Biology

Myosin heads directly use __________ to transition to their __________ conformation, which enables them ready to bind to actin.

ATP; energized Recall that the hydrolysis of ATP causes a conformational change in myosin heads, which energizes them in preparation for contraction. Refer to concept overview 10 module 3 step 3 for an interactive review of this concept.

What is the name of the neurotransmitter used at a skeletal neuromuscular junction?

Acetylcholine Skeletal muscle fibers have specific receptors for a neurotransmitter. When the neurotransmitter binds to the receptor, a series of events occur that result in skeletal muscle contraction.

Troponin and tropomyosin are associated with what other protein within the muscle fibers?

Actin Contractile proteins are arranged into the thin and thick filaments within the myofibrils.

Describe how action potentials travel from the sarcolemma into the interior of the muscle fibers, resulting in the release of calcium ions into the sarcoplasm.

Action potentials travel along T tubles. A mechanism is needed that links the stimulation of a muscle fiber with the release of calcium, which in turn leads to shortening of sarcomeres and, eventually, muscle contraction.

Which disorder results in degeneration of motor neurons and a resulting lack of muscular stimulation?

Amyotrophic lateral sclerosis There is no cure for this condition. Most treatments are designed to relieve symptoms and improve quality of life.

Proper functioning of skeletal muscle requires not just normal functioning of the muscle cells, but also of the components of the nervous system that stimulate skeletal muscles and the neuromuscular junctions. Diminished functioning of skeletal muscle can result from either neuromuscular diseases or neurotoxins. Neuromuscular diseases include a variety of disorders that affect either the nervous system or the neuromuscular junctions. Neurotoxins are toxins that disrupt nervous system function, some of which affect muscle stimulation. The activities that follow will focus on examples of neuromuscular diseases and neurotoxins that alter the functioning of skeletal muscle cells.

Amyotrophic lateral sclerosis (ALS) is a neurological disease that involves the degeneration of the motor neurons that serve to stimulate skeletal muscle. The cause of this disease is unknown, but may be associated with overproduction of a neurotransmitter (glutamate) in the central nervous system. Another possibility is that it is an autoimmune disorder in which the body's immune system attacks its own cells, resulting in neuron damage.

Muscles that flex the wrist are found on the __________ side of the forearm while those that extend the wrist are found on the __________ side of the forearm.

Anterior; Posterior Flexion involves decreasing the angle of a joint. Therefore, muscles that act as flexors would be positioned so that their contraction decreases the joint angle. The opposite applies to extensors.

Question 4: What term describes the change in muscle structure that results from lack of innervation, use, and/or stimulation?

Atrophy Question 1: The term "motor" refers to "motion", referring to the role of motor neurons in muscle function. Question 2: Stimulation by a motor neuron is necessary for skeletal muscle contraction to occur. Question 3: Use of a muscle causes changes to muscle structure. Lack of innervation to a muscle reverses these changes. Question 4: Lack of stimulation or lack of use causes a decrease in muscle size.

Which condition is caused by a neurotoxin that damages neurons, thus reducing the release of acetylcholine?

Botulism This disorder is treated with an antitoxin and with supportive care until muscle function returns.

In what types of muscle are the myofilaments arranged into sarcomeres?

Cardiac muscle and skeletal muscle All muscle types rely on the sliding filament model of muscle contraction. (Shorten/strengthen; strecth/weaken during process of sliding) However, muscle types differ in the arrangement of the myofilaments.

In what type of muscle do muscle impulses pass freely from cell to cell, resulting in contraction of the entire muscle, not just individual fibers?

Cardiac muscle and visceral smooth muscle In some muscle types, impulses freely pass from cell to cell via gap junctions, allowing coordinated contractions.

Which of the following statements describes myofibrils?

Comprised of thick and thin filaments The myofibrils occupy most of the cytoplasm within the muscle fibers.

Which equation best describes how the muscle cell can replenish ATP when ATP levels are low?

Creatine phosphate + ADP → creatine + ATP Creatine phosphate includes a high-energy phosphate bond that the muscle cell can utilize to replenish ATP stores quickly.

Place the following excitation-contraction coupling events in the order that they occur.

