DIGESTIVE SYSTEM: INTRODUCTION AND GENERAL PLAN OF ALIMENTARY CANAL

UNIT 1 – DIGESTION AND ABSORPTION OF FOOD

SL – STRUCTURE AND FUNCTION OF DIGESTIVE GLANDS[i]

NERVOUS SYSTEM

The nervous system is divided into

1.       Central nervous system (CNS)  

2.       Peripheral nervous system (PNS)

2.1.  Somatic nervous system (SoNS)

2.2. Autonomic nervous system (ANS)

2.2.1.        Sympathetic nervous system (SNS)

2.2.2.        Parasympathetic nervous system (PsNS)

1.       Central nervous system (CNS)  - it comprises of the brain and spinal cord – also called as the body’s master control unit

2.       Peripheral nervous system (PNS) – it includes all the nerves arising out and going to the central nervous system – also called the body’s link to the outside world

2.1.  Somatic nervous system (SoNS): cerebrospinal- it is a part of the peripheral nervous system, is associated with the voluntary control of body movements via skeletal muscles. The somatic nervous system consists of (afferent nerves) sensory nerves and (efferent nerves) motor nerves. The somatic nervous system controls all voluntary muscular systems within the body, and the process of voluntary (somatic) reflex arcs.[1]

2.2. Autonomic nervous system (ANS): visceral  

2.2.1.        Sympathetic nervous system (SNS) – the action of most of the organs is accelerated by the SNS

2.2.2.        Parasympathetic nervous system (PsNS) – the activity of most of the organs are inhibited by PsNS

It must by however noted that, all the nervous activities are always controlled by the central nervous system; the peripheral nervous system are merely carriers of the information.

Only a small part of the body activities are under the willful (coluntary) control of the body. The centers for the control of voluntary activities are present in the thalamus and cerebral cortex of the brain. These are therefore called as the conscious areas of the brain. Most of the organs of the body are controlled by an automatic feedback circuit in which no conscious thinking is required. Such activities are termed as involuntary and are controlled by autonomic nervous system. The centers for involuntary actions are present in the medulla oblongata, pons and midbrain.

The SNS regulates actions that require quick action and PsNS calms down the actions of SNS.

The PsNS regulates the actions that do not require quick responsiveness.

1.       Motor functions

2.       Sensory functions

DIGESTIVE SYSTEM

Digestion is the mechanical and chemical breakdown of foods into forms that the cell membranes can absorb.

Mechanical digestion breaks large pieces into smaller ones without altering their chemical composition

Chemical digestion breaks food into simpler chemicals.

The digestive system carries out ingestion, propulsion, digestion, absorption and defecation.

The digestive system consists of the alimentary canal, extending from the mouth to the anus, and several accessory organs, which release secretions into the canal.

The alimentary canals includes the mouth, pharynx, oesophagus, stomach, small intestine, large intestine, and anal canal.

The accessory organs include the salivary glands, liver, gall bladder and pancreas.

The digestive system originates from the inner layer (endoderm) of the embryo, which folds to form the tube of the alimentary canal. The accessory organs develop as the buds from the tube.

DIGESTIVE SYSTEM

The digestive system consists of the alimentary canal, extending from the mouth to the anus, and several accessory organs, which release secretions into the canal. The alimentary canal includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine and anal canal. The accessory organs include the salivary glands, liver, gall bladder and pancreas. The digestive system originates from the inner layer (endoderm) of embryo, which folds to form tube of alimentary canal. The accessory organs develop as buds from the tube.

The mouth is followed by a muscular tube (Pharynx). The pharynx leads to a spacious chamber – stomach, through a narrow tube – oesophagus. The passage from the oesophagus to the stomach is guarded by a valve known as cardiac spinchter. The stomach is followed by small intestine which can be divided into three parts – duodenum, jejunum, ileum.

The distal end of the stomach opens through pyloric spinchter into the duodenum. The middle part is jejunum and the last part is ileum.

The small intestine (ileum) opens into the large intestine (colon) through ileo-colic valve.

A vermiform appendix is present at the site of ileo-colic valve which is vestigial in human beings.

The colon leads into the last part of intestine – rectum. The rectum opens outside the body, the rectal opening is guarded by anal spinchter.

