Muscle Tissue

Action potential propagation in a skeletal muscle fiber ceases when acetylcholine is removed from the synaptic cleft. Which of the following mechanisms ensures a rapid and efficient removal of acetylcholine?

Acetylcholine is degraded by acetylcholinesterase. Yes! Acetylcholinesterase is an enzyme that degrades acetylcholine. This degradation results in a rapid cessation of the acetylcholine signal and a swift removal from the cleft.

The force of muscle contraction is controlled by multiple motor unit summation or recruitment.

True

1. : The swollen chamber of an organelle that lies on either side of a T tubule 2. : A protein that changes shape due to an action potential 3. : A protein with heads that attach in cross bridging 4. : A structure that forms a path from the membrane to the cell interior 5. : The protein that binds calcium

1. ) Cistern: The swollen chamber of an organelle that lies on either side of a T tubule 2. ) Voltage-sensitive tubule protein: A protein that changes shape due to an action potential 3.) Myosin: A protein with heads that attach in cross bridging 4. ) T tubule: A structure that forms a path from the membrane to the cell interior 5. ) Troponin: The protein that binds calcium

1. Serves as the actual "trigger" for muscle contraction by removing the inhibition of the troponin molecules. 2. A neurotransmitter released at motor end plates by the axon terminals. 3. Diffusion across the cell membrane results in depolarization. 4. Activate synaptic vesicles in axon terminals. 5. Used to convert ADP to ATP by transfer of a high-energy phosphate group. A reserve high-energy compound. 6. Destroys ACh.

1. )Calcium ionsServes as the actual "trigger" for muscle contraction by removing the inhibition of the troponin molecules. 2. )AcetylcholineA neurotransmitter released at motor end plates by the axon terminals. 3.) Sodium ionsDiffusion across the cell membrane results in depolarization. 4. )Calcium ionsActivate synaptic vesicles in axon terminals. 5. )Creatine phosphateUsed to convert ADP to ATP by transfer of a high-energy phosphate group. A reserve high-energy compound. 6. )AcetylcholinesteraseDestroys ACh. Acetylcholinesterase returned

1. Depends on oxygen delivery and aerobic mechanisms. 2. Have very fast-acting myosin ATPases and depend upon anaerobic metabolism during contraction. 3. Red fibers, the smallest of the fiber types. 4. Contain abundant amounts of glycogen. 5. Abundant in muscles used to maintain posture. 6. A relatively high percentage are found in successful marathon runners.

1. )Slow (oxidative), fatigue-resistant fibersDepends on oxygen delivery and aerobic mechanisms. 2. )Fast (oxidative or glycolytic), fatigable fibersHave very fast-acting myosin ATPases and depend upon anaerobic metabolism during contraction. 3. )Slow (oxidative), fatigue-resistant fibersRed fibers, the smallest of the fiber types. 4.) Fast (oxidative or glycolytic), fatigable fibersContain abundant amounts of glycogen. 5.) Slow (oxidative), fatigue-resistant fibersAbundant in muscles used to maintain posture. 6.) Slow (oxidative), fatigue-resistant fibersA relatively high percentage are found in successful marathon runners.

1. The molecules released just before power stroke: 2. Actin behavior during the power stroke: 3. Myosin head energy status during the power stroke:

1. )The molecules released just before power stroke: ADP and Pi 2.) Actin behavior during the power stroke: ratchets toward the M line 3.) Myosin head energy status during the power stroke: pivots and returns to low energy

1. blocks myosin-binding sites on actin: 2. long cylindrical cells: 3. contractile unit: 4. made up of several sarcomeres:

1. )blocks myosin-binding sites on actin: tropomyosin 2. )long cylindrical cells: skeletal muscle fibers 3. )contractile unit: sarcomere 4. )made up of several sarcomeres: myofibrils

1. diffuses out of the muscle fiber through open chemically gated ion channels. 2. diffuses into the muscle fiber through open chemically gated ion channels. 3. binds to ACh receptors, causing them to open chemically gated ion channels. 4. The end plate potential is primarily, and most directly, caused by the movement of .

