NCERT-Based Explanation
The Assertion is true as a logical inference. A motor unit comprises a motor neuron and the muscle fibres innervated by it. Since neurons arise from the ectoderm and skeletal muscle fibres arise from the mesoderm, a motor unit is derived from two different tissues originating from two different germinal layers.
The Reason is also true as a logical inference. A motor unit consists of neurons and muscle fibres. This composition explains why it is derived from tissues of two different germinal layers.

Pedagogical Value
This question encourages students to integrate concepts across different chapters instead of learning them in isolation. By identifying the components of a motor unit, students are expected to infer their tissue types and developmental origins. It promotes the habit of connecting structure, function, tissue organisation, and embryological origin, thereby developing integrative biological thinking rather than rote memorisation.

NCERT-Based Explanation
The Assertion is true as a logical inference. NCERT states that red muscle fibres contain a large amount of myoglobin, whereas white muscle fibres contain less myoglobin. This implies that myoglobin is present in both types of muscle fibres, although in different amounts.
The Reason is also true as a logical inference. Since myoglobin imparts a red colour, the presence of myoglobin gives muscle fibres a reddish appearance. Red muscle fibres appear more reddish because they contain more myoglobin, whereas white muscle fibres appear less reddish (paler) because they contain less myoglobin. Therefore, the Reason explains the Assertion.
This information is not stated directly in NCERT. It is inferred from the relative myoglobin content described in NCERT.

Pedagogical Value
This question encourages students to distinguish between the presence of a biological molecule and its relative abundance. It promotes the understanding that biological traits often exist as a continuum rather than as an all-or-none phenomenon. Students learn that white muscle fibres are not devoid of myoglobin; they simply contain less of it than red muscle fibres.

NCERT-Based Explanation
The Assertion is false. Direct NCERT: The sequence is the reverse. The action potential spreads along the sarcolemma and through the T-tubules, which subsequently triggers the release of calcium ions from the sarcoplasmic reticulum. Thus, calcium release is a consequence of the spreading of the action potential, not its cause.
The Reason is true. Direct NCERT: An increase in the calcium ion level in the sarcoplasm initiates the interaction between actin and myosin, leading to muscle contraction.

Pedagogical Value
This question addresses the common misconception regarding the sequence of events in excitation–contraction coupling. It reinforces that action potential propagation precedes calcium ion release, whereas the increase in sarcoplasmic calcium concentration is the immediate trigger for muscle contraction.

NCERT-Based Explanation
The Assertion is true. Direct NCERT: During muscle contraction, the sarcomere shortens as the Z lines are drawn closer together due to the sliding of thin filaments over thick filaments.
The Reason is false. The return of the Z lines to their original position occurs during muscle relaxation, not during muscle contraction. Therefore, it does not explain the Assertion.

Pedagogical Value
This question addresses the common misconception of confusing the events of muscle contraction and muscle relaxation. Students often interchange the movement of the Z lines during these two phases. The question reinforces that shortening of the sarcomere and approximation of the Z lines are characteristic of contraction, whereas the return of the Z lines to their original position is associated with relaxation.

NCERT-Based Explanation

The Assertion is true as a logical inference. A motor unit consists of a motor neuron and skeletal muscle fibres. Both are capable of conducting electrical excitation in their respective forms.
The Reason is also true as a logical inference. Both neurons and skeletal muscle fibres are excitable cells, i.e., they can respond to an appropriate stimulus by generating an electrical signal. This property enables both constituents of a motor unit to transmit electrical potentials. Therefore, the Reason correctly explains the Assertion.

Pedagogical Value
This question encourages students to identify the common physiological property shared by neurons and muscle fibres. It promotes integrative thinking by connecting the concepts of excitability, electrical signalling, and the functional organisation of a motor unit, rather than viewing the two components as unrelated cell types.

NCERT-Based Explanation
The Assertion is true as a logical inference. Biomacromolecules include proteins. Since globular (G) actin is a protein, it is also a biomacromolecule.
The Reason is true. Direct NCERT: Each filamentous (F) actin is a polymer of monomeric globular (G) actins. Thus, G-actin is a protein subunit of the actin filament.
Since proteins are biomacromolecules, the Reason correctly explains the Assertion.

Pedagogical Value
This question addresses the common misconception that only polymers are biomacromolecules. It reinforces that a protein, even when present as a monomeric subunit such as G-actin, is itself a biomacromolecule.

NCERT-Based Explanation
The Assertion is true. Direct NCERT: The reaction time varies from muscle to muscle.
The Reason is also true as a logical inference. Reaction time refers to the interval between the application of a stimulus and the initiation of muscle contraction. However, this definition is not provided directly in NCERT.
The Reason helps explain the physiological meaning of the term reaction time, but it does not explain why the reaction time varies among different muscles. Therefore, the correct answer is B.

