Sensory Nervous System Week 11 Dr. Walid Daoud A. Professor

Pain Sensation It is a protective mechanism for the body. It is produced when there is any tissue damage. It initiates reflexes which aim to remove the painful stimulus.

Pain Receptors They are free nerve endings of 3 types: 1- Mechanosensitive pain receptors: Stimulated by mechanical changes to Stimulated by mechanical changes to tissues. tissues. 2- Thermosensitive pain receptors: Stimulated by extremes of heat or cold. Stimulated by extremes of heat or cold. 3- Chemosensitive pain receptors: Stimulated by injurious chemical substances Stimulated by injurious chemical substances

Types of Pain Sensation A- Cutaneous Pain: Slow chronic pain Fast acute pain By all types of receptors Felt after 1 sec or more Carried by C fibers In skin and deep tissues Dullaching burning throbbing Poorly localized Elicited by mechanical or thermal receptors 0.1 sec in Felt Carried by A gama fiber Not felt in deep tissues Sharp picking pain Well localized

B- Deep Pain It is produced in muscles, tendons, ligaments, joints and periosteum of bones. It is conducted along C fibers. Causes: 1- Inflammation in deep structures. 2- Ischemia. 3- Muscle spasm.

C- Visceral (referred) Pain Most viscera contain only pain receptors. It is transmitted along C fibers. Causes: 1- Ischemia: accumulation of metabolites 2- Spasm of hollow viscus. 3- Overdistension of hollow viscus. 4- Infalmmation of peritoneal covering. 5- Chemical irritation as perforated peptic ulcer. Characters:1- Dull aching. 2- Accompanied by nausea and vomiting 2- Accompanied by nausea and vomiting 3- Usually referred to surface area. 3- Usually referred to surface area.

Referred Pain Pain is not felt in the diseased viscus but in the body surface somatic structures supplied by the same posterior roots as the diseased viscus. 1-Cardiac pain: ischemia of cardiac muscle pain is referred to base of neck or left arm pain is referred to base of neck or left arm 2-Gastric pain: referred to body surface between umbilicus & xiphoid process. between umbilicus & xiphoid process. 3-Gall bladder pain: felt in mid-epigastrium and tip of right scapula. of right scapula. 4-Appendicitis pain: felt around the umbilicus.

Mechanism of Referred Pain Convergence-projection theory: Pain impulses from diseased viscus converge on the same cells in the SGR in spinal cord which receive nerve impulses from a particular skin dermatome that activate the same cortical neuron. Sensory area in cerebral cortex is accustomed to receive pain sensation from skin so pain impulses from viscera are projected to skin are of the same dermatomal nerve supply.

Pathway of Pain Sensation 1- Neospinothalamic tract. 2- Paleospinothalamic tract.

Pain Control A-Pain Control System: 1-Analgesia System: supraspinal level of pain inhibition: supraspinal level of pain inhibition: - -B endorphin from hypothalamus or pituitary - -Enkephalins from periaqueductal grey area --- Fibers of raphe magnus nucleus secrtete serotonin –Serotonin stimulates pain inhibitory complex area in dorsal horn of spinal cord that secretes enkephalins causing presynaptic inhibition by Ca channel blocking preventing release of substance P

Pain Control 2- Gate inhibition theory: Spinal level of pain inhibition.SGR cells in layers II & III act as gate for pain impulses to reach lateral spinothalamic tract. It can be closed by: A-Impulses from: 1- A beta fibers: rubbing of skin inhibits pain. 1- A beta fibers: rubbing of skin inhibits pain. 2- A gama fibers: counter irritants inhibit pain. 2- A gama fibers: counter irritants inhibit pain. B-Endogenous opioid peptides: 1- Secreted from interneurons. 1- Secreted from interneurons. 2- Circulating endorphins. 2- Circulating endorphins.

Pain Control B- Medical treatment: 1- Using painkillers. 1- Using painkillers. 2- Treating the cause of pain. 2- Treating the cause of pain. C- Surgical treatment of pain inhibition: 1- Antelateral cordotomy to cut 1- Antelateral cordotomy to cut spinothalamic tract. spinothalamic tract. 2- Gyrectomy in frontal lobe. 2- Gyrectomy in frontal lobe. D- Electric stimulation for pain inhibition: 1- Electric stimulation for analgesia system 1- Electric stimulation for analgesia system 2- Electric stimulation of large sensory fibers 2- Electric stimulation of large sensory fibers

Headache A type of referred pain to the surface of the head from deep structures. Causes of intracranial headache: 1- Meningitis. 2- Meningeal trauma. 3- Brain tumors. 4- Migraine headache. 5- Low cerebrospinal fluid. 6- Constipation headache.

Headache Headache of extracranial origin: 1- Muscular spasm. 2- Inflammation of nasal sinuses. 3- Errors of eye refraction. 4- Tooth ache. 5- Otitis media.

Sensory Areas of Cerebral Cortex Somatic Sensory Area I: Lies in postcentral gyrus of cerebral cortex Receives impulses from posteroventral nucleus of thalamus. Characterized by: -Crossed representation. -Inverted body presentation. -Size of represented area is directly proportional to number of receptors in this proportional to number of receptors in this area of the body. area of the body.

Functions of Somatic Sensory Area I It is the center of: 1- Fine touch. 2- Discrimination of weights. 3- Vibration sense. 4- Sense of position and movements of joints. 5- Discrimination of various grades of temperature. temperature.

Somatic Sensory Area II Lies posterior and inferior to lower end of postcentral gyrus. Receives information from dorsal column, spinothalamic tract, visual, auditory and somatic sensory area I. Functions: It begins to make meaning of sensory signals.

Somatic Association Area Lies behind the somatic sensory area I and above somatic sensory area II. Receives impulses fromsomatic sensory area I and II, from posteroventral nucleus of thalamus. Functions: It combines information fromsomatic sensory areas & interpret this information to have a meaning.