletal muscle
E) cardiac muscle
A) smooth muscle
Which of the following mechanisms does NOT account for the decrease in force development by muscle cells during sustained force development?

A) Compression of muscle during contraction can reduce blood flow to the muscle.

B) Oxidative fibers can be depleted of glycogen, their primary energy source.

C) The crossbridges can become resistant to ATP.

D) Glycolytic fibers produce lactic acid and inhibit enzyme activity with their hydrogen ion.

E) Repeated action potentials along the same neuron can lead to neuromuscular fatigue.
C) The crossbridges can become resistant to ATP.
Which of the following is NOT an adaptation of skeletal muscle that would be observed in response to aerobic training?

A) an increase in the number of oxidative fibers

B) an increase in the concentration of oxidative enzymes

C) an increase in the diameter of the skeletal muscle fibers

D) an increase in capillary density

E) an increase in mitochondrial density
C) an increase in the diameter of the skeletal muscle fibers
Where are gap junctions commonly found?

A) single-unit smooth muscle only

B) multi-unit smooth muscle only

C) cardiac muscle only

D) both single-unit smooth muscle and cardiac muscle

E) both single-unit and multi-unit smooth muscle
D) both single-unit smooth muscle and cardiac muscle
Which of the following is true of ALL muscle types?

A) They contain calmodulin.

B) Recruitment increases the strength of contraction.

C) They are striated.

D) They contract by using the sliding filament mechanism and crossbridge cycling.

E) They contain T tubules.
D) They contract by using the sliding filament mechanism and crossbridge cycling.
In smooth muscle, calcium triggers contraction by binding to what protein?

A) troponin

B) calmodulin

C) myosin light chain kinase

D) myosin light chain

E) DHP receptors
B) calmodulin
Calcium binding to calmodulin causes phosphorylation by myosin light chain kinase of what protein in what type of muscle?

A) troponin in skeletal and cardiac muscle only

B) troponin in smooth muscle only

C) actin in cardiac and smooth muscle

D) actin in smooth muscle only

E) myosin in smooth muscle only
E) myosin in smooth muscle only
Which of the following adaptations does NOT generally occur with aerobic exercise?

A) an increase in the number of mitochondria per muscle fiber

B) a conversion of fast glycolytic fibers to fast oxidative fibers

C) an increase in the number of myofibrils per muscle fiber

D) an increase in the blood supply to the muscles

E) an increase in the aerobic capacity of muscle
C) an increase in the number of myofibrils per muscle fiber
The interaction between actin and myosin in smooth muscle requires

A) that the calcium-calmodulin complex activates myosin light-chain kinase, which phosphorylates myosin thereby allowing it to bind with actin.

B) the interaction between calcium and troponin to expose the myosin binding site on the actin molecule.

C) that the calcium-calmodulin complex directly phosphorylates myosin light chain, allowing myosin to bind to actin.

D) the activation of troponin, which stimulates myosin light-chain kinase to phosphorylate myosin light chain, allowing myosin to bind with actin.

E) the tropomyosin must be moved out of the way by troponin before myosin can bind to actin.
A) that the calcium-calmodulin complex activates myosin light-chain kinase, which phosphorylates myosin thereby allowing it to bind with actin.
What is the source of variability in the response of smooth muscle cells from different organs to autonomic nervous activity (some relax to sympathetic nervous output while others contract)?

A) the type of receptor present on the effector organ

B) the duration of neurotransmitter release from the autonomic neurons

C) the presynaptic modulation of the autonomic neurons

D) the amount of neurotransmitter released into the synaptic cleft

E) the neurotransmitter released by the autonomic nervous system
A) the type of receptor present on the effector organ
Which of the following could cause a pacemaker potential?

A) spontaneous closing of calcium channels

B) spontaneous opening of potassium channels

C) spontaneous opening of sodium channels

D) spontaneous opening of chloride channels

E) All of the answers are correct.
C) spontaneous opening of sodium channels
Which of the following stores calcium to be released for muscle contraction?

A) motor end plate
B) sarcomeres
C) tendons
D) sarcoplasmic reticulum
E) actin and myosin
D) sarcoplasmic reticulum
Which of the following transmits action potentials to the interior of the muscle cell to trigger calcium release?

A) sarcoplasmic reticulum
B) motor end plate
C) sarcomeres
D) tendons
E) actin and myosin
B) motor end plate
Which of the following generates the mechanical force of a muscle through crossbridge cycling?