1. K+ diffuses out of the muscle fiber through open chemically gated ion channels. 2. Na+ diffuses into the muscle fiber through open chemically gated ion channels. 3. ACh binds to ACh receptors, causing them to open chemically gated ion channels. 4. The end plate potential is primarily, and most directly, caused by the movement of Na+. The binding of ACh to ACh receptors increases the permeability of the sarcolemma to both Na+ and K+. The movement of Na+ into the muscle fiber is primarily responsible for the change in membrane potential known as the end plate potential.

1. : Neurotransmitter at neuromuscular junction 2. : Membrane surrounding muscle fiber 3. : Ion triggering exocytosis 4. : Portion of sarcolemma containing ACh receptors

1.) ACh : Neurotransmitter at neuromuscular junction 2.) Sarcolemma : Membrane surrounding muscle fiber 3. )Ca2+ : Ion triggering exocytosis 4.) Junctional folds : Portion of sarcolemma containing ACh receptors

Which of the following is/are mechanism(s) to end neural transmission at the neuromuscular junction?

1.) ACh is broken down into acetic acid and choline by the enzyme acetylcholinesterase (AChE). 2.) ACh diffuses away from the synaptic cleft. The chemical signal (ACh) at the neuromuscular junction is very short! Soon after ACh is released from the axon terminal, it is rapidly degraded by AChE or diffuses out of the synaptic cleft. These two mechanisms ensure that just enough ACh binds to its receptor to initiate an action potential and that the muscle fiber's response is quickly terminated.

Place the events that occur during excitation-contraction coupling in the correct order from left to right.

1.) AP propagates along sarcolemma 2.) AP travels downT tubules to triads 3.)Voltage-sensitive proteins open Ca2+ channels 4.) Sarcoplasmic reticulum releases Ca2+ 5.) Ca2+ levels in sarcoplasm increase Recall from the video that a single motor neuron can conduct action potentials to several skeletal muscle fibers within a muscle. Excitation-contraction coupling converts these action potentials in a muscle fiber to a contraction. As muscle fibers contract, the entire muscle shortens and pulls the tendons on either end of the muscle, producing muscle tension.

place in the correct order of occurrence from left to right the events that occur at the neuromuscular junction after the action potential reaches the axon terminal.

1.) Ca2+ enters the axon terminal. 2.) Synaptic vesicles release ACh. 3.) ACh binds to ACh receptors. 4.) Ligand-gated cation channels open. 5.) Na+ enters and K+ exits. 6.) Membrane potential is less negative. The neuromuscular junction is a chemical synapse. An electrical signal (known as an action potential) from the motor neuron is converted into a chemical signal (ACh release). The skeletal muscle fiber responds to the chemical signal (ACh) by depolarizing and initiating an action potential. This action potential is propagated along the sarcolemma of the muscle fiber, which triggers sliding of the myofilaments for muscle contraction.

1. : Weight lifting 2. : Marathons 3. : 25 meter swim

1.) Direct phosphorylation : Weight lifting 2. ) Aerobic pathway : Marathons 3. ) Anaerobic pathway : 25 meter swim

1. Location of higher concentration of Ca2+ needed for cross bridge formation and cycling: 2. Actin status to begin cross bridge formation: 3. Molecule(s) bound to myosin head at start of cross bridge cycle: 4. Energy state of myosin for beginning cross bridge formation:

1.) Location of higher concentration of Ca2+ needed for cross bridge formation and cycling: cytoplasm 2. )Actin status to begin cross bridge formation: actin binding sites exposed 3. )Molecule(s) bound to myosin head at start of cross bridge cycle: ADP and Pi 4. )Energy state of myosin for beginning cross bridge formation: energized/cocked myosin head

1. The stimulus above which no stronger contraction can be elicited, because all motor units are firing in the muscle. 2. Determined by alternating motor units of a muscle organ even when the muscle is at rest. 3. Continued sustained smooth contraction due to rapid stimulation. 4. The situation in which contractions become stronger due to stimulation before complete relaxation occurs. 5. How a smooth increase in muscle force is produced.

1.) Maximal stimulusThe stimulus above which no stronger contraction can be elicited, because all motor units are firing in the muscle. 2.) Muscle toneDetermined by alternating motor units of a muscle organ even when the muscle is at rest. 3. )TetanusContinued sustained smooth contraction due to rapid stimulation. 4. )Wave summationThe situation in which contractions become stronger due to stimulation before complete relaxation occurs. 5.) Multiple motor unit summationHow a smooth increase in muscle force is produced.

