Neurons are stimulated in various ways. There are some specialized neurons that convert the physical energy of our environment into a neural signal. These neurons are called receptors. The receptors of greatest interest to us in this web book are the ones found in the retina and are responsible for converting light energy to an electrochemical neural signal for vision.
An excellent discussion of the nervous system is presented in the Online Biology Book.
We have noted elsewhere that when light (a photon) is absorbed by photopigments in the outersegment of our receptors it causes the photopigment to change its shape; a process called isomerization. When this isomerization occurs, an electrophysiological process is initiated that results in signals being sent through our retina and up into the brain. The end result is that we say that "we saw something." Here we will discuss how such signals travel along a neuron and transfer from one neuron to another.
It is necessary to use the concept referred to as "ions."
Let's see how the concept of ions applies to the atoms on both sides of a cell membrane. It is important to understand that cell membranes are semi-permeable.
Movement of charged particles across a membrane causes an electrical current. If one places an electrode connected to the positive pole of a voltmeter inside the neuron and another electrode, which is connected to the negative pole of a voltmeter, on the outside of the neuron the voltage difference between inside and outside of the neuron can be measured. This voltage will be very small and is measured in millivolts (one millivolt is 1/1000 th of a volt). When one measures the voltage across a neural membrane that has not been stimulated they will find -70 mV. This is called the resting membrane potential and is found because there are more negative ions inside the neuron than outside due to large ions in the cell that can't get out and a membrane "pump" that pushes most of the positive sodium ions out.
Let's assume that a neuron has been stimulated. An action potential is formed and travels down an axon headed toward the junction between this neuron and another called a synapse. A synapse is a small gap between two neurons. The presynaptic ending contains synaptic vesicles that contain transmitter chemicals. When an action potential reaches the presynaptic ending it causes some of these vesicles to bond to the presynaptic membrane and to spew its transmitter chemical into the synaptic cleft. It migrates across the cleft and is received by the postsynaptic receptors. There are two kinds of postsynaptic receptors: 1. Those which when they receive the transmitter chemical exhibit an excitatory postsynaptic potential which results in a neuron depolarizing. 2. Those which when they receive the transmitter chemical exhibit an inhibitory postsynaptic potential which results in a neuron hyperpolarizing.
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