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Chapter 2

PSYC55 - Cognitive Neuroscience Ch. 2
PSYC55 - Cognitive Neuroscience Ch. 2

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University of Toronto St. George
Pare, Dwayne

PSYC55 - CHAPTER 2  dementia praecox is the same name for schizophrenia o discovered by Bleuler o disorder of cognition (sometimes called a thought disorder) o genetic  has biological component Cells of the Nervous System  Cajal observed that although neurons are close to each other, they are separated by small gaps  Cajal defined 2 main principles of neurons o Connectional specificity – cells are separate because cytoplasms are not in contact; circuits pass information through specific pathways o Dynamic polarization - some parts of the neuron are specialized for taking info in while others send it out  2 main classes of cells in nervous system o Neurons o Glial cells Structure of Neurons  Distinguished by form, function, location, interconnectivity  Take in info, make decision and pass it along  Consists of o Cell body – metabolic machinery to maintain cell o Dendrites – receive input from synapse (said to be postsynaptic since they come after the synapse) o Axons – said to be presynaptic since it appears before the synapse  Most neurons are both pre and postsynaptic  Activity within a neuron involves changes in electrical state while at the synapse the signal between neurons is usually mediated by chemical transmission  Dendrites take varied and complex forms o Can exhibit spines: little knobs attached by small necks to the surface of the dendrites; synapses are located on these spines  Axon terminals have specialized intracellular structures that enable communication via the release of neurotransmitters  Dendrites and axon are extensions of the cell body; filled with the same cytoplasm o Continuity of the intracellular space between these neuronal components is necessary for the electrical signaling that neurons perform  Four general morphological classifications of neurons o Unipolar  Only 1 process extending from the body  Common in invertebrates o Bipolar  Have 2 processes (one axon and one dendrite)  Participate in sensory processes  Called prototypical: info comes in one end via the dendrite and leaves through the other end down the axon  Ex. Cells of the retina o Pseudounipolar  Appear to be unipolar but are originally bipolar sensory neurons whose dendrites and axon have fused  Ex. Dorsal root ganglia of spinal cord o Multipolar  Have one axon but many dendrites  Participate in motor and sensory processing  Make up majority of neurons in the brain  Ex. Spinal motor neurons, cortical sensory neurons Role of Glial Cells (neuroglial cell)  More numerous than neurons (1:10)  Account for more than half of brains volume  Do not conduct signals but neurons need them to function  Literally means “nerve glue”  In the CNS (brain and spinal cord) and PNS (motor and sensory outputs to the brain and spinal cord)  CNS has 3 main types of glial cells o Astrocytes  Large, round, symmetrical  Surround neurons and come in close contact with brain’s vasculature  Make contact with blood vessels at end feet which allow the astrocyte to transport ions across and create barrier between the tissues of CNS and blood (blood-brain barrier) o Microglial cells  Small, irregularly shaped  Come into play when tissue is damaged  Serve a phagocytic role, devouring damaged cells  Can proliferate whereas neurons cannot o Oligodendrocytes  Forms myelin  In PNS, Schwann cells form myelin  The myelin is wrapped around the axon until the cytoplasm is squeezed out leaving just the glial cell membrane  1 oligodendrocyte in the CNS can form myelin sheath around several axons whereas Schwann cells can only form myelin only for a single axon in the PNS  Goal of myelin = to provide electrical insulation around the axon that changes the way intracellular electrical currents flow  In myelinated axons, the myelin is interrupted at the nodes of Ranvier Neuronal Signaling Overview of Neuronal Communication  Goal of neuronal processing = take in info, evaluate it, and pass a signal to the other neurons  Neurons first receive a signal that is either chemical or physical  Signal initiates changes in membrane of postsynaptic neuron  Current flow is mediated by ionic currents carried by electrically charged ions  Action potentials can be generated in a spike-triggering zone of the neuron that integrates the currents from many synaptic outputs  Signal travels down the axon to its terminals and eventually causes the release of neurotransmitters at synapse Properties of the Neuronal Membrane and the Membrane Potential  Neuronal membrane is a bilayer of lipid molecules that separates intracellular space from extracellular space  It does not dissolve in the watery environments inside and outside the neuron  The membrane has many transmembrane proteins including ion channels and active transporters or pumps  Resting membrane potential = the difference in voltage across the neuronal membrane  Ion channels are formed by transmembrane proteins that create pores  Thousands of ion channels exist in the neuronal membrane  Some ion channels are passive (nongated) and some are active (gated)  Permeability = extent to which a channel permits ions to cross the membrane  The neuronal membrane is selectively permeable  It is more permeable to K+ than to Na+ because there are more nongated K+ channels than the nongated Na+ channels  Active transporters can move ions across the membrane  ATP provides a form of fuel that neurons uses to operate these small transmembrane pumps  Every molecule of ATP can provide enough energy to move 2 K+ ions inside for every 3 Na+ extruded  Over time, pumping changes internal to external neuronal concentrations of Na+ and K+ and creates ionic concentration gradients  In the resting state, a higher concentration of Na+ exists outside the neuron and a higher concentration of K+ exists