ZOO 112 The Mammalian Body Skeletal and Muscular

ZOO 112 The Mammalian Body Skeletal and Muscular Systems Dr Aina O. Adeogun

Skeletal and Muscular Systems q. Skeletal systems q. The internal sructure of mammals include the design and arrangement of a bony framework known as an endoskeleton. Skeletons are supportive systems that provide rigidity to the body surfaces for muscle attachment and protection for delicate internal body organs. q. There are several kinds of supportive connective tissue serving various binding and weight bearing functions. In mammals, skeletons are rigid elements and usually jointed (to which muscles can attach). q. There are two types of rigid skeleton A). Exoskeletonarthropods, molluscs. B)Endoskeleton

q. The mammalian endoskeleton is composed of bone and some cartilage. The bone is a living tissue that differs from other connective and supportive tissues by having large deposits of calcium salts laid down in an extracellular matrix. q. Bone develops from 2 sources q. Eroding embryonic cartilage- endochondral (other bones: consists of bones of the skull and the long bones)/replacement bone q. Sheets of embryonic cells- Intramembranous bone (consists of bones of the face, cranium, clavicle) q. Bones also vary in density when fully formed qa. Cancellous (spongy) bone- consists of an open interlacing framework of bony tissue. All bones first develop as cancellous bone. qb. Compact bone- dense due to further deposition of bone salts; appears solid to the naked eyes.

The Mammalian Skeleton

Microscopic structure of the bone. q. Compact bone consists of a calcified bone matrix arranged in concentric rings. The rings contain cavities known as lacunae filled with bone cells Osteocytes. The Osteocytes are connected by many minute passages called canaliculi. -they distribute nutrients throughout the bone. q. Lacunae and canaliculi are arranged into an elongated cylinder called an osteon or harvesian systems. A typical bone consists of bundles of osteons meshed (connected) together and interconnected with blood vessels and nerves making the bone a living tissue capable of being restored to its formal state if broken and bone diseases can be painful.

Microscopic structure of the bone

Transverse section of the Human Bone

Transverse Section of the Bone

Histological (Transverse) Section of the Bone.

q. Bone growth is a complex process involving internal destruction by bone resorbing cells (osteoclasts) and deposition externally by bone building cells (osteoblasts). It is also under the control of several hormones e. g parathyroid hormone (stimulates bone resorption) and calcitonin (thyroid gland) inhibits bone resorption and a derivative of vitamin D (for maintaining a constant level of calcium in the blood). q. The mammalian skeleton (vertebrate skeleton) is divided into 2 parts. q. The Axial Skeleton and The Appendicular Skeleton. They both function in support, protection, movement and respiration.

The Axial Skeleton q. Is made up of the skull, vertebral column, sternum and the ribs. q. The movement from water to land resulted in dramatic and extensive remodeling in vertebrate evolution leading to increased cephalization and concentration of brain, sense organs, foodgathering and respiratory apparatus in the head. q. This made the skull the most intricate portion of the skeleton. Skull bones in early fishes (180), amphibians and lizards (50 -95). Mammals (35 or less), humans (29).

The Axial Skeleton Skull Vertebral column Sternum and ribs

The Vertebral Column q. This is the main stiffening axis of the postcranial skeleton and is made up of several small vertebraes. In mammals, the vertebraes are differentiated into cervical (neck) , thoracic (Chest), lumbar (back), sacral (pelvic) and caudal (tail) vertebrae. All members of the vertebral column have facets for articulation with other vertebraes known as zygaphophyses. q. The prezygapophyses faces upwards and is directed inwards (tapers away from you). It is found in the anterior view of the vertebra. q. The postzygapophyses faces downwards and is directed outwards (towards you). It marks the posterior view of the vertebra.

q. In humans, the caudal vertebrae are reduced in number and size while the sacral vertebrae are fused. The number of vertebraes vary among humans (mammals) for e. g in humans there are 33 in a young child but in adults 5 are fused to form the sacrum and 4 to form the coccyx (26). q. There are 7 cervical, 12 thoracic, 5 lumbar, 1 sacral and 1 coccyx. The number of cervical vertebrae is constant in almost all mammals. (dolphins-short neck giraffes-long neck) q. The first 2 cervical vertebrae – atlas and axis are modified to support the skull and permit pivotal movements. The atlas bears the globe of the head while the axis permits the head to turn form side to side. Ribs are long/short skeletal structures that articulate medially with vertebrae and extend into the body wall.

