PDF Cell Structure and Function - Table of Contents

Table of Contents Cell Structure and Function.................................................................................................................... 2 Musculoskeletal System.....................................................................................................................

Table of Contents Cell Structure and Function.................................................................................................................... 2 Musculoskeletal System.......................................................................................................................... 9 Respiratory System................................................................................................................................ 13 Cardiovascular System.......................................................................................................................... 16 Gastrointestinal System........................................................................................................................ 21 Blood..................................................................................................................................................... 28 Urinary System...................................................................................................................................... 34 Cell Structure and Function Cell Membrane The cell membrane is a biological membrane that separates the interior of the cell from the outside environment. It comprises of a phospholipid bilayer with other molecules such as proteins, lipids (glycolipids, sterols, cholesterol) and carbohydrates (glycocalyx). The functions of the cell membrane include: - Maintaining the structure and integrity of the cell - Separating the interior of the cell from the external environment - Allows for diffusion of substances in and out of the cell - Controls the entry of other substances into and out of the cell via endocytosis, exocytosis and via carrier proteins and channels - Controls the signalling from the outside to the inside of the cell via cell receptors - Carries identifying signals and markers Cholesterol present in the cell membrane regulates its fluidity. When body temperature is low, cholesterol "increases" fluidity of cell membrane. When body temperature is high, cholesterol “reduces" fluidity of cell membrane. Voltage gated sodium channels are an example of protein channels that are found in the cell membrane. At resting potential, the intracellular space is negatively charged relative to the extracellular space. When the membrane is depolarized to a threshold value, VGSCs open and the rapid influx of sodium ions further depolarizes the membrane. This increase in intracellular potential activates proximal VGSCs, resulting in a chain of channels opening sequentially – an action potential. The VGSC closes automatically, typically a few milliseconds after opening. On red blood cells, carbohydrate molecules (glycoproteins) on the surfaces of their cell membrane that encode for the ABO blood group. Cytoplasm The cytoplasm is the things within the cell that are found between the cell membrane and the nucleus. The cytoplasm is made up of the cytosol, organelles and cytoplasmic inclusions. The function of the cytoplasm is to support: - Cellular respiration - Synthesis of proteins - Waste handling - Growth and repair - Cell division The cytosol is a complex solution consisting of water, ions, and macromolecules. These ions and macromolecules are not placed randomly within the cytosol. Instead, there are several levels of organization that localize the ions and macromolecules to specific sites within the cytosol. These levels include: - Ion concentration gradients - Protein complexes - Protein compartments eg proteasome - Physically managed by cytoskeleton Proteasomes are part of a major mechanism which cells regulate the concentration of particular proteins and degrade misfolded proteins. They tag proteins for degradation by a small protein called ubiquitin. The tagging reaction is catalysed by ubiquitin ligase. The cytosol has many functions and is the site where multiple cell processes occur such as: - Protein synthesis - Metabolic pathways such as pentose phosphate pathway, glycolysis and gluconeogenesis. - Transport metabolites - Transmit signals from the outside of the cell to the correct site within the cell - Site where a lot of steps in cell division occurs The cytoskeleton is found in the cytoplasm and is a complex network comprising of: - Microtubules (25nm) made of α and β tubulin monomer units - Intermediate (12nm) filaments - Microfilaments (7nm) made of actin units Microtubules are found in centrioles, cilia and flagella. They are thin, hollow tubes and have a negative end and positive end. Microtubules are involved in axonal transport, acting as a railroad track to move things up and down a neuron. The negative end is towards the nucleus of the neuronal cell and the positive end is oriented towards periphery of the neuronal cell. Intermediate filaments are the toughest and most resilient of the cytoskeletal proteins. It is also a tumour marker. Intermediate filaments have different names depending on the types of cells they are found in: nucleus – lamin, epithelial cell – keratin, fibroblast cell – vimentin, muscle cell – desmin, neuronal cell – neurofilaments. Intermediate filaments help maintain the shape of the cell despite compressive forces and are present in cell junctions such as desmosomes and hemi-desmosomes to ensure adherence of cells to other cells and the basement membrane respectively. Microfilaments are made up of G actin monomers which form F actin polymer which in turn forms a double helix microfilament. It is the smallest and most flexible. It maintains the shape of the cell. It is also involved in cell migration, cell division, cellular extensions, cell junctions, muscle contractions and membrane transport. Nucleus The nucleus is a membrane bound organelle found in all human cells except for mature red blood cells. The nucleus is made up of the nuclear envelope, nucleoplasm, nucleoli and chromatin/chromosomes. The main function of the nucleus is the storage of genetic code which is responsible for controlling gene expression and protein production. It is also involved in cell division and cell cycle. The nuclear envelope separates the nucleus from the cytoplasm and consists of a unit membrane, perinuclear space, nuclear pore and nuclear diaphragm spanning the nuclear pore. The nucleoplasm is a highly gelatinous substance different from the cytosol. It consists of water, proteins, carbohydrates and nucleotides. The nucleolus is responsible for the production of ribosomal subunits and ribosomal RNA. The chromatin/chromosomes are complex structures made up of DNA, RNA and basic proteins (histones). They can exist in two forms: - Heterochromatin are inactive genetic material made up of closed up DNA that does not facilitate gene activity. They are electron dense and show up dark coloured. - Euchromatin are active genetic material made up of opened up DNA that facilitates gene activity. They are electron lucent and show up lightly coloured. Mitochondria The mitochondria is a double membrane organelle consisting of an outer mitochondrial membrane and an inner mitochondrial membrane which encloses the intermembrane space. The inner membrane is folded to form cristae which contains the mitochondrial matrix, a fluid containing substances. The function of the mitochondria is to: - Release energy for the cells to perform work (osmotic, chemical, mechanical or electrical) via the Krebs Cycle and electron transport chain - Heat production - Storage of cations such as calcium and magnesium - Regulate cell proliferation and programmed cell death - Degradation functions such as fatty acid oxidation - Contains the mitochondrial genome - Other synthetic functions such as the synthesis of certain proteins Unlike normal genome, the mitochondrial genome of the offspring is only inherited from the mother. If the mother has a heredity disease encoded in the mitochondrial genome, all the offspring will also have the disease regardless of the genotype of the father and vice versa. Endoplasmic reticulum The endoplasmic