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Conduction of a Nerve Impulse Function of a Neuron Primary function is the generation of a nerve impulse Similar in many ways to the flow of electricity through an insulated wire Resting Membrane Potential A potential difference occurs when there is a separation of charge between two points (ex. Battery – potential difference between + and – poles; measured in volts) Occurs in cells due to an unequal distribution of ions across membrane The primary ions that we are concerned with are potassium ions and sodium ions At rest, there are approximately 10x more Na+ outside than inside At rest, there are approximately 10x more K+ inside than outside The resting membrane potential is maintained by the action of a particular ion channel referred to as the sodiumpotassium pump which forces Na out while forcing K in to establish a concentration gradient Neurilemma Animation If conditions were ideal, there would be no difference in the concentration of ions inside and outside the cell, therefore no potential difference (no difference in net charge) Two condition make diffusion conditions less than ideal – Na-K pump transports uneven ratio of ions: 3 Na out for each 2 K in – Plasma membranes are more permeable to K ions than Na ions: there is a tendency for K ions to leak out faster than Na ions leak in The result of these two conditions is that there are more positively charged ions on the outside of the plasma membrane than the inside; the potential difference can be measured at – 70 millivolts This condition of the cell is referred to as being “polarized” Neurilemma Animation Action Potential •Neurons can respond to changes in the environment, a property called excitability •When the change becomes great enough, an action potential in the cell can be generated by increasing the membrane’s permeability to sodium This is done by opening proteins referred to as sodium ion channels Resultantly, sodium ions rush into the cell This results in the cell becoming depolarized (a reversal of the distribution of charges by allowing the concentration of Na ions to return to equilibrium) Neurilemma Animation Repolarization The cell can quickly re-establish potential difference between inside and outside by opening potassium ion channels, while closing sodium ion channels Positively charged K ions flow out of the cell, restoring the distribution of charges, but not the concentration of the particular ions Neurilemma Animation The depolarization / repolarization of membranes can occur very quickly (in less than 1 / 1000th of a second) During this process, the number of ions moving is relatively small; the potential difference of charges in more important than the concentration of ions Several action potentials can be generated before concentrations of ions need to be restored to original levels Neurilemma Animation Nerve Impulse Conduction Neurilemma Animation Action Potential Movement Animation Continuous Conduction Continuous Conduction – nerve impulse travels through entire neurilemma (membrane) in one direction – Occurs only in unmyelinated neurons – Moves at about 10 m/s Saltatory Conduction Nerve impulse travels down portions of the neurilemma that are not covered by Schwann cells in one direction Skips large spaces due to presence of insulating myelin, from node of Ranvier to node of Ranvier – Much faster, up to 130 m/s – Adaptation for split-second responses to emergency situations All-or-None Response Like muscle cells, if a stimulus is strong enough to cause an action potential, the nerve impulse travels the entire length of a neuron at full strength Minimum strength of stimulus required to initiate an action potential is called the threshold stimulus Stimuli weaker than threshold, called subthreshold, cannot initiate an impulse Transmission of Impulses from Cell to Cell Occurs at the junction between adjacent neurons, called a synapse Impulse must pass this gap to move to next neuron Presynaptic neuron sends impulse; postsynaptic neuron receives impulse Axon of pre-synaptic neuron ends at a round bulb, called the synaptic end bulb and contains synaptic vesicles filled with neurotransmitters About 50 different types of neurotransmitters Postsynaptic neuron has a concave surface that forms a gap, called the synaptic cleft, across which neurotransmitters must diffuse in order to contact the postsynaptic neuron Transmission of Impulse Across a Synapse When a nerve impulse arrives at the synaptic end bulb of a presynaptic neuron, calcium ion channels in the plasma membrane open briefly, and calcium ions flow into the end bulb from the surrounding interstitial fluid Ca ions presence cause synaptic vesicles to fuse to the plasma membrane and the neurotransmitters are released into the synapse by exocytosis These chemicals then diffuse across the gap until they contact the plasma membrane of the postsynaptic neuron When contact is made the result is either excitatory or inhibitory, depending on the nature of the neurotransmitter and its receptor The NT is then deactivated by an enzyme or transported away by other enzymes This limits the effect of neurotransmitters to fractions of a second and readies the synapse for the next transmission Transmission of Impulse Across a Synapse Excitatory vs. Inhibitory Transmissions Both depend upon altering the permeability of the postsynaptic neuron to certain ions Excitatory neurotransmitters, such as acetylcholine (Ach) or norepinephrine increase the permeability to Na ions, increasing the likelihood of generating an action potential Inhibitory neurotransmitters, such as GABA, endorphins, and enkephalins, increase the permeability to potassium ions, decreasing the likelihood of the generation of an action potential Some inhibitory ions also open chloride ion channels which allow negative charges to flow into the cell, cause hyperpolarization Processing at the Synapse The postsynaptic membrane is located at the dendrites and cell body of a single neuron May receive thousands of presynaptic end bulbs from thousands of different neurons Some may send excitatory impulses where others may send inhibitory impulses The overall effect is determined by the sum of these individual effects This provides a means of gathering information from a bunch of different receptors (stimuli) and coming up with a response according to the individual neuron’s threshold of stimulus