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Neuronal Anatomy and Communication Cells of the Nervous System: Neurons Three types of neurons: Sensory neurons Motor neurons Interneurons Neuronal structure Soma Dendrites Axon Terminal buttons Synaptic cleft Neuronal classifications Multipolar neuron Bipolar neuron (a) Unipolar neuron (b) Internal structure Cell membrane Cytoplasm Mitochondria Nucleus Chromosomes Proteins Microtubules Cells of the Nervous System: Glia Glial cells support neural function Glial Cells Astrocytes Arms wrap around blood vessels and neuronal structures Isolate the synaptic cleft Maintain chemical composition of extracellular space Clean up following cell death Glial cells Oligodendrocytes (CNS) & Schwann cells (PNS) Provide support and insulation in the form of the myelin sheath Myelin Nodes of Ranvier Glial cells Microglia Smallest glial cells Brain’s immune cells Blood-Brain Barrier Composed of tightly-packed cells of the cerebral blood vessels. Regulates chemicals in the CNS Protects the brain from toxins Semipermeable The withdrawal reflex: an example of neuronal communication 1. Sensory neuron detects 2. Message is sent 3. Neurotransmitter is released 4. Interneuron 5. Motor neuron sends a message Communication within a neuron Based on changes in the membrane potential of the neuron. Neurons have two basic potentials Resting membrane potential The inside of a neuron is negatively charged relative to the outside Due to concentrations of positively and negatively charged ions in the brain Outside the cell Inside the cell Resting membrane potential A concentration gradient pulls the sodium, potassium and chloride ions toward the membrane; electrostatic forces prevent them from crossing it. The balance between potassium and sodium ions in and out of the neuron is maintained Membrane potential The membrane potential can change: Depolarization Hyperpolarization -40 -50 -60 Depolarization -70 -80 Hyperpolarization Resting Membrane Potential Action potential A hyperpolarization _________ communication within a cell. A depolarization _________ the cell, and _________ the chances of communication within the cell. Threshold of excitation Action potential A massive, momentary reversal of the membrane potential. Carried down the axon from the cell body to the terminal buttons. Results in the release of a chemical message into the synapse. Action potential Chemical messages from other neurons affect the neuron’s charge. Excitatory Post-Synaptic Potentials (EPSPs) Inhibitory Post-Synaptic Potentials (IPSPs) When the cell is depolarized to -65mV, an action potential begins. Steps of the action potential 1. 2. 3. 4. 5. 6. Ion channels in the membrane rapidly open and Na+ enters the cell (-65mV +40mV) As Na+ rushes in, K+ is forced out of the cell. As the action potential peaks, Na+ channels close, and no more Na+ enters the cell. K+ is forced out of the cell, which decreases the charge inside the cell and K+ channels close. K+ ions trapped outside of the cell result in a temporary hyperpolarized membrane potential. Ion channels reset and the Na+/K+ pump returns the ions to the normal gradients. All-or-None law An action potential either occurs or it doesn’t. Magnitude is the same. Does not diminish in strength. Rate law The strength of a response depends on the firing rate of the cell. More action potentials/second = strong response, fewer = weak response. Action potential conduction Action potentials depend on sodium influx from the extracellular fluid. Nodes of Ranvier. Saltatory conduction Communication between neurons Within-neuron communication: electrical signal Between-neuron communication: chemical signal Synaptic transmission Synaptic structure Presynaptic membrane Terminal button Vesicles Transporter molecules Synaptic cleft Postsynaptic membrane On the dendrite, soma or axon Receptors Neurotransmitter binding Binding sites Ligands Molecule that fits into a specific binding site Endogenous ligands Exogenous ligands Synaptic firing 1. 2. Initiated by an action potential in the cell Neurotransmitter (NT) binds to the receptor Prompting specific ion channels to open Types of receptors Ionotropic receptor Metabotropic receptor Neurotransmitter Ion Channel Joins the ion channel Receptor G-protein Enzyme Gate Second messenger Synaptic firing 3. Postsynaptic potentials are produced by the flow of ions in and out of the cell. Each NT produces a specific postsynaptic potential Excitatory NTs Inhibitory NTs Synaptic firing 4. Neural integration is the summation of all postsynaptic potentials. Determines the response to PSPs. Synaptic firing Remember – Each neuron has synaptic connections with hundreds of other neurons, and must summate all incoming PSPs thousands of times each second! Synaptic firing 5. Removal of NT from the synapse terminates PSPs Reuptake Enzymatic deactivation Autoreceptors Found on the presynaptic membrane Types of synapses Axodendritic Axosomatic Axoaxonic