11 Fundamentals of the Nervous System and Nervous Tissue: Part C The Synapse • A junction that mediates information transfer from one neuron: • To another neuron, or • To an effector cell The Synapse • Presynaptic neuron—conducts impulses toward the synapse • Postsynaptic neuron—transmits impulses away from the synapse Types of Synapses • Axodendritic—between the axon of one neuron and the dendrite of another • Axosomatic—between the axon of one neuron and the soma of another • Less common types: • Axoaxonic (axon to axon) • Dendrodendritic (dendrite to dendrite) • Dendrosomatic (dendrite to soma) Electrical Synapses • Less common than chemical synapses • Neurons are electrically coupled (joined by gap junctions) • Communication is very rapid, and may be unidirectional or bidirectional • Are important in: • Embryonic nervous tissue • Some brain regions Chemical Synapses • Specialized for the release and reception of neurotransmitters • Typically composed of two parts • Axon terminal of the presynaptic neuron, which contains synapticvesicles • Receptor region on the postsynaptic neuron Synaptic Cleft • Fluid-filled space separating the presynaptic and postsynaptic neurons • Prevents nerve impulses from directly passing from one neuron to the next Synaptic Cleft • Transmission across the synaptic cleft: • Is a chemical event (as opposed to an electrical one) • Involves release, diffusion, and binding of neurotransmitters • Ensures unidirectional communication between neurons Information Transfer • AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca2+ channels • Synaptotagmin protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membrane • Exocytosis of neurotransmitter occurs Information Transfer • Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the postsynaptic neuron • Ion channels are opened, causing an excitatory or inhibitory event (graded potential) Termination of Neurotransmitter Effects • Within a few milliseconds, the neurotransmitter effect is terminated • Degradation by enzymes • Reuptake by astrocytes or axon terminal • Diffusion away from the synaptic cleft Synaptic Delay • Neurotransmitter must be released, diffuse across the synapse, and bind to receptors • Synaptic delay—time needed to do this (0.3–5.0 ms)• Synaptic delay is the rate-limiting step of neural transmission Postsynaptic Potentials • Graded potentials • Strength determined by: • Amount of neurotransmitter released • Time the neurotransmitter is in the area • Types of postsynaptic potentials 1. EPSP—excitatory postsynaptic potentials 2. IPSP—inhibitory postsynaptic potentials Excitatory Synapses and EPSPs • Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+ and K+ in opposite directions • Na+ influx is greater that K+ efflux, causing a net depolarization • EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltage-gated channels Inhibitory Synapses and IPSPs • Neurotransmitter binds to and opens channels for K+ or Cl– • Causes a hyperpolarization (the inner surface of membrane becomes more negative) • Reduces the postsynaptic neuron’s ability to produce an action potential Integration: Summation • A single EPSP cannot induce an action potential • EPSPs can summate to reach threshold • IPSPs can also summate with EPSPs, canceling each other out Integration: Summation • Temporal summation • One or more presynaptic neurons transmit impulses in rapid-fire order • Spatial summation • Postsynaptic neuron is stimulated by a large number of terminals at the same timeIntegration: Synaptic Potentiation • Repeated use increases the efficiency of neurotransmission • Ca2+ concentration increases in presynaptic terminal and ostsynaptic neuron • Brief high-frequency stimulation partially depolarizes the postsynaptic neuron • Chemically gated channels (NMDA receptors) allow Ca2+ entry • Ca2+ activates kinase enzymes that promote more effective responses to subsequent stimuli Integration: Presynaptic Inhibition • Release of excitatory neurotransmitter by one neuron may be inhibited by the activity of another neuron via an axoaxonic synapse • Less neurotransmitter is released and smaller EPSPs are formed Neurotransmitters • Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies • 50 or more neurotransmitters have been identified • Classified by chemical structure and by function Chemical Classes of Neurotransmitters • Acetylcholine (Ach) • Released at neuromuscular junctions and some ANS neurons • Synthesized by enzyme choline acetyltransferase • Degraded by the enzyme acetylcholinesterase (AChE) Chemical Classes of Neurotransmitters • Biogenic amines include: • Catecholamines • Dopamine, norepinephrine (NE), and epinephrine • Indolamines • Serotonin and histamine • Broadly distributed in the brain • Play roles in emotional behaviors and the biological clock Chemical Classes of Neurotransmitters • Amino acids include:• GABA—Gamma ( )-aminobutyric acid • Glycine • Aspartate • Glutamate Chemical Classes of Neurotransmitters • Peptides (neuropeptides) include: • Substance P • Mediator of pain signals • Endorphins • Act as natural opiates; reduce pain perception • Gut-brain peptides • Somatostatin and cholecystokinin Chemical Classes of Neurotransmitters • Purines such as ATP: • Act in both the CNS and PNS • Produce fast or slow responses • Induce Ca2+ influx in astrocytes • Provoke pain sensation Chemical Classes of Neurotransmitters • Gases and lipids • Nitric oxide (NO) • Synthesized on demand • Activates the intracellular receptor guanylyl cyclase to cyclic GMP • Involved in learning and memory • Carbon monoxide (CO) is a regulator of cGMP in the brain Chemical Classes of Neurotransmitters • Gases and lipids • Endocannabinoids • Lipid soluble; synthesized on demand from membrane lipids • Bind with G protein–coupled receptors in the brain • Involved in learning and memory Functional Classification of Neurotransmitters • Neurotransmitter effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing) • Determined by the receptor type of the