Atlas Axis Typical cervical vertebra Vertebral column Sacral vertebra with coccyx Thoracic vertebra Lumbar vertebra

In mammals the ribs act together to form the thoracic basket and supports the chest wall to prevent collapse of the lungs. Horses – 18 pairs; Primates have 13 pairs of ribs, humans have 12 pairs (1 in 20 people have a 13 th pair).

The Appendicular skeleton Comprises of the pectoral girdle (scapula, coracoid(fused sometimes), the clavicle (collar bone)); the pelvic girdle, the fore and hind limbs (pentadactyl arrangement)

Appendicular Skeleton

The Pectoral Girdle q. Is made up of 2 halves attached firmly (together) by muscles (not fused). There are 3 bones in each half of the pectoral girdle. Those are the scapula (shoulder blade), the coracoid and the clavicle (collar bone). In mammals, the scapula and coracoid are fused while the clavicle still remains a single unit visible on the anterior view (plane). The scapula is a triangular bone with the base of the triangle being adjacent to the vertebral column while the narrow apex lies at the shoulder (glenoid cavity found at the apex where the humerus of the fore limb fits into).

Pectoral girdle

The pelvic girdle q Consists of 2 halves each of which lies at either side of the vertebral column. It supports the hind limbs. The right and left halves of the pelvic girdle are fused in the median line ventral to the vertebral column. This line of fusion is the pubic symphysis and each half is called the innominate bone and has 3 parts/bones – illium (largest, longest), ischium and pubis. There is also the obturator foramen (hole) and the acetabulum (where the head of the femur fits in).

Pelvic girdle

The Pentadactyl limb q The fore and hind limbs of all vertebrates (mammals included) are built on a basic pattern known as the pentadactyl pattern (hence the name). q. It consists of a proximal long bone followed by a pair of long bones placed side by side, a set of nine small bones in 3 rows, 5 thin long bones and finally 5 digits (pentafive, dactyl- digits). Each digit is made up of a few small bones.

Pentadactyl limb

The Fore limb q. Humerus: The upper proximal part has a non-distinct head while the lower distal part ends in a grooved pulley – like surface called the trochlea q. Ulna and radius – adjacent to the humerus, the radius is bigger than the ulna and has a notch/cavity known as the sigmoid cavity. The ulna extends behind the sigmoid cavity to form the olecranon process at the elbow. q. Wrist - comprises of 8 small irregular bones arranged in 3 rows. The proximal rows have 3 bones that articulate with the radius and ulna. Middle row has 1 small bone and distal row has 4 bones (5 actually, 2 are fused). Wrist bones are carpals (wrist – carpus). Following the carpus are 5 long metacarpals which is the first part of the hand. Each metacarpal has a digit made up of smaller bones known as phalanges (singular – phalanx). Each digit ends in a tough claw.

The Hind limb Is arranged in a similar pattern as the fore limb where the femur is longer than the humerus. It has a distinct head that fits into the acetabulum of the pelvic girdle. Near the head of the femur are 3 projections called trochanters (serve for attachment of certain leg muscles). The distal end has 2 knobs (condyles) which articulate with the tibia and fibula. Tibia and fibula : is fused in the rabbit to a tibio – fibula. The tibia is the longest bone in the mammalian skeleton (bigger than the fibula). The proximal end of the fibula is independent while the distal end is fused to the tibia. Knee cap/patella : small triangular bone which occurs at the junction of the tibia and fibula.

