Laboratory 8: Medulla to Pons MCB 163 Fall 2005 Slide #124 Fourth Ventricle: This area contains the vagus nucleus, vestibular nuclei, and other nearby nuclei such as the nucleus solitarius. Bilateral damage to this region is not compatible with life. Trauma to this region can cause damage to the MLF among other regions. Damage to the MLF results in an inability to coordinate eye movements with head movements.
Dentate Nucleus: One would expect the dentate to be larger in primates that rodents due to the larger primate cortex. The dentate nucleus is located in the lateral cerebellar hemisphere, the region that receives input from the cortex. The dentate projects to the red nucleus and the thalamus via cerebellorubrothalamocortical and cerebellothalamocortical tracts respectively. Given that this is a lateral nucleus, it is likely a phylogenetically newer structure.
Interposed Nucleus: The interposed nucleus is closely related to the intermediate zone of the cerebellum. This nucleus would be larger in non-primate mammals as compared to humans, as more of their movements are mediated via non-corticospinal tracts. These fibers also project to the red nucleus and thalamus. Given its location between the other cerebellar nuclei, this nucleus evolved after the fastigial- and before the dentate nucleus.
Fastigial Nucleus: The fastigial nucleus is most closely related to the flocculus and vermis. One would expect this nucleus to be larger in humans due to its important role in posture and eye gaze. This nucleus projects to the vestibular nuclei in the brainstem via cerebellovestibulospinal pathways. Phylogenetically this is the oldest cerebellar nucleus.
Lateral Vestibular Nucleus: These nuclei are large because they have long projections, which potentially indicate motor functions. This nucleus is the sole source of the vestibulospinal tract, synapsing on anterior horn cells to mediate trunk and limb reflexes. This nucleus receives fibers from the cerebellum as part of the cerebellovestibulospinal tract, as well as direct innervation from the vestibular ganglia. These cells have a high resting firing rate.
Raphe Nuclei: Raphe nuclei are serotinergic neurons that receive descending input from the periaqueductal gray matter. These neurons then send descending projections to C-fibers in laminae II and III of the spinal cord. The raphe also send ascending fibers to the forebrain. The midline location of this nucleus suggests an older phylogenetic origin. It’s location next to and within the reticular formation indicates a role in the reticular modulation of pain. The raphe slow reticular formation transmission during pain suppression.
Gigantocellular Reticular Nucleus: These neurons are responsible for the reticulospinal tract. This tract runs almost entirely ipsilaterally, projecting to all motoneurons in the spinal cord without somatotopy. This tract has direct effects on motoneurons as well as indirect effects regularing spinal reflex sensitivity. Given their large size, one would expect their conduction velocity to be very high.
Facial Motor Nucleus: The facial motor nucleus cells are motor cells analogous to anterior horn cells. Like cell groups in the anterior horn, the cells in the facial motor nucleus are grouped according to the muscles they innervate. Given the larger muscle size and the greater number of facial muscles (leading to a wide array of facial expressions), humans would be expected to have larger neurons in greater numbers than a rat. Since these are lower motoneurons, damage to these cells would lead to ipsilateral flaccid paralysis. Concussive injury would cause damage to these cells, resulting in weakness of the facial muscles.
Spinal V Nucleus Oralis: Spinal V neurons are first-order sensory for the ipsilateral face and head. Destruction of this nucleus would lead to loss of pain and temperature sensation to the ipsilesional side of the face. This would deprive the VPL/VPM of the thalamus as well as the postcentral gyrus of their inputs.
Spinal Tract V: This tract is so long because it contains the output fibers from the spinal V nucleus, along whose side it runs. Its somatotopic organization is inverted from the organization of the head, indicating its early evolution as the head evolved from the neck. The axons of this tract are second-order, decussated axons, making this a lemniscus. Traction on this tract would cause sensation of pain on the contralateral side of the face.
Vestibulocochlear Nerve: Nerves are fiber tracts emanating from the CNS; peduncles are huge fiber tracts that connect two large regions of the CNS (there are two sets of peduncles, cerebellar and cerebral). A lemniscus is a second-order, decussated axon tract whereas a peduncle is simply a huge, mixed-order, mixed-information bundle. The auditory vestibulocochlear nerve carries both auditory and vestibular information. This nerve projects information from both ipsi- and contralateral auditory and vestibular hair cells to the cochlear and vestibular nuclei. The receptive fields of the sensory axons vary from neuron to neuron. Severing this nerve would result in hearing and balance deficits; the nerves may regenerate depending on the type of damage, though it is unlikely. The afferent fibers maintain tonotopy.
