Motor Unit Function — MCQs
What is one of the specific functions of the primary motor cortex located on the anterior edge of the pre-central gyrus?
Which of the following statements is true about red muscle fibers?
Hilton's law primarily relates to which of the following?
Latent period of muscle twitch is 10 milliseconds, contraction period is 40 milliseconds, and the relaxation time is 50 milliseconds. What would be the tetanizing frequency?
Golgi tendon organs are innervated by which type of nerve fibre?
UMN palsy is characterized by all except
Variation in size and shape of muscle fibers, combined with degenerative changes and intramysial fibrosis, is typical of
Most common nerve used for nerve conduction study in H reflex -
Which of the following structures passes through Guyon's canal?
Arrange the following parts of sarcomere from periphery to center. 1. Z line 2. M line 3. A band 4. H zone
Motor Unit Function Indian Medical PG Practice Questions and MCQs
Question 1: What is one of the specific functions of the primary motor cortex located on the anterior edge of the pre-central gyrus?
- A. Control of voluntary movement (Correct Answer)
- B. Increase extensor muscle tone
- C. Perception of pain
- D. Inhibition of stretch reflex
Explanation: ***Control of voluntary movement*** - The **primary motor cortex (M1)**, located in the **precentral gyrus**, is critically involved in generating neural impulses that control the execution of **voluntary movements**. - It plays a key role in **planning and executing complex, skilled movements**, especially of the distal musculature. *Increase extensor muscle tone* - While motor pathways influence muscle tone, the primary motor cortex's most specific role is not simply increasing extensor tone; rather, it coordinates a wide range of movements involving both flexors and extensors. - **Spasticity** or increased muscle tone (often extensor) is more commonly associated with damage to the **corticospinal tracts (upper motor neuron lesions)**, which *prevents* the fine-tuning inhibitory control from the cortex. *Perception of pain* - **Pain perception** is primarily processed in the **somatosensory cortex** (postcentral gyrus), limbic system, and insula, not the primary motor cortex. - The primary motor cortex is responsible for **motor output**, not sensory interpretation. *Inhibition of stretch reflex* - While descending motor pathways can modulate spinal reflexes, the direct and primary function of the primary motor cortex is not the specific inhibition of the stretch reflex. - The **gamma motor system** and other spinal interneurons are more directly involved in modulating the sensitivity of the stretch reflex.
Question 2: Which of the following statements is true about red muscle fibers?
- A. Contain fewer mitochondria than white muscle fibers
- B. Have less myoglobin than white muscle fibers
- C. Exhibit more oxidative capacity (Correct Answer)
- D. Utilize glycolytic metabolism
Explanation: ***Exhibit more oxidative capacity*** - **Red muscle fibers**, also known as **slow-twitch fibers**, are rich in **mitochondria** and enzymes for aerobic respiration, allowing for sustained contractions and high oxidative capacity. - Their high oxidative capacity is crucial for activities requiring **endurance**, such as long-distance running or maintaining posture through efficient **ATP production** via the **electron transport chain**. *Contain fewer mitochondria than white muscle fibers* - **Red muscle fibers** contain **more mitochondria** than white muscle fibers to support their greater reliance on **aerobic metabolism** for sustained energy production. - **Mitochondria** are the primary sites of **oxidative phosphorylation**, which is essential for the continuous ATP supply needed by these endurance specialized fibers. *Utilize glycolytic metabolism* - While red fibers can perform some glycolysis, their primary metabolic pathway is **oxidative phosphorylation**, utilizing **fatty acids** and **glucose** aerobically. - **Glycolytic metabolism** is more characteristic of **white muscle fibers (fast-twitch)**, which rely on anaerobic pathways for rapid, high-intensity contractions. *Have less myoglobin than white muscle fibers* - **Red muscle fibers** are characterized by a **high content of myoglobin**, which gives them their characteristic red color and high oxygen storage capacity. - **Myoglobin** is crucial for oxygen delivery to the mitochondria, supporting the sustained aerobic metabolism of these fibers, in contrast to white fibers which have less myoglobin.
Question 3: Hilton's law primarily relates to which of the following?
- A. Nerve innervation only
- B. Nerve supply of joints, muscles moving them, and overlying skin (Correct Answer)
- C. Blood supply to joints
- D. None of the above
Explanation: ***Nerve supply of joints, muscles moving them, and overlying skin*** - **Hilton's law** states that the nerve supplying a joint also supplies the muscles that move the joint and the skin overlying the insertions of those muscles - This anatomical principle is clinically significant as it explains **referred pain patterns** from joints to surrounding structures - The law demonstrates the **functional integration** between joint innervation, muscle control, and cutaneous sensation *Nerve innervation only* - While Hilton's law involves nerve innervation, this option is too vague and incomplete - The law specifically describes the **relationship between three components**: joint nerves, muscle nerves, and cutaneous nerves - Simply stating "nerve innervation only" misses the **clinical significance** of the anatomical pattern *Blood supply to joints* - This refers to the vascular supply of joints (articular arteries), which is important for joint nutrition - However, **Hilton's law** specifically addresses **nerve supply patterns**, not vascular anatomy - Blood supply to joints follows different anatomical principles *None of the above* - This is incorrect because Hilton's law clearly relates to the integrated nerve supply pattern described in the correct option - The law is a fundamental principle in anatomy explaining the **functional relationship** between joint, muscle, and skin innervation
Question 4: Latent period of muscle twitch is 10 milliseconds, contraction period is 40 milliseconds, and the relaxation time is 50 milliseconds. What would be the tetanizing frequency?