reticulum is a system where the continuous membrane forms flattened sacs. In cells, there are 2 types of endoplasmic reticulum: the smooth endoplasmic reticulum and rough endoplasmic reticulum. The rough endoplasmic reticulum has ribosomes present on the membrane. Its function includes: - Protein synthesis - Protein modification via core glycosylation and post-translational modifications of newly formed proteins - Protein folding - Protein quality control - Protein sorting based on where they are supposed be transported to (to other organelles or outside of the cell) - Limited proteolysis of newly synthesised proteins Some organs rich in rough endoplasmic reticulum are the exocrine pancreas which produces large amounts of digestive enzymes, goblet cells in the respiratory and gastrointestinal tract which produces mucinous proteins. Plasma cells which produce antibodies. Nissil body in neuronal cells which produce neurotransmitters. The smooth endoplasmic reticulum has no ribosomes present on the membrane. Its function includes: - Lipid synthesis - Steroid synthesis - Breakdown of endogenous and exogenous compounds Some organs rich in smooth endoplasmic reticulum is the testis which produces testosterone, ovaries which produces estrogen and progesterone, adrenal cortex which produces cortisol, aldosterone and DHEA. Hepatocytes are also rich in smooth endoplasmic reticulum as they produce bile acids and are involved in the breakdown and detoxification of endogenous and exogenous compounds such as glycogen and alcohol respectively. Golgi apparatus The Golgi apparatus are found between the endoplasmic reticulum and the cell membrane. They have membranes that form saccules (cisternae). The face closer to the endoplasmic reticulum is called the forming (cis) face while the face closer to the cell membrane is called the maturing (trans) face. The functions of the golgi apparatus includes: - Protein modification such as protein glycosylation catalysed by glycosyl-transferases and conversion of proproteins into smaller mature proteins. - Concentration, packing, franking and sorting of proteins. - To deliver the proteins to the correct destination either within the cell or to be secreted out of the cell. Sections of the golgi apparatus are biochemically distinct. This means that the enzymatic content of each segment is markedly different. Secretory vesicles which contain proteins to be secreted out of the cell commonly contain protease enzymes which modify the secretory proteins by cutting them along specific amino acid sequences. One example is insulin which is synthesised as an inactive pre-proinsulin molecule before it is modified into a mature insulin molecule. Lysosomes Lysosomes are spherical sacs made by the rough endoplasmic reticulum. It comprises of a single membrane which encloses a lumen that is filled with enzymes and an acidic liquid. The function of the lysosomes are mainly the intra-cellular digestion of waste such as: - Damaged organelles found within the cell - Substrates found within the cell or taken in from outside the cell - Destruction of the entire cell (autolysis) Some of the enzymes that are present in lysosomes include proteases, nucleases, glycosidases, lipases and phosphatases. The lysosomal membrane is impermeable to enzymes and substrates. The lysosomal membrane also has large amount of proton ATPase present which pumps H+ ions into the lysosome lumen. This ensures that the pH in the lysosome is kept low to ensure it is the optimal pH for the enzymes to act on macromolecules. Some processes which involve lysosomes are receptor-mediated endocytosis, phagocytosis, autophagy and autolysis. Cell imaging To create an image of a cell, several different equipment can be used such as light micrographs and electron micrographs. These equipment will give different magnifications (LM gives 10 to 100 times while EM gives 1000 to 10000 times) Different stains are also added to visualise the cells better. The most widely used stain is hematoxylin and eosin (H&E) stain. Hematoxylin stains the nucleus of the cell in blue or purple while eosin stains the proteins in the cytoplasm as pink or red. Cytoplasmic inclusions are metabolic products that are stored in the cytoplasm of long-lived cells such as hepatocytes, neurons and cardiac muscle cells. These cytoplasmic inclusions often appear in micrographs and are useful in identifying cell types. Some examples include glycogen granules seen in hepatocytes, melanin pigment granules seen in skin cells and lipofuscin granules also seen in liver cells. Musculoskeletal System Bones form the skeletal framework of the body for support, locomotion and protection of internal organs. The bone marrow can be classified into red bone marrow and yellow bone marrow. The red bone marrow forms blood cells such as red blood cells, white blood cells and platelets while the yellow bone marrow mainly store energy in the form of fats. Red bone marrow is more prevalent in infants compared to adults who mainly have yellow bone marrow. The bone matrix surrounds the bone marrow. It has a system of canals present. The vertical channels are called Haversian canal while the horizontal channels are called Volkmann’s canal. Blood vessels and nerves make use of these channels to provide nourishment to the bone matrix cells and to innervate the bone. In the cross-section of the bone, concentric arrangements of cells called osteocytes are present. These cells, along with the matrix, and Haversian canal containing a bone marrow, forms the functional unit called an osteon. These osteons are produced through the continuous process of bone remodelling. They are very dynamic structure. In the bone matrix, osteoprogenitor cells are stem cells population in the bone and these cells form the osteoblasts. The osteoblasts are key players that drive bone remodelling and in the process of bone formation gets transformed into osteocytes and is embedded in the calcified bone matrix. Another player for bone remodelling is the osteoclast. Its origin is from the monocyte (phagocytic cell). It is multinucleated and when stimulated, will eat up the bone. The bones in the body can be categorised into axial skeletons and appendicular skeleton. The bones of the axial skeleton make up the head and torso while the bones of the appendicular skeleton make up the limbs. Some important bones of the axial skeleton include: Skull Mandible (jaw) Sternum Ribs Cervical vertebrae Thoracic vertebrae Lumbar vertebrae Sacrum Coccyx Some important bones of the appendicular skeleton include: Clavicle (collar bone) Scapula (shoulder Humerus Radius blade) Ulna Pelvic bone Femur Patella (knee cap) Tibia Fibula Joints are found between different connecting bones. A typical joint has articular surfaces which is made of hyaline cartilage. The space between the two joints is filled with a fluid called synovial fluid. The synovial fluid is secreted by a synovial membrane that lines the joint cavity from the inside. The outer layer of the joint capsule is lined by a protective fibrous layer. There are also different types of joints that are present in the body. One example is the ball and socket joint found in shoulder and hip joints. t’s a ball which can fit inside a socket and can move around three axes. There is higher freedom of movement. Another example is the hinge joint found in elbow and knee joints. They can only move in one axis and have a lower freedom of movement. The type of joint present between the bones can indicate the movements that the joint is able to undergo. For a ball and socket joint, the joint can undergo flexion & extension, abduction & adduction as well as medial rotation & lateral rotation. For a hinge joint, they are only able to undergo flexion & extension. Muscles are connected to the bone by a fibrous structure called a tendon. Muscles found in the body can be divided into 3 types: skeletal muscle, smooth muscle and cardiac muscle. Skeletal muscles are packaged as such: Epimysium (muscle) -> perimysium (fascicles) -> endomysium (muscle cell) -> myofibril The cells present in a muscle, muscle cells, are long cylindrical and multinucleated cells. The skeletal muscles are mainly responsible for voluntary, rapid and powerful contractions. There are striations present on the skeletal muscle. The functional unit of a