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4/25/11 General • Exchanging materials with environment happens at cellular level via diffusion • Therefore aqueous environment essential – easy in aquatic, unicellular or simple multicellular animals – simple diffusion inadequate for larger animals Transport of internal fluids & Gas Exchange • Not differentiated in many organisms • Highly interconnected in all. • Circulatory system: hemolymph, blood • Respiratory system: transport of O2 and CO2: body ⇔environment via gills or lungs, trachea systems on land 1 4/25/11 Circulation 1. Gastrovascular cavity – Cnidarians, Flatworms: few cell layers, all tissues bathed directly Circulation 2. Open – Mollusks (except cephalopods), Arthropods – Hemolymph baths internal organs directly in sinuses 2 4/25/11 Circulation 3. Closed – Annelids, Cephalopods, vertebrates – blood confined to vessels, and transported to tissues/cells Basic structure of closed system 1. vessels: arteries ⇒arterioles ⇒ capillaries ⇒ tissues ⇒ venules ⇒ veins 2. pump: muscular walls of arteries (heart) 3. Unidirectional: heart ⇒gills ⇒ systemic circuit 4. Double circulatory system of tetrapods – pulmonary – systemic 3 4/25/11 Basic structure of closed system 4. Double circulatory system of tetrapods – pulmonary – systemic Heart - Tetrapod • Chambers • Atrium: receives blood returning from systemic & pulmonary circuits • +O2 from pulmonary • -O2 from systemic 4 4/25/11 Heart - Tetrapod • Ventricle: receives blood from atrium, contracts (very muscular) to push blood out • Pulmonary to lungs back to atria ⇒ventricle ⇒ systemic Heart - Tetrapod • Ventricle undivided in amphibians (3 chambers) • partially divided in most reptiles (3ish chambers) • completely divided in birds & crocs, mammals (i.e. four chambers, no mixing of unoxygenated and oxygenated blood) 5 4/25/11 Heart - Tetrapod • Ventricle undivided in amphibians (3 chambers) • partially divided in most reptiles (3ish chambers) • completely divided in birds & crocs, mammals (i.e. four chambers, no mixing of unoxygenated and oxygenated blood) Heart - Tetrapod • Ventricle undivided in amphibians (3 chambers) • partially divided in most reptiles (3ish chambers) • completely divided in birds & crocs, mammals – (i.e. four chambers, no mixing of unoxygenated and oxygenated blood) 6 4/25/11 Heart - Tetrapod • Valves: assure one-way flow: atrioventricular (AV); semilunar valves (base of pulmonary artery, aorta) Cardiac cycle 1. Heart beat: • Diastole: relaxation of the heart muscles (atria and ventricles fill) 7 4/25/11 Cardiac cycle 1. Heart beat: • Diastole: relaxation of the heart muscles (atria and ventricles fill) • contraction = systole – FIRST, atrial systole – lub - recoil of blood against closed AV valve Cardiac cycle 1. Heart beat: • Diastole: relaxation of the heart muscles (atria and ventricles fill) • contraction = systole – FIRST, atrial systole – lub - recoil of blood against closed AV valve • contraction = systole – SECOND, ventricular systole – “dub - recoil against semilunar valve • cycle ca 0.8 sec. in human at rest 8 4/25/11 Cardiac cycle 2. Heart rate = beats/min – affected by oxygen debt, temperature, hormones etc. 3. Stroke volume = the amount of blood pumped in a single contraction 4. Cardiac output = vol blood/min by left ventricle (rate & vol/beat dependent) Cardiac cycle 5. SA node (pacemaker) in wall of right atrium sets pumping rhythm • AV node acts as relay • Pacemaker 9 4/25/11 Blood flow 1. Obeys laws of fluid dynamics; flows under pressure, faster in larger vessels, slowest in capillaries 2. Elastic quality of arteriole walls helps smooth out pressure 3. Blood pressure: peripheral resistance of arterial walls, measured in mm mercury Blood flow • The critical exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries • The difference between blood pressure and osmotic pressure drives fluids out of capillaries at the arteriole end and into capillaries at the venule end 10 4/25/11 Lymphatic System • Collects blood lost through capillary action (ca. 1%) • Collects interstitial fluid (lymph) • Lymph vessels drain into veinous circulation near the neck • Lymph nodes filter lymph Blood • Connective tissue: cells in a liquid matrix “plasma , • solutes (salts, electrolytes for osmotic balance) dissolved in water (90%) • Average human 4-6 liters 11 4/25/11 Blood • • • • • RBCs - Erythrocytes most numerous mammal RBC lack nuclei all RBCs lack mitochondria major function: O2 transport via hemoglobin, an iron carrying protein that binds O2, also nitric oxide (NO) Formed in red marrow of bone (esp. ribs, vertebrae) Blood WBCs - Leukocytes • travel in interstitial fluid to fight infections • 5 kinds: – Monocytes: migrate & differentiate as part of immune response – Neutrophils: first defense against microbes – Basophils: allergic reactions (anticoagulant & vasodilator) – Eosinophils: allergic reactions & parasites – Lymphocytes: vertebrate adaptive immune response 12 4/25/11 Blood Platelets • chips of cells (pinched off cytoplasm from marrow cells) • function: clotting; fibrinogen a plasma protein seals leaks in vessels; multiple clotting factors RESPIRATION • Mitochondrial respiration consumes O2 and produces CO2. • Animals need to exchange gasesà respiration • Animals that are larger than a few cells cannot rely on diffusion (remember this occurs slowly over long distances) • Animals rely on bulk flow then diffusion: there is an intimate relationship between the circulatory system and the respiratory system. 13 4/25/11 Structure • Only requires a thin, moist simple squamous epithelium w/rich blood supply, interfaces with medium (air or water) • Many animals have specialized respiratory organs with large surface areas (to maximize gas exchange): gills (outpocketings, water) or lungs (infolding, air) • Remember the skin is also an important gas exchange surface for some animals (cutaneous gas exchange). Gills • External extensions of pharynx, feathery, delicate • Advantages: water keeps gills constantly moist • Disadvantages: [O2] in water is low compared to air 14 4/25/11 Air • high [O2] , easier to ventilate • but respiratory surface looses H2O through evaporation • air utilized via invaginating respiratory surface into body Tracheae - Insects • tiny air tubes via spiracles • carry gases directly to tissues, do not use circulatory system 15 4/25/11 Lungs • vascularized mantel of land snails • booklungs - spiders • air sacs of terrestrial vertebrates Mammalian Lung • Paired invaginations restricted to single location, circ system must bridge gap to other parts of body • Trachea, bronchus, bronchiole, alveoli • Ventilation: negative pressure breathing rib cage and diaphragm 16 4/25/11 Birds • anterior and posterior air sacs • two cycles inhalation and exhalation required Temperature and Thermal Environment • Temperature: a measure of the amount of heat energy present. – Organisms are largely governed by the response of water to temperature. • Macroclimate: The climate at the level of biomes. • Microclimate: The climate that an organism experiences: can vary considerably over short distances. 