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Neurophysiology Keri Muma Bio 6 The Nervous System The master controlling and communicating system of the body Cells communicate by electrical signals that are rapid and cause immediate responses Functions Sensory input – monitoring stimuli occurring inside and outside the body Integration – interpretation of sensory input Motor output – response to stimuli by activating effector organs Overall Organization Neural Tissue The two principal cell types of the nervous system are: Neuroglial – cells that surround and support neurons Neurons – excitable cells that transmit electrical signals Neuroglia Anatomy of Neurons Cell body – contains nucleus and organelles Dendrites – branching extensions Receptive to neurotransmitters from pre-synaptic neurons and transmit graded potential towards cell body Anatomy of Neurons Axon hillock – where cell body tapers into the axon, site where action potential originates Axon – single process extending from the cell body, transmits action potential away from cell body Anatomy of Neurons Myelin sheath – formed by schwann cells wrapping around the axon resulting in concentric layers of plasma membrane Nodes of Ranvier – gaps in the myelin sheath Anatomy of Neurons Telodendrites – distant branches of the axon Axon terminals – enlarged distal ends containing secretory vesicles filled with neurotransmitters Synapses Synapses – junctions between neurons Function as a control or decision point since they can be excitatory or inhibitory Occurs between axon terminals and a cell body, dendrite, axon hillock, muscle or gland Structure of Chemical Synapses Presynaptic neuron – transmits impulse towards the synapse, axon terminal with vesicles containing neurotransmitters Synaptic cleft – fluid filled space between pre and post synaptic neuron Postsynaptic neuron - transmits impulse away from synapse, contains receptors for neurotransmitters Types of Ion Channels found in Neurons Ligand-gated channels – chemically gated, open when neurotransmitters bind. Found on dendrites, cell bodies, and axon hillocks Mechanically gated channels – open in response to physical forces Voltage-gated channels – open or close in response to changes in membrane potential. Found along axon Leaky channels – always open, non-gated, found everywhere Neurophysiology Electricity When opposite charges are separated they contain potential energy and when they come together energy is released as electrical energy In cells, the separation of charges by the plasma membrane is referred to as the “membrane potential” Membrane has no potential Membrane has potential Membrane Remainder of fluid electrically neutral Separated charges responsible for potential Remainder of fluid electrically neutral Principles of Electricity Voltage – the measurement of potential energy created by charge separation In neurons voltage is measured in millivolts (1 mV = 1/1000 V) The voltage depends on the quantity of charges and the distance between the charges Membrane Potentials Resting Membrane Potential – potential difference across the membrane in a resting neuron (-70mV) Chemical gradient – higher concentration of Na+ in the extracellular fluid and a higher concentration of K+ in the intracellular fluid Electrical gradient – the inside of the membrane is negatively charged and the outside is slightly positive EFC IFC Permeability Na+ 150 15 1 K+ 5 150 50-75 65 0 Proteins (A- 0 ) Resting Membrane Potential Factors contributing to the resting membrane potential Membrane is 50 – 75X more permeable to K+ so K+ ions leak out faster than Na+ leak in Intracellular proteins - fixed anions inside the cell Sodium-Potassium pump maintains the chemical and electrical gradient – 3 Na+ out for every 2 K+ in Membrane Potentials Stimuli will trigger disruptions in RMP Can be stimulated by neurotransmitters binding to ligand gated channels, mechanical stress, or temperature change Triggers a graded potential – a localized change in membrane potential Short lived and dissipates as it travels Examples: receptor potentials, post-synaptic potentials, motorend plate potentials Changes in Membrane Potential If the stimulus is excitatory it will cause depolarization of the membrane Depolarization – the membrane potential becomes less negative When neurons are stimulated Na+ channels open and Na+ rushes into the cell down its electrochemical gradient Graded Potentials Magnitude of the stimulus depends on