. d. Fast-twitch fibers Fast-twitch fibers are skeletal muscle fibers that contract quickly and powerfully but fatigue rapidly. They rely primarily on anaerobic metabolism. e. Caul A caul is a membrane, part of the amniotic sac, that may cover a newborn's head or face at birth. It is rare and usually harmless. Mononuclear agranulocytes are white blood cells with a single, non-lobed nucleus and no visible cytoplasmic granules. The two main types are: a. Lymphocytes - Types : B cells, T cells, and Natural Killer (NK) cells. - Roles : B cells produce antibodies for humoral immunity. - T cells mediate cellular immunity (e.g., cytotoxic and helper functions). - NK cells destroy virus-infected and tumor cells without prior sensitization. b. Monocytes - Role :Circulate in the blood and migrate to tissues where they differentiate into macrophages or dendritic cells . - Macrophages perform phagocytosis , antigen presentation, and cytokine secretion. - Dendritic cells act as antigen-presenting cells to initiate immune responses. Together, mononuclear agranulocytes play crucial roles in adaptive and innate immunity, inflammation, and tissue repair. (2 marks each) - Vascular Spasm Immediate vasoconstriction of injured blood vessels to reduce blood loss. - Platelet Plug Formation Platelets adhere to exposed collagen, become activated, release chemical signals, and aggregate to form a temporary plug. - Coagulation (Blood Clotting) A cascade of clotting factors activates the conversion of fibrinogen to fibrin, stabilizing the plug into a solid clot. (3 marks each) Neuro-glial metabolic coupling refers to the metabolic interaction between neurons and astrocytes , ensuring energy supply during neuronal activity. - Glucose uptake : Astrocytes take up glucose from the blood. - Lactate production : Astrocytes metabolize glucose to lactate via glycolysis. - Lactate shuttle : Lactate is released and taken up by neurons as a preferred energy source during activation. - Regulation : This supports neurotransmission and prevents excitotoxicity through neurotransmitter clearance (e.g., glutamate uptake by astrocytes). - To convert lipophilic xenobiotics into hydrophilic forms for easier excretion. - To detoxify potentially harmful substances. - To sometimes activate prodrugs into their active pharmacological forms. --- Non-shivering Thermogenesis (8 Marks) - Thermoreceptor Activation :Cold exposure triggers thermoreceptors in the skin, sending signals to the hypothalamus to initiate heat production. - Sympathetic Nervous System Activation :The sympathetic nervous system (SNS) is activated and releases norepinephrine into brown adipose tissue (BAT) . - Norepinephrine Effect : Norepinephrine binds to β3-adrenergic receptors on BAT cells, activating adenylate cyclase and increasing cAMP levels. - Hormone-Sensitive Lipase Activation : cAMP activates protein kinase A (PKA) , which stimulates hormone-sensitive lipase , leading to lipolysis in BAT. - Fatty Acid Release :Lipolysis breaks down triglycerides (TAGs) into free fatty acids (FFAs) and glycerol , which are released into the bloodstream. - Fatty Acid Transport : FFAs are transported into the mitochondria of BAT for oxidation. - Beta-Oxidation :In the mitochondria, FFAs undergo beta-oxidation , producing acetyl-CoA , which enters the citric acid cycle . - Electron Transport Chain (ETC) :The citric acid cycle generates high-energy electrons, which fuel the electron transport chain (ET C) , creating a proton gradient across the inner mitochondrial membrane. - Uncoupling by UCP1 : Uncoupling Protein 1 (UCP1) in BAT uncouples the proton gradient, allowing energy to be released as heat instead of ATP, a process known as non-shivering thermogenesis . --- - Fuel Substrate Shifting : Low-intensity exercise : Muscles primarily use fatty acids as they are abundant and provide long-lasting energy. - High-intensity exercise : The body switches to glycogen , which provides a faster ATP supply for rapid energy production. - Oxygen Availability : Aerobic exercise : Sufficient oxygen allows muscles to use fatty acids and glucose for ATP production through oxidative phosphorylation. - Anaerobic exercise : When oxygen is limited, muscles rely on glycogen which is converted into lactate through anaerobic glycolysis. - Energy Efficiency : Fatty acids generate more ATP per molecule but are slower to metabolize, making them suitable for sustained, lower-intensity exercise. - Glycogen , while limited in storage, provides a faster ATP supply, making it ideal for high-intensity, short-duration exercise. - Hormonal Regulation : Insulin promotes lipogenesis , leading to fat storage in adipocytes. - Cortisol and catecholamines (e.g., epinephrine ) activate lipolysis , promoting the breakdown of fat for energy. - Dietary Factors : High carbohydrate intake increases insulin levels, which stimulates fat storage in adipose tissue. - Fatty acids from dietary fats are stored directly in adipose tissue, influencing the type of fat (saturated or unsaturated) stored. - Physical Activity : Exercise , especially aerobic activities, increases lipid oxidation by enhancing mitochondrial function and improving the enzyme activity involved in fat metabolism. - Genetic Factors : Genetic predisposition determines how much fat is stored, the type of fat (visceral or subcutaneous), and the efficiency of lipid metabolism . - Environmental/Lifestyle Factors :Factors such as sleep , stress , and a sedentary lifestyle can disturb hormonal regulation, leading to increased fat storage and metabolic disorders like obesity. - Passive Diffusion :Lipid-soluble molecules such as oxygen , carbon dioxide , and steroid hormones can diffuse directly across the blood-brain barrier (BBB) due to their ability to pass through the lipid-rich membrane. - Facilitated Diffusion :Larger, hydrophilic molecules like glucose and amino acids require specific transporters (e.g., GLUT1 for glucose, LAT1 for amino acids) to move across the BBB. This process doesn’t require energy but depends on the concentration gradient. - Active Transport :Active transport mechanisms, using ATP-powered pumps , actively transport essential ions (e.g., Na+ , K+ ) and molecules like dopamine and neurotransmitters into or out of the brain. This maintains ion balance and proper brain function. - Endocytosis :Certain large molecules, such as insulin and some proteins , are transported into the brain via receptor-mediated endocytosis . These molecules bind to specific receptors on the BBB endothelial cells, triggering internalization and transcytosis across the barrier. - Efflux Transport : P-glycoprotein and other efflux pumps actively remove unwanted or harmful substances (e.g., toxins, drugs) from the brain into the bloodstream, protecting the brain from harmful compounds. This process requires energy and helps maintain the integrity of the BBB. - Glycolysis Control Point : Phosphofructokinase-1 (PFK-1) - This enzyme is regulated by ATP (inhibits) and AMP (activates), controlling the rate of glucose metabolism. - Gluconeogenesis Control Point : Fructose-1,6-bisphosphatase - This enzyme is inhibited by AMP and fructose-2,6-bisphosphate, regulating the synthesis of glucose. - Citric Acid Cycle Control Point : Isocitrate dehydrogenase - Activated by ADP and inhibited by ATP and NADH, controlling the pace of the Krebs cycle. - Fatty Acid Synthesis Control Point : Acetyl-CoA carboxylase - Activated by insulin and inhibited by AMP-activated protein kinase (AMPK) and glucagon. - Fatty Acid Oxidation Cont