Neurotransmission PDF

Neurotransmission Dr R Dangarembizi  Learning objectives By the end of this lecture we should be able to: 1. Discuss the importance of neuronal signaling 2. Describe the resting membrane potential 3. Describe the development of an action potential 4. Describe the factors a...

Neurotransmission Dr R Dangarembizi  Learning objectives By the end of this lecture we should be able to: 1. Discuss the importance of neuronal signaling 2. Describe the resting membrane potential 3. Describe the development of an action potential 4. Describe the factors affecting the propagation of an action potential 5. Describe the events occurring at a chemical synapse Electrical signaling is the fundamental neuronal process that underlies all aspects of brain function… The neuronal membrane is a phospholipid bilayer The resting membrane potential Graded potentials Graded potentials When a stimulus is SUBTHRESHOLD -no Action potential results When a stimulus depolarizes the membrane to THRESHOLD - an action potential occurs - the spike is initiated at the axon hillock - then it is conducted along the axon When a stimulus is SUPRATHRESHOLD - an action potential occurs which is the same amplitude as that of a threshold stimulus. *The all-or-none principle  Action potentials An AP fires when threshold potential is reached Threshold is caused by: – Cumulative graded potentials from a persistent stimulus – Large enough graded potentials from one stimulus Phases of the Action Potential 1. At threshold,– all Na+ channels open and there is an influx of Na+ and the depolarization phase occurs. 2. Na+ channels close and K+ channels open and there is an efflux of K+ and the repolarization phase occurs 3. K+ channels are slow closing therefore the membrane becomes more negative than the RMP and this is called the hyperpolarization phase The Na+/ K+ pump restores the membrane potential back to RMP Propagation of an action potential Saltatory vs continuous propagation  Synapses When AP arrives at presynaptic terminal: – the terminal can synapse with different regions of other neurons (or other structures e.g. gland, muscle etc.) through either a(n): 1. ELECTRICAL SYNAPSE 2. CHEMICAL SYNAPSE –possess gap junctions with connexon –unidirectional proteins – connect adjacent cells –no direct connection between cells –rapid communication –possesses a synaptic cleft –synchronous connection –AP can’t cross synaptic cleft –2-way transmission between cells –uses chemical messengers to transport –mainly in smooth & cardiac muscle AP across cleft The chemical synapse  The chemical synapse Summary of events: 1. Arrival of an action potential 2. Calcium influx into presynaptic knob 3. Release of neurotransmitter by exocytosis 4. Neurotransmitter binds to ligand gated channels 5. Sodium influx into postsynaptic membrane 6. Depolarization of postsynaptic neuron 7. Reuptake of neurotransmitter from synaptic cleft Postsynaptic potentials Postsynaptic potentials 2 types of neurotransmitter receptors: 1. Ionotropic receptors –opened by binding of specific neurotransmitter –allow ions exchange 2.Metabotropic receptors –no channel –like a “lock and key” –neurotransmitter binds to receptor → –activates G-protein → –activates a secondary messenger molecule → –opens ions channel in various ways –initiating a cascade which opens channel