A) tendons
B) sarcoplasmic reticulum
C) sarcomeres
D) motor end plate
E) actin and myosin
E) actin and myosin
Which of the following attaches muscle to bone?

A) sarcomeres
B) actin and myosin
C) sarcoplasmic reticulum
D) tendons
E) motor end plate
D) tendons
What is the functional unit of skeletal muscle?

A) sarcomeres
B) tendons
C) actin and myosin
D) motor end plate
E) sarcoplasmic reticulum
A) sarcomeres
Which of the following contains ryanodine receptors?

A) tendons
B) actin and myosin
C) motor end plate
D) sarcomeres
E) sarcoplasmic reticulum
E) sarcoplasmic reticulum
Which of the following contains DHP receptors?

A) motor end plate
B) tendons
C) transverse tubules
D) sarcomeres
E) sarcoplasmic reticulum
C) transverse tubules
Which of the following contains nicotinic cholinergic receptors?

A) tendons
B) sarcomeres
C) motor end plate
D) sarcoplasmic reticulum
E) actin and myosin
C) motor end plate
Which of the following extends the length of the thick filaments?

A) A band
B) I band
C) M line
D) H zone
E) Z line
A) A band
What region of thick filaments has no overlap with thin filaments?

A) I band
B) M line
C) A band
D) H zone
E) Z line
D) H zone
What is the region of thin filaments that has no overlap with thick filaments called?

A) M line
B) A band
C) H zone
D) Z line
E) I band
E) I band
What anchors thin filaments together?

A) M line
B) A band
C) H zone
D) Z line
E) I band
D) Z line
What anchors thick filaments together?

A) H zone
B) I band
C) Z line
D) M line
E) A band
D) M line
Which of the following appears as light bands under the microscope?

A) M line
B) A band
C) I band
D) Z line
E) H zone
C) I band
Which of the following appears as dark bands under the microscope?

A) A band
B) H zone
C) Z line
D) M line
E) I band
A) A band
Which protein binds calcium in smooth muscle cells?

A) tropomyosin
B) troponin
C) myosin light-chain kinase
D) calmodulin
E) titin
D) calmodulin
What binds calcium in skeletal muscle cells?

A) titin
B) myosin light-chain kinase
C) tropomyosin
D) troponin
E) calmodulin
D) troponin
What binds calcium in cardiac muscle cells?

A) troponin
B) calmodulin
C) titin
D) myosin light-chain kinase
E) tropomyosin
B) calmodulin
What are the elastic fibers that anchor thick filaments in place?

A) calmodulin
B) troponin
C) titin
D) myosin light-chain kinase
E) tropomyosin
C) titin
What is the enzyme that modulates the ability of myosin to bind actin in smooth muscle?

A) troponin
B) calmodulin
C) myosin light-chain kinase
D) tropomyosin
E) titin
C) myosin light-chain kinase
Which type of skeletal muscle fiber contains a high concentration of mitochondria?

A) slow
B) fast
C) glycolytic
D) fermentative
E) oxidative
E) oxidative
Which type of skeletal muscle fiber contains high shortening velocities?

A) fast
B) oxidative
C) slow
D) fermentative
E) glycolytic
A) fast
Which type of skeletal muscle fiber catalyzes hydrolysis of ATP slowly?

A) glycolytic
B) oxidative
C) slow
D) fast
E) fermentative
C) slow
Which type of skeletal muscle fiber fatigues rapidly?

A) slow
B) glycolytic
C) oxidative
D) fast
E) isotonic
B) glycolytic
Which type of skeletal muscle fiber contains myoglobin?

A) fast
B) glycolytic
C) oxidative
D) fermentative
E) slow
C) oxidative
Which type of skeletal muscle fiber has most of its energy come from substrate-level phosphorylation?

A) fast
B) oxidative
C) glycolytic
D) slow
E) fermentative
C) glycolytic
Which type of skeletal muscle fiber is white muscle?

A) fermentative
B) fast
C) glycolytic
D) slow
E) oxidative
C) glycolytic
Which muscle type contains actin and myosin?

A) skeletal muscle only

B) skeletal, smooth, and cardiac muscle

C) smooth muscle only (single-unit or multi-unit)

D) skeletal and smooth muscle only

E) smooth and cardiac muscle only
B) skeletal, smooth, and cardiac muscle
Which muscle type contracts by the sliding-filament mechanism?

