Concept Practice
Sense of Smell

Chapter 51. Sense of Smell

axon
part of a neuron specialized to send impulses to other neurons (or to muscles or glands)
olfactory cortex
part of the brain that processes information about smell; located at the lower edges of the frontal and temporal lobes
olfaction
the sense of smell
olfactory receptor cells
specialized cells in the nose that detect chemicals floating in the air; located at the top of the nasal cavity
olfactory bulb
tube-shaped structure at the lower front of the brain that receives signals from the olfactory receptor cells
Sense of Smell
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Photo: Close-up photo of a person smelling a flower
Aleksandr Markin/Shutterstock
Learning Objective:

Describe the sequence of events that allows a person to smell an odor.

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The figure is a side image of the head and brain. Odor molecules travel from flowers inside the nose and up the nasal cavity, a large area going from the nose to the throat. At the top of the nasal cavity the odor molecules stimulate the olfactory receptors, which connect the nasal cavity to the front of the brain.

1. How do we smell the odors around us? Many substances, especially foods, release molecules into the air. The act of breathing draws these molecules into our nostrils, where they stimulate olfactory receptor cells located high in the nasal cavity.

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The figure shows an enlarged view of the olfactory receptors at the top of the nasal cavity. There are three layers to the image. At the bottom are the olfactory receptor cells, which match to specific odor molecules. The messages from the olfactory receptors travel up through the next layer, which is bone. Then the messages travel up to the last layer to arrive in the olfactory bulb in the brain.

2. The axons from these olfactory receptor cells travel through small openings in the bone that divides the nasal cavity from the brain. They enter the olfactory bulb, where the signals from many receptor cells converge to create messages that can be carried to the olfactory cortex in the brain.

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The image lists common distinctive odors. The list on the left includes perfume, chocolate, fruit, cooking food, and flowers. The list on the right includes sewage, sweat, smoke, gasoline, and garbage.

3. Humans have several hundred different types of olfactory receptor cells, and the brain combines the messages from these cells to detect about 10,000 different odors. There is no specific odor for “spaghetti” or any other food. Instead, each recognizable odor represents a specific profile of many olfactory cells responding simultaneously.

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Photo: dog (beagle preferred) sniffing the ground or an object
iStock/Getty Images

4. Although human olfaction (sense of smell) is impressive, most animals are much more sensitive to odors. Each human olfactory receptor cell is about as good as a single cell from animals with a superior sense of smell. But, those animals have many more receptors, with a larger portion of their brains dedicated to analyzing smells.

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Practice: Detecting and Identifying Odors

Play the animation to watch the sequence of events that allow this person to smell the flower.

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      The animation starts with a side image of the head and brain. A flower releases chemical molecules into the air. Breathing pulls these molecules into the nose and brings them into contact with the receptor cells for smell. An enlarged view of the receptor cells shows these receptors are clustered in a membrane at the top of the nasal cavity. At the end of each receptor cell there are tiny hairs that extend into the nasal cavity. These tiny hairs come in contact with odor molecules The actual receptors for smell are located on these tiny hairs. Each receptor will only accept a specific family of odor molecules. When an odor molecule binds with a receptor, this activates the receptor cell.The signals from the receptor cells then travel to the olfactory bulb. The message then goes to higher brain centers, which identify the odor and interpret its meaning.

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      Quiz 1

      Drag each event to arrange the events in the proper sequence. After all the events have been placed, select the CHECK ANSWER button.

      Select the NEXT button and move to Quiz 2.
      Perhaps you should go back to review the sequence of events that allows a person to smell an odor.
      Receptor cells are activated
      Odor molecules drawn into nose
      Odor molecules bind to receptors
      Signal goes to olfactory bulb
      Message goes to higher brain centers
      Odor molecules released by flower
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      Select the NEXT button and move to the Conclusion.
      Try to respond to the statements again.

      Quiz 2

      For each statement, select one of the buttons to indicate whether the statement is True or False. When a response has been placed for all statements, select the CHECK ANSWER button.

      TrueFalse

      The sense of smell is called olfaction.

      The olfactory bulb is located near the tip of the nose.

      The olfactory receptor cells detect the various molecules in the air we breathe.
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      Close-up photo of a person smelling a flower
      Aleksandr Markin/Shutterstock
      Congratulations!
      You have completed the activity Sense of Smell.