Exam 1 Study Guide
Exam 1 Study Guide ZOOL 4380
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This 16 page Study Guide was uploaded by Tiffany Schweda on Saturday February 6, 2016. The Study Guide belongs to ZOOL 4380 at University of Texas at El Paso taught by DR. ZAINEB AL-DAHWI in Spring 2016. Since its upload, it has received 194 views. For similar materials see Vertebrate Physiology in Animal Science and Zoology at University of Texas at El Paso.
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Date Created: 02/06/16
Exam 1 Study Information Overview of Chapter 3: Cellular Basis of Animal Physiology Everything that an animal does is due to the communication between the cells Communication happens when a signaling cell sends a signal to the target cell Chemical messengers can travel from signaling cells to nearby target cells by diffusion in a process called paracrine communication Messengers can even affect the signaling cell in a process called autocrine communication For long-distance cell to cell communication, animals use the endocrine system and nervous system In endocrine system the chemical messenger travels from the signaling cell to the target cell carried by the circulatory system Endocrine messengers are called hormones Types of cell signaling/cell communication o Direct cell signaling o Autocrine and paracrine cell signaling o Endocrine cell signaling o Neural signaling o Exocrine signaling Exocrine communication system is when animals send chemical messengers between individuals (pheromones) Biochemical Basis of Cell Signaling Cells are separated from their environment by a phospholipid membrane Most chemicals are either hydrophilic or hydrophobic which means that sending a chemical messenger from one cell to another can present a challenge General Features of Cell Signaling Gap junctions o composed of interlocking cylindrical proteins (for vertebrates: connexins) assembled in groups of 4 or 6 to form doughnut-like pores (for vertebrates: hemichannels or connexons) o specialized protein complexes hemichannels of two adjacent cells come together to form a hollow tube, connecting the two cells via an aqueous bridge gap junctions are between two adjacent cells in most physiological situations direct communication via gap junctions involves the movement of ions between cells Figure 1: (a) direct cell signaling (b) autocrine and paracrine signaling (c) endocrine signaling (d) neural signaling Figure 2: Gap junctions. Proteins called connexins (for vertebrates) or innexins (for invertebrates) form the structure of the gap functions Feature Autocrine/Paracrine Nervous Endocrine Exocrine Secretory Cell Various Neural Endocrine Various Target Cell Most cells in body Neuron, muscle, Most cells in Sensory and neural endocrine body Signal Type Chemical Electrical and chemical Chemical Chemical Maximum Short Long intracellularly, short Long Very long Signaling Distance across synapse Transport Extracellular Synapse Circulatory External system environment Speed Rapid Rapid Slower Various Duration of Short Short Longer Various Response Chart 1: Comparison of systems 3 Major Characteristics 1: Release of a chemical messenger from the signaling cell into the extracellular environment 2: Transport of chemical messenger through extracellular environment to the target cell 3: Communication of the signal to the target cell via receptor binding Indirect Signaling Systems Autocrine and paracrine communication the messenger simple diffused through the extracellular fluid from the signaling cell to the target cell Intracellular signaling also occurs across short distances in the nervous system at a structure call the synapse, a region where the signaling cell and the target cell are very close together Endocrine system can regulate the activities of distant cells, tissues and organs by sending chemical signals through the blood in the form of hormones In exocrine communication chemicals called pheromones are released by one individual and travels through the external environment to exert its effects on a different individual Only neurons act as the secretory cells in nervous communication Some neurons can secrete neurotransmitters directly into the circulatory system Secretory cells of exocrine and endocrine tissues are often grouped into structures called gland Figure 3: Structure of exocrine and endocrine glands. Exocrine glands secrete chemicals into ducts that lead to the surface of the body. Endocrine glands secrete hormones directly into the circulatory system Structure of Messenger Determines Type of Signaling Mechanism Six main classes of chemicals that are known to participate in cellular signaling 1. Peptides 2. Steroids 3. Amines 4. Lipids 5. Purines 6. Gases Feature Hydrophilic Messengers Hydrophobic Messengers Storage Intracellular vesicles Synthesized on demand Secretion Exocytosis Diffusion across membrane Transport Dissolved in extracellular fluids Short distances: dissolved in extracellular fluid Long distances: bound to carrier proteins Receptor Transmembrane Intracellular or transmembrane Effects Rapid Slower or rapid Chart 2: Comparison of Hydrophilic and Hydrophobic Chemical Messengers Mechanisms for hydrophilic or hydrophobic messengers o Peptides Released by exocytosis Most peptide hormones and neurotransmitters are synthesized in advance and stored for later use Peptide hormones are often synthesized as large and inactive polypeptides known as preprohormones Signal sequence is cleaved from preprohormone prior to being packed into secretory vesicles (forming prohormone) Secretory vesicle contains proteolytic enzymes that cut prohormone into the active hormone(s) Paracrine peptides, such as cytokines, are synthesized only on demand Dissolve in extracellular fluids Rate of breakdown is measured as the messenger’s half-life Bind to transmembrane receptors Hydrophilic signaling molecules such as peptides and proteins can’t pass through the membrane of target cell Extracellular portion of transmembrane receptor contains the ligand binding domain Figure 4: Synthesis of peptide hormones. Peptide hormones synthesized by ribosomes on the rough endoplasmic reticulum Figure 5: Synthesis of arginine vasopressin (AVP). AVP is synthesized on the rough endoplasmic reticulum as a large polypeptide (preprovasopressin), which contains a signal peptide (SP), neurophysin (NHP), and a glycoprotein (GP). Provasopressin passes to the Gogli apparatus where it is packed into vesicles. In secretory vesicles the provasopressin is cleaved into three peptide: AVP, NPH and GP Figure 6: Structure of a transmembrane receptor (a) transmembrane receptors have extracellular ligand-binding domain, membrane-spanning domain and intracellular domain (b) when ligand binds to receptor the conformation of receptor changes o Steroids Derived from the molecule cholesterol Important hormones for both vertebrates and invertebrates Mineralocorticoids are involved in regulating sodium uptake by the kidneys Important for fluid and electrolyte balance in body Glucocorticoids have widespread actions Increasing glucose production Increasing breakdown of proteins into amino acids Increasing release of fatty acids from adipose tissue Regulating immune system Inflammatory responses Glucocorticoids are known as the stress hormones Bind to carrier proteins Steroids can easily pass through biological membranes and can’t be stored within the cell o Must be synthesized on demand Diffuse across short distances dissolved in extracellular fluids Long distances they are bound to carrier proteins Some steroids bind to specific proteins Others bind to generalized proteins such as albumin (principle carrier protein in blood) Bind to intracellular receptors Lipophilic steroids can easily cross the membrane of target cells o Can bind either to transmembrane receptors or receptors in cell Intracellular steroid receptors act as transcription factors Figure 7: Transport of hydrophobic chemical messengers Communication of the Signal to the Target Cell When a ligand binds to its receptor the receptor undergoes a conformational change Change in the shape of the receptor sends a signal to the target cell Ligand-receptor interactions are specific Ligand-receptor interactions are extremely specific The binding site has a particular shape Only molecules sharing related structures are allowed to bind efficiently to the receptor Chemicals that bind to and activate the receptors are agonists Chemicals that bind to but don’t activate the receptors are antagonists Many drugs are either agonist or antagonist Figure 8: Ligand-receptor interactions Receptor type determines cellular response A target cell can respond to a ligand only if the appropriate receptor is expressed on/in the target cell Two cells that are side by side can be bathed in a chemical signal but only the cell that possesses the appropriate receptor will respond Hundreds of chemical messengers found in animals can be used in millions of combinations But any given cell responds to only a fraction of these signals – depends on the types of receptors that are present in the cell Receptors have several domains Receptors are large proteins that are composed of several domains Ligand-binding domain contains the binding sire for the chemical messenger Remaining domains of the protein convey its functional activity by interacting with signal transduction molecules within the cell The structure of the domain determines the nature of the ligands that can interact with the receptor A ligand may bind to more than one receptor Many receptors are part of large gene families These genes are transcribed into similar proteins Ligand-receptor binding obeys the law of mass action Ligand-receptor interactions are governed by the law of mass action Natural ligands usually bind reversibly to their receptors As ligand concentration increases, the balance shifts to the right and the proportion of receptor bound to ligand increases Receptor number can vary Target cells vary in number of receptors they possess The more receptors on a cell the more likely it is that a ligand will bind to the receptor at any given concentration of ligand o Leads to greater response in target cell Numbers of receptors on target call can change over time o This can be seen through the observation of drug administration When a person regularly consumes a drug the number of opiate receptors on the target cell decreases in an attempt to reduce the intensity of the pleasure signal and maintain homeostasis o Called down-regulation Receptors can also be up-regulated o Example: caffeine binds to the receptors for the neurotransmitter adenosine, the caffeine is an antagonist for the receptors, the end result is that the caffeine acts as a stimulant by removing adenosine’s calming effect Strength of binding between ligand and receptor can be expressed using the dissociation constant Strength of receptor-ligand interactions can be expressed with the affinity constant Figure: Effects of receptor concentration and affinity on the percentage of bound receptors. A: cells that have higher concentration of receptors have larger number of bound receptors at any given concentration of messengers. B: at a given concentration of messenger, cells with high-affinity receptors have a higher percentage of bound receptors and a greater response than cells with low-affinity receptors. Figure: Effects of Messenger Concentration Ligand signaling must be inactivated As long as a ligand remains bound to receptor it will continue to activate that receptor and cause a response in the target cell The signal must be terminated in order for the body to be able to respond to the changing conditions There are six ways in which the ligand signaling can be inactivated o A: ligand removed by distant tissue o B: ligand taken up by adjacent cells o C: ligand degraded by extracellular enzymes o D: ligand-receptor complex removed by endocytosis o E: Receptor inactivation o F: Inactivation of signal transduction pathway Figure: Termination of ligand- receptor signaling. Signal Transduction Pathway Transducers are devices that convert signals from one form to another All transducers have four components o Receiver o Transducer o Amplifier o Responder Intracellular receptors are located inside the cell and interact with hydrophobic chemical messengers Ligand-gated ion channels initiate a response in the target cell by changing the ion permeability of the membrane Receptor-enzymes induce a response by activating or inactivating intracellular enzymes G-protein coupled receptors send signals to an associated G protein which then initiates a signal transduction pathway that causes a response in the target cell Introduction to Endocrine System In a negative feedback loop the effector brings the variable back toward a predetermined set point Negative feedback loops help to maintain homeostasis by maintaining a regulated variable within a small range around the set point Feedback regulation Occurs at both local level and across long distances Paracrine and autocrine signals are responsible for local physiological control Reflex control mediates long-distance regulation In animals nervous and endocrine systems are responsible for regulating physiological systems across long distances Direct feedback loop o Endocrine cell itself senses a change in the extracellular environment and releases chemical messenger that acts on target cells First-order feedback loop o Provide slightly more sophisticated level of regulation involving the nervous system o Sensory organ perceives a stimulus and sends a signal via nervous system to an integrating center that interprets the signal Second-order feedback loop o Sensory organ perceives a stimulus and sends a signal via nervous system to an integrating center which sends a signal via neuron that secretes either a neurohormone or neurotransmitter that acts on an endocrine gland Third-order feedback loop o Sensory organ perceives a stimulus and sends a signal via nervous system to an integrating center which sends a signal via neuron that secretes either a neurohormone or neurotransmitter that acts on an endocrine gland, the gland then secretes a hormone that binds to a receptor on a second endocrine gland and triggers the secretion of a second hormone which then induces a response in target cells Figure: Feedback loops Pituitary hormones provide examples of several types of feedback loops Pituitary gland is closely associated with a part of the brain called the hypothalamus Connected to the hypothalamus by a narrow stalk called the infundibulum Pituitary gland is divided into two distinct sections o Anterior pituitary o Posterior pituitary Figure: hypothalamus and the posterior pituitary gland Posterior pituitary secretes neurohormones Posterior pituitary isn’t really an independent organ Extension of the hypothalamus Neurons that originate in the hypothalamus terminate in the posterior pituitary Oxytocin is involved in a positive feedback loop Most hormonal regulation involves negative feedback loops but oxytocin is an example of a hormone that is involved in a positive feedback loop It binds to receptors on the smooth muscle cells of the uterus causing them to contract Uterine contractions push the fetus against the cervix, increasing the stimulus on the stretch- sensitive cells This positive feedback loop continues until the fetus is delivered, releasing the pressure on the cervix and terminating the signal Hypothalamic neurohormones regulate anterior pituitary hormones Figure: Anterior pituitary and the hypothalamic-pituitary portal system Hypothalamus controls the secretion of hormones from the anterior pituitary by secreting neurohormones into a specialized microcirculation o Hypothalamic-pituitary portal system Figure: Relationship between the hypothalamic hormones and the hormones of the anterior pituitary Regulation of Glucose Actions of insulin illustrate the principles of negative feedback Insulin is one of several hormones involved in homeostatic regulation of blood glucose in mammals Pancreas secrets the peptide hormone insulin when blood glucose rises Dispersed among the exocrine tissue are small clumps of cells (islets of Langerhans) which perform the endocrine functions of the pancreas Pancreatic β cells within the islets secrete insulin when blood glucose rises Multiple types of feedback control can regulate blood glucose Regulation of blood glucose by insulin is an example of a direct feedback loop because the pancreas secretes insulin when it senses increases in blood glucose, without involving integrating centers (like the brain) This kind of direct control of hormone secretin by the nervous system is an example of a second- order feedback loop Figure: interaction of pathways regulating insulin secretion Insulin and glucagon illustrate the principle of antagonistic control Second major hormone involved in glucose homeostasis in mammals is the peptide hormone glucagon It is secreted from α cells in the pancreatic islets of Langerhans When blood glucose falls α cells release glucagon into circulation Glucagon binds to receptors on target cells initiating pathways that cause them to release glucose which raised blood glucose levels When blood glucose concentration rises above set point, pancreas secretes insulin, causing target cells to take up and store glucose, lowering blood glucose levels Hormones can demonstrate additivity and synergism Like glucagon, the hormones epinephrine and cortisol can increase blood glucose When both glucagon and epinephrine are injected together the increase in blood glucose is larger and equivalent to the sum of the increase in blood glucose in response to epinephrine plus the increase in blood glucose in response to glucagon o Phenomenon is called additivity When glucagon, cortisol and epinephrince are injected in combination the net effect is much greater than the sum of the effects observed when any one hormone is injected alone o Phenomenon is called synergism The vertebrate stress response Stressful stimuli activate the sympathetic nervous system If the brain decides that the stimulus represents a threat it sends out a signal via motor neurons Causes muscles to contract causing the animal to run away or fight as necessary At same time hypothalamus activates a portion of the nervous system called the sympathetic nervous system Sympathetic nervous system sends out signals to target organs including the heart, vascular smooth muscle and other tissues These responses help to increase blood flow and redirect it toward the working muscles and away from tissues such as the gut The sympathetic nervous system stimulates the adrenal medulla The sympathetic nervous system also affects the activity of several endocrine glands Example: stimulation of the sympathetic nervous system reduces the release of insulin from the pancreas and increases the release of glucagon Sympathetic nervous system also stimulates the adrenal glands Adrenal glands are compact organs located adjacent to each kidney and consist of two types of tissue o Adrenal cortex located on the outside of the gland is compose of interrenal tissue and secrets mineralocorticoid and glucocorticoid hormones o Adrenal medulla is located on the inside and is compose of chromaffin cells that secrete catecholamines, epinephrine and norepinephrine
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