A 14-year-old girl with acute asthma in the paediatric casualty is given nebulised salbutamol, and her wheeze settles within minutes. Through which pathway does salbutamol relax her bronchial smooth muscle?

Beta-2 receptor, cAMP rise and reduced MLCK activity

Beta-2 receptor, cGMP rise and calcium entry

Direct opening of potassium channels by the drug

Muscarinic block with less IP3 formation

A 58-year-old man who takes isosorbide mononitrate for angina uses sildenafil before intercourse and collapses with a BP of 70/40 mmHg. Both drugs relax vascular smooth muscle by raising cGMP. Through which final step does the raised cGMP relax the smooth muscle?

Dephosphorylation of myosin light chain by its phosphatase

Phosphorylation of myosin light chain by MLCK

Blocking calcium binding to troponin C

Opening of ryanodine receptors in the SR

A 45-year-old tailor has tingling in the thumb and index finger at night. In the nerve conduction lab, the median nerve is stimulated at the elbow and at the wrist, 240 mm apart, and the response is recorded from abductor pollicis brevis. Onset latencies are 8.0 ms from the elbow and 4.0 ms from the wrist (normal distal latency below about 4.2 ms in this lab). What is the forearm motor conduction velocity, and what does it suggest?

60 m/s; forearm conduction is normal

60 m/s; a generalised neuropathy is likely

30 m/s; forearm segment is demyelinated

120 m/s; the forearm reading is invalid

A 22-year-old student sits cross-legged on the floor through a long exam. When he stands up, his foot feels 'dead': he cannot sense the position of his toes or light touch, but a pinprick still feels painful. Which principle explains this pattern?

