ANIMAL PHYSIOLOGY MOYES CH 4 PPT

Neuron Structure and Function Neurons Vary in structure and properties Use same basic mechanisms to send signals Neural Zones Four functional zones Signal reception Dendrites and the cell body (soma) Incoming signal received and converted to change in membrane potential Signal integration Axon hillock Strong signal is converted to an action potential (AP) Signal conduction Axon (some wrapped in myelin sheath) AP travels down axon Signal transmission Axon terminals Release of neurotransmitter Electrical Signals in Neurons Neurons have a resting membrane potential (like all cells) Membrane potential is negative at rest Neurons are excitable Can rapidly change their membrane potential Depolarization – membrane potential becomes less negative Repolarization – membrane potential returns to resting value Hyperpolarization – membrane potential becomes more negative than resting value Electrical Signals in Neurons Changes in membrane potential act as electrical signals Membrane Potential Factors contributing to membrane potential Distribution of ions across the membrane Relative permeability of the ions Charges of the ions Goldman equation for the calculation of membrane potential (Em) The Goldman Equation Em = membrane potential R = gas constant T = temperature (Kelvin) F = Faraday’s constant Px = relative permeability of ion [X] = ion concentration outside or inside membrane Other ions (Ca++, Mg++, etc.) are ignored in this simplified form of the equation because their permeabilities are very low. Gated Ion Channels Neurons depolarize or hyperpolarize by selectively altering permeability Gated ion channels open or close in response to a stimulus Example: neurotransmitter Channels only allow specific ions to pass through the membrane Ion moves down its electrochemical gradient Only relatively small numbers of ions move across As permeability to a specific ion increases, membrane potential will approach that ion’s equilibrium potential (Nernst equation) Changes in Membrane Potential Signals in the Dendrites and Cell Body Incoming signal Example: neurotransmitter Membrane-bound receptors bind to neurotransmitter Receptors transduce the chemical signal to an electrical signal by changing ion permeability of membrane Change in ion permeability causes change in membrane potential (graded potential) Graded Potentials Vary in magnitude depending on strength of stimulus More neurotransmitter ? more ion channels open ? larger magnitude of graded potential Depolarization Na+ or Ca2+ channels open Hyperpolarize K+ and Cl– channels open Stimulus Strength and Graded Potentials Graded Potentials Travel Short Distances Conduction with decrement Magnitude of graded potential decreases with increasing distance from opened ion channel Decrement due to: Leakage of charged ions across membrane Electrical resistance of cytoplasm Electrical properties of membrane Electrotonic current spread Positive charge spreads through cytoplasm causing depolarization of adjacent membrane Conduction with Decrement Action Potentials Travel Long Distances Characteristics of Action Potentials: Triggered by net graded potential at axon hillock (trigger zone) Do not degrade over time or distance Travel long distances along membrane All-or-none Must reach threshold potential to fire Depolarizations below threshold will not initiate an action potential Action Potentials Integration of Graded Signals Many graded potentials can be generated simultaneously Many receptor sites Many types of receptors Some graded potentials are depolarizations, some are hyperpolarizations Spatial summation Graded potentials from different sites influence the net change Temporal summation Graded potentials that occur at slightly different times influence net change Spatial Summation Temporal Summation Graded Potentials vs. Action Potentials Action Potentials (AP) Occur only when membrane potential at axon hillock reaches threshold Three phases: Depolarization Repolarization Hyperpolarization Absolute refractory period Cell incapable of generating a new AP Relative refractory period More difficult to generate new AP Voltage-Gated Channels Change shape due to changes in membrane potential Closed at resting potential Positive feedback Influx of Na+ ? local depolarization ? more Na+ channels open ? more depolarization Na+ channels open first (depolarization) K+ channels open more slowly (repolarization) Na+ channels close K+ channels close slowly relative refractory period caused by open K+ channels Action Potentials (AP) Ion Movement Relatively small number of ions move into and out of cell Single action potential has no measurable affect on ion concentrations inside and outside cell Na+/K+ ATPase restores concentration gradients following repeated action potentials Na+ Channels Have Two Gates Activation gate Voltage dependent Opens when membrane reaches threshold Inactivation gate Time-dependent Closes after brief time Na+ Channels Have Two Gates Voltage-Gated Channels and the AP Action Potentials Travel Long Distances “All-or-none” Occurs or does not occur All APs are same magnitude Self propagating An AP triggers the next AP in adjacent areas of membrane without degradation Electronic current spread Charge spreads along membrane Regenerative cycle Ion entry ? electronic current spread ? triggering of AP Action Potentials Travel Long Distances Myelination Vertebrate neurons are myelinated Myelin Insulating layer of lipid-rich Schwann cells wrapped around axon Reduce “leakage” of charge across membrane Schwaan cells are a type of Glial cell Cells other than neurons that support neuron function Myelination Nodes of Ranvier Areas of exposed axonal membrane between Schwann cells Internodes The myelinated region Saltatory conduction APs “leap” from node to node APs occur at nodes of Ranvier, and electrotonic current spread through internodes This type of conduction is very rapid Myelination Unidirectional Signals Action potentials start at the axon hillock and travel towards the axon terminal “Up-stream” Na+ channels (just behind the region of depolarization) are in the absolute refractory period The absolute refractory period prevents backward (retrograde) transmission and summation of APs Relatively refractory period also contributes by requiring a very strong stimulus to cause another AP Information Transfer by AP AP frequency carries information AP frequency increases with stronger stimuli Magnitude of each AP does not change Maximum frequency is limited by the absolute refractory period Mammalian nerves can conduct 500–1000 action potentials per second Action Potential Frequency The Synapse Signal transmission from neuron to another cell Synapse Presynaptic cell, synaptic cleft, and postsynaptic cell Synaptic cleft Space between the presynaptic and postsynaptic cell Postsynaptic cell May be a neuron, muscle cell, or endocrine cell Neuromuscular junction Synapse between a motor neuron and a skeletal muscle cell Signal Transmission at a Chemical Synapse Amount of Neurotransmitter Released [Ca2+]i is affected by AP frequency More open voltage-gated Ca2+ channels ? [Ca2+]i Factors that lower intracellular [Ca2+]i Binding with intracellular buffers ? [Ca2+]i Ca2+ ATPases ? [Ca2+]i High AP frequency ? Ca2+ influx is greater than removal ? ? [Ca2+]i ? many synaptic vesicles release their contents ? high [neurotransmitter] in synapse Acetylcholine Postsynaptic Cells Postsynaptic cells have specific receptors for neurotransmitters Example: nicotinic ACh receptors Similar to specific hormone receptors on target cells Binding of neurotransmitter to receptor alters ion permeability of postsynaptic cell Change in membrane potential of postsynaptic cell Transmission of Signal Strength at Synapse Response of postsynaptic cell influenced by amount of neurotransmitter