skeletal muscle is called a sarcomere. They can be identified by the striations on the myofibril. In the myofibril, there is the thin filament made of actin and thick filament made of myosin. The thin filament causes the light bands while the thick filaments causes the dark bands. During muscle contraction, the sarcomere length shortens. Some important muscle groups include: Pectoralis major Biceps brachii Triceps Deltoid muscle Trapezius Latissimus dorsi Rectus abdominus Quadriceps femoris Hamstring Gastrocnemius Soleus Gluteus maximus Things to note: - The quadriceps femoris is a group of 4 muscles that make up the thigh area. The fourth muscle is behind the first and can only be seen if the first muscle is removed. - The hamstring is a group of 3 muscles that are found at the back of the leg. They are called semimembranosus, semitendinosus and biceps femoris. - The soleus is a group of muscles that is found behind the soleus. The dysfuction of bones and muscles can lead to several clinical considerations. Bone fractures can be caused by traumas like falls, accidents or sports injuries. However, people of older age are more susceptible to bone fractures due to their bone density decreasing causing the bones to become weaker and more fragile. Paralysis can be caused when nerve signals to the muscles are disrupted. These muscles will be unable to perform any action. Some common causes of paralysis include stroke, spinal cord injury and nerve disorders. Arthritis is the damage to the joints. The joint usually becomes painful, stiff and inflamed in these individuals. In some cases, the inflammation can result in damage to the joint. Respiratory System Nasal cavity The nasal cavity is found further down from the nostrils and inside the head. It is lined by pseudostratified ciliated columnar epithelial cells. One of the main functions of the nasal cavity is the conditioning of the air. This is done by the highly vascularised concha which increases the surface for conditioning of respired air, warming it. The meatus are the passageways made by the concha. They create air turbulences in the nasal cavity to mix the air around, ensuring particles are spread around for olfaction and immune/filtering purposes. There are also olfactory cells that are present in the nasal cavity responsible for the sense of smell. Paranasal sinuses with ciliated and mucous secreting respiratory mucosa, opening up to the nasal cavity. The 4 paranasal sinuses are the frontal sinus, maxillary sinus, ethmoidal sinus and sphenoidal sinus. Pharynx The pharynx is the passage between oral and nasal cavities to the larynx and esophagus in the neck. The nasopharynx is lined by pseudostratified columnar epithelium while the oropharynx and laryngopharynx is lined by stratified squamous epithelium since it is shared with the GIT. Larynx The larynx links the pharynx to the trachea. 3 cartilages form the larynx: thyroid, cricoid and epiglottis. The thyroid and cricoid are hyaline cartilages while the epiglottis is an elastic cartilage. The thyroid and cricoid cartilage protects the vocal cords while the epiglottis acts as a valve to prevent the entry of food and water. The vocal cords are found in the larynx and comprises of vestibular fold (false vocal cord), vocal fold (true vocal cord) and other cartilages in the larynx. The rubbing and vibration of the vocal folds causes sound to be produced. Trachea The trachea links the larynx to the bronchus. The wall of the trachea is strengthened by C-shaped cartilage. There is trachealis muscle present at the back of the cartilage to allow for the oesophagus to expand when food is passing through. The trachea is lined with pseudostratified ciliated columnar epithelium. Bifurcates into left and right primary bronchi entering the lung at the hilum. Bronchial tree The trachea branches into the primary bronchi. The primary bronchi branches into lobar bronchi based on how many lobes each lung has. The lobar bronchi branches into segmental bronchi. The segmental bronchi branches into bronchioles, then to terminal bronchioles, then to respiratory bronchioles and finally alveoli. In the lobar and segmental bronchi, the C-shaped cartilages are replaced by cartilage plates. The amount of smooth muscles present in them also increases. From the bronchioles onwards, there are no cartilage present. A sympathetic signal from the body will lead to bronchodilation while a parasympathetic signal from the body will lead to bronchoconstriction. Alveoli Inhaled air travels from the alveolar ducts to alveolar sac and finally to individual alveoli, where gaseous exchange occurs between the air and the blood. Alveoli have a thin wall lined with simple squamous epithelium known as pneumocytes. There are two types of pneumocytes, pneumocyte I which is responsible for gaseous exchange and pneumocyte II which is responsible for secreting a surfactant that prevents alveoli collapse. Alveoli are also wrapped with capillaries to enable a more efficient exchange of gases. There are also alveolar macrophages present in the alveoli known as dust cells that clear foreign particles that may have reached the alveoli. Pleura There are two types of pleura that are associated with the lungs. The parietal pleura is the outer layer that is in contact with the thoracic wall surface. The visceral pleura is the inner layer that is in contact with the lungs. In between the two layers, there is a pleural cavity that contains pleura fluid, a serous secretion that acts to connect the two surfaces and also acts as a lubricant. Lungs The lungs comprise of the apex, the base, 3 borders and 3 surfaces. The 3 borders of the lung include the anterior border, posterior border and inferior border. The 3 surfaces of the lung include the costal surface, medial surface and the diaphragmatic surface. The right lung is made up of 3 lobes: the superior lobe, middle lobe and inferior lobe. The superior lobe is separated from the inferior lobe by the horizontal fissure. The inferior lobe is separated from the other two lobes by the oblique fissure. The left lung is made up of 2 lobes: the superior lobe and the inferior lobe. The two lobes are separated by the oblique fissure. The left lung is also where the cardiac notch which houses the heart can be found. This is the reason for the left lung having fewer lobes. The lingula is a flap of excess skin that is found on the lateral edge of the lungs. Pulmonary circulation Blood travels from the right atrium -> right ventricle -> pulmonary artery -> bronchial artery -> capillary -> bronchial vein -> pulmonary vein -> left atrium -> left ventricle -> aortic arch The breast and anterior chest is supplied by the internal thoracic artery. Ribs The human body has 12 ribs present. Ribs 1 to 7 are true ribs as they are directly connected to the sternum. Ribs 8 to 12 are false ribs as they are indirectly connected to the sternum. Ribs 11 and 12 are also called floating ribs as they are not attached to the sternum at all. Respiratory muscles Respiratory muscles are responsible for controlling the movement of the ribcage and lungs. The changes to air pressure caused by this movement is able to force air into and out of the lungs. Primary respiratory muscles include the diaphragm and external intercostal muscles. There are also accessory respiratory muscles for changing of depth and frequency. Cardiovascular System Heart The heart is located in the ribcage, superior to the diaphragm and between the lungs in the cardiac notch. The heart extends more into the left lung than the right. At the anatomical position, the heart is deviated to the left. This means that the right side of the heart (right atrium and right ventricle) is towards the front while the left side of the heart (left atrium and left ventricle) is more towards the back. In the walls of the atrium, there are ridges present known as musculi pectinate. They are formed from the embryonic parts while the smooth sides of the heart are the newly formed part. In the walls of the ventricles, there are also ridges present called trabeculae carneae. There are also papillary muscles present connected to chordae tendineae which in turn are connected to the valves. The papillary muscles along with the chordae tendineae ensures that the valve is able to close properly. The papillary muscles