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Wednesday, 27 June 2018

TRANSMISSION OF THE ACTION POTENTIAL BETWEEN CELLS





TRANSMISSION OF THE ACTION
POTENTIAL BETWEEN CELLS


 When action potential travels along an axon after it reaches the branching end of axon terminal called the end bulb. The synapse is the space between the axon of one neuron and the dendrite of another neuron or effector cell. The space (junction) between the end bulb and the dendrite of the next neuron is the synaptic cleft.
 The neuron carrying the action potential toward a synapse is the presynaptic (“before the synapse”) neuron. It initiates a response in the segment of a postsynaptic (“after the synapse”) neuron leading away from the synapse. The presynaptic cell is a neuron, but the postsynaptic cell can be always neuron, gland cell or muscle cell.

Types of Synapses

Synapses can be electrical or chemical.

Electrical synapse

Electrical synapse, nerve impulses transmit nerve impulses, directly from neuron to neuron when positively charged ions move from one neuron to the next. These ions depolarize the postsynaptic membrane, as though the two neurons were electrically coupled. An electrical synapse can rapidly transmit impulses in both directions. Electrical synapses are common in fishes and that give the their ability to dart swiftly away from a threatening predator.

Chemical synapse

In a chemical synapse, two cells communicate through the chemical agent called a neurotransmitter, that release from the presynaptic neuron. A neurotransmitter changes the resting potential in the plasma membrane of the receptive segment of the postsynaptic cell, creating an action potential in that cell, which continues the transmission of the impulse.





When a nerve impulse reaches an end bulb, it causes storage vesicles (containing the chemical neurotransmitter) to fuse with the plasma membrane. The vesicles release the neurotransmitter
by exocytosis into the synaptic cleft (figure ). One common neurotransmitter is the chemical acetylcholine; another is norepinephrine. (More than 50 other possible transmitters are known.)
When the released neurotransmitter (e.g., acetylcholine) binds with receptor protein sites in the postsynaptic membrane, it causes a depolarization similar to that of the presynaptic cell. As a
result, the impulse continues its path to an eventual effector. Once acetylcholine has crossed the synaptic cleft, the enzyme acetylcholinesterase quickly inactivates it. Without this breakdown, acetylcholine would remain and would continually stimulate the postsynaptic cell, leading to a diseased state. You have probably created a similar diseased state at the synapses of the fleas on your dog or cat. The active ingredient in most flea sprays and powders is parathion. It prevents the breakdown of acetylcholine in the fleas, as well as pets and people. However, because fleas are so small, the low dose that immobilizes the fleas does not affect pets or humans.


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