A&P CH 9 Muscles

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The plasma membrane surface of a muscle fiber


Contraction in which the muscle does not shorten, "the load is too heavy" but its internal tension increases

Isometric contraction

The nonfibrillar cytoplasm of a muscle fiber


Movable attachment of a muscle


Sheath of fibrous connective tissue surrounding a muscle


Period of time between stimulation and the onset of muscle contraction

Latent period

Bundle of nerve or muscle fibers bound together by connective tissue


Extension of the muscle cell plasma membrane "sarcolemma" that protrudes deeply into the muscle cell

T tubule ‘transverse tubule’

The length of time a muscle can continue to contract using aerobic pathways

Aerobic endurance

Fibrous or membranous sheet connecting a muscle and the part it moves




Cord of dense fibrous tissue attaching muscle to bone


Reduction in size or wasting away of an organ or cell resulting from disease or lack of use


Accumulation of effects, especially those of muscular, sensory, or mental stimuli


Spindle-shaped cells with one centrally located nucleus and no externally visible striations (bands). Found mainly in the walls of hollow organs

Smooth muscle

Specialized muscle of the heart

Cardiac muscle

Abnormal protrusion of an organ or a body part through the containing wall of its cavity


Attachment of a muscle that remains relatively fixed during muscular contraction


Not requiring oxygen


Thin connective tissue surrounding each muscle cell


One of the principal contractile proteins found in muscle


Energy-yielding conversion of glucose to lactic acid in various tissues, notably muscle, when sufficient oxygen is not available

Anaerobic glycolysis

Filament that constitutes myofibrils. Of two types: actin and myosin


A contractile protein of muscle


The force exerted by a contracting muscle on some object

Muscle tension

Glycolysis: Breakdown of glucose to pyruvic acid-an anaerobic process


Enzyme that catalyzes the transfer of phosphate from creatine phosphate to ADP, forming creatine and ATP; important in muscle contraction

Creatine kinase

The response of a muscle to a single brief threshold stimulus

Muscle twitch

Layers of fibrous tissue covering and separating muscle


Loss of a state of polarity; loss or reduction of negative membrane potential


Progressive, wavelike contractions that move foodstuffs through the alimentary tube organs. or that move other substances through other hollow body organs


Enzyme present at the neuromuscular junction and synapses that degrades acetylcholine and terminates its action

Acetylcholinesterase (AChE

Connective tissue that bundles muscle fibers into fascicles


A graphic recording of mechanical contractile activity produced by an apparatus that measures muscle contraction


The smallest contractile unit of muscle; extends from one Z disc to the next


Chemical transmitter substance released by some nerve endings

Acetylcholine (ACh

Respiration in which oxygen is consumed and glucose is broken down entirely; water, carbon dioxide, and large amounts of ATP are the final products

Aerobic respiration

Movement of the membrane potential to the initial resting "polarized" state


Sequence of events by which transmission of an action potential along the sarcolemma leads to the sliding of myofilaments

Excitation-contraction (E-C) coupling

Muscle cell’s ability to move by shortening


Embryonic mesoderm cells from which all muscle fibers develop


Type of smooth muscle; its cells contract as a unit and rhythmically, are electrically coupled by gap junctions, and often exhibit spontaneous action potentials. Also called unitary smooth muscle

Visceral muscle

A group of inherited muscle-destroying diseases

Muscular dystrophy

Specialized endoplasmic reticulum of muscle cells that stores calcium

Sarcoplasmic reticulum (SR

Low levels of contractile activity in relaxed muscle; keeps the muscle healthy and ready to act

Muscle tone

Rodlike bundle of contractile filaments "myofilaments" found in muscle fibers (cells


Product of anaerobic metabolism, especially in muscle

Lactic acid

The point at which muscle metabolism converts to anaerobic glycolysis

Anaerobic threshold

Variations in the degree of muscle contraction by changing either the frequency or strength of the stimulus

