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What is a Neuron? - Definition, Parts & Function

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  • 0:37 Neurotransmitters
  • 1:18 Neuron Parts
  • 2:09 Action Potential
  • 3:04 Synaptic Gap
  • 4:29 Refractory Period
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Lesson Transcript
Instructor: Polly Peterson
What are the parts of a neuron? You'll watch Neuron Garciaparra work up a sweat as he throws baseballs to demonstrate the structures and functions of the billions of neurons that reside in your body.

Neurons are nerve cells that are constantly sending signals to your brain, muscles and glands. You have over 100 billion neurons in your brain sending signals. The signals help the different parts of your body communicate with each other. Thanks to neurons, you're able to swat a mosquito if you feel it land on your arm or wave to a friend if you see her walking towards you. Neurons send chemical signals called neurotransmitters, and they work quickly to help you react to everything going on around you.

Think of these neurons as little baseball players throwing balls to each other. The baseballs are the chemical signals called neurotransmitters. So, when you see a ball flying through the air towards you, sensory neurons send signals to your brain. This sets off a chain reaction of signals, which are fired off to motor neurons that cause your muscles to react so you can catch the ball.

Now, let's take a journey inside the human body to see what happens when we see, hear or touch something. Neuron Garciaparra is warming up. He has dendrites, like catching arms, that receive signals. He also has a pitching arm that fires off signals. This pitching arm is called an axon. Like a pitcher's power sleeve, a myelin sheath covers the axon, or pitching arm, and boosts the speed at which Neuron can fire off signals. The terminal branches at the end of the axon make up the pitching hand. This is where Neuron fires off signals.

Here's how it works. When the signal (ball) comes in, it excites Neuron into action. Positively charged sodium ions begin to enter the cell membrane. There are sodium ions in sports drinks, so think of this process as Neuron drinking a sports drink to increase electrolytes like sodium ions. A neural impulse, or electric current, travels from the dendrites (catching arms) through the axon (pitching arm) to the terminal branches (pitching hand) to be fired off to another neuron. This process is called the action potential. Then he winds up his pitch and...BAM!...fires off the signal.

The all-or-none law states that, once the neuron receives the signal, it has to fire it off. Like a pitcher who winds up and starts delivering the pitch, the neuron can't balk. There's no stopping once the nerve impulse has been activated. But, if your neurons don't get the signal, they don't fire. So, you'll swat a mosquito only if you feel it land on your arm. Either you feel it and the neurons deliver the signals so you can react, or you don't.

The signal (ball) flies over the synaptic gap (field) to another neuron (player) and the process repeats itself. The dendrite receives the chemical signal or neurotransmitter > excites the neuron > sodium ions enter the neuron and charge it > an electric current travels through the axon > and the terminal branches fire off the signal over the synaptic gap to the next neuron.

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