17 4/25/11 Microclimate • • • • • • Altitude Aspect Vegetation Color of ground Exposure Topography Strategies for responding to thermal environments • Some confusing terminology… – – – – – – – – – Endotherm Ectotherm Poikilotherm Warm-blooded Cold-blooded Homeotherm Heterotherm* Stenotherm* Eurytherm* * terms not used in your textbook 18 4/25/11 Strategies for responding to thermal environments Classic distinctions • Poikilothermy: body temperature fluctuates with environment. • Homeothermy: body temperature remains constant • Ectothermy: body temperature regulated by external sources. • Endothermy: body temperature regulated by internal metabolic sources. • Poikilothermy/Homeothermy & ectothermy/endothermy vary on a continuum. Strategies for responding to thermal environments More distinctions • • Homeothermy: Body temperature remains constant. Heterothermy: Body temperature varies. • Note that endothermy & ectothermy are distinguished by source of heat, not body temperature. • Organisms can be poikilothermic homeotherms. How? • Organisms can be endothermic heterotherms. How? 19 4/25/11 Strategies for responding to thermal environments More terminology • • Eurythermic: wide tolerance range. Stenothermic: narrow tolerance range. • Examples: Tropical vs. Arctic terrestrial animals. • Examples: Polar fish vs. intertidal fish. Antarctic Ice Fish thermal tolerance range 6 ºC (-1.8º to 4ºC) Intertidal goby thermal tolerance range >30 ºC (8º to 40ºC) Acclimation • Changes in physiological or biochemical processes in response to some environmental factor. • Permits organisms to tolerate temperatures one season that would be fatal or sub-optimal in another. 20 4/25/11 Acclimation • Involves biochemical or physiological adjustments. • Changes are relatively short term and are reversible. • Not all organisms can acclimate. – Depends on the amount of variation in the environment. – In what environments/ organisms would you expect to see acclimation? Hibernation & Estivation • State of reduced metabolism that may last several months. • To avoid cold: hibernation. • To avoid heat: estivation. • Must rely on stored energy reserves. • Lower metabolic rate reduces loss of these reserves. Hibernating ground squirrels may have core temperatures as low as -2ºC Estivating lungfishes seal themselves in a mud/mucus ball as lake beds begin to dry up Check out this website on frozen frogs! It s very, very cool 21 4/25/11 Why thermoregulate? • Biochemistry. • Maintain a metabolically active temperature when external temperatures vary. • Why would thermoregulation evolve? (Later) Why thermoregulate? • • • • Biochemistry Increased rate of many chemical reactions. Affects solubility. Too high temperatures denatures proteins. Too low temperatures result in freezing. 22 4/25/11 Why thermoregulate? • • • • Thermal neutral zone The range of environmental temperatures over which the metabolic rate of a homeothermic animal does not change. Not a problem for poikilothermic homeotherms (external environment does not vary). Big problem when external environment varies considerably. Permits endothermic homeotherms to live in environments with higher temperature fluctuations. Thermoregulation • Regulation of body temperature. • Must manipulate heat gain and loss. • Via energy transfer processes: Hs = Hm ± Hcd ± Hcv ± Hr - He • What are these variables? 23 4/25/11 Hs = Hm ± Hcd ± Hcv ± Hr - He • Hs: Heat stored in the body (this is what is being thermoregulated!) • Hm: Metabolic heat (heat gain through cellular respiration) • Hcd: Conduction (transfer of heat between two objects). • Hcv: Convection (transfer of heat between solid and liquid or air). • Hr: Radiation (transfer through electromagnetic radiation). • He: Evaporation (heat loss due to evaporation). Temperature regulation in ectotherms • Temperature dependent upon external environment. – What variables of Hs = Hm ± Hcd ± Hcv ± Hr - He can we ignore? • How then, do they thermoregulate? – Which variables can they manipulate? 24 4/25/11 Temperature regulation in ectotherms • Can manipulate conduction, convection, and radiation. • Behavioral patterns – Habitat selection – Posture • Color • Growth form Temperature regulation in ectotherms • When it is too cold the problem is to increase heat storage (maximize the + aspect of the equation; minimize the aspect). Hs = Hm ± Hcd ± Hcv ± Hr - He 25 4/25/11 Temperature regulation in ectotherms • When it is too hot the problem is to keep heat storage low (minimize the + aspect of the equation; maximize the - aspect). Hs = Hm ± Hcd ± Hcv ± Hr - He Temperature regulation in endotherms • Endotherms can maintain body temperatures using cellular respiration. • This is energetically expensive (high metabolic costs). • Can mitigate this using other mechanisms that have negligible metabolic costs. – Behavioral & Physiological – These manipulate Hcd, Hcv, and Hr. • Can mitigate this using strategies that decrease Hm and He. Hs = Hm ± Hcd ± Hcv ± Hr - He 26 4/25/11 Negligible metabolic costs • • • • Vasomotor responses Postural changes Insulation adjustments Microclimate choices How can conductance and convection change? • Insulation – Pilomotor response (goosebumps) – Seasonal fur – Fat/blubber • Vasoconstriction & vasodilation • Burrow or huddle (can also decrease Hr) • Counter-current exchange. 