M1 | Neuronal Mechanisms for Information Transmission (3/14) -- KINES 531: Neural Control of Movement

M1 | Neuronal Mechanisms for Information Transmission How information is transmitted between neurons Structure of the synapse Axons have terminal swellings called boutons, which contain synaptic vesicles. Synaptic vesicles contain the chemical signal in the form of molecules called neurotransmitters or transmitters. They are called transmitters because they transmit the neural signal from the signaling cell, which is called the presynaptic cell, to the receiving cell, which is called the postsynaptic cell. The narrow space between the membrane of the signaling cell and the membrane of the receiving cell is called the synaptic cleft (20 nm). Together, the presynaptic membrane, the synaptic cleft, and the postsynaptic membrane are called the synapse. Watch: Synaptic Transmission – Narrated Animation (3:37) Neurotransmitter release: sequence of events - Action potential invades presynaptic terminal - Depolarization opens Ca2+ channels - Increased intracellular Ca2+ concentration mediates release of neurotransmitter via exocytosis of synaptic vesicles - Neurotransmitter diffuses through the synaptic cleft and combines with receptors on the postsynaptic membrane - Changes in membrane potential of the postsynaptic membrane: post-synaptic potentials (PSPs) - Reuptake or degradation of neurotransmitter Synaptic Potentials Excitatory post-synaptic potential (EPSP) - Neurotransmitter increases permeability for Na+ - Depolarization: Excitatory PostSynaptic Potential (EPSP) - EPSP brings membrane potential closer to threshold Inhibitory post-synaptic potential (IPSP) - Neurotransmitter increases permeability for K+ or Cl- - Hyperpolarization: Inhibitory PostSynaptic Potential (IPSP) - IPSP brings membrane potential further away from threshold Postsynaptic receptors Postsynaptic receptors, not neurotransmitter, determine whether a PSP is excitatory or inhibitory. All postsynaptic receptors influence the opening and closing of ion channels. All postsynaptic receptors are membrane-spanning proteins. Receptors that gate ion channels directly (ionotropic) - Structurally part of the ion channel - Neurotransmitter affects ion channel directly - Fast PSPs (range: few ms) - Function in circuits that directly mediate behavior Receptors that gate ion channels indirectly (metabotropic) - Spatially separate from ion channels - Neurotransmitter acts indirectly via second messengers (e.g. cAMP) - Slow PSPs (range: seconds or minutes) - Neuromodulatory (e.g. involved in learning) Neurotransmitters Criteria For a molecule to be considered a neurotransmitter, it has to fullfil the following criteria: - Synthesized in neuron - Present in presynaptic terminal - When applied to synapse experimentally, it mimics effects of transmitter exactly - Specific mechanism exists to remove it from synaptic cleft Classification There are two broad classes of neurotransmitters: small molecule and large molecule transmitters. Small molecule transmitters Amino acid transmitters - Major excitat