* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The Neuron - Austin Community College
Endocannabinoid system wikipedia , lookup
Neural coding wikipedia , lookup
Multielectrode array wikipedia , lookup
Clinical neurochemistry wikipedia , lookup
Psychophysics wikipedia , lookup
Feature detection (nervous system) wikipedia , lookup
SNARE (protein) wikipedia , lookup
Signal transduction wikipedia , lookup
Spike-and-wave wikipedia , lookup
Neuromuscular junction wikipedia , lookup
Channelrhodopsin wikipedia , lookup
Patch clamp wikipedia , lookup
Node of Ranvier wikipedia , lookup
Nonsynaptic plasticity wikipedia , lookup
Synaptic gating wikipedia , lookup
Synaptogenesis wikipedia , lookup
Neurotransmitter wikipedia , lookup
Nervous system network models wikipedia , lookup
Action potential wikipedia , lookup
Neuropsychopharmacology wikipedia , lookup
Membrane potential wikipedia , lookup
Biological neuron model wikipedia , lookup
Single-unit recording wikipedia , lookup
Electrophysiology wikipedia , lookup
Chemical synapse wikipedia , lookup
Resting potential wikipedia , lookup
Molecular neuroscience wikipedia , lookup
BIO2305 Neurophysiology The Neuron The functional and structural unit of the nervous system There are many, many different types of neurons but most have certain structural and functional characteristics in common Function Neurons are excitable cells (responsive to stimuli) specialized to conduct information (communicate) from one part of the body to another via electrical impulses (Action Potentials) conducted along the length of axons 1 Electric Current Nerve impulse: flow of ions across membrane Basic Concepts Review Ions – charged particles Anions – Negatively charged particles Cations – Positively charged particles Electrostatic forces Opposite charges attract, same charges repel Ions flow along their electrochemical gradient when they move toward an area of opposite charge Concentration forces Diffusion – movement of ions through semipermeable membrane Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration Together, the electrical and chemical gradients constitute the electrochemical gradient Review: Passive and Active Transport 2 Sodium-Potassium Pump Is one type of active transport system It is an electrogenic pump that generates the voltage across a membrane Ion Channels – Membrane Potential Membrane potential is the voltage difference across a membrane Resting membrane potential (when the cell is not depolarizing) is a -70mV difference between the inside and the outside - the membrane is polarized When voltage-gated ion channels open, ions diffuse across the membrane following their electrochemical gradients This movement of charge is an electrical current and can create voltage (measure of potential) energy change across the membrane. The sodium-potassium pump reestablishes the -70mv difference This electrical charge across the membrane is the resting membrane potential 3 Resting Membrane Potential The resting potential exists because ions are concentrated on different sides of the membrane: Na+ and Cl- outside the cell K+ and organic anions inside the cell Due to different membrane permeabilities of the passive ion channels and operation of the sodiumpotassium pump Electrical Signals: Ion Movement Resting membrane potential determined by K+ concentration gradients Cell’s resting permeability to K+, Na+, and ClImpermeability of anions – amino acids, proteins Gated channels control ion permeability Mechanically gated Chemical gated Voltage gated Threshold voltage varies from one channel type to another Membrane Potentials: Signals Neurons use changes in membrane potential to receive, integrate, and send information Two types of signals are produced by a change in membrane potential: Graded potentials (short-distance) Action potentials (long-distance) Signals Carried by Neurons a) Resting neuron – membrane is polarized; cytoplasmic side is negatively charged b) Stimulation of the neuron depolarization; Na+ enters the cell, making the inside positive compared to the outside. The charges switch. c) Positive on the inside, negative on the outside. Impulse propagates down the length of the axon d) Membrane repolarizes with efflux of K+ and with the help of Na+/K+ pumps 4 Graded Potentials Short-lived, local changes in membrane potential Currents decrease in magnitude with distance Their magnitude varies directly with the strength of the stimulus – the stronger the stimulus the more the voltage changes and the farther the current goes Sufficiently strong graded potentials can initiate action potentials if they maintain threshold by the time reach the trigger zone Action Potentials Supra-threshold stimuli cause voltage-gated Na+ channels to open Na+ to enters the cell down its electrochemical gradient to produce depolarizing currents that are translated into action potentials Threshold Voltage – membrane is depolarized by ~ 15 mV stimulus (from -70 to -55mV) The AP is a brief reversal of membrane potential with a total amplitude of 100 mV (from -70mV to +30mV) APs do not decrease in strength with distance All-or-None phenomenon – action potentials either happen completely, or not at all Action Potentials Action potentials consist of three main phases: Depolarization Repolarization Hyperpolarization Depolarization Phase Na+ activation gates open quickly and Na+ enters causing local depolarization which opens more activation gates and cell interior becomes progressively less negative. Threshold – a critical level of membrane potential (~ -55 mV) where depolarization becomes self-generating 5 Repolarization Phase Positive intracellular charge reduces the driving force of Na+ to zero. Na+ inactivation gates of Na+ channels close. After depolarization, the slower voltage-gated K+ channels open and K+ rapidly leaves the cell following its