DEALING WITH AN AXON WITHOUT MYELIN Synapses Concentration gradient is the amount of ions in a given area Ions crossing a membrane want to go to an area of low concentration from an area of high concentration Electrical gradient involves positive and negative charges Sodium and potassium have positive charges Chloride has negative charge Neurons have selective permeability Some things can cross the membrane with no problem Other things can t cross like sodium and potassium At rest inside the cell is more potassium outside the cell is more sodium In the absence of any outside influence a neuron has a slightly negative charge inside the cell meaning outside is rather positive This negative state is the resting potential of the neuron It s due to the uneven distribution of sodium and potassium At rest the potassium and a little of chloride might be allowed in through the gates of the neuron only a LITTLE Sodium gates are shut tight Sodium potassium pump kicks 3 sodium ions out of the neuron while bringing in 2 potassium neurons Concentration gradient would drive the sodium inside the cell electrical gradient would also drive the sodium inside the cell At rest the concentration gradient would drive the potassium out but the electrical gradient would keep the potassium inside because the inside of the cell is more negative than the outside This system responds quickly If you make the neuron even more negative inside it s even less likely to fire That s hyperpolarizing the membrane When you depolarize the membrane you make the cell more likely to fire that means making the inside of the cell more positive If the threshold is reached that neuron is fired and an action potential is created Once threshold is reached the sodium gates open slightly and it doesn t take long for the gates to open wide The more sodium enters the more positive the cell becomes until it ACTUALLY becomes positive The potassium gates also open as the sodium gates open wide to let sodium ions in Once the cell inside is positive sodium s electrical gradient wants them out but the concentration gradient still wants them in because there s more outside But once the cell is positive the sodium gates close Potassium gates stay open With the cell inside positive concentration and electrical gradients want potassium out Once the right amount of potassium leaves hyperpolarization is reached Somehow the neuron rights itself and returns to its resting state how is unknown With the potassium out it s picked up by some glial cells and it dilates blood vessels Refractory period prevents neuron from firing again after it has one action potential Two parts to the refractory period absolute no amount of stimulation gets that neuron to fire again and relative the neuron might fire again but it would require massive stimulation Action potentials start at the axon hillock No matter what causes the action potential the size and shape are the same once it hits threshold All or none law means that an action potential either happens or it doesn t and if it does all action potentials are exactly the same Propagation spurs the action potential down the axon At every point on that axon a new action potential occurs AXON WITH MYELIN Myelin speeds up information transmission across an axon Nodes of Ravier is where the myelin sheath almost meets the axon and that s where the action potential occurs Action potential then doesn t occur at every point but at the nodes This is faster Saltatory conduction is what it s called instead of propagation Something like Novocain blocks sodium gates and that means no action potentials can fire Scorpion venom causes sodium gates to remain open and potassium gates to remain closed NEURONS WITH NO AXONS Neurons don t produce action potentials but produce graded potentials These lose strength over time and space So cells far away from these neurons aren t as affected as the ones closer to them An axon sends the information in one direction Graded potentials release and go everywhere GENERAL On a neuron dendrites receive information axons transmit information The space between the terminal bulbs of the neuron transmitting and the dendrites of the neuron receiving information is called a synapse synaptic bridge The neuron sending the information is called the pre synaptic neuron the neuron receiving information is the post synaptic neuron The synaptic bridge is the point of communication between the two neurons The transmission of information is very different between two neurons than it is across the axon Information released from the transmitting neuron s terminal bulbs is a graded potential as it jumps across the synapse to reach the next neuron Stimulation neuron receives can be excitatory post synaptic potential EPSP or inhibitory post synaptic potential IPSP The combination of all the information a neuron receives that determines whether or not it will fire More IPSP means it doesn t fire more EPSP means it does fire Most neurons have a spontaneous firing rate What you typically see is an increase in EPSPs or IPSPs that changes that spontaneous firing rate Ignore the hormone stuff on pages 63 65