McKenna's Pharmacology for Nursing, 2e

C H A P T E R 2 6 Opioids, opioid antagonists and antimigraine agents 399

Pain receptors Opioid receptors are receptor sites that respond to naturally occurring peptides, the endorphins and the encephalins. These receptor sites are found in the CNS, on nerves in the periphery and on cells in the gastro- intestinal (GI) tract. In the brainstem, opioid receptors help to control blood pressure, pupil diameter, GI secre- tions and the chemoreceptor trigger zone (CTZ) that regulates nausea and vomiting, cough and respiration. In the spinal cord and thalamus, these receptors help to integrate and relate incoming information about pain. The endorphins and encephalins normally modulate the pain information coming into the brain. Endor- phins are released during stress to block the sensation of pain. Professional athletes may be injured during an important game and have no sensation of pain or injury because their stress reaction is highly activated, and the endorphins are blocking pain transmission into the brain. In the hypothalamus, stimulation of the opioid receptors may interrelate the endocrine and neural responses to pain. In the limbic system, the receptors

According to the gate control theory , the transmission of these impulses can be modulated or adjusted all along these tracts. All along the spinal cord, the interneurons can act as “gates” by blocking the ascending trans­ mission of pain impulses. It is thought that the gates can be closed by stimulation of the larger A fibres and by descending impulses coming down the spinal cord from higher levels in such areas as the cerebral cortex, the limbic system and the reticular activating system. The inhibitory influence of the higher brain centres on the transmission of pain impulses helps to explain much of the mystery associated with pain. Several factors, including learned experiences, cultural expecta- tions, individual tolerance and the placebo effect, can activate the descending inhibitory nerves from the upper central nervous system. These other factors need to be considered and incorporated into pain management strategies, which usually involve the use of drugs. For example, the placebo effect, stress reduction, acupunc- ture and back rubs (which stimulate the A fibres) can all play important roles in the effective management of pain.

Pain signals to brain

Hypothalamus, limbic system Thalamus

Reticular formation

Cerebral cortex

Response from brain

Cross-section of medulla

Spinothalamic tracts

Pain receptors in skin C fibre— unmyelinated, slow conducting A-delta fibre— small, myelinated acute pain

Spinal cord

Reflex withdrawal tract

Spinal nerve

FIGURE 26.1  Neural pathways of pain.