11.7 The Synapse

11.7 The Synapse (1 of 2) • Nervous system works because information flows from neuron to neuron • Neurons are functionally connected by synapses, junctions that mediate information transfer – From one neuron to another neuron – Or from one neuron to an effector cell 11.7 The Synapse (2 of 2) • Presynaptic neuron: neuron conducting impulses toward synapse (sends information) • Postsynaptic neuron: neuron transmitting electrical signal away from synapse (receives information) – In PNS may be a neuron, muscle cell, or gland cell • Most function as both Synapses (1 of 3) Figure 11.15a Synapses. Synapses (2 of 3) Figure 11.15b Synapses. Synapses (3 of 3) • Synaptic connections – Axodendritic: between axon terminals of one neuron and dendrites of others – Axosomatic: between axon terminals of one neuron and soma (cell body) of others – Less common connections:  Axoaxonal (axon to axon)  Dendrodendritic (dendrite to dendrite)  Somatodendritic (dendrite to soma) – Two main types of synapses:  Chemical synapse  Electrical synapse Chemical Synapses (1 of 7) • Most common type of synapse • Specialized for release and reception of chemical neurotransmitters • Typically composed of two parts – Axon terminal of presynaptic neuron: contains synaptic vesicles filled with neurotransmitter – Receptor region on postsynaptic neuron’s membrane: receives neurotransmitter  Usually on dendrite or cell body – Two parts separated by fluid-filled synaptic cleft • Electrical impulse changed to chemical across synapse, then back into electrical Chemical Synapses (2 of 7) • Transmission across synaptic cleft – Synaptic cleft prevents nerve impulses from directly passing from one neuron to next – Chemical event (as opposed to an electrical one) – Depends on release, diffusion, and receptor binding of neurotransmitters – Ensures unidirectional communication between neurons Animation: Neurotransmitters Click here to view ADA compliant Animation: Neurotransmitters Chemical Synapses (3 of 7) • Information transfer across chemical synapses – Six steps are involved: 1. AP arrives at axon terminal of presynaptic neuron 2. Voltage-gated Ca2+channels open, and Ca2+ enters axon terminal – Ca2+ flows down electrochemical gradient from ECF to inside of axon terminal Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters (1 of 6) Focus Figure 11.3 Chemical Synapse. Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters (2 of 6) Focus Figure 11.3 Chemical Synapse. Chemical Synapses (4 of 7) 3. Ca2+ entry causes synaptic vesicles to release neurotransmitter – Ca2+ causes synaptotagmin protein to react with SNARE proteins that control fusion of synaptic vesicles with axon membrane – Fusion results in exocytosis of neurotransmitter into synaptic cleft – The higher the impulse frequency, the more vesicles exocytose, leading to a greater effect on the postsynaptic cell Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters (3 of 6) Focus Figure 11.3 Chemical Synapse. Chemical Synapses (5 of 7) 4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane – Often chemically gated ion channels 5. Binding of neurotransmitter opens ion channels, creating graded potentials – Binding causes receptor protein to change shape, which causes ion channels to open • Causes a graded potential in postsynaptic cell • Can be an excitatory or inhibitory event • Some receptor proteins are also ion channels Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters (4 of 6) Focus Figure 11.3 Chemical Synapse. Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters (5 of 6) Focus Figure 11.3 Chemical Synapse. Chemical Synapses (6 of 7) 6. Neurotransmitter effects are terminated – As long as neurotransmitter is binding to receptor, graded potentials will continue, so process needs to be regulated – Within a few milliseconds, neurotransmitter effect is terminated in one of three ways • Reuptake by astrocytes or axon terminal • Degradation by enzymes • Diffusion away from synaptic cleft Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters (6 of 6) Focus Figure 11.3 Chemical Synapse. Chemical Synapses (7 of 7) • Synaptic delay – Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors  Can take anywhere from 0.3 to 5.0 ms – Synaptic delay is rate-limiting step of neural transmission  Transmission of AP down axon can be very quick, but synapse slows transmission to postsynaptic neuron down significantly  Not noticeable, because these are still very fast Electrical Synapses • Less common than chemical synapses • Neurons are electrically coupled – Joined by gap junctions that connect cytoplasm of adjacent neurons – Communication is very rapid and may be unidirectional or bidirectional – Found in some brain regions responsible for eye movements or hippocampus in areas involved in emotions and memory – Most abundant in embryonic nervous tissue