A) smooth and cardiac muscle only

B) skeletal and smooth muscle only

C) smooth muscle only (single-unit or multi-unit)

D) skeletal muscle only

E) skeletal, smooth, and cardiac muscle
E) skeletal, smooth, and cardiac muscle
Where does calcium bind to troponin?

A) smooth and cardiac muscle only

B) skeletal muscle only

C) skeletal and cardiac muscle only

D) skeletal and smooth muscle only

E) smooth muscle only (single-unit or multi-unit)
C) skeletal and cardiac muscle only
Calcium binds to calmodulin where?

A) skeletal muscle only

B) smooth and cardiac muscle only

C) skeletal, smooth, and cardiac muscle

D) smooth muscle only (single-unit or multi-unit)

E) skeletal and smooth muscle only
D) smooth muscle only (single-unit or multi-unit)
Some muscle cells have pacemaker activity. These include

A) skeletal, smooth, and cardiac muscle.

B) smooth and cardiac muscle only.

C) smooth muscle only (single-unit or multi-unit).

D) skeletal and smooth muscle only.

E) skeletal muscle only.
B) smooth and cardiac muscle only.
Which muscles are innervated by autonomic nervous system?

A) skeletal and smooth muscle only

B) smooth and cardiac muscle only

C) skeletal, smooth, and cardiac muscle

D) smooth muscle only (single-unit or multi-unit)

E) skeletal muscle only
B) smooth and cardiac muscle only
Gap junctions are present between which cells?

A) skeletal muscle only

B) skeletal and smooth muscle only

C) skeletal, smooth, and cardiac muscle

D) smooth muscle only (single-unit or multi-unit)

E) smooth and cardiac muscle only
E) smooth and cardiac muscle only
Calcium for contraction comes entirely from the sarcoplasmic reticulum in which of the following?

A) skeletal, smooth, and cardiac muscle

B) smooth and cardiac muscle only

C) skeletal muscle only

D) smooth muscle only (single-unit or multi-unit)

E) skeletal and smooth muscle only
C) skeletal muscle only
In which muscle type do thick and thin filaments organize into sarcomeres?

A) smooth muscle only (single-unit or multi-unit)

B) skeletal muscle only

C) smooth and cardiac muscle only

D) skeletal and cardiac muscle only

E) skeletal and smooth muscle only
D) skeletal and cardiac muscle only
What is the structure indicated by the number 1 in Figure 12.1 and what is its functional significance

A) M line : where the tails of the myosin molecules are bound to one another

B) A band : length of myosin attached to an M line

C) Z line : anchors the thin filaments together

D) sarcomere : the functional unit of muscle contraction whose length is changed as a muscle shortens and lengthens

E) H zone : area of myosin not overlapped by actin
A) M line : where the tails of the myosin molecules are bound to one another
What is the structure indicated by the number 2 in Figure 12.1 and what is its functional significance?

A) M line : where the tails of the myosin molecules are bound to one another

B) A band : length of myosin attached to an M line

C) Z line : anchors the thin filaments together

D) H zone : area of myosin not overlapped by actin

E) sarcomere : the functional unit of muscle contraction whose length is changed as a muscle
shortens and lengthens
C) Z line : anchors the thin filaments together
What is the structure indicated by the number 3 in Figure 12.1 and what is its functional significance?

A) M line : where the tails of the myosin molecules are bound to one another

B) A band : length of myosin attached to an M line

C) sarcomere : the functional unit of muscle contraction whose length is changed as a muscle
shortens and lengthens

D) Z line : anchors the thin filaments together

E) H zone : area of myosin not overlapped by actin
E) H zone : area of myosin not overlapped by actin
What is the structure indicated by the number 4 in Figure 12.1 and what is its functional significance?

A) M line : where the tails of the myosin molecules are bound to one another

B) H zone : area of myosin not overlapped by actin

C) A band : length of myosin attached to an M line

D) sarcomere : the functional unit of muscle contraction whose length is changed as a muscle shortens and lengthens

E) Z line : anchors the thin filaments together
C) A band : length of myosin attached to an M line
What is the structure indicated by the number 5 in Figure 12.1 and what its functional significance?