Graded muscle responses

Oxygen-binding pigment in muscle


A motor neuron and all the muscle cells it stimulates

Motor unit

To shorten or develop tension, an ability highly developed in muscle cells


Region where a motor neuron comes into close contact with a skeletal muscle cell

Neuromuscular junction

Contraction in which muscle tension remains constant at a given load, and the muscle shortens

Isotonic contraction

Weakest stimulus capable of producing a response in an irritable tissue

Threshold stimulus

Muscle composed of cylindrical multinucleate cells with obvious striations; the muscle(s) attached to the body’s skeleton; voluntary muscle

Skeletal muscle

Compound that serves as an alternative energy source for muscle tissue

Creatine phosphate (CP

High-intensity exercise in which the muscles are pitted against high resistance or immovable forces and, as a result, muscle cells increase in size

Resistance exercise

#1) A smooth, sustained muscle contraction resulting from high-frequency stimulation; #2) an infectious disease caused by an anaerobic bacterium


Knoblike swellings of certain autonomic axons containing mitochondria and synaptic vesicles


Ability to respond to stimuli

Excitability "responsiveness"

A large transient depolarization event, including polarity reversal, that is conducted along the membrane of a muscle cell or a nerve fiber

Action potential

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


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).

Muscle tissue does

produce movement ….maintain posture ….generate heat ….stabilize joints

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

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

In a bedridden patient recovering from a badly fractured femur, disuse atrophy in the thigh muscles is caused by

decreased synthesis of muscle proteins and/or increased breakdown of muscle proteins. Muscle tissue grows and heals in response to stress. Without the stress of exercise and normal daily activities, muscle tissue degenerates

skeletal muscle cell is commonly termed a

muscle fiber

The dense connective tissue of the epimysium is continuous with

the tendon that connects the muscle to a bone

A fascicle

is an organized group of muscle fibers bounded by a perimysium

The striated appearance of muscle fibers is due to

the arrangement of myofilaments within the myofibrils

The smallest contractile unit within skeletal muscle is

a sarcomere, the smallest contractile unit of a muscle fiber. Z line to Zline

Actin and myosin are components of thin and thick filaments, respectively.

The A band includes the area of overlap between thin and thick filaments.The A band is occupied by the entire length of thick myofilaments

The I band

is found between A bands around the junction between two successive sarcomeres. Essentially, the light I band would span from point 6 in one sarcomere to point 2 in the next sarcomere.

Neurotransmitters stored in synaptic vesicles are released into the synaptic cleft by


Nerve impulses arriving at the axonal terminus trigger the opening of Ca2+ channels,

which allows for the diffusion of Ca2+ into the terminus. This in turn leads directly to the release of neurotransmitter by exocytosis

the neurotransmitter used exclusively at the neuromuscular junctions of skeletal muscle


Binding of acetylcholine to its receptor opens

chemically ligand gated ion channels that allow Na+ and K+ to diffuse across the sarcolemma.

wave of depolarization

Excitation of the muscle fiber involves the opening of sodium channels in the sarcolemma. Diffusion of sodium into the muscle fiber causes the inside of the cell to become more positively charged or depolarized.

Myosin tail groups form the extended regions of the thick myofilament.

Globular regions of the myosin protein also form the head groups

Cross bridges between thin and thick myofilaments is formed by

the specific interaction between actin and myosin head groups.

protein functions as a motor protein

The myosin head groups hydrolyze ATP to power molecular movement along the actin subunits of the thin myofilaments

Binding of calcium by troponin

removes the blocking action of tropomyosin along the thin myofilament. This allows myosin to bind to actin and form the cross bridge

Contraction of skeletal muscle is controlled by

somatic motor neurons

A motor unit consists of

a motor neuron and all the muscle fibers it innervates

If both of the neurons were activated, more muscle fibers would contract than if either neuron alone were active. This mechanism for control of the force of muscle contraction is known as

Recruitment; refers to the increased force generated by the activation of increasing numbers of motor units

Myosin heads catalyze the hydrolysis of ATP to power the sliding of myofilaments. This event would be at a maximum during

the period of contraction

During the period of relaxation

Calcium is transported by active transport into the SR. Decreased Calcium concentrations in the sarcoplasm leads to detachment of cross bridges and, consequently, decreased contractile force

What result would be expected if an additional stimulus, equal in intensity to the first, were to be applied to the muscle at the 60 millisecond (ms) time point?

