a Synapse- functional connection between a neuron(presynaptic) and another cell (postsynaptic) Requirements for Synapse to work: (all presynaptic) 1. production of the neurotransmitters 2. storage of the neurotransmitters 3. release of the neurotransmitters (all postsynaptic) 4. binding of neurotransmitter 5. generation of a new action potential 6. removal of neurotransmitters from the synapse Two kinds of synapse connections: 1.chemical 2. electrical Chemical Synapse: -synaptic cleft separates presynaptic from postsynaptic cell -neurotransmitters are in synaptic vesicles -more common Electrical Synapse: - no chemical messenger -gap junctions (connexons-direct connection between pre and post) -response in post cell is the same polarity as response in pre cell -more rapid action potential transmission (.2ms v. 2ms) -not as versatile -more common in muscle cells where synchronization is important (heart)
Chemical Neurotransmitter release -synaptic vesicles (containing neurotransmitter) congregate the presynaptic membrane -AP causes VG Ca channels to open -Ca ions come in and bind to a sensor protein -Ca allows the vesicles to move toward the presynaptic membrane and bind to SNARE proteins -vesicle contents enter into cleft (via exocytose)
-neurotransmitter diffuses across synaptic cleft -amount of neurotransmitters released depends on the frequency of action potentials Where do Neurotransmitters bind? 1. ionotropic receptors 2. metabotropic receptors Ionotropic Receptors: -NT diffuses across cleft -binds to Ligand-gated ion channels -channel opens -ion goes in
Metabotropic Receptors: -NT diffuses across cleft -binds to Second Messenger-linked receptors -activate G protein couple receptor which them transmits the signal
Speed (fast to slow):
1. Gap Junctions 2. Iontropic (Ligand-gated ion channels) 3. Metabotropic (Second messenger-linked receptors) Post Synaptic Responses: 1. Excitatory (EPSP-excitatory post synaptic potentials) - the bring membrane closer to threshold for generating action potential (more positive) -depolarize the post synaptic neuron can occur by ligand gated or G-protein receptors *no threshold *not all or nothing *summate *have no refractory period 2. Inhibitory (IPSP) -takes the membrane potential away from the action potential (more negative) -hyperpolarizing the post synaptic neuron *All neurotransmitters are derived (or are) amino acids Synaptic Transmission -if the membrane potential in the post cell reaches threshold at the axon hillock, a new action potential is generated -axon hillock- site where action potentials are normally initiated Summation 1. Temporal Summation- occurs because EPSPs that occur closely in time and can sum before they fade *occurs so close in time that it doesn't come all the way down before the next one starts 2. Spatial Summation- occurs when EPSPs from different synapses occur in postsynaptic cell at the same time Nicotinic ACh Channel Ligand Gated
-Na comes in -depolarizing= EXCITATORY Muscarinic ACh Channel G protein
-closes some= depolarizing= EXCITATORY -opens some= hyperpolarizing=INHIBITORY ex: drugs hit the same receptor and you have 2 different side affects overactive bladder --> dry mouth
Synaptic Inhibition 1. Postsynaptic inhibition -GABA and glycine produce IPSPs -IPSPs dampens EPSPs - makes it harder to reach threshold 2. Presynaptic inhibition -occurs when 1 neuron synapses onto axon of another neuron, inhibiting release of its neurotransmitter Synaptic Plasticity
-ability of the connection or synapse between 2 neurons to change in strength -synaptic facilitation=repeated use of a synapse can increase ease of transmission -synaptic depression=decrease ease of transmission -long term potential= a long lasting enhancement in signal transmission between 2 neurons -long-term potentiation= high frequency stimulation can cause enhanced excitability -key process underlying learning and memory