A) H zone : area of myosin not overlapped by actin

B) sarcomere : the functional unit of muscle contraction whose length is changed as a muscle shortens and lengthens

C) Z line : anchors the thin filaments together

D) A band : length of myosin attached to an M line

E) M line : where the tails of the myosin molecules are bound to one another
B) sarcomere : the functional unit of muscle contraction whose length is changed as a muscle shortens and lengthens
What is the structure indicated by the number 6 in Figure 12.1 and what is its functional significance?

A) I band : area of actin not having a crossbridge

B) sarcomere : the functional unit of muscle contraction whose length is changed as a muscle shortens and lengthens

C) Z line : anchors the thin filaments together

D) A band : length of myosin attached to an M line

E) H zone : area of myosin not overlapped by actin
A) I band : area of actin not having a crossbridge
Which structure(s) in Figure 12.1 would be altered by muscle shortening?

A) 2, 3, and 6
B) 4, 5, and 6
C) 1, 3, and 5
D) 3, 5, and 6
E) 1, 2, and 4
D) 3, 5, and 6
The continuation of the sarcolemma that penetrates the interior of the muscle fiber and thereby facilitates the release of calcium from the sarcoplasmic reticulum is called what?

A) troponin
B) sarcoplasmic reticulum
C) tropomyosin
D) actin
E) transverse tubule
E) transverse tubule
What is the protein component of the thin filament that blocks the myosin-binding site on the actin monomer?

A) sarcoplasmic reticulum
B) tropomyosin
C) troponin
D) actin
E) transverse tubule
B) tropomyosin
Troponin is a complex of three proteins that bind to tropomyosin, calcium, and

A) transverse tubule.
B) sarcoplasmic reticulum.
C) tropomyosin.
D) actin.
E) troponin.
D) actin.
Calcium is stored in what region of skeletal muscle cells?

A) troponin
B) tropomyosin
C) actin
D) transverse tubule
E) sarcoplasmic reticulum
E) sarcoplasmic reticulum
The three proteins found to make up the thin filaments are actin, tropomyosin, and

A) tropomyosin.
B) troponin.
C) transverse tubule.
D) sarcoplasmic reticulum.
E) actin.
B) troponin.
Heavy intensity exercise generates what byproduct that contributes to fatigue?

A) glycogen
B) lactic acid
C) CO2
D) pyruvic acid
E) fatty acids
B) lactic acid
The time between the occurrence of an action potential in skeletal muscle and the onset of tension is called what?

A) reticence
B) relative refractory period
C) synaptic delay
D) absolute refractory period
E) latent period
E) latent period
Mechanisms whereby force of skeletal muscle fibers can be altered include all of the following EXCEPT

A) fiber diameter (sarcomeres in parallel).

B) recruitment.

C) frequency of stimulation (summation).

D) fiber length.

E) conversion of myosin ATPase type.
E) conversion of myosin ATPase type.
The two types of sensory endings found in muscle spindles are the annulospiral endings and

A) Pacinian corpuscle.
B) Ruffini's corpuscle.
C) free endings.
D) flower-spray endings.
E) Merkel's disks.
D) flower-spray endings
What type of motor neuron innervates intrafusal muscle fibers?

A) alpha
B) beta
C) gamma
D) delta
E) epsilon
C) gamma
What is the antagonistic muscle to the triceps?

A) posterior deltoid
B) quadriceps
C) biceps
D) coracobrachialis
E) brachialis
C) biceps
What are the three types of muscle fibers that are found in all skeletal muscles?

A) fast oxidative, slow glycolytic, and fast glycolytic

B) slow oxidative, fast oxidative, and slow glycolytic

C) slow oxidative, fast oxidative, and fast glycolytic

D) slow oxidative, slow glycolytic, and fast glycolytic

E) fast oxidative, fast oxidative, and fast glycolytic
C) slow oxidative, fast oxidative, and fast glycolytic
Postural muscles of the legs and back tend to have a high proportion of what fiber type?

A) fast glycolytic
B) slow oxidative
C) slow glycolytic
D) fast oxidative
E) fast and slow glycolytic
B) slow oxidative
In smooth muscle, calcium binds to calmodulin and then activates the enzyme

A) calmodulin ATPase.

B) adenylate cyclase.

C) myosin light-chain phosphatase.

D) tyrosine kinase.

E) myosin light-chain kinase.
E) myosin light-chain kinase.
The decreased ability of a muscle to maintain a constant force of contraction during repetitive stimulation is called what?

