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Laboratory 7: Medulla MCB 163 Fall 2005 Slide #152 1) This is the gracile nucleus. The fibers coming in to this nucleus are the fibers of the gracile fasciculus, which are first order and uncrossed. 2) This is the cuneate nucleus. The fibers coming into it are the fibers of the cuneate fasciculus. There is a somatotopic organization to this structure, just as there is in the cuneate fasciculus. 3) This is the hypoglossal nucleus, which is the source of the twelfth cranial nerve. The twelfth cranial nerve innervates the tongue, and plays a role in the reflex movements of the tongue in swallowing, chewing, and sucking in response to gustatory stimuli and other stimuli from the oral and pharyngeal mucosae (Barr, 151) 4) This is the solitary nucleus. It receives input from cranial nerves VII, IX, X, and the cranial part of nerve XI (Can you give these nerves their common names?). This nucleus is concerned with visceral sensation and taste (www.wikipedia.org). It is a major coordinator of autonomic nervous system regulation of cardiovascular, respiratory, gustatory, gastrointestinal, and chemoreceptive aspects of homeostasis (http://fred.hmc.psu.edu/ds/retrieve/fred/meshdescriptor/D017552). 5) This is the spinal trigeminal tract. It is analogous to Lissauer's tract in the spinal cord. In the trigeminal system for pain, the top of the head is represented at the bottom, and the bottom of the head is represented at the top. Since we're fairly caudal at this level (evidence is the presence of the gracile and cuneate nuclei, which are present near the spino-medullary junction), the top of the head is represented here. 6) This is the spinal nucleus of V (spinal trigeminal nucleus). Think of this structure as a continuation of the external aspect of the dorsal horn of the spinal cord (Rexed lamina I-III): C fibers terminate here before decussating and ascending. And just like in the spinal cord, these fibers are from the ipsilateral side of the body. 7) This is the reticular formation. Reticular means “net-like.” You can see why it is named this now, yes? A lesion here would likely disrupt sleep/wake cycles, and can produce either insomnia or narcolepsy. 8) This is the source of the lateral reticulospinal tract. A lesion here would do what to muscle tone? 9) This is the pyramidal decussation, where the pyramidal tract decussates. The pyramidal tract is also known as the corticospinal tract. These axons are from upper motor neurons on the motor cortex, and will ultimately end in the spinal tract on alpha and gamma motoneurons as well as interneurons. Primates are the only animals that have monosynaptic corticospinal influence on alpha motor neurons (humans have the most developed monosynaptic projection); rats and cats, for example, have only disynaptic influence through interneurons. These neurons are heavily myelinated, and have a somatotopic organization, with fibers destined for the upper spinal segments descending most medially, and fibers destined for lower segments more lateral. 10) This is the dorsal spinocerebellar tract. Proprioceptive recepters such as muscle spindles, golgi tendon organs, and other somatosensory receptors feed into it. This tract reaches the cerebellum, which is typically thought of as an unconscious pathway. The equivalent of this tract for the head itself comes from the vestibular system? The cerebellum does not project back into the spinal cord directly, but does modify the response of the vestibulospinal and reticulospinal tracts. Slide #142: 1) A communicating hydrocephalus (hydrocephalus resulting from blockage of the subarachnoid space, which is where most of the CSF leaving the fourth ventricle goes) would impact the nuclei along the floor of the fourth ventricle. Such nuclei include the facial colliculus, the hypoglossal nuclei, and more! (Better answers can be found in the Paxinos atlas) 2) This is the vagal motor nucleus, which is the source of the 10th cranial nerve. The conduction velocity and degree of myelination are lower than skeletomotor axons. The nuclei immediately around this include the solitary and the hypoglossal, which all have visceromotor functions. 3) This is the medial longitudinal fasciculus. This tract extends from the midbrain tegmentum to the cervical levels of the spinal cord. The MLF plays an important role in coordinating movements of the eyes with respect to the position of the head in space. It also plays a role in coordinating eye movements between the various oculomotor nuclei (abducens, trochlear, oculomotor nuclei; which cranial nerves are these?), and plays a role in the vestibuloocular reflex (which is why you can continue to read this page while shaking your head in bewilderment). 4) The dorsal column nuclei are more caudal to this section. Now the fibers form the medial lemniscus, which means they are Decussated, Second Order Sensory Afferent axons. These axons represent the contralateral side of the body, and are fairly highly myelinated. Solitary nucleus influences would be directed toward the thoracic segments of the spinal cord? 5) This is the inferior olive. This nucleus sends climbing fiber input to the cerebellum, which innervate Purkinje cells in a complimentary manner to the mossy fibers of other spinocerebellar axons. There is 1 climbing fiber that synapses on a Purkinje cell, while there can be thousands of mossy fibers. Climbing fibers also cause complex spikes in Purkinje cells. 6) This is the pyramidal tract. At this level, the axons in this tract are uncrossed. They are fast conducting, corticospinal axons. Damage to these axons results in a loss of power and precision of muscle movements. 7) This is the spinal trigeminal nucleus. The second order fibers project to the posterior thalamus, the reticular formation, and the central gray, just as the neo- and paleospinothalamic fibers do. Slide #134: 1) This is the choroid plexus, which secretes cerebrospinal fluid. 2) This is the medial vestibular nucleus. 3) This is the spinal vestibular nucleus. Together these are the source of the vestibulospinal tract. A lesion here would cause the patient to lean to the side of the lesion. 4) This is the nucleus ambiguous. This nucleus supplies muscles of the soft palate, pharynx, and larynx, together with striated muscle fibers in the upper part of the esophagus (Barr 149). 5) The chief source of descending influence to the lateral parts of the cerebellar hemisphere is the cerebropontocerebellar system, which arises in the cerebral cortex. The corticofugal impulses go through the pons to arrive at the cerebellar cortex as mossy fibers. The output of the cerebellar cortex is directed to the deep cerebellar nuclei. The lateral-most parts send their output through the Dentate Nucleus. A unilateral lesion to the lateral cerebellar hemispheres would not have much of an effect on posture and locomotion, but it would have an effect on coordinated movements of the ipsilateral limb. 6) These tiny cells are the granule cells. They are incredibly numerous; some estimates place their number at 10 to the 8th power (100,000,000). They use glutamate as their transmitter, and synapse on Purkinje cells as parallel fibers.