The inhibitory inter-neurons in the olfactory system are the granule and periglomerular cells. They establish reciprocal synapses with the dendrites of mitral cells. When a mitral cell is active, it depolarizes these inhibitory cells, which then inhibit the original and adjacent glomeruli. This mechanism enhances stimulus contrast and allows adaptation to continuous stimulation. Similar to horizontal cells in the retina, this process helps in enhancing stimulus contrast. The olfactory chemoreceptor cells and supporting cells are illustrated in Figure 8.30, while Figure 8.31 shows a drawing of a Sagittal Section Through an Olfactory Bulb, indicating the terminations of the olfactory chemoreceptor cells in the olfactory glomeruli and the intrinsic neurons of the olfactory bulb.

The axons of mitral cells exit the olfactory bulb and enter the olfactory tracts. The olfactory connections become more complex at this stage. Within the olfactory tracts, there is a nucleus called the anterior olfactory nucleus, which receives input from the olfactory bulb. It then projects to the contralateral olfactory bulb through the anterior commissure. As the olfactory tract approaches the base of the brain, it splits into the lateral and medial olfactory striae. The lateral olfactory stria synapses in the primary olfactory cortex, including the prepiriform cortex and, in many animals, the piriform lobe. The medial olfactory stria projects to the amygdala and the basal forebrain. These structures are part of, or directly connected to, the limbic system.

It is important to note that the olfactory pathway is unique among sensory systems as it does not require an obligatory synaptic relay in the thalamus before signals reach the cortex. However, olfactory information does reach the mediodorsal nucleus of the thalamus, which then transmits it to the prefrontal and orbitofrontal cortex. Besides conscious perception of odor, olfaction also plays a functional role in providing subtleties of taste by enhancing the overall sensory experience.

Moving on to the visual system, light enters the eye through the cornea and lens and is focused on the retina, which lines the back of the eye. The cornea acts as the most powerful refractive surface, while the lens has variable power to focus images of near objects onto the retina. The iris regulates depth of field and controls the amount of light entering the eye.

The outer segments of the photoreceptor cells are responsible for transducing light. These photoreceptors synapse on retinal bipolar cells, which then synapse on other interneurons and ganglion cells. The ganglion cells transmit visual information to the brain through the optic nerve. The optic disc, located where the optic nerve exits the retina, does not contain any photoreceptors and is therefore a blind spot. The region of the retina with the highest spatial resolution is the fovea and the surrounding macula.

Rod photoreceptors have high sensitivity, do not distinguish colors, and function best in low light conditions. Cone photoreceptors, on the other hand, have lower sensitivity but higher spatial resolution. Color vision relies on the three types of cones that have different spectral sensitivities.
The granule and periglomerular cells are inhibitory inter-neurons. They form dendrodendritic reciprocal synapses with the dendrites of mitral cells. Activity in a mitral cell depolarizes these inhibitory cells, and they in turn inhibit the original and adjacent glomeruli. Because each glomerulus is specialized by being the target of afferent fibers for a unique combination of odor qualities, this appears to be a way of enhancing stimulus contrast, much the way horizontal cells do in the retina. In addition, it provides a mechanism for adaptation to continuous stimulation.
• Fig. 8.30 Olfactory chemoreceptors and supporting cells. (Redrawn from de Lorenzo AJD. In: Zotterman Y [ed]. Olfaction and Taste. Elmsford, NY: Pergamon; 1963.) • Fig. 8.31 Drawing of a Sagittal Section Through an Olfactory Bulb, Showing Terminations of the Olfactory Chemoreceptor Cells in the Olfactory Glomeruli and the Intrinsic Neurons of the Olfactory Bulb. The axons of the mitral cells are shown exiting in the olfactory tract to the right. (Modified from House EL, Pansky B. A Functional Approach to Neuroanatomy. 2nd ed. New York: McGraw-Hill; 1967.)
CHAPTER 8 The Special Senses
The axons of mitral cells leave the olfactory bulb and enter the olfactory tracts. From here, the olfactory connections become highly complex. Within the olfactory tracts is a nucleus, called the anterior olfactory nucleus, that receives input from the olfactory bulb and projects to the contralateral olfactory bulb through the anterior commissure. As each olfactory tract approaches the base of the brain, it splits into the lateral and medial olfactory striae. Axons of the lateral olfactory stria synapse in the primary olfactory cortex, which includes the prepiriform cortex (and, in many animals, the piriform lobe). The medial olfactory stria includes projections to the amygdala, as well as to the basal forebrain. These structures are portions of, or directly connected to, the limbic system (see
Of note is that the olfactory pathway is the only sensory system that does not have an obligatory synaptic relay in the thalamus before signals reach the cortex. However, olfactory information does reach the mediodorsal nucleus of the thalamus, and it is then transmitted to the prefrontal and orbitofrontal cortex. The functional roles of olfaction, in addition to the conscious perception of odor, include providing much of the subtleties of taste by enhancing the 1.
Light enters the eye through the cornea and lens and is focused on the retina, which lines the back of the eye. The cornea is the most powerful refractive surface, but the lens has a variable power that allows images of near objects to be focused on the retina. The iris regulates depth of field and the amount of illumination that enters the eye.
2.
The outer segments of the photoreceptor cells transduce light. Photoreceptors synapse on retinal bipolar cells, which in turn synapse on other interneurons and on ganglion cells. The ganglion cells project to the brain through the optic nerve. The optic disc, where the optic nerve leaves the retina, contains no photoreceptors and is therefore a blind spot. The portion of the retina with the highest degree of spatial resolution is the fovea and the surrounding macula.
3.
Rod photoreceptors have high sensitivity, do not discriminate among colors, and function best under low light levels. Cone photoreceptors have lower sensitivity but higher spatial resolution. Color vision relies on the three types of cones that have different spectral sensitivities.
4.