electrochemical gradient restoring resting membrane potential The neuron is insensitive to stimulus and depolarization during this time (refractory period) Hyperpolarization The slow K+ gates remain open longer than needed to restore the resting state. This excessive efflux causes hyperpolarization of the membrane The neuron may depolarize during this time but it would require a much greater stimulus. Ion Permeability during an AP 6 Propagation of an Action Potential The action potential is self-propagating and moves away from the stimulus (point of origin) Role of the Sodium-Potassium Pump Repolarization restores the resting electrical conditions of the neuron, but does not restore the resting ionic conditions Ionic redistribution is accomplished by the sodium-potassium pump following repolarization Refractory Periods Absolute refractory period is the time from the opening of the Na+ activation gates until the closing of inactivation gates; the neuron cannot respond to another stimulus Relative refractory period follows the absolute refractory period. Na+ gates are closed (almost all Na+ gates are reset), K+ gates are open, and hyperpolarization is occurring. Only a strong stimulus can generate an AP 7 Refractory Periods Axon Conduction Velocities Conduction velocities vary widely among neurons, determined mainly by: Axon Diameter – the larger the diameter, the faster the impulse (less resistance) Presence of a Myelin Sheath – myelination increases impulse speed (Continuous vs. Saltatory Conduction) Myelin Sheath A Schwann cell envelopes and encloses the axon with its plasma membrane. The concentric layers of membrane wrapped around the axon are the myelin sheath Neurilemma – outermost cytoplasm and exposed membrane of a Schwann cell 8 Saltatory Conduction Gaps in the myelin sheath between adjacent Schwann cells are called nodes of Ranvier (neurofibral nodes) Voltage-gated Na+ channels are concentrated at these nodes Action potentials are triggered only at the nodes and jump from one node to the next Much faster than conduction along unmyelinated axons Trigger Zone: Cell Integration and Initiation of AP A graded potential above threshold (above -55 mV) reaches the trigger zone and becomes an action potential a) A graded potential starts above threshold (T) at its initiation point, but decreases in strength as it travels through the cell body. At the trigger zone, it is below threshold and therefore does not initiate an action potential. b) A stronger stimulus at the same point on the cell body creates a graded potential that is still above threshold by the time it reaches the trigger zone, so an action potential results. 9 Synapse As the AP reaches the axon terminals the signal is relayed to target cells at specialized junctions known as synapses Synapse is a junction that mediates information transfer from one neuron to another neuron or to an effector cell Synaptic Cleft: Information Transfer An AP reaches the axon terminal of the presynaptic cell and causes V-gated Ca2+ channels to open Ca2+ rushes in, binds to regulatory proteins on vesicles, & initiates NT exocytosis NTs diffuse across the synaptic cleft and binds to receptors on the postsynaptic membrane and initiate some sort of response on the postsynaptic cell 10 Synaptic Transmission Neurotransmitter Removal NTs are removed from the synaptic cleft via: Enzymatic degradation Diffusion Reuptake 11 Effects of the Neurotransmitter Different neurons can contain different NTs Different postsynaptic cells may contain different receptors -thus, the effects of an NT can vary Some NTs cause cation channels to open, which results in a graded depolarization (excitatory) Some NTs cause anion channels to open, which results in a graded hyperpolarization (inhibitory) EPSPs & IPSPs Typically, a single synaptic interaction will not create a graded depolarization strong enough to migrate to the axon hillock and induce the firing of an AP However, a graded depolarization will bring the membrane potential closer to threshold. Thus, it’s often referred to as an excitatory postsynaptic potential or EPSP. Graded hyperpolarizations bring the membrane potential farther away from threshold and thus are referred to as inhibitory postsynaptic potentials or IPSPs. Excitatory And Inhibitory Neurotransmitters Excitatory - a neurotransmitter depolarizes the post-synaptic neuron, Inhibitory – a neurotransmitter hyperpolarizes the post-synaptic neuron Whether a neurotransmitter is excitatory or inhibitory depends on its receptor 12 Excitatory And Inhibitory Neurotransmitters KEY: ICR = ion channel receptor GPLR = G Protein Linked Receptor Excitatory And Inhibitory Neurotransmitters Acetylcholine is excitatory because its receptor is a ligand-gated Na+ channel GABA is inhibitory because its receptor is a ligand-gated Cl- channel Other transmitters (e.g. vasopressin, dopamine) have G-protein-linked receptors Effects depend on the signal transduction pathway and cell type 13 Summation One EPSP is usually not strong enough to cause an AP However, EPSPs may be summed Temporal summation The same presynaptic neuron stimulates the postsynaptic neuron multiple times in a brief period. The depolarization resulting from the combination of all the EPSPs may be able to cause an AP Spatial summation Multiple neurons all stimulate a postsynaptic neuron resulting in a combination of EPSPs which may yield an AP Synaptic Organization Communication between neurons is not typically a one-to-one event, but often a combination of convergence and divergence Convergence - as many as 10,000 neurons converge upon a single postsynaptic neuron, increasing its sensitivity Divergence – a single presynaptic neuron branches; its collaterals synapse on multiple target neurons, allowing sensory info, for example, to reach multiple target nuclei in the brain and spinal cord 14