in synapse and number of receptors Amount of neurotransmitter Rate of release – rate of removal Release determined by frequency of APs Removal determined by Passive diffusion out of synapse Degradation by synaptic enzymes Uptake by surrounding cells Number of receptors Density of receptors on postsynaptic cell Diversity of Neurons All neurons have three functions: Receive and integrate incoming signals Conduct the signal along the neuron Transmit the signal to other cells Neurons differ in their ability to receive incoming signals Different receptors Neurons differ in mechanism of signal conduction and synaptic transmission Structural Diversity of Neurons Functional Classes of Neurons Afferent neurons (sensory) Conduct action potentials towards the central nervous system Efferent neurons (motor) Conduct action potentials from the central nervous system to the organs Interneurons Conduct action potentials between neurons in the central nervous system Neuron Classification Based on Function Structural Classes of Neurons Multipolar Many dendrites One axon Bipolar One dendrite (may have branches) One axon Unipolar Single process extending from cell body May split to form afferent and efferent branches Neuron Classification Based on Structure Glial Cells More abundant than neurons 90% of cells in human brain are glial cells Do not generate or conduct APs Do not form synapses with neurons Types of Glial Cells Five main types of glial cells in vertebrates Schwann cell Forms myelin on motor and sensory neurons of PNS Oligodendrocyte Forms myelin on neurons in CNS Astrocyte Transport nutrients, remove debris in CNS Types of Glial Cells Microglia Remove debris and dead cells from CNS Ependymal cells Line fluid-filled cavities of CNS Glial Cells Diversity of Signal Conduction Diverse mechanisms of signal conduction Electrotonic Action potentials Saltatory conduction Chemical and electrical synapses Additional diversity in AP physiology: Shape and speed of action potential due to properties of Na+ and K+ channels Function of channels Number of channels Ion Channel Isoforms Channel isoforms encoded by different genes Voltage-gated K+ channels are highly diverse 18 genes encode for 50 isoforms in mammals Voltage-gated Na+ channels are less diverse 11 isoforms in mammals Each isoform has distinct functional characteristics Ion Channel Isoforms Channel Density Density of voltage-gated Na+ channels affects signal conduction Increased density of channels lowers threshold Increased density of channels shortens relative refractory period Voltage-Gated Ca2+ Channels Presence of voltage-gated Ca2+ channels affects AP Open at the same time or instead of voltage-gated Na+ channels Ca2+ enters the cell causing depolarization Ca2+ influx is slower and more sustained than Na+ influx Slower maximal frequency of APs due to longer refractory period Voltage-gated Ca2+ channels play key role in function of cardiac muscle Conduction Speed of Axons Two ways to increase speed: Myelination Increasing diameter of axon Cable Properties of Axons Similar physical principals govern current flow through axons and undersea telephone cables Current (I) Amount of charge moving past a point at a given time A function of the voltage (V) drop across circuit and the resistance (R) of circuit Cable Properties of Axons Voltage (V) Difference in electrical potential Resistance (R) Rorce opposing flow of electrical current Ohm’s law: V = I ? R Cable Properties of Axons An axon behaves like an electrical circuit Ions moving through voltage-gated channels cause current across membrane Current spreads electrotonically along axon Some current leaks out of axon and flows backwards along outside of axon, completing circuit Current Flow In Axons Cable Properties of Axons Each area of axon consists of an electrical circuit Three resistors: Extracellular fluid (Re) Membrane (Rm) Intracellular fluid (Ri) A capacitor (Cm) Stores electrical charge; Two conducting materials (ICF and ECF) Insulating layer (phospholipids) Cable Properties of Axons Voltage Decreases with Distance Change in membrane potential (voltage) during AP decreases over distance due to resistance Conduction with decrement Higher resistance of intracellular and extracellular fluids causes greater decrease in voltage along axon Lower resistance of membrane causes greater decrease in voltage along axon K+ leak channels (always open) Some + charge leaks out Number of K+ leak channels will affect current loss and voltage decrease along axon Length Constant (l) of Axons Distance over which membrane potential will decrease to 37% (1/e) of its original value Variables affecting length constant: Resistance of cell membrane (rm) Resistance of intracellular fluid (ri) Resistance of extracellular fluid (ro) ro is usually low and constant; and is often ignored l is largest when rm is high and ri is low Length Constant (l) of Axons l and the Speed of Conduction Axonal conduction is a combination of electrotonic current flow and ions flowing through voltage-gated channels during AP Electrotonic current flow much faster than opening of voltage-gated channels Electronic current flow decreases over distance Higher l allows more electrotonic current flow and faster speed of conduction Axon Membrane Capacitance Capacitance Quantity of charge needed to create a potential difference between two surfaces of a capacitor Depends on three features of the capacitor: Material properties Generally the same in cells (lipid bilayer) Area of two conducting surfaces Larger area increases capacitance Thickness of insulating layer Greater thickness decreases capacitance Axon Membrane Capacitance Time Constant (t) Time over which membrane potential will decay to 37% of its maximal value How well does the membrane “hold” its charge? Variables affecting time constant: Resistance of cell membrane (rm) Capacitance of the cell membrane (cm) t = rmcm Low rm or cm result in low t Capacitor becomes full faster Faster depolarization Faster conduction Time Constant (t) Giant Axons Easily visible to naked eye (up to 1 mm diameter) Not present in mammals Giant Axons Have High Conduction Speed rm inversely proportional to surface area Large diameter axons have greater surface area and more leak channels; therefore low resistance ri inversely proportional to volume Large diameter axons have greater volume; therefore low resistance As axon diameter increases, rm and ri both decrease Giant Axons Have High Conduction Speed Low rm reduces the length constant and decreases conduction speed Low ri increases the length constant and increases conduction speed Do not cancel each other out: rm is proportional to radius, ri is proportional to radius2 Net effect of increasing axon radius is to increase speed of conduction Axon Diameter and the Length Constant Myelinated Neurons in Vertebrates Disadvantage of large axons Take up a lot of space which Limits number of neurons that can be packed into nervous system Large volume of cytoplasm makes them expensive to produce and maintain Myelin enables rapid signal conduction in compact space Myelin Increases Conduction Speed Increased membrane resistance Insulators decrease current loss through leak channels, increasing the length constant Decreased membrane capacitance Increased thickness of insulating layer reduces capacitance, decreasing the time constant High length constant and low time constant increase conduction speed Nodes of Ranvier are needed to boost depolarization Synaptic Transmission Transfer of electrical signal from presynaptic cell to postsynaptic cell Electrical synapse Gap junction Chemical synapse Chemical messenger crosses synaptic cleft Electrical and Chemical Synapses Electrical and Chemical Synapses Structural Diversity of Chemical Synapses Neurotransmitters Characteristics of neurotransmitters