1. : myofilament with a knob-like head 2. : myofilament stiffened and stabilized by tropomyosin 3. : the cytoplasm of a skeletal muscle fiber 4. : bundle of skeletal muscle fibers enclosed by connective tissue called perimysium 5. : membrane of muscle cell

1.) Myosin: myofilament with a knob-like head 2. )Actin: myofilament stiffened and stabilized by tropomyosin 3.) Sarcoplasm: the cytoplasm of a skeletal muscle fiber 4.) Fascicle: bundle of skeletal muscle fibers enclosed by connective tissue called perimysium 5.) Sarcolemma: membrane of muscle cell

1. Status of ATP: 2. Status of myosin head: 3. Status of actin and myosin:

1.) Status of ATP: hydrolyzed to ADP and Pi 2. )Status of myosin head: returns to the energized position 3. )Status of actin and myosin: actin and myosin are detached

The sarcomere shortens when the myosin heads of the thick filaments, in a cocked position, form cross bridges with the actin molecules in thin filaments. This activity will test your understanding of the steps that occur in one complete cross bridge cycle. Place the steps that occur during a single cross bridge cycle in the correct order from left to right.

1.) The activated myosin head binds to actin, forming a cross bridge. 2.) ADP is released and myosin slides the thin filament toward the center of the sarcomere. 3.) ATP binds to the myosin head and detaches it from actin. 4.) ATP is hydrolyzed toADP and Pi and the energy released re-cocks the myosin head. The thin filaments are pulled toward the center of the sarcomere by the myosin heads of the thick filament. This shortens the sarcomeres in the muscle fiber and causes the whole skeletal muscle to contract. The cross bridge cycle ends when Ca2+ are actively transported back into the sarcoplasmic reticulum. The cycle repeats as long as the binding sites on actin remain exposed, and both Ca2+ and ATP are available.

1. The bond between the actin and myosin head weakens due to . 2. Only of the myosin heads are detached from actin at any one time while calcium is present and cycling continues. 3. is not bound to myosin during the detachment step.

1.) The bond between the actin and myosin head weakens due to ATP. 2.) Only one-half of the myosin heads are detached from actin at any one time while calcium is present and cycling continues. 3.) ADP is not bound to myosin during the detachment step.

1. : Location of acetylcholinesterase 2. : Container of many neurotransmitter molecules 3. : Ion entering muscle fiber through open chemically gated ion channels 4. : Protein in sarcolemma

1.)Synaptic cleft : Location of acetylcholinesterase 2. )Synaptic vesicle : Container of many neurotransmitter molecules 3. )Na+ : Ion entering muscle fiber through open chemically gated ion channels 4. )ACh receptor : Protein in sarcolemma

The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge?

A myosin head bound to actin Yes! As soon as the activated myosin head forms a cross bridge with actin, the power stroke begins

Which of the following statements best summarizes excitation-contraction coupling?

A series of events in which an electrical stimulus is conveyed to a muscle fiber to enact contraction.

A triad is composed of a T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum. How are these components connected?

A series of proteins that control calcium release Yes! When action potentials propagate along T-tubules, a voltage-sensitive protein changes shape and triggers a different protein to open it's channels, resulting in the release of calcium from the terminal cisternae.

Which of the following best describes how ACh changes the ion permeability of the sarcolemma?

ACh binds to receptors in the junctional folds. The junctional folds of the muscle fiber's sarcolemma contain ACh receptors. These receptors are chemically gated ion channels. When ACh binds to these receptors, they open, allowing ions to diffuse across the sarcolemma. This represents an increase in the sarcolemma's ion permeability.

After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment?

ATP binds to the myosin head. Yes! The binding of ATP to the myosin head weakens the bond between myosin and actin, forcing the myosin head to detach. ATP also provides the energy for the next power stroke.

The neuromuscular junction is a well-studied example of a chemical synapse. Which of the following statements describes a critical event that occurs at the neuromuscular junction?

Acetylcholine is released by axon terminals of the motor neuron. Yes! Acetylcholine is released into the synaptic cleft via exocytosis.

Which of the following best describes the role of acetylcholinesterase molecules at the neuromuscular junction?

