From these images, he proposed the "reticular theory" suggesting that the neurons are not discrete cells, but instead are continuous with each other and form a syncytium.
#Most common synapse type dendrite axon full#
Around the turn of the nineteenth century, the Italian physician Camillo Golgi (1873) developed a silver staining technique that revealed the full extent of dendritic and axonal arbors. The binding of a neurotransmitter to its receptor is reversible, and for good reason.Neurons give rise to processes known as dendrites and axons that form a complex network of interconnections throughout the brain at specialized sites called synapses. Only the ion channels in very close proximity to the action potential are affected, causing the action potential to be regenerated along the axon-this creates the movement of the action potential down the axon. The action potential occurs in a wave-like motion down the axon until it reaches the terminal button. This exchange of Na+ and K+ ions happens very rapidly, in less than one millisecond. The cell returns to the resting membrane potential, and the excess extracellular K+ diffuses away. In addition, the sodium-potassium pump is pushing Na+ out of the cell. With the Na+ channels closed, electrostatic pressure from a K+ charge gradient continues to push K+ out of the cell. The movement of K+ out of the cell causes the cell potential to return back to the resting membrane potential this is the falling or hyperpolarizing phase of the action potential (see Module 5.2).Ī short hyperpolarization occurs partially due to the gradual closing of the K+ channels. With the inside of the cell very positive relative to the outside of the cell (depolarized) and the high concentration of K+ within the cell, both the force of diffusion, along concentration gradients, and the force of electrostatic pressure, along charge gradients, drive K+ outside of the cell. The refractory period also ensures the action potential can only move in one direction down the axon, away from the soma.Īs the cell becomes more depolarized, a second type of voltage-dependent channel opens this channel is permeable to K+. Thus, a new action potential cannot occur during the refractory period. This means the Na+ channels cannot reopen again until after the cell returns to the resting membrane potential. At this point, the Na+ channels close and become refractory.
The inside of the cell becomes very positively charged, from about +30mV to about +60mv, depending upon the particular neuron. This is responsible for the rising or depolarizing phase of the action potential (see Module 5.2). Once these channels are opened when trigger threshold has been reached, Na+ rushes inside the cell, causing the cell to become very positively charged relative to the outside of the cell. However, Na+ cannot permeate the membrane when the cell is at rest. As we learned earlier, both diffusion from concentration gradients and electrostatic pressure from charge gradients are pushing Na+ ions toward the inside of the cell.
When the cell is at resting membrane potential, these voltage-dependent Na+ channels are closed. A voltage-dependent ion channel is a channel that opens, allowing some ions to enter or exit the cell, depending upon when the cell reaches a particular membrane potential (i.e. When the cell becomes depolarized (more positively charged) and reaches the threshold of excitation, this causes a voltage-dependent Na+ channel to open. Not surprisingly, some of these same ions are involved in the action potential. We discussed previously which ions are involved in maintaining the resting membrane potential. by Mariana Ruiz LadyofHats public domain by its author, LadyofHats). (Image from Wikimedia Commons File:Gap cell junction-en.svg by Mariana Ruiz LadyofHats. Two adjacent neurons with electrical synapse between them can communicate through hydrophilic channels Note that the gap between cell membranes of pre and post-synaptic neurons at electrical synapses is much smaller than the synaptic gap at chemical synapses which is about 10 times larger. \): Gap junction at an electrical synapse.
0 kommentar(er)
