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SUBJECT: PHYSIOLOGY TOPIC: Neurophysiology of Somatic Sensations LECTURER: DR. SIMBULAN DATE: FEBRUARY 2011
Discussion Outline: i. Overview of somatic sensations ii. Cutaneous Sensory Physiology a. Skin Mechanoreceptors b. Thermoreceptors iii. Proprioceptors also are somatosensory (will be discussed during the Reflex Lecture) a. Muscle spindle b. Golgi tendon organs *these two are the most studied proprioceptors, though they are also somatosensory receptors *There are also other proprioceptors in skin and joint/muscle, but we will not be dealing with them much anymore iv. Cutaneous & Visceral Pain Receptors (Nociceptors & Intro to Pain Physiology) Pain receptors and various ascending fibers which may take the fast pain / slow pain, and recent advances in introductory pain physiology
phantom limb/phantom pain and peripheral and central mechanisms theories to explain this phenomenon itch vs. tickle vs. pain
Review: Dorsal Root Ganglion (DRG) 1st order neuron of the somatosensory system, whether it is for crude touch, pressure, pain, temperature, proprioception, fine touch, 2 point determination) If from face region, what is the 1st order sensory neuron? 2 general classes of DRG neurons (innervate various somatosensory receptors): 1. Large DRG neurons Large soma, large axon (from single branching from soma, divides into 2 processes) o Peripheral innervates the sensory receptors o Central enter the dorsal horn (posterior gray horn) of spinal cord they are myelinated (not all myelinated are large, refer to Type A delta fibers described below) Ex. Neurons that give rise to Type A beta, Type A delta, and Type A gamma fibers Sensory modalities involved: Proprioception, Fine Touch, Pacinian Corpuscles, etc. 2. Small DRG Neurons Neurons that give rise to: o C Fibers o Type A delta fibers (myelinated) = if compared with other Type A afferent fibers, A delta are smaller Sensory Modalities involved: Most from Anterolateral system (but not all, only majority) some experimental literatures state that fine touch afferents send some collateral to ALS; also, pain too sends some collaterals to the medial lemniscal pathway. * keyword: somePage 1
Two Well Studied Endogenous Pain Control Systems: 1. Gate control system by Dr. Walden Melzack discovered in 1960s - know the lateral inhibition mechanism behind this GCC/spinal gating - you may read more about this in your book 2. Descending pain inhibition system (late 19701980s) **these two are briefly described in handout diagram Objectives: Know the features & functions of receptors Know how adequate stimuli is converted to changes membrane conductance (receptor potential) Know the size of receptive field (small/large; contribution in characterizing sensory stimuli Know how increasing stimulus intensity is expressed in terms of increasing spike discharges Know the Central & Peripheral Mechanisms of pain and analgesia distinguish fast pain, slow pain, referred pain, projected pain gating theory, opiod and non-opiod mechanism of endogenous pain control short definitions: hyperalgesia, allodynia, muscle radiating spasm, congenital insensitivity to pain,PHYSIOLOGY: Neurophysiology of Somatic Sensations
Fast Pain = generally part of small DRG, with Type A delta fibers, myelinated but smaller in diameter (smaller, as compared with large DRG axons) Slow pain = C fibers innervated Crude Touch = difficulty to localize; innervated by C fibers Fine Touch = uses vibratory sensors, those specialized for precise stimulus localization such as Merkels disks, Meissners, Ruffinis, Pacinian, Hair Cells *however based on actual experimental observation (ex: histological), fine touch modalities are discovered to have some Type A delta fibers (but still, mostly have larger diameters) *thermal receptors = predominantly a delta and c fibers Dr. Simbulan: this can be confusing in terms of innervation. Generally, small myelinated and unmyelinated C fibers predominate nociceptors; proprioception generally A beta and A alpha Still Confused? Just remember this: Proprioceptors: o large diameter, myelinated o A, A Crude touch, pain, temperature: o small, unmyelinated o Group 4 or Type C fibers Fine touch, vibration, pressure: o large, myelinated o Group 2, or the rest of Type A A few Thermoreceptors: o A, C *somatic motor neurons (ventral horn): o A Diameter (m) 12-20 5-12 3-6 2-5 Severely damaged Area 312 = contralateral loss of sensation; however, based on experiments, actually: tactile sensations all fine tactile is generally abolished (esp. at contralateral side); crude touch is recovered after a period of time Pain, temperature left generally intact
A A A A B C Proprioception Fine touch, Pressure Muscle Spindles Pain, Cold, crude touch Preganglionic Autonomic Pain, Temperature, etc.