Acetylcholinesterase breaks down ACh, which allows chemically gated ion channels to close. The chemical signal at the neuromuscular junction is quite brief. As soon as ACh is released from the axon terminal, acetylcholinesterase molecules in the synaptic cleft begin to break it down. This ensures that ACh receptors are open only for the brief amount of time required to initiate an action potential in the muscle fiber.

Consider how the action potential that initiates contraction is delivered to the muscle cell. Which of the choices below correctly describes how an action potential generated at the neuromuscular junction (NMJ) is converted to excitation in the muscle fiber?

An action potential in the motor neuron causes ACh to be released into the synaptic cleft. Binding of ACh to sarcolemma receptors initiates graded potentials. Correct Yes! An action potential traveling down a motor neuron arrives at the axon terminal and causes exocytosis of the neurotransmitter ACh into the synaptic cleft. ACh diffuses into the synaptic cleft, binds to the receptor proteins on the junctional folds of the muscle sarcolemma (motor end plate), and initiates graded potentials. These graded potentials sum to an action potential, thus initiating excitement of the muscle fiber.

Signals flowing through the neuromuscular junction pass through several structures in a single direction. Arrange the structures below in the order in which signals travel through them.

Beginning 1.) Axon 2.) Axon terminal 3.) Synaptic cleft 4.) Sarcolemma End Overall, an electrical signal in the motor neuron's axon leads to an electrical signal in the muscle fiber's sarcolemma. However, the electrical signal is briefly converted to a chemical signal as the information crosses the synaptic cleft.

The cross bridge cycle starts when _________.

Ca2+ from the sarcoplasmic reticulum binds to troponin The release of Ca2+ is triggered by the propagation of an action potential along a skeletal muscle fiber. Ca2+ is released from the sarcoplasmic reticulum and into the sarcoplasm of the muscle fiber. When Ca2+ concentration is high in the sarcoplasm, Ca2+ binds to troponin causing change in its shape. This shape change alters the position of tropomyosin and moves it away from myosin binding sites on actin, thus allowing the myosin head to bind actin and form a cross bridge.

Which selection correctly describes the role of calcium in coupling?

Calcium binds to troponin, which moves tropomyosin and exposes the myosin-binding sites on actin. The thin filament in a sarcomere is composed of actin, troponin, and tropomyosin. Troponin and tropomyosin are attached to one another, both overlaying actin. When a muscle is relaxed, tropomyosin blocks actin's myosin-binding sites. Calcium binds to troponin, initiating a shape change that removes the blocking action of tropomyosin. This exposes the myosin-binding sites on actin to the myosin heads for cross bridging.

Calcium ions couple excitation of a skeletal muscle fiber to contraction of the fiber. Where are calcium ions stored within the fiber?

Calcium ions are stored in the sarcoplasmic reticulum. Yes! Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle fibers. The sarcoplasmic reticulum is very elaborate in skeletal muscle fibers, allowing for significant storage of calcium ions.

What specific event triggers the uncovering of the myosin binding site on actin?

Calcium ions bind to troponin and change its shape Yes! The shape change caused by the binding of calcium to troponin shifts tropomyosin away from the myosin binding sites on actin.

Excitation of the sarcolemma is coupled or linked to the contraction of a skeletal muscle fiber. What specific event initiates the contraction?

Calcium release from the sarcoplasmic reticulum initiates the contraction. Yes! Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle cells. It is especially abundant and convoluted in skeletal muscle cells. It functions in the storage, release, and re uptake of calcium ions.

Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers?

Calcuim ions Yes! Action potentials propagating down the T-tubule cause a voltage-sensitive protein to change shape. This shape change opens calcium release channels in the sarcoplasmic reticulum, allowing calcium ions to flood the sarcoplasm. This flood of calcium ions is directly responsible for the coupling of excitation to contraction in skeletal muscle fibers.

When does cross bridge cycling end?

Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin. Yes! The sarcoplasmic reticulum contains Ca2+-ATPases that actively transport Ca2+ into the SR. Without Ca2+, troponin returns to its resting shape, and tropomyosin glides over and covers the myosin binding sites on actin.

A person dies and within hours, the skeletal muscles develop a locked contraction known as rigor mortis. Calcium ions leak from the sarcoplasmic reticulum into cytoplasm. From your knowledge of cross-bridge cycling, what best explains this rigor?

