Intro | A-Delta Fiber | C Fiber | Somatosensory Cortex | Midbrain | Midline Nucleus of the Thalamus | Ventrobasal Nucleus of the Thalamus | Descending Neural Pathway | Spinal Cord | Thalamus
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Neurons within the Midbrain project down to the pain fibers located in the spinal cord via the Descending Neural Pathway. This information is able to inhibit the transmission of pain information to somatosensory cortex, and thereby prevent the perception of pain.
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The earliest support of the descending neural pathway came from effects of stimulating a region in the midbrain called the periaqueductal gray (PAG) area. When this region is stimulated, the perception of pain is blocked. The next line of supportive evidence came with the discovery of receptors in PAG that respond to opiate-like, analgesic substances such as morphine. The third line of supportive evidence emerged with the isolation of naturally occurring, opiate-like analgesic substances called endorphins. These opiate-like endorphins block the perception of pain. Evidence has now suggested that the projections of endorphin neurons in PAG excite the serotonin neurons of the raphe' nucleus, located at the base of the medulla. The raphe' nucleus in turn projects downward to the dorsal columns of the spinal cord where the blocking of pain-transmission occurs. Stimulation of electrodes implanted in the periaqueductal gray has provided relief to people suffering from severe pain.
The endorphin neurotransmitters are composed of two categories of molecules. Beta-endorphin and dymorphin are produced by the pituitary gland, and are released into the blood stream as hormones. These same two substances act as neurotransmitters. Met-enkephalin and leu-enkephalin are also endorphins. These two small peptides are dissimilar to the opiates in structure, but as neurotransmitters they stimulate the same pain inhibiting neurons.
The Spinal Cord contains the pain pathways that project from free nerve ending in the skin to the thalamus and somatosensory cortex. The spinal cord is also the location where signals descending from the midbrain above, are able to inhibit the ascending transmission of pain information.