Describe the tissue composition and the general function of each of the four layers of the alimentary canal

Submucosa is external to the mucosa. It's moderately dense connective tissue containing blood & lymphatic vessels, lymphoid follicles, & nerve fibers.

The rich supply of of elastic fibers allow the stomach to recoil after holding a meal, and the vascular network supplies surrounding tissues of the GI wall.

Describe the tissue composition and the general function of each of the four layers of the alimentary canal

Muscularis externa is deep to the submucosa & it's responsible for segmentation & peristalsis. It has an inner circular layer & an outer longitudinal layer of smooth muscle cells.

Sphincters: areas along tract where circular layer thickens and acts valves to prevent back flow; control food passage from organ to organ

Describe the tissue composition and the general function of each of the four layers of the alimentary canal

Serosa is the protective outermost layer of the intraperitoneal organs (visceral peritoneum). It's formed of areolar connective tissue covered w/ mesothelium (single layer of squamous epithelial cells).

Adventitia: ordinary fibrous connective tissue that binds the esophagus to surrounding structures .

Retroperitoneal organs have serosa (side facing peritoneal cavity) & adventitia (side abutting dorsal body wall)

Explain the dental formula

It is a shorthand way of indicating the numbers & relative positions of different types of teeth in the mouth. It is written as a ration (uppers over lowers for 1/2 of the mouth). The dental formula is multiplied by 2 since the other half of the mouth mirrors the first half.

Primary Dentition: 2 incisors (I), one canine (C), & 2 molars (M) on each side of the jaw.

2I, 1C, 2M (UJ) / 2I, 1C, 2M (LJ) x 2 = 20 teeth

Permanent Dentition: 2 incisors (I), one canine (C), 2 premolars (PM) & 3 molars (M) on each side of the jaw.

2I, 1C, 2PM, 3M / 2I, 1C, 2PM, 3M x 2 = 32 teeth

Describe the function of saliva & explain how saliva is regulated

Function: cleans the mouth, dissolves food chemicals (for tasting) & moistens food & aids in compacting it into a bolus, and contains enzymes that begin the chemical breakdown of starchy food.

Saliva is produced by extrinsic salivary glands that lie outside the oral cavity & empty their secretions into it. Output is augmented by intrinsic salivary glands (buccal glands) scattered throughout the oral cavity mucosa.

Regulation: The intrinsic salivary glands secrete saliva continuously to keep the mouth moist. When food enters the mouth, the extrinsic glands are activated & copious amounts of saliva pour out (1000-1500 ml daily).

Salivation is controlled by parasympathetic division of ANS. Chemoreceptros & mechanoreceptors in the mouth send signals to the salivary nuclei in the pons & medulla --> the PNSactivity increases & impulses sent via motor fibers in the facial (VII) & glossopharyngeal (IX) nerves trigger an increase output of watery, enzyme rich saliva.

The sympathetic division (T1-T3) cause the release of a thick, mucin-rich saliva, & can inhibit saliva release.

Describe the mechanisms of chewing

- "Mastication" or the mechanical breakdown of food.

- The cheeks & closed lips hold food between the teeth, the tongue mixes food w/ saliva to soften it, & teeth cut/grind food into smaller morsels.

-Partly voluntary: by putting food into our mouths & contract the muscles that close our jaw

-Partly reflexive: the pattern of jaw movements are controlled mainly by stretch reflexes & in response to pressure inputs from receptors in the cheeks, gums, and tongue.

Describe the mechanisms of swallowing

- "deglutination" is a complicated process involving coordination of 22 separate muscle groups in two phases

- food is first compacted by the tongue into a bolus then swallowed.

1. Buccal Phase: occurs in the mouth & is voluntary. We place the tip of the tongue against the hard palate, then contract the tongue to force the bolus into the oropharynx. Food enters the pharynx & stimulates tactile receptors & passes into the involuntary reflex activity.
2. Involuntary Pharyngeal-esophageal phase: is triggered by saliva or food reaching the receptors in the posterior pharynx. It is controlled by the swallowing center located in the medulla & lower pons. Motor impulses from this center are transmitted via various cranial nerves (vagus nerve) to the muscles of the pharynx & esophagus. When food enters the esophagus, respiration momentarily stops & all routes are blocked except for the digestive tract.