contract during ventricular contraction which pulls the chordae tendineae down along with the valves, preventing blood from returning into the atria. The chordae tendineae is only present for the atrio-ventricular valve and not the semi-lunar valves as the vessel is small enough to allow the valves to shut completely without the assistance of chordae tendineae. Cardiac Cycle Deoxygenated blood from systemic circulation flows to the superior vena cava (from head and upper limbs), inferior vena cava (from abdomen, pelvis and lower limbs) and coronary sinus (from heart muscles). The deoxygenated blood from these blood vessels is then emptied into the right atrium during atrial diastole. When the atrium contracts during atrial systole, the blood in the right atrium is forced into the right ventricle through the tricuspid valve. When the ventricle contracts during ventricular systole, the blood in the right ventricle is forced into the pulmonary trunk through the semi-lunar valves and eventually reaches the pulmonary arteries. The deoxygenated blood is then transported to the pulmonary capillaries where gaseous exchange occurs. Oxygenated blood is then returned to the heart by the pulmonary veins. The oxygenated blood is emptied into the left atrium through the 4 pulmonary veins. When the atrium contracts during atrial systole, the blood in the left atrium is forced into the left ventricle through the bicuspid valve. When the ventricle contracts during ventricular systole, the blood in the left ventricle is forced into the aorta through the semi- lunar valves and eventually reaches systemic circulation. At the aorta, instead of returning into systemic circulation, a small amount of blood enters the left and right coronary arteries at the coronal section of the aorta. The blood that enters is responsible for supplying cardiac muscles with oxygen and other nutrients. The deoxygenated blood from the cardiac muscles then enters the coronary veins and then to the coronary sinus before it returns to the right atrium. Pericardial sac The pericardial sac comprises of 2 layers of pericardium which encloses the heart: the outer fibrous pericardium and the inner serous pericardium. The serous pericardium has a parietal layer that is attached to the fibrous pericardium and a visceral layer that is attached to the heart tissue. Between the two layers of the serous pericardium, there is a pericardial cavity that contains lubricant. The function of the pericardial sac is: - Position the heart between the lungs in the chest cavity - Lubricates the surface of the heart to reduce friction when beating - Prevents overfilling of the heart Conduction system of the heart There are 4 parts that are involved in the conduction of electrical impulses in the heart. 1. Sino atrial node (SA node) which acts as the natural pacemaker 2. Atrioventricular node (AV node) 3. Atrioventricular bundle (AV bundle or bundle of His) which can be further divided into the left and right bundle branch 4. Purkinje fibre Electrical impulses are initiated at the SA node. This leads to the depolarisation of the left and right atria which causes contraction of both atria. After atrial depolarisation, the electrical impulses are delayed at the AV node. This allows time for the atrium to completely contract and blood to fully fill the ventricles. After this delay, the electrical impulses travel from the AV node, down the AV bundle and to the apex of the heart. From the apex of the heart, impulses move upwards along the walls of the ventricles and begins depolarisation and ventricular contraction from bottom up. The bottom-up contraction allows the ejection of blood upwards into the pulmonary trunk and aorta. Meanwhile, the atria repolarise which results in the atria relaxing which is then followed by the ventricles repolarise which results in the ventricles relaxing. The cycle then repeats. Histology of Heart Muscles One unique structural feature of heart muscles are the presence of intercalated discs. There are 2 unique components in the intercalated discs. - Desmosomes present function as an anchor for adjacent cardiac cells and prevent them from separating from each other during contraction. - Gap junctions present serve as a pathway which allow ions to pass from one cell to another, allowing electrical impulse to spread through the myocardium. This causes a contraction as one unit also termed as functional syncytium. Histology of blood vessels The blood vessels are made up of 3 layers: - Tunica intima is the innermost layer. o It has an endothelium formed by simple squamous epithelium o It contains a basement membrane and loose connective tissue to support the endothelial lining. o It also contains an internal elastic lamina - Tunica media is the middle layer. o It has smooth muscles present that is responsible for vasoconstriction and vasodilation depending on the input given. o It also contains an external elastic lamina - Tunica externa is the outermost layer. o It has a layer of collagen fibres to provide structural support and protection to the blood vessel. o It has its own blood supply from its own vessels called vasa vasorum. Arteries and veins are commonly found adjacent to each other. On the other hand, capillaries are significantly different and are found at the end of the arteries and the start of veins. They only comprise of a single layer of tunica intima made up of simple squamous endothelium and a basement membrane. There are also unique blood vessels present in the body that make up a portal system. A portal system is a connection in which blood passes through two sets of capillary systems before returning to the heart. In humans, the only system is the hepatic portal system. This system connects the capillaries in the digestive system with the capillaries in the liver. Gastrointestinal System The gastrointestinal system has several functions which include: - Ingestion - Mechanical digestion and processing - Chemical digestions - Secretion - Absorption - Defecation - Defence The organs can be main organs which come into direct contact with the food (oral cavity, esophagus, stomach, intestines) and assessory organs which does not come into direct contact with the food but are involved in digestion (liver, pancreas). Oral cavity The first section of the GIT is the oral cavity. The main functions of the oral cavity are the mastication and lubrication of ingested food. Limited chemical digestion of glucose and lipid by amylase and lipase respectively also occurs in the oral cavity. At the roof of the oral cavity lies the hard palette and soft palette. The hard palette helps with mastication while the soft palette directs ingested food into the pharynx. At the bottom of the oral cavity lies the tongue. The tongue is responsible for mechanical digestion of food, manipulation of food, gustation (sensory of taste and temperature) of food and lubrication. The oral mucosa is lined by stratified squamous epithelium. The epithelium of the hard palette which frequently comes into contact with food is lined by keratinised epithelium while the bottom of the tongue which rarely comes into contact with food is lined by non-keratinised epithelium. Teeth are also present in the oral cavity for mastication. During developmental stages, there is usually 20 milk teeth while in adults, there are 32 permanent teeth. Saliva and Salivary glands Saliva produced has several functions which include: - Lubrication and moisten food bolus - Limited digestion - Dissolving chemicals for gustation - Buffer - Control oral bacteria population - Salivary reflex The saliva is produced by 3 main glands. - The parotid gland produces a serous secretion that contains enzymes such as amylase. The secretions are released into the vestibules of the oral cavity via the parotid duct.  - The sublingual gland produces a mucus secretion that contains mucin. It is released into the frenulum of the tongue via the submandibular duct. - The submandibular gland produces a mucus secretion that contains mucin, buffer and amylase. It is also released into the frenulum of the tongue via the submandibular duct. Histologically, the mucous cells and serous cells of the glands can be differentiated by staining with eosin. Mucous cells commonly contain a gel-like substance called mucin and is not stained by eosin. Serous cells commonly contain proteins such as enzymes and is thus heavily stained by eosin. Pharynx The pharynx is made up of the nasopharynx, oropharynx and laryngopharynx. However, the nasopharynx is not involved in the GIT. The parts of the pharynx involved in the GIT is lined by stratified squamous epithelium. There are 4 steps of swallowing a bolus of food that is ingested. 