Cross bridge detachment cannot occur. Detachment requires ATP, which is only produced during life. After ATP attaches to the myosin head, the bond between actin and myosin is weakened and the cross-bridge breaks.

Excitation-contraction coupling is a series of events that occur after the events of the neuromuscular junction have transpired. The term excitation refers to which step in the process?

Excitation, in this case, refers to the propagation of action potentials along the sarcolemma Yes! These action potentials set off a series of events that lead to a contraction.

Click and drag the description of the part of the excitation-contraction coupling to the correct order of occurrence from left to right.

First step 1.) Action potential is generated at the sarcolemma. 2.) Action potential goes down the T tubule. 3.) Voltage-gated tubule protein changes shape. 4.) Calcium ion channels open. 5.) Calcium exits the cistern into sarcoplasm. 6.) Calcium binds to troponin. 7.) Actin sites are exposed. 8.) Cross-bridges form between actin and myosin. Last step Coupling begins with an action potential after the arrival of ACh and graded potentials, and ends before cross bridge cycling.

The action potential propagates along the sarcolemma. As the action potential spreads down the T tubules of the triads, voltage-sensitive tubule proteins change shape. How does the shape change of these proteins lead to contraction?

It allows calcium to exit the sarcoplasmic reticulum and enter the cytosol. As the action potential propagates, it changes the shape of T tubule proteins. These proteins are linked to calcium channels in the terminal cisterns of the sarcoplasmic reticulum. When these proteins calcium channels open, a massive amount of calcium flows into the cytosol.

Which best describes the initial event in contraction?

Myosin heads bind to the newly exposed myosin-binding sites on actin to form cross bridges. Contraction begins when myosin binds to actin and forms cross bridges

Calcium entry into the axon terminal triggers which of the following events?

Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine. Yes! When synaptic vesicles fuse to the plasma membrane, acetylcholine is released via exocytosis.

How does the myosin head obtain the energy required for activation?

The energy comes from the hydrolysis of ATP. Yes! Myosin is a large, complex protein with a binding site for actin. It also contains an ATPase. The energy released during the hydrolysis of ATP activates the myosin head.

Sodium and potassium ions do not diffuse in equal numbers through ligand-gated cation channels. Why?

The inside surface of the sarcolemma is negatively charged compared to the outside surface. Sodium ions diffuse inward along favorable chemical and electrical gradients. Yes! The resting membrane potential of all cells is negative (inside compared to outside). Therefore, given the direction of the chemical and electrical gradients, more sodium ions diffuse inward than potassium ions diffuse outward.

What structure is the functional unit of contraction in a skeletal muscle fiber?

The sarcomere Yes! A sarcomere is a regular arrangement of thin and thick myofilaments that extends from one Z disc to the next. A myofibril consists of a series of sarcomeres.

What is the role of tropomyosin in skeletal muscles?

Tropomyosin serves as a contraction inhibitor by blocking the myosin binding sites on the actin molecules.

A contraction in which the muscle does not shorten but its tension increases is called isometric contraction.

True

A resting potential is caused by a difference in the concentration of certain ions inside and outside the cell.

True

An increase in the calcium ion level in the sarcoplasm starts the sliding of the thin filaments. When the level of calcium ions declines, sliding stops.

True

Muscle cells store more creatine phosphate than ATP resulting in the muscle having a reserve source of energy.

True

One of the important functions of skeletal muscle contraction is production of heat.

True

The effect of a neurotransmitter on the muscle cell membrane is to modify its ion permeability properties temporarily.

True

What is the relationship between the number of motor neurons recruited and the number of skeletal muscle fibers innervated?

Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron. Yes! There are many more skeletal muscle fibers than there are motor neurons. The ratio of neurons to fibers varies from approximately one to ten to approximately one to thousands.

The sliding filament model of contraction involves ________.

actin and myosin sliding past each other and partially overlapping

A muscle cell runs out of ATP. Even though these are cyclic reactions, what step of the cross bridge cycle given is most directly inhibited or terminated?

cross bridge detachment When ATP is added to the myosin, myosin detaches from actin.

The connective tissue sheaths of skeletal muscle, in order from internal to external, are the ________.

endomysium, perimysium, and epimysium These are the three connective tissue sheaths in order (from internal to external). Endomysium surrounds individual muscle fibers; perimysium surrounds fascicles; and epimysium surrounds the entire muscle organ.