27 4/25/11 How can conductance and convection change? • Insulation – Pilomotor response (goosebumps) – Seasonal fur – Fat/blubber • Vasoconstriction & vasodilation • Burrow or huddle (can also decrease Hr) • Counter-current exchange. Decreasing metabolic costs • Avoidance and habitat selection • Nocturnal activity • Hibernation or estivation • Torpor 28 4/25/11 Decreasing metabolic costs • Avoidance and habitat selection • Nocturnal activity • Hibernation or estivation • Torpor Decreasing metabolic costs • Avoidance and habitat selection • Nocturnal activity • Hibernation or estivation • Torpor 29 4/25/11 Decreasing metabolic costs • Avoidance and habitat selection • Nocturnal activity • Hibernation or estivation • Torpor: Facultative decrease of metabolic rate. – Are hummingbirds homeothermic??? Hummingbirds can have an active body temperature of 40ºC (=104ºF). This is the highest body temperature of any bird. During torpor they may lower this to 12ºC! Manipulating He • Sweat (animals). • Breathing. – Panting. • All are tied in with vasomotor control. – Evaporative surfaces often heavily vascularized. 30 4/25/11 Why did endothermy evolve? • Metabolically expensive--costs involved. – Difficult if food is scarce. – Metabolic costs increase 10X--need to eat more. • What are the benefits? Why did endothermy evolve? • Metabolically expensive--costs involved. • What are the benefits? – Increased locomotor activity via increased aerobic ability. – Increased responsiveness in varying environments. – Ability to exploit more diverse environments. 31 4/25/11 Summary • Animals and plants use numerous mechanisms to cope with temperature variation. • There is a broad continuum of thermoregulatory strategies. • Thermoregulation can be accomplished through numerous behavioral, morphological, and physiological pathways. Osmoregulation • Balances the uptake and loss of water and solutes • Cells & tissues are bathed in internal fluids, cannot tolerate dramatic changes in osmotic content – Regardless of external environment: – Marine, freshwater, terrestrial; transitional, fluctuating 32 4/25/11 Osmolarity • Concentrations of solutes in fluids – Measured in milliosmoles/ L • Hyperosmotic: higher solute concentration • Hypoosmotic: lower solute concentration • Isoosmotic: equal osmolarity Osmolarity • Water and salts will move down their concentration gradients • This presents different challenges for marine, freshwater, and terrestrial organisms. 33 4/25/11 Transport Epithelium • Is the specialized tissue that regulates solute movement. • Active transport of ions, small molecules. • Controls permeability to water. Osmoconformers • Body fluids are isoosmotic with environment. • Most marine invertebrates. • Any freshwater? • Do not lose or gain water. • Are there costs to this? 34 4/25/11 Osmoregulators • Adjust internal osmolarity: body fluids have different osmolarity from environment. • What they do depends on habitat, adaptation, and phylogeny. Tolerance • Most animals (whether osmoconformers or osmoregulators) cannot tolerate broad changes in external osmolarity: Stenohaline (what does this word remind you of?) 35 4/25/11 Tolerance • Euryhaline animals can tolerate broad changes. • Example: salmon travel from freshwater to saltwater to freshwater to breed Osmoregulators: Chondrichtyes • Which animals are these? • Where do they live (for the most part)? • Sharks and relatives are hyperosmotic (gain water through osmosis). – Because retain urea dissolved in body fluids Urine gets rid of excess water. Salt gland gets rid of excess sodium. 36 4/25/11 Osmoregulators: Marine Osteichthyes • What kind of animals are these? • Bony fishes evolved in freshwater, maintain ancestral freshwater osmolarity • Are therefore strongly hypoosmotic (lose water through osmosis). Drinking constantly. Get rid of salts through chloride cells in gills and urine Osmoregulators: Marine Reptiles • Have salt glands • Marine birds: nasal salt glands • Crocodile tears • Marine turtles and cloacal salt glands • Marine iguanas: nasal salt glands 37 4/25/11 Freshwater animals • Problems are opposite to marine • ALL animals are hyperosmotic to freshwater, so osmotically take in water • Constantly urinating • Expend energy to maintain salts Terrestrial animals • Desiccation greatest threat to life on land • Humans die if water loss exceeds 12% • Helps explain why so few colonizations of terrestrial environments 38 4/25/11 Water balance in terrestrial animals • Water balance is acquisition vs loss • Acquisition – Wd = drinking – Wf = food – Wa = absorption • Loss – We = evaporation – Ws = secretions • Overall – Wia = Wd + Wf + Wa - We - Ws Adaptations to terrestriality involve maximizing acquisition while minimizing loss Water balance in terrestrial animals • Acquisition strategies when water is limiting: – Condense fog. – Metabolic water from food (oxidation of glucose). • Water conservation: – Prevent evaporation with waterproofing cuticle. – Restrict time or place of activity. – Concentrate urine or feces. 39 4/25/11 Water balance in terrestrial animals • Acquisition strategies when water is limiting: – Condense fog. – Metabolic water from food (oxidation of glucose). • Water conservation: – Prevent evaporation with waterproofing cuticle. – Restrict time or place of activity. – Concentrate urine or feces. Water balance in terrestrial animals • Acquisition strategies when water is limiting: – Condense fog. – Metabolic water from food (oxidation of glucose). • Water conservation: – Prevent evaporation with waterproofing cuticle. – Minimize evaporation through behavior or morphological structures. – Restrict time or place of activity. – Concentrate urine or feces. 40 4/25/11 Water balance in terrestrial animals • Acquisition strategies when water is limiting: – Condense fog. – Metabolic water from food (oxidation of glucose). • Water conservation: – Prevent evaporation with waterproofing cuticle. – Restrict time or place of activity. • Estivation, nocturnality – Concentrate urine or feces. Water balance in terrestrial animals • Acquisition strategies when water is limiting: – Condense fog. – Metabolic water from food (oxidation of glucose). • Water conservation: – Prevent evaporation with waterproofing cuticle. – Minimize evaporation through behavior or morphological structures. – Restrict time or place of activity. – Concentrate urine or feces. 41 4/25/11 Nitrogenous wastes • Nitrogenous wastes come from the metabolic breakdown of proteins or DNA. • Byproduct is ammonia, which is highly toxic to tissues. • Not a problem for aquatic animals: simply rid ammonia into aqueous environment Nitrogenous wastes • Terrestrial animals cannot do this. • Expend energy to convert ammonia into less toxic form. • Mammals produce urea – Does not require dilution by lots of water – Water soluble, removed from fetus via placenta 42 4/25/11 Nitrogenous wastes • Terrestrial animals cannot do this. • Expend energy to convert ammonia into less toxic form. • Why does this not work for animals with shelled eggs? • Reptiles (including birds) and insects produce uric acid – Precipitate, paste-like, not water-soluble – Concentrations in pocket in egg, does not contaminate interstitial fluids The Excretory System • • These nitrogenous wastes are removed from the bloodstream by the excretory system Four components: 1. 