Neurophysiology of Nerve Impulses PDF

Neurophysiology of Nerve Impulses PEx 3 1 Electrical Potentials and Currents A nerve pathway is not a continuous “wire” but a series of separate cells Neuronal communication is based on mechanisms for producing electrical potentials and currents – Electri...

Neurophysiology of Nerve Impulses PEx 3 1 Electrical Potentials and Currents A nerve pathway is not a continuous “wire” but a series of separate cells Neuronal communication is based on mechanisms for producing electrical potentials and currents – Electrical Potential Difference in concentration of charged particles between different parts of the cell – Electrical Current Flow of charged particles from one point to another within the cell 2  The Structure of Neurons Neurons (nerve cells) – Specialized cells that conduct messages in the form of electrical impulses throughout the body – Consist of Cell body Axon Dendrites Each neuron has a single axon that generates and conducts nerve impulses away from the cell body to the axon terminals 3 Structural Components of a Neuron 4  Transmembrane Potential The interior (cytoplasmic side) of the plasma membrane is slightly negative, in relation to the outside (extracellular fluid side) Transmembrane Potential – The unequal charge across the plasma membrane – It is due to differences in the permeability of the membrane to various ions Resting Potential – The transmembrane potential in an undisturbed cell – Only leakage channels for Na+ and K+ are open – All gated Na+ and K+ channels are closed The average resting membrane potential varies by cell type, but averages -70 mV 5  Transmembrane Potential Extracellular Fluid – Contains a high concentration of Na+ and Cl- Cytosol – Contains a high concentration of K+ and negatively charged proteins Negatively charged proteins inside the cell can not cross the cell membrane It is easier for K+ to diffuse out of the cell through K+ leakage channels, down it’s concentration gradient, than it is for Na+ to enter the cell through a Na+ leakage channel – Therefore, the inner plasma membrane surface has an excess of negative charges relative to the outer surface 6  Resting Membrane Potential Explanation for -70 mV Resting Membrane Potential – Plasma membrane very permeable to K+ K+ leaks out until electrical gradient created attracts it back in Cytoplasmic anions can not escape due to size or charge (PO42-, SO42-, organic acids, proteins) – Plasma membrane is much less permeable to Na+ – Na+/K+ Pumps Pump out 3 Na+ for every 2 K+ it brings in Work continuously and require a great deal of ATP Necessitate glucose and oxygen be supplied to nerve tissue 7  Transmembrane Channels Membrane Channels – Control movement of ions across the plasma membrane Na+ and K+ – The primary determinants of the transmembrane potential of neurons and many other cell types The two main types of membrane channels – Leak Channels (passive) – Gated Channels (active) Chemically Gated Voltage Gated Mechanically Gated 8  Transmembrane Channels Leak Channels (passive) – Non-gated – Always open – Permeability can vary based on local conditions – Important in establishing resting membrane potential 9  Transmembrane Channels Gated Channels (active) – Open and close in response to specific stimuli – Chemically Gated (ligand-gated) Channels Open/close when they bind specific chemicals (i.e.neurotransmitters) Most abundant on dendrites and cell body of a neuron – Voltage-Gated Channels Open and close in response to changes in membrane potential Participate in the generation and conduction of action potentials Found in neural axons, skeletal muscle sarcolemma, cardiac ms – Mechanically Gated Channels Open and close in response to physical deformation of receptors (i.e. touch, pressure, vibration) The force distorts the channel, causing the gates to open 10  Transmembrane Channels Chemically (ligand) Gated Voltage-gated Ion Channels Ion Channels 11  Resting Membrane Potential Depolarization and hyperpolarization are changes in the resting membrane potential 12  Depolarization Any shift from the resting membrane potential toward a more positive potential Occurs when the resting membrane is exposed to a stimulus that opens the Na+ chemical channels – Na+ enters the cell Creates a voltage change – The positive charge of Na+ shifts the transmembrane potential toward 0 mV » This is called depolarization 13  Depolarization The maximum change in transmembrane potential is proportional to the size of the stimulus – The greater the stimulus The greater the number of chemical channels to open The more Na+ that enters the cell The greater the membrane area affected The greater the degree of depolarization 14 Transmembrane Potential Changes Changes in membrane potential can produce two types of signals – Graded Potentials – Action Potentials 15 Transmembrane Potential Changes Graded Potentials – Also called local potentials – A short-lived localized change in the resting membrane potential – They are changes in the transmembrane potential that can not spread far from the site of stimulation These changes cause current flows that decrease in magnitude with distance – Magnitude varies with strength of stimulus » The greater the stimulus, the greater voltage change and the farther the current will flow – Any stimulus that opens a chemical-gated channel will produce a graded potential 16 Chemical Excitation 17  Action Potential A brief reversal of the membrane potential Propagated changes in the transmembrane potential that, once initiated, affect the entire excitable membrane – It is an electrical impulse that travels along the cell membrane and does not diminish as it moves away from its source For an action potential to occur – Depolarization has to be great enough to reach the membrane threshold causing the voltage-gated channels to open Threshold – The minimum voltage to stimulate an action potential Varies, but average is about -55mV in many neurons Only cells with excitable membranes (neurons and muscle cells) can generate action potentials 18  Action Potentials The principle way neurons communicate Follow the all-or-none principle – A stimulus either triggers an action potential or does not produce one at all You can not have a partial action potential – If threshold is reached, an action potential will occur It is irreversible – Once started goes to completion and can not be stopped – The impulse generated will