q. Ankle or tarsus- made up of 6 bones collectively known as the tarsals. 4 metatarsals in rabbits (digits have 3 phalanges). In other mammals + man 5 metatarsals, 5 digits make up the toes. Tarsals were actually 9 reduced to 5 by fusion of 2 pairs of bones and a reduction in one bone q Modifications of the basic pentadactyl limb for life in different environments involve distal portions more than proximal portions. Generally bones are lost or fused than for new ones to be added e. g horses (odd toed ungulates and their relatives) evolved a foot structure for fleetness by elongation of the 3 rd toe , so the horse stands on the 3 rd fingernail (hoof) like a ballet dancer standing on the tips of the toes. • The arrangement of constituent bones • Synarthroses (immovable joints) e. g. pubic symphysis • Diarthroses (movable joints) e. g. shoulder joints • Amphiarthroses (combination of a and b) e. g. vertebrate joints (with the exception of atlas and axis

The Muscular System q. Muscular tissue is derived from the mesoderm. It is specialized for contraction and made up of elongated units called muscle fibres. These fibres are bound in a framework of vascular connective tissue which provides anchor for the skeleton and skin. q. Muscular tissue is classified into 3 different types of fibres based on the appearance of muscle fibres and type of contraction.

Muscular system (Anterior View)

Muscular system (Posterior View)

Muscle fibers

1. Skeletal muscle: When viewed with a microscope, appears transversely striped (striated) with alternating dark and light bands. It is organized into sturdy compact bundles/bands and is attached to skeletal systems. Skeletal muscle is responsible for movements of the trunk, appendages, respiratory organs, eyes, mouthparts and other structures. Skeletal muscle fibres are very long, cylindrical multinucleate cells that reach from one end of the muscle to the other. They are packed into bundles called fascicles ( singular: fasciculus small bundle). Skeletal muscles contract with rapidity (powerfully and quickly) during locomotion and fatigues more rapidly than smooth muscles. It is stimulated by motor fibres and is under conscious cerebral control hence the name voluntary muscle.

Skeletal muscle

Skeletal muscle

Transverse section of Skeletal muscle

2. Smooth muscle: Does not have striations and the cells are long tapering strands, each containing a single nucleus. Smooth muscle cells are organized into sheets of muscle circling the walls of the alimentary canal, blood vessels, respiratory passages, urinary and genital ducts. Smooth muscle is slow acting and can maintain prolonged contractions with very little energy expenditure (slow rhythmic contractions) - peristalsis/peristaltic movement of the wall of the alimentary canal. It is controlled by the autonomic nervous system with contractions that are involuntary and unconscious hence the name involuntary muscle. They function to push materials into a tube (along the way by active contraction) e. g. the intestine through contractions or to regulate the diameter of a tube e. g blood vessel by sustained contraction.

Smooth muscle

Transverse Section of the Smooth muscle

q. Cardiac muscle: intermediate between the skeletal muscle and smooth muscle. It is fast acting and striated like the skeletal muscle but is under the control of involuntary autonomic nervous system like the smooth muscle. (beats fast but rhythmically). Found in the heart, it is composed of closely opposed, separate, uninucleate cell fibres. q. Muscle contraction: The contraction of muscle fibres is brought about by a change in the arrangement of the protein molecules known as contractile proteins (actomyosin system – composed of actin and myosin) arranged to contract when powered by ATP (energy derived from the chemical energy of food). q. Food and oxygen are supplied in blood circulating in an extensive network of capillaries within the muscles.

Cardiac muscle

Cardiac muscle

Transverse Section of the Cardiac

A CHART TO COMPARE THREE TYPES OF MUSCLE FIBRES

Structure of a muscle Skeletal (striped) muscle contracts in response to motor impulses from the central nervous system. (Each motor nerve fibre comprises from ten over 100 branches and each fibre terminates at a motor end portion (plate) on a muscle fibre). Each muscle cell or fibre is a multinucleated tube containing numerous myofibrils. These are packed together (in a bundle) and surrounded by a cell membrane known as the sarcolemma.

Structure of the muscle

A myofibril contains 2 types of myofilaments (a) thick filaments is composed of the protein myosin. The myosin molecule in each thick filament is composed of 2 polypeptide chains, each with a club -shaped head (and the double heads of each myosin molecule face outward from the centre of the filament) and these heads acts as molecular cross bridges that interact with thin filaments during contraction. (b) thin filaments is composed of the protein actin collectively known as contractile proteins. The thin filaments are held together by a dense structure called the Z line. The functional unit of a myofibril is the sarcomere and is extensive between successive Z lines.

