Mastication Biology Quiz

Chewing

• Mastication is the process of mechanically breaking down food.
• Involves action of teeth: incisors for cutting, molars for grinding.
• Jaw muscles are regulated by higher brain centers.
• Individuals without teeth (edentulous) require a soft diet.

Functions of Mastication

• Aids digestion by increasing surface area for enzyme action.
• Mixes food with saliva and lubricates it for easier swallowing.
• Prevents damage (excoriation) to the gut wall.
• Facilitates food emptying and movement through the digestive tract.

Regulation of Chewing

• Controlled by nuclei located in the brainstem, cerebral cortex, hypothalamus, and amygdala.
• Motor branch of the trigeminal nerve (cranial nerve V) innervates the muscles of chewing.

Chewing Reflex

• Initiated by the presence of food in the mouth.
• This triggers a reflex inhibition of mastication muscles, causing the jaw to drop.
• The jaw muscles stretch, resulting in a rebound contraction that raises the jaw.
• The teeth close, compressing food against the mouth lining.
• Further reflex inhibition of the mastication muscles occurs, restarting the cycle.

Deglutition (Swallowing)

• A reflex action moving air, liquids, and solids from the mouth to the stomach.

Regulatory Pathways for Swallowing

• Controlled by the swallowing center in the brainstem reticular formation.
• Involves the nucleus ambiguus, tractus solitarius, and dorsal motor nucleus of the vagus nerve.
• Afferent (sensory) nerves from the pharynx and cranial nerves V, VII, and XII carry information about the bolus.
• Efferent (motor) nerves, including V, VII, IX, X, and the hypoglossal nerve, control muscle action during swallowing.

Vagal Integrative Center for the Upper GI Tract

• Includes the dorsal motor nucleus of the vagus, nuclei of ambiguus, tractus solitarius, and area postrema.
• Known as the dorsal vagal complex, located in the medulla oblongata.
• Responsible for integrating vagal control of the esophagus, stomach, duodenum, pancreas, and gallbladder.

Muscles Involved in Swallowing

• Oral/Facial muscles: Masseter, temporalis, pterygoideus (lateral & medial), tensor palatini, digastricus, stylohyoideus, facial muscles.
• Pharyngeal Muscles: Palatopharyngeus, palatoglossus, levator palatini, salpingopharyngeus, superior, middle & inferior constrictors.
• Laryngeal Muscles: Cricothyroideus, cricoarytenoideus, interarytenoideus.
• Tongue Muscles: Geniohyoideus, thyrohyoideus (extrinsic) - posterior intrinsic tongue muscles, hypoglossus, genioglossus, styloglossus.
• Esophageal Muscles: Esophageal muscles (smooth & striated)

Swallowing Stages

• Voluntary: Food is moved posteriorly by the tongue and pressed against the hard palate, then soft palate into the oropharynx.
• Involuntary:
  • Pharyngeal Phase (< 1 second): Closure of the nasopharynx, respiration inhibited, bolus propelled through the pharynx by pharyngeal peristalsis.
  • Esophageal Phase (8-10 seconds): Bolus moves from pharynx to stomach via peristalsis.

Pharyngeal Phase

• Nasopharynx closure accomplished by contraction of the tensor palatini and levator palatini muscles, elevating the soft palate.
• Superior constrictor muscle contraction forms the ridge of Passavant, creating a seal and initiating pharyngeal peristalsis.
• Palatopharyngeal folds approximate to form a slit, allowing for passage of properly masticated food.
• Larynx closure: hyoid bone is raised & moved forward by geniohyoid & mylohyoid muscles, bringing the larynx under the tongue.
• Vocal cords approximate.
• Epiglottis swings backward over the larynx opening to prevent food entry.

Esophageal Phase

• UOS (upper esophageal sphincter) closure at the top of the esophagus.
• Cricopharyngeal, or pharyngoesophageal, sphincter is a striated muscle.
• A primary peristaltic wave (OPW) is initiated in the pharynx and propels the bolus distally at 4 cm/sec over 8-10 seconds.
• Gravity assists bolus movement.
• Secondary OPW is triggered by retained food distending the myenteric plexus.
• Tertiary waves can indicate pathology.

Lower Esophageal Sphincter (LOS)

• A physiological sphincter created by:
  • Smooth muscle thickening at the stomach junction.
  • Crural portion of the diaphragm surrounding the esophagus.
  • Oblique stomach fibers form a flap valve.
• LOS closes during high intragastric pressure to prevent reflux.

LOS Function

• High resting tone of the UOS and LOS act as a barrier against air and gastric contents.
• The esophageal body is flaccid at rest.
• Sequential body contractions propel the bolus distally.
• Contractions are coordinated centrally and peripherally.

LOS Contraction and Relaxation

• Contraction is induced by myogenic, neural (acetylcholine), and humoral factors.
• Smooth muscle contracts when stretched.
• Relaxation is neurally mediated by vagal efferent fibers stimulating enteric inhibitory neurons (EIN).
• EIN release vasoactive intestinal peptide (VIP) and nitric oxide (NO) to relax smooth muscle.

Esophageal Coordination and Receptive Relaxation

• Vagus nerve coordinates esophageal contractions with somatic and visceral motor nerves.
• Vago-vagal reflexes are triggered by esophageal body stretching.
• Receptive relaxation of the stomach allows the bolus to enter and complete swallowing.

Disorders of Chewing & Swallowing

• Sphincter disorders: Achalasia (LES failure to relax), Incompetent LOS ("heartburn").
• Nerve Damage: Cranial nerves V, IX, X affected by conditions like poliomyelitis or encephalitis.
• Muscle dysfunction: Myasthenia gravis, muscle dystrophy, diffuse esophageal spasm.
• Glottis remains open: Drug-induced.