Biological Basis of Psychology
Biological Basis of Psychology
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Date Created: 04/18/14
Tuesday April 15 2014 Biopsychology Week 3 Lecture 3 continued Neurophysiology Cell membrane lipid bilayer two layers of linked fatty molecules within which specialized proteins float One important type of membranespanning protein is called an ion channel Ion an atom or molecule that has acquired an electrical charge by gaining or losing one or more electrons Ion channel a pore in the cell membrane that permits the passage of certain ions through the membrane when channels are open selectively permeable the property of a membrane to allow some substances to pass through but not others Some ion channels are gated and can open and close in response to changes in voltage This kind of opening and closing is a passive process does not require energy ATP Some ion channels remain open ie CI The Resting Membrane Potential of a Neuron At rest neuron is not communicating the inside of the cell is more negative than the outside Typically the electrical potential difference across the membrane is 70 millivolts mV due to concentrations of these ions sodium more on outside of neuron potassium more on inside of neuron calciummore on outside of neuron Chloride more on outside Also protein more on inside Tuesday April 15 2014 4 factors establish resting membrane potential 1 Concentration gradient particles in random motion tend to move from areas of high concentration to areas of low concentration simple diffusion to areas of lower concentration 2 electrostatic pressure like charges repel each other and opposite charges attract each other ie chloride ion charge inside neuron will want to move to outside 3 permeability of membrane to ions the ion channels contributing to the membrane potential are voltagegated open and close when the membrane potential reaches a particular voltage at rest Na channels are mostly closed require membrane potential to be about 50mV to open K channels are mostly open opened at 90mV or higher Cl channels are open opened by 70 mV or higher 4 Active ion transport NaK pump a transporter that uses energy provided by mitochondria to pump 3 Na out of the neuron for every 2K into the neuron the NaK pump counteracts the forces driving Na in by quickly pumping it back out the NaK pump counteracts the forces driving K out by quickly pumping it backin At rest Na want to enter the neuron Passive concentration gradient more Na outside electrostatic pressure bc Na is positive and attracted to the negative inside of the neuron most Na stay out of the neuron bc the Na channels are closed and what does get in is pumped out by the NaK pumps K want to leave the neuron Tuesday April 15 2014 passive concentration gradient more K inside electrostatic pressure to keep them in more negative on inside K channels are eaky so less resistance to them leaving the neuron but Nal K pump keeps pumping them back inside Cl wants to remain in equilibrium As C accumulates outside passive concentration gradient moves it back inside electrostatic pressure forces C out more negative inside Cl channels are open at rest so no resistance to them crossing the membrane Polarizing the membrane of a neuron when neurotransmitters are released from the axon terminal of the presynaptic neuron and diffuse across the synapse connecting with receptors in the dendrite of the postsynaptic neuron they can change the voltage or electrical potential of the postsynaptic neuron Postsynaptic Potential or PSP 2 different kinds of PSP Excitatory Postsynaptic Potentials or EPSPs some PS Ps can make the inside of the cell more positive relative to the outside gt depolarization movement of positive charge inside or negative charge outside Inhibitory Postsynaptic Potentials or lPSPs some PS Ps can make the inside of the cell more negative relative to the outside gt hyperpolarization movement of positive charge outside or negative charge inside Depends on the neurotransmitters if it is excitatory or inhibitory important in pharmacology and drug treatments PSPs are graded their size depends upon the intensity of the stimuli that elicited them strength depends on the amount of neurotransmitters released number of receptors activated PSPs degrade as the travel along the dendrite or cell body Tuesday April 15 2014 Spatial Summation the integration of PSPs arriving at different parts of the neuron Scenario 4 presynaptic neurons synapsing on a postsynaptic neuron Neurons A and B activate excitatory receptors at the same time they will add together to create a larger EPSP neurons C and D activate inhibitory receptors they add together to make a much bigger IPSP if neurons A and C activate they can cancel each other out Temporal Summation the integration of PSPs arriving at different times Scenario 2 presynaptic neurons synapsing on a postsynaptic neuron neurons A activates excitatory receptors and fires rapidly Neurons B activates inhibitory receptors and fires rapidly Review of EPSPs and PSPs the size type and effect of PSPs small changes in voltage across the membrane depend on several factors frequency of firing and neurotransmitter release from the presynaptic neuron higher frequency greater build up of PSPs caused by that neurotransmitter number and identity of presynaptic neurons firing at the same time on a postsynaptic neuron many excitant inputs gt buildup of EPSPs many inhibitory inputs gt buildup of PSPs the neurotransmitterreceptor combination inhibitory receptors gt IPSP excitatory receptors gt EPS P Number of a particular type of receptor on the postsynaptic neuron more receptors gt more signal detection gt greater PSPs Distance of