ZOO 120.docx

Neuronal Physiology Action Potential Brief, rapid and large changes in membrane potential. Conduction in non-decremental Graded potential is formed when the voltage Na+ gated channel is opened and an influx of Na+ is seen. When the graded potential reaches a threshold potential, which can either be...

Neuronal Physiology Action Potential Brief, rapid and large changes in membrane potential. Conduction in non-decremental Graded potential is formed when the voltage Na+ gated channel is opened and an influx of Na+ is seen. When the graded potential reaches a threshold potential, which can either be -50 to -55 mv (threshold potential), it will trigger the formation of an action potential. Two main types of channels Voltage gated Na+ channel Activation gate – a hinge door that will open and close depending on the changes in the voltage or potential in the membrane. Inactivation gate – a ball and chain sequence of amino acids that are facing the ICF (intracellular fluid). Three Stages: Closed but capable of opening: Activation gate may open depending on the change in the resting membrane potential. Open: Influx of Na+ ions inside the cell. Closed and not capable of opening/ Inactivated: Ball and chain sequence of amino acids binds to a receptor in the voltage gated channel, which closes the channel. Voltage gated K+ channel Closed state: channel is closed. Open state: channel is open. Permeability Changes and Ion Fluxes During an Action Potential Activation gate opens at the depolarization event. Voltage gated K+ channel. The action potential is a 100mV chain. Two events that happen when the threshold potential is reached. rapid opening of activation gate slow closing of inactivation gate slow opening of voltage gated K+ channel After reaching back to resting membrane potential it returns to its original state – the closed but capable of opening. Hyperpolarization – caused by slow closing of voltage gated K+ channels. Voltage gated K+ channels will fully close once it reaches the resting membrane potential. Na+ -- K+ pump brings back the concentration of the ions in and out of the cell. Phases of Action Potential Phase 1 – influx of Na+ Phase 2 – efflux of K+ Phase 3 – further efflux of K+ Permeability changes for Na+ and K+ action potential As the threshold potential is reached (-50 to -55 mv), it will cause a rapid explosion in the opening of the voltage gated Na+ channel. When the peak is reached, a decrease in the permeability is seen. The permeability of K+ ions will increase at the peak of the action potential. Graded vs Action Potential Graded potential is the dependent on the triggering potential. Action potential is an all or none potential (if it reaches the threshold, it will undergo action potential, if it does not reach threshold). Neuron Main Parts Cell body – input zone Dendrites – input zone Axon – trigger or conducting zone or passageway of information from the cell body. axon hillock – trigger zone and initiates action potential. branches known as collaterals can end up in axon terminals and may innervate other neurons in the cell body, or muscle cells. Types of Action Potential Propagation Contiguous conduction: spread of action potential will be through every patch. Action potential initiation triggers the influx of Na+ ions in the axon influx. Saltatory conduction: the transfer of action potential is relatively faster. Leaps or jumping of information exists. Myelin sheath. Nodes of Ranvier where action potential will occur. should be reached on voltage gated channels. Two main types that will form myelin sheaths: Schwann cell: will myelinate in the peripheral nervous system. Oligodendrocytes: they will be myelinating axons in the central nervous system. What ensures the one-way propagation of action potential? Refractory Periods Absolute Refractory Period: Another action potential cannot be initiated. They cannot be bothered by any stimulus until the closed but opening state is reached. Relative Refractory Period: after the absolute refractory period. At this point, this portion of the axon can be stimulated to produce an action potential. A greater stimulus is required because of hyperpolarization. What happens when the action potential reaches the axon terminal? Synapses – junctions between two neurons or a neuron to muscle. Neuron-to-neuron synapse Presynaptic Axon terminal is called Synaptic knob: contains synaptic vesicles. Inside the synaptic vesicles are the neurotransmitters. Releases information Post synaptic Receives information. Receptor for the neurotransmitters. Receptors are located in the subsynaptic membrane. When the action potential reaches the synaptic knob, the voltage gated Ca+ channel. Ca+ moves towards the inside of the cell and triggers the release of the synaptic vesicles via exocytosis, thus releasing the neurotransmitters in the synaptic cleft. The neurotransmitters binds to the receptor for from subsynaptic membrane, which are chemically gated. Non-specific.