End plate potential reaches threshold Action potential sent throughout the sarcolemma Voltage-sensitive DPH receptors are activated Conformational change opens RyR1 channels Release of calcium into sarcoplasm

Which muscle group is composed of mainly slow-twitch (type I) fibers?

Erector spinae muscles of the back Slow-twitch fibers are fatigue-resistant due to their highly oxidative nature. Muscles responsible for posture are more likely to be composed mostly of slow-twitch fibers.

Which muscle fiber type is most fatigable?

Fast-twitch (Type IIb) Muscle fibers that have substantial oxidative capacity are considered fatique-resistant.

Which fiber type is most prone to muscle fatigue?

Fast-twitch fibers Muscle fibers that rely on glycolysis for ATP production are more readily fatigable than muscle fibers that utilize oxidative metabolism (aerobic respiration).

Which class of lever has the fulcrum in the center with resistance and the force being on opposite ends?

First-class lever There are three classes of levers that differ in the arrangement of resistance, fulcrum, and force.

What is the normal response of a muscle cell to the change in membrane permeability caused by binding of acetylcholine to its receptors?

Generation of muscle impulse The change in permeability in the muscle cell membrane will eventually lead to muscle cell contraction.

Which of the following describes smooth muscle contraction but not skeletal muscle contraction?

Hormones can inhibit muscle contraction.

The triad in a skeletal muscle fiber is composed of a centrally located T-tubule flanked on both sides by the terminal cisternae of the sarcoplasmic reticulum. The spatial relationship between the DHP receptors and RyR1 embedded within at the T-tubules and SR membranes, respectively, has been the focus of scientific inquiry for several decades. It has been determined that voltage-induced conformational changes are responsible for sequentially activating these two receptors. The sequence of events involved in excitation-contraction coupling is somewhat analogous to falling dominoes. In this scenario, the action potential propagating along the sarcolemma initiates the sequence of events in the same way the movement of your hand does. Just as your hand pushes over the first domino, the voltage change across the T-tubule membrane causes a conformational change in the DHP receptor. The first falling domino knocking over the subsequent dominoes represents the conformational change of the DHP receptor initiating a subsequent allosteric change in RyR1. When RyR1 is activated, calcium is released from the sarcoplasmic reticulum, leading to muscle contraction.

Hundreds of mutations have been identified in RyR1 that contribute to multiple muscular diseases. Assume that a new mutation was discovered that causes this receptor to continually remain open. Based on what you know about the function of this receptor, how would you expect this new mutation to impact contraction? This mutation will increase levels of calcium in the sarcoplasm, which will induce continued muscle contraction. Recall that when activated, RyR1 allows an efflux of calcium from the sarcoplasmic reticulum into the sarcoplasm. Reference concept overview 10 module 2 step 4 for an interactive review of this concept.

The condition called myasthenia gravis is an autoimmune disease, in which the patient's immune system is producing autoantibodies against his/her own body. With myasthenia gravis, the antibodies destroy or block the acetylcholine receptors at the neuromuscular junction.

In normal skeletal muscle tissue, what results from the binding of acetylcholine to its receptors at the neuromuscular junction? Increased permeability of the membrane to sodium ions Binding of acetylcholine to its receptors triggers an electrical impulse at the muscle cell surface.

Describe the role of acetylcholinesterase in stimulation of a muscle fiber.

It is the enzyme that breaks down acetylcholine in the synaptic cleft, ending stimulation of the muscle fiber. The activity of acetylcholinesterase leads to the relaxation of a muscle fiber.

What is the result of the lack of ACh in the synaptic cleft due to the botulinum toxin?

Lack of muscular contraction (called flaccid paralysis) Botulism causes paralysis (loss of muscle function) because of the lack of the neurotransmitter acetylcholine.

Question 1: What is the role of motor neurons in skeletal muscle function?

Motor neurons release neurotransmitters that stimulate muscle contraction

Muscles with attachment points on the pelvic girdle and on the femur will cause what type of movement?

Movement of the thigh Muscles typically cross joints. As they contract, they pull closer together the bones on either side of the joint.

Which type of muscle is most likely found in the walls of large blood vessels?

Multiunit smooth muscle The body needs to be able to contact portions of the blood vessels to regulate blood flow, rather than the entire blood vessel.

Place in order, from largest to smallest, the components of a skeletal muscle.

