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116 Lecture 10.S10 - CBNS 116 Lecture 10 Brainstem III...

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CBNS 116 Lecture 10 - Brainstem III: Cranial nerves: Focus on special senses Lecture notes by Todd Fiacco ASSIGNMENT: No new assignment. Finish reading Nolte Chapter 12: “Cranial nerves and their nuclei”. FOR NEXT LECTURE: Read Nolte Ch. 23 pp. 580-591: “hypothalamus and pituitary”. TODAY'S LECTURE: The big picture: Just like the rest of the body, the head has skin, muscle, and autonomic viscera, which are innervated by the cranial nerves, the head’s equivalent of the spinal nerves. However, unlike the rest of the body, the head contains several specialized sensory structures that are innervated by special sensory nerves. The emphasis of this lecture is on the cranial nerves through which special sensory information (olfactory, optic, gustatory, auditory, and vestibular) reaches the central nervous system. For all of these special senses, the adequate stimulus (light, sound, odorants etc.) is transduced by specialized neural sensory structures into electrical signals that then propagate via specific cranial nerves to the CNS. The intensity of the adequate stimulus is encoded by the frequency of action potentials that are propagated centrally; this is called frequency coding . Although Nolte goes into some detail with the special senses (separate chapters for olfaction and taste, auditory and vestibular, and visual system), for this course our primary focus will be on the central anatomical pathways for each. The visual system overview will be cursory, since this was such a major focus of the second half of CBNS 106. I. The olfactory nerve (CN I) carries olfactory special sensory information to the CNS. Realize that the structures you see on the base of the brain, the olfactory bulb and olfactory tract , are actually extensions of the CNS. So where is the olfactory nerve? The “olfactory nerve” is a collection of very short axons of the olfactory receptor neurons (a component of the olfactory epithelium lining the roof and adjacent walls of the nasal cavity) that pass through the cribriform plate of the ethmoid bone to synapse onto second order mitral cells of the olfactory bulb [slides 80-83]. The olfactory bulb is therefore a nucleus containing second order olfactory neurons. The axons of the mitral cells form the olfactory tract. The olfactory system is unique in that, unlike virtually all other sensory information, it does not relay first through the thalamus. It proceeds directly to olfactory cortex, all located in and around the medial temporal lobe [slides 84, 85]:
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A. Primary olfactory cortex , called piriform cortex B. Parahippocampal gyrus , part of the limbic cortex C. the amygdala , part of the limbic system Through these initial cortical structures, olfactory information passes to the hippocampus, hypothalamus, and to orbitofrontal association cortex via the thalamus (the dorsomedial nucleus) [slide 86]. The orbitofrontal cortex also receives taste information , and therefore may be an important area for the perception of flavor . The direct input of olfactory information to
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