Thin filaments are more complex and composed of 3 different proteins – (a) a double strand of the protein actin (like double helix). (b) 2 thin strands of another protein tropomyosin (surrounds actin filaments double helix arrangement ). (c) a complex of 3 globular proteins called troponin located at intervals along the filament. Troponin is a calcium dependent switch that acts as the control point in contraction.

The sliding filament model of muscle contraction q. The sliding filament model of muscle contraction was proposed in the 1950’s by 2 English physiologists (scientists) (independently at the same time). A. F. Huxley and H. E. Huxley. They proposed that the thick and thin filaments become linked together by molecular cross bridges acting as levers to pull the filaments past each other. q. During muscle contraction, cross bridges on the thick filaments swing rapidly back and forth, alternately attaching to and releasing from special receptor sites on the thin filaments. These act together to draw the thin filaments past the thick filament and as contraction continues, the Z lines are pulled closer together shortening the sarcomere. q. Since all sarcomere units shorten at the same time, the muscle contracts. Muscle relaxation is brought about when cross bridges between the thick and thin filaments are released thereby freeing the sarcomeres and they become elongated (requires some force usually supplied by antagonistic muscle/force of gravity).

Sliding filament model of muscle contraction

Control of contraction q. Muscles contract in response to nerve stimulation. (if the nerve supply to a muscle is severed, the muscle atrophies/ waste away). Skeletal muscle fibres are served (innervated) by motor neurous whose cell bodies are located in the spinal cord. q. Each cell body gives rise to a motor axon that leaves the spinal cord and travel through a peripheral nerve trunk to a muscle where it branches several times into many terminal branches with each terminal branch serving a single muscle fibre.

q. A single motor axon may serve (innervate) as few as 3 -4 muscle fibres (where precise control is needed e. g muscles that control the eye movement) or as many as 200 muscle fibres (where precise control is not needed e. g large leg muscles). q. A motor axon (motor neuron) and all the muscle fibres it serve (innervates) constitutes a motor unit. (and is a functional unit of skeletal muscle). q. The Myoneural junction is a place where a motor axon terminates on a muscle fibre. At this junction, there is a tiny gap known as the synaptic cleft and this (thinly) seperates a nerve fibre from a muscle fibre. The neuron stores a chemical – acetylcholine at this juncton in minute vessicles known as synaptic vessicles.

q. Acetylcholine is released when a nerve impulse reaches a synapse. q. Acetylcholine is a chemical mediator that diffuses across the narrow junction and acts on muscle fibre membrane to generate an electrical depolarization. The depolarization spreads rapidly along the muscle fiber causing it to contract. Thus the synapse is a special chemical bridge that couples together the electrical activities of nerve and muscle fibres.

Control of muscle contraction

Energy for contraction The process that brings about muscle contraction requires large amounts of energy and this energy is sourced from 3 levels: q. First Level : ATP is the immediate source of energy, amount present will sustain muscle contraction for 1 – 2 seconds. q. After ATP is rapidly used up, muscle cells immediately call on 2 nd level of energy reserve, creatine phosphate Creatine phosphate is a high energy phosphate compound that stores bond energy during periods of rest. As ADP is produced during contraction, creatine phosphate releases its stored bond energy to convert ADP to ATP. Creatine phosphate + ADP ATP + creatine.

The reserves of creatine phosphate are depleted within 30 seconds or less of muscle contraction depending on pace/rapidity of contraction (fast/slow). Contracting muscle must now be fueled at level 3. • The third level of energy is the largest store of energy – the glycogen. Glycogen is a polysaccharide chain of glucose molecules stored in both liver and muscle. The muscle stores ¾ of all glycogen in the body. The use of glycogen is advantageous for muscle contraction because • It is relatively abundant • It can be mobilized quickly • It can provide energy under anoxic conditions

q As soon as the muscle’s store of creatine phosphate declines, enzymes break down glycogen converting it to glucose-6 -phosphate (this is the first stage of glycolysis that leads to mitochondrial respiration and ATP generation). q If muscular contraction is not too prolonged or rigorous, the glucose released from glycogen can be completely oxidized to carbon dioxide and H 2 O by aerobic metabolism. q During prolonged/heavy exercise however, blood flow to the muscles (although greatly used above the resting level) cannot supply oxygen to the mitochondria rapidly enough to complete oxidation of glucose.