receptors from the axon hillock which is the area where the cell body becomes the axon more distance gt weaker impact Tuesday April 15 2014 The Starting Site of an Action Potential Axon Hillock Initial Segment high in voltagegated Na channels at the hillock highly sensitive to ionic flux where the action potential is generated The Ionic Basis of the Action Potential can be measured with electrodes placed in the axon terminal Action Potential a massive momentary reversal of the membrane potential from 70mV to 50 mV not graded it s all or none once it s started can t be stopped propagated down the entire axon Action potentials are generated when depolarization reaches a particular threshold of excitation a depolarization of about 5 to 20 mV around the axon hillock the depolarization is a change in voltage that opens voltagegated Na channels in the area of the axon hillock gt Na comes rushing in making the membrane potential depolarize all the way up to 50 mV rising phase then overshoot All ornone property either it tires at its full amplitude or it doesn t fire at all The Ionic Basis of the Action Potential the influx of Na Na moving into the neuron activates additional K gates to open allowing K to flow out the influx of Na is so large that the efflux moving out of K does not alter the effect of Na on the membrane potential rising phase will be more positive When the neuronal membrane becomes very depolarized about 50mV the Na channels close voltagegated channels have thresholds for opening as well as closing K continues to leave the neuron gt depolarization because you re losing positive charge from the inside of the cell falling phase Tuesday April 15 2014 unlike Na channels K channels are slow to close gt keep spitting out K dropping the membrane potential below the resting membrane potential gt hyper polarization undershoot the resting potential then levels out to resting potential when the neuronal membrane is hyper polarized ie below resting potential the neuron is now that much further away from the threshold of AP the period of time when the neuron is hyperpolarizaed is called the refractory period 2 aspects or refractory period absolute refractory period time when it is impossible to fire another AP relative refractory period time when it is possible to fire another AP but only if you apply greater than normal stimulation the refractory period keeps the AP traveling in one direction cannot travel backwards Mediation of the action potential by voltagegated sodium channels 1 open K channels create the resting potential Na want to enter K want to exit 2 Some Na channels open depolarizing the cell to threshold 3 At threshold additional voltagegated Na channels open causing a rapid change in polarity 4 Na channels are inactivated gated K channels open repolarizing then hyper polarizing 5 All gated channels close the cell returns to its resting potential Propagation of the Action Potential once generated at the axon hillock the action potential travels without degrading down the length of the axon when an action potential is generated in one area of the axon it alters the membrane potential in the adjacent area triggering the threshold for activation of Na channels there and the massive depolarization happens again in the adjacent area of the axon the action potential does not degrade because there are so many sodium channels covering the axon right up to the terminal boutons Tuesday April 15 2014 Conduction of the Action Potential slow 10ms conduction ofAP along unmyelinated axon Rapid saltatory conduction of AP along myelinated axon saltatory conduction Na channels open generating an action potential myelin prevents K leakage out channeling the depolarization down the axon interior depolarization spreads within the axon very rapidly like electricity through a wire The action potential is triggered at the new Node of Ranvier and continues from node to node as fast as 150ms up to 15 times faster than unmyelinated axons Disorder affecting myelination Multiple Sclerosis MS an auto immune disease that is characterized by the degeneration of meylin resulting in the neuronal data and the formation of plaques hardeing in brain and spinal cord disruption in fast saltatory conductance sometimes loss of conduction altogether results in weakness paralysis or spasms impaired coordination visual problems etc Thursday April 17 2014 Lecture 4 The Synthesis Transport amp Storage of Neurotransmitters Neurotransmitter A chemical gas or hormone that is synthesized in and released from a neuron Large neurotransmitters ie peptides and hormones are synthesized in the cell body and then transported down the axon to the terminal small neurotransmitters ie amino acids monoamines gases are synthesized in the terminals In the terminal most neurotransmitters are stored in membranes called vesicles Tuesday April 15 2014 Release of a neurotransmitter occurs when vesicles dump neurotransmitters into the synaptic cleft or gap Neurotransmission 1 action potential travels down the axon of the presynaptic neuron 2 the depolarization of the membrane in the terminals activates voltagegated calcium Ca2 channels gt Ca2 enters 3 Ca2 turns on machinery to stimulate vesicles to move to the active zone the site where vesicles dock prior to neurotransmitter release 4 the membrane of the vesicles fuses joins with the cell membrane in the active zone and neurotransmitters are released into the synaptic cleft 5 The neurotransmitters interact with receptors on the postsynaptic neuron gt PSPs either EPSPs or PSPs Exocytosis Ca2 turns on machinery to stimulate vesicles to move to the active zone the site where vesicles dock prior to neurotransmitter release The