The second-order neurons in the dorsal column nuclei receive input from ascending branches of large primary afferent fibers. These second-order neurons then project through the medial lemniscus to the contralateral thalamus, where they synapse on third-order neurons in the VPL nucleus. The trigeminal pathway follows a similar relay, with synapses occurring in the main sensory nucleus before projecting to the contralateral VPM nucleus.

The dorsal column spinal cord pathways are responsible for signaling sensations of flutter-vibration, touch-pressure, proprioception, and visceral sensation, including visceral pain.

In contrast, the spinothalamic tract consists of nociceptive, thermoreceptive, and tactile neurons. The cells of origin for this tract are mainly located in the dorsal horn of the spinal cord. The axons cross over to the contralateral side, ascend through the ventrolateral funiculus, and synapse in various thalamic nuclei such as VPL, VPI, posterior, and intralaminar nuclei. The trigeminal pathway follows a similar route, with synapses occurring in the descending trigeminal nucleus before projecting to the contralateral VPM and intralaminar nuclei.

The relay of the spinothalamic tract in the VPL, VPM, and VPI nuclei contributes to the sensory-discriminative aspects of pain. Additionally, there are parallel nociceptive pathways in the ventrolateral funiculus known as the spinoreticular and spinomesencephalic tracts, which, along with the spinothalamic projection to the medial thalamus, play a role in the motivational-affective aspects of pain.

Convergent input from both the body wall and viscera onto spinothalamic tract cells explains referred pain.

The VPL and VPM nuclei are organized based on somatotopic maps and also contain inhibitory circuits. These nuclei have multiple maps, each corresponding to a specific submodality of somatosensory information. The somatosensory cortex, specifically the S-I and S-II regions, also exhibit somatotopic organization.

The S-I cortex consists of columns of neurons with similar receptive fields and response properties. Some S-I neurons are involved in feature extraction.

Descending control systems regulate transmission in somatosensory pathways. The endogenous analgesia system, which utilizes transmitters like endogenous opioid peptides, norepinephrine, and serotonin, plays a role in regulating nociceptive transmission.

For further reading, refer to Lau BK, Vaughan CW. Descending modulation of pain: the GABA disinhibition hypothesis of analgesia. Curr Opin Neurobiol. 2014;29:159-164., Coste B, Xiao B, Santos JS, et al. Piezo proteins are pore-2012;483:176-181., and Squire L, Berg D, Bloom FE, et al., eds. Fundamental Neuroscience. 4th ed. Waltham, MA: Academic Press; 2013.

After studying this chapter, the student should be able to answer the following questions:

1. Explain the concept of dark current and how the absorption of a photon affects it.
2. Describe the synaptic pathways for the central and surround portions of the receptive field in on-center bipolar cells and off-center bipolar cells.
3. Discuss the receptive field properties of simple and complex cells in the visual cortex.
4. Explain the frequency theory of sound encoding and why the place theory is also necessary.
5. Identify the stimuli that hair cells in the semicircular canals and otolith organs normally transduce.
6. Discuss the functional consequences of the different numbers of receptor molecules between olfactory and gustatory receptor cells.
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?