1. mouth and tongue
2. pharynx
3. esophagus
4. stomach
5. small intestine --> (duodenum, jejunum, ileum)
6. colon
7. rectum
8. anus

1. salivary glands
2. liver
3. pancreas
4. gallbladder

1. ingestion
2. motility
3. secretion
4. digestion
5. absorption
6. elimination

1. Propulsive movements: move material from one end of the tract toward the other. Can occur in an orthograde (aboral) direction (from mouth to anus) or in retrograde (orad) direction as in the case of vomiting. 
2. Mixing movements: back and forth movements of material that result in mixing of contents with digestive juices. 

1. lubricate (mucus)
2. liquify (water)
3. buffer (bicarbonate)
4. digest (enzymes)

The breakdown of large substances into their constituents.

1. mechanical: chewing
2. chemical: enzymes

1. mucosa
2. submucosa
3. muscularis externa
4. serosa

1. epithelium (specialized cells lining the lumen of the GI tract)
2. lamina propia (connective tissue to which epithelia attach. It contains blood vessels, lymph nodes and some glands)
3. muscularis mucosae (thin layer of smooth muscle)

Circular layer (innermost part)

Longitudinal layer (outermost part)

Intrinsic innervation: formed by the Enteric Nervous System (ENS).

Extrinsic innervation: formed by the Autonomic Nervous System (ANS)

Myenteric plexus (Auerbach's plexus)

Located between the circular and longitudinal muscle layers.

Primarily concerned with motility control.

Submucosal plexus (Meissner's plexus)

Located inside the submucosal layer.

Concerned with the control of secretions from glands.

Formed by postganglionic axons coming from cells in the paravertebral and prevertebral ganglia.

Most end in the myenteric and submucosal plexuses where they act to inhibit activity. A few fibers end on cell glands and blood vessels where they inhibit secretion and produce vasoconstriction.

The activation of this system:

a) inhibits contraction in muscularis externa

b) stimulates contraction of sphincters and the muscularis mucosa. 

Supplied by preganglionic fibers from the vagus (GI tract to level of transverse colon) and pelvic nerves (descending colon to rectum). 

Fibers end on ganglion cell in the myenteric and submucosal plexuses. The axons of the ganglion cells innervate the smooth muscle and glands of the GI tract. 

Activation increases contractions and secretions

75% of vagal fibers are afferent and about 50% of fibers in sympathetic nerves to the gut are afferents. 

GI has both intrinsic and extrinsic afferent fibers 

Receptors in the GI tract include: mechanoreceptors, chemoreceptors, and nociceptors. 

Cell bodies of some of these sensory neurons lie in submucosal and myenteric plexuses (local reflex arc).

Cell bodies of the other sensory neurons are located in the dorsal root ganglia or cranial ganglia (nodose). (central reflex arcs).

Visceral nociceptive fibers of the thorax and abdomen travel to the CNS through sympathetic nerves while non-nociceptive fibers do so through parasympathetic nerves.

Are low frequency fluctuations in membrane potential found from stomach to colon. 

Different regions of GI tract have different frequencies.

They are generated by pacemakers cells (interstitial cells) located between circular and longitudinal smooth muscle of muscularis externa. Their amplitude is modulated by intrinsic and extrinsic innervation: sympathetic decreases and parasympathetic increases amplitude of slow waves.

Slow waves can elicit contractile activity:

in the stomach and small intestine, if sufficient amplitude, contraction is produced.

Relationship b/w slow waves and contractile activity not clear in colon.

In the stomach and small intestine, the frequency of slow waves represents the maximum frequency that contractions can occur.  

GI smooth muscle has a resting tension which results from slightly elevated levels of intracellular calcium.

This tone can be modified by neurotransmitters, hormones, or drugs. 

1. Enjoyment of food.
2. Reduction of particle size and break down of cellulose walls.
3. Mixing of food with saliva which: 

Control of mastication involves both, voluntary and involuntary components. 