how many Na+ channels open This determines the distance that the graded potential will travel Amount of Na+ channels affected by the stimulus depends on the: Frequency of stimuli summation Amplitude of stimuli - strength Strong graded potentials can initiate action potentials if the threshold potential is reached at the trigger zone Graded Potentials Membrane Potentials Threshold potential = -55mV The critical level the membrane potential must reach to open voltage-gated Na+ channels on the axon to produce an action potential Membrane Potentials Action Potential – brief reversal of the membrane potential Propagated away from the cell body down the entire length of the axon without diminishing (all or none) Wave of depolarization followed by repolarization Frequency of axon potentials increases to reflect stronger stimuli Active area at peak of action potential Adjacent inactive area into which depolarization is spreading; will soon reach threshold Remainder of axon still at resting potential Local current flow that depolarizes adjacent inactive area from resting to threshold Direction of propagation of action potential Membrane Potentials Repolarization - the membrane returns to its resting membrane potential Voltage gated Na+ channels close Voltage gated K+ channels fully open and K+ efflux restores the resting membrane potential Membrane potential becomes more negative as K+ rushes out Previous active area returned to resting potential Adjacent area that was brought to threshold by local current flow; now active at peak of action potential New adjacent inactive area into which depolarization is spreading; will soon Remainder of axon still at resting potential reach threshold Membrane Potentials Hyperpolarization - the inside of the membrane becomes more negative than the resting potential Voltage gated K+ channels are sluggish to close K+ permeability lasts longer and membrane potential dips below resting potential (between -80 mV and -90mV) Membrane Potential Restoring the Resting Membrane Potential: Repolarization restores the electrical gradient Na/K pump restores resting ionic concentrations Voltage gated Na+ Channels Voltage Gated Na+ Channels Voltage gated K+ Channels Refractory Periods Refractory Periods – amount of time required for a neuron to generate another action potential Refractory Period Absolute Refractory Period - when another AP cannot be generated From the opening of the Na+ activation gates until the resetting of the activation gates Ensures that each action potential is separate Enforces one-way transmission of nerve impulses Previous active New active area area returned to at peak of action resting potential potential New adjacent inactive area into which depolarization is spreading; will soon reach threshold “Forward” current flow excites new inactive area “Backward” current flow does not re-excite previously active area because this area is in its refractory period Direction of propagation of action potential Refractory Period Relative Refractory Period The interval following the absolute refractory period Threshold is raised so only exceptionally strong stimuli will trigger another action potential : Sodium gates are reset Potassium gates are still open Hyperpolarization is occurring Summary Factors Influencing Conduction Velocity Myelination of axon – increases impulse rate Acts as insulator preventing charge leakage Saltatory conduction – voltage gated channels are concentrated at the nodes so electrical impulses jump from node to node instead of having to travel down the entire axon Factors Influencing Conduction Velocity Diameter of the axon – the larger the diameter the quicker the impulse travels, less resistance to current flow so adjacent membranes depolarize quicker Alcohol, sedatives, and anesthetics – slow or block nerve impulses by reducing permeability to Na+. Insufficient blood flow – slows impulses, caused by cold or pressure Transmission Across the Synapse Action potential reaches the axon terminal Voltage-gated Ca2+ channels open and Ca2+ floods into the terminal Synaptic vesicles fuse with the plasma membrane and release neurotransmitters into the synaptic cleft Neurotransmitters diffuse across the synaptic cleft and bind to receptors on chemical gated channels initiating a postsynaptic potential Neurotransmitter effects on Postsynaptic Potentials Binding of neurotransmitters cause a graded potential (localized change in the membrane) Depending on how the