Muscular System

What is the defining anatomical location and connection of skeletal muscle?

It attaches directly to the skeleton to facilitate bodily movement.

How are the individual cells of skeletal muscle structured microscopically?

They are long, cylindrical, striated, and contain multiple nuclei.

Which statement accurately describes the control mechanism of skeletal muscle?

It is under voluntary control and innervated by the somatic nervous system

What is the unique anatomical location and functional organization of cardiac muscle?

It is exclusively located in the heart and forms functional syncytia to coordinate contractions.

How are individual cardiac muscle cells structurally characterized under a microscope?

They are short, branching, mononucleated, and faintly striated.

Which statement accurately describes how cardiac muscle contractions are controlled and initiated?

Control is involuntary, and contractions are myogenic, originating within the muscle itself.

Where is smooth (visceral) muscle primarily located within the human body?

It is found in blood vessel walls and the gastrointestinal, respiratory, reproductive, and urinary tracts.

What are the microscopic structural features of smooth muscle cells?

They are small, oblong cells that contain only a single nucleus.

Which statement accurately describes the control and coordination of smooth muscle tissue?

It is involuntary, can be myogenic, and may form a functional syncytium.

Which of the following correctly lists the primary physiological functions of skeletal muscle?

Movement, posture maintenance, heat generation, joint stabilization, and blood/lymph circulation.

Which statement accurately captures the core physiological characteristics defining skeletal muscle?

They are excitable tissues that contract to pull, extend when stretched, and elastically recoil to their resting length.

What does the characteristic of excitability mean for skeletal muscle tissue?

It means the muscles possess the ability to be stimulated by both chemical and electrical means.

Which statement accurately describes the contractility of skeletal muscle?

Muscles respond to stimuli by shortening forcibly or generating tension, allowing them to actively pull but never push.

How do tonus and flaccidity differ in skeletal muscle tissue?

Tonus is a state of continuous partial muscle contraction, whereas flaccidity is a complete lack of muscle tone.

What is meant by the characteristic of muscle extensibility?

It is the capacity of muscles to be stretched safely beyond their normal resting length.

How is the characteristic of elasticity demonstrated in skeletal muscle?

Muscles show a natural tendency to recoil back to their original resting length after being stretched.

What core set of traits defines the overall functional characteristics of skeletal muscle?

Excitability, contractility (including tonus), extensibility, and elasticity.

What best defines a tendon?

Dense regular connective tissue cords that attach muscle to bone

How are aponeuroses characterized in skeletal muscle anatomy?

Broad, flat, tendinous attachments

In skeletal muscle anatomy, how is the "origin" defined?

The relatively fixed point of attachment

During contraction, what is the directional pull of a skeletal muscle action?

Pulls the insertion toward the origin

Which muscle group is primarily responsible for producing a specific, desired movement?

Agonist

What is the primary action of an antagonist muscle during movement?

Resisting an agonist or executing the opposing action

Which classification describes muscles that cooperate to perform the exact same action?

Synergists

What is the primary role of a fixator muscle in functional movements?

Immobilizing a bone or origin to maximize insertion tension

What is the specialized cell membrane that encloses the sarcoplasm of a muscle fiber?

Sarcolemma

What is the primary function of T-tubules within a skeletal muscle fiber?

Conducting electrical impulses deep into the fiber

Which membranous network of tubules is responsible for storing the $\text{Ca}^{2+}$ needed to initiate contraction?

Sarcoplasmic reticulum

What specific structural combination makes up a muscle triad?

One T-tubule and two terminal cisternae

Which specialized $\text{Ca}^{2+}$-binding protein is found inside the terminal cisternae?

Calsequestrin

What best describes the terminal cisternae within a muscle fiber?

Expanded chambers of the sarcoplasmic reticulum

What is the smallest functional unit of a skeletal muscle contraction?

Sarcomere

What is the direct physical result when sarcomeres shorten within a muscle fiber?

Muscle contraction

What causes the shortening of sarcomeres during contraction?

Actin and myosin myofilaments sliding past each other

What happens to the actual length of actin and myosin myofilaments during muscle contraction?

They do not shorten

During the sliding filament process, how do the Z lines behave?

They move closer together

How do the zones of overlap change during sarcomere contraction?

They get larger

Which specific structural regions get smaller as the sarcomere contracts?

H zones and I bands

What happens to the length of the A bands during muscle contraction?

They remain constant

What prevents myosin heads from attaching to actin filaments at rest?

Tropomyosin blocks active sites on actin

What is the specific role of troponin during a muscle's resting state?

Holds tropomyosin in position

What happens when $\text{Ca}^{2+}$ is released into the sarcoplasm?

Troponin shifts and tropomyosin uncovers active sites on actin

What is the ultimate result when $\text{Ca}^{2+}$ uncovers the active sites on actin?

Muscle contraction occurs

Which myofilament is classified as the "thin" filament in a skeletal muscle sarcomere?

Actin

Which myofilament is classified as the "thick" filament in a skeletal muscle sarcomere?

Myosin

During muscle contraction, what do myosin heads form with the exposed active sites on thin actin filaments?

Cross-bridges

What movement of the myosin head is referred to as the "power stroke"?

Pivoting

What happens as the myosin head pivots during the power stroke?

Myofilaments slide past each other

What is the specific function of ATP regarding myosin heads during contraction?