Large myelinated A fibres are most sensitive to pressure

Unmyelinated C fibres are most sensitive to pressure

Small fibres are blocked first by local ischaemia

B fibres are most sensitive to hypoxia from compression

Nerve and muscle physiology

14 NEET-PG practice questions on Nerve and muscle physiology, part of Physiology. The first 14 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
A 14-year-old girl with acute asthma in the paediatric casualty is given nebulised salbutamol, and her wheeze settles within minutes. Through which pathway does salbutamol relax her bronchial smooth muscle?
1. ABeta-2 receptor, cAMP rise and reduced MLCK activity
2. BBeta-2 receptor, cGMP rise and calcium entry
3. CDirect opening of potassium channels by the drug
4. DMuscarinic block with less IP3 formation
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Answer: A · Beta-2 receptor, cAMP rise and reduced MLCK activity
Salbutamol stimulates beta-2 receptors, which act through Gs to raise cAMP. Protein kinase A then reduces cytosolic calcium and phosphorylates myosin light chain kinase, lowering its activity, so the myosin light chains are less phosphorylated and the muscle relaxes. Smooth muscle contraction depends on calcium-calmodulin activating MLCK, rather than on troponin. Ipratropium, a muscarinic blocker, relaxes bronchi by a different route and is added in severe attacks.
- B: Beta-2 receptors act through cAMP, not cGMP, and relaxation needs less calcium, not more.
- C: Salbutamol acts through its receptor and second messenger, not by opening channels directly.
- D: Muscarinic blockade is the mechanism of ipratropium, not salbutamol.
Question 2 · Medium
A 58-year-old man who takes isosorbide mononitrate for angina uses sildenafil before intercourse and collapses with a BP of 70/40 mmHg. Both drugs relax vascular smooth muscle by raising cGMP. Through which final step does the raised cGMP relax the smooth muscle?
1. APhosphorylation of myosin light chain by MLCK
2. BBlocking calcium binding to troponin C
3. CDephosphorylation of myosin light chain by its phosphatase
4. DOpening of ryanodine receptors in the SR
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Answer: C · Dephosphorylation of myosin light chain by its phosphatase
Nitrates release nitric oxide, which activates soluble guanylyl cyclase, while sildenafil blocks phosphodiesterase 5, which breaks down cGMP; together they produce a very large rise in cGMP and profound vasodilatation. cGMP activates protein kinase G, which lowers cytosolic calcium and activates myosin light chain phosphatase. Dephosphorylated myosin light chains cannot cycle with actin, so the muscle relaxes. This is why the combination is contraindicated.
- A: MLCK phosphorylation causes contraction, the opposite effect.
- B: Smooth muscle has no troponin; calcium acts through calmodulin.
- D: Opening ryanodine receptors would release calcium and promote contraction.
Question 3 · Hard
A 45-year-old tailor has tingling in the thumb and index finger at night. In the nerve conduction lab, the median nerve is stimulated at the elbow and at the wrist, 240 mm apart, and the response is recorded from abductor pollicis brevis. Onset latencies are 8.0 ms from the elbow and 4.0 ms from the wrist (normal distal latency below about 4.2 ms in this lab). What is the forearm motor conduction velocity, and what does it suggest?
1. A60 m/s; forearm conduction is normal
2. B60 m/s; a generalised neuropathy is likely
3. C30 m/s; forearm segment is demyelinated
4. D120 m/s; the forearm reading is invalid
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Answer: A · 60 m/s; forearm conduction is normal
Conduction velocity is the distance between the two stimulation points divided by the difference in latency: 240 mm / (8.0 - 4.0) ms = 60 mm/ms, which equals 60 m/s, a normal value for a median motor nerve in the forearm. Using the difference cancels the time taken by neuromuscular transmission and muscle activation, which both latencies share. The distal latency here is at the upper limit, and in carpal tunnel syndrome slowing is found across the wrist while the forearm velocity is normal. Sensory studies across the wrist are more sensitive for early compression.
- B: A normal forearm velocity argues against, not for, a generalised neuropathy.
- C: Dividing by the full elbow latency (8 ms) instead of the difference gives the wrong figure of 30 m/s.
- D: 120 m/s comes from halving the latency difference; the reading itself is valid.
Question 4 · Easy
A 40-year-old man in the medicine OPD has his knee jerk tested. Tapping the patellar tendon stretches the quadriceps muscle spindles, and the impulse travels to the spinal cord in the largest, fastest-conducting sensory fibres. Which fibres are these?
1. AGroup Ia (A-alpha) afferents
2. BGroup II (A-beta) afferents
3. CA-delta afferents
4. DGroup IV (C) afferents
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Answer: A · Group Ia (A-alpha) afferents
The primary (annulospiral) endings of muscle spindles are supplied by group Ia fibres, which are A-alpha fibres: the thickest myelinated fibres, conducting at about 70-120 m/s. They synapse directly on alpha motor neurons of the same muscle, making the stretch reflex monosynaptic and very fast. Group II fibres supply the secondary (flower-spray) endings. A reduced or absent jerk points to a break anywhere in this arc, such as an L3-L4 radiculopathy or a peripheral neuropathy.
- B: Group II fibres supply the secondary spindle endings and conduct more slowly than Ia fibres.
- C: A-delta fibres carry fast pain and cold, not spindle stretch.
- D: Group IV (C) fibres are unmyelinated and carry slow pain.
Question 5 · Medium
A 22-year-old student sits cross-legged on the floor through a long exam. When he stands up, his foot feels 'dead': he cannot sense the position of his toes or light touch, but a pinprick still feels painful. Which principle explains this pattern?
1. AUnmyelinated C fibres are most sensitive to pressure
2. BSmall fibres are blocked first by local ischaemia
3. CLarge myelinated A fibres are most sensitive to pressure
4. DB fibres are most sensitive to hypoxia from compression
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Answer: C · Large myelinated A fibres are most sensitive to pressure
Nerve fibres differ in their sensitivity to blocking agents. Pressure blocks the large myelinated A fibres (touch, proprioception, motor) first, while the small unmyelinated C fibres carrying slow pain are the last to fail, so pain sensation survives. Local anaesthetics act in the reverse order, blocking small C fibres first, which is why pain goes before touch and motor power in a nerve block. B fibres are the most sensitive to hypoxia.
- A: C fibres are the least sensitive to pressure and the most sensitive to local anaesthetics.
- B: If small fibres failed first, pain would be lost while touch was kept.
- D: B fibres are most sensitive to hypoxia, but they are autonomic and do not explain this sensory pattern.
Question 6 · Medium
A 48-year-old man who had paralytic poliomyelitis of the right leg in childhood has an EMG of the right quadriceps. It shows motor unit potentials of very large amplitude and long duration, and a reduced number of units on maximal effort. Why are these potentials so large?
1. AEach muscle fibre has hypertrophied after denervation
2. BNerve conduction has slowed, spreading out the potential
3. CSurviving axons have sprouted to take over denervated fibres
4. DRemaining motor neurons fire at a faster rate
Answer and explanation in LearnFree account
Question 7 · Medium
Muscle biopsies are compared in a sports medicine study at a Pune institute. A marathon runner's calf muscle is rich in fibres that look dark red, with many mitochondria and dense capillaries. Which other property do these fibres have?
1. AFast myosin ATPase and rich glycogen stores
2. BSlow myosin ATPase and high resistance to fatigue
3. CLarge motor units that are recruited last
4. DMainly glycolytic metabolism with little myoglobin
Answer and explanation in LearnFree account
Question 8 · Medium
A 55-year-old man with hypertension is started on verapamil. Over the next weeks his heart rate and myocardial contractility fall slightly, but the strength of his skeletal muscles is unaffected. What explains this difference?
1. ASkeletal fibres have no L-type (dihydropyridine) receptors
2. BSkeletal fibres release SR calcium without needing calcium entry
3. CSkeletal fibres lack a sarcoplasmic reticulum store
4. DSkeletal fibres rely on calcium binding to calmodulin
Answer and explanation in LearnFree account
Question 9 · Medium
A 30-year-old man develops weakness of the legs that spreads to the arms over a week, two weeks after an upper respiratory infection. Tendon reflexes are absent and sensation is mildly reduced. Nerve conduction studies show markedly slowed conduction velocities with conduction block. What is the physiological basis of the slowing?
1. ALoss of saltatory conduction between nodes of Ranvier
2. BLoss of voltage-gated sodium channels from axon terminals
3. CDeath of anterior horn cells in the spinal cord
4. DBlock of acetylcholine release at the end plate
Answer and explanation in LearnFree account
Question 10 · Medium
A 35-year-old man has ptosis and diplopia that worsen through the day, and proximal arm weakness after repeated effort. His acetylcholine receptor antibody result is awaited. Which electrodiagnostic finding would support the diagnosis of myasthenia gravis?
1. AMarked slowing of motor nerve conduction velocity
2. BDecrement of the CMAP on 3 Hz repetitive stimulation
3. CIncrement of the CMAP after 10 s of maximal exercise
4. DFibrillations and positive sharp waves at rest
Answer and explanation in LearnFree account
Question 11 · Easy
A 28-year-old woman has drooping eyelids and double vision that are mild in the morning and worse by evening, and she has trouble chewing towards the end of a meal. Pupils are normal. Antibodies against which structure are most likely present?
1. AVoltage-gated potassium channels on the axon
2. BPresynaptic voltage-gated calcium channels
3. CMuscarinic receptors on the motor end plate
4. DNicotinic acetylcholine receptors at the end plate
Answer and explanation in LearnFree account
Question 12 · Medium
A 16-year-old boy having an appendicectomy under sevoflurane develops a rising end-tidal CO2, masseter rigidity, tachycardia and a temperature of 40°C within 30 minutes of induction. Which drug is the specific treatment, and how does it act?
1. ANeostigmine, inhibiting acetylcholinesterase
2. BDantrolene, blocking calcium release via RyR1
3. CDantrolene, blocking end-plate acetylcholine receptors
4. DVerapamil, blocking L-type calcium channels
Answer and explanation in LearnFree account
Question 13 · Medium
A 29-year-old woman with relapsing-remitting multiple sclerosis notices that her old blurring of vision in the right eye returns for a short time after a hot bath or a game of badminton, then clears as she cools down. What is the physiological basis of this?
1. AHeat blocks transmission at retinal synapses
2. BHeat breaks down further myelin in the plaque
3. CHeat raises intraocular pressure in that eye
4. DHeat lowers the safety factor in demyelinated fibres
Answer and explanation in LearnFree account
Question 14 · Hard
A 29-year-old woman at 31 weeks is admitted to the labour room with regular painful contractions and is given oral nifedipine to suppress them. Consider four statements about her myometrium: 1. It behaves as single-unit smooth muscle, with its cells linked by gap junctions. 2. Contraction starts when calcium binds troponin C on the thin filaments. 3. Nifedipine reduces calcium entry through L-type channels in the cell membrane. 4. Cross-bridge cycling needs phosphorylation of the myosin regulatory light chain by MLCK. Which statements are correct?
1. A2, 3 and 4 only
2. B1 and 4 only
3. C1, 3 and 4 only
4. D2 and 3 only
Answer and explanation in LearnFree account

