1) The gastrointestinal tract is approximately 9 meters long and runs from the mouth to the anus, mechanically and chemically breaking down food. 2) The mouth contains taste buds that detect the five basic tastes and contains salivary glands that produce saliva to moisten food for swallowing. 3) Chewing and swallowing propel food through the esophagus and into the stomach through peristalsis, where further digestion will occur.

1. Gastro intestinal tract –part I
Dr. Radhakrishna G Pillai
Department of Life Sciences
University of Calicut

2. Introduction
• The gastrointestinal (GI) tract,
also called the gut
• Muscular tube approximately
9m (30 feet) in length
• Running from the mouth to
the anus
• Mechanically and chemically
break down foods
• Into simple components that
can be absorbed by the body

3. GI Tract
• It produces various enzymes that help in the
chemical digestion of food
• It produces various hormones that play a role in
regulating digestion, appetite and satiety
• Different parts with different functions
• Home to myriad micro-organisms (mostly
bacteria): many are essential to health
• These form a large part of the human
microbiome

4. Mouth
• Opening of the GI tract : receives food, tastes it and prepares it for swallowing
• The average volume of the adult mouth is 72 ml in men and …. ml in women
• 55
• The mouth is lined by mucous membranes and consists of two major regions:
– Vestibule – the space between the inner surface of the cheeks/lips and the teeth
– Oral cavity proper – the space inside the teeth, where food is chewed and mixed
with saliva before being swallowed.

5. Tongue and sense of taste
• The tongue is a muscular structure
• Approx 10cm long and weighs 60-70g
• It is anchored posteriorly
to the hyoid bone via small
muscles and
• Attached to the floor of
the mouth by a thin flap of
tissue, the lingual frenulum

6. Tongue and sense of taste
• Tiny extensions called papillae make its surface
rough textured
• Papillae help to grip food as it is moved
around the mouth and mixed with saliva
• The papillae contain taste buds
Taste buds contain
gustatory cells that act as
taste receptors

7. Taste buds
• Food molecules dissolve in saliva and enter
taste buds
• Taste receptors are activated
Nerve impulses are relayed
to the gustatory cortex of
the brain, where they are
perceived as distinct tastes

8. Sensing tastes
• In humans, there are five primary tastes
• Each taste has only one corresponding
type of receptor
• Each receptor is specific to its stimulus
• The five tastes are sensed through different
mechanisms based on the molecular composition
of the tastant
• A salty substance (containing NaCl) provides the
sodium ions (Na+) that enter the taste neurons and
excite them directly

9. Sensing tastes
• Map of the tongue showing the distribution of
different taste receptor types are misleading –
distributed across
• Sour molecules are acids
• Binding of an acid or other sour-tasting molecule
triggers a change in the ion channel and these
increase hydrogen ion (H+) concentrations in the
taste neurons, thus depolarizing them
• Sweet, bitter, and umami tastants require a G-
protein coupled receptor

10. • These molecules bind to their respective
receptors, thereby exciting the specialized
neurons associated with them

11. Taste and age
• Tasting abilities change with age
• In humans, the senses decline dramatically by age 50
and continue to decline
• A child may find a food to be too spicy, whereas an
elderly person may find the same food to be bland
and unappetizing

12. Olfaction and taste
• The sense of taste is intimately linked to
olfaction (sense of smell)
• Food cannot be tasted properly if the nose is
blocked
• As soon as the olfactory apparatus detects the
smell of food, the parasympathetic nervous
system is activated
• Salivary glands stimulated - release extra saliva,
preparing the mouth to receive the food

13. Retronasal and ortho nasal olfaction
• Retronasal olfaction -perception of odours
emanating from the oral cavity during eating and
drinking
• The retronasal olfactory pathway, which
contributes to the flavor of foods or drinks, is
commonly associated with the sense of taste
• Also named as “gustatory olfaction”
• Orthonasal olfaction- during sniffing- through
olfactory epithelium of nose

14. Retronasal and ortho nasal olfaction

15. Gustatory pathway
• Taste neurons project from taste cells in the
tongue, esophagus, and palate to the medulla
in the brainstem
• From the medulla, taste signals travel to the
thalamus
• Then to the primary gustatory cortex
• Information from different regions of the
tongue is segregated in the medulla, thalamus,
and cortex

17. Dentition
• Adults normally have 32 teeth, which are evenly
distributed between the upper and lower jaws
• The teeth comprise:
• Eight incisors – flat, chisel-like teeth that bite into food
and break it into small manageable pieces
• Four canines – sharp, fang-like teeth just outside the
incisors that grip and tear food
• Eight premolars – small teeth between the canines and
molars that grind up and cut into food
• Eight molars – square teeth in the rear of the mouth
that grind food to increase its surface area

18. Dentition

19. General structure of teeth
• Their central portion, the
pulp cavity, is composed of
living tissue – mostly blood
vessels and sensory nerve
endings
• Teeth are susceptible to
erosion as a result of wear
and tear as well as
exposure to acidic foods
Teeth are mainly composed of a bony material
(dentine) covered by a layer of white enamel

20. Mastication
• Carried out by the teeth
• The tongue moves food towards the premolars
and molars
• If teeth are missing, the gums can also chew
food- requires much more effort
• Chewing makes food easier to swallow
• Increases the surface area that is available to
the digestive enzymes-enhance chemical
digestion

21. Salivary glands
• The mouth has three pairs of salivary glands
• Sublingual glands – located below the tongue
• Submandibular glands – located below the
mandible
• Parotid glands –
located to the side of
the earlobes
• SGs innervated by the
parasympathetic branch
of the autonomic NS

22. Saliva
• The salivary glands continuously produce small amounts of
saliva to keep the mouth moist (basal secretion)
• In a typical 24-hour period 0.5-1.5L of saliva is secreted
• Eating induces a significant increase in saliva production
(induced secretion)
• The submandibular glands do most of the basal secretion
• The parotid glands are the major contributors to secretion
induced by the presence of food
• The sublingual glands only produce around 5% of both
basal and induced saliva secretion
• The volume of saliva that is produced can be increased or
decreased according to the nature of the food

23. Swallowing
• Towards the end of mastication
• The tongue and roof of the mouth gather and
shape the chewed food into a pellet called bolus
• Moisture and mucus bind and coat the bolus
• Pushed towards the back of the mouth and into
the oropharynx
• Bolus in the posterior pharyngeal wall trigger the
swallowing reflex (deglutition)

24. Muscular contractions in swallowing
• Involves precisely coordinated muscular
contractions
• The pharynx walls contract around the bolus
• The epiglottis closes over the airway, preventing
food from entering the trachea and bronchial
tree
• The bolus is then funnelled, through the upper
oesophageal sphincter, into the oesophagus

25. Movement of food through oesophagus

26. • Voluntary nervous system help to swallow
• Involuntary nervous system takes over
swallowing once the liquid or food enters the top
of your esophagus
• Stretching from food stimulates the smooth
muscles on the walls of esophagus
• This stimulates network of nerves inside muscles
known as intrinsic nerves, start a coordinated
muscle action called peristalsis
Peristalsis

27. Peristalsis
• Peristalsis squeezes esophageal
muscles from top to bottom
• This pushes food and liquid along
• Sphincters act like one-way valves
• They relax to let food and liquid
pass into the stomach and then
tightens to prevent backflow,
called reflux
• Food enters stomach