2. 3. 4. Filtration: nonselective, water and solutes Reabsorption: returns valuable substances to body fluids Secretion: eliminates toxins and excess salts Excretion: altered filtrate (urine) leaves body 43 4/25/11 Excretory Systems • • • • Varies widely across animal taxa BUT similar theme where form meets function: network of tubules that provide a large surface area for the exchange of water and solutes Protonephridia in flatworms Simple tubular system with flame bulb filtration – Interstitial fluid ⇒ flame bulb filtration ⇒ external environment Excretory systems • Metanephridia: most annelids – Excretory tubules with internal opening. – Closed circulatory system: metanephridia surrounded by capillaries. – Metanephridia take in filtrate, selectively reabsorb solutes. – Terrestrial & freshwater annelids (?) live in hypoosmotic environment, metanephridia eliminate large amounts of water. – Marine annelids isoosmotic, eliminate ammonia wastes. 44 4/25/11 Excretory systems • Malphighian Tubules: Terrestrial arthropods • Open circulatory system • Dead end tips open into hemolymph • Transport epithelium secretes nitrogenous wastes and solutes into tubules (water follows, how?) • Transports to rectum, water and solutes reabsorbed, uric acid eliminated Excretory systems • • • • • Vertebrate kidney Paired compact organs, principal site of water balance and salt regulation Each kidney is supplied with blood by a renal artery and drained by a renal vein Urine exits each kidney through a duct called the ureter Both ureters drain into a common urinary bladder, and urine is expelled through a urethra 45 4/25/11 Kidney • The mammalian kidney has two distinct regions: an outer renal cortex and an inner renal medulla Kidney • The nephron is the functional unit of the kidney – Single long tubule and a ball of capillaries called the glomerulus – Bowman s capsule surrounds and receives filtrate from the glomerulus 46 4/25/11 Kidney • • • Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman s capsule Filtration of small molecules is nonselective The filtrate contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules Kidney • • From Bowman s capsule, the filtrate passes through three regions of the nephron: the proximal tubule, the loop of Henle, and the distal tubule Fluid from several nephrons flows into a collecting duct, all of which lead to the renal pelvis, which is drained by the ureter 47 4/25/11 Kidney • Vasa recta are capillaries that serve the loop of Henle • The vasa recta and the loop of Henle function as a countercurrent system Kidney • The mammalian kidney conserves water by producing urine that is much more concentrated than body fluids • The cooperative action and precise arrangement of the loops of Henle and collecting ducts are largely responsible for the osmotic gradient that concentrates the urine • NaCl and urea contribute to the osmolarity of the interstitial fluid, which causes reabsorption of water in the kidney and concentrates the urine 48 4/25/11 Urine Concentration in the Kidney • Loop of Henle maintains interstitial gradient of NaCl – Increases in the descending limb – Decreases in the ascending limb – Energy is expended to maintain the gradient between the medulla and cortex Urine Concentration in the Kidney • Urea diffuses into the interstitial fluid of the medulla from the collecting duct • Filtrate makes three trips between the cortex and medulla • As filtrate flows past interstitial fluid of increasing osmolarity, more water moves out by osmosis, conserving water and concentrating urine. 49 4/25/11 Diversity of the Kidney • Reptiles tend to have shorter loops of Henle, conserve by elimination of uric acid (energetically expensive) • Mammals in arid environments have extremely long loops Hormonal control of the kidney • How are water and solute levels regulated? • Decision: conserve or flush… 50 4/25/11 Hormonal control of the kidney • Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney • An increase in osmolarity triggers the release of ADH, which helps to conserve water Hormonal control of the kidney • Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney • An increase in osmolarity triggers the release of ADH, which helps to conserve water 51 4/25/11 Hormonal control of the kidney • ADH binds to collecting duct cells • Signals activation of aquaporin water channels • Increases permeability of cell wall to water Hormonal control of the kidney • Why do caffeine and alcohol make us need to urinate (or insert favorite euphemism here)? 52 4/25/11 Summary • Maintaining homeostasis in osmolarity presents a challenge for all animals. • Habitat (marine, freshwater, terrestrial) and phylogeny affect the solutions found in the diversity of animals. • Homeostasis and excretion of waste are intricately linked. Neurons are excitable cells • A cell can rapidly alter its membrane potential in response to an incoming signal. • A change in membrane permeability to certain ions causes the change in membrane potential. • Neurons use this change in membrane potential to carry electrical signals… often across longggg distances. 53 4/25/11 Motor Neurons • We ll first focus on motor neuronsthese communicate between the central nervous system and the skeletal muscles. Why do I keep saying neuron instead of nerve ? • A nerve is a bundle of axons encased in sheath found in peripheral nervous system. • Nerves can have mixed functions – May contain sensory fibers as well as motor fibers, for example. 54 4/25/11 Neuron Structure • Neurons usually receive signals at one end and conduct a signal in one direction to the other end, so they have a certain polarity. • They receive signals at the dendrites and the cell body. • The signal is converted into an electrical signal and passed to the cell body. Neuron Structure • Signal integration occurs in a specialized region between the cell body and the axon, called the axon hillock. • If the signal is sufficiently large (above a particular threshold), an action potential is initiated. 