Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Sensory systems
Olfactory sensory system

Although humans can distinguish thousands of odors through the SENSE OF SMELL, they are classified as microsmatics because this system is significantly less developed in humans than in animals that rely on it to navigate their environment.

The peripheral part of the olfactory sensory system consists of receptor Cells in the epithelial (olfactory) lining of the Nasal cavity. Located in the superior nasal concha and the adjacent part of the nasal septum, it has a yellowish tint (due to pigment in the cells) and covers an area of about 2.5–5 cm² in the nasal cavity.

The nasal mucosa in the region of the olfactory lining is slightly thicker than the rest of the mucous membrane. It is composed of receptor and supporting cells (Atl. Fig. 157). Olfactory receptor cells are primary sensory cells. Their apical region features a long, slender dendrite ending in a knob-like Swelling. Numerous cilia of typical Structure project from this swelling and are immersed in mucus. This mucus is secreted by supporting cells and glands located beneath the epithelial layer (Bowman's glands). A long axon extends from the basal part of The Cell. The unmyelinated axons of many receptor cells form relatively thick bundles beneath the epithelium, known as olfactory fibers (fila olfactoria). These axons pass through the foramina of the cribriform plate of the Ethmoid bone and travel to the olfactory bulb, situated on the Inferior surface of the Brain (see Fig. 3.15). Excitation of the receptor cells occurs when a stimulus interacts with the cilia, and is then transmitted along the axon to the brain. Although olfactory cells are Neurons, unlike typical neurons, they are capable of regeneration. The lifespan of these cells is approximately 60 days, after which they degenerate and undergo phagocytosis. Replacement of receptor cells occurs through the division of basal cells in the olfactory lining.

The pathway and central Divisions of the olfactory sensory system. The olfactory bulb consists of five concentrically arranged layers (Fig. 3.72):

Layer 1 is formed by the fibers of the Olfactory nerve—the processes of the olfactory receptor cells;

Layer 2 is formed by glomeruli 100–200 µm in diameter, where synaptic contact occurs between olfactory fibers and the processes of second-order neurons;

Layer 3—the external plexiform layer, is formed by periglomerular cells, each contacting several glomeruli;

Layer 4—the internal plexiform layer, contains the largest Cells of the olfactory bulb, the mitral cells (second-order neurons). These are large neurons whose apical dendrites each form a single glomerulus in layer 2, while their axons form the olfactory tract. Within the bulb, the axons of mitral cells give off collaterals that contact other cells. Electrophysiological experiments have shown that odor stimulation evokes varying activity in mitral cells. Cells located in different Regions of the olfactory bulb respond to specific types of odors;

Layer 5—the granule cell layer, is formed by granule cells, where efferent fibers coming from the brain centers terminate. These cells are capable of modulating The activity of mitral cells.

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Fig. 3.72. Olfactory bulb:

A—drawing from a histological specimen; B—diagram; 1—granule cells; 2—granule cell layer; 3—mitral cells; 4—internal and 5—external plexiform layers; 6—periglomerular cells; 7—glomeruli; 8—processes of olfactory receptor cells

The olfactory tract, formed by the axons of mitral cells, extends from the olfactory bulb. Along this tract, olfactory signals travel to other regions of the brain (Atl. Fig. 156). The tract terminates in the lateral and medial olfactory striae. Via the lateral olfactory stria, impulses primarily reach the paleocortex of the olfactory trigone, where the third-order neuron is located, and then proceed to the amygdala. The fibers of the medial olfactory stria terminate in the archicortex of the subcallosal area, the septum pellucidum, and the Gray matter cells deep within the sulcus of the corpus callosum. After looping around the latter, they reach the hippocampus. This is the Water/144.html">Origin of the fibers of the fornix—the projection System of the archicortex—which terminates partly in the septum pellucidum and the mammillary body of the Hypothalamus. From there, the mammillothalamic tract, leading to one of the nuclei (anterior) of the thalamus, and the mammillotegmental tract, terminating in the interpeduncular Nucleus of the tegmentum of the Midbrain, originate, from which impulses are conducted to other efferent nuclei of the Central Nervous system. From the anterior thalamic nucleus, impulses are directed to the cortex of the limbic region. In addition, nerve fibers from the primary olfactory cortex reach the medioventral nucleus of the thalamus, which also receives inputs from the gustatory system. The axons of the neurons in this nucleus project to the frontal cortex, which is considered the highest integrative center of the olfactory system.

The hypothalamus, hippocampus, amygdala, and limbic cortex are interconnected; they are part of the Limbic System and participate in The formation of emotional responses, as well as in regulating the activity of Internal Organs. The connection of olfactory pathways with these structures explains the involvement of the sense of smell in feeding behavior, emotional status, and other Functions.