Bio 3324 1nd Edition Lecture 6Outline of Last LectureI. System reflex II. HormonesIII. Pituitary glandIV. HypothalamicOutline of Current Lecture II. StimulusIII. Action PotentialIV. SynapseV. Neurotransmitter ReceptorsVI. Postsynaptic response Current LectureStrength of stimulus• Action potentials always have the same magnitude, therefore varying magnitudes of stimuli code for varying frequencies of action potentials• Two-fold:– On a single neuron, can cause more APs to be fired– More neurons can be stimulated to fireAction potentials are conducted• A single action potential involves only a small patch of total surface membrane• The action potential is initiated in one part of the cell membrane that results in self-perpetuating propagation along the rest of the fiber.– Depolarization causes positive charges to spread through the cytoplasm away from the site of depolarization and simultaneously on the outside of the cell towards the site of depolarization’s called local current flow– The local current flow causes membrane potential changes resulting in the opening of voltage-gated Na+ channels which causes further depolarizationAction potential propagation• Once an action potential is triggered the impulse is propagated along the length of the axon without further stimulation• Two methods of propagation:– Contiguous: the spreading of an action potential down the entire length of an axon– Saltatory: the action potential jumps betweens nodes to conduct the AP down the axonsSpeed of an action potential• Dependent on two factors:These notes represent a detailed interpretation of the professor’s lecture. Grade Buddy is best Used as a supplement to your own notes, not as a substitute.• The diameter of the fiber– Resistance hinders electrical charge movement (current) thus as diameter increases, resistance decreases– Some signals need to move so fast that the size of the fibers would be too large to support• Presence of myelin– Composed primarily of lipids– Acts as an insulator to prevent the movement of ions across the myelinated portions of a membrane– Not part of the neuron itself, but from “support” cells that wrap themselves around the axons– Impulses travel 50x faster– Consumes less energy• Na+-K+ pumps only need to work at the nodal regions, therefore less ATP consumedMyelinated regions are separated from one another by small areas called the Nodes of Ranvier• Expose the axon to the ECF• Only at the Nodes of Ranvier can current flow to produce an AP• Distance between each node is short enough to allow current to flow between adjacent nodes• Thus an AP is able to “jump” from node to node, skipping the myelinated sections along an axon fiber– Called Saltatory conduction• Saltatory conduction is faster than contiguous conduction because an action potential only needs to be regenerated at each node rather than along the entire length of the axonNeurons communicate at synapses• Synapses is a junction between an axon terminal of the presynaptic cell and the membrane of the postsynaptic cell• Two types of synpases:– Electrical synapse – pass electrical current between cells via gap junctions (rarer)– Chemical synapses – the electrical signal in the presynaptic cell is transduced intoa chemical message that is passed to the postsynaptic cell • Typically the junction is between the terminal of one neuron and the dendrites of the second– Three types of structures where axons terminate• Muscle (contract)• Gland (excrete)• Neuron (convey an electrical message)The synapse• The pre-synaptic neuron’s terminals end with a synaptic knob• The synaptic knob contains synaptic vesicles which store neurotransmitter (chemical messenger)• Synaptic knob comes in close proximity but doesn’t touch the post-synaptic terminal• The space between the pre-synaptic junction and the post-synaptic junction is called the synaptic clef• Current does not directly spread from the pre- to post-synaptic neuron because no channels are present in the pre-synaptic membrane for the passage of Na+/K+ ions• The action potential in the pre-synaptic neuron stimulates an graded potential in the post-synaptic neuron by chemical means• The signal between two neurons is uni-directional• Pre-synaptic neuron brings about change in membrane potential in the post-synaptic neuron, but the opposite is not trueSynaptic events(a 5-step program) 1) An action portential depolarizes the axon terminal2) The depolarization opens voltage gated Ca+2 channels and Ca2+ enters the cell3) Calcium entry triggers exocytosis of synaptic vesicle contents4) Neurotransmitter diffuses across the synaptic clef and binds with receptors on the postsynaptic cell5) Neurotransmitter binding initiates a response in the postsynaptic cell.Neurotransmitters&neuromodulators• The chemical signals released by neurons that act in a paracrine fashion– Neurotransmitters (NT) act at the synapse and elicit a rapid response– Neuromodulators act at both synapses and non-synaptic sites and are slow acting; bring about long-term changes at the synapse • May also act in an autocrine signal• 7 major classes based on structure– Acetylcholine (ACh)– Amines – catecholamines (Epi, NorE, Dopamine)– Amino Acids – Glutamate, GABA, Aspartate, Glycine– Purines – AMP, ATP– Gases – NO (as well CO & H2S)– Peptides – Substance P, opiods (enkephalins and endorphins), etc.; ofen co-secreted with other NTs– Lipids – Eicosanoids (long chain fatty acids)Neurotransmitter Receptors• All NT bind to one or more receptor types (except NO)– Every receptor type may have multiple subtypes• Fall into two categories: GPCR or Ligand-gated ion channel– GPCRs are metabotropic receptors– LGICs are ionotropic receptors• Cholinergic receptors are activated by ACh– Found in CNS, ANS of the PNS, skeletal muscle• Nicotinic receptors - Monovalent cation channels (allow Na+ and K+ pass through)• Muscarinic receptors - five subtypes all linked to G-proteins• Adrenergic receptors are activated by Catecholamines– Divided into 2 subclasses (a & b) and multiple subtypes– Linked to G-proteins• Glutameric receptors bound by the neuromodulator glutamate– NMDA receptor is a metabotropic receptor– AMPA receptor is an ionotropic receptor• Purinergic receptors bound by AMP & ATP; found in the CNS and heart• Cannabinoid receptors bound by lipidsHow