8.1 Electrical Synapses Electrical synapses occur between 2 neurons or a neuron and glial cell. Plasma membranes link together by gap junctions. Ions flowing through the gap junction allows the electric signals to transfer from one cell to the other. Second messenger’s also travel through gap junctions. Important for development of CNS and transmitting signals in the adult brain. Electrical synapses allow fast communication and synchronizes the electrical activity in the cells. Gap junction is usually bidirectional but sometimes only one direction of communication. o Can be excitory or inhibitory. Neurons that release tropic hormones in the hypothalamus are connected to other neurons that release tropic hormones by gap junctions. The junction synchronizes their activity which leads to burst of tropic hormones. 8.2 Chemical synapses Almost all neurons uses chemical synapses to communicate. Chemical synapse- neuron secretes neurotransmitter into e-fluid b/e of action potential in axon terminal. Neurotransmitter binds to receptors on PM of 2 nd cell, triggering an electrical signal that may start an action potential. Unidirectional Neuron can make a synapse with another neuron or an effecter cell (muscle cell or gland cell.) Neuroeffector junction- synapse between neuron and effecter organ. Rare- non neural cells form synapse with neurons(receptor cells in taste buds on the tongue.) Functional Anatomy of Chemical synapses. Neuron to neuron synapses o Presynaptic neuron-1 st neuron transmits signals to 2 nd neuron. o Postsynaptic neuron-2 nd neuron receives signal from 1 st neuron. o Synaptic cleft- space between pre and post neuron.(30-50 nm wide) o Axodendritic synapse- presynaptic neurons axon terminal forms synapse with dendrite of post synaptic. o Axosomatic synapse- axon terminal and soma of postsynaptic cell. o Axoaxonic synapse- Presynaptic neurons axon terminal forms a synapse with the post synaptic neurons axon terminal Function- modulate communication at axodendritic and axosomatic synapses. Transmitting signals across a membrane Axon terminal for presynaptic neuron releases a neurotransmitter into a synaptic cleft b/e of calcium levels o Neurotransmitters are made in the cytosol of the axon terminal and stored in synaptic vesicles. o When depolarized by action potential the calcium channels open and calcium flows down the axon terminal which causes synaptic vesicles to fuse to attachment sites on the inner surface of the axon terminal membrane where they undergo exocytosis into the synaptic cleft. o The amount of neurotransmitter that is released depends on how much calcium is released which depends on frequency of action potential.
Neurotransmitters diffuses through the synaptic cleft and binds to receptors of the postsynaptic neuron producing response in the post neuron by signal transduction mechanisms.
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- Fall '08
- axon hillock, post synaptic neuron