membrane of the vesicles fuses with the cell membrane in the active zone and neurotransmitters are released into the synaptic cleft small molecule neurotransmitters are released in pulses every time calcium enters the neuron bc of an action potential large molecule neurotransmitters are only released when there are high rates of action potentials and calcium levels rise substantially in the terminal Receptors just as a particular key can open different does a molecule of the correct shape called a ligand can fit into a particular receptor protein and activate or block the channel lonotropic receptors directly control ion channels when they receive a particular neurotransmitter Metabotropic receptors receive particular transmitters but do not directly control ion channels Instead they activate molecules known as G proteins These G proteins can then activate neighboring ion channels when released Tuesday April 15 2014 Autoreceptors a metabotropic receptor that is located in the presynaptic membrane not postsynaptic They tell the axon terminal how much neurotransmitter has been released and they regulate calcium channels and the machinery involved in exocytosis normal behavior depends upon precise neurotransmission auto receptors ensure that neurotransmitter levels do not get too high Ending neurotransmission need a system that stops release of neurotransmitters in the gap as well as clean up after transmission 3 different ways this occurs 1 reuptake transporters in the terminals or the dendrites that take the neurotransmitters back inside the neuron to be re packaged or degraded 2 enzymatic degradation enzymes found in the synaptic cleft break down the neurotransmitter after it is released 3 diffusion neurotransmitters simply move out of the synaptic cleft Different Types of Neurotransmitters Know Amines amino acids neuropeptides peptide hormones gases neuropeptides larger molecules that more resemble proteins over 100 different peptide neurotransmitters in the brain endorphins pain and pleasure runners high opiate receptors slower signaling than the small molecule neurotransmitters broken down by degradative enzymes in synaptic cleft amino acids small molecule neurotransmitters building blocks of proteins include glutamate glycine aspartate from food excitatory more likely to trigger an action potential GABA made from glutamate in neurons and astrocytes inhibitory causes hyperpolarization less likely to fire action potential found in majority of fast acting directed synapses ie eye movements use reuptake mechanisms IO Tuesday April 15 2014 SoIubIe gases small molecule neurotransmitter nitric oxide and carbon monoxide made in the cell body and diffuse across the membrane not released in vesicles believed to play a role in retrograde signaling signaling from postsynaptic neuron to presynaptic neuron very short lived broken down inside cells quickly by enzymes Acetylcholine ACh small molecule neurotransmitter in a class on its own made from putting an acetyl group onto choline hence name found at neuromuscular junctions degraded in the synapse by acetylcholinesterase chops the acetyl group off the choline Cholinergic pathways in basal forebrain project throughout the cortex and to the hippocampus and amygdala Neurotransmitters are located in different regions of the brain have different effects depends on where neurotransmitters are made up and then projected to monoamines small molecule neurotransmitter made from a single amino acid hence name more diffuse effects than amino acid neurotransmitters monoamine neurons have highly branched axons and their cell bodies tend to be clustered in small groups in the brain stem dopamine epinephrine and norepinephrine are made from tyrosine catecholamines Mesostrial dopamine system is thought to play a crucial role in motor control Mesolimbocortical dopamine system is thought to play ac uncial role in several critical thinking processes Noradrenergic activity has been implicated in many diverse functions ie mood overall arousal sexual behavior Tuesday April 15 2014 Serotonin a monomine made from tryptophan in milk makes you sleepy serotonergic pathways serotonergic activity has been implicated in the control of sleep states mood anxiety and many other functions Be familiar with the table of neurotransmitters for test Drug Effects Agonist amp Antagonists the endogenous ligand is a naturally occurring molecule such as a neurotransmitter that binds and activates the receptor opens up the ion channel some drugs resemble the endogenous ligand and affect the receptor in the same way Both endogenous and exogenous ligands are known as agonists agonist facilitating neurotransmission other Agonist effects other ways to facilitate neurotransmission by using agonists have a drug that increases the production of the neurotransmitters drug increases the release of neurotransmitters at the terminal Drug blocks re uptake keep more neurotransmitters in the gap to allow more neurotransmission to occur agonist examples Morphine binds to endorphin receptors and mimics the action of endorphins which cause euphoric feelings Cocaine a serotoninnorepinephrinedopamine reuptake inhibitor antagonists other molecules that bind the receptor too but fail to activate it so they block agonists antagonists prevent neurotransmission other antagonists effects drug blocks the synthesis of neurotransmitters Drug activates auto receptors amp inhibits neurotransmitter release ex of antagonists naloxone binds to endorphin receptors and blocks the action of endorphins used to counteract effects of overdosing on heroine etc antihistamines blocks the synthesis of histamine 11
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