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Pain receptors or nociceptors are free sensory nerve endings that are present in most tissues of the body (Fig.Two types of afferent fibers conduct pain impulses: The myelinated A delta fibers that transmit impulses very rapidly and the unmyelinated C fibers that transmit impulses slowly, Acute pain-the sudden, sharp, localized pain related to thermal and physical stimuli primarily from skin and mucous membranes-is transmitted by the A delta fibers, whereas chronic pain-often experienced as a diffuse, dull, burning or aching sensation --is transmitted by C fibers.Thermal means refer to extremes of temperature, mechanical means could refer to pressure, and chemical sources could include acids or compounds produced in the body, such as bradykinin, histamine, or prostaglandin. 4-1).

Pain receptors or nociceptors are free sensory nerve endings that are present in most tissues of the body (Fig. 4-1). These sensory nerves may be stimulated by thermal, chemical, or physical means. Thermal means refer to extremes of temperature, mechanical means could refer to pressure, and chemical sources could include acids or compounds produced in the body, such as bradykinin, histamine, or prostaglandin.
The pain tareshold refers to the level of stimulation required to activate the nerve ending sufficiently for the individual to perceive pain. The associated nerve fibers then transmit the pain signal to the spinal cord and brain. The pain threshold is relatively constant over time and between individuals. The ability to withstand pain or the perception of its intensity is referred to as pain tolerance; this varies considerably with past pain experience and overall state of health.
Two types of afferent fibers conduct pain impulses: The myelinated A delta fibers that transmit impulses very rapidly and the unmyelinated C fibers that transmit impulses slowly, Acute pain-the sudden, sharp, localized pain related to thermal and physical stimuli primarily from skin and mucous membranes-is transmitted by the A delta fibers, whereas chronic pain-often experienced as a diffuse, dull, burning or aching sensation
—is transmitted by C fibers. C fibers recelve thermal, physical, and chemical stimull from muscle, tendons, the myocardium, and the digestive tract as well as from the skin. The peripheral nerves transmit the afferent pain impulse to the dorsal root ganglia and then into the spinal cord through the dorsal horn or substantia gelatinosa (see Chapter 14).
Each spinal nerve conducts impulses from a specific area of the skin called a dermatome (see Fig. 14-22, which illustrates the areas of the skin innervated by each spinal nerve), and the somatosensory cortex is "mapped" to correspond to areas of the body so that the source of the pain can be interpreted In the brain (see Fig. 14-3 for a map of the brain). The dermatomes can be used to test for areas of sensory loss or pain sensation and thus determine the site of damage after spinal cord injuries.
At the spinal cord synapse, a reflex response to sudden paln results in a motor, or efferent, impulse back to the muscles that initiates an involuntary muscle contraction to move the body away from the source of pain. After the sensory Impulse reaches the synapse, connecting neurons also transmit it across the spinal cord to the ascending tracts to the brain. There are two types of tracts in the spinothalamic bundle: The fast Impulses for acute sharp pain travel in the neospinotbalamic tract, whereas the slower impulses for chronic or dull pain use the paleaspinothalamic tract. This double pathway explains the two stages of pain one often experiences with an Injury to the skin, the initial sharp severe pain, followed by a duller but persistent throbbing or aching pain. These tracts connect with the reticular formation in the brain stem, hypothalamus, thalamus, and other structures as they ascend to the somatic sensory area in the cerebral cortex of the parietal lobe of the brain. It is here that the location and characteristics of the pain are perceived. The many branching connections from the ascending tracts provide information to other parts of the brain, forming the basis for an integrated response to pain.