55 4/25/11 Neuron Structure • The action potential passes down the axon which is specialized for signal conduction. • Axons may be very short or VERY long. • Vertebrate axons are wrapped with a myelin sheath which speeds up conduction. Neuron Structure • The action potential passes down the axon to the axon terminal where the signal is transmitted to the target cell (in the case of motor neurons, this is skeletal muscle). • The axon terminal forms a synapse with the target cell, and the electrical signal is transduced into a chemical signal (neurotransmitter). • The neurotransmitter diffuses across the synapse and binds to receptors on the muscle cell membrane. 56 4/25/11 Electrical Signals • Neurons can rapidly alter their membrane potential in response to a signal. • Most neurons have a resting potential of -70mV. • This means that when the neuron is not involved in sending signals, the inside of the cell membrane is more negatively charged than the outside of the membrane. Electrical Signals • Depolarization occurs when the charge difference between the inside and outside decreases and becomes less negative. • This can happen when positively charged ions enter the cell or negatively charged ions leave the cell. 57 4/25/11 Electrical Signals • Hyperpolarization occurs when the membrane potential becomes more negative. • Negative ions may enter the cell or positive ions may leave. Membrane Potential (Vm) • Only two factors are required to establish a potential difference across a membrane: a concentration gradient for an ion and a membrane that is permeable to that ion (also need to know the ion s charge). Here we have more K+ inside, so it would favor outward movement of K+. And we have more Na+ outside, so this would favor inward movement of Na+. There is electroneutrality across the membrane. 58 4/25/11 Membrane Potential • To create a membrane potential, we insert K+ channels. This allows K+ to move out, but no other ions may move across the membrane. Membrane Potential • When K+ moves out, it creates a local region of negativity on the inner surface of the membrane and positivity on the outer surface. 59 4/25/11 Membrane Potential • The negative charge inside generates an electrical force that tends to draw positive charges back inside. • So, the K+ is battling the force of its concentration gradient as well as this electrical gradient. Membrane Potential • K+ ions continue to move across the membrane, but their inward and outward movements exactly balance each other. • The potential difference (voltage) under these equilibrium conditions is called the equilibrium potential. 60 4/25/11 Membrane Potential • The actual number of ions needed to create this membrane potential is VERY SMALL! • This is a local phenomenon right at the cell membrane. • The ion movements we ll be talking about (across the cell membrane) are not enough to change the overall concentrations of these ions inside the cytoplasm or in the extracellular fluid. Membrane Potential • That example was SIMPLE, considering movement of K+ only. • In a neuron, resting Vm is determined by 2 factors: 1) Large chemical gradients for K+ and Na+ across plasma membrane Na+ / K+ ATPase is required to maintain this chemical gradient but does NOT generate the resting electrical membrane potential AND 2) Permeability of plasma membrane to ions Membranes of resting cells are permeable to K+ K+ flux via passive (leaky) K+ channels is most important contributor to Vm Na+ flux also contributes to Vm 61 4/25/11 Membrane Potential • Neurons depolarize or hyperpolarize by selectively altering the permeability of their membranes to ions (by opening and closing gated ion channels). • Dependent on the membrane permeability to an ion that is at different concentrations on either side of the membrane. Membrane Potential • Open Na+ channels and Na+ flows IN due to electrochemical gradient until it reaches the equilibrium potential. • Membrane depolarizes due to + ions flowing in. • Open K+ channels and K+ flows OUT. • Membrane hyperpolarizes due to + ions flowing out. 62 4/25/11 So how do neurons communicate? • Remember the different parts of the neuron: – Signal reception – Signal integration – Signal conduction – Signal transmission Our example: vertebrate motor neuron • Signal is a neurotransmitter (remember in other neurons, this can be electrical, chemical, mechanical, etc.) • Neurotransmitter must bind to a receptor. • Receptor is a ligand-gated ion channel. • These receptors are concentrated on the dendrites and cell body. 63 4/25/11 Graded Potentials • The neurotransmitter may be in high or low concentrations resulting in varying responses in our neuron. • At low concentrations, only a few channels will open (permeability does not change very much). • At high concentrations, many channels open, increasing ion permeability a LOT… This is considered a graded potential changing membrane because the membrane potential will vary depending on the strength of the potential. incoming stimulus (the amount of NT). Graded Potentials • NT binds to a ligand-gated Na + channel causing it to open. • Na+ flows into channel causing depolarization. • That depolarization can spread along the membrane, but will decrease with distance due to resistance within the cell and current leakage through the cell membrane. 64 4/25/11 Graded Potentials • Graded potentials can be excitatory or inhibitory. • HOW? • If a receptor is more likely to depolarize the membrane it is excitatory; if a receptor is more likely to hyperpolarize the membrane it is inhibitory. • Na+ and Ca2+ channels are excitatory, K+ and Clchannels are inhibitory. Graded Potentials and Action Potentials • Because these graded potentials cannot be transmitted across long distances without degrading, the cell needs another way to transmit information across distances of more than a couple of mm. • Action potentials are triggered at the axon hillock when the voltage caused by graded potentials is high enough (above the threshold potential). 65 4/25/11 Graded Potentials and Action Potentials • Threshold potential is ~ -55mV. • The axon hillock must depolarize 15mV from resting potential in order to initiate an AP. Graded Potentials and Action Potentials • Graded potentials can result in action potentials in numerous ways… 1) A very strong graded potential may cause a large enough depolarization to pass to the axon hillock and cause an AP. 2) Graded potentials from different sites can interact with each other and add up to a large enough depolarization at the axon hillock to cause an AP: spatial summation. 