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GENERAL PLAN OF THE ALIMENTARY CANAL

The alimentary canal is a muscular tube about 8 meters long that passess through the thoracic and abdominopelvic cavities. The structure of its walls, function and innervations are similar throughout its length with slight modifications at places.

Structure of the wall

The wall of alimentary canal consists of four distinct layers that are developed to different degrees from region to region. The four distinct layers persists throughout the alimentary canal, but certain regions are specialized for particular functions. Beginning with innermost tissues, the layers are as follows.

1.       Mucosa layer

This layer is formed of surface epithelium, underlying connective tissue (lamina propria), and a small amount of smooth muscles (muscularis mucosa). In some regions the mucosa is folded with tiny projections towards lumen. This increases the absorptive surface area.

The mucosa has tubular invaginations, which are lined by cells that secrete mucus and digestive enzymes.

2.       Submucosa layer

Next to the mucosa layer is the submucosa layer. It contains considerable loose connective tissue as well as glands, blood vessels, lymphatic vessels, and nerves. Its blood vessels nourish the surrounding tissues and carry away absorbed materials.

3.       Muscular layer

This layer, which provides movement of the tube, consists of two layers of smooth muscle tissues. The outer is the layer of longitudinal muscle. The inner is the layer of circular muscle. The contraction of circular muscle fibers reduces the diameter of lumen of the alimentary canal; the contraction of longitudinal muscle layers shortens the length of the tube.

4.       Serosa

The serosa layer is the outer covering of the tube; it is composed of the visceral peritoneum, which is formed of epithelium on the outside and connective tissue beneath. The cells of serosa protect the underlying tissue and secrete serous fluid, which moistens and lubricates the tubes outer surface so that the organs (which are lined by parietal peritoneum) slide freely inside the body cavity and against one another.

5.       Gut associated lymphoid tissues (GALT) – Peyer’s patches

Innervation [6]of the tube

Branches of the sympathetic and parasympathetic divisions of autonomic nervous system extensively innervates the alimentary canal. These nerve fibers, mainly associated with tube’s muscular layer, maintain muscle and regulate the strength, rate and velocity of muscular contractions.

Many of the postganglionic fibers are organized into a nerve plexus [7]within the wall of the canal.

The submucosal (Meissner’s plexus) plexus is important in controlling secretions of gastrointestinal tract.

The myenteric plexus of the muscular layer controls the gastrointestinal motility

The nerve plexus of the gastrointestinal tract are so extensive, that it is some times said to have a ‘second brain’.

** a great advance in our knowledge of gastric digestion, particularly in man, was made through the observations of Beaumont on his patient, Alexis St. Martin who in 1822, following a gunshot wound was left with an opening from the stomach through the abdominal wall to the exterior. Through this fistula, Beaumont found it possible to follow the course of gastric digestion of different food under varying conditions of health and obtained pure gastric juice for digestion experiments outside body

Movements of the tube

The motor functions of the alimentary canal are of two basic types – mixing movements and propelling movements

Mixing occurs when smooth muscles in particular segments of tube contract and relax rhythmically. For example – when the stomach is full, waves of the muscular contractions move along its wall from one end to the other. These waves occur every twenty seconds of so. They mix the digestive juices secreted by the mucosa with food.

Propelling movements include a wavelike motion called – peristalsis. During peristalsis a ring of contraction appears in the wall of the tube. At the same time the the muscular wall just ahead of the ring relaxes – a phenomena called as receptive relaxation.

Law of gut: when a segment of the intestinal tract is excited by distention and thereby initiates peristalsis (peristaltic reflex), the contractile ring causing the peristalsis normally begins on the orad [8]side of the distended segment and moves towards the distended segment, pushing the intestinal contents in the anal direction for 5 – 10 centimeters before dying out. At the same time the gut relaxes several centimeters downstream toward the anus (receptive relaxation), thus allowing the food to be propelled easily towards the anus.

This complex movement occurs only in the presence of the myenteric plexus. Therefore the movement is called the myenteric reflex or peristaltic reflex.

The peristaltic reflex and the movement of peristalsis towards anus is called the “law of gut”

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[1] There are two types of reflex arcs; the autonomic reflex arc – affecting inner organs and the somatic (voluntary) reflex arc – affecting skeletal muscles.