What is the functional role of the T tubules?

enhance cellular communication during muscle contraction

Which muscle characteristic describes the ability of muscle to respond to a stimulus?

excitability Excitability, also termed responsiveness, is the ability to receive and respond to a stimulus, For example, skeletal muscle contracts in response to receiving chemical stimulation by a neurotransmitter called acetylcholine (ACh).

Action potentials travel the length of the axons of motor neurons to the axon terminals. These motor neurons __________.

extend from the brain or spinal cord to the sarcolemma of a skeletal muscle fiber Yes! The cell bodies of motor neurons to muscles in the head and neck are located in the brain. The cell bodies of motor neurons to the rest of our muscles are located in the spinal cord.

Myasthenia gravis is sometimes treated medically by a treatment that involves ________.

inhibiting the action of acetylcholinesterase This enzyme breaks down acetylcholine into acetic acid and choline.

During vigorous exercise, there may be insufficient oxygen available to completely break down pyruvic acid for energy. As a result, the pyruvic acid is converted to ________.

lactic acid

Cross bridge formation between myosin heads and actin molecules is caused by the elevation of calcium ion concentration in the cytosol. During rigor mortis, this elevation of calcium ion concentration in the cytosol is permanent because ________.

mitochondria stop producing ATP molecules required by the sarcoplasmic reticulum's calcium ion pumps A pump is considered an active transport process. Active transport moves molecules against their concentration gradient, which requires energy in the form of ATP.

The oxygen-binding protein found in muscle cells is ________.

myoglobin

Thick myofilaments are made of ________.

myosin Myosin, a contractile protein, is the principal component of the thick myofilaments.

Rigor mortis occurs because ________.

no ATP is available to release attached actin and myosin molecules

Muscle tissue does NOT ________.

produce blood cells Producing blood cells is a function of red bone marrow, not a function of muscle tissue.

Slow oxidative muscle fibers are best suited for ________.

running a marathon Slow oxidative muscle fibers are best suited for endurance activities, such as long distance running, cycling, or rowing.

Muscle tissue has all of the following properties except ________.

secretion

Which type of muscle CANNOT contract without being stimulated by the nervous system?

skeletal In order to contract, skeletal muscle fibers must be stimulated by the nervous system. The site of muscle stimulation, where the nerve fiber communicates with the muscle fiber, is called the neuromuscular junction.

Which of the following is CORRECTLY paired?

skeletal muscle: voluntary control

Which muscle fiber type is best suited for endurance activities, such as long-distance jogging?

slow oxidative fibers Slow oxidative fibers are best suited for endurance (long duration) activities because they produce ATP aerobically, and are thus, fatigue-resistant. Fast glycolytic fibers, and to a lesser extent fast oxidative fibers, rely more on anaerobic glycolysis, and thus produce more lactic acid, which is a major cause of fatigue.

Acetylcholine binds to its receptor in the sarcolemma and triggers __________.

the opening of ligand-gated cation channels Yes! These channels permit sodium ions to diffuse inward and potassium ions to diffuse outward.

Smooth muscle is characterized by all of the following except ________.

there are more thick filaments than thin filaments

What is name given to the regularly spaced infoldings of the sarcolemma?

transverse or T tubules Yes! T tubules penetrate a skeletal muscle fiber and provide a pathway for excitation into the interior.

Which protein inhibits interaction between actin and myosin to prevent skeletal muscle contraction; and which ions remove the inhibition?

tropomyosin; calcium ions Tropomyosin, a regulatory protein, blocks the myosin binding sites on the actin molecules, preventing myosin heads from binding to actin to form cross bridges. When calcium ions bind to the TnC subunit of a troponin complex, troponin changes shape and moves tropomyosin away from its inhibitory position. As a result, the energized myosin heads (of thick myofilaments) can bind to the actin molecules (of thin myofilaments) and begin the cross bridge cycle, which shortens (contracts) the muscle fiber.

The type of muscle found in the walls of most hollow organs is ________.

unitary smooth muscle Smooth muscle is generally classified as being either unitary smooth muscle or multi unit smooth muscle. The type of muscle found in the walls of most hollow organs is unitary smooth muscle.