Neuromuscular & Muscle Physiology MCQ

The neuromuscular junction (NMJ) is the gateway where motor nerves meet skeletal muscle. Acetylcholine is released, opens nicotinic receptors, and starts a cascade that releases Ca2+ from the sarcoplasmic reticulum and slides actin past myosin. Cardiac and smooth muscle work differently, but the NMJ governs general anesthesia (succinylcholine, non-depolarizing blockers), myasthenia gravis, organophosphate poisoning, and chair-side reflex responses. Masticatory muscle physiology adds the bite-force, TMD, bruxism, and trismus layer dentists see every day.

The Neuromuscular Junction

• An action potential reaches the motor nerve terminal and opens voltage-gated Ca2+ channels; Ca2+ entry drives vesicle fusion and ACh release into the synaptic cleft.
• ACh binds NICOTINIC receptors (N-M subtype on muscle), opening cation channels that depolarize the end-plate; once threshold is reached, voltage-gated Na+ channels in the muscle fiber propagate the action potential.
• ACETYLCHOLINESTERASE (AChE) in the synaptic cleft rapidly hydrolyzes ACh, terminating the signal; AChE inhibitors (neostigmine, pyridostigmine, edrophonium, donepezil; organophosphates irreversibly) raise synaptic ACh.
• Myasthenia gravis is an autoimmune disease in which antibodies against the nicotinic ACh receptor reduce receptor numbers; characteristic fatigability is the hallmark, and AChEIs (pyridostigmine) are first-line.

Neuromuscular Blocking Drugs

• SUCCINYLCHOLINE is a DEPOLARIZING neuromuscular blocker: it activates the nicotinic ACh receptor like ACh but is not broken down by AChE (it is metabolized by plasma pseudocholinesterase more slowly), so the end-plate stays depolarized and further action potentials cannot fire (phase I block); brief fasciculations precede paralysis.
• Side effects include hyperkalemia (especially in burns, crush injury, denervation), increased intraocular and intragastric pressure, and MALIGNANT HYPERTHERMIA (in susceptible patients with RYR1 mutations); treatment of MH is dantrolene (which blocks SR Ca2+ release) and cooling.
• NON-DEPOLARIZING neuromuscular blockers (rocuronium, vecuronium, cisatracurium, atracurium; curare is the prototype) competitively block the nicotinic receptor; reversal uses AChE inhibitors (neostigmine + glycopyrrolate to prevent muscarinic side effects) or sugammadex (for rocuronium and vecuronium specifically).
• Pseudocholinesterase deficiency (autosomal recessive) prolongs succinylcholine paralysis; the dibucaine number quantifies it.

Excitation-Contraction Coupling and Muscle Types

• In SKELETAL muscle, the action potential travels along the T-tubule, activates the dihydropyridine receptor (DHPR; a voltage sensor), which mechanically opens the ryanodine receptor (RyR1) in the sarcoplasmic reticulum to release Ca2+; Ca2+ binds TROPONIN C, exposes actin binding sites, and myosin cross-bridges cycle (sliding filament theory) using ATP.
• In CARDIAC muscle, the L-type Ca2+ channel opens and brings extracellular Ca2+ into the cell; this small Ca2+ entry triggers a much larger Ca2+ release from the SR via RyR2 (Ca2+-induced Ca2+ release); the syncytium of cardiac muscle uses gap junctions to spread the AP cell to cell.
• In SMOOTH muscle, Ca2+ (from extracellular space or SR) binds calmodulin; the Ca2+-calmodulin complex activates myosin light-chain kinase (MLCK), which phosphorylates myosin and allows contraction; there is no troponin, and the contraction is slow and sustained.
• Skeletal muscle fibers are classified as SLOW-TWITCH (type I; fatigue-resistant; oxidative; postural muscles) and FAST-TWITCH (type II; glycolytic; rapid powerful contractions; phasic muscles); the masseter is mixed but rich in type II fibers for rapid biting.

Motor Units, Reflexes, and Proprioception

• A MOTOR UNIT is one motor neuron and all the muscle fibers it innervates; small motor units (extraocular muscles, intrinsic hand muscles) allow fine control; large motor units (quadriceps) generate force.
• The MONOSYNAPTIC STRETCH REFLEX (myotatic reflex) uses muscle spindle Ia afferents to directly excite alpha motor neurons; the JAW-JERK is the dental example (tap to the chin → masseter Ia afferent → trigeminal motor nucleus → masseter contraction); an EXAGGERATED jaw-jerk suggests upper motor neuron pathology.
• MUSCLE SPINDLES (intrafusal fibers + Ia afferents) sense MUSCLE LENGTH; GOLGI TENDON ORGANS (Ib afferents in tendon) sense MUSCLE TENSION and inhibit contraction when load is high (autogenic inhibition).
• Withdrawal reflexes are polysynaptic and include the crossed-extensor reflex (flexes the affected limb and extends the opposite limb to maintain balance).

Masticatory Muscles, Bite Force, TMD, Bruxism, and Trismus

• The masticatory muscles include the MASSETER (elevates mandible; powerful biting), TEMPORALIS (elevates and retracts mandible), MEDIAL PTERYGOID (elevates and contributes to lateral movement), and LATERAL PTERYGOID (protrudes mandible; the inferior head opens the jaw); all are innervated by the mandibular division of CN V (V3).
• Bite force varies by position (highest at first molar, lowest at incisors), gender, age, and dentition status; routine measurements help in occlusal analysis but vary widely.
• TMD (temporomandibular disorder) often involves masticatory muscle dysfunction (myofascial pain), joint disease, or both; bruxism (parafunctional clenching/grinding, often nocturnal) is a recognized contributor; first-line care includes patient education, soft diet, jaw rest, NSAIDs, occlusal appliance therapy, and physical therapy.
• TRISMUS (limited mouth opening) has a broad differential: muscular (masticatory muscle spasm, myofascial pain), joint (arthritis, ankylosis, disc displacement), inflammatory (odontogenic infection, pericoronitis), traumatic (mandibular fracture, hematoma after IAN block), neoplastic (oral, oropharyngeal cancer), and systemic (scleroderma, tetanus).