[2] https://www.newhealthguide.org/Pyloric-Sphincter-Function.html

[3] https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcR09b3hIlAQsXZmPGhKUKzlHCgt4sKEX3DSz8Fe0bL6bs6P6IUB

[4] https://thumbs.dreamstime.com/z/gallbladder-duct-anatomy-pancreas-liver-duodenum-s-stomach-detailed-description-86699426.jpg

[5] https://i.pinimg.com/originals/b8/b7/39/b8b739c5b9e4618fe9bbb7ec7e0013e8.jpg

[6] Supply nerves to; to put the nerves into

[7] network

[8] Towards the mouth; oral direction

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[i] Guyton, Hall, Textbook of Medical Physiology, 11^th Edition, Elsevier

Shier D, Butler J, Lewis R – Hole’s Human anatomy and Physiology, 11^th Edition

Agarwal, Srivastava, Kumar – Animal Physiology and Biochemistry, 5^th Edition, 2013, S. Chand & Company Ltd

Verma, Tyagi, Agarwal – Animal Physiology, 2015, S. Chand & Company Ltd

Asim Kumar Datta – Functional Histology, 1^st Edition, Current Books International

ORIGIN & EVOLUTION OF REPTILES

ORIGIN & EVOLUTION OF REPTILES

Reptiles evolved from amphibians of Carboniferous period[1], which depended on water bodies for laying eggs and development of larval stages and hence could not exploit arid/terristrial habitats far away from water bodies.

They developed a large yolk-laden [2]shelled egg that could be laid on land and in which an amniotic[3] sac contained fluid in which embryo could develop to an advanced stage, capable of fending for itself when hatched. The following anatomical changes transformed the ancestral amphibians into land adapted reptiles:

• Body developed a covering of epidermal scales to prevent loss of body moisture, and skin became cornified and devoid of glands.
• Skull became monocondylic[4] for better movement and flexibility. Atlas and axis vertebrae together permitted skull movement in all directions.
• Limb bones and girdles became stronger but limbs were attached on the sides of body, and belly touched the ground during creeping mode of locomotion.
• Sacral region involved two strong and fused vertebrae to support the body weight on hind legs.
• Pentadactyle limbs developed claws that helped in climbing on rocks and trees.
• Respiration through lungs became more efficient to use the oxygen available in the air.
• As a water conservation strategy, metanephric kidneys excreted uric acid which did not require water for excretion.
• Reptiles continued to be ectothermal since ventricle was not completely partitioned[5] by a septum and blood mixed in heart.
• Internal fertilization evolved as a large cleioid[6] shelled egg was laid on land.
• Embryonic membranes amnion, allantois and yolk sac evolved to enable embryonic development in arid conditions.

ANCESTORS OF REPTILES

1.       THE ANAPSIDS (THE COTYLOSAURS)

They were the most primitive stem reptiles that evolved from the labyrithodont amphibians (Embolomeri) in Carboniferous period.

Seymoria was a lizard-like animal, with pentadactyle limbs and a short tail. It had homodont labyrinthine teeth on the jaw bones as well as on vomer and palatine bones. Presence of lateral line indicates its amphibious habits. Skull was monocondylic for better movement of head. Seymoria indicates gradual transition from labyrinthodont amphibians to reptiles. Another 5 foot long cotylosaur fossil, Limnoscelis was found in Mexico that had large premaxillary teeth and long tail.

2.       THE PARAPSIDS

They possessed superior temporal vacuity in the skull and were adapted for aquatic mode of life.

Plesiosaurus was marine long-necked, fish-eating animal with 15 metre long fusiform body, short tail and paddle-like limbs modified for swimming. The skull was euryapsid type with a superior temporal vacuity. The fossils are from lower Jurassic (about 180 million years) and they are believed to have become extinct in end-Cretaceous mass extinction.

Ichthyosaurus  had fish-like body with fore limbs modified into paddle-like fins and hind limbs disappeared. There was a fleshy dorsal fin too. Caudal fin was large and bilobed. Jaws projected into an elongated snout and teeth were homodont, an adaptation for fish-catching. Skull was parapsid type with additional postfrontal and supratemporal bones behind the eye orbit. Vertebral column became secondarily simplified with amphicoelous vertebrae.

