Pain messages are two-way traffic. Inhibitory effects are achieved through the descending pathways, which reach from the conscious brain down to the gates in the subconscious brain and the spinal cord. The reason for this is that the gates are places where the flow of pain messages can be controlled or influenced (Wells & Nown 1998). By sending responses back to the periphery, the brain can ordered the release of chemicals that have analgesic effects, which can reduces or inhibit pain sensation.
Pain generally starts with a physical event; a cut, burn, tear, or bump (Catalano, 1987). The sensation of pain usually depends on the activation of a set of neurons that includes primary afferent nociceptors, interneurons in the spinal cord, cells of the ascending tracts, thalamic neurons and neurons of the cerebral cortex. Hence, the pain system involves a set of ascending pathways that convey nociceptive information from peripheral nociceptors to higher levels of the central nervous system, as well as descending pathways that modulate that information (Bromm & Desmedth, 1995).
The term nociception refers to the process by which pain information is carried from the periphery sense receptors in the skin and in the viscera to the cerebral cortex through network of neuronal relays (Karoly & Jensen 1987). Exteroceptors on the body surface and propioceptors within the body are specialized neurons that receive stimulation; mechanical (e.g. pressure), chemical, electrical, or thermal (i.e. hot-cold sensitive).
The body is equipped with mechanical nociceptors at the periphery (so-called first-order neurons), which project to second-order neurons in the spinal cord and medulla, which then carries the sensory information (in the form of electrical impulse) to the thalamus, where it synapses with third-order neurons that transmit the impulse to the cortex.
Second-order neurons sends their sensory inputs to the thalamus via two ascending pathways: the dorsal column medial-lemniscal system and the anterolateral system (includes the spinothalamic, spinoreticular, and spinotectal fibers). The former transmits impulse involving position sense, touch, and pressure. The latter pathway is involved in pain transmission (Karoly & Jensen 1987).
The spinal cord is the central concourse along which all pain messages travels to and from the brain (Catalano, 1987). For example, when you stub your toe and your peripheral nerves register alarm, this acute pain is immediately relayed along the nerve fibers of your foot and leg to the substantia gelatinosa located within the dorsal horn of the spinal cord. The cells in the substantia gelatinosa relay this "fast pain" message along the neospinothalamic and terminating in the thalamus and the cortex (see diagram 4) (Catalano, 1987). The cortex is the region in which thoughts are processed.
In contrast, chronic pain moves along a different and slower tract, called the paleospinothalamic tract. This "slow pain" is generally dull, aching, burning, and cramping (Catalano, 1987). Slow pain follows the same path as the fast pain through the spinal cord, but once in the brain, it separates and terminate in the hypothalamus and the limbic structures (Catalano, 1987). The hypothalamus is responsible for stimulating the release of stress hormones. The limbic structures are the places where emotions are processed.
Just as there is an ascending pain pathway from the body to the brain, there is a descending pathway that allows the brain to modulate pain sensory. The brain uses this pathway to send chemical substances and nerve impulses back down to the cells in the spinal cord to act against the pain message sent up by the pain receptors. Hence, the primarily role of the descending pathway is to send chemical messages from the brain to close the gates in the spinal cord to ascending messages (Catalano, 1987, Wells & Nown, 1998).
Descending inhibitory processes are of great interest in the research arena. Hence, it has been extensively studied by scientists. For instance, descending inhibitory processes have been investigated in anesthetized animals (Zimmerman 1984). It was found that the firing of dorsal horn neurons in response to noxious skin heating can be inhibited by stimulation in the periaqueductal gray (PAG) and the lateral reticular formation (LRF) in the midbrain. In addition, inhibition of the spinal cord neurons can also be achieved by electrical stimulation in other regions of the brain, such as the raphe nuclei, the locus coeruleus, and various regions of the medullary reticular formation (for review, see Willis 1982; Mokha 1983; Morton et al. 1983; Gebhart et al. 1984), as well as sites in the hypothalamus, septum, orbital cortex, and sensorimotor cortex (Zimmerman 1984). At the present it is not clear to what extent these different descending systems cooperate and interact, what their normal physiological functions are, and how they can be activated other than by focal electrical stimulation.
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Emotion dimension of Pain