1. Buccal phase. The bolus of food is located in the region of the soft palette, tongue and uvula. At this step, movement of the bolus of food is still voluntary. 2. Pharyngeal phase. The tongue pushes the bolus of food down the pharynx. The bolus of food is now located in the region of the epiglottis. At this step and the rest of the steps, movement of food becomes involuntary. 3. Esophageal phase. The bolus of food enters the esophagus through the upper esophageal sphincter which prevents the entry of air into the esophagus. 4. Reaching the stomach. The bolus of food travels down the esophagus via peristalsis and triggers the opening of the lower esophageal sphincter before reaching the stomach. Histology of the GIT After the pharynx, the parts of the GIT share a similar histology. They comprise of the following 4 layers: 1. Mucosa. It comprises of the mucosal epithelium, lamina propria and muscularis mucosae. a. The mucosal epithelium can still vary based on the organ b. The lamina propria comprise of blood vessels, lymphatic vessels and muscles. In some regions, secretory cells can be found. c. The muscularis mucosae comprise of smooth muscle and elastic fibre. There are two layers of smooth muscle present, the inner circular layer and the outer longitudinal layer. They work together to regulate the shape of the lumen. 2. Submucosa. It functions to connect the mucosa and muscular layer. a. The connective tissue present helps the submucosa perform its function. b. There are blood vessels, lymphatic vessels and nerve fibres present. c. In some organs, there are also exocrine glands that secrete enzymes and buffers into the lumen of the digestive tract. 3. Muscular layer. It is made up of circular and longitudinal smooth muscle for mechanical digestion and movement. a. The movement of these muscles are coordinated by the enteric nervous system, a localised nervous system that is specific to the GIT.  4. Serosa. It comprises of a serous membrane a. In organs not lined with the visceral peritoneum, the serosa is also called the adventitia. Esophagus The epithelium of the esophagus comprises of stratified squamous epithelium. In the submucosa, there are esophageal glands present that secrete mucus in order to facilitate smooth passage of food bolus down the esophagus. In the muscular layer, skeletal muscle is mainly found in the upper esophagus and slowly transitions to smooth muscle in the lower esophagus. There is an outer layer of adventitia that anchors the esophagus to the posterior body wall. There are two sphincters: upper esophageal sphincter and lower esophageal sphincter to prevent regurgitation of the food bolus and stomach acid. Peritoneum The peritoneum is a serous membrane that functions to keep the organs in the abdomen in place. The peritoneum can be divided into the parietal peritoneum which is attached to the abdominal wall and the visceral peritoneum which surrounds an organ. The visceral peritoneum is also referred to as the serosa of the organ that it surrounds. Between he two layers of peritoneum, there is a peritoneal cavity. The cavity secretes a serous fluid that acts as a lubricant. Mesenteries The mesenteries are double sheets of peritoneal membrane that stabilises the digestive tract. It also acts as a route for blood vessels, lymphatic vessels as well as nerves for the digestive tract. Some examples are: - Falciform ligament which attaches the liver to the abdominal wall as well as separate the liver into the left and right lobes - Mesentery proper which surrounds the jejunum and ileum and attaches them to the posterior abdominal wall. The duodenum is not surrounded by it as it is retroperitoneal.  - Mesocolon which surrounds the colon and rectum and attaches them to the abdominal wall. - Lesser omentum which is found between the stomach and the liver.]. - Greater omentum which covers the intestines from the stomach to serve as protection and insulation Stomach The main function of the stomach is the storage of food as well as the mechanical and chemical digestion of ingested food into chyme. There is rugae present on the surface of the stomach when the stomach is empty to allow for the stomach to expand when large amounts of food enter. Additionally, there is an extra oblique layer of muscle in the muscular layer to allow for better mechanical digestion of ingested food. The stomach can be divided into 4 sections: - Cardia which is the esophagus entry. There are mucosal glands present to protect the esophagus from acid and enzymes. - Fundus which is the superior region of the stomach. - Pyloric part which is the entry to the duodenum - Body which is found between the fundus and the pyloric. The epithelium of the stomach is simple columnar epithelium that is covered by an alkaline mucus. The epithelium caves into the mucosa layer to form a gastric pit which leads to a gastric gland. Near the fundus, there are parietal cells which secrete intrinsic factor B12 as well as HCl in the from of H+ ions into the stomach. It is responsible for the killing of microorganisms, denaturing proteins and breaking down connective tissues in food. There are also chief cells which secrete pepsinogen which is responsible for digesting proteins. Pepsinogen requires an acidic environment in order to carry out its function. Near the pyloric, there are G cells present which secrete gastrin that stimulates gastric glands and contraction of the muscle wall. There are also D cells which secrete somatostatin that inhibits G cells. Pancreas In the pancreas, there are 2 types of cells: exocrine cells and endocrine cells. The pancreatic islets are made up of endocrine cells that secrete insulin and glucagon directly into the bloodstream. The pancreatic acini have simple cuboidal epithelium and secrete pancreatic juice directly into the duodenum via the pancreatic duct. Pancreatic juice is made up of the following: - Amylase - Lipase - Nuclease - Inactive proteolytic enzymes such as trypsinogen, chymotrypsinogen, procarboxypeptidase and proelastase that are activated in the small intestine by enteropeptidases. Liver The liver comprises of 4 lobes: the left lobe, caudate lobe, right lobe and the quadrate lobe. The liver is one of the organs that is able to regenerate itself as long as the blood supply is not affected. Some of the functions of the liver includes: - Metabolism of glucose, lipids, amino acids and drugs - Storage of minerals (iron) and vitamins (A, D, E, K, B12) - Waste and toxin removal - Hematological function o Largest blood reservoir o Synthesis of blood proteins o Removal of old and damaged red blood cells - Bile production and secretion Histology of the liver In the liver, each functional unit is called a liver lobule. Each lobule is surrounded by 6 portal triads comprising of the hepatic portal vein, hepatic artery and bile duct. In the centre of the liver lobule, there is the central vein present. Within the liver lobule, there are sinusoids lined by fenestrated endothelial cells which separates the hepatocytes from the sinusoids. These sinusoids empty into adjacent veins which eventually reaches the central vein. On the other hand, bile produced are secreted into bile canaliculi which is then emptied into the bile duct at the portal triad. Gall bladder After bile is produced by the liver and secreted by the bile duct, it is then stored in the gall bladder where it is concentrated before being secreted into the duodenum. The secretion of bile is regulated by Cholecystokinin and is used to buffer chyme and emulsify the fats present. Bile is made from cholesterol, lipids, bilirubin, water and ions. It is then recycled after being secreted into the duodenum. Small intestine The small intestine is the portion of the GIT where 90% of absorption takes place. The small intestine can be divided into 3 regions: duodenum, jejunum and ileum. Most of the small intestine is surrounded by the mesentery proper which has blood