neurotransmitter affects the membrane potential determines if it will excite or inhibit the postsynaptic neuron Postsynaptic Potentials Excitatory postsynaptic potentials (EPSP) – binding of neurotransmitter opens Na+ channels and causes depolarization Membrane potential becomes less negative and closer to reaching threshold potential therefore closer to firing an action potential Postsynaptic Potentials Inhibitory Postsynaptic Potentials (IPSP) – binding of neurotransmitters cause hyperpolarization of the membrane therefore moving away from threshold and reducing the ability to initiate an action potential Causes K+ or Cl- channels to open K+ rushes out or Cl- rushes in, both causing the inside to become more negative Summation A single EPSP cannot induce an action potential but they can be summed The axon hillock keeps score of all graded potentials received Postsynaptic Potentials Temporal summation – a presynaptic neuron increases the frequency of impulses and more neurotransmitters are released in quick succession Postsynaptic Potentials Spatial summation – postsynaptic neuron is stimulated by multiple presynaptic neurons at the same time IPSPs and EPSPs can also be summed and cancel each other out Summary of Summation Summary of Summation Modulator Neurons The effectiveness of the presynaptic input can be affected by another neuron (see neuron B in picture below) Allows for the selective inhibiting/enhancing of a specific presynaptic neuron without affecting the input from other neurons or effecting all targets Modulator Neurons Presynaptic inhibition – the amount of neurotransmitter released from neuron “A” is decreased Presynaptic facilitation – the amount of neurotransmitter released from neuron “A” is enhanced Presynaptic Inhibition vs. Postsynaptic Inhibition Effects of Neurotransmitters Neurotransmitter receptors mediate changes in membrane potential according to: The amount of neurotransmitter released The amount of time the neurotransmitter is bound to receptors Neurotransmitters will affect the membrane potential as long as they are bound so they must be deactivated Deactivation of Neurotransmitters Three ways neurotransmitters are inactivated: By enzymes Through reuptake by presynaptic axon terminals or astrocytes They diffuse away from synapse Termination of Neurotransmitter Effects Acetylcholine Degraded by the enzyme acetylecholinesterase found in the synaptic cleft Ach Acetate + Choline Choline is actively transported back into the presynaptic terminal and recycled Choline + acetyl CoA Ach Choline acetyltransferase Termination of Neurotransmitter Effects Norepinephrine, dopamine, serotonin Taken back up by presynaptic terminal Repackaged or broken down by monoamine oxidase (MAO) Catechol-Omethytransferase is used by liver and kidney cells to break down NE & E in the circulation Classification of Neurotransmitters by Chemical Structure Acetylcholine (ACh) Biogenic amines – catecholamines, serotonin Amino acids – glutamate, glycine, GABA Peptides – endorphins, substance P Messengers: ATP and dissolved gases NO Classification by Function Excitatory neurotransmitters (e.g., glutamate) Inhibitory neurotransmitters (e.g., GABA and glycine) Some neurotransmitters have both excitatory and inhibitory effects Determined by the receptor type of the postsynaptic neuron Example: acetylcholine Excitatory at neuromuscular junctions with skeletal muscle (nicotinic receptor) Inhibitory in cardiac muscle (muscarinic receptor) Neurotransmitter Receptor Mechanisms Direct: neurotransmitters that open ion channels Promote rapid responses “fast synapses” Examples: ACh and amino acids Neurotransmitter Receptor Mechanisms Indirect: neurotransmitters that act through second messengers Promote long-lasting effects, “slow synapses” Examples: biogenic amines, peptides, and dissolved gases Fast vs. Slow Responses in Postsynaptic Cells Types of Circuits in Neuronal Pools Divergent – one incoming fiber stimulates ever increasing number of fibers, often amplifying circuits Figure 11.25a, b Types of Circuits in Neuronal Pools Convergent – opposite of divergent circuits, resulting in either strong stimulation or inhibition Figure 11.25c, d Types of Circuits in Neuronal Pools Reverberating – chain of neurons containing collateral synapses with previous neurons in the chain Figure 11.25e Types of Circuits in Neuronal Pools Parallel after-discharge – incoming neurons stimulate several neurons in parallel arrays Figure 11.25f