Spinothalamic tract

Collaterals from neurons of neighbouring specific thalamo-cortical relay nuclei. Because these neurons are themselves activated by sensory inputs transmitted along the spinothalamic tract, this provides one way in which sensory stimuli can influence cortical activity generally, as well as specifically. 

Stimulation of a nociceptor in the periphery of the body elicits action potentials in the first-order neuron, which transmits the signal to the second-order neuron in the dorsal horn of the spinal cord. From the spinal cord, the signal is transmitted to several regions of the brain. The most prominent ascending nociceptive pathway is the spinothalamic tract. Axons of the second-order sensory neurons project to the contralateral (opposite) side of the spinal cord and ascend in the white matter, terminating in the thalamus (see Figure 8.1). The thalamus contributes to the basic sensation or awareness of pain only it cannot determine the source of the painful stimulus. 

Small quantities of opiate injected intrathecally or epidurally produce segmental analgesia. This observation led to the clinical use of spinal and epidural opiates during surgical procedures and for the relief of postoperative and chronic pain. As with local anesthesia, analgesia is confined to sensory nerves that enter the spinal cord dorsal horn in the vicinity of the injection. Presynaptic opioid receptors inhibit the release of substance P and other neurotransmitters from primary afferents, whereas postsynaptic opioid receptors decrease the activity of certain dorsal horn neurons in the spinothalamic tracts. 

Stage I—analgesia Loss of pain sensation results from interference with sensory transmission in the spinothalamic tract. The patient is conscious and conversational. A reduced awareness of pain occurs as Stage II is approached. 

Neurons at the origin of several ascending somatosensory pathways have been examined for the presence of glutamatergic inputs. Westlund et al. (1992) investigated inputs to three intracellularly labeled spinothalamic tract neurons in the deep dorsal horn. Of the... 

Blomqvist A, Ericson AC. Craig AD, Broman J (1996) Evidence for glutamate as a neurotransmitter in spinothalamic tract terminals in the posterior region of owl monkeys. Exp Brain Res 108 33-44. 

Ericson AC, Blomqvist A, Craig AD, Ottersen OP, Broman J (1995) Evidence for glutamate as neurotransmitter in trigemino- and spinothalamic tract terminals in the nucleus submedius of cats. Eur J Neurosci 7 305-317. 

Lekan HA, Carlton SM (1995) Glutamatergic and GABAergic input to rat spinothalamic tract cells in the superficial dorsal horn. J Comp Neurol 36/ 417-428. 

Westiund KN, Carlton SM, Zhang D, Willis WD (1992) Glutamate-iramunoreactive terminals synapse on primate spinothalamic tract cells. J Comp Neurol 522 519-527. 

The classical ascending pathway (Fig. 1) is the spinothalamic tract, a contralaterally projecting fiber bundle that ascends in the anterolateral aspect of the spinal white matter to the ventral posterolateral thalamus with extensive collateralization to brainstem structures prominent among these being the periaqueductal gray (PAG). 

Stage 1 (analgesia) is bronght about by a decrease in the activity of the dorsal horn of the spinal cord, which interferes with the sensory transmission in the spinothalamic tract. 

Carlton, S. M., Rees, H T.suruoka, M.. and Willis, W. D. (1998). Memantine alicnuaies responses of spinothalamic tract celks to cutaneous stimulation in neuropathic monkeys. Eur. J. Pain 2, 229-238. 

Yukioka H, Yoshimoto N, Nishimura K, Fujimori M (1985) Intravenous lidocaine as a suppressant of coughing during tracheal intubation, Anesth Analg 64 1189-1192 Zou X, Lin Q, Willis WD (2000) Enhanced phosphorylation of NMDA receptor 1 subunits in spinal cord dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats. J Neurosci 20 6989-6997... 

Davidson, S., X Zhang, C.H. Yoon, et al. 2007. The itch-producing agents histamine and cowhage activate separate populations of primate spinothalamic tract neurons. /. Neurosci. 27(37) 10007-10014. 

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