Introductory Neurobiology Week 10 Day 1 Notes
Introductory Neurobiology Week 10 Day 1 Notes Biol 3640
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This 9 page Class Notes was uploaded by lucy allen on Thursday March 10, 2016. The Class Notes belongs to Biol 3640 at University of Denver taught by Dr. John C Kinnamon in Fall 2016. Since its upload, it has received 13 views. For similar materials see Introductory Neurobiology in Biology at University of Denver.
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Date Created: 03/10/16
-final exam -100 questions, multiple choice -study old exams 1st, then power points, then notes, and then the text last -review on Thursday 5pm, Olin 105 The Somatic Sensory System -functions of a sensory system -the function of a sensory system is to provide the CNS with a representation of the physical world, external and/or internal -we have receptors for both external and internal stimuli -external: skin -internal: proprioception -information extracted by a sensory system -modality: senses of Aristotle -somatic (touch, pressure, temperature and pain) -intensity: how strong the stimulus is -duration: how long the stimulus is presented -location: where the stimulus comes from -general organization -cellular components -receptor cells: either specialized (as in the sense of hearing or taste) or they can be primary afferent neurons (as in sensing pain or touch) -synapse is followed by a second order neuron -another synapse is followed by a third order neuron -then we are in the sensory cortex -slide 6: pathways for touch (red) and pain/temperature (blue) -stimulus to the finger -in the spinal cord the dorsal route ganglion cells represent where the cell bodies for the primary afferent neurons are located -the receptor cell is also the PAN in this case -touch -enters spinal cord -travels ipsilaterally (up the same side) of the SC until it reaches the medulla -this is where the first synapse is -after this synapse, the path crosses over to the contralateral side of the medulla and up to the mid brain/thalamus -thalamus is the gateway of the senses -except for olfactory system (directly from nose to olfactory bulb) -second synapse in thalamus, then we go to the somatosensory cortex -third synapse is located here -pain/temperature -enters spinal cord, first synapse is here -after the afferent fiber goes from the finger to the ipsilateral side of the spinal cord -then it crosses over in the SC to the contralateral side -it travels up this contralateral side through the medulla where there is no synapse until we reach the thalamus (where second synapse is) -similar pathway is then followed to the somatosensory cortex -two pathways begin and end at the same place, but the first synapse is located in a different place, and the cross over to the contralateral side is different for pain vs. touch -principles common to most sensory systems -receptor cells are specialized for the most part (chemo, thermo, etc.) -all neurons have a receptive field -all of the receptors innervated by a single neuron; may be large (on your back or thighs, large number of receptors innervated by a single sensory neuron) or may be small (tips of fingers, touch) -two point discrimination -specific pathways project information from receptors to distinct regions of the cortex Two-Point Discrimination- Why More Sensitive in Fingers and Lips? -higher density of mechanoreceptors -receptors have small receptive fields -little convergence of sensory input -if the receptive field is large, it becomes impossible to discriminate between two close points (back and calf) -fingers and lips have little or no convergence -back and calf have a lot of convergence Types of Somatosensory Receptors -touch/pressure -respond to external mechanical stimulation -receptors -Pacinian Corpuscles -make up 10-15% of innervation in the hand -rapidly adapting -located deep in the dermis, in the connective tissue -laminar capsule is like an onion, acting as a filter, only high frequency stimuli (350-450 Hz) activate nerve endings -lower response threshold -can sense a skin displacement of 10 nanometers (SENSITIVE) -BUT don't forget they have low resolving power (deep in skin) -important for sensing vibration and manipulating tools (to cut bread, handle a wrench, etc.) -Meissner's Corpuscles -mainly talk about tactile information associated with glabrous skin, meaning hairless (soles of feet, palms) -perceive high definition neural representation of objects -rapidly adapting fibers (phasic) -account for about 40% of the receptors in the human hand -located towards the surface of the skin -formed by connective tissue made of Schwann cells, surrounding free nerve endings -more than 4x more sensitive to skin deformation as Merkel's corpuscles -receptive fields are larger, so spatial resolution is reduced -good for sensing low frequency vibration (< 40 Hz) -ex: sliding fingers across desk surface -Merkel's disks -tonic, not phasic, slowly adaptive fibers -account for about 25% of mechanosensory afference in the hand -especially concentrated in the finger tips -lie in the epidermal ridges -specialized for touching and identification, have the highest spatial resolution of all of the sensory receptors -can resolve stimuli to about 0.5 mm -Ruffini's Corpuscles -slowly adapting fibers, least understood -located in the epidermis, oriented with the long axis parallel to the surface of the skin -particularly sensitive to stretching and internal movements of the hand -20% of the receptors in the hand -haptics: active touching (take something and manipulate it) -stereognosis: when you hold something in your hand and identify it by touch -all accomplished via receptor types -Meissner's and Pacinian's fire at the beginning and end of a stimulus -more rapid adaptation -Merkel's and Ruffini's generate a slower but more sustained response and may continue to fire after the stimulus has ended -slower adaptation -mechanisms for