2.3 Neuromuscular Physiology & Salivary Function

2.3 Neuromuscular Physiology & Salivary Function

Quick Answer: Skeletal muscle contracts when acetylcholine (ACh) from the motor neuron binds nicotinic receptors at the neuromuscular junction (NMJ), depolarizing the end plate and triggering a muscle action potential and Ca²⁺-dependent cross-bridge cycling. Saliva—mostly water plus buffers, mucins, enzymes, and IgA—protects teeth and mucosa; parasympathetic input drives watery flow. Xerostomia from drugs, Sjögren disease, radiation, or systemic disease sharply raises caries and candidiasis risk.

This section closes the physiology chapter by linking nerve–muscle transmission (mastication, facial expression, tongue, and the targets of local anesthesia) with salivary gland function (caries prevention, taste, swallowing, and oral medicine).

The Neuromuscular Junction

Sequence of transmission

1. Motor neuron action potential invades the presynaptic terminal
2. Voltage-gated Ca²⁺ channels open; Ca²⁺ enters the terminal
3. Synaptic vesicles release ACh into the synaptic cleft
4. ACh binds nicotinic ACh receptors on the motor end plate (ligand-gated cation channels)
5. Na⁺ influx produces an end-plate potential (EPP)
6. If threshold is reached, a muscle action potential propagates along the sarcolemma and into T-tubules
7. Dihydropyridine receptors couple to ryanodine receptors on sarcoplasmic reticulum → Ca²⁺ release
8. Ca²⁺ binds troponin C, moves tropomyosin, allows actin–myosin cross-bridges → contraction
9. ACh is rapidly hydrolyzed by acetylcholinesterase in the cleft; Ca²⁺ is pumped back into SR → relaxation

High-yield NMJ pharmacology physiology (AFK overlap)

Remember the distinction: local anesthetics stop axonal conduction; they do not act as the primary NMJ ACh-receptor story. Botulinum toxin and myasthenia do live at the NMJ.

Excitation–contraction coupling essentials

• ATP is required both for cross-bridge cycling and for SR Ca²⁺-ATPase (relaxation)
• Rigor patterns appear when ATP is depleted (conceptual physiology; postmortem rigor is the extreme teaching example)
• Motor units: one motor neuron + all muscle fibers it innervates; fine control (extraocular, some facial/tongue units) uses small motor units; force recruitment adds larger units

Muscle Types Relevant to Dentistry

Masticatory muscle physiology notes

• Masseter, temporalis, medial pterygoid — elevators; lateral pterygoid — protrusion/depression contributions (anatomy chapter details attachments)
• Masticatory muscles are skeletal but exhibit specialized fiber-type mixes for force and endurance; bruxism is prolonged isometric-like activity that can cause pain and hypertrophy
• Stretch reflexes and periodontal mechanoreceptors help regulate bite force—protective unloading when a hard object is bitten unexpectedly
• TMJ loading is neuromuscular: muscle hyperactivity + joint anatomy interact (pain chapters expand clinical TMDs)

Autonomic contrast for glands and vessels

While skeletal muscle uses nicotinic NMJs, autonomic postganglionic endings use:

• Parasympathetic: mainly ACh on muscarinic receptors (glands, smooth muscle)
• Sympathetic: mainly norepinephrine on adrenergic receptors (vascular tone; glandular protein secretion modulation)

This receptor split explains why anticholinergic drugs dry the mouth (block muscarinic salivation) without paralyzing masseter the way a nicotinic NMJ blocker would.

Salivary Gland Physiology

Major glands and ducts (functional view)

Whole saliva is not just glandular fluid—it includes crevicular fluid, microbes, desquamated cells, and food debris. Glandular saliva is the protective core.

Composition

Saliva is approximately 99% water. Key solutes and macromolecules:

pH and buffering: resting saliva is often near neutral to slightly acidic; stimulated saliva is more bicarbonate-rich and better buffered—another reason chewing and gustatory stimulation protect teeth after acid challenges.