q. The contractile machinery (muscle and nerve) then receives its energy largely by anaerobic glycolysis - a process that does not require oxygen. (The ability to take advantage of this anaerobic pathway although not nearly as efficient as the aerobic one, is of great importance and without it, all forms of heavy muscular exertion would be impossible). q During anaerobic glycolysis, glucose is degraded to lactic acid with release of energy. This energy is used to resynthestize creatine phosphate which in turn passes the energy to ADP for the resynthesis of ATP. Lactic acid then accumulates in the muscle and diffuses rapidly into general circulation. If muscular exertion continues, the buildup of lactic acid causes enzyme inhibition and fatigue. In essence, the anaerobic pathway is a self – limiting one as continued heavy exertion leads to excavation. The muscles then incure an oxygen debt because accumulated lactic acid must be oxidized by extra oxygen. After a period of exertion, O 2 consumption remains elevated until all of the lactic acid has been oxidized/resynthesized to glycogen.

Energy for contraction

Fast and slow muscle fibres Skeletal muscles of vertebrates consists of more than one type of fibre – namely slow and fast fibres. Slow fibres Specialized for slow sustained contractions without fatigue. Important in maintaining posture in terrestrial vertebrates – commonly called red muscles because they contain an extensive blood supply, a high density of mitochondria for supplying ATP and an abundant stored myoglobin which supplies oxygen reserves. All these features give the muscle a red colour.

Fast fibres Capable of fast powerful contractions. There are 2 types of fast fibres. • Lacks an efficient blood supply, high density of mitochondria and myoglobin- muscles made up of those fibres are called white muscle. Usually pale in colour, function anaerobically and fatigue rapidly e. g white meat of chicken • Has extensive blood supply, high density of mitochondria and myoglobin, functions largely aerobically. Animals use this for rapid, sustained activities. Most muscles have a mixture of these different fibre types to provide for a range of activities. Dogs and ungulates (hoofed mammals) have limb muscles with a high percentage of fast aerobic fibres, and are capable of active locomotion for long periods of time.

Members of the cat family, however, have running muscles made up almost entirely of fast fibres that operate anaerobically. During a chase, such muscles build up a substantial oxygen debt that is replenished after the chase. E. g a cheetah after a high-speed chase lasting 50 seconds (less than a minute) will pant heavily for 30 – 40 minutes before its O 2 debt is paid off.

Digestive System and Nutrition ZOO 112 Dr Aina O. Adeogun

Digestive System and Nutrition For all organisms, there is a basic requirement of energy to maintain their highly ordered and complex structure. This energy known as Chemical Bond Energy is released by transforming complex compounds acquired from the environment into simpler ones. Almost all animals are heterotrophic organisms because they depend on already synthesized organic compounds of plants and other animals to obtain the materials they need for growth, maintenance and reproduction. The food of animals (the complex tissue of other organisms) is usually too bulky to be absorbed directly by cells; hence it must be broken down or digested into soluble molecules small enough to be utilized. The ingestion of foods and their simplification by digestion are initial steps in nutrition foods reduced by digestion to soluble molecular form are absorbed into the circulatory system and transported to the body tissues. They are then assimilated into the structure of cells and oxidized (by oxygen transported by blood to the tissues) or burned to yield energy and heat. Food not used immediately is stored for future use and wastes produced by oxidation are excreted while food products unsuitable for digestion are egested in the form of faeces.

