Two cell types in the NS: 10% Neurons & 90% Glial Cells A “typical” motor neuron is multipolar Cell body (soma) is the “life hub” mediates cell metabolism, growth, division, contains DNA Axons are specialized to send information to other neurons Dendrites are specialized to receive information from other cells
A “typical” sensory neuron is unipolar or bipolar
Most neurons in the brain are interneurons Most interneurons don’t need an axon… WHY?
Communication between neurons: Step 1: the neuron at rest (polarized) Inside is more negative than outside Outside has more Sodium (+) and Cholride (-) Inside has more Potassium (+) and anions (-) lipid molecules Ion channels
What keeps the cell at -70 mV when at rest? 1.Concentration gradient ions move from an area of high to low concentration 2. Electrical gradient ions with the same sign (polarity) repel each other ions with the opposite sign attract each other
Step 2: the neuron’s membrane potential begins to depolarize (inside becomes less negative) If neighboring cells stimulate our neuron, sodium (+) will rush into the cell, making it less and less negative. If it reaches approximately -55mV, our neuron will fire That is, it will generate an action potential.
Step 3: the neuron’s membrane potential begins to repolarize (inside starts to become negative) Eventually, the cell reaches its positive peak, causing sodium gates to close and potassium (+) gates to open… K+ rushes out, returning the cell to - ive Oops! Too far! No worries, the Na – K pump will bring us back to normal
Step 4: action potentials sweep down the axon The first action potential begins at the AXON HILLOCK This creates a dominos effect of action potentials which end at the axon terminals
Once NT’s bind to receptors on the postsynaptic cell, what then? The NT will general either an IPSP or EPSP IPSP’s make the cell more negative, decreasing the prob that the cell will fire EPSP’s make the cell less negative, increasing the prob that the cell will fire PSP’s are “local potentials” which are distinct from action potentials
Local potentials vs. action potentials Analogous to a gentle nudge vs. a explosive push Local potentialsAction potential Decremental Graded Initiated in dendrite or soma Non-decremental All-or-none Initiated at axon hillock
From local potential to action potential Once an IPSP or EPSP is generated, it travels to the axon hillock One IPSP or one EPSP is not enough to generate an action potential But, at any given time, MANY IPSP’s and EPSP’s are being generated The axon hillock “adds up” all the PSP’s… if the sum is ≥ -55mV …
How can local potentials have more “umph”? Temporal summation & Spatial summation
What determines the firing rate for a cell? Absolute Refractory Period Immediately after the AP, the cell cannot fire How does the neuron code the intensity of the EPSP? Relative Refractory Period This occurs after the absolute refractory period Only a large EPSP can get the cell to fire
What Gial cells do 1.They are the glue that hold neurons in place 2.They increase speed of conduction of the neural impulse Local potentials travel faster down The axon than action potentials Between schwann cells are gaps “Nodes of Ranvier” AP’s occur at the nodes, local Potentials occur under the myelin Saltatory conduction
3.Glial cells provide energy to neurons 4.They remove cellular debris 5.They contribute to the development & maintenance of connections between neurons 6. During fetal development, they help guide neurons to their destination What Gial cells do continued…