3) Graded potentials can occur at two slightly different times, but can combine to create a large enough depolarization at the axon hillock to cause an AP: temporal summation. 66 4/25/11 Spatial Summation Graded potentials from different sites can interact with each other and add up to a large enough depolarization at the axon hillock to cause an AP: spatial summation. Temporal Summation Graded potentials can occur at two slightly different times, but can combine to create a large enough depolarization at the axon hillock to cause an AP: temporal summation. 67 4/25/11 Spatial and Temporal Summation Of course these two phenomena are happening at the same time! Action Potentials • Action potentials can be transmitted across LONG distances without degrading… so there s something else going on (compared to graded potentials). 68 4/25/11 Action Potentials • Three stages: Depolarization Repolarization After-hyperpolarization or undershoot Neuron can generate a new action potential only during certain phases of the AP. Absolute refractory period: NO NEW AP Relative refractory period: Need a very large stimulus to overcome the hyperpolarization. Action Potentials • AP starts when threshold potential is reached at the axon hillock. • Voltage-gated Na+ channels open. These are FAST opening. • Voltage-gated K+ channels also open, but they are SLOW… so we ll concentrate on Na+ first. 69 4/25/11 Action Potentials • Voltage-gated Na+ channels open and Na+ flows INTO the cell causing the membrane to depolarize (move toward the Na+ equilibrium potential). • As more Na+ flows in, the membrane depolarizes more, causing more voltage-gated Na+ channels to open, causing more Na+ to flow in… positive feedback. Voltage-gated Na+ channels • Na+ channels do not remain open forever. • There are two gates on this channel- an activation gate, and an inactivation gate. • At resting Vm, the activation gate is closed, and the inactivation gate is open. • As Vm increases, the activation gate is more likely to be open, allowing Na+ in, causing depolarization, resulting in more activation gates opening. 70 4/25/11 Voltage-gated Na+ channels • The inactivation gate closes after a certain amount of time (this may be different for different isoforms of the channel). • This happens to coincide with the Vm approaching +30mV (but it is NOT voltage sensitive). • With the inactivation gate closed, Na+ can no longer enter, terminating the depolarization phase of the AP. • Finally the channel returns to its initial conformation. Action Potentials • Remember that the K+ channels are also voltage sensitive, and they opened as well (slowly) when threshold was reached at the axon hillock. • They start to open in large numbers only shortly before the Na+ channels start to close. • K+ leaves the cell, making the inside more negative, causing repolarization. 71 4/25/11 Action Potentials • The difference in opening and closing kinetics of the Na+ and K + channels in response to a threshold depolarization results in repolarization occurring after depolarization. • Because K+ channels close slowly too, K+ ions continue to move out of the cell until it hyperpolarizes. • Na+/K+ ATPase pumps ions back across the membrane to restore Vm. Refractory Periods • Excitability changes across the AP. 72 4/25/11 APs travel long distances • It is not actually individual APs that flow down the axon. • Each AP triggers an AP in adjacent regions of the axonal membrane… like a knocking over dominoes. • So, each AP is just as strong as the last one- no degradation. • AP can theoretically move up or down the axon, but in the organism, the AP is initiated at the axon hillock, so it always moves from there to the axon terminal. The Synapse • The neuron must transmit the signal (AP) across the synapse to the target cell. • Presynaptic cell, postsynaptic cell, synaptic cleft. • For a motor neuron, the synapse is the neuromuscular junction. 73 4/25/11 The Synapse • When the AP reaches the axon terminal, voltagegated Ca2+ channels open. • Ca2+ flows into the cell. • Ca2+ is a signaling molecule that causes vesicles to bind with cell membranes. • Synaptic vesicles contain neurotransmitter. • Each vesicle contains a certain amount of NT. (NT is not released in a smooth, graded fashion) The Synapse • NT must diffuse across the synaptic cleft. • Ca2+ is buffered in the presynaptic cell, keeping [Ca2+] low, limiting release of NT. • If there are many APs arriving, [Ca2+] rises, providing a stronger signal for exocytosis, and more NT release. 74 4/25/11 How is the signal terminated? • The neurotransmitter must be removed from the synaptic cleft, or it will over-stimulate the postsynaptic cell. • NT may diffuse away. • NT may be destroyed by enzymes in the synaptic cleft. • NT may be taken up by the presynaptic cell and other surrounding cells. How can the signal be sped up? • Invertebrates use large diameter neurons. • With a large diameter, resistance decreases, allowing faster currents (remember Ohm s law V=IR) • Vertebrates insulate the neurons with myelin. • This has the advantage of allowing thinner axons (not giant axons like inverts, so you can pack more into small spaces); and energetic savings (without the large volume of cytoplasm). 75 4/25/11 Myelination • Myelin acts as insulation, reducing current loss through leak channels. • AP jumps from Nodes of Ranvier (saltatory conduction). • Placement of nodes is critical for function because current carries AP from node to node. Why do animals need FAST axons? • Why have these two methods evolved: (large diameter axons (giant fibers) and myelinated neurons)? • Escape response in inverts • Vertebrates evolved very complex nervous systems so in order to integrate all of that information, rapid conduction is necessary. 76 4/25/11 Functional organization of the Nervous System • Most animals are bilaterally symmetrical • Allows for cephalization: concentration of sense organs and nervous integration centers at the anterior end of the body. • Animals move in a particular direction. 77 4/25/11 • Vertebrate CNS (central nervous system) is encased in bone and cartilage (brain and spinal cord). • The rest is the peripheral nervous system. The brain and the spinal cord are made up of gray matter (neuron cell bodies and dendrites) and white matter (bundles of axons and associated myelin sheaths). 