Like taste sensation, smell is also a chemical sensation. For taste, the substance must be in soluble form, and for smell it must be in gaseous form.
1. Impulses do not pass through thalamus.
2. Impulses do not pass to neocortex (no neocortical representation).
3. Receptors are first order neurons in this case and in other sensations the first order neurons are in the posterior root ganglion.
The olfactory receptors are bipolar in nature. The dendrites have an expanded end called olfactory rods. Over this surface area, a number of cilia projecting upwards. They act as antenna. About 10-20 million receptors are present in the nose. The supporting cells secrete some sort of mucus. The cilia are found within the mucous layer.
Olfactory area is confined to a small area (500 mm2) in nose in human beings. Human beings are considered as microsomatic. Lower animals have a wide area of representation in the cortex. In macrosomatic animals, olfactory sensation plays protective role and sexual instinct.
In the olfactory bulb, two different types of neurons are seen:
1. Majority—mitral cell
2. Minority—tufted cell
Axon from tufted cells crosses to the opposite side by passing through anterior commissure. They end in the olfactory bulb of the opposite side through the olfactory tract of opposite side. Some of the fibers pass through the intermediate olfactory striae and end in the anterior perforated substance and region of diagonal band. From here they are relayed to the limbic system and hypothalamus.
Axons from most of the mitral cells pass through the lateral olfactory striae and go to prepyriform cortex (Fig. 10.35).
Amygdaloid nuclear complex and periamygdaloid region form the higher centre for smell.
Mechanism of Stimulation of Receptors:
The substances have a particular odor (odoriferous substance) gets dissolved in the mucus.
Normally, as you breathe in, the substance entering the nostrils will not reach the olfactory region. When some eddy currents are produced and then only substances are able to reach the olfactory region and stimulate the receptor. Sniffing brings about production of eddy currents. Odoriferous substances should get dissolved in the water and lipid lining of the membrane and if it does so, it can stimulate the receptors more effectively.
Adaptation of these receptors is extremely fast. Receptors show adaptation only for a particular odor.
Abnormalities of Olfactory Sensation:
1. Anosmia:
Loss of sense of smell, e.g. common cold.
Reasons for anosmia:
a. During the inflammatory condition, the mucus lining is inflamed. The edematous cells compress the nerve fibers. Hence, from the receptor, thus impulses cannot be transmitted.
b. Fracture of ethmoidal bone.
c. Tumor pressing upon olfactory nerve fibers.
2. Parosmia:
Altered/perverted sense of smell sensation, e.g. hysteria.
3. Hyperosmia:
Smell and taste sensation greatly increased in adrenal cortical insufficiency. It also occurs in raised intracranial pressure, etc.