Pain

 

Pain is a protective mechanism. Pain receptors in the skin are free nerve endings. There are two qualities

  • Fast pain -Fast pain is felt at about 0.1 seconds after pain stimulus. Fast pain may also be called pricking, acute, and electric pain. Neospinothalamic tract is for fast pain.

  • Slow pain- Slow pain is felt about 1 second or more after pain stimulus Slow pain may be called slow burning, aching, throbbing, nauseous and chronic pain. It is usually associated with tissue destruction and can occur in skin and almost any deep tissue or even organs. Paleospinothalamic tract carries slow pain

Three types of stimuli excite pain receptors-mechanical, thermal and chemical. Fast pain can be elicited mechanical and thermal. Slow pain can be elicited by the three. Chemicals agents are as follows bradykinin, serotonin, histamine, potassium ions, acids, acetylcholine, proteolytic enzymes. Also prostaglandins and substance P enhance sensitivity of pain endings but do not excite them directly.

Pain receptors are free nerve endings but have two pathways within the central nervous system called slow chronic pain pathway and fast-sharp pain pathway. The fast type pain fibers are type Aδ fibers which terminate in lamina I called lamina marginalis of the dorsal horn of spinal cord. They stimulate second order neurons in the neospinothalamic tract called lateral spinothalamic tract of the lateral funiculus of white matter.  They will reach the ventral posterolateral nucleus of the thalamus. The second type of pain fibers are the slow pain fibers called C pain fibers. They terminate in Rexed laminae II and III of the dorsal horn of spinal cord called the substantia gelatinosa. They also end in lamina V. They run in the paleospinothalamic pathway which are part of protopathic sensation. Their second order neurons enter the reticular formation in the pons and medulla  running in the fasciculus gracilis and fasciculus cuneatus, which terminate in the nucleus gracilis and nucleus cuneatus. Some reach the midbrain in the periaqueductal gray region. From this region fibers project into the thalamus through mainly ventral posteromedial (VPM) nucleus. From the thalamus 3rd or 4th order neurons project into sensory cortex through the thalamic radiation or thalamoneocortical projections. Awareness of pain comes from projection into the neocortex and this can be blocked by the so called dissociation anesthesia as in ketamine hydrochloride.

Analgesia system in brain and spinal cord include the following

  1. Periaqueductal gray and periventricular areas of mesencephalon and upper pons
  2. Raphe magnus nucleus
  3. Pain inhibitory complex at the dorsal horn of spinal cord

Other areas include

  1. Periventricular nuclei in the hypothalamus
  2. Medial forebrain bundle also in the hypothalamus
  3. Enkephalin and serotonin are mainly involved in analgesic system as neurotransmitters.

Brainís opiate system

Endorphins and enkephalins

Opiaite peptides are derived from breakdown products of the large protein- propopiomelanocortin, proenkephalin and prodynorphin. Most important of the opiate peptide are β-endophin, met-enkephalin, leu-enkephalin and dynorphin.

 
Brainstem Thalamus Forebrain Medulla Midbrain Pons
Temporal lobe Basal ganglia Epithalamus Cerebral operculum Remove operculum Wall of lateral ventricle 1
Wall of lateral ventricle 2 Wall of lateral ventricle 3 Lateral ventricle 1 Lateral ventricle 2 Base Rhomboid fossa

 

Neuroanatomy of pain
Neurophysiology of pain
Neurochemistry of pain
Neuropathology of pain
Neuropharmacology of pain
Neurology of pain
Neurosurgery of pain

 

 
Gating theory of pain

 

 

 

 


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