- A. 50 Hz
- B. 75 Hz
- C. 25 Hz (Correct Answer)
- D. 100 Hz
Explanation: ***25 Hz*** - **Tetanizing frequency** is the minimum stimulation frequency required to produce tetanus (sustained muscle contraction without complete relaxation between stimuli). - For **incomplete tetanus** to occur, the next stimulus must arrive during the relaxation phase, before the muscle fully relaxes. - The critical time window is the **latent period + contraction period** = 10 ms + 40 ms = 50 ms. However, to ensure summation occurs reliably during relaxation, stimuli typically arrive at a slightly higher frequency. - **Practical tetanizing frequency** = approximately 1/(40 ms) = **25 Hz**, which ensures stimuli arrive during the latter part of contraction or early relaxation phase, producing sustained tension. - This frequency allows sufficient overlap for tetanic fusion while accounting for the physiological requirements of the muscle twitch cycle. *50 Hz* - This frequency (one stimulus every 20 ms) would produce a **complete tetanus** with no visible relaxation between stimuli. - This is higher than the minimum tetanizing frequency required for this muscle with its 100 ms total twitch duration. - While this would produce tetanus, it exceeds the minimum frequency needed. *75 Hz* - This very high frequency (one stimulus every 13.3 ms) would produce a **smooth, complete tetanus**. - This is approximately 3 times the minimum tetanizing frequency and represents excessive stimulation. - Such high frequencies are well beyond what is needed to prevent relaxation in this muscle. *100 Hz* - This extremely high frequency (one stimulus every 10 ms, equivalent to the latent period alone) would produce **maximal tetanic fusion**. - This is 4 times the minimum tetanizing frequency needed for this muscle. - While physiologically possible, this represents supramaximal stimulation frequency for tetanus production in this scenario.
Question 5: Golgi tendon organs are innervated by which type of nerve fibre?
- A. Ia
- B. Ib (Correct Answer)
- C. II
- D. III
Explanation: ***Ib*** - **Golgi tendon organs (GTOs)** are encapsulated sensory receptors located in the musculoskeletal junction that monitor **muscle tension**. - They are innervated by **Ib afferent nerve fibers**, which are large diameter, myelinated nerve fibers with a high conduction velocity that transmit information to the central nervous system. *Ia* - **Ia afferent nerve fibers** innervate **muscle spindles**, which detect changes in **muscle length** and the rate of change of muscle length. - While both Ib and Ia fibers are involved in proprioception, their specific sensory receptors and functions differ. *II* - **Type II afferent nerve fibers** also innervate **muscle spindles**, primarily sensing sustained changes in **muscle length** (static stretch). - They do not innervate Golgi tendon organs; their role is distinct in providing information about muscle position. *III* - **Type III afferent nerve fibers** are smaller, thinly myelinated fibers that respond mainly to **nociceptive (pain)** and **temperature stimuli** in muscles and joints. - They are not involved in sensing muscle tension or length and do not innervate Golgi tendon organs.
Question 6: UMN palsy is characterized by all except
- A. Fasciculations (Correct Answer)
- B. Spasticity
- C. Exaggerated tendon reflexes
- D. Extensor plantar response
Explanation: ***Fasciculations*** - **Fasciculations** are characteristic of **lower motor neuron (LMN) lesions**, representing spontaneous firing of motor units due to denervation [1]. - They are generally absent in **upper motor neuron (UMN) lesions**, where the motor neuron itself is intact [1]. *Spasticity* - **Spasticity** is a hallmark of **UMN lesions**, characterized by a velocity-dependent increase in muscle tone and exaggerated stretch reflexes [2]. - It results from the loss of supraspinal inhibitory control over spinal reflexes. *Exaggerated tendon reflexes* - **Exaggerated (hyperactive) deep tendon reflexes** are a classic sign of **UMN pathology**, due to the removal of inhibitory control from higher centers [3]. - This contrasts with LMN lesions, which typically cause diminished or absent reflexes [3]. *Extensor plantar response* - An **extensor plantar response (Babinski sign)**, where the great toe extends and other toes fan out upon plantar stimulation, is indicative of a **UMN lesion**. - This reflex is normally absent in adults and signifies damage to the corticospinal tract.
