4 Two cell types in the NS: 10% Neurons & 90% Glial Cells A “typical” motor neuron is multipolarCell body (soma) is the “life hub” mediates cell metabolism, growth, division, contains DNAAxons are specialized to send information to other neuronsDendrites are specialized to receive information from other cells
5 A “typical” sensory neuron is unipolar or bipolar
6 Most neurons in the brain are interneurons Most interneurons don’t need an axon… WHY?
7 Communication between neurons: Step 1: the neuron at rest (polarized) Inside is more negative than outsideOutside has moreSodium (+) andCholride (-)lipid moleculesIon channelsInside has morePotassium (+) and anions (-)
8 What keeps the cell at -70 mV when at rest? Concentration gradientions move from an area of high to low concentration2. Electrical gradientions with the same sign (polarity) repel each otherions with the opposite sign attract each other
9 Step 2: the neuron’s membrane potential begins to depolarize (inside becomes less negative) If neighboring cells stimulate our neuron, sodium (+) will rush intothe cell, making it less and less negative.If it reaches approximately -55mV, our neuron will fireThat is, it will generate an action potential.
10 Step 3: the neuron’s membrane potential begins to repolarize (inside starts to become negative) Eventually, the cell reaches its positive peak, causing sodium gatesto close and potassium (+) gates to open… K+ rushes out, returningthe cell to - iveOops! Too far! Noworries, the Na – Kpump will bring usback to normal
11 Step 4: action potentials sweep down the axon The first action potential begins at the AXON HILLOCKThis creates a dominos effect of action potentials which endat the axon terminals
16 The NT will general either an IPSP or EPSP Once NT’s bind to receptors on the postsynaptic cell, what then?The NT will general either an IPSP or EPSPIPSP’s make the cell more negative, decreasing the probthat the cell will fireEPSP’s make the cell less negative, increasing the probPSP’s are “local potentials” which are distinct fromaction potentials
17 Local potentials vs. action potentials Analogous to a gentle nudge vs. a explosive pushLocal potentials Action potentialDecrementalGradedInitiated in dendrite or somaNon-decrementalAll-or-noneInitiated at axon hillock
18 From local potential to action potential Once an IPSP or EPSP is generated, it travels to the axon hillockOne IPSP or one EPSP is not enough to generate an action potentialBut, at any given time, MANY IPSP’s and EPSP’s are being generatedThe axon hillock “adds up” all the PSP’s… if the sum is ≥ -55mV …
19 How can local potentials have more “umph”? Temporal summation & Spatial summation
20 What determines the firing rate for a cell? Absolute Refractory PeriodImmediately after the AP,the cell cannot fireHow does the neuron code the intensity of the EPSP?Relative Refractory PeriodThis occurs after theabsolute refractory periodOnly a large EPSP can get thecell to fire
24 What Gial cells do They are the glue that hold neurons in place They increase speed of conduction of the neural impulseLocal potentials travel faster downThe axon than action potentialsBetween schwann cells are gaps“Nodes of Ranvier”AP’s occur at the nodes, localPotentials occur under the myelinSaltatory conduction
25 What Gial cells do continued… Glial cells provide energy to neuronsThey remove cellular debrisThey contribute to the development & maintenance ofconnections between neurons6. During fetal development,they help guide neurons totheir destination