Synthesized in neurons Released at presynaptic cell following depolarization Bind to a postsynaptic receptor and cause an effect Neurotransmitters More than 50 known substances Categories Amino acids Neuropeptides Biogenic amines Acetylcholine Miscellaneous (gases, purines, etc.) A single neuron can produce and release more than one neurotransmitter Neurotransmitters Neurotransmitter Action Inhibitory neurotransmitters Cause hyperpolarization of membrane Inhibitory postsynaptic potential (IPSP) Make postsynaptic cell less likely to generate an AP Excitatory neurotransmitters Cause depolarization of membrane Excitatory postsynaptic potential (EPSP) Make postsynaptic cell more likely to generate an AP Neurotransmitter Receptor Function Ionotropic receptors Ligand-gated ion channels Fast Example: nicotinic Ach receptor Neurotransmitter Receptor Function Metabotropic receptors Receptor changes shape Formation of second messenger Alters opening of ion channel Slow May lead to long-term changes via other cellular functions Receptors for Acetylcholine Cholinergic receptors Nicotinic receptor Ionotropic Muscarinic receptor Metabotropic Linked to ion channel function via G-protein Receptors for Acetylcholine Receptors for Acetylcholine Receptors for Norepinephrine Adrenergic receptors Alpha (?) Several isoforms Metabotropic Linked to ion channel function via G-protein Beta (?) Several isoforms Metabotropic Linked to ion channel function via G-protein Receptors for Norepinephrine Adrenergic Receptors Synaptic Plasticity Change in synaptic function in response to patterns of use Synaptic facilitation Repeated APs result in increased Ca2+ in terminal Increased neurotransmitter release Synaptic depression Repeated APs deplete neurotransmitter in terminal Decreased neurotransmitter release Synaptic Plasticity Post-tetanic potentiation (PTP) After train of high frequency APs there is increased neurotransmitter release Exact mechanism unknown, but believed to involve changes in Ca2+ in terminal Post-tetanic Potentiation (PTP) Evolution of Neurons Only metazoans have neurons Other organisms have electrical signaling Algae have giant cells that can generate APs using Ca2+ activated Cl– channels Plants have APs involving Ca2+ that travel through the xylem and phloem Paramecium can change direction as a result of APs produced by Ca2+ channels Only metazoans have voltage-gated Na+ channels Copyright © 2008 Education, Inc., publishing as Benjamin Cummings PowerPoint® Lecture Slides prepared by Stephen Gehnrich, Salisbury University Click to edit Master title style 4 C H A P T E R Figure 4.1 Figure 4.4 Figure 4.5 Figure 4.6 Figure 4.7 Figure 4.8 Figure 4.9 Table 4.1 Figure 4.10 Figure 4.11 Figure 4.12 Figure 4.13 Figure 4.14 Figure 4.15 Figure 4.16 Figure 4.17 Figure 4.18a Figure 4.18b Figure 4.18c Figure 4.19 Table 4.2 Table 4.3 Figure 4.20a Figure 4.20b,c Figure 4.21 Figure 4.20b and Figure 4.22 Figure 4.23 Figure 4.24 Figure 4.25 Figure 4.26 Postsynaptic signal can be different Postsynaptic signal is similar to presynaptic Excitatory or inhibitory Excitatory Unidirectional Bi-directional Slow Fast Rare in simple animals Common in simple animals Common in complex animals Rare in complex animals Chemical synapse Electrical synapse Figure 4.27 Table 4.4 Figure 4.28a Figure 4.28b Figure 4.29 Table 4.5 Figure 4.31 Table 4.6 Figure 4.32 Neuron Structure and Function Neurons Vary in structure and properties Use same basic mechanisms to send signals Neural Zones Four functional zones Signal reception Dendrites and the cell body (soma) Incoming signal received and converted to change in membrane potential Signal integration Axon hillock Strong signal is converted to an action potential (AP) Signal conduction Axon (some wrapped in myelin sheath) AP travels down axon Signal transmission Axon terminals Release of neurotransmitter Electrical Signals in Neurons Neurons have a resting membrane potential (like all cells) Membrane potential is negative at rest Neurons are excitable Can rapidly change their membrane potential Depolarization – membrane potential becomes less negative Repolarization – membrane potential returns to resting value Hyperpolarization – membrane potential becomes more negative than resting value Electrical Signals in Neurons Changes in membrane potential act as electrical signals Membrane Potential Factors contributing to membrane potential Distribution of ions across the membrane Relative permeability of the ions Charges of the ions Goldman equation for the calculation of membrane potential (Em) The Goldman Equation Em = membrane potential R = gas constant T = temperature (Kelvin) F = Faraday’s constant Px = relative permeability of ion [X] = ion concentration outside or inside membrane Other ions (Ca++, Mg++, etc.) are ignored in this simplified form of the equation because their permeabilities are very low. Gated Ion Channels Neurons depolarize or hyperpolarize by selectively altering permeability Gated ion channels open or close in response to a stimulus Example: neurotransmitter Channels only allow specific ions to pass through the membrane Ion moves down its electrochemical gradient Only relatively small numbers of ions move across As permeability to a specific ion increases, membrane potential will approach that ion’s equilibrium potential (Nernst equation) Changes in Membrane Potential Signals in the Dendrites and Cell Body Incoming signal Example: neurotransmitter Membrane-bound receptors bind to neurotransmitter Receptors transduce the chemical signal to an electrical signal by changing ion permeability of membrane Change in ion permeability causes change in membrane potential (graded potential) Graded Potentials Vary in magnitude depending on strength of stimulus More neurotransmitter ? more ion channels open ? larger magnitude of graded potential Depolarization Na+ or Ca2+ channels open Hyperpolarize K+ and Cl– channels open Stimulus Strength and Graded Potentials Graded Potentials Travel Short Distances Conduction with decrement Magnitude of graded potential decreases with increasing distance from opened ion channel Decrement due to: Leakage of charged ions across membrane Electrical resistance of cytoplasm Electrical properties of membrane Electrotonic current spread Positive charge spreads through cytoplasm causing depolarization of adjacent membrane Conduction with Decrement Action Potentials Travel Long Distances Characteristics of Action Potentials: Triggered by net graded potential at axon hillock (trigger zone) Do not degrade over time or distance Travel long distances along membrane All-or-none Must reach threshold potential to fire Depolarizations below threshold will not initiate an action potential Action Potentials Integration of Graded Signals Many graded potentials can be generated simultaneously Many receptor sites Many types of receptors Some graded potentials are depolarizations, some are hyperpolarizations Spatial summation Graded potentials from different sites influence the net change Temporal summation Graded potentials that occur at slightly different times influence net change Spatial Summation Temporal Summation Graded Potentials vs. Action Potentials Action Potentials (AP) Occur only when membrane potential at axon hillock reaches threshold Three phases: Depolarization Repolarization Hyperpolarization Absolute refractory period Cell incapable of generating a new AP Relative refractory period More difficult to generate new AP Voltage-Gated Channels Change shape due to changes in membrane potential Closed at resting potential Positive feedback Influx of Na+ ? local depolarization ? more Na+ channels open ? more depolarization Na+ channels open first (depolarization) K+ channels open more slowly (repolarization) Na+ channels close K+ channels close slowly relative refractory period caused by open