Thirteen Flashcards
What are some functions of mastication? Is it voluntary or reflex? How is the chewing cycle initiated? Describe the chewing cycle.
Liquids are swallowed immediately but solids are usually reduced in size and mixed with
saliva by chewing (mastication) before initiation of swallowing. Chewing is not essential, but it
facilitates the digestive process. Functions of mastication include:
Stimulation of saliva
Lubrication of food for swallowing
Reduction in food particle size (this increases the surface area available for enzyme
Chewing is both a voluntary and an involuntary (reflex) activity. Centers in the hindbrain
send fibers along the trigeminal nerve to supply the muscles of mastication. After ingestion of a
food bolus, the mouth is closed and the pressure of food against the tongue, teeth, gums, and
hard palate stimulates receptors, which initiate the chewing cycle:
When the mouth opens, stretch receptors in the jaw closing muscles (masseter, medial
pterygoid, and temporalis muscles) reflexly contract to close the mouth. When the mouth
closes, food comes into contact with buccal receptors eliciting relaxation of the jaw
closing muscles and contractions of the jaw opening muscles (lateral pterygoid, digastric,
and other muscles) causing the mouth to open. When the jaw drops, the stretch reflex
causes the cycle to be repeated again. The muscle spindles coordinate the chewing cycle,
adjusting tension of the jaw closing muscles according to the consistency of food (hard
versus soft and particle size). The cycle ends as the consistency of food is perceived as
ready for swallowing.
action)
What is swallowing? How often does it occur? what are its functions? What are the 3 phases?
Deglutition or swallowing is an orderly process that transports saliva or ingested material
from the mouth to the stomach. Swallowing occurs once a minute (to clear saliva) in awake
subjects, ceases during sleep and may occur up to 1,000 times per day. A small amount of air is
swallowed with each bolus of liquid or food. Functions of swallowing include:
Transportation of food, liquid (including saliva) from mouth to stomach
Protection from laryngeal aspiration
Clearance of acid from the esophagus
The process of swallowing in traditionally divided into three phases: Oral, pharyngeal and
esophageal.
Describe the 3 phases of swallowing.
- The process of swallowing begins with the voluntary process of tongue elevation. The
tongue pushes the bolus posteriorly making contact first with the hard and then with the soft
palate. This forces the bolus into the pharynx, stimulating tactile receptors of the soft palate,
base of the tongue and pharynx that send afferent impulses via V, IX and X to the
swallowing center in the medulla. This stimulates the swallowing reflex.
- Closure of the Nasopharynx The nasopharynx is closed by the soft palate and superior
pharyngeal constrictor.
- Upper Esophageal Sphincter Relaxation (UES) The UES relaxes as a result of inhibition of
its neural impulses (receptive relaxation).
- Closure and Protection of Airway Reflux of food bolus into the airway is prevented through:
a) reflex inhibition of respiration; b) closure of the glottis (lumen of the larynx), c) elevation
and anterior displacement of the larynx; and d) deflection of the bolus away from the
laryngeal vestibule by the glottis as it tilts upward.
- Pharyngeal Peristalsis Sequential contractions of the pharyngeal constrictors clear the
pharynx and propel the bolus into the esophagus. The UES then relaxes to prevent reflux of
the bolus into the hypopharynx.
Esophageal Phase
After the bolus passes through the UES, this sphincter reflexly contracts. A peristaltic wave then transports the bolus through the esophagus.
Where is the swallowing center? What are its inputs? Outputs?
The swallowing center regulates swallowing. In the medulla, the nucleus tractus solitarus (NTS)
and ventromedial reticular formation (VMRF) receive input from afferent fibers and cerebral
higher midbrain fibers. It connects with the respiratory and vomiting center. The swallowing
center is more precisely two half centers in the medulla and each half center receives afferents
from the ipsilateral side and its efferent output controls the muscles on the same side. During
swallowing respiration is inhibited centrally.
The major peripheral sensory inputs to the swallowing center are:
- The maxillary and lingual branches of the trigeminal nerve (V)
- The glossopharyngeal nerve (IX)
- The superior laryngeal branch of the vagus nerve (X).
The motor output of the swallowing center is mediated via the motor branches of the trigeminal V, facial VII, glossopharyngeal IX, vagus X, and hypoglossal nerves XII.
Describe the efferent innervation of the esophagus.
In the proximal esophagus, each striated muscle fiber is stimulated by a single nerve fiber
via a motor endplate as is true in all other striated muscle. Relaxation is achieved by cessation of
neural impulses to the muscle. The smooth muscle portion of the esophagus has a dual
interacting innervation. The swallowing center sends impulses via the vagus nerve, and the
smooth muscle of the esophagus has local reflex arcs that function through the myenteric plexus.
