the synapses method of communication between neurons
THE SYNAPSES METHOD OF COMMUNICATION BETWEEN NEURONS Synapses -‐ Means of communication between neurons Structure -‐ Presynaptic membrane – the membrane of the presynaptic terminal button -‐ Postsynaptic membrane – the membrane of the postsynaptic neuron -‐ Dendritic spine – a ridge on the dendrite of a postsynaptic neuron, with which a terminal button from a presynaptic neuron forms a synapse -‐ Synaptic cleft – the tiny gap between the presynaptic and postsynaptic membrane (approximately 20 nanometres wide; a nanometre is a billionth of a metre) -‐ Synaptic vesicles – tiny balloons filled with neurotransmitter molecules; found in the release zone of the terminal button -‐ Microtubules – long tubes that run down the axon and guide the transport of synaptic vesicles from the soma to the axon terminal -‐ Release zone – part of the interior of the presynaptic membrane to which synaptic vesicles fuse in order to release their neurotransmitter into the synaptic cleft Three Types 1) Axodendritic – the terminal button synapses with a dendrite of the postsynaptic neuron 2) Axosomatic -‐ the terminal button synapses with the cell body (soma) of the postsynaptic neuron 3) Axoaxonic -‐ the terminal button synapses with the axon of the postsynaptic neuron Release of Neurotransmitters -‐ Vesicles contain neurotransmitter molecules -‐ Action potential in presynaptic cell triggers vesicles (guided by protein cells) to move to a cell membrane -‐ Protein structures act like ropes pulling presynaptic membranes -‐ Influx of calcium à induces fusion of two membranes
Activation of receptors on postsynaptic neurons -‐ Ionotropic receptors have own binding sites -‐ Neurotransmitter attaches to binding site, opening ion channel Four key ions -‐ Are crucial to the resting membrane potential, two with a positive charge (cations) and two with a negative charge (anions) 1) Sodium (Na+) – higher concentrate outside neuron 2) Chloride (Cl-‐) – higher concentrate outside neuron 3) Potassium (K+) – higher concentrate inside neuron 4) Organic anions (which are proteins, A-‐) – don’t leave neuron (intracellular) Movement of Ions a) Inflow of Na+ causes depolarization (excitatory post synaptic potential EPSP) b) Outflow of K+ causes hyperpolarization (internal post synaptic potential IPSP) c) Inflow of Cl-‐ causes hyperpolarization (IPSP) -‐ EPSPs increase the likelihood that and action potential will be triggered in the postsynaptic neuron. -‐ IPSPs decrease the likelihood that an action potential will be triggered. The combination of EPSPs and IPSPs is called neural integration. Termination of postsynaptic potentials -‐ Reuptake: any free neurotransmitter can be reabsorbed -‐ Enzymatic deactivation: acetylcholine (ACh) only Seven step summary of neurotransmitter action at the synapses -‐
