Sensory CN: I, II, V, VII, VIII, IX, and X Flashcards
Early in development, the neural tube is separated into a dorsal (posterior) alar plate and a ventral (anterior) basal plate by what?
the sulcus limitans; Nuclei dorsal to the sulcus limitans are sensory (afferent) and nuclei ventral to the sulcus limitans (in basal plate) are motor (efferent).
This is a cross-section through the neural tube. The red arrows indicate the sulcus limitans. Sensory nuclei (green, blue, purple) are dorsal to the sulcus limitans (in the alar plate). Motor nuclei (shown yellow, gold, red) are located ventral to the sulcus limitans in the basal plate.
This is a posterior view of the adult brainstem with the cerebellum removed to show the floor of the fourth ventricle (within red square).
In the adult the neural tube has opened dorsally, thus the alar plate is displaced laterally, relative to the basal plate. This is as if you took a hose (neural tube), used a knife to slit the top (dorsal) side along the length of the hose, and then grabbed the cut ends and pulled them laterally (see next slide)
Note the rhomboid shape of the floor of the ventricle. The sulcus limitans can be seen as the outer margin of the floor of the fourth ventricle.
The yellow line indicates the plane of section for the next slide.
This is a cross section of the adult rostral medulla taken at about the level of the yellow line in the previous slide. The dotted lines indicate the sulcus limitans (border of the floor of the fourth ventricle) and mark the boundaries between the alar plate (dorsal) and the basal plate (ventral). Sensory nuclei are in the alar plate and motor nuclei in the basal plate.
Note that there are three sensory nuclei for the Trigeminal nucleus, namely:
chief sensory, spinal, and mesencephalic nuclei of V
The chief sensory and mesencephalic nuclei of CN V are typical discrete nuclei in the _____ and _________, respectively.
pons and mesencephalon
The spinal nucleus of CN V extends as a column from the mesencephalon to the spinal cord, where it is continuous with what?
the substantia gelatinosa of the spinal cord (layers I and II). Several cranial nerves (V, VII, IX, X) project to this nucleus.
Also in the pons, the vestibular nuclei and cochlear nuclei are concerned with body position and hearing, respectively. They are associated with _____
CN VIII.
Several cranial nerves project to the solitary nucleus (nucleus solitarius). The rostral part of the nucleus is concerned with what?
taste (CNs VII, IX, X) and the caudal part of the nucleus is concerned with cardiorespiratory inputs.
Cranial nerve I is completely sensory. It serves olfaction. The sensory receptors reside where?
in the nasal epithelium. These cells send very fine axons (olfactory nerves or filae) through the cribriform plate to synapse in the olfactory bulb.
Axons of projection neurons in the olfactory bulb project where?
travel as the olfactory tract to synapse in olfactory cortex, amygdala, and related forebrain structures.
Cranial nerve II is the Optic Nerve. It is completely sensory - it transmits visual input to the brain. The receptors are located in the retina, at the rear of the eye. The optic nerve is made up of axons of retinal ganglion cells. Describe the route of the optic nerves
The optic nerves from each eye converge just outside the mesencephalon. At this point, about 50% of the fibers from each eye cross to the opposite side of the body at the optic chiasm (“cross”). Central to the chiasm, the fibers run as the optic tract.
Describe the route of the optic tract
The optic tract runs along the outside of the mesencephalon to the lateral geniculate body (the visual thalamus: LGN), which in turn projects to primary visual cortex (banks of calcarine fissure).
NOTE: Some optic nerve fibers bypass the LGN and instead travel in the brachium (“arm”) of the superior colliculus to synapse in the superior colliculus (tectum of mesencephalon). This input is important for visual “startle” reflexes.
Cranial Nerve V is the Trigeminal nerve. This system is the primary one for somestheses for the face. Peripherally, there are three main branches to the nerve:
V1 = Ophthalmic, V2 = Maxillary, and V3 = Mandibular.
In addition to motor innervation of muscles of mastication (and a few other muscles), the trigeminal nerves carry fine touch, proprioception, and pain and temperature afferents from the skin of the face region.
Note that the back of the head is mostly innervated by spinal nerves from C2. Also note the complicated arrangement for innervation of the external ear (pinna) by multiple cranial nerves.
There are three sensory nuclei associated with the trigeminal nerve, namely:
The chief sensory nucleus of V, the mesencephalic nucleus of V, and the spinal nucleus of V.
Where is the chief sensory nucleus found? What does it relay?
The chief sensory nucleus is located in the rostral pons and receives epicritic inputs from afferents for fine touch and vibration sense from the face.
It is thus analogous to the posterior column nuclei (chief sensory nucleus of V is to fine touch for the face as posterior columns are for fine touch from the body).
Epicritic afferents in the trigeminal nerve have their cell bodies where?
in the trigeminal (or Gasserian) ganglion. This ganglion is located outside of the brainstem and is analogous to the dorsal root ganglia associated with the spinal cord.
The central process of these cells synapse in the chief sensory nucleus.
It is important to note that fine touch afferents that run in other cranial nerves (VII, IX, X) have cell bodies in different peripheral ganglia but also synapse in the chief sensory nucleus of V. The chief sensory nucleus is thus the first integration center for all fine touch and vibration inputs for the face region.
The mesencephalic nucleus of V contains the cell bodies of what?
proprioceptive afferents (spindle organs, Golgi tendon organs, etc.) from the muscles of mastication (and probably extraocular muscles and muscles of the tongue). It is essentially a displaced peripheral ganglion in the tegmentum of the mesencephalon. The peripheral processes of these cells form the mesencephalic tract of V.
The spinal nucleus of V extends from the mesencephalon to the spinal cord, where it becomes continuous with what?
the substantia gelatinosa (layers I and II of dorsal horn).
The spinal tract of CN V receives inputs from what sensations?
pain and temperature afferents for the face that run in cranial nerve V, as well as cranial nerves VII, IX, and X.
Pain and temperature afferents in branches of V have their cell bodies in the _______ ganglion.
trigeminal.
The central processes of these cells project into and synapse in the spinal nucleus of V. These axons, as well as second order axons from the spinal nucleus of V, run in the associated spinal tract of V.
What is the trigeminal lemniscus?
Path that runs from the chief sensory nucleus of V to the ventroposteromedial nucleus (VPM) of thalamus. It is analogous to the medial lemniscus for spinal pathways.
What is the trigeminothalamic tract?
Tract that runs from the spinal nucleus of V to the VPM. It is analogous to the spinothalamic tract for spinal pathways.
Afferents from the different cranial nerves and nerve branches remain segregated within the spinal tract and nucleus of V (somatotopic organization). This is indicated in the figure by the color coding.
Afferents from CNs VII, IX and X are located most dorsal.
Afferents from the mandibular branch of V (V3) are next most dorsal (posterior), followed by afferents from V2 (maxillary), with ophthalmic (V1) afferents the most ventral (anterior).
This pattern can be useful for localizing brain stem lesions.
