Ascending branches of large primary afferent fibers synapse on second-order neurons in the dorsal column nuclei. These second-order neurons project via the medial lemniscus to the contralateral thalamus and synapse on third-order neurons of the VPL nucleus. The equivalent trigeminal pathway is relayed by the main sensory nucleus to the contralateral VPM nucleus.
5.
The dorsal column spinal cord pathways signal the sensations of flutter-vibration, touch-pressure, and proprioception. They also contribute to visceral sensation, including visceral pain.
6.
The spinothalamic tract includes nociceptive, thermoreceptive, and tactile neurons; its cells of origin are mostly in the dorsal horn, and the axons cross, ascend in the ventrolateral funiculus, and synapse in the VPL, VPI, and posterior and intralaminar nuclei of the thalamus. The equivalent trigeminal pathway is relayed by the descending trigeminal nucleus and projects to the contralateral VPM and intralaminar nuclei.
7.
The spinothalamic relay in the VPL, VPM, and VPI nuclei helps account for the sensory-discriminative aspects of pain. Parallel nociceptive pathways in the ventrolateral funiculus are the spinoreticular and spinomesencephalic tracts; these tracts and the spinothalamic projection to the medial thalamus contribute to the motivational-affective aspects of pain.
8.
Referred pain is explained by convergent input to spinothalamic tract cells from the body wall and from viscera.
9.
The VPL and VPM nuclei are somatotopically organized and contain inhibitory circuits. These nuclei contain multiple somatotopic maps, one for each somatosensory submodality. The somatosensory cortex includes the S-I and S-II regions; these regions are also somatotopically organized.
10.
The S-I cortex contains columns of neurons with similar receptive fields and response properties. Some S-I neurons are involved in feature extraction.
11.
Transmission in somatosensory pathways is regulated by descending control systems. The endogenous analgesia system regulates nociceptive transmission, and it uses transmitters such as endogenous opioid peptides, norepinephrine, and serotonin.
Additional Readings Lau BK, Vaughan CW. Descending modulation of pain: the GABA disinhibition hypothesis of analgesia. Curr Opin Neurobiol. Coste B, Xiao B, Santos JS, et al. Piezo proteins are pore-2014;29:159-164. forming subunits of mechanically activated channels. Nature. Squire L, Berg D, Bloom FE, et al., eds. Fundamental Neuroscience. 4th ed. Waltham, MA: Academic Press; 2013.
2012;483:176-181.
Upon completion of this chapter, the student should be able to answer the following questions: 1.
What is the dark current, and how does the absorption of a photon change it?
2.
What are the synaptic pathways for the central and surround portions of the receptive field of an on-center bipolar cell? Of an off-center bipolar cell?
3.
What are the receptive field properties of simple and complex cells in the visual cortex?
4.
What is the frequency theory of sound encoding? Why is the place theory also required?
5.
What are the stimuli that are normally transduced by the hair cells in the semicircular canals and otolith organs?
6.
What are the functional consequences of the differing numbers of different receptor molecules between olfactory and gustatory receptor cells?