K+ channels Action Potentials (AP) Ion Movement Relatively small number of ions move into and out of cell Single action potential has no measurable affect on ion concentrations inside and outside cell Na+/K+ ATPase restores concentration gradients following repeated action potentials Na+ Channels Have Two Gates Activation gate Voltage dependent Opens when membrane reaches threshold Inactivation gate Time-dependent Closes after brief time Na+ Channels Have Two Gates Voltage-Gated Channels and the AP Action Potentials Travel Long Distances “All-or-none” Occurs or does not occur All APs are same magnitude Self propagating An AP triggers the next AP in adjacent areas of membrane without degradation Electronic current spread Charge spreads along membrane Regenerative cycle Ion entry ? electronic current spread ? triggering of AP Action Potentials Travel Long Distances Myelination Vertebrate neurons are myelinated Myelin Insulating layer of lipid-rich Schwann cells wrapped around axon Reduce “leakage” of charge across membrane Schwaan cells are a type of Glial cell Cells other than neurons that support neuron function Myelination Nodes of Ranvier Areas of exposed axonal membrane between Schwann cells Internodes The myelinated region Saltatory conduction APs “leap” from node to node APs occur at nodes of Ranvier, and electrotonic current spread through internodes This type of conduction is very rapid Myelination Unidirectional Signals Action potentials start at the axon hillock and travel towards the axon terminal “Up-stream” Na+ channels (just behind the region of depolarization) are in the absolute refractory period The absolute refractory period prevents backward (retrograde) transmission and summation of APs Relatively refractory period also contributes by requiring a very strong stimulus to cause another AP Information Transfer by AP AP frequency carries information AP frequency increases with stronger stimuli Magnitude of each AP does not change Maximum frequency is limited by the absolute refractory period Mammalian nerves can conduct 500–1000 action potentials per second Action Potential Frequency The Synapse Signal transmission from neuron to another cell Synapse Presynaptic cell, synaptic cleft, and postsynaptic cell Synaptic cleft Space between the presynaptic and postsynaptic cell Postsynaptic cell May be a neuron, muscle cell, or endocrine cell Neuromuscular junction Synapse between a motor neuron and a skeletal muscle cell Signal Transmission at a Chemical Synapse Amount of Neurotransmitter Released [Ca2+]i is affected by AP frequency More open voltage-gated Ca2+ channels ? [Ca2+]i Factors that lower intracellular [Ca2+]i Binding with intracellular buffers ? [Ca2+]i Ca2+ ATPases ? [Ca2+]i High AP frequency ? Ca2+ influx is greater than removal ? ? [Ca2+]i ? many synaptic vesicles release their contents ? high [neurotransmitter] in synapse Acetylcholine Postsynaptic Cells Postsynaptic cells have specific receptors for neurotransmitters Example: nicotinic ACh receptors Similar to specific hormone receptors on target cells Binding of neurotransmitter to receptor alters ion permeability of postsynaptic cell Change in membrane potential of postsynaptic cell Transmission of Signal Strength at Synapse Response of postsynaptic cell influenced by amount of neurotransmitter in synapse and number of receptors Amount of neurotransmitter Rate of release – rate of removal Release determined by frequency of APs Removal determined by Passive diffusion out of synapse Degradation by synaptic enzymes Uptake by surrounding cells Number of receptors Density of receptors on postsynaptic cell Neuron Structure and Function Neurons Vary in structure and properties Use same basic mechanisms to send signals Neural Zones Four functional zones Signal reception Dendrites and the cell body (soma) Incoming signal received and converted to change in membrane potential Signal integration Axon hillock Strong signal is converted to an action potential (AP) Signal conduction Axon (some wrapped in myelin sheath) AP travels down axon Signal transmission Axon terminals Release of neurotransmitter Electrical Signals in Neurons Neurons have a resting membrane potential (like all cells) Membrane potential is negative at rest Neurons are excitable Can rapidly change their membrane potential Depolarization – membrane potential becomes less negative Repolarization – membrane potential returns to resting value Hyperpolarization – membrane potential becomes more negative than resting value Changes in membrane potential act as electrical signals Membrane Potential Factors contributing to membrane potential Distribution of ions across the membrane Relative permeability of the ions Charges of the ions Gated Ion Channels Neurons depolarize or hyperpolarize by selectively altering permeability Gated ion channels open or close in response to a stimulus Example: neurotransmitter Channels only allow specific ions to pass through the membrane Ion moves down its electrochemical gradient Only relatively small numbers of ions move across As permeability to a specific ion increases, membrane potential will approach that ion’s equilibrium potential (Nernst equation) Changes in Membrane Potential Signals in the Dendrites and Cell Body Incoming signal Example: neurotransmitter Membrane-bound receptors bind to neurotransmitter Receptors transduce the chemical signal to an electrical signal by changing ion permeability of membrane Change in ion permeability causes change in membrane potential (graded potential) Graded Potentials Vary in magnitude depending on strength of stimulus More neurotransmitter ? more ion channels open ? larger magnitude of graded potential Depolarization Na+ or Ca2+ channels open Hyperpolarize K+ and Cl– channels open Stimulus Strength and Graded Potentials Graded Potentials Travel Short Distances Conduction with decrement Magnitude of graded potential decreases with increasing distance from opened ion channel Decrement due to: Leakage of charged ions across membrane Electrical resistance of cytoplasm Electrical properties of membrane Electrotonic current spread Positive charge spreads through cytoplasm causing depolarization of adjacent membrane Conduction with Decrement Action Potentials Travel Long Distances Characteristics of Action Potentials: Triggered by net graded potential at axon hillock (trigger zone) Do not degrade over time or distance Travel long distances along membrane All-or-none Must reach threshold potential to fire Depolarizations below threshold will not initiate an action potential Action Potentials Integration of Graded Signals Many graded potentials can be generated simultaneously Many receptor sites Many types of receptors Some graded potentials are depolarizations, some are hyperpolarizations Spatial summation Graded potentials from different sites influence the net change Temporal summation Graded potentials that occur at slightly different times influence net change Spatial Summation Temporal Summation Graded Potentials vs. Action Potentials Action Potentials (AP) Occur only when membrane potential at axon hillock reaches threshold Three phases: Depolarization Repolarization Hyperpolarization Absolute refractory period Cell incapable of generating a new AP Relative refractory period More difficult to generate new AP Voltage-Gated Channels Change shape due to changes in membrane potential Closed at resting potential Positive feedback Influx of Na+ ? local depolarization ? more Na+ channels open ? more depolarization Na+ channels open first (depolarization) K+ channels open more slowly (repolarization) Na+ channels close K+ channels close slowly relative refractory period caused by open K+ channels Action Potentials (AP) Ion Movement Relatively small number of ions move into and out of cell Single action potential has no measurable affect on ion concentrations inside and outside cell Na+/K+ ATPase restores concentration gradients following repeated action potentials Na+ Channels Have Two Gates Activation gate Voltage dependent Opens when membrane reaches threshold Inactivation gate Time-dependent Closes after brief time Na+ Channels Have Two Gates Voltage-Gated Channels and the AP Action Potentials Travel Long Distances “All-or-none” Occurs or does not occur All APs are same magnitude Self propagating An AP triggers the next AP in adjacent areas of membrane without degradation Electronic current spread Charge spreads along membrane Regenerative cycle Ion entry ? electronic current spread ? triggering of AP Action Potentials Travel Long Distances Myelination Vertebrate neurons are myelinated Myelin Insulating layer of lipid-rich Schwann cells wrapped around axon Reduce “leakage” of charge across membrane Schwaan cells are a type of Glial cell Cells other than neurons that support neuron function Myelination Nodes of Ranvier Areas of exposed axonal membrane between Schwann cells Internodes The myelinated region Saltatory conduction APs “leap” from node to node APs occur at nodes of Ranvier, and electrotonic current spread through internodes This type of conduction is very rapid Myelination Unidirectional Signals Action potentials start at the axon hillock and travel towards the axon terminal “Up-stream” Na+ channels (just behind the region of depolarization) are in the absolute refractory period The absolute refractory period prevents backward (retrograde) transmission and summation of APs Relatively refractory period also contributes by requiring a very strong stimulus to cause another AP Information Transfer by AP AP frequency carries information AP frequency increases with stronger stimuli Magnitude of each AP does not change Maximum frequency is limited by the absolute refractory period Mammalian nerves can conduct 500–1000 action potentials per second Action Potential Frequency The Synapse Signal transmission from neuron to another cell Synapse Presynaptic cell, synaptic cleft, and postsynaptic cell Synaptic cleft Space between the presynaptic and postsynaptic cell Postsynaptic cell May be a neuron, muscle cell, or endocrine cell Neuromuscular junction Synapse between a motor neuron and a skeletal muscle cell Signal Transmission at a Chemical Synapse Amount of Neurotransmitter Released [Ca2+]i is affected by AP frequency More open voltage-gated Ca2+ channels ? [Ca2+]i Factors that lower intracellular [Ca2+]i Binding with intracellular buffers ? [Ca2+]i Ca2+ ATPases ? [Ca2+]i High AP frequency ? Ca2+ influx is greater than removal ? ? [Ca2+]i ? many synaptic vesicles release their contents ? high [neurotransmitter] in synapse Acetylcholine Postsynaptic Cells Postsynaptic cells have specific receptors for neurotransmitters Example: nicotinic ACh receptors Similar to specific hormone receptors on target cells Binding of neurotransmitter to receptor alters ion permeability of postsynaptic cell Change in membrane potential of postsynaptic cell Transmission of Signal Strength at Synapse Response of postsynaptic cell influenced by amount of neurotransmitter in synapse and number of receptors Amount of neurotransmitter Rate of release – rate of removal Release determined by frequency of APs Removal determined by Passive diffusion out of synapse Degradation by synaptic enzymes Uptake by surrounding cells Number of receptors Density of receptors on postsynaptic cell Neuron Structure and Function Neurons Vary in structure and properties Use same basic mechanisms to send signals Neural Zones Four functional zones Signal reception Dendrites and the cell body (soma) Incoming signal received and converted to change in membrane potential Signal integration Axon hillock Strong signal is converted to an action potential (AP) Signal conduction Axon (some wrapped in myelin sheath) AP travels down axon Signal transmission Axon terminals Release of neurotransmitter Electrical Signals in Neurons Neurons have a resting membrane potential (like all cells) Membrane potential is negative at rest Neurons are excitable Can rapidly change their membrane potential Depolarization – membrane potential becomes less negative Repolarization – membrane potential returns to resting value Hyperpolarization – membrane potential becomes more negative than resting value Changes in membrane potential act as electrical signals Membrane Potential Factors contributing to membrane potential Distribution of ions across the membrane Relative permeability of the ions Charges of the ions Gated Ion Channels Neurons depolarize or hyperpolarize by selectively altering permeability Gated ion channels open or close in response to a stimulus Example: neurotransmitter Channels only allow specific ions to pass through the membrane Ion moves down its electrochemical gradient Only relatively small numbers of ions move across As permeability to a specific ion increases, membrane potential will approach that ion’s equilibrium potential (Nernst equation) Changes in Membrane Potential Signals in the Dendrites and Cell Body Incoming signal Example: neurotransmitter Membrane-bound receptors bind to neurotransmitter Receptors transduce the chemical signal to an electrical signal by changing ion permeability of membrane Change in ion permeability causes change in membrane potential (graded potential) Graded Potentials Vary in magnitude depending on strength of stimulus More neurotransmitter ? more ion channels open ? larger magnitude of graded potential Depolarization Na+ or Ca2+ channels open Hyperpolarize K+ and Cl– channels open Stimulus Strength and Graded Potentials Graded Potentials Travel Short Distances Conduction with decrement Magnitude of graded potential decreases with increasing distance from opened ion channel Decrement due to: Leakage of charged ions across membrane Electrical resistance of cytoplasm Electrical properties of membrane Electrotonic current spread Positive charge spreads through cytoplasm causing depolarization of adjacent membrane Conduction with Decrement Action Potentials Travel Long Distances Characteristics of Action Potentials: Triggered by net graded potential at axon hillock (trigger zone) Do not degrade over time or distance Travel long distances along membrane All-or-none Must reach threshold potential to fire Depolarizations below threshold will not initiate an action potential Action Potentials Integration of Graded Signals Many graded potentials can be generated simultaneously Many receptor sites Many types of receptors Some graded potentials are depolarizations, some are hyperpolarizations Spatial summation Graded potentials from different sites influence the net change Temporal summation Graded potentials that occur at slightly different times influence net change Spatial Summation Temporal Summation Graded Potentials vs. Action Potentials Action Potentials (AP) Occur only when membrane potential at axon hillock reaches threshold Three phases: Depolarization Repolarization Hyperpolarization Absolute refractory period Cell incapable of generating a new AP Relative refractory period More difficult to generate new AP Voltage-Gated Channels Change shape due to changes in membrane potential Closed at resting potential Positive feedback Influx of Na+ ? local depolarization ? more Na+ channels open ? more depolarization Na+ channels open first (depolarization) K+ channels open more slowly (repolarization) Na+ channels close K+ channels close slowly relative refractory period caused by open K+ channels Action Potentials (AP) Ion Movement Relatively small number of ions move into and out of cell Single action potential has no measurable affect on ion concentrations inside and outside cell Na+/K+ ATPase restores concentration gradients following repeated action potentials Na+ Channels Have Two Gates Activation gate Voltage dependent Opens when membrane reaches threshold Inactivation gate Time-dependent Closes after brief time Na+ Channels Have Two Gates Voltage-Gated Channels and the AP Action Potentials Travel Long Distances “All-or-none” Occurs or does not occur All APs are same magnitude Self propagating An AP triggers the next AP in adjacent areas of membrane without degradation Electronic current spread Charge spreads along membrane Regenerative cycle Ion entry ? electronic current spread ? triggering of AP Action Potentials Travel Long Distances Myelination Vertebrate neurons are myelinated Myelin Insulating layer of lipid-rich Schwann cells wrapped around axon Reduce “leakage” of charge across membrane Schwaan cells are a type of Glial cell Cells other than neurons that support neuron function Myelination Nodes of Ranvier Areas of exposed axonal membrane between Schwann cells Internodes The myelinated region Saltatory conduction APs “leap” from node to node APs occur at nodes of Ranvier, and electrotonic current spread through internodes This type of conduction is very rapid Myelination Unidirectional Signals Action potentials start at the axon hillock and travel towards the axon terminal “Up-stream” Na+ channels (just behind the region of depolarization) are in the absolute refractory period The absolute refractory period prevents backward (retrograde) transmission and summation of APs Relatively refractory period also contributes by requiring a very strong stimulus to cause another AP Information Transfer by AP AP frequency carries information AP frequency increases with stronger stimuli Magnitude of each AP does not change Maximum frequency is limited by the absolute refractory period Mammalian nerves can conduct 500–1000 action potentials per second Action Potential Frequency The Synapse Signal transmission from neuron to another cell Synapse Presynaptic cell, synaptic cleft, and postsynaptic cell Synaptic cleft Space between the presynaptic and postsynaptic cell Postsynaptic cell May be a neuron, muscle cell, or endocrine cell Neuromuscular junction Synapse between a motor neuron and a skeletal muscle cell Signal Transmission at a Chemical Synapse Amount of Neurotransmitter Released [Ca2+]i is affected by AP frequency More open voltage-gated Ca2+ channels ? [Ca2+]i Factors that lower intracellular [Ca2+]i Binding with intracellular buffers ? [Ca2+]i Ca2+ ATPases ? [Ca2+]i High AP frequency ? Ca2+ influx is greater than removal ? ? [Ca2+]i ? many synaptic vesicles release their contents ? high [neurotransmitter] in synapse Acetylcholine Postsynaptic Cells Postsynaptic cells have specific receptors for neurotransmitters Example: nicotinic ACh receptors Similar to specific hormone receptors on target cells Binding of neurotransmitter to receptor alters ion permeability of postsynaptic cell Change in membrane potential of postsynaptic cell Transmission of Signal Strength at Synapse Response of postsynaptic cell influenced by amount of neurotransmitter in synapse and number of receptors Amount of neurotransmitter Rate of release – rate of removal Release determined by frequency of APs Removal determined by Passive diffusion out of synapse Degradation by synaptic enzymes Uptake by surrounding cells Number of receptors Density of receptors on postsynaptic cell Neuron Structure and Function Neurons Vary in structure and properties Use same basic mechanisms to send signals Neural Zones Four functional zones Signal reception Dendrites and the cell body (soma) Incoming signal received and converted to change in membrane potential Signal integration Axon hillock Strong signal is converted to an action potential (AP) Signal conduction Axon (some wrapped in myelin sheath) AP travels down axon Signal transmission Axon terminals Release of neurotransmitter Electrical Signals in Neurons Neurons have a resting membrane potential (like all cells) Membrane potential is negative at rest (-70mV) Neurons are excitable Can rapidly change their membrane potential Depolarization – membrane potential becomes less negative Hyperpolarization – membrane potential becomes more negative than resting value Repolarization – membrane potential returns to resting value Changes in membrane potential act as electrical signals Membrane Potential Factors contributing to membrane potential Distribution of ions across the membrane Relative permeability of the ions Charges of the ions Gated Ion Channels Neurons depolarize or hyperpolarize by selectively altering permeability Gated ion channels open or close in response to a stimulus Example: neurotransmitter Channels only allow specific ions to pass through the membrane Ion moves down its electrochemical gradient Only relatively small numbers of ions move across As permeability to a specific ion increases, membrane potential will approach that ion’s equilibrium potential (Nernst equation) Changes in Membrane Potential Signals in the Dendrites and Cell Body Incoming signal Example: neurotransmitter Membrane-bound receptors bind to neurotransmitter Receptors transduce the chemical signal to an electrical signal by changing ion permeability of membrane Change in ion permeability causes change in membrane potential (graded potential) Graded Potentials Vary in magnitude depending on strength of stimulus More neurotransmitter ? more ion channels open ? larger magnitude of graded potential Depolarization Na+ or Ca2+ channels open Hyperpolarize K+ and Cl– channels open Stimulus Strength and Graded Potentials Graded Potentials Travel Short Distances Conduction with decrement Magnitude of graded potential decreases with increasing distance from opened ion channel Decrement due to: Leakage of charged ions across membrane Electrical resistance of cytoplasm Electrical properties of membrane Electrotonic current spread Positive charge spreads through cytoplasm causing depolarization of adjacent membrane Conduction with Decrement Action Potentials Travel Long Distances Characteristics of Action Potentials: Triggered by net graded potential at axon hillock (trigger zone) Do not degrade over time or distance Travel long distances along membrane All-or-none Must reach threshold potential to fire Depolarizations below threshold will not initiate an action potential Action Potentials Integration of Graded Signals Many graded potentials can be generated simultaneously Many receptor sites Many types of receptors Some graded potentials are depolarizations, some are hyperpolarizations Spatial summation Graded potentials from different sites influence the net change Temporal summation Graded potentials that occur at slightly different times influence net change Spatial Summation Temporal Summation Graded Potentials vs. Action Potentials Action Potentials (AP) Occur only when membrane potential at axon hillock reaches threshold Three phases: Depolarization Repolarization Hyperpolarization Absolute refractory period Cell incapable of generating a new AP Relative refractory period More difficult to generate new AP Voltage-Gated Channels Change shape due to changes in membrane potential Closed at resting potential Positive feedback Influx of Na+ ? local depolarization ? more Na+ channels open ? more depolarization Na+ channels open first (depolarization) K+ channels open more slowly (repolarization) Na+ channels close K+ channels close slowly relative refractory period caused by open K+ channels Action Potentials (AP) Ion Movement Relatively small number of ions move into and out of cell Single action potential has no measurable affect on ion concentrations inside and outside cell Na+/K+ ATPase restores concentration gradients following repeated action potentials Na+ Channels Have Two Gates Activation gate Voltage dependent Opens when membrane reaches threshold Inactivation