Hence, we can conclude that: in fine touch: cortex is very important in pain, temp: awareness center is below cortex (can even be at thalamus), though there are no definite studies as to where it is exactly. It is just somewhere below the cortex. The role of cortex perhaps is mainly on the localization of the pain or temperature stimulus. > Lesions to Area 5 AND 7 = Sensory Association Cortex Lesion) = astereognosis; inability to identify an object if touched when blindfolded
PHYSIOLOGY: Neurophysiology of Somatic Sensations
Somatic Senses Pathways (ALS or Medial Lemniscus) Ventrobasal Nuclei (VPM if from head, neck) (VPL if from rest of the body) Area 312 (Anterior Parietal Cortex) Sensory Association Cortex (Posterior Parietal Cortex) - helps identify objects only by touching via higher order processing (stereognosis) - makes holistic image of the object II. Sensory Receptors of Skin Cutaneous Receptors can be found in both hairy/hairless skin studied via microneurography (thus, it was studied in isolated nerves from animals/humans, without actually exposing the nerves. They just stimulate nerve ending above skin with some pressure stimuli (or other stimuli). Microelectrodes are inserted to actually impinge on the isolated nerve itself. Action potentialns are recorded. It is with this technique that we were able to identify the thresholds for various mechanoreceptors, as well as their receptive fields, and whether the receptors are slowly or rapidly adapting. Receptor potentials as stimulus intensity is increased (ex: from 0.1 to 0.6 Newton) = increasing amplitude. When the action potentials were recorded from the axon itself where checked, they have increasing discharge frequency In short: Increase stimulus intensity, increase amplitude of receptor potential, increase discharge frequency of sensory nerve.
Scientists were also able to identify via microneurography whether the receptors have small or large receptive fields. Which size of receptive field gives more exact information about the location of the stimulus? Small!Naming Convention for Mechanoreceptors Tactile Receptor 1. 2. 3. 4. 5. Pacinian Corpuscle Meissners Corpuscle Merkels Disk Hair Follicle Endings Ruffinis Endings Adaptation Fast Fast Slow Fast Slow Receptive Field large small small small large Code FA-II FA-I SA-I FA-I SA-II
FA = Fast Adapting SA = Slowly Adapting I = small receptive field II = large receptive field >>>>>Slowly vs. Fast Adapting Receptors - see last page
(want tabular form? See final page) 1. Pacinian Corpuscle (FA-II) Rapidly adapting Studied well via microneurographic techniques Onion-like lamellar capsule (about 1mm in diameter) Speed of Adaptation: very rapid Receptive Field: Large (hence, not accurate in localizing stimulus source) Encoded Sensation: touch, vibration (acceleration detector) Afferents, location: generally A (some A); skin A = large, myelinated, group 2Increase in stimulus intensity (skin deformation is the stimulus)
Stimulus intensity maintained constant. There is only discharge only when there is increase in intensity. After that, there is no more response
PHYSIOLOGY: Neurophysiology of Somatic Sensations
Pacinian Corpuscles only become really active (with higher discharge frequency) when stimulus frequency is increased within this frequency range: 200-300 Hz. (Create stimulus within this range, and this is the only time the sensory nerve will have higher discharge frequency) 2. Meissners Corpuscle (FA-I) Rapidly adapting Studied well via microneurographic techniques Speed of Adaptation: very rapid Receptive Field: small (hence, accurate in localizing stimulus source) Encoded Sensation: speed of stimulus application, low freq vibration sensors (like pacinian, though meissners have lower sensibility: 30-40Hz) Afferents, location: A (group2); fingertips, lips, skin, even at sexual erectile tissues, glans penis, etc.
Speed of Adaptation: slow Receptive Field: small (hence, accurate in localizing stimulus source) Encoded Sensation: localization of stimulus (touch pressure) Afferents, location: A (group2); high density in fingertips, lips, many skin areas
5. Hair Follicle Endings Rapidly adapting Studied well via microneurographic techniques Practical example: when you wear your clothes, at first you feel them on you, but as time passes, you wont even feel them (you wont be constantly conscious of it) Of course, when you move, you stimulate these hair endings a bit to produce some spike discharges so you can feel it as you move Speed of Adaptation: rapid Receptive Field: small (hence, accurate in localizing stimulus source) Encoded Sensation: light touch flutter; low vibration sensors (30-40 Hz, like Meissners) Afferents, location: A (group2) 6. 7. Tactile Discs Other Cutaneous Receptors
Ruffini Endings (SA-II) Slowly adapting Studied well via microneurographic techniques Speed of Adaptation: slow Receptive Field: large (henc