The muscle would increase in tension to a level greater than that measured at the beginning of the period of relaxation.The second, more forceful contraction that occurs before the muscle has completely relaxed is an example of wave summation

The myofibrils contains

the contractile elements of skeletal muscles, the sarcomeres. The sarcomeres contain even smaller rod-like structures called myofilaments

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 can bind to the actin molecules and begin the cross bridge cycle, which shortens or contracts the muscle fiber

Excitation-contraction coupling includes

propagation of an action potential along the sarcolemma and down T tubules … release of calcium ions from the terminal cisterns ….binding of calcium ions to troponin, which removes the blocking action of tropomyosin

What is a cross bridge cycle?

is the series of events during which energized, ATP-activated myosin heads bind to actin molecules and pull them toward the center of the sarcomere, called a power stroke. The myosin heads must bind to ATP in order to detach from actin and re-energize for the next cycle or the next power stroke

Which pathway for regenerating ATP provides the majority of the energy used for muscle activity during 30 minutes of light to moderate exercise?

Aerobic respiration occurs in the mitochondria, requires oxygen, and involves a sequence of chemical reactions in which bonds of fuel molecules are broken and the energy released is used to make ATP.

Aerobic respiration provides

a high yield of ATP for hours of low-to-moderate intensity muscle activity, whereas the other energy production processes provide energy for short duration, seconds to minutes intense muscle activity.

Which of the following factors influence the velocity and duration of muscle contraction?

load placed on the muscle….Speed or velocity of shortening is a function of load and muscle fiber type. Contraction is fastest when the load on the muscle is zero; a greater load results in a slower contraction and a shorter duration of contraction

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

Slow oxidative fibers are best suited for endurance or 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.

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

Muscle cells, just like other cells, use ATP to do work

Creatine phosphate is used by muscle cells to directly phosphorylate i.e., donate a high-energy phosphate group, to ADP to resynthesize ATP

Skeletal muscle cells are multinucleated.

Skeletal muscle cells have T tubules. Skeletal muscle cells contain myoglobin. Skeletal muscle cells have glycosomes.

The sarcomere is the smallest contractile unit of a muscle fiber,

and the smallest functional unit of muscle. A sarcomere is the region of a myofibril between two successive Z discs; it primarily consists of thin and thick myofilaments

The major role of the sarcoplasmic reticulum is to regulate

intra-cellular Calcium levels. The sarcoplasmic reticulum stores ionic calcium when the muscle is relaxed and releases it when the muscle fiber is stimulated to contract.

During muscle contraction, the distance between Z discs decreases

as the thin myo-filaments slide across thick myo-filaments, toward the M line in the center of each sarcomere. As the sarcomeres shorten, the myo-fibrils and, thus the myo-fibers shorten or contract.

The sliding filament model of contraction states

that during contraction, the thin myo-filaments slide past the thick myo-filaments so that actin and myosin myo-filaments overlap to a greater degree. In a relaxed muscle fiber, the thick and thin myo-filaments overlap only at the ends of the A band

The first step toward generating a skeletal muscle contraction is

nervous stimulation of the muscle fiber in order to generate an action potential. The site of muscle stimulation, where the nerve ending communicates with the muscle fiber, is called the neuromuscular junction.

During isotonic, eccentric contractions muscle lengthens as is generates tension, but not enough force to overcome the load

We use eccentric contractions to lower objects such as lowering a book from the shelf, or lowering the barbell during a bench press.