Muscle Fascicle Muscle fiber Myofibril Filaments

What is the term for the inability of a muscle to contract after rigorous exercise?

Muscle fatigue After a prolonged period of vigorous exercise, muscles are no longer able to contract due to a variety of factors including electrolyte imbalances and decreased ATP availability.

Which condition is an autoimmune disorder in which acetylcholine receptors are blocked or destroyed?

Myasthenia gravis This disorder is helped by treating the patient with antiacetylcholinesterase or by taking measures to reduce the amount of autoantibodies in the blood.

What generates the force that shortens the sarcomeres to bring about muscle contraction?

Myosin cross-bridges pulling on the actin filaments

During the contraction cycle, what is the result of ATP binding to myosin?

Myosin heads release from the binding site on actin.

A specific sequence of events follow the arrival of a nerve impulse at a neuromuscular junction. The result is the production of an electrical impulse along the sarcolemma. This, in turn, will lead to muscle contraction.

Nerve impulse arrives at distal end of motor neuron axon. Some vessels release acetylcholine Acetylcholine diffuses across synaptic cleft Acetylcholine binds to receptors on motor end plate Membrane permeability to sodium is increased (opening of sodium channels) Sodium enters muscle cell Triggers an electrical impulse along sarcolemma

Name the type of synapse that occurs between a motor neuron and a muscle fiber.

Neuromuscular junction Synapses between motor neurons and muscle fibers are the site of neurotransmitter release and stimulation of the muscle.

With myasthenia gravis, the acetylcholine receptors are blocked or destroyed by autoantibodies. How will this affect muscle cell function?

No muscle impulse will be generated, so the muscle cell will not contract If neurotransmitter receptors are blocked or destroyed, the neurotransmitter will be unable to bind and, therefore, unable to cause a normal response.

At what level of organization is a skeletal muscle such as the biceps brachii?

Organ Organs are composed of different types of tissues. Tissues are composed of similar cells working together.

Place the following muscles in order based on the proximity of their insertions to the axial skeleton. Begin with the most proximal insertion (on the coracoid process).

Pectoralis minor -inserts on coracoid process Supraspinatus -inserts on greater tubercle Psiform and hamteectoralis major -inserts on lateral ridge of intertuburcular groove Deltoid -inserts on deltoid tuberosity Brachialis -inserts on coronoid process Biceps Brachii -inserts on radial tuberosity Pronator teres -inserts on proximal shaft of radius Brachioradialis -inserts on styloid process of radius Flexor carpi ulnaris- inserts on pisiform and hamate Flexor digitorum profundus -inserts on distal phalanges II-V

Name the connective tissue layer that separates a skeletal muscle into fascicles.

Perimysium Connective tissue is found dispersed throughout a muscle, separating the muscle fibers.

two motor units Correct, each consisting of one motor neuron Correctand all muscle fibers with which it synapses.

Recruitment is the process by which multiple motor units are stimulated, resulting in stronger muscle contractions.

Place the following muscles in order based on the proximity of their insertions to the axial skeleton. Begin with the most proximal insertion (on the pubic symphysis).

Rectus adominis -inserts on pubic symphysis Gluteus medius -inserts on greater trochanter lliopsoas -inserts on lesser trochanter Adductor longus -inserts on distal linea aspera Adductor magnus -inserts on supracondylar of femur Biceps femoris -inserts on fibular head soleus -inserts on calcaneus Fibularis longus - inserts on medial cuneiform Flexor digitorum -inserts on distal phalanges Insertions are usually located more distal in the appendicular skeleton. The specific location of the insertion allows one to understand the functional importance and actions of each muscle.

Name the functional units of contraction in a muscle fiber.

Sarcomeres Thick and thin filaments are precisely arranged within myofibrils, forming the units that contract in response to stimulation of the muscle cell.

Compare skeletal muscle to smooth muscle.

Smooth muscle takes longer to contract and can stay contracted longer than skeletal muscle. While skeletal muscle and smooth muscle share some of the same characteristics, their development of contraction differs.

The electrical impulse that moves over the surface of a muscle fiber and into the T tubules results from increased membrane permeability to what ion?

Sodium The electrical impulses result from the opening of ion channels following the binding of acetylcholine to receptors on the motor end plate.

