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Anatomy and Physiology of the Central and Peripheral Nervous System

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Which of the following sensory pathways projects outside the thalamus?

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(A) The visual pathway

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(B) The auditory pathway

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(C) The vibration sense pathway

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(D) The olfactory pathway

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(E) The temperature sense pathway

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(D) The olfactory pathway is the only sensory pathway that does not project to the thalamus. The olfactory nerve penetrates the cribriform plate of the ethmoid bone and enters the olfactory bulb to synapse with the second-order neurons: mitral and tufted cells. The axons of the second-order neurons course posteriorly as the olfactory tract in the orbital surfaces of the frontal lobe and project to the primary olfactory cortex in the temporal lobe. In the visual pathway, axons of the ganglion cells in the retina gather together at the optic disk to form the optic nerve. The two optic nerves come together at the optic chiasma, where a partial crossing of optic nerve fibers takes place. The crossed and uncrossed fibers from the optic nerves join caudal to the optic chiasma to form the optic tracts, which extend from the dorsolateral corners of the chiasma to the lateral geniculate bodies. These constitute a thalamic nucleus that provides a relay station for all the axons of the retinal ganglion cells subserving vision. The lateral geniculate nucleus is laminated into six layers. Not all parts of the retina are represented equally in it. The central area of the retina has larger representation than does the periphery of the retina. Axons of neurons of the lateral geniculate nucleus project to the visual cortex in the occipital lobe via the geniculocalcarine tract. Geniculocalcarine fibers project to the visual cortex.

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The auditory pathway is described as a four-tiered neuronal network. The auditory cochlear nerve, extending from the organ of Corti to the cochlear nucleus, generates action potentials that travel in the afferent nerve fibers via the central components (axons) of bipolar neurons in the spiral ganglion to reach the cochlear nuclei in the pons. The second-order neurons of the auditory pathway are formed by fibers of cochlear nuclei crossing to the contralateral inferior colliculus. The latter contains the third-order neurons and serves as the central relay nucleus in the auditory pathway. The projections from the inferior colliculus terminate in the medial geniculate body, a thalamic auditory relay nucleus. The fourth-order neurons are formed by the geniculotemporal fibers that project to the primary auditory cortex. Vibration sensation is mediated by Merkel disk receptors and Meissner's corpuscules. Fibers mediating vibration terminate in the deeper layers of the dorsal horn. Second-order neurons from the dorsal horn ascend through the ipsilateral dorsolateral funiculus, terminating on neurons in the nuclei of the posterior ...

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