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Western University
Physiology 1021

The Excitable Cell Homeostasis, Membrane Transport, and Excitable Cells - Definitions o Physiology study of function in living organisms mechanisms by which the organisms control their internal environments and attempts to explain the physical and chemical factors responsible for both normal function and pathology o Homeostasis maintenance of relatively stable conditions in the internal environment self-regulatory mechanisms involving all of the organs and tissues of the body o Negative feedback system operates to maintain homeostasis: Set Point Control Centre Effector Sensor Controlled Variable - Body Fluid Components o Total Body Water (TBW) 42L o Intracellular 28L (67% TWB) o Extracellular 14L (33% TBW) o Interstitial 11L (26.4% TBW) o Plasma 3L (6.6% TBW) - Function of Membrane Proteins o Receptors o Enzymes o Ion channel (pores) o Membrane carriers (transport of glucose) - Membrane Transport o Endocytosis, exocytosis o Diffusion through lipid bilayer (fat-soluble molecules) o Diffusion through protein channels (water-soluble molecules) o Facilitated diffusion o Active transport - Simple Diffusion o The movement of molecules from high concentration to low concentration down the concentration gradient due to the molecules random thermal motion - Factors influencing Rate of Diffusion o Concentration gradient (larger faster) o Temperature (higher - faster) o Surface area (larger faster) o Size of molecule (smaller faster) o Viscosity (lower faster) - Osmosis o The net movement of water down its concentration gradient due to random thermal motion (water between 2 solutions) o Osmosis across the cell membrane is affected by: Permeability Concentration of gradients of the solutes Pressure gradient across cell membrane o The osmotic pressure of a solution is the pressure which would have to be applied to the solution in order to stop the net influx of water into the solution depends of the number rather than the size/type of particle in the solution - Tonicity o Ability of a solution to cause osmosis across biological cell membranes o Isotonic same osmolarity as body fluids same not net movement of water o Hypotonic lower osmolarity less cell swell/burst (water in) o Hypertonic higher osmolarity more cell shirks (water out) - Chemical and Electrical Gradients o Molecules tend to move from areas where they are in high concentration to areas of low concentration they move down their concentration gradient o Ions tend to move towards area of opposite change down their electrical gradient o If the chemical and electrical gradients are in opposite directions, the movement of the ion will depend on the balance of two gradients down its electrochemical gradient, until an electrochemical equilibrium is established - Resting membrane potential o Establishment of an electrical potential difference across the membrane with the inside negative with respect to the outside since this potential difference across the plasma membrane is present, even in resting cells, it is sometimes referred to as the resting membrane potential All cells have a r.p.m Caused by selective permeability to particular ions Almost all cells have a -70mv, but can vary slightly from cell to cell The membrane is permeable to a variety of ions Each ion affects r.p.m depending on the membrane permeability and concentration gradient for each ion Ion movement depends on both concentration and electrical gradient - Sodium Potassium (ATPase) Pump o Integral membrane protein, form of active transport, requires ATP for energy o Moves Na+ out and K+ in, responsible for maintaining and establishing resting membrane potential - Excitable Cells o Excitable cells generate or respond to electrical signals o 2 types Nerve cell (neuron) Muscle cell (skeletal, smooth, cardiac) - Action Potential o Depolarization Stimulate nerve, membrane depolarizes opening some voltage gated Na+ channels, if depolarization reaches threshold of -55mv, then all v.g. Na+ channels open and action potential is initiated, Na+ rushes into cell, membrane depolarizes rapidly to +45mv o Repolarization K+ v.g channels begin to open while Na+ v.g channels become inactivated, eventually all K+ voltage gated channels are open and all Na+ channels are inactivated, K+ rushes out of the cell o Hyperpolarization K+ continues to leave the cell, membrane hyperpolarizes (-90mv), membrane potential returns to normal as channels return to resting configuration K+ channels close, Na+ channels switch from inactivated to closed state - Functional organization of the Cerebral Cortex o Parietal lobe sensation o Frontal lobe movement and cognitive function o Temporal lobe hearing o Occipital lobe vision - Hemispheric Specialization o Cortical hemispheres have somewhat different functions o Language is highly lateralized; Wernickes area temporal lobe, Brocas area frontal lobe are located in the left hemisphere of most people o Damage to either of these areas results in aphasia (language deficit) o Wernickes Area impaired comprehension nonsense words o Brocas area good comprehension effortful, non-fluent speech - Glial Cells o Many roles myelination, scaffolding, immune system, assist in production of cerebrospinal fluid (CSF) o Helps establish blood-brain barrier o 4 types: Astrocytes help form blood-brain barrier Oligodenrocytes myelinate CNS axons Ependymal cells contribute to production of CSF Microglia CNS immune system scavenger - Protection of CNS o Skull and spinal column provide suit of armour for support and protection o Inner lining of CSF allows the brain to float within skull and spinal column o CSF has some density of the brain acts as a shock absorber - The Blood-Brain Barrier o Protects CNS from blood-borne substances that could disrupt or harm neuronal function o Astrocytes secrete hormones that tightly seal gaps between endothelial cells in the CNS capillaries substances must travel thru endothelial cells to enter or exit CNS o Few parts of the brain do not have a blood-brain barrier o The absence of a BBB relates to an areas physiological function Posterior pituitary secretes hormones Circumventricular organs sample blood chemistry Vomiting area in medulla monitors blood for toxins and initiates vomiting reflex if necessary - Basic Principles of Sensation o The afferent division of the PNS tells us about our internal and external environment o Only certain types of energies or stimulus modalities can be detected by our bodies o Afferent division of PNS has specialized receptors that transduce stimuli into action potentials, which are relayed to the CNS - Sensory Receptors o Each type of receptor is specialized to be most sensitive to one type of stimulus o Receptors can respond weakly when activated by a different stimulus cross talk o Perceived sensation depends on the stimulated receptor, not on the type of stimulus labeled line coding o 6 types: Photoreceptors wavelengths in visible spectrum Mechanoreceptors mechanical energy Chemoreceptors specific chemicals Nociceptors pain/tissue damage Thermoreceptors temperature Osmoreceptors solute concentration and osmotic activity - Two mechanisms for Sensory Transduction o Complex neural receptors Specialized nerve ending Myelinated axon Cell body o Special Senses Receptor Specialized receptor Synapse Myelinated axon Cell body - Transduction at Complex o Application of stimulus o Alternation of receptor membrane o Local current flow within receptor o Alter frequency of action potential o AP propagation to CNS - Transduction at Special o Application of stimulus o Alternation of receptor membrane o Local current flow within receptor o Release NT o Alteration of post-synaptic membrane potential
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