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Download 4. Nervous System: Synapses
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Transcript
Synapses Synapse • Location where information is transmitted from presynaptic neuron to postsynaptic neuron- gap where axon terminal meets dendrite of next neuron • Neurotransmitter molecules released into this space • Synapses can be electrical or chemical • Electrical synapse: electrical current flows between 2 cells – Via Gap Junction – Cell membranes are continuous – in cardiac muscle and some smooth muscle cells • Chemical synapse – Neurotransmitters: chemicals that move from one cell to the next to carry the signal across a space to the next cell • Examples: acetylcholine serotonin norepinephrine histamine dopamine 3 parts of synapse: • 1. synaptic knob—bulge at end of one axon terminal of presynaptic neuron • 2. synaptic cleft-tiny (25 nm) gap between two neurons • 3. plasma membrane of post synaptic neuron– usually at the dendrite or cell body- contains protein receptors Synaptic transmission • Action potential itself cannot cross cleft • Instead neurotransmitters released from knob, attach to receptors in next neuron, and cause response in next neuron Neurotransmitters • Two categories: – Excitatory: cause depolarization in postsynaptic neuron (stimulate next neuron) – Inhibitory: cause hyperpolarization of postsynaptic membrane that inhibits/stops the potential from moving on • Drugs can be inhibitory or excitatory • Endorphins are inhibitory- block pain Neurotransmitters • Neurotransmitters cause ion channels to open – Some channels let Na+ ions in depolarization – Other channels let K+ ions out and Cl- in hyperpolarization that inhibits action potential Summation • Summation: amount released by one knob won’t start AP in next neuron—may need several working together or “rapid fire” of repeated stimulation= summation • Does all sensory information received by sensory neurons get transmitted to conscious part of brain? Summation… • Much sensory information gets stopped at synapses where threshold was not reached • Summation= mechanism for making decisions about what info is important to move on Synapses and Memory • Theory: info is stored in form of increased flow of info at synapses in particular pathways • Certain neurotransmitters and structural changes in synapses may affect short vs. intermediate vs. long term memories • Ex: more synapses and receptors are built when we create long term memories
