BIO 141 Lecture 2 notes
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This 7 page Class Notes was uploaded by Camryn McCabe on Saturday January 30, 2016. The Class Notes belongs to Biol 141 at a university taught by Janelle Malcos in Spring 2016. Since its upload, it has received 33 views.
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Date Created: 01/30/16
BIOL 141 notes 3: 1/22-1/29 The Nervous System o Regulation- maintaining homeostasis (stable internal environment) o Systems that work for regulation Endocrine- sends chemical messages Works slowly; hormones Nervous- sends electrical signals and chemical messages Works fast; neurotransmitters o Structure o Two major subdivision Central nervous system (CNS)- brain and spinal cord Integration center Peripheral nervous system (PNS)- nerves Delivers/sends information to CNS o Nervous tissue o Neurons- specialized cells that make up nervous tissue Main characteristics: Excitability (uses ions) o Cells respond to stimuli from environment OR other cells o Results in electrical signal (action potential) – form of communication o Major evolution pressure to maintain this function for communication Conductivity- sending the message o Neurons propagate an electrical signal over a distance from its origin Secretion- passing on the signal o Neurons translate electrical signal to chemical signal Neuron structure Cell body (Soma) o Contains ONE nucleus Dendrites- short and thick extensions off cell body o Receives signals from other neurons Axon- extension away from cell body o Some are covered by myelin sheath (myelinated)- improves conductivity Synapse- site of communication between neurons or between neuron and target tissue o Secretion BIOL 141 notes 3: 1/22-1/29 Structural classes Multipolar- cell body at the end o Most common Bipolar- cell body is more centrally located Unipolar- cell body is off to the side o Axon comes after the cell body Functional classes Afferent neurons (sensory) o Sometimes called receptors o Specialized to detect stimuli o Start in PNS and travel to CNS o Delivers information about environment/condition of body to CNS Internuerons o In CNS o Receives and integrates information Efferent neurons (motor) o Send signals from CNS to effectors (targets) Muscles, organs, glands All of these pathways work by communication through electrical and chemical signals o Neural circuits Allows the body to monitor the environment (or itself) and decide on an appropriate response to maintain homeostasis o Electrical/chemical function of neurons Use electrical currents to communicate This speedy diffusion of ions across the membrane has been favored as form of rapid communication throughout evolution No concentration gradient = no communication o If potential is 0 no flow of ions no communication Electrical potential (voltage) Voltage- concentration gradient of charged particles (the separation) Requires potential to create a current Electrochemical gradient- concentration gradient of charged particles Electrical current Flow of charged particles from one point to another BIOL 141 notes 3: 1/22-1/29 Current- flow of charged particles from high to low o Example of passive transport In a battery… Flow of electrons in a battery circuit produces an electrical current A semipermeable membrane separates charges, creating voltage Flow of ions in/out of a cell (through protein channel) produces an electrical current Extracellular fluid (ECF)- outside the cell Intracellular fluid (ICF)- inside the cell EFC and ICF are separated by semipermeable membrane Main ions separated: Na and K + o Electrochemical gradient o Consists of a concentration gradient of charged particles Na and K + o 2 components Chemical gradient- uneven distribution of molecules Electrical gradient- uneven distribution of charge Because molecules are ions Movement of ions through protein channels Driven by chemical gradient (diffusion) AND Electrical gradient (attraction/repulsion of charges) o How electrochemical gradient is established Chemical gradient- created by Na /K protein pumps The pump (active; requires energy) + + Moves 3 Na ions out and 2 K ions in Creates slightly more positive environment outside the cell, and slightly more negative environment inside the cell The pump works to move Na ions out of the cell, but they can leak back in if there are leak channels + o Same goes for K ions; they will leak back out of the cell if there are leak channels These leak channels counteract the pump o But the pump works hard enough that the gradient is still there o The leak channels keep the gradient from getting bigger and bigger Electrical gradient- permeability and trapped anions BIOL 141 notes 3: 1/22-1/29 More K leak channels than Na channels o More K leaves the cell than Na entering the cell + Cell is more permeable to K Inner membrane is more negative than outer membrane Trapped anions inside the cell o More anions are proteins and huge o They can’t get through trapped inside make inside more negative o Sodium- chemical and electrical gradient go in the same direction; enforce each other Wants to get into the cell because of chemical gradient (+) is attracted to (-) electrical gradient inside o Potassium- chemical and electrical gradient go in the opposite direction Coming out because of the chemical gradient But goes in because of the electrical gradient (+) attracted to (-) o Resting Membrane Potential (RMP)- balance of diffusion between chemical and electrical gradients o At rest; unexcited o Balance between chemical and electrical gradients = Electrochemical gradient o RMP = -70mV in neurons o All cells have a resting membrane potential o Changes in RMP lead to currents Currents are the communication (flow of charged molecules from high to low)- local action potential o Local action potential- when a stimulus causes a change in RMP o Usually occurs at dendrite/cell body At stimulus sight, communication btw neuron 1 and 2, communication btw neuron 2 and 3 Effects are close to the origin, not down axon o If local potential is large enough (reaches the threshold) it activates action potential Threshold = -55mV o Usually excitartory in PNS From stimuli in the environment OR from organs inside the body o Excitatory or inhibitory in CNS Communication btw neurons via neurotransmitters o Ion movement is regulated by gated protein channels BIOL 141 notes 3: 1/22-1/29 Certain stimulus opens it, allows current o Reversible to a certain point (like when dust lands on you) o Stimulus local potential reach threshold action potential communication to CNS o Ion movement during Local Potentials o Regulated by gated protein channels Respond to stimuli All passive o These function in addition to pumps and leak channels already working o Chemical gated channels (ligand) Usually a chemical binds and causes channel to open Ex. Taste, smell o Voltage gated channels (potential) Open in response to change in voltage o Mechanical gated channels Respond to force (vibration, touch); forces channel to open Ex. Hearing (vibrations), touch o Can result in excitatory response depolarization Na+ channels open and Na+ ions diffuse INTO cell Membrane potential becomes more POSITIVE (closer to zero) (closer to neutral) o Can result in inhibitory response hyperpolarization K+ channels open and K+ ions diffuse OUT of the cell Membrane potential becomes more NEGATIVE (farther from zero/neutral) o Type of gated channel depends on location in NS In PNS Usually ligand or mechanical At interneurons Usually ligand o Ligands are neurotransmitters In CNS Chemical (ligand) o Example of Local potential o Depolarization This example is taking place at interneurons (chemical communication) Ligand binds Na+ chemical-gated channel Na+ channel opens and Na+ enters cell Membrane potential becomes LESS NEGATIVE (towards 0) BIOL 141 notes 3: 1/22-1/29 If potential reaches -55mV (threshold) in cell body, action potential is triggered Electrical signal transmitted down axon o Action potentials o Always depolarize o Regulated by voltage-gated channels Located at axon hillock AP goes down axon Secretion at axon terminal o In LP- lots of (+) coming in, going to reach action potential o Na+ voltage-gated channels open at -55mV Na+ rushes in (to the first part of the axon only) o Membrane depolarizes (becomes LESS NEGATIVE) o Membrane potential reaches +30mV (peak depolarization) Na+ channels close K+ voltage-gated channels open K+ diffuses out of the cell Cell starts to repolarize (become MORE NEGATIVE) o Repolarization K+ channels stay open over a longer voltage More K+ leaves the cell than Na+ enters Hyperpolarization o Na+/K+ pumps actively transport ions back to original state Na+ comes out, K+ back in back to RMP Necessary to get back to RMP before another action potential can occur Absolute refractory period- from when it hits -55 til it gets back to RMP Relative refractory period- when it’s below RMP Very strong local potential needed to overcome this o Conduction o Dendrites/cell body axon o After initial action potential Sodium diffuses in cytoplasm from entry point Causes adjacent voltage-gated channels to open Continues down axon One-way bc refractory period o Myelin Sheath o Electrically insulates axons o In CNS- made of oligodendrocytes o In PNS- made of Schwann cells Lipid/protein dense BIOL 141 notes 3: 1/22-1/29 o Important for conductivity Allows for fast propagation o Action potentials only occur at nodes of Ranvier Myelin sheath allows ions to diffuse further bc they can’t be transported to ECF by pumps o Saltatory propagation- allows action potential (signal) to jump down axon between nodes o Not all neurons are myelated That would take up too much space Only ones that need fast conduction are o LP v. AP o LP Triggered by receptors and neurotransmitters Can depolarize or hyperpolarize the membrane Are graded (proportional to strength of stimuli) Reversible (to a certain point…threshold!) Local (effects are close to the origin) Decremental (signal gets weaker as it moves away from origin) o AP Triggered by voltage-gated channels Always depolarizes the membrane Are all-or-nothing (exhibits same peak regardless of stimulus strength Irreversible Self-propagating (effects have a great distance from origin) Non-decremental (signal is the same each time)
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