Resets myosin heads and allows the power stroke cycle to continue

Which division of the nervous system stimulates skeletal muscle cells via motor neurons?

Somatic (voluntary) division

Where do motor neuron axons extend from to stimulate skeletal muscles?

Central nervous system (CNS)

What is a neuromuscular junction?

A specialized intercellular connection between a motor neuron synaptic terminal and a muscle fiber

What do synaptic terminals (synaptic end bulbs) contain inside their vesicles?

Acetylcholine (ACh)

Where is the motor end plate located on a skeletal muscle fiber?

On the sarcolemma adjacent to the synaptic terminal

What fluid-filled space separates the synaptic terminal from the motor end plate and contains acetylcholinesterase?

Synaptic cleft

What event first arrives at the synaptic terminal to initiate neuromuscular stimulation?

An electrical impulse called an action potential (depolarization wave)

What process is directly triggered when the action potential reaches the synaptic terminal?

Exocytosis of ACh into the synaptic cleft

Once ACh binds to receptors on the motor end plate, what ion diffuses into the sarcoplasm?

Sodium

What is generated on the sarcolemma as a result of the sodium influx through chemically-gated ion channels?

An action potential

Along which pathway does the action potential travel to reach the interior of the muscle fiber?

Inward along the T-tubules

What triggers the sarcoplasmic reticulum to release{Ca}^{2+} into the sarcoplasm?

When the action potential reaches the sarcoplasmic reticulum

What is the function of acetylcholinesterase (AChE) in the neuromuscular junction?

Breaks down ACh so the junction returns to its initial state

What is the term for the electrochemical gradient established across the cell membrane of resting muscle cells and neurons?

Resting membrane potential

Which cellular mechanism is responsible for establishing the resting membrane potential?

Sodium-potassium pump

How is the chemical gradient distributed across the resting muscle cell membrane?

More Na+ outside the cell and more K+ inside the cell

What characterizes the electrical gradient of a resting muscle cell membrane?

More positive ions outside the cell and more negative proteins inside

What are action potentials defined as in excitable membranes?

Changes in the transmembrane potential that affect the entire membrane

What is produced when ACh binding at the motor end plate causes chemically-regulated Na+ channels to open?

A local depolarization

What defines a threshold stimulus during an action potential?

A local depolarization large enough to open nearby voltage-regulated Na+ channels

What happens when voltage-regulated Na+ channels open in response to reaching the threshold?

Further depolarization occurs as Na+ rushes into the cell

What type of chain reaction propagates the action potential down the entire length of the muscle fiber?

An all-or-none reaction opening adjacent voltage-regulated Na+ channels

What triggers K+ channels to open and K+ to rush out, repolarizing the cell?

The depolarization wave

What mechanism restores the original ion concentrations and resting membrane potential after an action potential?

Na+/K+ pump

What are the multiple branches given off by a motor neuron called?

Axonal (synaptic) terminals

How many muscle fibers does each individual synaptic terminal synapse with?

A single muscle fiber

What is a single motor neuron and ALL the muscle fibers it controls called?

A motor unit

What functional property characterizes the behavior of a motor unit?

All-or-none

What happens when a motor neuron fires?

All muscle fibers of the motor unit contract

How many cells per motor unit are found in fine control muscles like the eyes and fingers?

Less than 10 cells per motor unit

What is the cell count range per motor unit for average control muscles?

10 to 100 cells per unit

How many cells per motor unit are characteristic of gross control postural muscles?

Greater than 100 cells per unit

While individual motor unit function is all-or-none, how is whole muscle contraction classified?

It is not all-or-none

Which factor influences the strength or tension of a whole muscle contraction?

Number of motor units involved (recruitment)

What is the second factor that influences contraction strength?

Frequency of stimulation

What is the third factor influencing muscle contraction strength?

Length of the muscle and sarcomeres relative to resting length

What does recruitment, or multiple motor unit summation, involve?

Increasing the number of active motor units

What is the direct effect of motor unit recruitment on muscle contraction?

Increases strength (tension) of contraction

When is maximal strength, or peak tension, achieved in a muscle?

When ALL motor units are recruited simultaneously

What happens during asynchronous recruitment of motor units?

Different motor units cycle between contracting and relaxing

What is defined as a single contraction of a muscle fiber in response to a single stimulus?

A twitch

hat occurs during the latent period between stimulation and the onset of contraction?

Excitation-contraction coupling occurs

What happens during the contraction phase, which spans from the onset of contraction to peak tension?

Cross-bridge interactions occur within sarcomeres

What happens during the relaxation phase when muscle tension declines?

Cross-bridges detach and muscle fiber recoils passively

What is the term for the phenomenon where frequent stimulations of a muscle result in increased tension step-by-step?

Treppe

What occurs when the time intervals between stimulations are shorter than the overall twitch duration?

Wave summation, producing even greater tension

What is produced by a significant increase in stimulus frequency that eliminates the relaxation phase?

A sustained contraction called tetanus

When do muscles exert maximum tension relative to their resting length?

When muscle and sarcomere length is within about 10% of its optimal resting length

What determines whether muscle contractions are classified as isotonic or isometric?

Whether the muscle changes length or not

What occurs during an isotonic contraction where movement takes place?

Muscle changes length and moves a load

What are the two subcategories of isotonic contractions?

Concentric or eccentric