Question 7: Variation in size and shape of muscle fibers, combined with degenerative changes and intramysial fibrosis, is typical of
- A. Denervation of muscle with reinnervation
- B. Denervation atrophy
- C. Muscular dystrophy (Correct Answer)
- D. Mitochondrial myopathy
Explanation: ***Muscular dystrophy*** - The combination of **variation in muscle fiber size and shape**, **degenerative changes**, and **intramysial fibrosis** is a hallmark of muscular dystrophies [1]. - These conditions are characterized by progressive muscle weakness and degeneration, often with cycles of degeneration and attempted regeneration, leading to fibrosis [1]. *Denervation of muscle with reinnervation* - This typically leads to **fiber type grouping** as reinnervated fibers take on the characteristics of the reinnervating nerve. - While there may be some variation in fiber size, the prominent **degenerative changes** and **intramysial fibrosis** described are less characteristic of this process alone. *Denervation atrophy* - Primarily features widespread **atrophy of muscle fibers**, often angular in shape. - It lacks the prominent combination of **fiber size variation** and **significant degenerative changes with fibrosis**. *Mitochondrial myopathy* - Characterized by the presence of **ragged red fibers** on Gomori trichrome stain and often **lipid accumulation** due to mitochondrial abnormalities. - While there may be some fiber size variation, the description of widespread degenerative changes and intramysial fibrosis isn't the primary defining feature. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Peripheral Nerves and Skeletal Muscles, pp. 1244-1245.
Question 8: Most common nerve used for nerve conduction study in H reflex -
- A. Median nerve
- B. Tibial nerve (Correct Answer)
- C. Peroneal nerve
- D. Ulnar nerve
Explanation: ***Tibial nerve*** - The **tibial nerve** is the most commonly used nerve for H-reflex studies because it reliably activates the **soleus muscle's Ia afferents**, which are crucial for generating a strong and measurable H-reflex. - The H-reflex of the **tibial nerve** is the most robust and easily elicitable, making it a standard for assessing **monosynaptic reflex arcs** and **S1 radiculopathy**. *Median nerve* - While the **median nerve** can elicit an H-reflex, it is less common and harder to obtain consistently compared to the tibial nerve due to its smaller Ia afferent input to distal muscles. - H-reflexes in the upper limbs, including those of the median nerve, are generally smaller and more difficult to interpret clinically. *Peroneal nerve* - The **peroneal nerve** is primarily a motor nerve to the anterior and lateral compartments of the leg; it does not produce a significant or clinically useful H-reflex. - An H-reflex is typically an electromyographic response to stimulation of **Ia afferent fibers**, and the peroneal nerve lacks the prominent Ia afferent pool necessary for a robust reflex. *Ulnar nerve* - Similar to the median nerve, the **ulnar nerve** can produce an H-reflex, but it is typically small and less reliable than the tibial nerve due to fewer Ia afferent fibers involved in a monosynaptic arc compared to the soleus. - Its clinical utility for H-reflex studies is limited due to the difficulty in eliciting a consistent and large response.
Question 9: Which of the following structures passes through Guyon's canal?
- A. Median nerve
- B. Radial nerve
- C. Flexor carpi radialis
- D. Ulnar nerve (Correct Answer)
Explanation: Ulnar nerve - The **ulnar nerve** along with the **ulnar artery** passes through Guyon's canal at the wrist [1]. - Compression of the ulnar nerve in Guyon's canal can lead to **ulnar neuropathy**, characterized by sensory and motor deficits in the hand. *Median nerve* - The **median nerve** passes through the **carpal tunnel**, not Guyon's canal [1]. - Compression of the median nerve in the carpal tunnel results in **carpal tunnel syndrome**, affecting the thumb, index, middle, and radial half of the ring finger [1]. *Radial nerve* - The **radial nerve** typically passes through the **spiral groove of the humerus** and then divides into superficial and deep branches in the forearm. - It does not traverse Guyon's canal or the carpal tunnel at the wrist. *Flexor carpi radialis* - The **flexor carpi radialis tendon** is a muscle tendon located in the forearm that inserts on the base of the second metacarpal. - Although it crosses the wrist, it primarily runs over the **anterior aspect of the carpal bones** and does not pass through Guyon's canal.
Question 10: Arrange the following parts of sarcomere from periphery to center. 1. Z line 2. M line 3. A band 4. H zone
- A. 2,3,4,1
- B. 4,2,3,1
- C. 3,1,4,2
- D. 1,3,4,2 (Correct Answer)
Explanation: ***1,3,4,2*** - The **Z line** is found at the **periphery** of the sarcomere, defining its boundaries and anchoring the **actin filaments**. - Moving inwards, the **A band** is next, representing the entire length of the **myosin filament**, which may also overlap with actin. - The **H zone** is located within the A band, comprising only **myosin filaments** without actin overlap. - Finally, the **M line** is at the **center** of the sarcomere, bisecting the H zone and anchoring the myosin filaments. *2,3,4,1* - This sequence is incorrect because the **M line** is at the **center** and the **Z line** is at the **periphery**, which is the reverse of the expected order for from periphery to center. - Such an arrangement would place the innermost structure first and outermost last, not reflecting the correct spatial organisation. *4,2,3,1* - This order is incorrect as the **H zone** and **M line** are more central, while the **Z line** is peripheral. - Placing structures like the H zone and M line at the beginning does not align with arrangement from periphery to center. *3,1,4,2* - This option is incorrect because the **A band** includes both actin and myosin filaments, while the **Z line** is at the periphery of the sarcomere. - The given order does not represent a progression from the periphery to the center of the sarcomere.
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