A motor unit’s response to a single, brief threshold stimulus a single action potential is a

muscle twitch

Under anaerobic conditions, oxygen deficient conditions

most of the pyruvic acid produced during glycolysis or breakdown of glucose, will be converted to lactic acid rather than enter the mitochondria to participate in aerobic respiration.

The force of a muscle contraction is Not

affected by the amount of ATP stored in the muscle cells. Instead of relying on storage of ATP, muscle cells use ATP regenerating pathways, such as glycolysis, to meet the ATP demands of muscle contraction

Slow oxidative muscle fibers are best suited for

endurance activities, such as long distance running, cycling, or rowing

skeletal muscle fibers contain sarcomeres and are thus striated

smooth muscle fibers do not contain sarcomeres and are thus nonstriated.

An isometric contraction generates tension or force, but not enough to move the load

Overall muscle length does not change during iso means same -metric is length, contractions.

isotonic contractions

result in movement of the load.

Stretching of smooth muscle provokes contraction; however, the increased tension persists only briefly, and soon the muscle adapts to its new length and relaxes, while still retaining the ability to contract on demand.

The stress-relaxation response of smooth muscle allows a hollow organ to fill or expand slowly to accommodate a greater volume without promoting strong contractions that would expel its contents.

Binding of calcium to calmodulin is a step in excitation-contraction coupling of smooth muscle cells

The calcium calmodulin system of contraction regulation is found only in smooth muscle. Excitation-contraction coupling in skeletal muscle and cardiac muscle involves the binding of calcium to troponin not calmodulin.

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

Cardiac muscle generates most of its ATP via

aerobic pathways or cellular respiration. The anaerobic pathways, namely glycolysis, produce lactic acid, a major cause of fatigue. Cardiac muscle cannot afford to fatigue, it must continue to contract with no more than a fraction of a second of rest between contractions for your entire life.

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?

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 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 via exocytosis

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

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.

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…When synaptic vesicles fuse to the plasma membrane, acetylcholine is released via exocytosis

Acetylcholine binds to its receptor in the sarcolemma and triggers

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

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..The resting membrane potential of all cells is negative, inside compared to outside. Given the direction of the chemical and electrical gradients, more sodium ions diffuse inward than potassium ions diffuse outward.

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…These action potentials set off a series of events that lead to a contraction

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

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 reuptake of calcium ions.

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..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.

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

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

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.

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. 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.

a muscle stimulated at high-frequency with some short relaxation time between, such that sustained but quivering contractions occurs

unfused or incomplete tetanus

a muscle that is stimulated so frequently that the relaxation phase is completely eliminated and smooth, sustained contraction results

fused or complete tetanus

ability to be stretched or extended


ability of a muscle to resume its resting length after being stretched


contraction of muscle during which the muscle changes in length and the tension remains constant through most of the contractile period

isotonic contraction

contraction of muscle during which the tension continues to increase but the muscle neither shortens nor lengthens

isometric contraction

contraction of the muscle in which the muscle shortens and does work

concentric contraction

contraction of muscle in which the muscle contracts as it lengthens

eccentric contraction

involuntary; intrinsic system regulation, hormones, and autonomic nervous system controls

cardiac muscle

involuntary, autonomic nerves, hormones, local chemicals

smooth muscle

sustained spasm, or tetanic contraction


inflammation of a muscle, its connective tissue coverings and tendons, and capsules of nearby joints


excessive stretching and possible tearing of a muscle caused by muscle overuse or abuse


the time between the stimulus or the electrical event and the mechanical event of contraction

latent period

the very brief time after one stimulus during which the muscle is unresponsive to a second stimulus. the time when fiber cannot be stimulated until repolarization is complete

refractory period

propagation of an electrical current along the sarcolemma

action potential

the initial polarized state of the membrane

resting potential

restoration of membrane potential to resting potential


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