Conduction of the Action Potential PDF

PSYC 2210 Biological Psychology Conduction of the Action Potential Prof. Dr. Hakan Çetinkaya Yaşar University – Department of Psychology  Conduction of the Action Potential ▪ Let’s describe the movement of the message down the axon by using the giant squi...

PSYC 2210 Biological Psychology Conduction of the Action Potential Prof. Dr. Hakan Çetinkaya Yaşar University – Department of Psychology  Conduction of the Action Potential ▪ Let’s describe the movement of the message down the axon by using the giant squid axon. ▪ As the action potential travels, it remains constant in size. ▪ All-or-none law: An action potential either occurs or does not occur. Once it has been triggered, it is transmitted down the axon to its end. ▪ All-or-none law The principle that once an action potential is triggered in an axon, it is propagated, without decrement, to the end of the fiber. ▪ Rate law The principle that variations in the intensity of a stimulus or other information being transmitted in an axon are represented by variations in the rate at which that axon fires. Conduction of the action potential. 5  Conduction of the Action Potential ▪ If the action potential is an all-or-none event, how can it represent information that can vary in a continuous fashion? (e.g., Strength of a stimulus). ▪ Rate of firing: The rate of the production of action potentials (e.g., Strong-weak muscular contraction, perception of bright-dull light). The Rate Law The strength of a stimulus is represented by the rate of firing of an axon. The size of each action potential is always constant.. 6  Conduction of the Action Potential ▪ Subtreshold depolarization: It is too small to produce an action potential. It is passive, neither sodium channels, nor potassium channels are opening or closing. The axon acts like an electrical cable. The signal gets smaller because of leakage and resistance in the wire. ▪ Since the signal decreases in size, it is referred to as decremental conduction. Decremental conduction. When a subtreshold depolarization is applied to the axon, the disturbance in the membrane potential is largest near the stimulating electrode and gets progressively smaller at distances farther along the axon. 7  Conduction of the Action Potential ▪ Sultatory conduction: Since the myelinated parts of the axon is sealed from the extracellular fluid and the only naked part is the node of Ranvier, an action potential is regenerated at nodes of Ranvier by the enterance of Na+. ▪ The decremental conduction is followed by regeneration of action potential Advantages of saltatory conduction 1. Saltatory conduction is economically advantageous: Much less Na+ has to be pumped out. 2. Saltatory conduction is fast: Transmission between the nodes occurs by means of the axon’s cable properties which is very fast. Saltatory conduction Conduction of action potentials by myelinated axons. The action potential appears to jump from one node of Ranvier to the next. 8  Synaptic Transmission Direction of transmission (by axoplasmic transport) Synaptic cleft is 200Å (angstroms) wide Postsynaptic density is caused by the presence of receptors 10  Synaptic Transmission Release of Transmitter Substance The release of the transmitter substance is a very rapid event; it takes only a few milliseconds to occur. This event can be frozen to study by a pure copper block that had been cooled to 4 K (-270 C). Photographs from an electron microscope showing a cross section of a synapse. Synaptic vesicle A small, hollow, beadlike structure found in terminal buttons; contains molecules of a neurotransmitter. Release zone A region of the interior of the presynaptic membrane of a synapse to which synaptic vesicles attach and release their neurotransmitter into the synaptic cleft. 11 Synaptic Transmission Release of Transmitter Substance 12  Synaptic Transmission Release of Transmitter Substance When the membrane of Release the terminal zone of the button is presynaptic depolarized by membrane an arriving contains action voltage- potential, the dependent calcium calcium channels channels. open. Release of neurotransmitter. An action potential opens calcium channels. Ca+2 ions enter and bind with the protein embedded in the membrane of synaptic vesicles docked at the release zone. The fusion pores open and the transmitter substance is released into the synaptic cleft. The membrane of vesicles fuses with that of the terminal button. 13  Activation of Receptors ▪ Postsynaptic receptor A receptor molecule in the postsynaptic membrane of a synapse that contains a binding site for a neurotransmitter. ▪ Ionotropic receptor A receptor that contains a binding site for a neurotransmitter and an ion channel that opens when a molecule of the neurotransmitter attaches to the binding site. ▪ Neurotransmitter-dependent ion channel An ion channel that opens when a molecule of a neurotransmitter binds with a postsynaptic receptor. 