stimulating receptor cell -1 -ion channels themselves are stretch activated -pressure on the skin opens the ion channel which depolarizes the primary afferent neuron -2 -part of the ion channel is attached to the cytoskeleton, other attached to the extracellular matrix -deformation of the extracellular matrix moves the ion channel, deforms/opens it so it depolarizes the primary afferent neuron -mechanoreceptive protein in the cell membrane, attached to cytoskeleton and extracellular matrix, when this is deformed it somehow sends a message to the ion channel to open it and depolarize the cell -not sure if it is a mechanical or GPCR connection at this point -proprioception: muscle spindles in the muscles and Golgi tendon organs in the tendons -responds to internal mechanical stimulation -receptors -muscle spindle -Golgi tendon organs -thermal -signaled by TRP channels -TRPV1 -stimulated by temperatures above 42 C o -stimulated by chemicals like chili peppers (capsaicin) -stimulated by acids (low pH) -TRPV2 -activated in A-delta nerve fibers, very high temperatures (>52 C) -painfully hot temperatures -TRPV3 and TRPV4 o -V3: 32-39 C -V4: 27-34 C -example of range fractionation: when different receptors tuned to different portions of the spectrum of possible sensory stimuli -TRPM8 -activated by modest cooling -expressed in a subpopulation of C-fibers -TRPA1 -some believe this is activated by painfully cold stimuli (<20 C) -also activated by a host of pungent chemicals (mustard oli, garlic, wasabi) -pain -signaled by nociceptors -stimuli end up in the somatosensory cortex, in the parietal lobe -somatotopic map -mapping of stimuli onto the somatosensory cortex -feed and genitalia in center -as we go out there is a representation of the entire body -parts of the body with small receptive fields will have more representation on the somatosensory cortex -the homunculus -representation of somatosensory cortex range fractionation in the form of the organism -dermatome: region of skin innervated by the dorsal route from one spinal segment -different dermatomes can be represented as stripes in a human on all fours -virus from chicken pox remains in primary sensory neurons after outbreak (for life), can cause an irritation and an increase in the excitability of the neurons particularly in one dermatome -leads to a low threshold of firing -pain is a constant burning and stabbing -herpes virus usually only reactivates neurons in one dorsal root ganglion Proprioception -sensation of body position and movement using sensory input from receptors in muscles ,joints and skin -Golgi tendon organs: respond to muscle tension (transmit this information to the spinal cord via type 1b axons) -located in series with the muscle fibers, not parallel -in the tendon itself -stretch (myotatic) reflex -after you've taken a final, you may go to your favorite bar and have a pitcher of beer -as the beer is poured, it will apply more and more weight onto your bicep, which stretches the muscle spindle in the bicep -arm will come back up, and the bicep will shorten to where it was due to this reflex (due to passive stretch of the biceps) -series of motor neurons innervating the homonymous muscle (containing the muscle spindle), a synergist (in parallel with the bicep, assists in flexing the elbow), and an agonist (triceps, will cause extension of the arm) -stimulation of muscle spindle and biceps by a passive stretch causes a signal by the 1A afferent into the spinal cord, motor fiber going to the alpha motor neuron will be stimulated, causing contraction -also a motor neuron going to the synergist, causing it to contract -another pathway in the spinal cord goes to an inhibitory interneuron in the spinal cord, inhibiting the antagonistic muscles (triceps) so that when the biceps and the synergist are contracting, the triceps are relaxing -inverse myotatic reflex -signaled by the Golgi tendon organ -you go to the gym, doing curls to build up bicep muscles -try to curl too much weight, impossible -as hard as you try you will do damage to yourself -thanks to this reflex, the damage is prevented -when there is enough tension on the GTO, where the flexor muscle (bicep) attaches to the forearm, the 1V axon goes into the spinal cord, stimulating the extensor muscles (triceps), telling you to extend rather than flex the arm -1V afferent stimulates an inhibitory interneuron the SC at the same time, inhibiting the main motor neuron going to the biceps -spinal reflexes (polysynaptic) -walking on the beach, step on a piece of glass -pick up your foot first, then react with "OUCH!" -pain receptor goes to the spinal cord -there it branches and stimulates four different interneurons -one is stimulated to stimulate the flexor muscle on the ipsilateral side to lift the foot up -one is an inhibitory interneuron on the ipsilateral side to inhibit the extensor muscle -simultaneously on contralateral side, the extensor muscle is extended, causing right food to form a steady post for you to stand on when other foot is raised -muscle spindles: detect changes in muscle length such as stretching (associated with 1A axons) -located in parallel with the muscle fibers -typically find a muscle spindle inside a muscle -1A nerve fiber: fastest conducting nerve fiber in our system -gamma motor neuron: separate from alpha which stimulates main muscle and muscular contraction -both types are involved in sensorimotor reflexes -how do they work? -removal of tension on the spindle (shortening of muscle) -spindle becomes shortened and relaxed, no tension, so it stops signaling -where the gamma efferent motor neuron comes in -this motor neuron, when stimulated ,despite shortened muscle, the muscle spindle will shorten -puts the muscle spindle back in the middle of its dynamic range -can signal further shortening or lengthening Acupuncture -fMRI shows reduced activity of pain after acupuncture sessions
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