A) O2 depletion
B) wasting
C) fatigue
D) tetanus
E) treppe
C) fatigue
In smooth muscle, the structures analogous to Z lines in skeletal muscle are called

A) intercalated disks.
B) dense bodies.
C) MLCK.
D) gap junctions.
E) S lines.
B) dense bodies.
Intercalated disks contain what type of special junctions?

A) gap junctions only

B) hemidesmosomes

C) desmosomes

D) tight junctions only

E) both gap junctions and tight junctions
E) both gap junctions and tight junctions
T/F Skeletal muscle fibers are formed by the fusion of multiple cells during embryonic development, resulting in the presence of numerous nuclei per fiber.
TRUE
T/F Thick filaments have many protrusions along their middle (head of the myosin filaments) but none at their ends.
FALSE
T/F Skeletal muscle and cardiac muscle are multinucleated cells, whereas smooth muscle cells each have one nucleus.
FALSE
T/F As skeletal muscle shortens with contraction, the length of the I band remains constant as the length of the A band decreases.
FALSE
T/F Binding of ATP to myosin increases myosin's affinity for actin.
FALSE
T/F A skeletal muscle cell produces force for only a small portion of its contractile cycle because only a small portion of the crossbridge cycle involves force generation.
FALSE
T/F The proximity of the sarcoplasmic reticulum to each sarcomere permits the nearly simultaneous delivery of calcium to all sarcomeres of the muscle fiber.
TRUE
T/F The primary source of calcium that drives skeletal muscle contraction is the sarcoplasmic reticulum.
TRUE
T/F Ryanodine receptors are calcium channels in the sarcoplasmic reticulum.
TRUE
T/F Creatine phosphate provides an immediate source of high energy phosphate to donate to ADP at the onset of muscle activity.
TRUE
T/F The contraction phase of a twitch lasts longer than the relaxation phase.
FALSE
T/F Skeletal muscle fibers operate at near-optimal sarcomere length in situ.
TRUE
T/F The velocity of shortening during an isometric contraction is zero.
TRUE
T/F The velocity of shortening of a muscle is greatest when there is no load on the muscle
TRUE
T/F Intrafusal muscle fibers contribute directly to the tension produced during muscle contraction.
FALSE
T/F Alpha motor neurons innervate extrafusal muscle fibers.
TRUE
T/F The biceps cause flexion of the arm, whereas the triceps cause extension.
TRUE
T/F Activation of Golgi tendon organs during overstretching of a muscle protects the muscle by inhibiting its contraction.
TRUE
T/F A skeletal muscle with slower ATPase activity can complete more crossbridge cycles per second.
FALSE
T/F There are no slow glycolytic fibers.
TRUE
T/F Hemoglobin in skeletal muscle acts as an oxygen buffer.
FALSE
T/F Contraction of smooth muscle cells is driven by binding of calcium to calmodulin.
TRUE
T/F Smooth and cardiac muscle are innervated by the autonomic nervous system, whereas skeletal 161) muscle is innervated by the somatic nervous system.
TRUE
T/F The speed with which smooth muscle cells contract is similar to fast glycolytic skeletal muscle cells.
FALSE
T/F Spontaneous depolarization of smooth muscle cells is mediated by pacemaker cells.
TRUE
T/F Slow-wave potentials are caused by cyclical fluctuations in opening and closing of potassium 164) channels.
FALSE
T/F Action potentials in cardiac muscle last longer than action potentials in skeletal muscle.
TRUE
T/F Pacemaker activity occurs in certain smooth muscle cells and cardiac muscle cells
TRUE
T/F There is no summation of cardiac muscle.
TRUE
T/F Oxidative phosphorylation is the primary energy source for cardiac muscle.
TRUE
Force development by muscle cells requires an interaction between actin and myosin. Describe the proteins involved in the development of force through the process of the sliding-filament theory, including crossbridge cycling.
Force development in skeletal muscle requires the interaction between the head of the myosin molecule (crossbridge)
and actin. The head of the myosin molecule contains two important regions: 1) an actin-binding site, and 2) an ATPase that derives energy from ATP for force development. Actin monomers are arranged into two filaments (thin filaments) that are twisted into a double helix and are anchored to the Z-line. Myosin molecules are arranged side by side out of phase with one another. At the same time, myosin dimers are bound tail to tail such that the myosin heads on either side of the thick filament pull toward one another. The muscle is able to generate force by the cycling of crossbridges that bind to actin. The cycling starts with an ATP molecule binding to the ATPase portion of the myosin head that is still bound to actin and is in the low-energy form. This ATP binding initiates a conformational change that removes it from actin. The ATP molecule is then split into ADP and Pi, changing the myosin head into a high-energy form (cocked). Once this energized myosin head comes in contact with an actin molecule whose binding site is exposed, the myosin will bind to the actin molecule. The release of Pi allows the myosin head to return to the low-energy form, and in the process, the myosin head pivots, pulling the Z lines of the sarcomere toward one another. The bond between actin and myosin is maintained until ATP is able to bind to the myosin head. In the absence of ATP, the muscle is maintained in the state of rigor. The cycling of crossbridges causes the two thin filaments of the sarcomere to be pulled toward one another by the thick filament, thereby pulling the Z-lines closer together.