78 4/25/11 CNS development • The brain and spinal cord develop from a hollow tube (neural tube). • The posterior portion forms the spinal cord and the anterior portion swells and forms 3 (basic) sections of brain. • The brain and spinal cord are hollow (ventricles in the brain) filled with cerebrospinal fluid (CSF). Vertebrate Brains • Brain size and structure varies greatly among vertebrates (birds and mammals have much larger relative brain sizes than other tetrapods). • Presumably this allows for more complex integration. 79 4/25/11 Vertebrate Brains • All vertebrates have the same basic structure of the brain, no new structures, just enlarged sections of brain. • In birds and mammals, the forebrain is enlarged. 80 4/25/11 • FOREBRAIN: • Telencephalon →cerebrum • Diencephalon →hypothalamus, thalamus, epithalamus. • FOREBRAIN: • Telencephalon →cerebrum • Specialized for information processing, perception, voluntary movement, learning • Diencephalon →hypothalamus, thalamus, epithalamus. • Specialized for processing and integrating sensory information, coordinating behavior, and maintaining homeostasis. Corpus callosum connects the two hemispheres allowing them to communicate. 81 4/25/11 MIDBRAIN is greatly reduced in mammals. Important for some coordination and sensory integration. HINDBRAIN includes the medulla oblongata15, pons14 and cerebellum25-29. Often called the primitive brain . Supports vital body functions (breathing, circulation and coordination of movement). Cerebellum integrates sensory input from eyes, ears and muscleà coordination. 82 4/25/11 The mammalian cerebrum integrates and interprets sensory information and initiates voluntary movements. Highly folded- WHY? Four Regions (based on names of bones that overlie them) Frontal lobe: reasoning, planning of action and movement, and some aspects of speech. Parietal lobe: movement, orientation, recognition and perception of stimuli. Occipital lobe: visual processing Temporal lobe: perception and recognition of auditory stimuli, memory and speech. 83 4/25/11 Some regions of the brain are organized topographically (homunculus). The best examples of these are the somatosensory cortex and the primary motor cortex. In these regions, various parts of the body are represented by disproportionate areas of the brain. The size of the cortical region typically reflects the number of sensory or motor neurons present in that body part, rather than the size of that body part. 84 4/25/11 Peripheral Nervous System • Afferent neurons- carry sensory information to integrating centers. • Efferent neurons- carry signals from integrating centers to govern physiological responses and behaviors. Peripheral Nervous System 85 4/25/11 Autonomic Nervous System • Involuntary, homeostatic regulation of most physiological functions. • Made up of sympathetic, parasympathetic and enteric nervous systems. • Sympathetic: fight or flight , active during stress or physical activity. • Parasympathetic: rest and digest , most active during rest. Autonomic Nervous System • Maintain homeostasis via: – Dual innervation: parasympathetic vs. sympathetic – Creates antagonistic action (stimulate or inhibit) – Basal tone: even at rest they produce some action potentials so that increases and decreases in AP frequency can alter the response in the target organ. 86 4/25/11 Autonomic Nervous System • All autonomic pathways contain two (2) neurons in series: – Preganglionic (in the central nervous system) – Postganglionic (efferent neuron in the periphery) Anatomical Differences • Cell bodies of preganglionic neurons are in different regions of the CNS – Sympathetic arise in the thoracic lumbar region – Parasympathetic arise in the hindbrain, cranial and sacral regions. 87 4/25/11 Anatomical Differences • Locations of postganglionic neurons are different: – Sympathetic: close to spinal cord – Parasympathetic: close to effector organ • So, sympathetic have short preganglionic and long postganglionic neurons, and vice versa for parasympathetic. Anatomical Differences • Sympathetic preganglionic neurons synapse with 10 or more postganglionic neuronsà widespread effects • Parasympathetic preganglionic neurons synapse with 3 or fewer postganglionic neuronsà more localized effects. 88 4/25/11 Neurotransmitters • For both systems, the preganglionic neurons release stimulatory neurotransmitter. • Postganglionic in parasympathetic: • Inhibitory or stimulatory, but slower acting. Neurotransmitters • Sympathetic: postganglionic cell typically releases norepinephrine. • Results in flight or fight response. • For instance, the heart rate will increase and the pupils will dilate, energy will be mobilized, and blood flow diverted from other non-essential organs to skeletal muscle. 89 4/25/11 Regulation • Autonomic NS regulates primarily through reflex arcs- simple neural circuits that do not involve the conscious centers of the brain. • Sympathetic and parasympathetic often work antagonistically. • Ex: control of blood pressure 90 4/25/11 Somatic Motor Pathways • Control skeletal muscle. • Usually under conscious (voluntary) control… a.k.a. voluntary nervous system . • Can you think of an example when muscle control is not under conscious control? • Reflexes! 91 4/25/11 Efferent Motor Pathways are different from the Autonomic N.S. • Only control one type of effector organ: skeletal muscle. • Cell bodies of motor neurons are located in the CNS, never in ganglia outside of the CNS. • Monosynaptic- only one synapse between the CNS and effector organ- can be LONG neurons. • Neurotransmitter always excitatory. Efferent Motor Pathways are different from the Autonomic N.S. • Synapse at neuromuscular junction splits into a cluster of axon terminals that branch out over the motor end plate. This allows the neuron to contact more than one muscle fiber. • Synaptic cleft is very narrow- diffusion across of NT is very rapid. • All motor neurons release acetylcholine (ACh). • Effect of ACh on skeletal muscle is ALWAYS excitatory. 92 4/25/11 Reflex • Patellar tendon (knee jerk) reflexmonosynaptic stretch reflex. Communication: Nervous & Endocrine Systems • Signaling in the body occurs in more ways than just via neurons (direct signaling) 93 4/25/11 Indirect Signaling 1. Release chemical messenger from signaling cell into extracellular environment. 2. Transport of the messenger to the target cell. 3. Communication of the message to the target via receptor binding. Endocrine Signaling • Usually slower than autocrine, paracrine and nervous communication. • Any ideas why? • Hormones are transported through the circulatory system- takes a while to get around the whole body. • Hormones may be long-lived, increasing the time they can have an effect. 