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1 SYSTEMS NEUROBIOLOGY COURSE SOMATOSENSORY SYSTEM LECTURE – FEB. 21, 2002 Areti Tsiola ([email protected]) OVERVIEW OF NERVOUS SYSTEM: CNS – brain, spinal cord Autonomic – functionally and anatomically distinct and overlapping to different extents Sympathetic Parasympathetic Enteric PNS – (cranial and spinal) nerves and corresponding ganglia NS & communication with the outside world. Today’s topic: somatosensory system. Skin = largest organ of the human body. Modalities of somatic sensibility: 1. Discriminative touch (// size, shape, texture of objects, movement across skin) 2. Temperature sense 3. Nociception (// perception of noxious stimuli – pain or itch) 4. Proprioception (// static position and movement of body) Each modality has (morphologically and functionally) distinct Rs (neurons) and pathways. SENSORY NEURONS FAITHFULLY ENCODE STIMULI (Fig 21-8) For cranial structures (sensory part of) trigeminal neurons (# V) For rest of body DRG neurons DEFINE GANGLIA DRAW CROSS SECTION OF SPINAL CORD – point out dorsal and ventral horns (white, gray matter) DRG neurons: soma in ganglia Axon w/ 2 branches to CNS (spinal cord) to periphery: (specialized) nerve terminal (sensory fn) 1o afferent fiber (signal transmission) Peripheral terminals of DRG neurons: bare endings: pain (nociceptors) & temperature sensations. Small diameter, unmyelinated or thinly myelinated, slowly conducting axons. encapsulated: touch (mechanoRs) & proprioception. Large diameter, myelinated axons, fast conducting. I. TOUCH – MECHANO-Rs. Greatest sensitivity on hairless skin. High density of mechano-Rs in ridges/ papillae, which detect changes in contour of skin. 2 TYPES OF MECHANO-Rs ON HAIRY SKIN (Fig 22-2) 2 All rapidly adapting. Hair follicle R – respond to hair displacement Field R – primarily over joints, sense skin stretch Merkel Rs Bare nerve endings 4 TYPES OF MECHANO-Rs ON GLABROUS SKIN (Fig 22-2) In superficial skin layers: 1. Meissner’s corpuscle Rapidly adapting. Fine mechanical sensitivity due to mechanical coupling to papillary ridge. R = fluid-filled structure w/ flattened epithelial cells. 2. Merkel disk R Slowly adapting. Semi-rigid structure transmitting strain from skin to nerve ending. Found in clusters. Flat surface – continuous firing. Shape of object translated by changes in firing rate. The higher the curvature, the higher the increase in firing rate in a small # of Rs, and vice versa. Superficial touch mechanoRs and Braille reading: Increased sensitivity based on subsets of Rs converging to a single axon – Population coding. Deep subcutaneous tissue (fewer and larger): 3. Pacinian corpuscle (morphology slide, physiology slide) Responds to rapid indentation or vibration, not to steady pressure. Most sensitive mechanoR. 4. Rufini ending Slowly adapting. Senses stretches of skin, shape of grasped objects, gravitational forces (e.g. when lifting an object). Slowly adapting Rs Rapidly adapting Rs Detect shape of objects and pressure Detect motion of objects on skin Higher touch threshold Lower touch threshold RECEPTIVE FIELDS OF MECHANO-Rs (Re-)Define RF. Region of space from which a given R receives input. Fig 22-3. Superficial layer Rs – multiple Rs per afferent fiber. Small RFs. Finer sensitivity. Deep layers – 1 R to 1 afferent fiber. Larger RFs. Coarse sensitivity. CHANGES IN SENSITIVITY DUE TO DIFFERENCES IN R DISTRIBUTION (Fig 22-4). DEGREE OF SPATIAL RESOLUTION DEPENDS ON CONTRAST BTWN ACTIVE AND INACTIVE NERVE FIBERS. 3 TWO-POINT DISCRIMINATION – Explain it wrt RFs. SYNERGISTIC OPERATION OF ALL TOUCH MECHANO-Rs IN EVERYDAY LIFE. II. TEMPERATURE SENSING Humans recognize: cold, cool, warm, hot. Warm and cold Rs. Fire at rest. Respond to a range of temperatures. Change in firing rate in response to temperature change. Population coding. Sensory adaptation: Thermal Rs reduce their firing rate w/in a few secs. III. NOCICEPTION – PAIN Chemical Rs to a large extent – respond to substances released from injured tissue. 3 CLASSES OF NOCICEPTORS 1. Mechanical nociceptors Fire in response to strong tactile stimuli (Draw exp: blunt end vs. pinprick vs. forceps pinch and afferent fiber recording). Bare nerve endings and myelinated axons fastest-conducting nociceptive afferents. 2. Thermal nociceptors Respond to extreme heat (> 45 C) or extreme cold (< 5 C). 3. Polymodal nociceptors Respond to all of the above. Responsible for toothaches. IV. PROPRIOCEPTION 2 SUBMODALITIES: Limb-position sense Kinesthesia (limb movement sense) 3 TYPES OF MECHANO-Rs – in muscles and joints 1. Muscle spindle R – specialized stretch R in muscle 2. Golgi tendon organs – @ junction btwn muscle fibers and tendon 3. Rs in joint capsules Stretch Rs in skin also. VISCERAL MECHANOSENSORY AND CHEMOSENSORY Rs The gut has a mind of its own. Similar to skin mechanical nociceptors. Control of visceral activity. CONDUCTION VELOCITIES 4 Differences based on axon diameter and degree of myelination, Compound AP. FROM THE PERIPHERY TO THE CORTEX Show 1o, 2o somatosensory ctx – Subdivisions. Serial and parallel processing in order to perceive. Neurons in 1o somatosensory ctx are at least 3 synapses away from the peripheral Rs. RFs of cortical neurons are larger than RFs of peripheral Rs. Successful discrimination in the ctx is accomplished by the fact that a cortical neurons responds BEST at a particular spot. Integration of info for stimulus processing. Convergence – divergence in relay nuclei before reaching ctx. TOPOGRAPHIC ARRANGEMENT IN THE CTX. Preservation of location and modality. Describe columns. (Show cortical layers.) More evident in L4. Visualized with radioactive probes. Technique: extracellular recordings. Phantom limb pain Humunculus: somatotopic map in the human ctx. Not necessarily correlated with surface area. Criterion = innervation density.