travel the entire length of the membrane – It is nondecremental It does not get weaker with distance 19  Action Potential Generation of an Action Potential Involves – Depolarization – Repolarization – Hyperpolarization Only neurons and muscle cells generate action potentials 20 Generation of an Action Potential Depolarization – A stimulus that opens the chemical-gated channels The stimulus must cause a depolarization large enough to open voltaged-gated Na+ channels to initiate an action potential A more dramatic change in the membrane potential is produced where there is a high density of voltage-gated channels – Trigger zone has 500 channels/m2 (normal is 75) If the stimulus is great enough to cause the plasma membrane potential to reach the threshold potential (-55mV) – Voltage-gated Na+ channels open Na+ rushes into the cell generating an action potential Not all depolarizations lead to action potentials – The stimulus must be significant enough to cause the membrane potential to reach threshold 21 Generation of an Action Potential Depolarization – When the voltaged-gated Na+ channels open, the plasma membrane becomes much more permeable to Na+ – Due to their electrochemical gradient, Na+ rushes in and rapid depolarization occurs – The inner membrane surface now contains more + ions than – ions – The transmembrane potential changes from – 70 mV to a positive value 22 Generation of an Action Potential Repolarization – The process that occurs when the stimulus is removed and the transmembrane potential begins to return to normal resting levels – Is the re-establishment of the resting membrane potential As the membrane potential approaches +30mV – Repolarization begins Voltage-gated Na+ channels begin to close Voltage-gated K+ channels open – K+ exit the cell – K+ flows out, down the electrochemical gradient – The transmembrane potential shifts back towards its resting level Voltaged-gated K+ channels begin closing as the membrane potential approaches the normal resting potential (-70 mV) – It takes time for all of the voltage-gated K+ channels to close Therefore, the membrane potential passes the resting state (-70mV) 23 and produces hyperpolarization Generation of an Action Potential Hyperpolarization – K+ continues to exit the cell until all of the voltaged-gated K+ channels close They are closed at about - 90 mV – The leak channels and Na+/K+ pumps help to restore the plasma membrane to its resting membrane potential (-70mV) 24 Generation of an Action Potential Action Potential occurs 55 ------------------------------------- Hyperpolarization (-90mV) - 55 (-55mV) Stimulus occurs 25  The Refractory Period The time period from which an action potential begins until the normal resting potential has stabilized – During this period, the membrane will not respond normally to additional depolarizing stimuli It represents a period of resistance to stimulation Consists of 2 parts – Absolute Refractory Period As long as the voltage-gated Na+ channels are open, no stimulus will trigger an action potential – Relative Refractory Period As long as the voltage-gated K+ channels are open, only an especially strong stimulus will trigger new action potential The refractory period occurs only to a small segment of the plasma membrane at one time and quickly recovers 26  Myelin Sheath A whitish, protein-lipid substance made by – Oligodendrocytes in the CNS – Schwann cells in the PNS Protects and electrically insulates an axon Myelinated Internode – The area of the axon wrapped in myelin Nodes of Ranvier – Small gaps between the myelinated internodes Myelin Sheaths – Only associated with axons, not dendrites White Matter – Consists of regions of CNS with many myelinated nerves Gray matter – Consists of unmyelinated areas of CNS (short axons) 27  Schwann Cell Myelin sheath over a single axon 28  Propagation of Action Potentials Action Potentials – Spread (propagate) along the surface of the axon membrane – Travel along an axon by Continuous Propagation (unmyelinated axons) Saltatory Propagation (myelinated axons) 29  Propagation of Action Potentials Continuous Propagation (unmyeliated axons) – The action potential moves along the axon membrane in segments, starting at the initial segment of the axon – The local current spreads in all directions – As the next segment depolarizes and an action potential is generated, the previous segment enters the refractory period Therefore, the action potential can only move in one direction (forward) – The axon hillock can not respond with an action potential because it lacks voltage-gated Na+ channels – The action potential moves across the surface of the membrane in a series of tiny steps 30 Continuous Conduction in Unmyelinated Fiber 31 Propagation of Action Potentials Saltatory Propagation (myelinated axons) – Myelin increases resistance to the flow of ions across the membrane – Ions can only cross the plasma membrane at the nodes of Ranvier When an action potential is generated in a myelinated fiber – The local depolarizing current does not dissipate through the adjacent membrane regions, which are nonexcitable Instead, the current is maintained and moves rapidly to the next node of Ranvier where it triggers another action potential – Action potentials are triggered only at the nodes of Ranvier In saltatory conduction, the impulse jumps from node to node along the axon Nerve impulses are carried along myelinated axons (saltatory propagation) faster than in unmelinated axons (continuous propagation) 32 Saltatory Conduction in Myelinated Fibers 33 Saltatory Conduction in Myelinated Fiber Notice how the action potentials jump from node of Ranvier to node of Ranvier 34  Speed of Impulse Propagation Speed of nerve impulse transmission is affected by – Myelin – Diameter of Axon The larger the diameter, the lower the resistance, the faster the propagation speed – Because the cytosol offers less resistance than the plasma membrane Speeds – Small, unmyelinated fibers = 0.5 - 2.0 m/sec – Small, myelinated fibers = 3 - 15.0 m/sec – Large, myelinated fibers = up to 120 m/sec Functions – Slow signals supply the stomach and dilate pupil – Fast signals supply skeletal muscles and transport sensory signals for vision and balance 35  Action Potential Sodium Na+ channel Potassium channel Activation gates K+ Inactivation gate Na+ Na+ 1 Resting state K+ K+ 4 Hyperpolarization Na+ 2 Depolarization K+ 3 Repolarization 36 End of Chapter 37