3.       THE SYNAPSIDS

Synapsids split off from the primitive reptilian stock very early in evolution, perhaps in the middle carboniferous period. Synapsids had started developing mammalian characteristics that enabled them to be fleet-footed and active predators. Their legs commenced to move under the body. Heterodont dentition and false palate started developing in pelycosaurs and had been completely formed in therapsids. Two types of synapsids occurred from carboniferous to Permian, namely, the primitive Pelycosaurs and advanced therapsids.

Pelycosaurs  are represented by Dimetrodon whose fossils were discovered from North America and Russia from the late Carboniferous to Permian periods. They were primitive reptile-like animals in which limbs had moved under the body but not completely and each limb had 5 digits with claws. Neural spines on the back were excessively long stretching highly vascularized skin between them that formed a fin-like or sail-like structure. They had heterodont dentition with incisors, canines and molars clearly defined but the false palate had not been completely formed.

Therapsids  were more advanced and active synapsids which were perhaps endothermic animals with high rate of metabolism. Heterodont dentition with false palate allowed these animals to chew and grind food for quick digestion in the gut so that high metabolic demand of the body could be fulfilled. Jaw muscles were attached to zygomatic arch to make chewing effective. Carnivore therapsids were called Cynodonts (ex. Cynognathus) and herbivores were Dicynodonts.

4.       THE THECODONTS

They evolved from the sauropsid Archosauria, a group of insignificant lizard-like reptiles that survived the Triassic mass extinction. They evolved into bipedal and highly agile predators.

Euperkeria and Ornithosuchus fossils were unearthed from South Africa and Europe. They were about 2 ft long bipedal lizard-like animals with small head but very long tail for balancing while they chased flying insects by rapid running. Endothermy must have evolved in thecodonts to meet the extraordinary energy demands of their predatory life style.

5.       THE SAURISCHIANS

They were dinosaurs with lizard-like pelvic girdle in which ischium and pubis bones radiated away from each other. They were both bipedal and quadrupedal and carnivores as well as herbivores.

6.       THE ORNITHISCHIANS

They were dinosaurs with bird-like pelvic girdle in which ischium and pubis bones were directed towards posterior as found in modern birds. These were also highly diversified carnivores as well as herbivores and both bipedal and quadruped.

7.       THE PTEROSAURIA

They were flying or gliding dinosaurs of Mesozoic that varied in size from sparrow-sized to some species, like Pteranodon, having a wing span of 8 meters. They had pneumatic bones. Last digit of the fore limb was extraordinarily long and served to attach the membranous patagium between fore limb, hind limb and the body. Hind limbs were used for clinging on to the rocks and cliffs and 3 digits of fore limbs also had curved claws, an adaptation for clinging. Their jaws were modified into beak that possessed homodont dentition but Pteranodon did not have teeth

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[1] The carboniferous period is famous for its vast swamp forests. The swamps produced coal from which the term carboniferous is derived. It lasted from about 359.2 to 299 million years ago.

[2] The eggs of reptiles are macrolecithal, they contain large amount of yolk for development of embryo into miniature adults which can feed and defend themselves.

[3] The amniotic egg of reptiles and birds is surrounded by a tough outer shell that protects the egg from predators, pathogens, damage and from drying. Oxygen passes through tiny pores in the shell, so embryo doesn’t suffocate. Inside the shell are four sac. The first sac inside the shell is chorion, which carries oxygen from the shell to the embryo and waste carbon dioxide from the embryo to the shell. Within the chorion is amnion, the membrane for which the amniotic egg is named. The amnion keeps the embryo from drying out, so it’s critical to living on land. A third sac, the allantois, stores wastes from the embryo and also fuses with the chorion to form the chorioallantoic membrane, ehich carries oxygen and carbon dioxide to and from the embryo, just like lungs. A fourth membrane, the yolk sac, holds and digests nutritious yolk for the developing embryo.

[4] Monocondylic skull has one occipital condyle in skull, it provides high degree of movement.

[5] Two atria and one ventricle. The two atria and one partially divided ventricle. There is a mixing of oxygenated and de-oxygenated blood because the ventricle is not split completely.

[6] Birds lay hard-shelled eggs, but most reptiles lay soft-shelled eggs. Bird’s eggs are incubated by body heat, but reptile eggs are incubated by natural heat. The reptiles eggs are hidden, thus are all white. The birds eggs are incubated in nests and are exposed thus show colouring and camouflage. * amniotic egg