The Ohio State University
Proprioception is the sense of the relative position of a body part. Proprioceptors are located in the striated muscle and joints though out the body. The term was coined as early as 1557 by Julius Caesar Scalinger when studying the position movement of locomotion, but was redefined by Henry Charlton Bastian (1880) and the term kineaesthesia arose. Bastian studied the afferent pathway between the brain and the tendons, muscles and the skin. In 1906, Charles Sherrington believed that the muscle spindles played a role in proprioception, transmitting information of size of muscle length and speed, which ultimately sensed limb position and movement and sent the impulse to the brain. Phantom pain exists in eighty percent of all limbs amputated. It is believed that the afferent pathways that the proprioceptors inhibit are misfiring; creating a pain for up to five years after the limb is amputated. Though the brain contains billions of neurons, it does not contain any receptors for pain. For instance, when you have a migraine, the pain one feels is the blood vessels that supply blood to the brain, not the specific neurons that make up the brain. In reality, when the blood vessels dilate in an abnormal way, the pain receptors in the walls are translated into pain impulses, which are then perceived as pain by your brain. Pain is viewed by individuals and physicians as the transfer of noxious stimuli from the particular damaged tissue (the peripheral receptors) to the sensory cortex via specific pathways in the central and peripheral nervous system. Nociceptors have no specialized structure. They have free nerve endings that form dense networks with multiple branches that are regarded a nociceptors, that is sensory receptors for pain. These receptors respond only when there is a stimulus strong enough to threaten the body's integrity.
There are various types of nerve fibers (axons) whose free nerve endings form nociceptors. These structures connect the peripheral organ to the spinal cord, but can differ greatly both in the diameter and the thickness of the surrounding myelin sheath, which determines the conduction of the impulse. Nociceptors fibers can either be A-Delta or C. A-Delta fibers carry mechanical and thermal information. They are slightly mylinated, 1-5 micrometers in diameter, and has a conduction speed of 5-40 millisecond. C-fibers also transmit mechanical, chemical and thermal information. They are unmylinated bigger in diameter, and render a slower conduction speed of 0.5-2millisecond. The difference between the speeds of the A-Delta and C fibers explains why one feels a sharp specific pain that ultimately gives later to a more diffuse dull pain. This time lag attributes to the conduction speeds of the nerves fibers. Sharp, fast pain, which goes away quickly comes from the stimulation over the A-Delta fibers, where as the slow pain that persists longer are the C- fibers. C- fibers account for 70 percent of all nociceptors. There are two types of pain control pathways, the ascending and descending. Most pain pathways start from the affected or injured area and end in the somatosensory cortex in the brain. Each consists of a chain of 3 neurons that pass impulses to the brain. The difference in conduction appears where the neurons cross in the spinal cord. The neurons from the left side of the body terminate in the right hemisphere of the brain. Regardless of the modality (touch pain, thermal regulation and priopoceptiom) the first neuron resides in the spinal (dorsal root) ganglion. This neuron, which is T-shaped, and immediately divides into two branches of the ascending pathways, which conduct different nerve impulses. The Leminiscal Pathway is responsible for touch and priopoception. The first axon in this
pathway runs along the dorsal root of the spinal nerve and up the dorsal column of the