In order for skeletal muscle to develop force, a muscle cell must be able to convert the action potential generated at a neuromuscular junction into crossbridge cycling. Describe the process of excitation-contraction coupling in skeletal muscle, including all of the important ion channels and the structures involved in this process.
An action potential is generated at the motor end plate by the binding of acetylcholine to the nicotinic receptor. That action potential travels along the sarcolemma and down the transverse tubules. Located within the membrane of the transverse tubules are dihydropyridine receptors that are voltage sensitive. These receptors are usually activated by depolarization. The membrane of the transverse tubule comes into contact with the sarcoplasmic reticulum, allowing the dihydropyridine receptor to directly interact with the ryanodine receptor on the sarcoplasmic reticulum. When the dihydropyridine receptor is activated by membrane depolarization from the action potential, the ryanodine receptor is stimulated to release calcium from the sarcoplasmic reticulum. As intracellular calcium increases, the release of calcium is enhanced by the binding of calcium to another calcium channel on the sarcoplasmic reticulum. In addition to their proximity to the transverse tubule, the sarcoplasmic reticulum is positioned near contractile proteins of the sarcomere to facilitate the delivery of calcium to those contractile proteins. As calcium increases within the cell, it binds to a subunit of the troponin molecule. This binding causes a conformational change in the other two troponin subunits that move the filamentous tropomyosin. At rest, tropomyosin blocks the myosin binding site on the actin molecules. Thus, the movement of tropomyosin exposes the binding site that would allow the energized myosin to interact with actin.
Energy in the form of ATP is essential to the contractile process. Describe the cellular sources of energy for skeletal muscle fibers and how those energy sources play a role maintaining muscle activity at low- and high-intensities of activity.
Energy in the form of ATP is required for the release of the actin-myosin complex, in addition to energizing the
myosin head. Thus, in order for skeletal muscle to contract, there must be an adequate supply of energy. At rest, the concentration of ATP is relatively low. In order to prevent this ATP supply from being depleted during the first few seconds of muscle contraction, creatine phosphate is present to act as an energy buffer by providing the Pi necessary to re-energize the ATP and allows crossbridge cycling to continue to break the actin-myosin complexes. Creatine phosphate is in equilibrium with creatine, whose concentration is also limited within skeletal muscle. The muscle must then switch to metabolizing glucose from its storage as glycogen in the muscle or liver (glucose must travel through the blood in order to enter the muscle). If the intensity of exercise is maintained at moderate levels or below, the aerobic metabolism of fatty acids through oxidative metabolism will take over as the primary source of energy. The fatty acids provide substrate (acetyl CoA) for the Krebs cycle and the electron transport chain as long as enough oxygen is present to act as the final accepter of electrons. However, if exercise intensity increases further, the ability of aerobic metabolism to supply energy can be compromised by the limitations on blood flow (oxygen delivery) to the active muscle. In that case, oxygen delivery would be compromised and substrate-level metabolism would have to take over the production of ATP. The byproduct of this substrate-level metabolism is lactic acid, whose dissociated hydrogen ion can limit the ability of the muscle to generate force through a number of mechanisms.
While muscle cells respond in an all-or-nothing manner to a single action potential, the extent of force that they generate can be modified by a number of factors. Describe the mechanisms whereby individual skeletal muscle cells can modify force development.
There are a number of mechanisms that contribute to alterations in force generation by individual skeletal muscle cells. The first is the frequency of stimulation. A muscle response to frequency contains two components: the treppe phenomena and the summation of contraction. Treppe describes the phenomena where an increase in frequency of stimulation (with complete relaxation between pulses) will progressively increase the force developed by the muscle until force ultimately stabilizes. The explanation for this phenomenon involves an increasing concentration of intracellular calcium, due to incomplete removal of calcium during relaxation, which elevates the force developed by the muscle cell. Summation of contraction is observed as the frequency of stimulation increases further, such that the muscle cell does not completely relax between twitches. As the frequency of twitches increases, the first twitch