When a muscle fiber is stimulated at a high enough frequency that it doesn't have time to relax, the forces of the individual twitches combine. What is this process called?

Summation The force of contraction of a muscle varies due to variation in the frequency and/or strength of stimulation of the muscle.

What is the name of the space between a motor neuron and a muscle fiber at the neuromuscular junction?

Synaptic cleft The terminal end of an axon does not come in direct contact with the surface of a muscle fiber. There is a small gap through which neurotransmitter molecules diffuse.

Name the cordlike structures that connect muscles to bone.

Tendons

Excitation-Contraction Coupling

The excitation of a muscle fiber occurs when there is sufficient depolarization at the motor end-plate to produce an action potential. This electrical signal propagates along the sarcolemma and into the muscle fiber by way of the transverse tubules (T-tubules). At the triad, the electrical signal gets converted to a mechanical signal when calcium is released from channels found at the terminal cisternae of the sarcoplasmic reticulum

The three muscles of the abdominal wall have fibers running in different directions. First, correctly place the muscle name in each sentence describing fiber direction. Then place the sentences in order, starting with the muscle that is most superficial and ending with the deepest layer.

The fibers of the external oblique run down and forward from the lower ribs to the pelvic girdle and linea alba. The fibers of the internal oblique run up and forward from the pelvic girdle to the lower ribs. The fibers of the transversus abdominis run horizontally.

Question 3: What effect does lack of motor neuron innervation have on the structure of a skeletal muscle?

The muscle becomes smaller in size due to lack of stimulation.

Question 2: What effect does lack of motor neuron innervation have on skeletal muscle function?

The muscle is unable to contract

What treatment would make the most sense for a patient with myasthenia gravis?

Use anticholinesterase inhibitor to increase the presence of ACh in synaptic cleft Any treatment that increases the stimulation of available acetylcholine receptors will improve symptoms for someone with myasthenia gravis.

Place in order the sources of ATP used as an exercise bout continues.

Use of ATP molecules present in the cytoplasm Use of creatine phosphate as a means to convert ADP to ATP Production of ATP via anaerobic metabolism Production of ATP via aerobic metabolism A supply of ATP must be maintained in order for muscular activity to continue. The muscle cells have a variety of ATP sources to draw from, and they are used in specific order.

The neurotoxin released by the bacterium Clostridium botulinum causes the paralytic disease called botulism. This neurotoxin damages axons and prevents the release of the acetylcholine (ACh) at the neuromuscular junction. Botulism can result from ingesting contaminated food (foodborne botulism) or via contamination of a wound by spores of the bacterium. Treatment involves an antitoxin that neutralizes the neurotoxin and supportive care for symptoms until axons regenerate. Medically, the botulism toxin (Botox) can be used to treat a variety of spasmoticconditions. In addition, it is used cosmetically to reduce the appearance of wrinkles by paralyzing facial muscles.

What is the role of acetylcholine (ACh) at the neuromuscular junction? After release by the motor neuron, it crosses the synaptic cleft and binds to receptors, stimulating the muscle cell. ACh is the neurotransmitter that stimulates muscle cells.

An action, such as flexing the knee, is caused by the contraction of a muscle acting as an __________ and is opposed by a muscle acting as an __________.

agonist; antagonist Muscles involved in an action are classified as agonists, antagonist, or synergists, depending on the effect of their contractions

If there was a high concentration of acetylcholine within the synaptic cleft, but little to no response at the motor end-plate, you would expect __________ to be the responsible neuromuscular-blocking agent.

curare Recall that curare competitively binds to and prevents the opening of acetylcholine receptors located in the membrane of the motor end-plate, but it does not effect acetylcholine release from the axon terminals of motor neurons.

Rigor mortis occurs after death because __________.

detachment of crossbridges does not occur due to the lack of ATP Recall that the myosin head binds ATP, which causes it to detach from actin. Myosin subsequently hydrolyzes the molecule of ATP, which reenergizes the myosin head so that it is ready for future contraction. Refer to concept overview 10 module 3 step 5 for interactive review of this concept.

Muscle contractions that generate less force than the resistance and result in lengthening of the muscle are called __________ contractions.

eccentric Isotonic contractions, such as eccentric and concentric contractions, have unchanging force ("iso"=same; "tonus"=activity/tone) but the muscle length changes.