Dorsal Cochlear Nucleus: A tumor near this nucleus would result in auditory innervation in the form of tinnitus. The small cells in this nucleus resemble the stellate and granule cells of the cerebellum. There are no intrinsic inhibitory cells; instead cells from neighboring receptive fields (auditory frequencies) send lateral connections to cells representing adjoining receptive fields. The dorsal cochlear nucleus sends fibers to the inferior colliculus via the lateral lemniscus.
Inferior Cerebellar Peduncle: The inferior cerebellar peduncle carries fibers from the spinal cord, climbing fibers from the inferior olives, as well as the vestibulo-, reticulo-, and trigeminocerebellar tracts. Severing this cord would result in a balance and equilibrium deficits as well as ipsilateral ataxia.
Slide #112 Locus Coeruleus: The locus coeruleus can project to so many places because its axons give off many collaterals. This structure is related to maintenance of attention (vigilance) and novelty reaction. A lesion would result in attentional deficits. Lesions to this area in cats (and humans?) can result in REM sleep while awake, leading to a state known as “dream enactment”.
Medial Vestibular Nucleus: The medial vestibular nucleus is responsible for coordinating head and eye movements and the stabilization of head position. The efferents from the nucleus project bilaterally to the cervical spinal cord via the medial vestibulospinal tract.
Medial Longitudinal Fasciculus: The medial longitudinal fasciculus runs from the cervical spinal cord all the way to the midbrain, terminating in the oculomotor nuclei. Given its medial location it is phylogenetically an older tract. Functionally it helps coordinate head and eye movements, which are important in hunting so you would expect to find it across many species.
Abducens Nucleus: Cells in the somatic efferent column innervate muscles of the eye or tongue. Damage to the abducens nucleus would cause abduction (turning out) of the ipsilateral eye. These cells are lower motor neurons, since they are immediately presynaptic to motoneurons that innervate a muscle. This nucleus receives its primary input from the motor cortex. For unknown reasons, poliomyelitis does not seem to affect the eyes, so this nucleus would remain undamaged with polio.
Tectospinal Tract: The Tectospinal tract originates in the tectum, as its name implies; its specific origin is the superior colliculus. Fibers from this tract are sent only to the cervical spinal cord via the MLF. By responding to visual stimuli, this tract mediates reflex postural movements, reflex head turning in response to visual stimuli, and visual following. Damage would therefore cause deficits to tracking rapidly moving objects.
Periolivary Region: This region receives its inputs from the ipsilateral inferior colliculus and cochlear nuclei, and project ultimately to outer hair cells as part of the descending olivocochlear pathway. The tuning curves of these neurons are of many different types: broad, narrow, multimodal, etc.
A5 Medullary Cell Group: The medial forebrain bundle is a major reward pathway that runs between the ventral tegmental area and the hypothalamus. There are multiple noradrenergic inputs to different structures that have simply evolved over time—evolutionarily old structures and pathways become subsumed by newer structures and the older structures evolve to take on many roles.
CN VII: The cell bodies of the motor component of the facial nerve are located in the facial motor nucleus (#8, slide 124). This nerve is thick because it carries axons to the large number of muscles in the face. This nucleus receives input from the reticular formation, providing an alternate, somatotopically organized motor pathway for muscles of the face.
Flocculus: The flocculus receives input directly from primary vestibular afferents, as well as a large number of secondary fibers from the vestibular nuclei. Damage to this region causes disequilibrium and vertigo, as well as problems with coordinating slow eye movements.
Superior Cerebellar Peduncle: The superior cerebellar peduncle is the major output path of the cerebellum. The middle cerebellar peduncle is so large because it contains fibers originating from the entire cerebral motor cortex. All cerebellar output: cutaneous, vestibular, spindle, etc. information passes through this peduncle from the deep cerebellar nuclei. A lesion that damages the locus coeruleus, vestibular nuclei, flocculus, superior cerebellar peduncle, tectospinal tract, and/or CN VII is not life-threatening, but it would be a rather unfortunate to have.
Intermediate Lobe: Input to the intermediate (paravermal) lobe of the cerebellum arises from the spinal cord; for this reason this lobe is also known as the spinocerebellum. The afferents received by this region are from somatosensory receptors of the skin, muscles, and joints either through the spinocerebellar or reticulocerebellar pathways or via direct innervation by receptors. If this region were damaged the interposed and dentate nuclei would be deafferented. Ultimately the projections from this region reach the red nucleus and either the thalamus or the spine (rubrospinal), though the projections to the thalamus are located in different areas that the similar dentatorubrothalamic pathway described earlier. In primates this area of the brain is smaller because much of the functions subserved by this region have been supplanted by cerebrocerebellar inputs.