11.4 Nerves and Neurons
When you watch a show on TV or on your computer that show was carried in the form of electrical signals from somewhere to your TV or computer. Likewise when you see something your eye sends electrical signals to your brain. When you hear something your ears send electrical signals as does your nose when you smell something and your tongue when you taste something. We don't have wires inside us and we don't transmit signals through the air. Instead our bodies transmit signals through nerve cells. When we want to move our brain sends a signal through nerve cells to our muscles.
Here is a brief video introduction about nerves.
Here is a picture of a neuron or nerve cell.
Like all cells it has a nucleus. It has dendrites where the input signal comes in and an axon where the output signal travels to the destination which could be another neuron, or a muscle cell. There are multiple dendrites but only one axon. Whether or not the neuron sends out a signal depends on the signals coming in the dendrites. If the combination of signals is large enough the neuron will send out a signal down the axon
The brain is made of billions of neurons connected together. The structure of a neuron gives us a clue as to how the brain works. To understand this lets think about how thinking works. When we program a computer to think we use input statements, output statements and if then statements. That's a lot like the way we think. For example
Input: I see my food my mother baked for her guests on the table. The food is round. The food has brown things that look like chocolate chips. The food smells good
If then: If the food is round and has chocolate chips and smells good then I will ask mother if I can have one.
Output: "Mom can I have a cookie".
When we reasoned we had 3 inputs. Those are like a neuron that has 3 dendrites with a signal flowing in them. The decision to ask Mom permission if the food is round and has chocolate chips and smells good is like the neuron firing only if the combined signal from all three dendrites is large enough which means all three of them are inputing a signal. The output of the signal down the axon is like us speaking to our mother and asking her for a cookie.
The output of one neuron can be the input of another neuron just like the conclusion from one if then statement can be the input to another if statement. For example on if then statement could conclude that an object is an apple and another if then statement could decide to eat the object if it is an apple. You could have if thens that say don't eat the apple like an input that the apple has a worm in it. Reasons to eat the apple are drawn with red arrows and reasons not to are drawn with blue arrows in the diagram below. Dendrites can have signals that signal the neuron not to fire so they are similar in the way they work to the diagram below.
The
neuron doesn't just add input signals and then fire if the signals are above a
certain threshold. The neuron can adjust how much it responds to different
input signals. How it adjusts depends on its inputs. That adjustment is what makes it possible for the
neuron to
learn. The adjustments of the sensitivities of the dendrites of many
connected neurons is one way the brain learns. The brain also can grow new
dendrites depending on neural activity. That too contributes to learning.
Neurons can transmit inputs to other neurons. They do that through a gap called a synapse. They contain vessicles which are like little fluid containing bubbles that contain a chemical called the neurotransmitter. When the axon fires the vessicle membranes merge with the outer membrane of the axon and release the chemical into a gap called the synapse. The chemical travels across the gap and binds to a receptor on the other side. If it is an activation chemical it will bind the activation receptors and that will signal the next neuron to send a signal down its axon. If it an inhibitory chemical it will bind another receptor that signals the next neuron not to fire. Those chemicals are called neurotransmitters.
The target neuron has receptors. If the neurotransmitter shape fits into the receptor and it binds that will signal the target neuron to fire.
Whether the target neuron fires or not may depend on the signals from multiple neurons.
Here is a video of how a synapse works.
Here is a very cool video showing neurons communicating with each other in the brain.
Signal travels very rapidly down the axon. The way it does that is very interesting and I will only give a simplified explanation of it here. The axon membrane has sodium potassium pumps. Using energy from molecules called ATP 3 positively charged sodium ions are pushed outside the axon membrane for every two potassium ions that are pumped inside the axon membrane. That causes a voltage to build up along the membrane in which the outside is positive and the inside is negative. The membrane has a lot of closed channels along its length. If the channels were open positive ions would flow in to the negative side and negative ions would flow out to the positive side. There are chemically gated channels and voltage gated channels. Chemically gated channels open when a chemical binds to them and voltage gated channels open if the voltage changes. When an axon is fired chemically gated channels are opened at the beginning of the axon. That lets in a flood of ions that changes the voltage difference across the membrane. The change in voltage opens the voltage gated channels. The flood of ions through them opens the next voltage gated channels which keep opening voltage gated axons all the way down the axon. Once the change in voltage reaches the end of the axon it causes neurotransmitters to be released into the synapse with the next neuron or with a muscle cell. If it is a muscle cell, the muscle cell will contract and cause motion such as movement of an arm or a leg.
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