inside  Pumps establish concentration gradients such that there is more Na+ outside and more K+ inside o This creates a force of unequal distribution of ions that wants to push Na+ from an area of high concentration to an area of low concentration o Since the membrane is more permeable to K+ then to Na+, the force of concentration gradient pushes some K+ out causing an electrical gradient to develop o Electrical gradient = K+ ions carry one unit of positive charge out of the neuron as they move across the membrane, the environment outside the neuron becomes more positive than the inside  As the K+ leaves the cell, it gets harder for the K+ to leave because of the negative environment inside the neuron (positive charge attracts the negative charge)  Electrochemical gradient = When the force of the concentration gradient pushing the K+ out through the nongated K+ channels is equal to the force of the electrical gradient acting to keep the K+ in  Difference of concentrations of ions across the selectively permeable membrane of the neuron leads to the resting membrane potential  Nernst equation proves for the calculation of that potential when one ionic species is involved  Equilibrium potential = membrane potential at which a given ion has no net flux across the membrane; that is, as many of the ion move outward as inward  In neurons, the resting membrane potential is equal to the equilibrium potential of K+ (-75 mV) Electrical Conduction in Neurons  Neurons have 2 important properties o They are volume conductors – allow currents to flow through them and across their membrane o They generate a variety of electrical currents called receptor potential, synaptic potentials and action potentials  To record the transmembrane difference in potential, you need a small recording electrode inside a neuron and one outside ; the difference in potential between these 2 electrodes is the value of the membrane potential  The action of stimulating electrode can approximate the role of the synaptic potentials in a neuron  Neurons are essentially sacks of electrically conductive fluid (cytoplasm) bounded by an electrical insulator (the cell membrane) making them excellent conductors  Neurons and their environment can be broken down into conductors (cytoplasm and ECF) and insulators (membranes)  Membranes have high but variable resistance and the ability to store charge (capacitance)  Active VS. Passive currents o When synapse is activated, active electrical currents are generated across the cell membrane near the synapse o These currents generate synaptic potentials  Current flows across postsynaptic membrane in localized region causing a passively conducted current conducted throughout the neuron (electrotonic conduction)  Passive current can be depolarizations (EPSP) which make inside cell more positive and more likely to generate an action potential; or they may be hyperpolarizations (IPSP) which make inside cell less positive and less likely to generate an action potential o Passive currents conducted via volume conductance through the cytoplasm and pass through the dendrites and soma of the postsynaptic neuron o If the passive currents sufficiently depolarize the neuron, they can trigger action potentials in the spike-triggering zone at the axon hillock of the neuron o Action potential is an active process because it involves the changes in membrane conductances by opening and closing ion channels o These passive currents can depolarize nearby patches of the axon and cause them to generate new action potential, allowing the process to continue down and release the neurotransmitter  Movement of ions to the inside of the neuron is accompanied by the return of currents to the outside of the neuron, forming a complete circuit  The distance of current flow is a function of 3 main properties of the neuron o Amplitude of the original current o Resistance and capacitance of the neuronal membrane o Conductivity of the intracellular and extracellular fluid  Placing an electrode in the axon of the neuron, the current is strongest near the electrode and the passive electronic currents that flow down the axon decrease with the distance away from the source  Assuming the resistivity does not change, the change in membrane voltage is due to the diminished amplitude of the current at more distant loci and therefore refer to the electrotonic conduction as “decremental conduction”  The distance from the source that electrotonic currents can still be effective for communication depends in part on the size of the original current. (greater the current, further it will conduct)  Passive electrotonic conduction is not a sufficient signal since it diminishes with distance so it is not appropriate for long-distance communication but it can work well for short distances  Under normal physiological conditions, the amplitude of the current is determined by the physiological factors such as the intensity of a physical stimulus at a receptor, or the strength and the number of synaptic inputs onto the neuron  As the membrane resistivity increases, more current will be shunted down the axon and less will leak out  Conductivity affects how far through a neuron a current will flow o IC and EC have high conductivities because they are generally good conductors of electrical currents o The resistivity of dendrites, cell bodies, and axons changes as the function of their size o If axon is large, the current flow is greater o High amplitude receptor or synaptic currents, high membrane resistance, and low- resistance intracellular pathways enhance the electrotonic conduction of currents by permitting them to affect the neuronal membrane at loci more distant from their site of generation  Electrotonic conduction is good for short distance communication but fails for long distance  Long distance communication requires active or regenerative electrical signals called action potentials 
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