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Sensory Neurons: Definition & Function

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  • 0:00 What are Sensory Neurons?
  • 0:54 Smell and Taste
  • 1:55 Sight
  • 2:49 Touch
  • 4:20 Hearing
  • 5:37 Lesson Summary
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Lesson Transcript
Instructor: Devin Kowalczyk

Devin has taught psychology and has a master's degree in clinical forensic psychology. He is working on his PhD.

In this video, you'll learn about sensory neurons and how they give rise to your five senses. Also, you'll explore the brain's role in processing transmissions from sensory neurons.

What are Sensory Neurons?

If someone removed the top of your skull and poked your brain with their finger, would you feel it? Nope! Yet, if that same person pricked the tip of your finger with a pin, it would hurt. Why is this?

Your brain is entirely isolated, floating in fluid in your skull, and it relies on connections to special neurons, called sensory neurons, throughout the body to understand what's occurring in the environment. A neuron is a specialized type of cell that transmits signals. A series of neurons can send a signal from your little toe all the way to your brain, where the signal is processed.

Sensory neurons make up your five primary senses (smell, taste, sight, touch, and hearing) allowing you to smell a banana or feel scalding coffee that's splashed on your arm. Let's take a closer look at each of these senses.

Smell and Taste

When the aroma of a banana, in the form of odor molecules, floats into your nose, the molecules dissolve in your mucus and spread throughout it. The molecules then attach to projections called microvilli, which are like little fingers. In the microvilli are the branching tips of neurons, called dendrites. A single sniff of a banana can cause millions of odor molecules to flood your nose, which comes into contact with hundreds, if not thousands, of dendrites. This contact causes the neuron to send a signal to a part of the brain that deals with the sense of smell.

Taste buds, the little bumps on your tongue, are also sensory neurons. They work with olfactory receptors in your nose to detect different flavors in food. These highly sensitive neurons can detect chemical differences, such as a change in pH. Think acids and bases (which can affect sourness), changes in sodium ions (which can affect saltiness), and complex triggers to various other chemicals (which can affect bitterness).

Sight

Next, let's take a look at sight. In the back of the eyeball is an area called the retina. On the retina are rods and cones, which are specialized cells that have a chemical reaction when light hits them. Rods, found mostly on the edge of the retina, take less energy to activate and give poor resolution of an image. Cones are more central and activate to different wavelengths of light, which gives things color. These require more energy to work, which is why you have a hard time seeing at night. An interesting trick at night is to look slightly off center at something, so you use your rods instead of cones. The rods and cones are activated by light, which activates the retinal ganglion cells, which are specialized nerves that activate when the rods and cones are activated. The retinal ganglion cells pass their information to the optic nerve, which travels to the brain.

Touch

Now, let's touch on the skin. The skin has many types of neurons, but they're broadly divided into two categories: free nerve endings and corpuscles. To make this a little more visual, imagine a two-layered gel. The top layer is your epidermis, the outer skin, and the second layer is the dermis, or the deeper skin. Beneath the dermis is more stuff, like veins, hair follicles, and glands.

Free nerve endings are neurons that have the edge of their branches embedded in the dermis. Imagine after the first layer of gel, the dermis, you put a couple of pieces of string. This string in the gel is equivalent to the free nerve endings. They react to temperature, pain, and chemicals in or on the skin. When the skin is damaged, it causes the end of the neurons to activate. Some are dedicated to fast pain, while others are dedicated to aching pain. Other free nerve endings react to changes in temperature, some activating more when it's hot and some activating when it's cold. The last set of free nerve endings reacts to chemicals like bleach or pollen.

Corpuscles are multilayered bulbs with nerves inside of them, looking kind of like onions. So inside the gel are very small onions. When the bulb is moved, the nerve reacts, sending a signal that something is touching it. But the bulbs' layers allow fluids to shift, so the neuron is no longer triggered, and you don't feel it anymore. A good analogy here is the clothes you're wearing; after a while, you stop feeling them.

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