Introduction to Gastrointestinal system

Introduction to Gastrointestinal System (Digestive system)

Definition and Overview of Gastrointestinal System

• What is digestive system?

What are the organs of digestive system?

Introduction to Gastrointestinal system

Digestive system consists of the digestive tract(alimentary canal) which is a tube extending from the mouth, pass through the thorax, abdomen and pelvis and ends at the anus and its associated accessory organs

Organs of the digestive tract include:

• The mouth or oral cavity, which has salivary glands and tonsils as accessory organs
• The pharynx, or throat , with tubular mucous glands
• The oesophagus, with tubular mucous glands

The stomach, which contains many tube like glands The small intestine, consisting of the duodenum, jejunum and ileum, with the liver, Gallbladder and pancreas as major accessory organs The large intestine, including the cecum, colon, rectum and anal canal, with mucus glands The anus

Introduction to Gastrointestinal system

The digestive system also consists of accessory organs which include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas Teeth aid in the physical breakdown of food, and the tongue assists in chewing and swallowing.

The other accessory digestive organs produce or store secretions that flow into the GI tract through ducts to aid in the chemical breakdown of food

The gastrointestinal tract is made of four layers of tissues:

• Mucosa is the innermost layer made up of three layers
• The inner epithelium
• Lamina propria

Muscularis mucosae

Submucosa this layer is composed of connective tissue, it contains numerous small glands, blood vessels, and parasympathetic nerves that form the submucosal plexus (Meissner plexus)

Muscular is thick layer of smooth muscle tissue which consists of inner circular layer and outer longitudinal layer

Between the two layers of muscles there is a nervous plexus called myenteric plexus (Auerbach plexus) Serosa is the outermost layer of made up of loose connective tissue (visceral peritoneum) Layers of the GI tract have various modifications to enable it to perform various functions.

Functions of the Gastrointestinal System

What are the functions of the digestive tract?

Overall, the digestive system performs six basic processes:

Ingestion: This process involves taking foods and liquids into the mouth (eating) Secretion: Each day, cells within the walls of the GI tract and accessory digestive organs secrete a total of about 7 litres of water, acid, buffers, and enzymes into the lumen (interior space) of the tract Mixing and propulsion: Alternating contractions and relaxations of smooth muscle in the walls of the GI tract mix food and secretions and propel them toward the anus This capability of the GI tract to mix and move material along its length is called motility Digestion: Mechanical and chemical processes break down ingested food into small molecules

Introduction to Gastrointestinal system

In mechanical digestion the teeth cut and grind food before it is swallowed, and then smooth muscles of the stomach and small intestine churn the food As a result, food molecules become dissolved and thoroughly mixed with digestive enzymes In chemical digestion the large carbohydrate, lipid, protein, and nucleic acid molecules in food are split into smaller molecules by hydrolysis Digestive enzymes produced by the salivary glands, tongue (lingual lipase), stomach, pancreas, and small intestine catalyse these catabolic reactions. A few substances in food(vitamins, ions, cholesterol, and water) can be absorbed without chemical digestion Absorption. The entrance of ingested and secreted fluids, ions, and the products of digestion into the epithelial cells lining the lumen of the GI tract is called absorption.

Defecation. Wastes, indigestible substances, bacteria, cells sloughed from the lining of the GI tract, and digested materials that were not absorbed in their journey through the digestive tract leave the body through the anus in a process called defecation and the materials in form of feaces

Structure of the Oral Cavity

The oral cavity or mouth is that part of the digestive tract bounded by the lips anteriorly, the fauces throat; opening into the pharynx posteriorly, cheeks laterally, the palate superiorly, and a muscular floor inferiorly The oral cavity is lined throughout with mucus membrane consisting of stratified squamous epithelium containing small mucus secreting glands.

