Propagation of Nerve Impulse 2015

Propagation of Nerve Impulse

Arnab Ganguli The Neuron The Resting Membrane Potential Voltage Gated Ion Channel

Action potentials result from the presence in a cell's membrane of special types of voltage-gated ion channels . A voltage-gated ion channel is a cluster of proteins embedded in the membrane that has three key properties: •It is capable of assuming more than one conformation. •At least one of the conformations creates a channel through the membrane that is permeable to specific types of ions. •The transition between conformations is influenced by the membrane potential. The Action Potential

In physiology , an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells , called excitable cells , which include neurons , muscle cells , and endocrine cells, as well as in some plant cells . In neurons, they play a central role in cell-to-cell communication. In other types of cells, their main function is to activate intracellular processes. In muscle cells, for example, an action potential is the first step in the chain of events leading to contraction. In beta cells of the pancreas , they provoke release of insulin .[a] Action potentials in neurons are also known as " nerve impulses " or "spikes", and the temporal sequence of action potentials generated by a neuron is called its " spike train ". A neuron that emits an action potential is often said to "fire". The Action Potential (2)

As an action potential travels down the axon , there is a change in polarity across the membrane . The Na+ and K+ gated ion channels open and close as the membrane reaches the threshold potential , in response to a signal from another neuron . At the beginning of the action potential, the Na+ channels open and Na+ moves into the axon, causing depolarization . Repolarization occurs when the K+ channels open and K+ moves out of the axon. This creates a change in polarity between the outside of the cell and the inside. The impulse travels down the axon in one direction only, to the axon terminal where it signals other neurons. Components of Action Potential

Approximate plot of a typical action potential shows its various phases as the action potential passes a point on a cell membrane . The membrane potential starts out at - 70 mV at time zero. A stimulus is applied at time = 1 ms, which raises the membrane potential above -55 mV (the threshold potential). After the stimulus is applied, the membrane potential rapidly rises to a peak potential of +40 mV at time = 2 ms. Just as quickly, the potential then drops and overshoots to -90 mV at time = 3 ms, and finally the resting potential of -70 mV is reestablished at time = 5 ms. The threshold potential is the critical level to which the membrane potential must be depolarized in order to initiate an action potential As the sodium rushes back into the cell the positive sodium ions raise the charge inside of the cell from negative to positive. Once the interior of the cell becomes positively charged, depolarization of the cell is complete. In neuroscience, repolarization refers to the change in membrane potential that returns it to a negative value just after the depolarization phase of an action potential has changed the membrane potential to a positive value. Refractory period (physiology) , the amount of time it takes for an excitable membrane to be ready for a second stimulus once it returns to its resting state following excitation in the areas of biology, physiology, and cardiology Mechanism of Action Potential

Action potential generated by special type of voltage gated ion channels in pm. These are shut when the voltage is near resting membrane potential These channels rapidly begin to open when the membrane potential increases To a threshold value and allow an inward flow of sodium ions which changes the Electrochemical gradient. This causes further channels to open. This influx causes The polarity of the membrane to reverse and the ion channels inactivate. These channels rapidly begin to open when the membrane potential increases To a threshold value and allow an inward flow of sodium ions which changes the Electrochemical gradient. This causes further channels to open. This influx causes The polarity of the membrane to reverse and the ion channels inactivate. As the sodium channels close Sodium ions can no longer Enter the neuron and they are Actively transported back out Of the plasma membrane. Potassium channels are then Activated and then there is an out Ward current of potassium ions Returning the electrochemical Gradient to its resting state After an action potential has Occured there is a transient Negative shift called the After hyperpolarization or Refractory period, due to Additional potassium currents. This mechanism prevents the Action potential from travelling Back the way it came.

Types of Action Potential

In animal cells, there are two primary types of action potentials. One type is generated by voltage-gated sodium channels, the other by voltage-gated calcium channels. Sodium-based action potentials usually last for under one millisecond, whereas calcium-based action potentials may last for 100 milliseconds or longer. In some types of neurons, slow calcium spikes provide the driving force for a long burst of rapidly emitted sodium spikes. In cardiac muscle cells, on the other hand, an initial fast sodium spike provides a "primer" to provoke the rapid onset of a calcium spike, which then produces muscle contraction.

Cardiac action potentials

Basic ventricular myocyte action potential (with labels). 0: depolarization: voltage- gated sodium channels open allowing influx of sodium ions 1: initial repolarization: inactivation of voltage-gated sodium channels; voltage-gated potassium channels begin to open allowing minor efflux of potassium ions 2: plateau: voltage-gated calcium channels open allowing influx of calcium ions triggering calcium release from sarcoplasmic reticulum which results in myocyte contraction 3: rapid repolarization: voltage-gated slow potassium channels open allowing major efflux of potassium ions; voltage-gated calcium channels close 4: resting potential: potassium channels allow for potassium permeability Saltatory Conduction

Saltatory conduction (from the Latin saltare, to hop or leap) is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials. SYNAPSE-ligand gated channels Action potential comes across the membrane SYNAPSE-ligand gated channels SYNAPSE-ligand gated channels Chemical Synapse

At a chemical synapse, one neuron releases neurotransmitter molecules into a small space (the synaptic cleft) that is adjacent to another neuron. The neurotransmitters are kept within small sacs called vesicles, and are released into the synaptic cleft by exocytosis. Electrical Synapse An electrical synapse is a mechanical and electrical conductive link between two neighboring neurons that is formed at a narrow gap between the pre- and postsynaptic neurons known as a gap junction . At gap junctions, such cells approach within about 3.5 nm of each other, [1] a much shorter distance than the 20- to 40-nanometer distance that separates cells at chemical synapse .[2] In many animals , electrical synapse-based systems co-exist with chemical synapses .

Straczynski seems to think that all synapses are electrical in nature, but that is not true — in fact, chemical synapses are much more numerous;