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MCB 163: Mammalian Neuroanatomy Name: Key Fall 2005 Seat #______ LABORATORY EXAMINATION TWO 1. LUMBAR SPINAL CORD: Distinguishing features include the large size of the ventral horns and the presence only of the gracile fasciculus in the dorsal columns; the corticospinal tract is reduced relatively since many descending fibers have already terminated; the relative ratio of gray to white matter favors the gray, suggesting that many tracts are small. 2. GRACILE FASCICULUS: This tract contains ganglion cell axons representing cutaneous Merkel and Meissner and Pacinian corpuscles and muscle spindles (Ia) and Golgi tendon organ (Ib) receptors from the hindlimb and inferior trunk that are ascending towards the gracile nucleus in the medulla to form topographic excitatory connections before becoming and joining lemniscal axons. 3. LAMINA I-V: The sources of the neo- (Aδ) and paleospinothalamic (C fiber) projections to the pain and temperature system. In it, first-order primary afferents (glutamatergic for Aδ substance P-containing for C fibers) synapse and decussate, the second-order cells forming the spinothalamic lemniscus and projecting to the posterior thalamus or to the central gray and reticular formation, respectively. 4. LAMINA VIII, IX: These are lower motoneurons immediately presynaptic to muscle, including α and γ motoneurons and the associated Renshaw and other local cells. These motoneuron pools represent, respectively, flexors in more dorsal parts of the ventral horn, and extensors in the ventral part. Proximal or axial or trunk muscles are represented in more medial parts of the ventral horn (near lamina VIII), and distal muscles are represented more laterally. 5. TRACT OF LISSAUER: This is a small fiber tract that lies dorsal to Rexed’s lamina I. It contains first order spinothalamic axons and is continuous through the dorsoventral extent of the spinal cord, and it represents the entry point for pain and temperature fibers to ramify before entering a dorsal horn to synapses. Such fibers at any given spinal level can synapse within three segments of their specific entry; this means that the dermatomes for pain have coarser somatotopy than those for touch and pressure. 6. THORACIC SPINAL CORD: The reduced gray matter, the presence of a distinct intermediolateral cell column, and the tiny ventral horn each signify the representation of the intercostal muscles; concomitantly, the dorsal horn is small, too, and the dorsal column representation is limited to the lower leg and trunk. 7. INTERMEDIOLATERAL CELL COLUMN: A specialized pool of preganglionic sympathetic neuronal cell bodies in the thoracicolumbar spinal cord whose axons project to the sympathetic chain before terminating in their targets on smooth muscle. These inputs oppose the complementary craniosacral parasympathetic efferents that usually serve to reduce neural activity in smooth muscle. 8. VESTIBULOSPINAL AND RETICULOSPINAL TRACTS: These (and other) tracts project preferentially to the proximal axial muscles of the trunk and limbs; the tracts extend the entire 1 length of the spinal cord, are primarily ipsilateral, and have no known topography in their projection; the vestibulospinal tract is driven primarily by otolith organ input, reticulospinal by all modalities. 9. LATERAL CORTICOSPINAL TRACT: Upper motoneurons that project the length of the spinal cord to α motoneurons and which are responsible for rapid and precise muscle contractions and powerful movements, especially of the distal extremities; often damage by stroke, these neurons arise from motor and sensory cortex in the contralateral hemisphere and are unique to primates. 10. SPINOCEREBELLAR TRACT: These are second-order undecussated axons en route to the cerebellum (via the inferior cerebellar peduncle) or to the inferior olive and which carry subconscious proprioceptive information to Purkinje cells (as mossy fiber endings) or to inferior olivary cells; the latter terminate in the cerebellar cortex as climbing fibers and are active phasically during complex motor learning, but otherwise only active tonically. This spinocerebellar tract parallels the dorsal columns save that the information in it is subconscious. 11. GRACILE NUCLEUS: The axons representing the bottom of the foot, the ankle, the shin, the knee and the lower half of the body ascending as first-order primary afferents in the dorsal columns terminate in this nucleus before decussating and terminating in the contralateral ventrobasal complex. The nucleus as well as the fibers has a somatotopic arrangement. Check #11: Was not caudal medulla? (you could see the cerebellum) 12. SPINAL TRIGEMINAL NUCLEUS: This nucleus contains first-order spinotrigeminothalamic neurons whose axons decussate and terminate in the central gray, posterior thalamus, or reticular formation; these structures propagate nociceptive information to the hypothalamus and to insular and cingulate cortex. This structure is analogous to layers I-V of the dorsal horn, and includes a raphetrigeminal component for descending modulation of pain. 13. MEDIAL LONGITUDINAL FASCICULUS: a tract that spans the cervical spinal cord to the rostral midbrain to interconnect the vestibular (VIII) and oculomotor (III, IV, VI) systems using ascending and descending axons to provide for conjugate and consensual eye movements that are linked with the input from the labyrinths and otolith organs. 14. INFERIOR OLIVE: Source of climbing fibers that form a 1:1 relation to postsynaptic Purkinje cells; this fiber makes complex spikes that are essential for skilled motor learning; well developed in primates. 15. VESTIBULAR NUCLEI: The site of first order terminations from ganglion cell axons innervating the three semicircular canals or the saccular and utricular maculae. Consists of four nuclei, of which the most medial feed forward into the medial longitudinal fasciculus, while the lateral (Deiters’) nucleus is the primary source of reticulospinal fibers, which are undecussated, without topography, and project the length of the spinal cord. 16. NUCLEUS AMBIGUUS: A tiny group of cholinergic lower motoneurons innervating the oropharyngeal area and vocal cords and important in the control of vocal tone and in the 2 integration of behaviors such as swallowing, speech, and voice pitch; contains alpha motoneurons and is a target of significant corticobulbar projections. 17. FACIAL MOTOR NUCLEUS: Lower motor neurons responsible for innervating the muscles of facial expression; nucleus contains α and γ motoneurons, Renshaw cells, Ia and Ib interneurons responsible for stretch reflexes; often affected in Bell’s palsy with consequent weakness of facial muscles. 18. DENTATE NUCLEUS: This cerebellar relay nucleus is enormous in humans and one of the main sources of the lateral cerebellar system that travels to the parvocellular part of the red nucleus, and then to the ventroanterior-ventrolateral thalamic nucleus, and from there to the premotor and motor cortex for long term motor planning prior to the execution of serial movements. Did he really point to this? 19. FACIAL NERVE AND GENU: These axons of lower motoneuron represent the final common pathway of α and γ motoneurons en route to the muscles of facial expression for power and precision of their contractions. A few axons are also en route to the tensor tympani to support its withdrawal from the repeated effects of loud sounds, thus attenuating their impact on hearing. Damage to the VIIth nerve because of its long intracranial course is not uncommon; the motor nucleus itself receives bilateral, predominantly crossed, corticofugal input. 20. SUPERIOR CEREBELLAR PEDUNCLE: The output pathway from the cerebellar deep nuclei toward the parvocellular (small-celled) subdivision of the red nucleus. Sends axons from there to the ventral anterior/ventral lateral thalamic nucleus, which projects to motor cortex. Crucial for motor planning and for informing motor cortex of subconscious state of muscle, joint, and cutaneous receptors. 3