Control of secretion

• Parasympathetic (primary volume driver):
  • Parotid: CN IX (glossopharyngeal) via otic ganglion → auriculotemporal
  • Submandibular/sublingual: CN VII (facial) via chorda tympani → submandibular ganglion
  • Muscarinic receptors → watery, electrolyte-rich flow; vasodilation of glandular vessels
• Sympathetic: superior cervical ganglion fibers; tends to favor protein-rich, lower-volume secretion and modulates composition; also influences blood flow
• Stimuli: taste (especially acid), mastication, smell, higher centers (appetite); conditioned responses exist
• Inhibition: fear/dehydration/sleep lower flow; anticholinergic drugs block muscarinic drive

Primary vs secondary saliva: acini produce isotonic primary secretion; ducts reabsorb Na⁺/Cl⁻ and secrete K⁺/HCO₃⁻, yielding a normally hypotonic final saliva at low flow rates. At high flow rates, ductal modification time falls—composition approaches primary fluid (higher Na⁺).

Normal flow rates (order-of-magnitude teaching values)

Exact cutoffs vary by source; AFK cares that you know unstimulated vs stimulated, which gland dominates each, and that low flow = disease risk.

Xerostomia and Hyposalivation

Xerostomia is the symptom of dry mouth; hyposalivation is objectively reduced flow. They overlap but are not identical—some patients feel dry with normal measured flow and vice versa.

Major causes

1. Medications (most common in general practice populations)
   • Anticholinergics, antimuscarinics
   • First-generation antihistamines
   • Tricyclic and many other antidepressants
   • Antipsychotics
   • Antihypertensives (e.g., some diuretics, clonidine, others)
   • Opioids, polypharmacy in elderly
2. Head-and-neck radiotherapy — irreversible acinar damage when glands in field; severe, persistent hyposalivation
3. Sjögren disease — autoimmune destruction of exocrine glands; dry mouth + dry eyes; caries, candidiasis, parotid enlargement patterns
4. Uncontrolled diabetes mellitus — dehydration and altered fluid balance
5. Dehydration, mouth breathing, smoking, aging-related polypharmacy (aging alone is a weaker primary cause than drugs)
6. Chemotherapy, HIV, graft-versus-host disease, sarcoidosis, glandular obstruction/infection (sialolithiasis more often painful swelling than pure xerostomia)

Consequences dentists manage daily

• Rampant cervical and root caries
• Oral candidiasis, burning mouth complaints
• Dysgeusia, difficulty chewing/swallowing/speaking
• Denture instability and mucosal trauma
• Ascending sialadenitis risk when flow is very low

Management principles (physiology-based)

• Stimulate residual parenchyma: sugar-free gum/lozenges, gustatory sialogogues
• Pharmacologic sialogogues (pilocarpine, cevimeline) when residual tissue exists and systemic status allows—muscarinic agonists
• Topical saliva substitutes, high-fluoride regimens, dietary sugar control, antifungal therapy as needed
• Review drug list with physician when xerostomia is medication-driven
• Radiation patients: lifelong aggressive caries prevention; recognize irreversible acinar loss

Putting NMJ and Saliva Together for AFK

Expect integrated stems such as:

• A patient on multiple anticholinergic drugs develops new root caries → link muscarinic blockade → hyposalivation → loss of buffer/remineralizing ions
• Botulinum toxin for masseter hypertrophy → presynaptic ACh release block → reduced bite force
• Sjögren disease → lymphocytic gland destruction → low unstimulated flow (submandibular-dominant resting secretion lost) → candidiasis
• Local anesthetic failure vs true NMJ disease → Na⁺ channel block on nerve vs AChR problem at muscle

Rapid review list

• NMJ sequence: AP → Ca²⁺ in → ACh out → nicotinic EPP → muscle AP → SR Ca²⁺ → troponin → contraction
• AChE ends the signal; botulinum stops release; myasthenia attacks receptors
• Skeletal vs cardiac vs smooth table from memory
• Parotid = serous, IX; submandibular = mixed, VII; resting vs stimulated dominance
• Saliva buffers, remineralizes, lubricates, and provides innate/adaptive antimicrobials
• Xerostomia causes: drugs, Sjögren, radiation, diabetes, dehydration

Master this triad—NMJ, muscle type, saliva—and you can answer pure physiology items and the oral-medicine stems that depend on them.