Transcript Slide 1
The synapse: Where the impulse is passed from one cell to another Two basic kinds of synapses: 1. Electrical (gap junctions) • Very fast • Excitatory • Does not require neurotransmiters 2. Chemical • Requires a neruotransmitter of some sort • Fast (but slower than electrical) • Can excite or inhibit • Can modulate the permeability of a post synaptic element for an extended period of time The synaptic process: Key events of a chemical synapse 1. Action potential reaches the axon terminal where the presynaptic element resides. 2. Causes the opening of CA+ channels. 3. Ca+ forces the movement of microtubules onto synaptic vesicles pressing them to the presynaptic element. 4. Vesicles bind to specific sites on the presynaptic element and open, spilling their contents (a neurotransmitter) into the synaptic cleft 5. Neurotransmitters (the ligand) bind to receptors at specific binding sites on the post synaptic cell membrane causing either: • Deformation of the receptor protein which opens a ion channel • Deformation of the receptor protein which activates a second messenger (G-protein coupled receptors). • Ultimately both mechanisms can either cause • depolarization of the post synaptic element (EPSP) • hyperpolarizing of the post synaptic element (IPSP) Typical excitatory vs inhibitory synaptic events The neuromuscular junction: synapse from neuron to muscle Synapses can occur between 1. Sensory cells and neurons 2. Neurons and muscles 3. Neurons and glands G-protein coupled receptors can amplify transmitter effects and modulate cell sensitivity Synaptic complexity: Neurons have a variety of receptive (dendritic) fields Adendritic Spindle radiation Spherical radiation Laminar radiation: Planar Laminar radiation: Offset Laminar radiation: Multi Conical radiation Biconical radiation Fan radiation Synaptic complexity: Types of synapses. 1. Axo-dendritic 2. Dendro-dendritic 3. Dendro-axonic 4. Axo-axonic 5. Dendro-somatic 6. Axo-somatic Computational complexity of synaptic regions Electron micrograph image of a reciprocal dendro-dendritic synapse (D1, D2) and an axon from a third cell (A) makes an asymmetric synapse on D2. Arrows point to synaptic vesicles and presumed polarity of chemical transmission. From Cat Thalamus (x 33 000) Synaptic complexity: There not that simple inside either The take home message here is that a synapse is like a tiny computational compartment! Synaptic complexity: Spines represent pockets of synaptic computation Synapses change: synaptic plasticity Plasticity occurs for a number of reasons •Development & aging •Experience (learning, exhaustion) The net result of plastic nervous systems is that they can adapt! Electrophysiology: Direct method(s) for monitoring neurons Intracellular (glass electrode) •Patch electrode •Sharp electrode Extracellular (wires/metals) •Hook electrodes •Beveled wire •Silicon electrodes Examples of Indirect methods: •FMRI •CT •Optical imaging •Calcium imaging Standards of evidence to establish a causal neural basis of behavior Take home message: To say a neuron causes a behavior, you need to establish that a neuron (or group of neurons) is both necessary and sufficient! -a correlation just wont do!