Nerve and muscle physiology

A 14-year-old girl with acute asthma in the paediatric casualty is given nebulised salbutamol, and her wheeze settles within minutes. Through which pathway does salbutamol relax her bronchial smooth muscle?

A 58-year-old man who takes isosorbide mononitrate for angina uses sildenafil before intercourse and collapses with a BP of 70/40 mmHg. Both drugs relax vascular smooth muscle by raising cGMP. Through which final step does the raised cGMP relax the smooth muscle?

A 40-year-old man in the medicine OPD has his knee jerk tested. Tapping the patellar tendon stretches the quadriceps muscle spindles, and the impulse travels to the spinal cord in the largest, fastest-conducting sensory fibres. Which fibres are these?

A 22-year-old student sits cross-legged on the floor through a long exam. When he stands up, his foot feels 'dead': he cannot sense the position of his toes or light touch, but a pinprick still feels painful. Which principle explains this pattern?

A 48-year-old man who had paralytic poliomyelitis of the right leg in childhood has an EMG of the right quadriceps. It shows motor unit potentials of very large amplitude and long duration, and a reduced number of units on maximal effort. Why are these potentials so large?

Muscle biopsies are compared in a sports medicine study at a Pune institute. A marathon runner's calf muscle is rich in fibres that look dark red, with many mitochondria and dense capillaries. Which other property do these fibres have?

A 55-year-old man with hypertension is started on verapamil. Over the next weeks his heart rate and myocardial contractility fall slightly, but the strength of his skeletal muscles is unaffected. What explains this difference?

A 35-year-old man has ptosis and diplopia that worsen through the day, and proximal arm weakness after repeated effort. His acetylcholine receptor antibody result is awaited. Which electrodiagnostic finding would support the diagnosis of myasthenia gravis?

A 28-year-old woman has drooping eyelids and double vision that are mild in the morning and worse by evening, and she has trouble chewing towards the end of a meal. Pupils are normal. Antibodies against which structure are most likely present?

A 16-year-old boy having an appendicectomy under sevoflurane develops a rising end-tidal CO2, masseter rigidity, tachycardia and a temperature of 40°C within 30 minutes of induction. Which drug is the specific treatment, and how does it act?

A 29-year-old woman with relapsing-remitting multiple sclerosis notices that her old blurring of vision in the right eye returns for a short time after a hot bath or a game of badminton, then clears as she cools down. What is the physiological basis of this?