Peristalsis can occur by this intrinsic neural system in the absence of CNS input. In the smooth
muscle, there are no direct neuromuscular junctions. Stimulation is by the cholinergic
postganglionic excitatory nerves. Relaxation is achieved predominantly through the actions of
nitric oxide and VIP.
What are the parts of the esophagus? What are its functions?
The esophagus is a hollow, tubular organ measuring 25 cm in length with the primary
goal of remaining emptying. It extends from the oropharynx to the stomach. Components of the
esophagus include the upper esophageal sphincter, the esophageal body and the lower
esophageal sphincter. At rest the esophagus is closed above and below by the upper and lower
esophageal sphincters respectively.
The esophagus has two primary functions:
Transport food from the pharynx into the stomach
Prevent reflux of gastric contents
Describe the UES. What are its parts? Functions?
The UES separates the oropharyngeal cavity from the esophagus. This sphincter
normally remains closed except with deglutition and belching. This prevents air from entering
the gastrointestinal tract and prevents the reflux of gastric materials from entering the airway.
The cricopharyngeus muscle is generally thought to be responsible for the high pressure zone of
the UES. In addition to the UES, the resting tone of this sphincter is also generated with
contributions from the inferior pharyngeal constrictor muscle and the musculo-cartilaginous
structures of the lower hypopharynx.
The UES is recognized manometrically as a high pressure zone and produces a
distinctive pattern with swallowing. The UES is 2.5 – 4.5 cm in length. The resting pressure of
the UES is maintained by cervical sympathetic outflow through the pharyngeal plexus nerve.
The superior laryngeal nerve controls swallowing and sphincter relaxation.
Describe primary peristalsis and secondary peristalsis. What are the layers of the esophagus? How do they relate to function? What is deglutitive inhibition?
The upper 1/3 of the esophagus is comprised of skeletal muscle and the lower 2/3 of
smooth muscle. These muscles contract in a coordinated fashion to allow the passage of a bolus
from the mouth to the stomach. When this occurs in response to food or liquid it is call primary
peristalsis. The UES opens and the bolus is propagated by sequential contractions into the
stomach. The lower esophageal sphincter relaxes allowing bolus passage. On occasion, acid or
gastric contents reflux back up the esophagus. When this occurs, secondary peristalsis removes
the material through the generation of propagated muscle contractions unassociated with the act
of swallowing.
The wall of the esophageal body comprises (from inside to out): mucosa, muscularis
mucosa, submucosa, and muscularis propria. The muscularis propria is made up of two layers:
inner circular and outer longitudinal layers. The intrinsic nerves of the esophagus include the
submucosal (Meissner’s) and myenteric (Auerbach’s) plexuses. Nerves of the myenteric plexus
play a key role in esophageal peristalsis. Extrinsic nerves connect the intrinsic nerves of the
esophagus to the central nervous system. These nerves run primarily with the vagus
(parasympathetic control). Sympathetic control of the esophagus comes from the sympathetic
chain ganglia.
Deglutitive Inhibition: This phenomenon is unique to the esophagus and describes the
finding that repetitive swallowing inhibits all esophageal body activity until after the last
swallow. This is a normal occurrence with drinking liquids and is important in allowing only a
single peristaltic wave to progress along the esophagus during multiple swallows.
Describe the innervation that leads to peristaltic esophageal activity.
Esophageal peristaltic activity is controlled by central mechanisms (swallowing center)
and the intrinsic nerves of the esophagus. During primary peristalsis, vagal motor neurons are
sequentially activated in a manner that results in progressively distal activation of the esophageal
striated muscle. The initiation of this process is under voluntary control. The vagus is also involved in the activation of the smooth muscle portion of the esophagus. The peristaltic activity
of the smooth muscle of the esophagus is due to the intrinsic activity of the myenteric plexus.
Lower Esophageal Sphincter
Describe the LES. What controls it? When does reflux occur? What might stimulate this? What might prevent it?
The lower esophageal sphincter is a specialized segment of smooth muscle in the distal
esophagus. The LES is identified manometrically as a high pressure zone 2 – 4 cm in length and
is located at the level of the diaphragm. The LES is not a distinct anatomical entity. The LES is
tonically contracted at rest. The two main functions of the LES include relaxation in response to
swallowing and the prevention of the reflux of gastric contents. A portion of the LES resides in
the thorax and a portion resides in the abdomen. At rest, LES pressure is higher than gastric
pressure. With swallowing, the LES relaxes immediately and remains relaxed until the
peristaltic wave has passed. LES tone is maintained by a myogenic process. The intrinsic
nervous system is responsible for LES relaxation during peristalsis. The synaptic transmitters
involved in this relaxation process are thought to be VIP and nitric oxide.
