Somatic Sensation (MCB160 Lecture by Mu-ming Poo, Friday March 9, 2007) Introduction Adrian s work on sensory coding Spinal cord and dorsal root ganglia Four somatic sense modalities Touch Mechanoreceptors and signal transduction Receptive fields of DRG neurons Proprioception Nociception Thermal Sensation Central Projection and Somatotopic Maps 1 Origin of Sensory Physiology and Neuronal Information Coding Weak pressure Higher pressure very high pressure Some of Adrian s first recordings from a very small number of nerve fibers in the sensory nerves of cat s toe. Adrian s Findings: 1. The nerve impulse (action potential) is all-or-none 2. The strength of stimulus is coded by the firing frequency 3. There is adaptation of neuronal firing after stimulus onset 2 Weber-Fechner s Law I = k Log (S/S o ) where I= intensity of sensation (freq of firing) k, So (threshold intensity) are constants Power law: I = k (S-S o ) n, where n is a constant 3 1
Organization of the Spinal Cord 4 Spinal Cord and Dorsal Root Ganglia (DRG) (ventral horn) Ventral root Spinal nerve Dorsal root Dorsal root ganglia - spinal nerve - limb and trunk Trigeminal ganglia - cranial nerve - head and face 5 Stimuli: mechanical force, temperature change, tissue damage, or chemical action; mediated specifically by different receptors Nociceptors Thermoceptors Mechanoceptors Mechanoceptors Nociceptors, thermoceptors bare endings, small diameter unmyelinated axon 6 Mechanoreceptors: encapsulated (wrapped) endings, large diameter myelinated axon 2
Peripheral endings of DRG neurons 7 Transduction by mechanoreceptors Stress-gated ion channels open when membrane is deformed Na + influx Depolarization (generator potential) Initiation of action potential 8 Four types of mechanoreceptors glaborous (hairless) skin 1. 2. 3. 4. 9 3
Subcutaneous Pacinian Raffini Superficial Meissner Merkel disk 10 Receptive Field The area in the periphery within which sensory stimulus can modulate the firing of the sensory neuron. Spatial resolution of the RF: Size: smaller RF - higher resolution Density: higher density higher resolution Two-point discrimination test 11 RFs of mechanoreceptor neurons RF size: 2-10 mm Each DRG axon receives convergent inputs from 10-25 corpuscles RF size: several cm Each DRG axon receives input from single corpuscle or ending 12 4
Proprioception (Sense of position and movement of limb and body) Mechanoreceptors in muscle and joints 1. Muscle spindle receptors detect the extent and rate of muscle contraction, endings in parallel with muscle fibers 2. Golgi tendon organs detect tension exerted by the muscle, ending in series with muscle fibers 3. Joint capsule receptors detect flexion or extension of joints 13 Nociception (Pain Sensation) Nociceptors -- Respond to noxious (mechano-, thermo) stimuli directly or chemical released by damaged cells -- Chemicals: histamine, bradykinin, substance P, ATP, serotonin, acetylcholine, acids, high K + Three types: 1. Mechano-sensitive nociceptors (high threshold) 2. Thermo-sensitive nociceptors (high threshold) 3. Polymodal (chemo-sensitive) nociceptors 14 Thermal Sensation Thermoreceptors -- Ending of unmyelinated C-fibers -- Spontaneous firing at low-freq at skin T of 34 o C, respond only within innocuous range. Two Types: 1. Cold receptors fire when T decreases from 34 o C, maximal firing at 25 o C 2. Warmth receptors fire when T increases from 34 o C, maximal firing at 45 o C Hot sensation noxious stimulus detected by nociceptors, not thermal receptors 15 5
Projection to CNS (you should know these pathways, although not covered in the lecture, see textbook for detail) Dorsal Column-Medial Lemniscal System (Touch and proprioception) -- afferent to spinal cord laminae III-VI -- ascend ipsilaterally (dorsal column) -- cross midline in medulla -- ascend to midbrain via medial lemnicus -- to thalamus and somatosensory cortex Anterolateral System (Pain and Thermal Sensation) -- afferent to spinal cord laminae I & II -- cross midline to contralateral anterolateral column -- ascend via direct (spinothalamic) and indirect pathways to thalamus -- to somatosensory cortex 16 17 Somatotopic map in the cortex Sensory homunculus: This model shows what a man's body would look like if each part grew in proportion to the area of the cortex of the brain concerned with its sensory perception. 18 6