Zoology 101: Animal Biology Last Lecture Outline Lecture 24 1. Neuroscience• Dendrites • Nervous systems 2. How does a neuron transmit a signal?Current Lecture 1. Conduction of action potentials2. Synapses Conduction of action potentials • genetic mutations effect action potentials• toxins target ion channels; have evolved for a defense mechanism or hunting.◦ Tetrodotoxin: blocks sodium channels leading to paralysis. Pufferfish ◦ Alpha- and beta- toxins: shift opening and closing of sodium channels leading to scrambled signals. Scorpions ◦ Apomin: blocks potassium channels leading to prolonged action potentials (bees)◦ Scientists use toxins to study ion channels • Conduction of action potentials move in one direction◦ only moves from dendrites to terminal branches ◦ region “behind” the action is hyperpolarized and cannot respond (Refactory period) ◦ Depolarization of one region of the axon stimulates depolarization of next one • Conduction speed varies between organism◦ Larger Axon diameters → faster conduction speed (primarily invertebrates) ▪ important for behavioral responses that need great speed ◦ Myelination; myelin sheath → faster conduction speed. Nodes of Ranvier: site of action potential▪ unmyelination: sodium and potassium pumps must always keep going in order to keep action potential up▪ Myelination: few pumps, jumps signal• saltatory conduction: one node to next • Much more space efficient Synapse • Junctions where 2 neurons meet up between synaptic terminal and another cell ◦ electrical: faster, direct signaling, not common◦ chemical: slower, signaling via chemicals, most common◦ Both lead to a change in polarization in post synaptic cell.• Vesicles and neurotransmitters highly involved in chemical synapse • Vesicle: packed of neurotransmitters• Neurotransmitter: chemical messenger between neurons • Action potential depolarizes pre-synaptic membrane; calcium channels open and calcium enters the cell• Calcium causes vesicles to fuse to presynaptic membrane, exocytosis• Neurotransmitters bind to receptors on post synaptic membrane • Binding triggers the opening of ion channels (neurotransmitters bind to their own receptors• Isotropic receptor (ligand-gated ion channels)◦ NT bind to receptors that are part of ligand-gated ion channels ◦ open ion channel, changes membrane potential resulting in a postsynaptic potential ◦ Metabotropic ▪ bind receptor that isn't part of the ion channel ▪ Activates a signal transduction pathway▪ Slower, long lasting • Post synaptic potentials can be excitatory or inhibitory ◦ ESPs: depolarization, cell less polarized, voltage gated sodium channels ◦ ISPs: hyperpolarization, cell more polarized. Voltage gated potassium channels • Reuptake, degraded by enzymes → won't send more • Postsynaptic neurons may receive input◦ axon hillock of post synaptic neuron ◦ one input alone is not enough to elicit action potential → synaptic integration (summation) • Temporal summation: signal from same synaps• Spatial summation: signal from a different synapse • Neurotransmitters can be ESPs or ISPs depending on receptr • Ionotropic receptors: muscle and brain (ESP)• Metabotropic: muscle neurons • GABA: chief inhibitory transmitter in CNS• Serotonin:
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