Nervous System                       

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• Smell (see location of the various components in Figure 09):
  
   

  Nasal cavity - This a large air space above and behind the nostril. Three shelf-like ridges of bone (nasal conchae) are there to deflect air. In normal breathing, air flows through the lower part of the cavity, past the rear of the soft palate and into the throat (Figure 16). A good sniff sends the odor eddying up into the roof of the nasal cavity, where it comes into contact with the olfactory apparatus. Mucus layer - Only those odor molecules that can be dissolved in the mucus, become the stimulants to the receptor sites on the cilia.

  Figure 16 Sense of Smell [view large image]

  Cilia - These are the hairlike projections from the olfactory sensory cells, and trigger the sensory cells to generate nerve signals if the receptors recognize the shape of the odor molecules. It is estimated that the human nose contains about 1000 different types of olfactory neurons, each type able to detect a particular set of chemicals.

  Bowman's gland - This gland makes the mucus. Olfactory sensory cell - It is embedded in the olfactory epithelium. Nerve signals pass upward along the cell body, which narrows into a wire-shaped nerve fiber, or axon. The axons from thousands of sensory cells group into bundles and convey their nerve signals to the olfactory bulb. Olfactory bulb - In this structure, the axons form complicated ball-shaped sets of connections with the mitral cells. These connection area are olfactory glomeruli, and there are hundreds in each olfactory bulb (see Figure 17). Each glomerulus receives signals from more than 25000 sensory cells and has tens of thousands of connections from the mitral cells in the bulb itself. Much sorting and processing of the signals takes place in the glomeruli. The resulting nerve messages are sent along the olfactory tract to the olfactory area in the brain.

  Figure 17 Olfactory Bulb [view large image]