Animals are divided into 4 broad categories on the basis of dietary habitats: • Herbivorous animals- feed mainly on plant life. feed mainly on herbivorous other carnivores • Carnivorous animals • Omnivorous animals – feed on both plants and animals • Saprophagous animals – feed on decaying organic matter. Animals have evolved various methods for procuring and manipulating solid food. These adaptations are based on the food of the animal. True mastication (the chewing of food) as opposed to tearing or crushing is found only in mammals. Mammals have 4 different types of teeth, each adapted for specific functions • Incisors- with simple crowns and sharp edges. Designed for biting, cutting and stripping. • Canines- with long conical crowns for piercing, seizing and tearing. • Pre molars and molars- large bodies and variable cusp arrangement for grinding and crushing

This basic pattern is highly modified in animals with specialized food habits. For e. g. herbivores have suppressed canines but well developed molars with enamel ridges for grinding and the well developed self sharpening incisors of rodents throughout life and are constantly norm away by gnawing to keep pace with growth. An elephant’s tusk have become so highly modified (upper incisor used for defense, attack and rooting; male wild boar has modified canines used as weapons) that they are no longer useful for biting or chewing food. The primitive teeth formula for most mammals expresses the number of each tooth type in one half of the upper and lower jaw I=3/3; C = 1/1; PM = 4/4; M = 3/3 = 44 e. g. order insect, some carnivores, some omnivores. Most mammals grow 2 sets of teeth (reptiles replace teeth continously) • A temporary set- deciduous or milk teeth • Permanent set.

Only incisors, canines and premolars are deciduous. Molars are never replaced and the single permanent set must last a lifetime. The process of digestion involves mechanical and chemical breakdown of organic foods into small units for absorption. Although food solids consist of CHO, proteins, and fats (the very components that make up the body of the consumer) they must first be reduced to their simplest molecular units and dissolved before they can be assimilated. The evolution of the alimentary system in mammals allowed for extracellular digestion of large food masses because certain cells lining the lumen (cavity) of the alimentary canal specialize in forming various digestive secretions while others function largely or entirely in absorption. The appearance of complete mouth to anus alimentary systems resulted in (emphasized extracellular digestion) increasing regional specialization of the digestive tract for mammals. The mechanical processes of cutting and grinding by teeth and muscular mixing by the intestinal tract are important in digestion. However, reduction of foods into small, absorbable units relies principally on chemical breakdown by enzymes.

Digestive enzymes are Hydrolytic enzymes or Hydrolases (because food molecules are split by hydrolysis- breaking of a chemical bond by adding components of water across it). R- R + H 2 O Food molecule Digestive enzymes R- OH + H – R Split to 2 products E. g. Proteins individual amino acids before absorption. CHO – reduced to simple sugars Fats (lipids) – molecules of glycerol, fatty acids and monoglycerides. The gut of mammals is lined with 2 opposing layers of smooth muscle- a longitudinal layer and a circular layer with the muscle fibre embracing the circumference of the gut. Segmentation (an alternate construction of rings of smooth muscle of the intestine that constantly divides and squeeze the contents back and forth) of the gut mixes food substances together within the gut lumen. Food moves through the gut by peristalsis (sweeps food down the gut with waves of contraction of circular muscle).

Organization and Regional Function of the Alimentary Canal On the basis of function, the metazoan alimentary canal can be divided into 5 broad regions (a) Receiving region (b) conduction and storage region (c) Region of grinding and early digestion (d) Region of Terminal Digestion and absorption (the intestine) (e) Region of water absorption and concentration of solids. Receiving Region The receiving region- 1 st region of the alimentary canal consists of devices for feeding and swallowing. It includes the mouthparts (mandibles, jaws, and teeth), the buccal cavity (and mucular pharnx). Metazoans including mammals have salivary glands that produce lubricating secretions containing mucus to assist in swallowing. Salivary glands contain within their secretions(toxic secretion) for salivary enzymes to begin digestion e. g. salivary amylase is a CHO splitting enzymes that begins hydrolysis of plant and animal starches. It is found in primate

mammals and humans. It does not completely hydrolyze starch but breaks it down to 2 glucose fragments called maltose; free glucose and larger fragments of starch (also produced). When a food mass (bolus) is swallowed, salivary amylase acts on it for some time digesting approximately half of the starch before it is inactivated by the acidic environment of the stomach. Starch is further digested in the intestine. The tongue of vertebrates is a unique innovation attached to the floor of the mouth and used for other purposes like food capture e. g. chameleon, woodpeckers, anteaters) (olfactory sensor- many lizards and snakes). In humans, swallowing begins with the tongue pushing moistened food towards the pharynx. The nasal cavity closes reflexively by raising the soft palate. As the food slides into the pharnx, the epiglottis moves tips down over the trachea, nearly closing it. Sometimes, some food particles may enter the tracheal opening but contraction of laryngeal muscles prevents it from going further into the pharnx. Once food is in the oesophagus, peristaltic contraction of oesophageal muscles forces it smoothly towards the stomach. (b) Conduction and Storage Region