vessels, lymphatic vessels and nerves present. Within the small intestine, there are also circular folds present to increase the surface area of the small intestine for faster absorption rate. On the circular folds, there are villi present that are lined with simple columnar epithelium with microvilli. The area covered by microvilli is referred to as the brush border. There are also enzymes present on the brush border responsible for chemical digestion. On the mucosa of the intestines, there are also intestinal glands present. In these intestinal glands are several different cell types such as goblet cells which secrete mucin, Paneth cells that are responsible for innate immunity, enteroendocrine cells that secrete gastrin, cholecystokinin and secretin, and also stem cells. In the lamina propria of the intestinal mucosa, there is a large network of capillaries to transport digested food to the hepatic portal vein. There are also lacteals present to transport fatty acids. Duodenum The duodenum is the part of the small intestine that is connected to the stomach. The main function of the duodenum is to receive chyme from the stomach and neutralise the gastric acid that may enter to protect the small intestine lining. Thus, there are less circular folds present in the duodenum and it is also the area where bile and pancreatic secretion is released to neutralise the acid. Besides the intestinal glands, there are also special submucosal glands present called Brunner’s Patches. These glands secrete mucus which serves as pH protection from stomach acid and also urogastrone which inhibits gastric acid. Jejunum The bulk of chemical digestion and absorption of digested food takes place in the jejunum. There are thus more prominent circular folds especially at the middle part before decreasing. Ileum The main function of the ileum is to act as the border between the small intestine and the large intestine. The ileum ends with the ileocecal valve and the distal part closest to the large intestine also lack circular folds. Close to the distal part, there are also Peyer’s Patches present on the ileum. These are lymphoid nodules that has a large number of macrophages and act as the gatekeeper on the bacteria present in the small intestine. Large intestine The large intestine has a thinner wall and larger diameter than the small intestine. The functions of the small intestine include: - Water absorption - Waste excretion - Vitamin K, B5 and biotin absorption that is aided by the microbes present The large intestine can be divided into the cecum, colon and rectum. The cecum is the pouch attached to the small intestine where entry is controlled by the ileocecal valve. The appendix is also found at the cecum. Colon The colon is the main part of the large intestine and comprises of the ascending colon, transverse colon, descending colon and sigmoidal colon. There are haustra (pouches) present on the colon that allows it to elongate and expand for segmentation of waste to take place. These haustra are created by a smooth muscle called tinea coli which also allows churning to take place. Similar to the small intestine, there are intestinal glands present on the mucosa of the colon. These intestinal glands have goblet cells to produce mucus for lubrication, Paneth cells for innate immunity and stem cells. There is a lack of villi on the mucosa of the colon. Besides the Paneth cells, there are also large lymphoid nodules present in the lamina propria containing immune cells in order to better control the microbe population present in the colon. Rectum The main function of the rectum is faeces storage. There are two sphincter which controls the opening of the rectum to the anus: the internal anal sphincter muscle which is involuntary and the external anal sphincter muscle which is voluntary. The epithelium of the rectum transitions from columnar epithelium at the large intestine to stratified squamous epithelium. The epithelium closest to the anus is also keratinised. Blood The blood is a specialised fluid connective tissues that serves to perform the following functions: - Gaseous exchange - Transport of nutrients, hormones, antibodies, water and metabolic waste The components of blood such as red blood cells, white blood cells and platelets are mainly produced by the red bone marrow. Red bone marrow are mainly present in infants and children and start to become yellow bone marrow in adolescents and adults to serve as a storage of energy in the form of fats. For blood samples to be analysed, it is centrifugated into its components which include: - Plasma (55%) comprising of water, proteins, electrolytes and dissolved gases - Buffy coat (1%) comprising of platelets and white blood cells - Red blood cells (45%) Red blood cells Red blood cells, also called erythrocytes, makes up 45% of the blood. They contain abundant haemoglobin for them to carry out their function of gaseous exchange. They perform their function within the blood. Red blood cells are biconcave as the disc shape facilitates the gaseous exchange. This is because more haemoglobin is closer to the plasma membrane for more efficient gaseous exchange. This means that a shorter time is needed for oxygen to reach the haemoglobin in red blood cells. Haemoglobin is a protein consisting of 4 polypeptide chains each complexed with an iron-containing heme group. Each heme group is responsible for the binding of 1 molecule of oxygen. The lifespan of a red blood cell is approximately 120 days. There are also antigens present on red blood cells. These antigens determine the blood group of the person. There are also antibodies that are present in the plasma which must be different from the antigens present to prevent binding and agglutination. The respective Anti-antigen serum can be used to test the blood group of a person. A positive test is indicated by agglutination present when serum is added. There is also another protein on the surface of red blood cells called Rh factor. They can either be present or absent on red blood cells. The anti-Rh serum can be used to test for the presence of these Rh factors on red blood cells. During blood transfusions, either parts of the blood or whole blood is given to the patient. Whole blood transfusions are not common and red blood cells are the most common part of the blood that is transfused. The transfusion of red blood cells also depends on the compatibility of the patient and donor. When a patient has anaemia, there is either insufficient haemoglobin present in red blood cells or the number of red blood cells present is reduced. Causes for anaemia includes blood loss, insufficient iron, vitamin deficiencies and genetic factors. White blood cells White blood cells are found in the buffy coat of centrifuged blood and are also known as leukocytes. They use the blood as a means of transport and leave it to perform its chief function. White blood cells can be traditionally divided into groups based on nuclear shape (lobe) and cytoplasmic granules. Granulocytes are white blood cells that contain prominent cytoplasmic secretory granules which contain protein. Some examples are: - Neutrophils o Most common type of white blood cells o Highly lobulated nucleus o Neutrophilic and have small, specific granules that are difficult to visualise due to its size o Highly mobile and phagocytotic  o Lifespan of approximately 5 days o First immune cells to arrive at the site and acts as the first line of defence o Cause an acute inflammatory response at the site o Pus contains dead neutrophils and tissue debris - Eosinophils o Bilobed nucleus o 1-6% of circulating white blood cells o Cytoplasmic crystalloid granules contain basic proteins that stain red with eosin o Granules contain peroxidase, histaminase, arylsulfatase and other hydrolytic enzymes o Provides defence against parasites and phagocytosis of antigen-antibody complexes - Basophils o Bilobed nucleus that is obscured by granules o Least common white blood cells o Large, cytoplasmic granules contain hydrolytic enzymes, histamines, heparin sulfate and slow-reacting substances (SRS) of anaphylaxis o Histamine and SRS are vasoactive and causes small blood vessels to dilate o Associated with allergic and inflammatory response - Mast cells o Produced in the bone marrow and matures in the tissue where they will remain