• The oral cavity is divided into two regions:
• o The Vestibule; part of the mouth between Gums and Cheeks

o The oral cavity proper; which lies medial to the alveolar processes (gums)

Muscles of oral cavity

Orbicularis oris (kiss muscle)

Structure of the Oral Cavity (The Buccal Cavity)

• Lips:
• Are muscular structure formed mostly by the orbicularis oris muscles
• They are covered externally by skin and internally
• The junction between skin and mucous membrane is highly sensitive
• When lips are closed, line of contact is oral fissure
• Cheeks

The cheeks are formed in large part by buccinators muscles covered They form the lateral boundaries of the oral cavity, they are continuous with lips and lined by mucous membrane They contain mucus secreting glands which are placed between mucous membrane and buccinator muscle

Their ducts open opposite the last molar teeth

Hard and soft palates; forms the roof of mouth consists of two parts:

o The anterior bony part the hard palate consists of portions of four bones, two maxillae and two palatine bones o The posterior non bony part the soft palate forms partition between the mouth and nasopharynx and is made of muscles arranged in an arch Suspended from midpoint of the posterior border of the arch is the uvula The opening from the mouth into the oropharynx is called fauces

Teeth

The teeth are hard conical structures set in the dental alveoli (tooth sockets) of the upper and lower jaws and are used in mastication (chewing) and assisting in articulation These are organs of mastication

• Humans have two generations of teeth:
• o The deciduous (primary) dentition
• o The permanent (secondary) dentition

Deciduous Teeth

• They are temporary teeth, twenty in number, and ten on each jaw
• They start to emerge at 6 months after birth

They should all be present at 24-36 months

Permanent Teeth

They are thirty two in number, they begin to replace deciduous teeth at 6 years of age, and become complete when the third molars erupt at 18 – 24 years of age Of these 32 teeth there are 8 incisors, 4 canines, 8 Premolars and 12 Molars Incisors and Canines are for cutting and Premolars and molars for grinding of food o There are two incisors, a central and a lateral, in each half jaw or quadrant o Behind each lateral incisor is a canine tooth with a single cusp o Distal to the canines are two premolars, each with a buccal and lingual cusp o Posterior to the premolars are three molars whose size decreases distally

A typical tooth has the following parts:

Crown; exposed portion of a tooth, covered by enamel; ideally suited to withstand abrasion during mastication Neck; narrow portion that joins the crown to the root; surrounded Roots fits into the socket of the alveolar process and is suspended by fibrous periodontal membrane

In the centre of the tooth there is pulp cavity containing blood vessels, lymph vessels and nerves Surrounding the pulp cavity there is hard ivory like substance called dentine Outside the dentine of the crown is the thin layer of the very hard substance called enamel The root of the tooth on the other hand is covered by substance resembling the bone called the cement which secures the tooth in its socket

NERVE SUPPLY OF TOOTH

Structure and Functions of the Tongue

• The tongue is a highly muscular organ of deglutition, taste and speech
• It is made up of skeletal muscles covered by a mucous membrane

It has the root, tip and the body

It is attached by its base to the hyoid bone and by the lingual frenulum to the floor of the mouth There are two types of muscles forming the tongue;

o Intrinsic muscles; they originate and insert in the tongue itself An example of the intrinsic muscles includes Genioglossus They are responsible for changing the shape of the tongue such as flattening and elevating the tongue during drinking and swallowing o Extrinsic muscles are those muscles that insert into the tongue but originate from other structures such as hyoid bone . An example of the muscles includes hyoglossus They are responsible for protruding and retract the tongue, move it from side to side and change its shape

The dorsal mucosa is covered by numerous papillae; tiny projections, some of which bear taste buds

There are three types of papillae; vallate, fungiform and filariform Circumvallate papillae, arranged in an inverted V shape at the base of the tongue are the largest papillae Fungiform papillae; are situated mainly at the tip and the edge of the tongue Filiform papillae; are the smallest of all three numerous on the surface of the anterior two-thirds of the tongue

There are four basic taste sensations

• o Sweetness; apex (tip)
• o Saltiness; lateral margins
• o Sourness; lateral and posterior parts of the tongue

o Bitterness; posterior part of the tongue Although it is commonly stated that particular areas of the tongue are specialized to detect these different tastes, evidence indicates that all areas of the tongue are responsive to all taste stimuli Each afferent nerve fibre is connected to widely separated taste buds and may respond to several different chemical stimuli

TONGUE INNERVATION

TASTE AREAS OF TONGUE

Some respond to all four classic categories, others to fewer or only one

Blood supply of the tongue is by lingual branch of the external carotid artery and venous drainage is by lingual veins, which joins the internal jugular vein Nerve innervations of the tongue is through:

• o The hypoglossal nerve ( CN XII)
• o The lingual branch of the mandibular nerves

o The facial and glossopharyngeal nerves ( VII and IX respectively ) the nerves of taste o The parasympathetic innervation of the various glands of the tongue is from the chorda tympani branch of the facial nerve o The postganglionic sympathetic supply to lingual glands and vessels arises from the carotid plexus

Salivary Glands and its Constituents

Salivary glands are compound, tubulo- alveolar exocrine glands, whose ducts open into the oral cavity They secrete saliva, a fluid which lubricates food to assist deglutition, moistens the buccal mucosa, which is important for speech, and provides an aqueous solvent necessary for taste and a fluid seal for sucking and suckling They also secrete digestive enzymes, e.g. salivary amylase and antimicrobial agents e.g.

IgA, lysozyme and lacto ferrin, into saliva Conditions where there is a significant decrease in the production of saliva (xerostomia) may result in periodontal inflammation and dental caries Three pairs of compound tubulo-alveolar glands secrete approximately 1 litre of saliva each day.

The major salivary glands are the paired parotid, submandibular and sublingual glands

In addition, there are numerous minor salivary glands scattered throughout the oral mucosa and submucosa Parotid glands – largest of the paired salivary glands produce watery saliva containing enzymes o Submandibular glands – compound glands that contain enzymes and mucous producing element o Sublingual glands – smallest of the salivary glands produce a mucous type of saliva o Buccal glands are important for hygiene and comfort of oral tissue In the unstimulated state, the parotid gland contributes 20%, the submandibular gland 65%, and the sublingual and minor salivary glands 15% of the daily output of saliva When stimulated, the parotid contribution rises to 50%

Mechanism of Swallowing

• Swallowing: A mechanism used to accomplish primary function of digestive system
• Deglutition: A process for swallowing

It is a complex process requiring coordinated and rapid movement Swallowing transports the chewed food into the oesophagus, passing through the oropharynx and hypopharynx The mechanism for swallowing is coordinated by the swallowing center in the medulla oblongata and pons The reflex is initiated by touch receptors in the pharynx as the bolus of food is pushed to the back of the mouth This occurs in three stages after mastication is completed and the bolus has been formed It is initiated voluntarily but completed by a reflex action

Oral stage (mouth to oropharynx), voluntarily controlled

Formation of food bolus in the middle of the tongue; The mouth is closed, tongue presses bolus against the palate, and the voluntary muscles of the tongue and cheeks push the bolus backwards into the oropharynx Pharyngeal stage (oropharynx to oesophagus) involuntary contraction of muscles of the pharynx Propels the bolus down into the oesophagus It begins with the elevation of the soft palate, which closes the passage between the nasopharynx and oropharynx This is controlled by the medulla and lower pons in the brain stem The presence of the bolus in the pharynx stimulates a wave of peristalsis that propels the bolus through the oesophagus to the stomach

Movement; contractions and gravity moves bolus through oesophagus and into stomach

So the function of the oesophagus is to convey food from the pharynx to the stomach The chewed food is pushed down the oesophagus to the stomach through peristaltic contraction of these muscles It takes only about seven seconds for food to pass through the oesophagus and no digestion takes place Peristalsis; wave like ripple of the muscle layer of a hollow organ progressive motility that produces forward movement of matter along the GI tract

Key points

The gastrointestinal system is the portal through which nutritive substances, vitamins, minerals, and fluids enter the body The gastrointestinal tract is made of four layers of tissues: the mucosa, submucosa, muscular and serosa The tongue is a highly muscular organ of deglutition, taste and speech There are four basic taste sensation; sweetness, saltiness, sourness and bitterness Teeth are organs of mastication and humans have two generations of teeth; the deciduous and permanent The oesophagus has 4 constrictions where adjacent structure produce impression

evaluation

• What is gastrointestinal system?
• What are the structures of the gastrointestinal tract?
• What are the functions of digestive system?
• What are the functions of salivary glands?

What are the structure of the tongue?

assignment

Describe structure and function of the tongue

CRANIAL NERVES

Circulatory system