gate Time-dependent Closes after brief time Na+ Channels Have Two Gates Voltage-Gated Channels and the AP Action Potentials Travel Long Distances “All-or-none” Occurs or does not occur All APs are same magnitude Self propagating An AP triggers the next AP in adjacent areas of membrane without degradation Electronic current spread Charge spreads along membrane Regenerative cycle Ion entry ? electronic current spread ? triggering of AP Action Potentials Travel Long Distances Myelination Vertebrate neurons are myelinated Myelin Insulating layer of lipid-rich Schwann cells wrapped around axon Reduce “leakage” of charge across membrane Schwaan cells are a type of Glial cell Cells other than neurons that support neuron function Myelination Nodes of Ranvier Areas of exposed axonal membrane between Schwann cells Internodes The myelinated region Saltatory conduction APs “leap” from node to node APs occur at nodes of Ranvier, and electrotonic current spread through internodes This type of conduction is very rapid Myelination Unidirectional Signals Action potentials start at the axon hillock and travel towards the axon terminal “Up-stream” Na+ channels (just behind the region of depolarization) are in the absolute refractory period The absolute refractory period prevents backward (retrograde) transmission and summation of APs Relatively refractory period also contributes by requiring a very strong stimulus to cause another AP Information Transfer by AP AP frequency carries information AP frequency increases with stronger stimuli Magnitude of each AP does not change Maximum frequency is limited by the absolute refractory period Mammalian nerves can conduct 500–1000 action potentials per second Action Potential Frequency The Synapse Signal transmission from neuron to another cell Synapse Presynaptic cell, synaptic cleft, and postsynaptic cell Synaptic cleft Space between the presynaptic and postsynaptic cell Postsynaptic cell May be a neuron, muscle cell, or endocrine cell Neuromuscular junction Synapse between a motor neuron and a skeletal muscle cell Signal Transmission at a Chemical Synapse Amount of Neurotransmitter Released [Ca2+]i is affected by AP frequency More open voltage-gated Ca2+ channels ? [Ca2+]i Factors that lower intracellular [Ca2+]i Binding with intracellular buffers ? [Ca2+]i Ca2+ ATPases ? [Ca2+]i High AP frequency ? Ca2+ influx is greater than removal ? ? [Ca2+]i ? many synaptic vesicles release their contents ? high [neurotransmitter] in synapse Acetylcholine Postsynaptic Cells Postsynaptic cells have specific receptors for neurotransmitters Example: nicotinic ACh receptors Similar to specific hormone receptors on target cells Binding of neurotransmitter to receptor alters ion permeability of postsynaptic cell Change in membrane potential of postsynaptic cell Transmission of Signal Strength at Synapse Response of postsynaptic cell influenced by amount of neurotransmitter in synapse and number of receptors Amount of neurotransmitter Rate of release – rate of removal Release determined by frequency of APs Removal determined by Passive diffusion out of synapse Degradation by synaptic enzymes Uptake by surrounding cells Number of receptors Density of receptors on postsynaptic cell Neuron Structure and Function Neurons Vary in structure and properties Use same basic mechanisms to send signals Neural Zones Four functional zones Signal reception Dendrites and the cell body (soma) Incoming signal received and converted to change in membrane potential Signal integration Axon hillock Strong signal is converted to an action potential (AP) Signal conduction Axon (some wrapped in myelin sheath) AP travels down axon Signal transmission Axon terminals Release of neurotransmitter Electrical Signals in Neurons Neurons have a resting membrane potential (like all cells) Membrane potential is negative at rest (-70mV) Neurons are excitable Can rapidly change their membrane potential Depolarization – membrane potential becomes less negative Hyperpolarization – membrane potential becomes more negative than resting value Repolarization – membrane potential returns to resting value Changes in membrane potential act as electrical signals Membrane Potential Factors contributing to membrane potential Distribution of ions across the membrane Relative permeability of the ions Charges of the ions Gated Ion Channels Neurons depolarize or hyperpolarize by selectively altering permeability Gated ion channels open or close in response to a stimulus Example: neurotransmitter Channels only allow specific ions to pass through the membrane Ion moves down its electrochemical gradient Only relatively small numbers of ions move across As permeability to a specific ion increases, membrane potential will approach that ion’s equilibrium potential (Nernst equation) Changes in Membrane Potential Signals in the Dendrites and Cell Body Incoming signal Example: neurotransmitter Membrane-bound receptors bind to neurotransmitter Receptors transduce the chemical signal to an electrical signal by changing ion permeability of membrane Change in ion permeability causes change in membrane potential (graded potential) Graded Potentials Vary in magnitude depending on strength of stimulus More neurotransmitter ? more ion channels open ? larger magnitude of graded potential Depolarization Na+ or Ca2+ channels open Hyperpolarize K+ and Cl– channels open Stimulus Strength and Graded Potentials Graded Potentials Travel Short Distances Conduction with decrement Magnitude of graded potential decreases with increasing distance from opened ion channel Decrement due to: Leakage of charged ions across membrane Electrical resistance of cytoplasm Electrical properties of membrane Electrotonic current spread Positive charge spreads through cytoplasm causing depolarization of adjacent membrane Conduction with Decrement Action Potentials Travel Long Distances Characteristics of Action Potentials: Triggered by net graded potential at axon hillock (trigger zone) Do not degrade over time or distance Travel long distances along membrane All-or-none Must reach threshold potential to fire Depolarizations below threshold will not initiate an action potential Action Potentials Integration of Graded Signals Many graded potentials can be generated simultaneously Many receptor sites Many types of receptors Some graded potentials are depolarizations, some are hyperpolarizations Spatial summation Graded potentials from different sites influence the net change Temporal summation Graded potentials that occur at slightly different times influence net change Spatial Summation Temporal Summation Graded Potentials vs. Action Potentials Action Potentials (AP) Occur only when membrane potential at axon hillock reaches threshold Three phases: Depolarization Repolarization Hyperpolarization Absolute refractory period Cell incapable of generating a new AP Relative refractory period More difficult to generate new AP Voltage-Gated Channels Change shape due to changes in membrane potential Closed at resting potential Positive feedback Influx of Na+ ? local depolarization ? more Na+ channels open ? more depolarization Na+ channels open first (depolarization) K+ channels open more slowly (repolarization) Na+ channels close K+ channels close slowly relative refractory period caused by open K+ channels Action Potentials (AP) Ion Movement Relatively small number of ions move into and out of cell Single action potential has no measurable affect on ion concentrations inside and outside cell Na+/K+ ATPase restores concentration gradients following repeated action potentials Na+ Channels Have Two Gates Activation gate Voltage dependent Opens when membrane reaches threshold Inactivation gate Time-dependent Closes after brief time Na+ Channels Have Two Gates Voltage-Gated Channels and the AP Action Potentials Travel