Pedagogical Value
This question encourages students to question and understand biological terminology instead of merely memorising NCERT statements. When students read that “reaction time varies from muscle to muscle”, they should naturally ask, “What exactly is reaction time?” Even though NCERT does not define the term, developing the habit of exploring the meaning of unfamiliar terminology promotes curiosity and conceptual understanding rather than rote memorisation. This question is designed to cultivate that scientific attitude.

NCERT-Based Explanation
The Assertion is true as a logical inference. During muscle relaxation, the actin filaments return towards the Z line as the overlap between actin and myosin decreases.
The Reason is false. Direct NCERT: According to the sliding filament theory, thin (actin) filaments slide over thick (myosin) filaments. NCERT does not describe the thick (myosin) filaments as moving towards the Z line.
The Assertion is a logical inference, whereas the Reason contradicts the sliding filament theory described in NCERT.

Pedagogical Value
This question encourages students to interpret conditional statements carefully instead of mechanically recalling NCERT sentences. It reinforces that actin filament movement determines contraction and relaxation, whereas myosin filaments remain essentially stationary during the sliding filament mechanism. Students learn to distinguish between the direction of actin movement and the absence of myosin movement, rather than simply memorising that “actin slides over myosin.

NCERT-Based Explanation
The Assertion is true as a logical inference. Cross-bridge formation and the subsequent rotation (power stroke) of the myosin head are energy-dependent events during muscle contraction.
The Reason is false. Direct NCERT: Binding of a new ATP molecule to the myosin head causes the cross bridge to break. Therefore, ATP is required for the dissociation of the actin–myosin cross bridge.

Pedagogical Value
This question addresses the common misconception that the recovery stroke or cross-bridge dissociation is a passive process that does not require ATP. Students often associate the term “recovery” with a resting state and therefore assume that energy is unnecessary. The question encourages them to analyse the role of ATP at each step of the cross-bridge cycle rather than associating ATP only with force generation. It also prompts them to carefully evaluate whether a seemingly related statement actually explains the Assertion.

NCERT-Based Explanation

The Assertion is false as a logical inference. Not all involuntary muscles are neurogenic. Cardiac muscle is involuntary but myogenic, whereas smooth muscle is involuntary and is regulated by the autonomic nervous system.
The Reason is also false as a logical inference. It is incorrect to state that involuntary muscles do not need a neural signal for contraction. While cardiac muscle can initiate contraction through spontaneous depolarisation of the nodal tissue, smooth muscle generally requires neural, hormonal, or other physiological stimuli for contraction.
This information is not stated directly in NCERT. It is inferred by integrating concepts from the Locomotion and Movement and Body Fluids and Circulation chapters.

Pedagogical Value
This question addresses the common misconception that voluntary/involuntary and neurogenic/myogenic are equivalent classifications. It encourages students to distinguish between the mode of control (voluntary or involuntary) and the origin of excitation (neurogenic or myogenic).

NCERT-Based Explanation
The Assertion is true as a logical inference. Voluntary (skeletal) muscles require neural stimulation to initiate contraction and are therefore neurogenic in nature.
The Reason is also true as a logical inference. Voluntary muscles require a neural signal for contraction, which explains why they are described as neurogenic.

Pedagogical Value
This question encourages students to relate the concepts of voluntary muscle and neurogenic instead of memorising them independently. It promotes conceptual thinking by recognising that muscles requiring neural stimulation can be described as neurogenic, while also distinguishing them from myogenic muscles such as the cardiac muscle.

Both are wrong.

Pedagogical Value
This question deliberately reverses the normal direction of signal transmission between neurons and muscles. It tests whether students are reading the statements consciously instead of relying on familiar keywords. A student with a clear understanding of the basic neuromuscular pathway should immediately recognise that motor neurons generate and transmit impulses to skeletal muscles, not the other way around. The question is designed to identify careless reading and weak conceptual awareness of one of the most fundamental principles of muscle physiology.

NCERT-Based Explanation
The Assertion is false. Direct NCERT: Each meromyosin consists of a globular head with a short arm and a tail. Therefore, the head is followed by the short arm, not vice versa.
The Reason is also false. Interpreting “it” as referring to the short arm of heavy meromyosin, the statement is incorrect because the globular head, not the short arm, contains the actin-binding site and forms the cross bridge.

Pedagogical Value
This question addresses two common misconceptions: the orientation of the head and short arm of heavy meromyosin, and the assumption that the short arm, rather than the globular head, directly binds to actin to form the cross bridge.

