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Chapter 2

Human Brain - Chapter 2.docx

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Ted Petit

Chapter 2 – Neuroanatomy MODULE: Cells of the Nervous System - humans > specialized cells that perform specialized functions - neurons [communicators] and glia [support] cells are specialized cells of the nervous system [structure and function] - neurons > react to stimuli and are the basis of beh Neurons and Glia: Structure and Function Gross anatomy of the neuron - shape of neuron > related to its function [receive, conduct, transmit signals] - 3 main compents: o dendrites > receives info from other neurons  increases surface area available for the reception of signals form axons of other neurons  info sent to the rest of the neuron as a electrical charge  extent of branching indicates of num of connections o soma [cell body] > contains genetic machinery and most f the metabolic machinery needed for celluar functions o axon > sends neural info to other neurons  info sender [only one per neuron]  can divide to increase num of synapses  long thin fiber > can send info to many cells simulataneously  covered with insulation, myelin  speeds rate of info transfer/ensures msg get to end  end of a axon, terminal button o info sent from button across synapse to dendrite  info sent across the axon to is in the form of neurotransmitter [can b transformed into electrical message within the dendrite] o synapse > gap that axons pass info to dendrites across - presynaptic > events that occur in the axon - postsynaptic > events that occur in the dendrite Internal Anatomy of the Neuron - neuron is covered w/ a membrane - plasma membrane: consists of bilayer of continuous sheets of phospholipids that separate the cytoplasm and the fluid enviro outside the cell - proteins/channels that allow the passage of materials in and out of the cell - organelles > nucleus [genetic], ribosome/endoplasmic reticulum [synthesis], mitochondria [metabolic] - nucleus > processes genetic info and contains all the genetic info needed to code proteins Structure and function of neurons - structurally named neurons > uniplor, bipolar, multipolar [most common] o uniploar > have only one process emanating from the cell body - neurons w/ no/short axons > inter-neurons > integrate info w/i structure - functionally can be classified by the types of signals they send - the type of info that is represented by neural sctivity related to the function of the neuron - afferent > bringing info to the CNS - efferent > sending info from the brain/away from a structure - neurons can change as a result of experience Glia - support functions, there are diff types of glia - satellite cells> support cells outside the brain/spinal cord - 3 types o astrocytes > largest glia, star shaped  fill the space btwn neurons > growth of neurons?  involved in brain-blood barrier  nutritive/metabolic functions for neurons  regulate how far neurotransmitters can spread  storage of neurotransmitters o oligodendrocytes > make myelin  wrap their processes around most axons in the brain/spinal  can provide many segments to many axons [schawann only 1] o microglia > smallest, are phagocytes that remove debris from the NS  are made outside the brain and spinal by microphages  excessive may = Alzheimer’s/MS Communication within the Neuron: The Action Potential - commun within neurons is largely electrical and rely of AP to transmit info - resting potential > initial state, electrical charge > -70millivolts o depends of the concentrations of ions across the neuron membrane - sodium ions [Na+] and potassium ions [K+] - at rest High > K inside cells, H – Na outside - brain > heterogeneous concentration of ions inside and outside cell > why? o 1 - permeability > some are allowed thru ion channels  at rest > K passes through while Na cannot easily o 2 – neuron active transport of ions [some K leaks out as Na goes in] - sodium-potassium pump >actively import K and export Na [3 Na for 2 K] - when a neurotransmitter diffuses > opens sodium ion channels o Na goes in > -70mv to +50mv, depolarization > action potential occur - action potential occur w/I one neuron they release NT > commun btwn neurons - then hyperpolarization > K channels open > result in charge beyond -70 - features of action potential > o absolute refactory period >period after a AP where no additional action potential can be produced [closing of sodium channels] o all of none > all are of the same size o nodes of Ranvier > gaps in the myelin [ion channels only occur here]  in myelinated axons ions can cross the membrane only here o salutatory conduction > jumping of AP from one node to another o since AP is actively propagated > neural transmission in myelineted neurons is faster that in neurons w/o myelinaton Communication between Neurons: The Synapse - communication between neurons is largely chemical - axosomatic synapses > made of axons forming synapses with the soma of the neurons [very common] - axoaxonix synapses > axons forming synapses w/ other axons - dendritic synapse > dendrites forming synapses w/ other dendrites - axodendritic > synapses that consist of axons that form synapses w/ dendritic spines [most common] o terminal buttons have small packages [vesicles] that contain NT - NT release: o NT release triggered by AP at TB o causes calcium [Ca2] channels to open > incre in Ca2 o causes NT released into the synapse by exocytosis [membrane of vesicle fuses w/ axonal membrane > opens vesicle allowing NT flw] o NT diffuses across synapse >  producing postsynaptic effects [when NT binds to a protein embedded in the postsynaptic membrane aka receptor] - specificity of receptors > key analogy - 2 types of receptors on postsynaptic membrane > o transmitter-gated ion channels > aka ionotropic receptors are proteins that control an ion channel > when NT binds = quick change in ion concentration o function > depends on the ion channel controlled [ie. Na > AP]  excitatory post-synaptic potential, EPSP > dendrite depolarized > toward AP > by release of NT from presynaptic  inhibitory post-synaptic potential, IPSP > dendrite hyperpolarized > away from AP > by release of NT from presynaptic o G-protein-coupled receptors aka metabotropic  slower, more diverse, sustained responses than above  more freq in NS  steps:  NT binding w/ receptor  subunit of g-protein break away o move and bind to ion channel OR trigger synthesis of other chemicals  can result in IPSP or EPSP or change in gene expression  therefore more diverse effects o both types of receptors bind diff NT - receptors of the presynaptic membrane > autoreceptors > metabotropic receptors that are located on presynaptic > bind the NT release by the presyn o thought to regulate the amount of NT in the synapse - mechanisms responsible for terminating activity of [removing] NT: o reuptake > more common, presyn neuron reabsorbing the NT from synapse and repacking in the vesicles to be used again o enzymatic degradation > Nt is broken down into a inactive form by an enzyme present in the synapse - postsynaptic potential unlike AP are not propagated and vary in size Neurotransmitters - NTs assoc w/ specific types of neurons and assoc specific beh - small/large molecule NT - small: acetylcholine, monoamine, soluble gases and amino acids - large: neuropeptides Acetylcholine Monoamines Soluble Gases - recent > gaseous NT > nitric oxide, carbon monoxide - rapidly synthesized and rapid degeneration - NO > small > pass membrane > no need for receptor to cause effects o retrograde messenger > released form postsyn and act on presyn o Viagra, ability to learn Amino Acids - 4 amino acid that act as NT > aspartate, glutamate [most prevalent excitatory], gylcine and gamma-aminobutyric acid [GABA] [most prevalent inhibitory] - receptors > ionotropic > fast responses - learning, memory Neuropeptides - endorphins > role in pain medications - drugs > heroin/codeine > act on endorphin receptors [relieve pain] - Substance P > sensory transmission - capsaicin > hot feeling - cholycystokinin and insulin > regulate hunger and ingestion - Large molecules/made, stored, and transported diff than small molecules - modulate the responses of the neurons > don’t induce AP themselves > adjust the sensitivity of the neurons - primary func > ability to modulate the
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