o Contains secretory granules with histamine o Degranulate when exposed to an antigen Histamine works as a vasodilator by binding to H1 receptors producing nitric oxide which is a potent vasodilator. Nitric oxide diffuses into vascular smooth muscle cells, causing them to relax. Histamine also increases the permeability of the blood vessel walls allowing for immune cells and more factors to pass through. Agranulocytes do not have any observable cytoplasmic granules. Some examples are: - Lymphocytes o Second most common type of white blood cell o Smallest in size o Round, densely stained nucleus and small amount of pale, non-granular cytoplasm o Present in both the blood and lymph o 3 functional subtypes: T-lymphocytes, B-lymphocytes and Null Lymphocytes (natural killer cells) ▪ T-lymphocytes have a long lifespan and are involved in cell-mediated immunity. Cytotoxic T-lymphocytes recognise cells with foreign antibodies and kill the cells Helper T-lymphocytes release factor to activate B-lymphocytes, CTLs and macrophages Suppressor T cells suppress the activity of B-lymphocytes and the immune response ▪ B-lymphocytes have variable lifespans and contribute to humoral immunity B-lymphocytes differentiate into antibody-producing plasma cells which are not considered white blood cells  ▪ Natural killer cells are part of innate immunity and attack cancer cells, foreign cells and virus infected cells - Monocytes o 2-10% of circulating white blood cells o Largest in size o Characteristic horseshoe shaped nucleus o Highly mobile and phagocytic o Leave the blood and transforms into macrophages in the tissues to carry out phagocytosis Immune response by these white blood cells can be divided into innate response and adaptive response. Innate response is the quick, non-specific response against pathogens while the adaptive response is the long-term, specific response against pathogens. The innate immune system is usually activated first and the adaptive immune system is activated when the innate immune system is unable to completely eradicate the foreign pathogens. The adaptive immune system is activated by the following mechanism: Platelets Platelets are small, non-nucleated cells. Most of the cytoplasmic organelles are found in the centre of the cell, thus they have a purple-stained granulated appearance while the periphery of the cell is poorly stained. Platelets are produced from the cytoplasm of a cell found in the bone marrow called megakaryocyte. Platelets bud from the periphery of the megakaryocyte and enters the bloodstream. Megakaryocytes divide via endomitosis which, unlike mitosis, does not involve cytokinesis as the final step. Platelets play a major role in blood clotting. Platelets adhere to the site of injury and are activated. These activated platelets attract more platelets to the site to form a clot. Thrombin present in the blood induces the formation of fibrin which stabilises the blood clot. However, not all blood clots are beneficial for the body. When a blood clot forms inside a blood vessel, the clot is called a thrombus. The thrombus can potentially break lose and move to other parts of the body. The thrombus is then called an embolus. A thrombus or embolus can partially or completely block blood flow in a blood vessel. Plasma Plasma is an aqueous solution of inorganic salts. There are also plasma proteins present such as: - Albumin which constitutes the bulk of plasma proteins and are responsible for the transport of fatty acids and other insoluble metabolites - Globulins which are the antibodies of the immune system and are responsible for the transport of lipids and heavy metal ions - Fibrinogen is a soluble protein which polymerises to form insoluble fibrin during blood clotting There are also lipoproteins present in the plasma which are soluble complex aggregates of lipids with specialised proteins circulating in the blood. They deliver insoluble lipids from tissues where they are synthesised to the tissues that utilise, degrade or store them. There are 2 types of lipoproteins: High-Density Lipoprotein (HDL) and Low-Density Lipoprotein (LDL) - HDL is commonly known as good cholesterol as it removes cholesterol from circulation to the liver where it is degraded  - LDL is commonly known as bad cholesterol as high levels of LDL can raise the risk of heart disease and stroke by forming atherosclerotic plaques Haematopoiesis Haematopoietic tissues give rise to RBCs, WBCs and platelets. They arise from haematopoietic stem cells. In adults, the red bone marrow is the major site of haematopoiesis. Urinary System The urinary system comprises of: - A pair of kidneys responsible for producing urine - A pair of ureters responsible for transporting urine to the urinary bladder - Urinary bladder responsible for temporarily storing urine - Urethra responsible for conducting urine to the exterior Kidney Besides producing urine, other functions of the kidney include elimination of nitrogenous waste and maintaining water and electrolyte balance. The kidneys also release renin responsible for regulating blood pressure and also erythropoietin to stimulate the production of red blood cells. The kidney is located behind the peritoneum and attached to the posterior abdominal wall. The right kidney is slightly lower than the left kidney due to the presence of the liver at the right side. The long axes of the kidneys are slightly oblique and moves with respiration. The external features of the kidney includes 2 poles, 2 surfaces, 2 borders and a hilum. The kidneys are covered by several layers of capsules/coverings: - Fibrous capsule (true capsule) is the innermost layer that surrounds the kidney - Perirenal fat (perinephric fat) - Renal fascia (false capsule) encloses the adrenal gland along with the kidney and fuses with the diaphragm - Pararenal fat The anterior relations of the kidneys are different while the posterior relations of the kidneys are the same with the exception that the right kidney is only related to one rib. The kidneys are attached to 4 muscles, which are mainly posterior abdominal wall muscles with the exception of the diaphragm. The other 3 muscles are psoas major, quadratus lumborum and transversus abdominis. The kidneys are innervated by 3 nerves: the subcostal, iliohypogastric and ilioinguinal nerve. Macroscopic structure of the kidney The structure of the kidneys comprises of the kidney proper and the sinus. The kidney proper comprises of an outer cortex located just below the renal capsule and extends between the renal pyramids as renal columns. The kidney proper also comprises of an inner medulla with 5-11 conical dark masses called renal pyramids. The apex of each renal pyramid form projections called renal papillae which invaginate the minor calyces. The renal sinus is a cavity of considerable size within the kidney. It opens at the medial border of the kidney as hilus. The renal sinus contains the greater part of the renal pelvis, major and minor calyces, renal vessels, lymphatics, nerves and fats. Histology of the kidney The nephron is the structural and functional unit of the kidney. Each nephron comprises of a glomerulus and a tubule system. The glomerulus is a tuft of capillaries surrounded by the Bowman’s capsule. The tubular system comprises of the proximal convoluted tubule, loop of Henle and distal convoluted tubule. Each collecting tubule begins from the distal convoluted tubule. Many collecting tubule unite to form the collecting duct which opens at the apex of the renal papillae. The nephron has 3 main functions: - Filtration of metabolic end products from the blood - Selective reabsorption of useful substances back into the blood - Secretion of some materials into renal tubules There are also different types of nephrons that have different functions. Cortical nephrons are confined mainly to the cortex and are responsible for sodium reabsorption. Juxta-medullary nephrons extend deeper into the medulla and are responsible for water reabsorption. The renal corpuscle is located in the cortical arches and consist of capillaries and the Bowman’s capsule. The structures intervening the glomerular capillary and Bowman’s space are the flattened endothelium of the capillary, a continuous basement membrane and a foot plate of podocyte cells found on the visceral layer of the Bowman’s