CBNS 116 Lecture 10 - Brainstem III: Cranial nerves: Focus on special senses
Lecture notes by Todd Fiacco
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”.
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
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
. 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.
(CN I) carries olfactory special sensory information to the CNS. Realize
that the structures you see on the base of the brain, the
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
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]: