Chapter 2 Structure and Function of the Nervous System Pages 23 48 Microscopic Level Neurons specialized cells that receive information and send it to other cells carry info within the brain and throughout the rest of the body About 100 billion neurons in the brain Structure soma body with nucleus dendrite receives signals stimuli axon signal travels out to send myelin sheath helps propagate action potential along axon presynaptic terminals what transmits signal to other cell s dendrites transmits to postsynaptic terminal dendrites Stimulus Input Brain Nervous system Interneuron Output Behavioral response Typical picture motor output neuron receive info from other neurons and send message to muscles gland cells don t necessarily produce movement glands in eyes for tears and glands that produce acid in the stomach are also connected to motor output neurons Others sensory input neurons receives a particular type of sensory info and carries info to other neuron s two types bipolar soma in middle separated by small outbranch with two branches off of it i and unipolar soma with two branches off of it o reflexes created by direct connection of input and output neurons like sneezing patellar knee reflex removing hand from hot surface etc innate Interneurons connect one neuron to another in a particular part of the Central Nervous System CNS most don t have much of an axon because they don t need to communicate over long distances organisms can now change due to environment Some behaviors of animals insects are innate but more complex than reflexes spiders building webs can t really adapt to the environment well but it works because they usually have shorter lives and produce more offspring Glial Cells cells that provide structural and functional support for neurons aid neurons in fundamental ways such as sending signals know less about them than neurons Glia Greek means glue Oligodendrocytes build myelin around axons in brain and spinal cord star like shape can build myelin around multiple interneurons Schwann cells build myelin around axons in the periphery nervous system only at one location Neural Membrane Critical for the neuron s ability to carry information Membrane phospholipid bilayer with protein molecules channels passive pumps active transmembrane self organizing property due to the different parts of phospholipids no information stored in DNA for construction Phospholipid fat derivatives molecules heads are attracted to water orient toward it tails are not attracted to water orient away from it Polarization difference in electrical charge voltage between the inside and outside of the cell inside cell is slightly more negative than the outside difference is 70 mV Resting Potential difference in electrical charge between the inside and outside of the membrane of a neuron at rest 70 mV due to unequal distribution of ions charged atom or molecule due to loss or gain of electron s on the two sides of the membrane organic anions A chloride anions Cl sodium cations Na potassium cations K 1 Organic anions are stuck inside the membrane because they re large and can t diffuse through membrane contribute largely to negative inside Sodium more concentrated outside the membrane potassium more concentrated inside membrane because membrane is impermeable right now due to protein channels being closed sodium potassium pumps create the difference with sodium out and potassium in and creates a gradient Membrane is selectively permeable some chemicals can pass through it more freely than others 2 Na channels are closed at rest K channels are slightly open at rest helps inside be more negative 3 Na K Pump repeatedly moves 3 Na out of the neuron and 2 K inside the neuron at rest Numbered why inside more negative than the outside K ions attracted inside by electrical gradient opposite charges attract more concentrated inside so it will move outwards if the channel is open these forces work almost equally against each other but concentration is slightly stronger Na ions attracted inside by electrical gradient and concentration gradient because of negative charge and not many inside both point inward When they rush inward the electrical signature changes and this creates the action potential which sends the signal through the neuron Na and K channels are voltage activated their permeability depends on the voltage potential across the membrane Changes in Membrane Potential Hyperpolarization increase in polarization headed away from zero more difference Depolarization decrease in polarization headed towards zero less difference Threshold of Excitation once reached you will see different behavior from neuron Neuron information generator of binary code either 0 or 1 anything below threshold is a 0 once it hits threshold it goes off like a gun because it either goes or it doesn t no grading Action Potential abrupt depolarization and slight reversal of the usual polarization of the membrane inside becomes more positive than the outside Hyperpolarization can be used to describe when more negative than the resting potential 5 6 Increase in Polarization away from zero may be called repolarization Allows the neuron to send signals over long distances Stimulus and Threshold is reached This happens 1 Na channels open fast and Na rushes inside due to threshold being reached 2 K channels open slowly K begins to leak outside 3 Na channels shut close no more Na enters 4 K continue to leave cell causing membrane potential to return toward resting level both concentration and electrical gradient are kicking it out 5 K channels close slowly K keeps leaking outside causing membrane potential to go below the resting level K more outside and Na more inside 6 Na K pump restores the normal distribution of ions inside and outside the membrane Absolute Refractory Period Na channels cannot reopen cannot produce other action potentials sets speed limit to how fast a neuron can fire about 3 msec 2 8 Relative Refractory Period only stimuli stronger than usual can produce action potentials during 6 Intensity of a stimulus is encoded by the rate of action potentials rate law Most neurons fire spontaneously at a relatively steady rate even when no stimulus is present are at rest Spontaneous Activity firing of a neuron in absence of environmental stimulation Occurs only in axons not in dendrites or soma graded potentials here Axon hillock very first part of membrane of neuron where you can detect an action potential Its strength