The oesophagus (of vertebrates) transfers food to the digestive region. (c) Region of Grinding and Early Digestion Stomach of most vertebrates provides initial digestion as well as storage and mixing of food with digestive juices. The stomach of carnivores and omnivores is a typical U-Shaped muscular tube provided with glands that produce proteolytic enzymes and strong acids (an adaptation that probably arose for killing prey and halting bacterial activity). When food is passed from the oesophagus to the stomach, the cardiac sphincter opens by reflex to allow food passage, and then closes to prevent regurgitation back into the esophagus. In humans, gentle peristaltic waves pass over a full filled stomach 3 times per minute. The most churning takes place at the intestinal end where food is steadily released into the duodenum (the 1 st region of the small intestine). The pyloric sphincter regulates flow of food into the intestine and prevents regurgitation into the stomach. Deep tubular glands in the stomach wall secrete gastric juice in humans approximately 2 litres per day. Two types of cells line these glands: Chief cells that secrete pepsin and parietal cells that secrete HCl. Pepsin is

a protease (protein splitting enzyme) that acts only in acidic medium (p. H 1. 6 – 2. 4) and splits large proteins by preferentially breaking down certain peptide bonds scattered along the peptide chain of the protein molecule. The specificity of pepsin prevents it from completely degrading proteins but it hydrolyses them into smaller polypeptides. Other proteases complete protein digestion in the intestine. Pepsin is present in the stomachs of all vertebrates. The stomach mucosa is prevented from being digested by its powerful acidic secretions by mucin (a gastric secretion) (highly viscous organic compound) that coats and protect the mucosa from chemical and mechanical injury. When the protective mucous coating fails sometimes (due to infection from a bacteria – Helecobacter pylori) that secretes toxins causing inflammation of the stomach lining. This may lead to gastric ulcer. Rennin ( not renin produced by kidney) is a milk – curdling enzyme found in the stomach of ruminant mammals, Probably other mammals. It clots and precipitates milk proteins and slows movement of the milk through the stomach. Rennin extracted from stomachs of calves is used in making cheese. Human infants lacking rennin digest milk protein and acidic pepsin just like adults.

Gastric juices are secreted intermittently although small volume is secreted continuosly (even during prolonged periods of starvation). Secretion is increased by the sight and smell of food, presence of food in the stomach, and emotional states e. g. anxiety and anger. d) Region of Terminal Digestion And Absorption The intestine of vertebrates including mammals is equipped for both digestion and absorption and devices for increasing internal surface area are highly developed. Among vertebrates the length of the intestine increases the absorptive surface of the gut. Coiling of the intestine is common in vertebrates and reaches its highest development in mammals where the length of the intestine may be over 8 times the length of the body. Mammals also have in addition minute finger- like projections called villi giving the inner surface of fresh intestinal tissue a velvet appearance. Also short delicate processes called microvilli + large villi and intestinal folds increase the internal surface area of the intestine. A million times facilitating absorption of food molecules.

Digestion in the Vertebrate Small Intestine. The pyloric sphincter releases food into the small intestine which relaxes at intervals to allow entry of acidic stomach contents into the initial segment of the small intestine- the Duodenum. Two secretions pour into the duodenum- pancreatic juice and bile. Both have a high bicarbonate content especially pancreatic juice that neutralizes gastric acid and raises the p. H of the liquefied food mass now called Chyme from 1. 5 -7 as it enters the duodenum. This change in p. H is essential because all intestinal enzymes are effective only in neutral or slightly alkaline medium. Pancreatic Enzymes The pancreatic secretions of vertebrates (mammals inclusive) contain several enzymes of major importance in digestion. Two powerful proteases: Trypsin and Chymotrypsin continue protein digestion initiated by pepsin (inactivated by alkaline nature of the intestine). These enzymes are highly specific proteases and split apart peptide bonds deep inside the protein molecule.