capsule. The main function is the generation of glomerular filtrate. The proximal convoluted tubule is the longest and most coiled. It is lined by low columnar to cuboidal epithelium with a microvilli brush border present on the luminal surface. The main function of the proximal convoluted tubule is the reabsorption of glucose, amino acids, sodium, calcium carbonate and the majority of water and also the secretion of ammonium and creatinine. The loop of Henle consists of a descending limb (thin segment) and a ascending limb (thick segment). The main function of the loop of Henle is to reabsorb water, sodium and chloride. The distal convoluted tubule begins from the vascular pole of the nephron. It is lined by cuboidal epithelium but does not have a brush border present in the lumen. The main function is to reabsorb sodium, chloride and water. The collecting duct is the final part of the nephron. Its main function is to reabsorb sodium, chloride and water as well as to secrete ammonium, hydrogen ions and potassium. The blood to the kidneys is supplied by the renal artery which branch from the abdominal aorta. The blood from the kidneys enter the renal vein and goes to the inferior vena cava. The juxtaglomerular apparatus associated with the nephrons is involved in the regulation of blood pressure. It is made up of 3 components, the juxtaglomerular cells, macula densa and Lacis/Polkissen cells. Juxtaglomerular cells are modified smooth muscle cells found in the tunica media of the afferent arteriole at the point of contact with the distal convoluted tubule. In the juxtaglomerular cells, there are secretory vesicles containing renin. These cells are sensitive to blood pressure in the afferent arteriole. The macula densa is a specialised region in the wall of the distal convoluted tubule that comes into contact with juxtaglomerular cells. These cells are taller with dense nuclei and are sensitive to the concentration of sodium ions in the fluids present in the distal convoluted tubule.\ Lacis/Polkissen cells are extraglomerular mesangial cells found in the vascular pole in close relationship with the macula densa. The juxtaglomerular apparatus is responsible for the activation of RAAS which regulates blood pressure via the reabsorption of sodium chloride and water as well as vasoconstriction when the blood pressure is too low. Ureter The ureter is a narrow, thick-walled, expansile muscular tube with a length of approximately 25cm. It is segmented into the abdominal part and the pelvic part depending on the location where the ureter is found. There are 3 sites of anatomical constrictions of the ureter when it presses onto another body part. 1. At the pelviureteric junction 2. At the pelvic brim where it crosses the common iliac artery 3. At the uretero-vesical junction where the ureter enters the bladder It is at these sites where kidney stones will commonly be lodged. The ureter derives its arterial blood supply from the branches of all the arteries related to it. This means that whatever artery that is close to the ureter will branch out in order to supply the ureter. The ureter gets its blood supply from different arteries depending on the location of the ureter. Histologically, the upper region of the ureter has 2 layers of muscle while the lower region has 3 layers of muscle. The epithelium of the urinary tract is made up of transitional epithelium/urothelium that is only found in the urinary tract. It is characterised by dome-shaped umbrella cells at the topmost layer. Urinary bladder It is located in the anterior part of the lesser pelvis, immediately behind the pubic symphysis. The location of the urinary bladder varies with the amount of urine it contains as well as with age. In the urinary bladder, there is rugae present when it is empty to allow for its expansion to hold urine. When the bladder is distended, the rugae will not be visible. At the posterior surface (base) of the bladder, there is an area called the trigone of the bladder that does not have any rugae present. Histologically, the urinary bladder is also lined by transitional epithelium/urothelium. When the bladder is full, the intermediate layer of the epithelium has lesser layers compared to when the bladder is empty. Urethra The structure of the urethra varies between males and females. The male urethra is made of 3 parts: prostatic urethra, membranous part of urethra and penile (spongy) part of urethra. The female urethra is much shorter and is not segmented into different parts. In both males and females, there is an internal urethral sphincter and an external urethral sphincter which control the passage of urine. The internal urethral sphincter is found at the bladder neck and is derived from the bladder musculature of the trigonal region. These muscles are smooth muscles innervated by sympathetic fibres, making them involuntary. The external urethral sphincter surrounds the membranous part of the urethra and is derived from the sphincter urethrae muscle. These muscles are skeletal muscles innervated by somatic fibres, making them voluntary. Nervous System The nervous system comprises of the central nervous system and the peripheral nervous system. The central nervous system includes the brain and the spinal cord while the peripheral nervous system includes nerves that branch off from the central nervous system. Brain The brain is located in the cranial cavity and is protected by the skull and the meninges and cerebrospinal fluid which acts as a shock absorber. The brain can be divided into the forebrain, midbrain and hindbrain. The forebrain includes the cerebrum and the diencephalon which includes the thalamus and hypothalamus. The hindbrain includes the medulla oblongata, pons and cerebellum. The midbrain is a short segment (~2cm) between the forebrain and hindbrain. The brainstem includes the midbrain, pons and medulla oblongata. Spinal cord The spinal cord is located in the vertebral column. It is also protected by the meninges and cerebrospinal fluid. The spinal cord is connected to the peripheral nervous system by 12 pairs of cranial nerves and 31 pairs of spinal nerves. Histology of the nervous system There are 2 types of cells present in the nervous system. - Neurons are the structural and functional unit of the nervous system. They are excitable and generate/conduct electrical impulses. - Glial cells are non-conducting cells. They mainly support and protect the neurons. Neuron Neurons have diverse morphological features that are based on the organ that they are found in and also the function that they serve. However, the features that are listed above are common for all neurons. There are several ways in which a neuron can be classified. - By morphology o Multipolar o Bipolar o Pseudounipolar - By information flow – determined by where the axon is projecting o Projection neurons – axons project away from where the neurons originate o Interneurons – axons are localised to the area where the neurons originate - By effects on target o Excitatory – most projection neurons are excitatory o Inhibitory – most interneurons are inhibitory - By neurotransmitter o Glutamatergic o GABAergic o Cholinergic o Dopaminergic The cell body of a neuron contains the nucleus and various cytoplasmic organelles, cytoskeletal elements and inclusions. The Golgi complex is located near the nucleus and the mitochondria can be found throughout the cytoplasm. The axons of a neuron conduct electrical impulses away from the cell body. Most axons are long, slender processes that arise from the axon hillock and branch at the distal end. The axonal cytoplasm lacks ribosomes, rough endoplasmic reticulum and Golgi complex. New proteins have to be transported from the cell body to the terminus (anterograde transport) while old proteins have to be transported from the terminus to the cell body (retrograde transport). Axons are also myelinated by glial cells for better conductivity. Each unit is called an internode and is surrounded by Node of Ranvier. The dendrites of a neuron conduct electrical impulses toward the cell body. Dendrites are relatively shorter but highly branched. They also contain all the cytoplasmic components found in the cell body with the exception of the Golgi complex. Synapses are found between 2 neurons or between a neuron and an effector cell.