14  Synaptic Transmission Activation of Receptors by Ionotropic Receptors: The Direct Method Ionotropic receptors. Ion channels opens when a molecule of neurotransmitter attaches to the binding site. 15 Activation of Receptors Ionic Movements During Postsynaptic Potentials 16 Activation of Receptors Metabotropic Receptors When a molecule of neurotransmitter binds with a receptor, a G protein activates an enzyme, which produces a second messenger (represented by black arrows) that opens nearby ion channels. 17  Synaptic Transmission Activation of Receptors by Metabotropic Receptors: The Indirect Method Metabotropic receptors. Receptors and ion channels are located apart from each other. When a molecule of neurotransmitter attaches to the binding site on the receptor, it initiates chemical events that open the ion channel. 18 Activation of Receptors Metabotropic receptor A receptor that contains a Excitatory postsynaptic potential (EPSP) An excitatory binding site for a neurotransmitter; activates an depolarization of the postsynaptic membrane of a enzyme that begins a series of events that opens an synapse caused by the liberation of a neurotransmitter ion channel elsewhere in the membrane of the cell by the terminal button. when a molecule of the neurotransmitter attaches to Inhibitory postsynaptic potential (IPSP) An inhibitory the binding site. hyperpolarization of the postsynaptic membrane of a G protein A protein coupled to a metabotropic synapse caused by the liberation of a neurotransmitter receptor; conveys messages to other molecules when by the terminal button. a ligand binds with and activates the receptor. Second messenger A chemical produced when a G protein activates an enzyme; carries a signal that results in the opening of the ion channel or causes other events to occur in the cell. 19 What is the advantage of a metabotropic receptor over an ionotropic receptor? Whereas ionotropic receptor- mediated responses generally last less than 1 second, metabotropic receptor- mediated responses last for considerably longer periods of time, from seconds to minutes. Nicotinic acetylcholine receptors (AChRs) are a family of acetylcholine-gated cation channels that form the predominant excitatory neurotransmitter receptors on muscles and nerves in the peripheral nervous system. Nicotinic receptors function within the central nervous system and at the neuromuscular junction. While muscarinic receptors function in both the peripheral and central nervous systems, mediating innervation to visceral organs. Recycling of the Membrane of Synaptic Vesicles ▪ Kiss and run ▪ Merge and recycle ▪ Bulk endocytosis 22 Termination of Postsynaptic Potentials Reuptake Molecules of a neurotransmitter that has been released into the synaptic cleft are transported back into the terminal button. The reentry of a neurotransmitter just liberated by a terminal button back through its membrane, thus terminating the postsynaptic potential. Enzymatic deactivation The destruction of a neurotransmitter by an enzyme after its release—for example, the destruction of acetylcholine by acetylcholinesterase 23 Termination of Postsynaptic Potentials acetylcholine (ACh) A neurotransmitter found in the brain, spinal cord, and parts of the peripheral nervous system; responsible for muscular contraction. acetylcholinesterase (AChE) The enzyme that destroys acetylcholine soon after it is liberated by the terminal buttons, thus terminating the postsynaptic potential. 24  Postsynaptic Potentials: Neural Integration Neural Integration (a) If several excitatory synapses are active at the same time, the EPSPs they produce (shown in red) summate as they travel toward the axon, and the axon fires. (b) If several inhibitory synapses are active at the same time, the IPSPs they produce (shown in blue) diminish the size of the EPSPs and prevent the axon from firing. 25  Synaptic Transmission Postsynaptic Potentials: Neural Integration EPSPs=Excitatory PostSynaptic Potentials IPSPs=Inhibitory PostSynaptic Potentials Measurement of the depolarizations and hyperpolarizations produced by excitatory and inhibitory synapses. 26  Synaptic Transmission EPSPs=Excitatory Postsynaptic Potentials: Neural Integration PostSynaptic Potentials IPSPs=Inhibitory PostSynaptic Potentials Neural integration. The effects of excitatory and inhibitory synapses on the production of action potentials in the postsynaptic neuron. 27 UBC Neuroanatomy Season:1/ Episode:4 https://youtu.be/ErpxEwlWww4 28