Concepts Of Skeletal Muscle Contractions - Massage and Bodywork Licensing Examination Mblex

What is recruitment (multiple motor unit summation) in skeletal muscle contraction?

Increasing force by activating additional motor units. By progressively engaging more units, the nervous system modulates overall muscle tension for graded force output.
What is tetanus in skeletal muscle physiology?

Sustained maximal contraction from very high-frequency stimulation. High-frequency inputs maintain elevated calcium levels, preventing relaxation and achieving fused, peak force production.
What is the immediate role of ATP binding to the myosin head during cross-bridge cycling?

Detaches myosin from actin (breaks the cross-bridge). ATP attachment induces a conformational change in myosin, releasing it from actin to reset for the next cycle after hydrolysis.
What neurotransmitter is released at the neuromuscular junction to start excitation?

Acetylcholine (ACh). It depolarizes the motor end plate by binding nicotinic receptors, initiating the action potential for muscle excitation.
Which protein blocks myosin-binding sites on actin in a resting muscle fiber?

Tropomyosin. In relaxed muscle, it wraps around actin filaments to sterically hinder myosin head attachment, preventing unintended contractions.
What ion binds to troponin to permit actin–myosin cross-bridge formation?

$Ca^{2+}$ . Its binding to troponin triggers the exposure of myosin-binding sites on actin, enabling the power stroke in excitation-contraction coupling.
Which band corresponds to the length of thick filaments and stays constant in contraction?

A band. It encompasses the full extent of myosin filaments, which maintain constant length regardless of sarcomere contraction state.
What organelle stores and releases $Ca^{2+}$ for skeletal muscle contraction?

Sarcoplasmic reticulum. This modified endoplasmic reticulum sequesters calcium ions and releases them via channels in response to T-tubule depolarization.
Which band contains only thick filaments and becomes narrower during contraction?

H zone. As the central region of the A band lacking actin overlap, it diminishes when thin filaments slide inward during sarcomere shortening.
What is the definition of an isometric contraction?

Tension changes while muscle length remains constant. It occurs when external resistance prevents shortening, allowing tension buildup without movement, as in static holds.
What is the primary mechanism that ends contraction by lowering cytosolic $Ca^{2+}$ ?

$Ca^{2+}$ reuptake into the sarcoplasmic reticulum by pumps (SERCA). Active transport via SERCA reduces cytoplasmic calcium, allowing troponin to revert and tropomyosin to inhibit cross-bridges.
What is the role of troponin in initiating skeletal muscle contraction?

Binds $Ca^{2+}$ and shifts tropomyosin to expose actin binding sites. Calcium-induced conformational change in troponin displaces tropomyosin, allowing myosin to bind actin and initiate cross-bridge cycling.
What is the name of the muscle fiber membrane that conducts the action potential?

Sarcolemma. As the specialized plasma membrane, it propagates electrical signals from the neuromuscular junction across the fiber surface.
What structure carries the action potential deep into the muscle fiber interior?

Transverse (T) tubules. These invaginations of the sarcolemma ensure rapid, uniform transmission of depolarization to trigger calcium release throughout the fiber.
Which band contains only thin filaments and shortens as the sarcomere shortens?

I band. Comprising solely actin filaments from Z line to A band edge, it narrows as Z lines are pulled closer together in contraction.
Which type of isotonic contraction occurs when the muscle shortens while producing force?

Concentric contraction. When internal force exceeds external resistance, the muscle shortens to perform work, such as in the upward phase of a bicep curl.
What is a motor unit in skeletal muscle physiology?

One motor neuron and all the muscle fibers it innervates. This organization enables precise control of muscle force through selective activation of neurons innervating varying numbers of fibers.
What is the definition of a skeletal muscle contraction in terms of tension and fiber length?

Development of tension in a muscle fiber, with or without length change. This definition captures the essence of contraction as force generation, applicable to both isometric (no length change) and isotonic (with length change) types.
What is the all-or-none principle as applied to a single muscle fiber?

A fiber contracts fully when threshold is reached, or not at all. Similar to neuronal action potentials, it ensures that suprathreshold stimuli elicit a complete twitch response in the fiber.
What is the Z line (Z disc) role within a sarcomere?

Anchors thin filaments; defines the boundary of each sarcomere. It serves as the attachment site for actin filaments and demarcates sarcomere ends, facilitating organized sliding during contraction.
Which type of isotonic contraction occurs when the muscle lengthens while producing force?

Eccentric contraction. Despite generating tension, the muscle yields to a greater load, elongating to control movement, like in controlled descents.
What is the name of the functional contractile unit of striated skeletal muscle?

Sarcomere. It is the smallest repeating segment capable of generating force through organized actin-myosin interactions in striated muscle.
What is temporal summation (wave summation) in skeletal muscle?

Increased tension from stimuli delivered before full relaxation. Repeated stimuli during the relaxation phase accumulate calcium, enhancing cross-bridge activity and thus tension.
What is the definition of an isotonic contraction?

Muscle length changes while tension remains relatively constant. Under constant load, the muscle adjusts length to move the load, enabling dynamic actions like lifting or lowering.

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