will not completely relax before the second twitch arrives, and so on. Thus, as frequency increases, the force generated by the muscle would continue to increase until a maximum is reached. As the frequency of stimulation increases, the muscle will eventually be able to maintain force with some oscillation around a constant value. This oscillation in force is termed tetanus. As frequency is increased further, force will eventually plateau (the trace flattens and there is no relaxation between twitches) into what is called fused (complete) tetanus or maximal tetanic tension. Prior to this force plateau (fusion), force fluctuates as the muscle cell partially relaxes between twitches, which is called unfused (incomplete) tetanus. Second, the force developed by a muscle fiber is dependent upon the diameter of that muscle fiber. As the diameter of a muscle fiber increases (number of parallel sarcomeres increases), the force generated by that muscle will increase. Finally, skeletal muscle length will affect the extent of tension development by the muscle cell. Typically, the muscle rests at near optimal length for force development. As the muscle is lengthened, the potential for interaction between actin and myosin is reduced as they slide past one another, thereby causing the force generated by the muscle to decrease. As the muscle is shortened, the extent of interaction will also be reduced by the thin filament blocking the binding of myosin and actin. In this case, force is also decreased as the muscle is shortened.
Irrespective of an individual muscle cell's ability to regulate the amount of force developed by that cell, the force generated by a skeletal muscle is also controlled at the whole muscle level. Describe the mechanism whereby force of a whole muscle can be modified, including a discussion on motor units.
In addition to the ability of individual muscle cells to modify their force development, the force of the entire muscle can be modulated. Every muscle cell is not contracted at the same time within a muscle. Since the functional unit of the muscle is the motor unit, the extent of force development will be determined by the number of muscle fibers associated with that motor unit and the way in which those motor units are recruited. As more motor units are simultaneously recruited, the force developed by the muscle will be increased. The extent of force developed depends upon the number of muscle cells present within a given motor unit, which can vary from several hundreds to thousands. Muscles that are involved in fine movement will have fewer muscle cells per motor unit than muscles involved in more gross movement. The specific motor units activated are determined by the size principle. This principle states that there is a correspondence between the size of motor units activated and their order of recruitment. The physiological determinant of which motor units are activated is the sensitivity of the motor neurons to action potential frequency; smaller motor neurons are stimulated to generate an action potential at a lower frequency of action potentials than larger diameter motor neurons. The smaller motor units are innervated by smaller diameter motor neurons. Thus, the size of the motor units corresponds with the diameter of the motor neuron. The larger motor units are resistant to depolarization. Thus, a higher frequency of action potentials is required to activate the larger motor units. At a lower frequency, the smaller motor units will be activated. As the force required to move an object is increased, the frequency of the action potentials from the motor center is increased, thereby recruiting more of the larger fibers that are capable of generating greater force.
In human skeletal muscle, a number of different fiber types exist. Describe the properties of the fibers that make them unique and how that is linked to motor units.
There are three major types of muscle fiber in human skeletal muscle, which are characterized by their metabolic profile and speed of contraction. With respect to their metabolic profile, the enzymes expressed in these cells can favor either a glycolytic (substrate-level phosphorylation) or oxidative metabolism. These alterations in enzyme profiles are supported by a number of structural differences within the muscle fibers. Muscle fibers with a greater oxidative enzyme profile will contain a greater density of capillaries, increased mitochondrial density, higher myoglobin concentration, and will have a smaller diameter than glycolytic fibers. In contrast, glycolytic fibers are thicker (have a greater diameter) than oxidative fibers, contain fewer capillaries, and have a greater capacity to produce ATP under anaerobic conditions. Thus, under high-intensity exercise, these muscles will actively produce an excess of pyruvic acid that is converted to lactic acid. The contractile properties are related to the ATPase portion of the myosin molecule. The rate of ATPase activity will greatly determine the speed with which a muscle cell can develop force. The fast fibers reach peak force sooner than the slow fibers. Putting these two properties together, the three major fiber types include slow oxidative, fast oxidative, and fast glycolytic.