Reflux occurs when gastric (intra-abdominal) pressure exceeds LES pressure. This occurs most
commonly when LES pressure is less than 10 mm Hg. Factors that decrease LES pressure
include: progesterone, fatty food, chocolate, alcohol, tobacco, caffeine, peppermint, theophylline
and anticholinergic agents. Cholinergic medications, gastrin and certain prokinetics such as
metoclopramide increase LES pressure.
Describe the motor pattern that occurs in the stomach and SI in a fasting state.
The motor functions of the stomach and small bowel differ greatly between the fasting
and postprandial periods. During fasting, cyclical motor events sweep through the stomach and
small bowel and are associated with similar cyclical secretion for the biliary tract and pancreas.
This cyclical motor activity is called the interdigestive migrating motor complex (MMC). The
MMC consists of a phase I of quiescence, phase II of intermittent pressure activity, and phase III
regular pressure activity at maximal frequency (3 per minute antrum, 12 per minute in the small
bowel) sweeping through the gut in an orderly fashion like a housekeeper, transporting
nondigestible residue, products of digestion, bacteria and epithelial debris towards the colon for
subsequent excretion.
Describe the motor pattern that exists in the SI and stomach in the fed state.
Postprandially, this cyclical activity is abolished and replaced by a fed pattern. This
response varies depending on the volume, physical state (solid, liquid), and nutrient content of
the meal. The different regions of the gastrointestinal tract subserve specific functions. Gastric
fundal tonic contractions result in the emptying of liquids, antral contractions sieve and triturate
solid food and propel particles that are less than 2 mm in size from the stomach. Irregular
frequent contractions in the postprandial period serve to mix food with digestive juices in the
duodenum and jejunum and to propel it aborally. The duration of the small bowel transit is on
average about 3 hours, and the ileum is a site of temporary storage of chyme, allowing salvage of
nutrients, fluids and electrolytes, that were not absorbed upstream. Residues are finally
discharged from the ileum to the colon in bolus transfers that probably result from prolonged
propagated contractions or restablished interdigestive cyclical motor activity.
Describe the motor functions of the stomach. What stimulates stomach motor activity?
Motor functions of the stomach include accommodation of ingested nutrients, trituration
of solids and emptying of solids and liquid materials. The stomach functions as a two-
compartment system. The proximal stomach consists of the fundus and proximal body. In
response to food ingestion, this region has a receptive relaxation and an accommodation
response. This allows the stomach to increase in volume without an undo increase in pressure.
Thus, the predominant function of the proximal stomach is relaxation for the temporary storage
of nutrients. The distal stomach grinds the ingested solids with contractions occurring every 20
seconds. When the particles are about 1mm in size, they are emptied from the stomach.
Emptying occurs when there is a positive pressure gradient between the stomach and the
duodenum. This process also requires the coordination of contractions between the antrum,
pylorus and duodenum.
The vagus nerve plays an important role in the regulation of gastric motility. The vagus
provides the innervation responsible for receptive relaxation, the accommodation response and
for antral contractions. In addition to the vagal control, gastric emptying is affected by the
amount and type of calories ingested through intestinal feedback and by sympathetic inhibitory
input.
Describe the motor functions and movements of the colon. What stimulates colonic motor activity?
The proximal colon (ascending and transverse regions) stores solid residue. The
ascending colon has variable patterns of emptying: relatively linear, or constant; intermittent; or
sudden mass movements. The descending colon is mainly a conduit and the rectosigmoid
functions as a terminal reservoir leading to the call to defecate and empty under voluntary
control. Eating and emotional stress are stimulants for colonic contraction. Eating results in an
increase in colonic motility within thirty minutes following meal ingestion. Two changes are
seen, increased left colon tone, and increased colonic phasic contractions. Large amplitude
propagating contractions typically precede defecation. A decrease in these contractions results in constipation and an increase number of contractions are seen in diarrhea. Meal related increased
in colonic activity are mediated through the vagus and spinal cord. Cholecystokinin (CCK)
stimulates colonic motility. CCK is normally increased postprandially and may be responsible
for part of the gastrocolonic response.
Describe the steps that lead to defecation. Describe what things maintain continence.
Defecation results from a well coordinated series of motor responses. The anorectal
angle is maintained in an acute position by the puborectalis muscle sling, pulling the distal
rectum forward. For defecation to occur, this sling relaxes, thereby opening the anorectal angle
to a straight conduit; the anal sphincters are inhibited by parasympathetic (S2,3,4) input and
intracolonicpressure increases predominantly by a rise in intraabdominal pressure associated
with straining. In contrast, continence is maintained by contraction of the puborectalis (pudendal
nerve), contraction of the internal sphincter (sympathetic lumbar colonic nerves) and contraction
of the external sphincter (parasympathetic pudendal nerve).