Pancreatic juice also contains carboxypeptidase that removes amino acids from carboxyl ends of polypeptides. Pancreatic lipase hydrolyses fats into fatty acids + glycerol. Pancreatic amylase (starch splitting identical to salivary amylase) + nucleases- degrade RNA and DNA to nucleotides. Membrane Enzymes. The cells lining the intestine have digestive enzymes embedded in their surface membrane that continue digestion of CHO, proteins and phosphate compounds. These enzymes are in the microvillus membrane and include aminopeptidase – splits terminal amino acids from the amino end of short peptides. Dissacharidases- enzymes that split 12 -carbon sugar molecules into 6 carbon units. They include maltase- splits maltose to 2 molecule of glucose. Sucrose – splits sucrose to fructose and glucose and galactose. Alkaline phosphatase- an enzyme that attacks a variety of phosphate compounds. Bile – secreted by the liver into bile duct that drains into the duodenum (upper intestine). Bile collects in gall bladder between meals (an expandable storage sac

that releases bile when stimulated by the presence of fatty food in the duodenum). Bile contains water, bile salts, pigments (no enzymes). Bile salts- mainly Sodiumtaunochlorate and Sodiumglycocholate are important for fat digestion. Fats (because of their tendency to remain in large, water insoluble globules) are especially resistant to enzymatic digestion. Bile salts reduce surface tension of fat globules to allow for the churning action within the intestine to break fats into tiny droplets (emulsification of fat). Increased total surface for fat exposure allows fatsplitting lipases (pancreatic lipases) to reach and hydrolyse the triglyceride molecules. Yellow-green colour of bile is due to bile pigments- breakdown products of haemoglobin from worn-out red blood cells. Bile pigments also give faeces its characteristic colour. Bile production is one of the livers many functions. Liver is a storehouse for glycogen , production center for plasma protein site for protein synthesis, detoxification of protein wastes, destruction of worn-out red blood cells and centre for metabolism of fat, amino acids and CHO.

Absorption Little food is absorbed in the stomach because digestion is incomplete and there is limited absorptive surface area. Although some materials like drugs and alcohol are absorbed in the stomach (making them act rapidly). Most digested food are absorbed from the small intestine as the villi provide a large surface area through which digested materials can pass from the intestinal lumen into circulation. CHO- absorbed as simple sugars/ monosaccharides e. g. glucose, fructose, galactose because intestine is impermeable to polysaccharides. Proteins- absorbed as amino acid sub units (though a limited amount of small proteins and polypeptide fragments are absorbed). Active and passive processes transfer simple sugars + amino acids across intestinal epithelium.

Passive processes- immediately after a meal decrease simple materials are in high cones in the gut that they readily diffuse into the bloodstream where cone is initially lower. Active transport- mechanism is located on the epithelial cells that transfer food molecules into the blood. Here materials are removed against their concentration gradient (requires energy expenditure) e. g. of nutrients transported are glucose, galactose, amino acids. Triglycerides are broken into fatty acids + monoglycerides which complex with bile salts to form minute droplets called micelles. When micelles contact the microvilli of the intestinal epithelium, fatty acids and monoglycerides are absorbed by diffusion. They then enter the endoplasmic reticulum of the absorptive cells and are resynthesized to triglycerides before passing into lacteals. Form lacteals, fat droplets enter the lymph system and pass into blood circulation through the thoracic duct.

Region of Water Absorption and Concentration of Solids Indigestible remnants of food items are concentrated in the large intestine and water is reabsorbed to form solid or semi solid faeces and removed from the body by defaecation. The colon of humans have numerous bacteria (first enter sterile colon of newborn infants with their food- in adults, 1/3 of the dry weight of faeces is bacteria) and they include harmless and harmful ones. These bacteria degrade organic waste in faeces and provide some nutritional benefits by synthesizing certain vitamins e. g. Vitamin K and small quantity of Vitamin B absorbed by the body.