Skeletal muscles generally contain differing proportions of all three skeletal muscle fiber types. However, the relative contribution of each fiber type can vary between muscles. Within a given motor unit, all of the muscle fibers are of the same fiber type, indicating that fiber type is determined by the motor neuron. The largest of the fibers are the fast glycolytic, while the smallest are the slow oxidative; the fast oxidative are intermediate. The same is true for the speed of contraction; the fast glycolytic are faster than fast oxidative, which are faster than slow oxidative. Motor units are also of different sizes; fast glycolytic are larger than fast oxidative or slow oxidative.
Training induces a number of changes within skeletal muscle that are specific to the type of training employed. Describe the changes that occur in skeletal muscle in response to training.
One reason why athletes train is to increase the capacity of their muscles to perform the work that their sport requires. Athletic training can be broken into two basic types: aerobic and anaerobic, depending upon the energy systems that are stressed by the specific training modality. In response to aerobic training, some of the fast glycolytic fibers are converted into fast oxidative fibers. This is accompanied by an increased mitochondrial density (size and number), an increase in the number of capillaries that surround each muscle fiber, and a decrease in the diameter of the muscle fiber (the last two would facilitate the delivery of oxygen to the active muscle fibers). For the slow oxidative fibers, they do not appear to be capable of converting to fast fiber types (because of the type of ATPase the myosin contains remains fairly constant). However, the changes in the slow oxidative fibers to aerobic training would be similar to the fast oxidative fibers. In contrast, high-intensity anaerobic exercise will increase the glycolytic capacity of the muscle fibers (switch some of the fast oxidative to fast glycolytic fibers). At the same time, the density of mitochondria (size and number) will be decreased, the concentration of glycolytic enzymes will be increased, and the diameter of the muscle fibers will be increased. The increased skeletal muscle girth in an individual who regularly weight trains is related to an increase in the diameter of muscle fibers (increased myofibrils) rather than an increase in the number of muscle fibers (hypertrophy rather than hyperplasia).
While contractile proteins are similar in skeletal and smooth muscle cells, the structure and regulation of muscle contraction is quite different. Describe the arrangement of contractile proteins in smooth muscle cells and how contraction is mediated.
Contractile proteins (actin and myosin) are the same in smooth muscle and skeletal muscle. However, their arrangement within those muscle cells is quite different. Skeletal muscles are characterized by the repeating striated arrangement of the contractile proteins into sarcomeres. This is not the case for smooth muscle, where the contractile proteins are not arranged in a striated pattern, hence the smooth appearance. Rather, contractile proteins are arranged in a variety of patterns that are oblique to the long axis of the smooth muscle cell. These contractile proteins are attached to the dense bodies where the force generated by the contractile proteins is transmitted to the cell's exterior. The regulation of excitation-contraction coupling is also quite different in smooth muscle cells. Calcium is still an important regulator of force development but is coupled to contraction in a different way than was observed in skeletal muscle cells. Calcium enters the cell through a voltage-sensitive calcium channel to bind with calmodulin. The calcium-calmodulin complex binds to the enzyme myosin light-chain kinase, causing it to become active. The myosin light-chain kinase phosphorylates the myosin light chain, causing it to become active and capable of binding to actin. Thereafter, the cycling of the crossbridges is the same as observed in skeletal muscle. In order to terminate crossbridge cycling, the smooth muscle cells must do more than remove calcium from the cytoplasm. The myosin light chain must be dephosphorylated by a phosphatase enzyme. These enzymes are always active within smooth muscle cells. Thus, the extent of force developed by smooth muscle cells is dependent upon the interaction between myosin light-chain kinase and phosphatase activity, each of which can be modulated.
Another name for a muscle cell is (myofibril / muscle fiber).
muscle fiber
Following death, rigor mortis occurs due to a lack of (calcium / ATP).
ATP
Receptor types at the motor end plate are (muscarinic / nicotinic) cholinergic receptors.
nicotinic
(Dihydropyridine / Ryanodine) receptors are located on the sarcoplasmic reticulum.
Ryanodine
During summation, calcium release is (greater than / less than) calcium reuptake.
greater than
The velocity at which a skeletal muscle contracts is (directly / inversely) related to load.
inversely
(Single-unit / Multi-unit) smooth muscle contains pacemaker cells.
single-unit
Cardiac muscle is (smooth / striated)s
striated
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