94 4/25/11 Secretory Cells of the Endocrine System • Endocrine tissues are often grouped into specific structures called glands. • Alternatively, hormones can be secreted by cells within specific tissues (not glands). • Hormones are not secreted only in blood; they can be released into lymph or extracellular fluids. What is a hormone? • Hormones are chemical messengers that are secreted by endocrine glands or cells. • They are transported to distant target tissues where they bind to specific receptors and exert effects at very low concentrations. • To maintain homeostasis, hormone action must be terminated (regulated by negative feedback loops). • All known hormones are either Peptides, Steroids or Amines 95 4/25/11 Hormones • Hormonal action is mediated via specific receptors located on or within each target cell. • Many hormones have several different types of receptors resulting in different physiological responses to the same hormone. • Action depends on both the types of hormones and the types of receptors. Endocrine Control • Neuroendocrine System: Neurosecretory neurons secrete neurohormones into blood or extracellular fluid serving as a link between the nervous system and the endocrine system. • Peripheral endocrine system: Non-neural tissues secrete hormones into blood or extracellular fluid. Secretion can be controlled by neurohormones, direct innervation or by peripheral factors such as hormones, nutrients, ions, temperature. 96 4/25/11 Neuroendocrine Control • Hypothalamus important in controlling body temperature, thirst, hunger and many other physiological processes, ALSO regulates the function of the pituitary. • Pituitary (a.k.a. Hypophysis): Located ventral to the brain. Attached to the brain by a stalk (infundibulum). – Anterior pituitary – true endocrine gland. – Posterior pituitary – extension of neural tissue. Posterior Pituitary • • • • • Extension of neural tissue. Hypothalamic neurohormones are polypeptide hormones synthesized, processed and packaged into secretory vesicles in cell body of neurosecretory neuron. Transported to axon terminal at a capillary bed at the posterior pituitary. Hormones are carried in blood to target tissues. Releases non-tropic hormones vasopressin and oxytocin. 97 4/25/11 Anterior Pituitary • Neurohormones from the hypothalamus control hormone secretion from anterior pituitary (AP). • These neurohormones arrive at the AP via a specialized microcirculation called the hypothalamic-pituitary portal system. This allows the signal from the hypothalamus to arrive at the AP without being diluted through the systemic circulation. • Hypothalamus secretes releasing or inhibiting neurohormones, stimulating or inhibiting release of AP hormones which are released into the systemic circulation. • Produces polypeptide hormones some of which serve as tropic hormones that control growth, metabolism and reproduction. 98 4/25/11 • Adrenal Gland (two parts)situated on top of your kidneys – Medulla - adrenal chromaffin tissue – produces epinephrine. Sympathetic Responses – Cortex - cortisol. Stress Response. – Gonads - ovaries and testes; Comprised of both endocrine and gametogenic tissue. Produce steroids for reproduction, sexual behavior, maintenance of secondary sex characteristics, brain differentiation, production of gametes. – Thyroid gland: Thyroxine (T4) and Triiodothyronine (T3) for Metabolism and thermoregulation; Growth and differentiation; Reproduction Pancreas - an exocrine and endocrine organ. Endocrine tissue referred to as the Islets of Langerhans is dispersed among the exocrine tissue. β cells - insulin α cells – glucagon Gastrointestinal tract – Secretes various hormones involved in digestion Thymus – hormones for lymphocyte development (immune) Pineal Hypothalamus Posterior Pituitary Anterior Pituitary Heart Thyroid Parathyroids Stomach Liver Pancreas Adrenal medulla Adrenal cortex Kidney Testes Pineal Hypothalamus Posterior Pituitary Anterior Pituitary Heart Thyroid Parathyroids Stomach Liver Pancreas Adrenal medulla Adrenal cortex Kidney Testes 99 4/25/11 Hormonal feedback Nervous system Feedback Regulation provides many steps for control Neurohormonal non-tropic feedback Neurohormonal tropic feedback Regulation of Glucose Metabolism • Remember that hormones play a role in regulation of the majority of physiological processes. • We will use glucose regulation as an example. 100 4/25/11 Regulation of Glucose Metabolism • Mammals regulate glucose levels precisely because the brain metabolizes glucose primarily. • If glucose levels fall too low, the brain cannot function. • If glucose levels are too high, the osmolarity of your blood will be altered affecting all of the cells in your body. • Regulation of Glucose Metabolism The pancreas is the endocrine gland that secretes the hormones needed for regulation of glucose metabolism. • The pancreas acts as an exocrine gland (secreting digestive enzymes) and an endocrine gland. 101 4/25/11 Regulation of Glucose Metabolism • Islets of Langerhans are the endocrine tissue in the pancreas that contain α and β cells. • β cells secrete insulin when blood glucose levels rise. • With ↑ glucose, β cell metabolism increases, resulting in exocytosis of vesicles containing insulin. Regulation of Glucose Metabolism • Insulin travels through the blood to target tissues (liver, muscle, adipose tissue). • Target tissues have receptor which activates a signal transduction cascade ultimately causing the uptake and storage of glucose. • This results in a decrease in blood glucose levels. • Once blood glucose ↓, β cells stop releasing insulin and insulin levels in blood decline. (negative feedback) 102 4/25/11 Regulation of Glucose Metabolism • Glucagon is secreted from α cells in the islets of Langerhans of the pancreas. • When blood glucose ↓, glucagon is released into the blood. • Target cells with glucagon receptors release glucose in response resulting in increased blood glucose levels. • Glucagon and insulin have antagonistic (opposite) effects on blood glucose levels. Summary • The nervous and endocrine systems are intimately connected in order to maintain homeostasis, regulate growth and development, and affect behavior. • Nervous system: direct signaling, rapid responses, short-term effectors. • Endocrine system: indirect signaling, slow responses, long-term effectors. 103