The presence of calcium in the sarcoplasm is directly responsible for __________.

exposing the binding sites on actin Recall that the binding of calcium to troponin causes a conformational change in the troponin-tropomyosin complex associated with the actin myofilament. This exposes the myosin binding sites on actin. Refer to concept overview 10 module 3 step 4 for interactive review of this concept.

The following layers are associated with skeletal muscles. Place them in order from superficial to deep.

fascia -seperates adjacent muscles epimysium -closely surrounds a skeletal muscle perimysium -seperates the muscles into fascicles endomysium -closely associated with each muscle fiber sarcolemma -muscle fibers plasma membrane

Only __________-twitch fibers can be glycolytic, using glycolysis to produce ATP.

fast

The metabolic process that generates a net of two ATP molecules in the cytosol of a muscle fiber is called __________ .

glycolysis Cellular respiration begins with anaerobic reactions occurring in the cytoplasm. The products of this process will be converted to lactic acid in the absence of oxygen, or will enter the aerobic pathways if oxygen is available.

If it were possible to move the terminal cisternae further away from the T-tubules, it would __________.

interfere with the signal required for the release of calcium from the sarcoplasmic reticulum Recall that the DHP receptor and RyR1 are tightly coupled and that the transfer of the signal from the T-tubules to the SR depends upon the allosteric interaction between each of these receptors. Reference concept overview 10 module 2 step 4 for an interactive review of this concept.

The antidote that holds the most promise to counteract the effects of tubocurarine is one that __________

modifies acetylcholine receptors so that they bind acetylcholine stronger than tubocurarine Recall that tubocurarine competitively binds to acetylcholine receptors.

A __________ includes a motor neuron and all of the muscle fibers that it controls.

motor unit An impulse from one motor neuron stimulates contraction of all of the muscle fibers that it controls. If a stronger contraction is needed, more motor neurons can fire, stimulating more muscle fibers to contract.

Botulinum toxin causes muscle paralysis because __________.

no acetylcholine enters the synaptic cleft Recall that synaptic vesicles at the axon terminal must release acetylcholine in order to cause an end-plate potential. Botulinum toxin directly interferes with the release of acetylcholine, thereby preventing muscle contraction.

Acetylcholine causes an end-plate potential by triggering the __________.

opening of sodium channels Recall that an end-plate potential is a local depolarization due to the influx of sodium. Refer to concept overview 10 module 1 step 4 for interactive review of this concept.

Muscles located on the _______ of the thigh will extends and/or rotate the thigh at the hip. The muscles located on the _____of the thigh will flex the thigh at the hip.

posterior; anterior Actions of muscles can be deduced by looking at their origins and insertions.

Facial wrinkles, such as glabellar or "frown" lines, appear with aging as a result of natural, repetitive muscle contractions. Botox® helps alleviate the appearance of these wrinkles by __________.

relaxing the facial muscles that cause the wrinkles Recall that Botox® is a dilute form of the botulinum neurotoxin, which inhibits the release of acetylcholine from motor neurons, thereby preventing muscle contractions.

Compared to skeletal muscle, smooth muscle is __________ to contract and __________ to relax.

slower; slower Differences in speed and duration of contraction of skeletal vs. smooth muscle enables smooth muscle to stretch as organs fill while maintaining constant pressure in the interior of the organ.

Action potentials spreading along the sarcolemma of a muscle fiber result from the opening of __________ channels in the sarcolemma.

sodium Voltage-gated ion channels are critical to the propagation of action potentials in both neurons and muscle fibers.

Acetylcholine (ACh) is released from motor neurons and enters the __________.

synaptic cleft Recall that ACh is released from synaptic vesicles and subsequently diffuses across the synaptic cleft. Refer to concept overview 10 module 1 steps 3 and 4 for interactive review of this concept.

Thin filaments connect to and extend from either side of a __________. These thin filaments are composed largely of the myofilament __________.

z disc; actin Recall that Z discs form the boundary of each individual sarcomere, and that multiple, thin, actin filaments extend perpendicularly from each Z disc. Refer to concept overview 10 module 3 step 2 for interactive review of this concept.