1. Module 1

1.22. Page 4

Lesson 4

Module 1—The Nervous System

Smell

 

Read

 

This photograph shows a young girl smelling a bunch of yellow daffodils.

© Terrie L. Zeller/shutterstock

How does scent or fragrance affect you? For example, what happens when you catch the scent of a fragrance as you approach somebody? That smell may make you want to get closer, but it may have the opposite effect on the friend you are with. In fact, he is sneezing and his nose and eyes are running. Are your senses communicating perceptions that are different than those of your friend? To learn more about smell and what it communicates, read “Smell” on page 426 in your textbook.

 

Smell, much like taste, detects chemicals in solution. Olfactory receptor cells are stimulated by chemicals only when in solution. A sensory nerve communicates this information to the frontal and temporal lobes of the brain. Cells in the olfactory epithelium produce a continuous secretion of mucous in which molecules of airborne chemicals dissolve. Mucous is continuously secreted to flush away old odour chemicals so that new odour chemicals may gain access to the receptors. Smells associated with dangers, such as smoke, natural gas, or skunk, trigger the sympathetic nervous system. The perception of unpleasant odours can trigger sneezing or a gag reflex. Because your perceptions are based on your unique experiences, your perceptions can be very different from your friend’s perceptions.

 

olfactory (receptor) cell: a neuron located in the olfactory epithelium that is specialized to receive chemical stimuli and to initiate a nerve impulse

 

olfactory epithelium: a patch of tissue located in the upper part of the nasal cavity that contains mucous-secreting cells and olfactory cells, or smell cells, that can detect different smells; a sense organ for smell


 

Self-Check

 

SC 6. Provide names for structures 1 to 14.

 

These images show detail of the olfactory system.

Inquiry into Biology (Whitby, ON: McGraw-Hill Ryerson, 2007), 427, fig. 12.26. Reproduced by permission.

 

SC 7. What has to happen to airborne chemicals, like those of a rose, before they can be smelled?

 

SC 8. What is the function of structure 10?

 

SC 9. What is the purpose of the olfactory cilia on the olfactory cell (receptor cell)?

 

SC 10. Where is the nerve impulse initiated?

 

SC 11. Outline the specific sensory pathway involved in smelling a rose.

 

Check your work.
Self-Check Answers

 

SC 6.

 

Structure 1: frontal lobe of cerebrum

 

Structure 2: olfactory bulb on underside of cerebrum

 

Structure 3: olfactory epithelium

 

Structure 4: airborne chemicals from rose

 

Structure 5: nasal cavity

 

Structure 6: olfactory bulb enlarged

 

Structure 7: sensory neuron

 

Structure 8: bone

 

Structure 9: sensory fibres

 

Structure 10: olfactory gland

 

Structure 11: olfactory (smell) cell

 

Structure 12: cilia (hairs) on olfactory cell

 

Structure 13: odour molecules dissolved in mucous

 

Structure 14: olfactory epithelium

 

SC 7. Airborne chemicals have to be dissolved before they can fit into receptors on olfactory cells.

 

SC 8. The olfactory glands (structure 10) continually produce mucous to dissolve airborne chemicals and to wash away old odours.

 

SC 9. The cilia (hairs) increase the surface area so that more odour molecules may reach more receptors.

 

SC 10. The nerve impulse is initiated at the cilia on the olfactory cell and transmitted to the neurons in the olfactory bulb.

 

SC 11. The main steps of smell perception include the following:

  • Odour molecules are breathed into the nasal cavity.

  • Odour molecules dissolve in mucous produced by olfactory glands of the nasal epithelium.

  • The molecules in solution attach to receptors on the cilia (hairs) of olfactory cells.

  • The sensory receptors or olfactory cells (specialized neurons) initiate nerve impulses that are sent via the sensory neuron to the olfactory bulb and then to the temporal lobe of the cerebrum where it is interpreted as a scent.