BISC 421 1st Edition Lecture 25 Outline of Current LectureI.Somatosensation Current LectureSomatosensation 1. Sensory systems "• somatosensation, pain, smell, taste, pheromones!2. Higher brain functions"• sleep, circadian rhythms, sex differences, memory …!3. Development" • Axon guidance, growth factorsSomatosensation!• Touch (light and deep pressure, vibration)!• Limb position (proprioception)!Next week!• Heat and Cold!• Pain!Cell types, molecules (receptors/channels), centralpathways, modulation/interesting factoids•Somatosensation: four basic senses (touch, proprioception, pain, temperature)•There are subsets within each•All are detected by the skin except proprioceptionBasic circuit for somatosensation!• Cell bodies of sensory neurons are in the DRG or trigeminal ganglion (face).• Cells are pseudounipolar – two axons, no dendrite.•We have sensory neurons that have cell bodies in the dorsal root ganglion (DRG) and trigeminal ganglion in the head•Within that cluster we have cell bodies that extend two processes•Nerve endings are in the skin and this is where most of the detection of stimuli occursDermatomes! Each ganglion innervates a specific region ofthe body, called a dermatome. Knowledge of dermatomes can aid in determining thelevel of a spinal cord lesion.•Organized in dermatomes-‐ along spinal cord we have our DRGs that are innervating particular areas of the torso in a reasonably logical way•Consequence is if you have spinal cord damage low you will effect legs and etc..•Don't need to memorize which dermatomes innervate which part of the bodyVariety of morphologically distinct mechanoreceptors•Many different types of mechanical sensations-‐ many different receptors that detect these•These are nerve endings wrapping around certain structuresCharacterizing responses!• Speed of conduction"(all touch is rapidly conducted)"• Adapting or non-adapting"• Receptive field size"• Threshold for mechanical stretch"•Somatosensory response characterization•1. Speed of conduction-‐ variable depending on the sensory modality•2. Adapting or non adapting-‐ if you still feel the chair underneath you or not (pain doesn't adapt)•3. Receptive field size-‐ how big the area of the skin being detected•4. Threshold•Know this chart-‐ merkel has smallest receptive field and pacinian has largeConduction velocity!•Many different kinds of axons in a nerve innervating the skin•Large ones are myelinated-‐ meaning they have higher conduction speed•Each axon corresponds to a certain sensory modality•Want to know right away that you’re touching something-‐ that's why proprioception has large axonsAdaptation•Second property•Some sensory response will continue to go on in a long applied stimulus whereasothers will only fire in the beginning – pain is non adapting and touch is adaptingReceptive felds•Third property•Receptive filed size varies in the body•Small receptive field size: hands•Large receptive field size: torso•How do we know this? Test illustrated at the bottom-‐ poked with one probe or twoprobes?Receptive feld sizes vary across the body •Fingers can discriminate 5 mm•This is a map of how far apart parts of the body and discriminate•Lots of variation-‐ depends on amount of innervation in each part of the bodybest discrimination (smallest receptive field) is in the fingersBraille!•Merkel cells important in Braille•Have monkey move finger over different patterns of shapes and record response activity in the merkel cells•The merkel cells do it really well but the others are messy because don't really have much spatial resolutionMechanosensory Transduction•How do we respond to these mechanical deformations?•At level of nerve ending there are ion channels that exist in the closed state prior to themechanical change and then when they are stretched they let Na+ inPiezo2 is a mechanically-activated ion channelPiezo1 and Piezo2 are essential components of distinct mechanicallyactivated cation channelsCoste et al., 2010 Science•On the verge of identifying these molecules•One ion channel that is mechanically sensitive has been identified and it is called piezo2•If put that ion channel into a cell and record and move the cell they can get a large current.The pathways for touch!Dorsal column- Medial lemniscal systemMechanosensory input:• ascends ipsilaterally through dorsal columns• synapses and crosses the midline near the brain (gracile and cuneate nucleus).• extends through the medial lemniscus to the thalamus and then cortex•One main point – mechanoreceptors enter the spinal cord and the axon travels up the spinal cord without synapsing until the brain•Mechanoresponses go up ipsilaterally•Pain pathways do the opposite (they cross at the spinal cord)Rroprioceptors•Bottom line is that they innervate the muscle spindle fibers and golgi tendon organs that tell us the position of our bodiesCentral pathway of proprioception!• Proprioceptive axons travel in the spinal cord with axons containing cutaneous information. "• Difference include:"• Axon collataterals penetrate dorsal horn where they synapse– this circuit underlies reflexes, such as “knee-jerk”"• Central projections include the cerebellum.•Already learned that proprioceptors unlike somatosensation is relayed only in the spinalcord-‐ this underlies the reflexes•Central projections of the somatosensory system has an interesting organization•The regions of the cortex that respond to different parts of the body-‐ the areas near each other on the body are near each other on the cortex•How much cortex is devoted to each part of the body-‐ some areas more represented•Notice that the thumb is near the eyes which is weirdPost central gyrus of parietal lobePlasticity in somatic sensory cortex! Expansion of receptive fieldsize after removal of a digitThere is no unoccupied real estate in the brain•The somatosensory system is very plastic•When you are born the regions of the cortex become devoted to different parts of the body-‐ but if you lose a limb the somatosensory cortex will rewire•There is never an area that is unrepresented in the cortex-‐ the cortex will expandWhy you should practice the piano! Expansion of a cortical representation by a repetitive behavioral task•Has