EXAM 2 will test your knowledge of THE LEMNISCAL SYSTEM Mechanoreceptors You should understand the anatomy and sensory physiology of the 4 kinds of mechanoreceptors emphasized in class Basket Cells Pacinian corpuscles muscle spindle fibers Golgi tendon organ and the stimuli that activate them 1 Basket Cells as soon as pressure comes on it will keep on firing till it s turned off Slowly adapting continues firing as long as the stimulus is there Tactile pressure detection 2 Pacinian Corpuscle specialized nerve ending associated with fast adapting fibers that have large receptive fields Responds to a vibrating texture stimulus Fires at onset at pressure then stops then fires at the offset Compression 3 Muscle spindle fibers stretch receptor contracts 4 Golgi tendon organ Tension Receptor Cannot contract a Receptors in muscles and tendons encode body position 5 Receptors can be characterized in terms of a Stimulation type pressure vibration temp damage b Size of receptive field amount of branching c Rate of adaption slow medium fast You should understand the fundamental similarity between inner hair cells sensory cells for hearing and mechanoreceptors in the skin muscles and tendons After all each of these receptor types transduces a mechanical stimulus into neural activity 1 There are also mechanoreceptors in hairy skin and the hair cells in the cochlea are the most sensitive mechanoreceptors transducing air pressure waves into nerve signals sent to the brain Both respond to pressure and basket cells react to hair movement 2 Cytoskeletal strands are like the tip links of inner hair cells Ion channels pulled open by mechanical force You should understand the different perceptual functions of different receptor types 1 Detection Basket Cells hair movement 2 Identification based on surface texture and shape Pacinian corpuscles 3 Kinesthesis muscle spindle fibers and Golgi tendon organs You should be able to describe differences in the distribution of Basket Cells and Pacinian Corpuscles across the skin surface and understand how these differences relate to human detection vs identification of tactile stimuli 1 Detection converging neurons less discrimination lower threshold Many to one 2 Identification less convergence more discrimination higher threshold Each receptive field connected to it s own Spinal cord organization and the lemniscal system be able to draw the pathway You should be able to describe how differences in the receptive field size and density of mechanoreceptors relate to distortions present in somatosensory maps in cortex You should be able to look at a homunculus from any animal species and make accurate predictions about mechanoreceptor receptive field size and density in that animal s skin 1 Lower face more detection because of little hairs on face Hands and lower half of the face identify fine texture and have the greatest level of fine motor control Necessary for haptic perception to work Most animals have very little motor control in the lower half of the face The homunculus effects of experience on somatosensory maps in cortex don t forget developmental young vs old differences in the effects of experience on somatosensory maps You should be able to describe the 1 Experiences change cortical maps 2 What happens if you a lose a finger What happens to all the neurons They find new receptive fields in the skin of the adjacent finger Will be more sensitive for the inside of the side fingers because of new sensory receptors However whenever you stimulate those receptors it s now going to feel like the middle finger is being touched like you still have a finger Not really phantom because in cortex it never went away Cortex of adjacent fingers will start to encroach Readjust but always going to have phantom sensation of lost finger 3 If you lose a hand stump will be very sensitive because all sensory receptors are trying to find sense from little stump Cortical maps will readjust and experience alone can readjust Example of violin training Young vs Old a Left hand is someone who didn t use left hand much Middle is one who started playing violin when older And right started when they were young Other cortical region had to give up some space so we could use it to process input from the left hand What is the result of all this experience Variation in cortical map b What activity would maximize all parts of the brain 1 Music is an activity that uses all of your cortical regions playing an instrument You should understand what Haptic Perception is using all of your mechanoreceptors at the same time to actively explore the texture and or shape of an object in order to identify that object Knowledge of the world that is derived from sensory receptors in skin muscles tendons and joints usually involving active exploration See Chapter 13 and Lecture Slides Skin THE SPINOTHALAMIC SYSTEM Free nerve endings Here you should focus on the anatomy and sensory physiology of free nerve endings nociceptors and thermal receptors Why are they called free 1 Tissue damage pain nociceptor detects Pain is a perception the stimulus is tissue damage Nociceptors respond to and release chemical stimuli the basis of inflammation you do not perceive pain until it reaches the brain What s being sensed is injury or tissue damage Nociceptors have branches when stimulation is placed on a single dendrite of a Nociceptors it s going to send a signal back out to these branches Cause blood vessels to dilate which causes us to get more blood into the cite of injury causing greater stimulation causing a stronger signal to be sent to the brain causing more dilation of the local vasculature a A delta fiber responds primarily to strong pressure or heat and are myelinated so they can conduct signals very rapidly Sharp burst of pain b C Fibers are unmyelinated and respond to intense stimulation of various sorts pressure heat or cold or noxious chemicals Dull throbbing 2 Temperature hot Cold thermal receptor Identification a Warmth fibers fire when temp of the skin surrounding the fibers rises b Cold fibers which outnumber warmth fibers by a ratio of about 30 1 fire in response to decreases in skin temp 3 This pathway is gated in the spinal and there is a mechanism in your spinal cord to prevent the injury sensation 4 They re called free because they don t have anything on the end no doodad Why are there so many of free nerve endings in human skin and how does their distribution differ across the skin surface 1
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