• tongue blocks off mouth
• soft palate rises to close off nasopharynx
• larynx rises so epiglottis covers opening into respiratory passageway & the upper esophageal sphincter relaxes.

Food moves along through the pharynx & into the esophagus via peristaltic contractions. Just before the peristaltic wave reaches the end of the esophagus, the gastroesophageal sphincter relaxes reflexively to allow food to enter the stomach. After food entry, the sphincter closes, preventing regurgitation.

Identify the structural modifications of the wall of the stomach that enhance the digestive process

Cardiac region: small & surrounds the cardiac orifice through which food enters the stomach from the esophagus.

Fundus: a dome-shaped part, tucked beneath the diaphragm, that bulges superolaterally to the cardia.

Body: the mid portion of the stomach, is continuos inferiorly w/ the funnel-shaped pyloric region.

The stomach wall contains the 4 tunics typical of most of the alimentary canal but its muscularis & mucosa are circular & longitudinal layers of smooth muscle, the muscularis externa has an innermost smooth muscle layer that runs obliquely.

• oblique layer allows stomach to move churn, mix, & pummel food into smaller fragments.
• lining epithelium of stomach mucosa is simple columnar epithelium composed of goblet cells that produce a protective layer of alkaline mucus. Contains gastric pits, which lead into gastric glands that produce gastric juice
• Rugae: large, longitudinal folds found in stomach when empty

Pyloric region: it is continuous with the duodenum. Its sphincter empties food into the small intestine

Identify the structural modifications of the wall of the small intestine that enhance the digestive process

Modifications for absorption: length provides a large surface area. Its wall has structural modifications:

• plicae circulares: "circular folds"; are deep permanent folds of the mucosa & submucosa. Force chyme to spiral through the lumen, slowing movement & allowing time for nutrient absorption
• villi: fingerlike projections of the mucosa. Epithelial cells are absorptive columnar cells. Lacteals are found in their core & serve lymph capillaries,
• microvilli: tiny projections of plasma membrane of the absorptive cells of the mucosa. Contain brush border cells that contain enzymes, which complete carbohydrate & protein digestion in small intestine.

-All amplify absorptive surface area

- most absorption occurs in the proximal part of the small intestine

- Peyer's patches are found toward the end of the SI

- the SI is composed of simple columnar absorptive cells bound by tight junctions & endowed with microvilli.

- Contains many goblet cells & T cells called Intraepithelial lymphocytes, that release cytokines that kill infected target cells upon encounter.

-Villi contain pits in between that lead to tubular Intestinal crypts that are lined by epithelial cells that secrete intestinal juice (carrier fluid for absorptive nutrients from chyme)

-Paneth Cells: deep in crypts & fortify the SI's defenses by releasing antimicrobial agents (defenses & lysozyme)

Describe the composition of gastric juice.

Composition: corrosive; about 100000 times more acidic

Secretory Cell type: gastic pits (i.e. goblet cells) -> gastric glands (produce gastric juice)

Gastric juice substances:

-Pepsin: enzyme; digests proteins
-Hydrochloric acid: Denatures proteins; decreases pH (for optimum pepsin function
-Intrinsic factor: protects and allows later absorption of vitamin B12
-Mucus: lubricates chyme; protects stomach lining
-Water: Dilutes food and other substances; facilitates mixing

Mucous neck cells: found in neck of stomach; secrete a thin mucous

Parietal Cells: found in the middle region of the glands, scattered among chief cells secrete Hydrochloric Acid & Intrinsic factor (microvilli). Makes the stomach acidic which is necessary to activate pepsin

Chief Cells: produce pepsinogen, which is an inactive form of pepsin (protein-digesting enzyme). Are found in the basal region of gastric glands. Also secrete insignificant amounts of lipases.

Enteroendocrine Cells: release various chemical messengers into interstitial fluid of lamina propia. E.g.: histamine, seratonine somatostatin, & gastrin.

Indicate the importance of each component in stomach activity