NCERT-Based Explanation
The Assertion is true as a logical inference. Not all muscle contractions depend on a neural signal because cardiac muscle possesses intrinsic rhythmicity.
The Reason is true as a logical inference. NCERT states that the nodal tissue of the heart has the ability to spontaneously generate an action potential due to spontaneous depolarisation. This enables the initiation of cardiac contraction without requiring a neural signal. Therefore, the Reason correctly explains the Assertion.

Pedagogical Value
This question addresses the common misconception that all muscle contractions require neural stimulation. It reinforces that certain muscles, such as the cardiac muscle, possess specialised nodal tissue capable of spontaneous depolarisation, highlighting the concept of myogenic muscle contraction through the integration of concepts from different NCERT chapters.

NCERT-Based Explanation
The Assertion is false. Direct NCERT: The globular head of meromyosin (myosin) is an active ATPase and possesses ATP-binding sites. ATP does not bind to the globular head of globular actin (G-actin).
The Reason is false. The actin-binding site is present on the globular head of myosin, enabling myosin to bind to actin. It is not an active site of the actin protein.

Pedagogical Value
This question addresses two common misconceptions: confusing the globular head of myosin with globular actin (G-actin), and incorrectly assigning ATP-binding and actin-binding sites to actin instead of the globular head of myosin.

NCERT-Based Explanation
The Assertion is false. Direct NCERT: Each filamentous (F) actin is a polymer of monomeric globular (G) actins.
The Reason is false. Direct NCERT: Tropomyosin runs close to the F-actins throughout their length, and troponin is distributed at regular intervals on the tropomyosin. NCERT does not state that F-actin itself contains troponin and tropomyosin. These proteins are associated with the thin filament, not with F-actin itself.

Pedagogical Value
This question addresses the common misconception of treating F-actin and the thin filament as the same structure. It reinforces that F-actin is a polymer of G-actin, whereas the thin filament consists of actin along with the associated regulatory proteins tropomyosin and troponin.

NCERT-Based Explanation
The Assertion is true. Direct NCERT: The mechanism of muscle contraction is best explained by the sliding filament theory.
The Reason is false. Direct NCERT: During muscle contraction, the thin filaments slide over the thick filaments. The thick filaments do not slide over the thin filaments.

Pedagogical Value
This question addresses the common misconception regarding which filament slides during muscle contraction. It reinforces that, according to the sliding filament theory, thin filaments slide over thick filaments, while the lengths of both filaments remain unchanged.

NCERT-Based Explanation
NCERT states that the globular head is connected to the short arm, and the head and the arm project outward from the surface of the polymerised myosin filament. Therefore, the assertion is false because both the head and the arm, not the head alone, project outwards.
The cross arm (head together with the short arm) plays an essential role during muscle contraction by interacting with actin and bringing about filament sliding. Hence, the reason is true.

Pedagogical Value
This question tests whether students have carefully read the structural description of the myosin filament in NCERT. A common mistake is to remember only the globular head as projecting outward, whereas NCERT clearly states that both the head and the arm project from the filament. It also reinforces the functional role of the cross arm during muscle contraction.

NCERT-Based Explanation
The Assertion is true as a logical inference, and the Reason is also true as a logical inference. However, neither statement is given directly in NCERT. NCERT mentions acetylcholine as the neurotransmitter released at the neuromuscular junction but does not describe its biochemical synthesis from acetyl-CoA or state that the synthesis is enzyme-mediated. Therefore, this question cannot be answered solely from direct NCERT information and requires biological reasoning beyond the text.

Pedagogical Value
This question is not intended to assess direct NCERT knowledge. Instead, it is designed to stimulate curiosity. On encountering the term acetylcholine, students should naturally begin to think about related biomolecules such as acetyl-CoA and explore whether they are chemically or biologically connected. The objective is to encourage students to relate concepts across different topics and develop the habit of questioning the origin, relationship, and synthesis of biological molecules, even when the answer is not explicitly provided in NCERT.

NCERT-Based Explanation
The Assertion is false. Direct NCERT: The globular head of meromyosin is an active ATPase and has ATP-binding sites. NCERT does not state that the entire heavy meromyosin (HMM) is an active ATPase.
The Reason is also false. Interpreting “It” as referring to the entire heavy meromyosin (HMM), the statement is incorrect. The globular head is the catalytic ATPase region that hydrolyses ATP during muscle contraction, not the entire HMM.

Pedagogical Value
This question addresses the common misconception of attributing the properties of a specific domain to the entire protein complex. It reinforces that ATPase activity is confined to the globular head of meromyosin, rather than the whole heavy meromyosin molecule.