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As undesirable as it might seem, PAIN is actually a very important defense mechanism. It WARNS the body about potential or actual injuries or diseases, so that protective actions can be taken. Basically, noxious signals send impulses to the spinal cord, which relays the information to the brain. The brain interprets the information as pain, localizes it, and sends back instructions for the body to react.
Pain sensation is mediated by pain receptors, or nociceptors, which are present in the skin, superficial tissues and virtually all organs, except for the brain. These receptors are essentially the nerve endings of so-called “first-order neurons” in the pain pathway. The axons of these neurons can be myelinated, A type, or, unmyelinated, C type. Myelinated A fibers conduct at FAST speeds and are responsible for the initial SHARP pain perceived at the time of injury. Unmyelinated C fibers conduct at SLOWER speeds and are responsible for a longer-lasting, dull, diffusing pain.
First-order neurons travel by way of spinal nerves to the spinal cord, where they synapse with second-order neurons in the dorsal horn. These second-order neurons cross over to the OTHER side of the cord, before ascending to the brain. This is how information of pain on the left side of the body is transmitted to the right side of the brain, and vice versa.
There are two major pathways that carry pain signals from the spinal cord to the brain:
– The spinothalamic tract: second-order neurons travel up within the spinothalamic tract to the thalamus where they synapse with third-order neurons; third-order neurons then project to their designated locations in the somatosensory cortex. This pathway is involved in LOCALIZATION of pain.
– The spinoreticular tract: second-order neurons ascend to the reticular formation of the brainstem, before running up to the thalamus, hypothalamus, and the cortex. This tract is responsible for the EMOTIONAL aspect of pain.
Pain signals from the face follow a DIFFERENT route to the thalamus. First-order neurons travel mainly via the trigeminal nerve to the brainstem, where they synapse with second-order neurons, which ascend to the thalamus.
Pain from the skin, muscles and joints is called SOMATIC pain, while pain from INTERNAL organs is known as VISCERAL pain. Visceral pain is often perceived at a DIFFERENT location in a phenomenon known as REFERRED pain. For example, pain from a heart attack may be felt in the left shoulder, arm or back, rather than in the chest, where the heart is located. This happens because of the CONVERGENCE of pain pathways at the spinal cord level. In this example, spinal segments T1 to T5 receive pain signals from the heart as well as the shoulders and arms, and the brain canNOT tell them apart. Because the superficial tissues have MORE pain receptors and are MORE often injured, it’s common for the brain to make an assumption that the pain comes from the shoulder or arm instead of the heart.
