Physiology Of Pain PDF

Physiology Of Pain Dr. Mai Adawi Lecturer of physiology FOMSCU  Introduction  Pain is unpleasant sensation occurs whenever tissues are being damaged and causes the individual to react to remove the pain stimulus....

Physiology Of Pain Dr. Mai Adawi Lecturer of physiology FOMSCU  Introduction  Pain is unpleasant sensation occurs whenever tissues are being damaged and causes the individual to react to remove the pain stimulus. pain somatic visceral internal Deep superficial visceral somatic organs  1. Somatic pain: I. Cutaneous pain (superficial)  is felt in superficial structures, such as the skin and subcutaneous tissues. A pinprick is an example of cutaneous pain. It is a sharp pain with a burning quality that may be easily localized. II. Deep somatic pain  is generated in deep body structures, such as the periosteum, muscles, tendons, joints, and blood vessels.  This type of pain is more diffuse than cutaneous pain. It may be elicited by strong pressure, ischemia, and tissue damage. 2. Visceral pain  Visceral pain occurs in the organs and tissues of the thoracic and abdominal cavities. Types of pain: 1. Fast pain is felt within about 0.1 second after a pain stimulus is applied 2. slow pain begins only after 1 second or more and then increases slowly over many seconds and sometimes even minutes  Fast pain: is felt when a needle is stuck into the skin, when the skin is cut with a knife, or acutely burned. This type of pain is not felt in most of the deeper tissues of the body. Superficial pain may often have two components: an immediate, sharp, and highly localizable initial pain and, after a latency of about 1 second, a longer lasting and more diffuse delayed pain  Slow pain: is usually associated with tissue destruction. It can lead to prolonged, almost unbearable suffering. Slow pain can occur both in the skin and in almost any deep tissue or organ. Fast pain Slow pain Occurs first Occurs second and persist longer Sharp pricking sensation Dull aching, throbbing sensation; more unpleasant A-delta fibers C fibers Easily localized poorly localized mechanical and thermal types of all three types of stimuli stimuli Usually associated with tissue destruction. Types of pain receptors  Free nerve ending  These receptors can detect mechanical, thermal, or chemical changes  Non adapting: it allows the pain to keep the person apprised of a tissue-damaging stimulus as long as it persists. Types nerve fibers  Pain pathway  afferent nerve fibers are different: 1. Fast pain sensation is carried by A δ fibers (myelinated) 2. slow pain sensation is carried by C nerve fibers (unmyelinated) Pain pathway Painful stimulation releases substance P or glutamate from afferent fibers in the dorsal horn of the spinal cord. From there, signals are relayed to the somatosensory cortex  VISCERAL PAIN  Not all tissues evoke visceral pain (e.g., liver, lung parenchyma).  It is diffuse and poorly localized.  It often generates referred pain  It is typically accompanied by autonomic nervous system responses (e.g., nausea, vomiting, sweating, pallor, increased blood pressure). Causes Of Visceral Pain 1. Inflammation Mechanical receptors are activated by the tissue distension associated with inflammation. In addition, inflammatory mediators, such as histamine and bradykinin, may activate nociceptors. e.g. appendicitis 2. Ischemia Substances released during ischemic reactions such as lactic acid, bradykinin and proteolytic enzymes stimulate the pain receptors of viscera. 3. Chemical Stimuli Chemical substances like acidic gastric juice reach the esophagus through incontinent lower esophageal sphincter and produce pain. 4. Spasm and Over distention of Hollow Organs Spastic contraction of smooth muscles in gastrointestinal tract and other hollow organs of viscera cause pain by direct stimulating mechanical nociceptors. Muscle spasm also causes tissue ischemia.Over distention of hollow organs also causes pain with the same mechanism.  Referred Pain  is a term to describe the pain phenomenon that is perceived at a site adjacent to the site of injury  One of the best examples of referred pain is during an episode of ischemia brought on by a myocardial infarction (heart attack) in which pain is frequently experienced in the neck, left arm, shoulders, and back rather than in the chest, the original site of injury.  Mechanism Of Referred Pain  Convergence-projection theory: This states that the pain is caused by convergence of afferent information of the visceral organs and those of somatic origin on the same segment. This causes hyperreactivity of the dorsal horn neurons which is interpreted as coming from the same dermatome  Referred pain occurs because both visceral and somatic afferents often converge on the same neurons in the spinal cord kidney  Analgesic Pathway  Analgesic pathway that interferes with pain transmission is often considered as descending pain pathway, the ascending pain pathway being the afferent fibers that transmit pain sensation to the brain.  Analgesic Pathway  Role of Analgesic Pathway in Inhibiting Pain Transmission:  1. Fibers of analgesic pathway arise from frontal lobe of cerebral cortex and hypothalamus  2. These fibers terminate in the gray matter surround ing the third ventricle and aqueduct of Sylvius (periaqueductal gray matter)  3. Fibers from here descend down to brainstem and terminate on:  i. Nucleus raphe magnus, situated in reticular formation of lower pons and upper medulla  ii. Nucleus reticularis, paragigantocellularis situated in medulla  4. Fibers from these reticular nuclei descend through lateral white column of spinal cord and reach the synapses of the neurons in afferent pain pathway situated in anterior gray horn  Synapses of the afferent pain pathway are between:  i. Aδ type afferent fibers and neurons of marginal nucleus  ii. C type afferent fibers and neurons of substantia gelatinosa of Rolando.  5. At synaptic level, analgesic fibers release neuro-transmitters and inhibit the pain transmission before being relayed to brain.  Analgesic Pathway  Neurotransmitters of Analgesic Pathway:  Neurotransmitters released by the fibers of analgesic pathway are serotonin and opiate receptor substances namely enkephalin, dynorphin and endorphin.  GATE CONTROL THEORY  The pain stimuli transmitted by afferent pain fibers are blocked by gate mechanism  Mechanism of Gate Control at Spinal Level  1. When pain stimulus is applied on any part of body, besides pain receptors, the receptors of other sensations such as touch are also stimulated  2. When all these impulses reach the spinal cord through posterior nerve root, the fibers of touch sensation send collaterals to the neurons of pain pathway  GATE CONTROL THEORY  3. Impulses of touch sensation passing through these collaterals inhibit the release of glutamate and substance P from the pain fibers  4. This closes the gate and the pain transmission is blocked  GATE CONTROL THEORY  Role of Brain in Gate Control Mechanism :  1. If the gates in spinal cord are not closed, pain signals reach thalamus through lateral spinothalamic tract  2. These signals are processed in thalamus and sent to sensory cortex  3. Perception of pain occurs in cortical level in context of the person’s emotional status and previous experiences  4. The person responds to the pain based on the integration of all these information in the brain. Thus, the brain determines the severity and extent of pain.  GATE CONTROL THEORY  Role of Brain in Gate Control Mechanism :  5. To minimize the severity and extent of pain, brain sends message back to spinal cord to close the gate by releasing pain relievers such as opiate peptides  6. Now the pain stimulus is blocked and the person feels less pain.  Significance of Gate Control  Gating of pain at spinal level is similar to pre-synaptic inhibition. It forms the basis for relief of pain through rubbing, massage techniques, application of ice packs, acupuncture and electrical analgesia. All these techniques relieve pain by stimulating the release of endogenous pain relievers (opioid peptides), which close the gate and block the pain signals. Thank you