Coordination of muscle activity during chewing is the result of complex interactions between several motor and sensory nuclei in the brain stem.

The brain stem contains a Neural Pattern Generator (masticatory center)= basic oscillatory pattern of movements during mastication.

Cortical input: initiate and modify masticatory movements.

Sensory input from oral cavity: modify chewing movements

1. Voluntary phase (Oral phase)
2. Involuntary phase (Pharyngeal phase)

The tongue moves the bolus of food upward and backward in the mouth, eventually forcing it against the pharynx.

The bolus of food in the pharynx stimulates mechanoreceptors and initiates the swallowing reflex. 

The soft palate is pulled upward closing the nasopharynx. 

Respiration is inhibited, the vocal cords are pulled together, the epiglottis covers the opening of the larynx and larynx is moved upward against the epiglottis.

Upper esophageal sphincter relaxes and a peristaltic wave of contraction begins in the superior constrictor muscles of the pharynx and forces the bolus of food into the esophagus. 

1. Conduit to get food form the mouth to the stomach.
2. Sphincters act as barriers 

Follows a general scheme of an inner circular layer and an outer longitudinal layer of muscle.

Upper circular sphincter and upper 1/3 of esophagus is composed of skeletal muscle.

Lower sphincter and distal 1/3 of esophagus is composed of smooth muscle.

Intermediate section of esophagus is mixed smooth and skeletal muscle.

1. Primary peristalsis: the peristaltic wave that began in the pharynx during swallowing continues through esophagus taking about 10 s to go from upper esophagus to stomach. Controlled mainly by the swallowing center through the vagus.
2. Secondary peristalsis: if primary wave fails to clear esophagus of food, a second wave occurs. Controlled mainly by the intrinsic nervous system.

If a second swallow occurs within 5 seconds of the first, peristalsis associated with the first bolus is inhibited until the second bolus catches up and then both move in tandem.

If a series of rapid swallows occurs, peristalsis is inhibited until the last swallow.

During quiescent (non-peristaltic) periods, the LES tone is high due to myogenic mechanisms (remainder of esophagus is flaccid); however, resting tone can be increased by both, neuronal (ACh) and hormonal (gastrin) influences.

Relaxation of LES is mediated by the vagus primarily through Nitric Oxide and perhaps Vasoactive Intestinal Peptide (VIP).

1. Achalasia: insufficient relaxation of LES to allow food into stomach.
2. GERD: in people with GERD, LES relaxations may be more prolonged or more frequently in duration allowing reflux of gastric contents into esophagus. May produce ulceration.
3. Diffuse Esophageal Spasm: prolonged painful contraction of esophagus instead of normal peristalsis.

1. Reservoir for ingested food
2. Digestion

1. outer - longitudinal
2. middle - circular (most prominent)
3. inner - oblique (present on anterior and posterior sides)

1. Parasympathetic innervation from the vagus stimulates motility and secretion.
2. Sympathetic innervation from the splanchnic inhibits motility and secretions.

1. Gastric distention
2. Intragastric pressure
3. pH
4. Pain

1. Cardia: region surrounding superior opening (LES) 
2. Fundus: superior portion of stomach above the cardia
3. Body or Corpus: main portion of stomach
4. Antrum: 2 parts

Fundus and body of stomach relax. 

It can be initiated as part of the reflex relaxation of LES induced by primary esophageal peristalsis.

It can also be induced by directly filling stomach with gas or liquid (vagovagal reflex involving stretch receptors in stomach lining).

Reflex is dependent on intact vagal innervation.

It allows accommodation of 1-2L without a significant increase in intragastric pressure.

Physical and chemical characteristics of its contents.

Inert, isotonic solutions (glucose) leave the stomach rapidly.

Solutions containing nutrients (amino acids, fats) empty more slowly reflecting feedback from receptors in the small intestine.

Solids (regular meal) will empty even slower with a half time of 1-2 hrs