Long Distances “All-or-none” Occurs or does not occur All APs are same magnitude Self propagating An AP triggers the next AP in adjacent areas of membrane without degradation Electronic current spread Charge spreads along membrane Regenerative cycle Ion entry ? electronic current spread ? triggering of AP Action Potentials Travel Long Distances Myelination Vertebrate neurons are myelinated Myelin Insulating layer of lipid-rich Schwann cells wrapped around axon Reduce “leakage” of charge across membrane Schwaan cells are a type of Glial cell Cells other than neurons that support neuron function Myelination Nodes of Ranvier Areas of exposed axonal membrane between Schwann cells Internodes The myelinated region Saltatory conduction APs “leap” from node to node APs occur at nodes of Ranvier, and electrotonic current spread through internodes This type of conduction is very rapid Unidirectional Signals Action potentials start at the axon hillock and travel towards the axon terminal “Up-stream” Na+ channels (just behind the region of depolarization) are in the absolute refractory period The absolute refractory period prevents backward (retrograde) transmission and summation of APs Relatively refractory period also contributes by requiring a very strong stimulus to cause another AP Information Transfer by AP AP frequency carries information AP frequency increases with stronger stimuli Magnitude of each AP does not change Maximum frequency is limited by the absolute refractory period Mammalian nerves can conduct 500–1000 action potentials per second Action Potential Frequency The Synapse Signal transmission from neuron to another cell Synapse Presynaptic cell, synaptic cleft, and postsynaptic cell Synaptic cleft Space between the presynaptic and postsynaptic cell Postsynaptic cell May be a neuron, muscle cell, or endocrine cell Neuromuscular junction Synapse between a motor neuron and a skeletal muscle cell Amount of Neurotransmitter Released [Ca2+]i is affected by AP frequency More open voltage-gated Ca2+ channels ? [Ca2+]i Factors that lower intracellular [Ca2+]i Binding with intracellular buffers ? [Ca2+]i Ca2+ ATPases ? [Ca2+]i High AP frequency ? Ca2+ influx is greater than removal ? ? [Ca2+]i ? many synaptic vesicles release their contents ? high [neurotransmitter] in synapse Acetylcholine Postsynaptic Cells Postsynaptic cells have specific receptors for neurotransmitters Example: nicotinic ACh receptors Similar to specific hormone receptors on target cells Binding of neurotransmitter to receptor alters ion permeability of postsynaptic cell Change in membrane potential of postsynaptic cell Transmission of Signal Strength at Synapse Response of postsynaptic cell influenced by amount of neurotransmitter in synapse and number of receptors Amount of neurotransmitter Rate of release – rate of removal Release determined by frequency of APs Removal determined by Passive diffusion out of synapse Degradation by synaptic enzymes Uptake by surrounding cells Number of receptors Density of receptors on postsynaptic cell Neuron Structure and Function Neurons Vary in structure and properties Use same basic mechanisms to send signals Neural Zones Four functional zones Signal reception Dendrites and the cell body (soma) Incoming signal received and converted to change in membrane potential Signal integration Axon hillock Strong signal is converted to an action potential (AP) Signal conduction Axon (some wrapped in myelin sheath) AP travels down axon Signal transmission Axon terminals Release of neurotransmitter Electrical Signals in Neurons Neurons have a resting membrane potential (like all cells) Membrane potential is negative at rest (-70mV) Neurons are excitable Can rapidly change their membrane potential Depolarization – membrane potential becomes less negative Hyperpolarization – membrane potential becomes more negative than resting value Repolarization – membrane potential returns to resting value Changes in membrane potential act as electrical signals Membrane Potential Factors contributing to membrane potential Distribution of ions across the membrane Relative permeability of the ions Charges of the ions Gated Ion Channels Neurons depolarize or hyperpolarize by selectively altering permeability Gated ion channels open or close in response to a stimulus Example: neurotransmitter Channels only allow specific ions to pass through the membrane Ion moves down its electrochemical gradient Only relatively small numbers of ions move across As permeability to a specific ion increases, membrane potential will approach that ion’s equilibrium potential (Nernst equation) Changes in Membrane Potential Signals in the Dendrites and Cell Body Incoming signal Example: neurotransmitter Membrane-bound receptors bind to neurotransmitter Receptors transduce the chemical signal to an electrical signal by changing ion permeability of membrane Change in ion permeability causes change in membrane potential (graded potential) Graded Potentials Vary in magnitude depending on strength of stimulus More neurotransmitter ? more ion channels open ? larger magnitude of graded potential Depolarization Na+ or Ca2+ channels open Hyperpolarize K+ and Cl– channels open Stimulus Strength and Graded Potentials Graded Potentials Travel Short Distances Conduction with decrement Magnitude of graded potential decreases with increasing distance from opened ion channel Decrement due to: Leakage of charged ions across membrane Electrical resistance of cytoplasm Electrical properties of membrane Electrotonic current spread Positive charge spreads through cytoplasm causing depolarization of adjacent membrane Conduction with Decrement Action Potentials Travel Long Distances Characteristics of Action Potentials: Triggered by net graded potential at axon hillock (trigger zone) Do not degrade over time or distance Travel long distances along membrane All-or-none Must reach threshold potential to fire Depolarizations below threshold will not initiate an action potential Action Potentials Integration of Graded Signals Many graded potentials can be generated simultaneously Many receptor sites Many types of receptors Some graded potentials are depolarizations, some are hyperpolarizations Spatial summation Graded potentials from different sites influence the net change Temporal summation Graded potentials that occur at slightly different times influence net change Spatial Summation Temporal Summation Graded Potentials vs. Action Potentials Action Potentials (AP) Occur only when membrane potential at axon hillock reaches threshold Three phases: Depolarization Repolarization Hyperpolarization Absolute refractory period Cell incapable of generating a new AP Relative refractory period More difficult to generate new AP Voltage-Gated Channels Change shape due to changes in membrane potential Closed at resting potential Positive feedback Influx of Na+ ? local depolarization ? more Na+ channels open ? more depolarization Na+ channels open first (depolarization) K+ channels open more slowly (repolarization) Na+ channels close K+ channels close slowly relative refractory period caused by open K+ channels Ion Movement Relatively small number of ions move into and out of cell Single action potential has no measurable affect on ion concentrations inside and outside cell Na+/K+ ATPase restores concentration gradients following repeated action potentials Na+ Channels Have Two Gates Activation gate Voltage dependent Opens when membrane reaches threshold Inactivation gate Time-dependent Closes after brief time Na+ Channels Have Two Gates Voltage-Gated Channels and the AP Action Potentials Travel Long Distances “All-or-none” Occurs or does not occur All APs are same magnitude Self propagating An AP triggers the next AP in adjacent areas of membrane without degradation Electronic current spread Ch