Understanding the Transmission of Nerve Impulses

 

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Transmission of a nerve impulse: Resting potential and action potential.

Like the gaps between the Schwann cells on an insulated axon, a gap called a synapse or synaptic cleft separates the axon of one neuron and the dendrites of the next neuron. Neurons don't touch. The signal must traverse the synapse to continue on its path through the nervous system. Electrical conduction carries an impulse across synapses in the brain, but in other parts of the body, impulses are carried across synapses as the following chemical changes occur:

1. 
   

   Calcium gates open.

   
   

   At the end of the axon from which the impulse is coming, the membrane depolarizes, gated ion channels open, and calcium ions (Ca2+) are allowed to enter the cell.

   
   


2. 
   

   Releasing a neurotransmitter.

   
   

   When the calcium ions rush in, a chemical called a neurotransmitter is released into the synapse.

   
   


3. 
   

   The neurotransmitter binds with receptors on the neuron.

   
   

   The chemical that serves as the neurotransmitter moves across the synapse and binds to proteins on the neuron membrane that's about to receive the impulse. The proteins serve as the receptors, and different proteins serve as receptors for different neurotransmitters — that is, neurotransmitters have specific receptors.

   
   


4. 
   

   Excitation or inhibition of the membrane occurs.

   
   

   Whether excitation or inhibition occurs depends on what chemical served as the neurotransmitter and the result that it had. For example, if the neurotransmitter causes the Na+ channels to open, the neuron membrane becomes depolarized, and the impulse is carried through that neuron. If the K+ channels open, the neuron membrane becomes hyperpolarized, and inhibition occurs. The impulse is stopped dead if an action potential cannot be generated.

   
   

   If you're wondering what happens to the neurotransmitter after it binds to the receptor, you're really getting good at this anatomy and physiology stuff. Here's the story: After the neurotransmitter produces its effect, whether it's excitation or inhibition, the receptor releases it and the neurotransmitter goes back into the synapse. In the synapse, the cell "recycles" the degraded neurotransmitter. The chemicals go back into the membrane so that during the next impulse, when the synaptic vesicles bind to the membrane, the complete neurotransmitter can again be released.

   
   

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