How does the pain signal reach the brain?
Following the excitation of pain receptors, the message to the brain is sent via one of two pathways. The fast pathway is characterized by myelinated, thin fibers that send messages quickly, giving a sensation of bright pain. The slow pathway is characterized by unmyelinated fibers that send messages much slower, generating a nagging pain sensation.
The fast pathway allows the organism to directly detect where the injury is located, the severity of the injury and for how long the pain has been present. In this sense, the fast pathway is also a warning system, reacting in immediate terms. This system connects to the thalamus, making synaptic connections via A-delta fibers that in turn project onto the motor and sensory areas of the cortex. A-delta fibers are intermediate sized, myelinated sensory nerve fibers that transmit pain and temperature signals. These synapses with the myelination of its neurons explain the quick speed of transmission.
The slow pathway is a reminder system where the brain stays aware of a previous injury, provides continuous recognition or the presence of pain and thus restricts normal activity. Emotional pain is also attributed to the slow pathway because the injured person associates qualities to the painful sensations due to the more continuous, nagging aspect of the pain. The limbic system and prefrontal cortex are believed to mediate this emotional aspect of pain sensation.
Small non-myelinated C-fibers detect tissue damage due to a drop in pH and a release of chemicals (such as histamines and bradykin). C-fibers mainly respond to chemical sensors, as well as pressure and temperature. Peripheral sensitization causes serotonin, prostaglandins, thromboxane and leucotrienes in the damaged tissue to signal for up-regulation of C-fibers. C-fibers once activated then generate an electrical impulse. These electrical impulses travel along the nerve of the dorsal horn of the spinal cord. Once the dorsal horn is activated in the process of pain sensitization, it is called central sensitization, whereas previously the sensitization was only peripheral. Both mechanisms are present with chronic pain.
Role of the Dorsal Horn
Substance P is released in the dorsal horn which is located in the central nervous system is a regulator of different functions including mood, anxiety/stress, respiratory rhythm and pain among others. Substance P channels pain impulses from the peripheral receptors to the central nervous system. In the dorsal horn C-fibers also release other excitatory neurotransmitters. The dorsal horn, put simply is where fibers from the slow pathway converge. Spinal neuron discharge from the dorsal horn can be 10 times as strong as that of a single pain receptor and high neuronal activity can occur 100 seconds after removal of the pain inducing stimulus. Neurons released from the dorsal horn continue to the brain where the sensation is recognized in multiple areas. The limbic region and prefrontal cortex are believed to mediate this emotional aspect of pain sensation. Neural connection occurs in the slow pathway to the hypothalamus and amygdala as well as diffusing to other parts of the brain. View here for an extensive list defining all the parts of the brain to which the slow pathway connects. The below diagram illustrates the relation of many of the affected regions.
Reticular Formation- Network of neurons on the brain stem involved in consciousness,
breathing regulation, sensory stimuli transmission to higher brain centers and
constant shifting of muscular activity.
Medulla- A relay station in the crossing of motor tracts between the spinal cord and the
brain. The medulla also contains respiratory, vasomotor and cardiac centers and control of reflex activities.
Pons- Bridge-like structure linking parts of the brain, and also serving as a relay station
from the medulla to higher cortical structures of the brain. The pons also contains the respiratory system.
Midbrain- Nerve pathway of the cerebral hemisphere, containing auditory and visual
Hypothalamus- Located between the thalamus and the midbrain, the hypothalamus functions as a control center for the autonomic nervous system. Regulation of sleep cycles, body temperature and appetite are all controlled here.
Thalamus- Translator of inputs processed into a form that is readable by the cerebral cortex. Process and relays sensory information.