Chandan Singh | Sensory Input

sensory input

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notes from Neuroscience, 5th edition + Intro to neurobiology course at UVA

9- somatosensory

cheat sheet

  • vocab
    • nerve - bundle of axons
    • tract - bundle of axons in CNS
    • nucleus - bundle of neurons related to some function
    • midline - center of nervous system
      • brain tends to be lateralized - one side is given control
      • ex. speak almost exclusively from left side of brain
  • information processing
    1. feedback (gain)
      • almost always with glutamatergic / GABA
    2. feedforward - anticipation
      • estimate things before they happen
      • adjust your behavior in advance of the world (ex. lean before you hit a table)
    3. center-surround inhibition (spatial gain)
      • if you touch yourself, brain enhances sensitivity of one point by suppressing information from around it

sensory system overview

  • we have dorsal root ganglia (DRG) on spinal cord
    • axon goes to CNS
    • dendrites go everywhere
    • pseudounipolar - born polar but become uni-polar
      • dendrite goes straight into axon with cell body off to the side
    • do very little processing
  • dorsal horn - top layer that controls sensory information
  • in the brain stem, these are called cranial ganglia
    • special one is trigeminal ganglia (sensory receptors for face)
  • oxytocin important clinically
  • Trp channels - connected mechanically into membrane
  • dermatomes
    • map of sensory parts to brain
    • segments of spinal cord correspond to stripes across your body
    • brain to feet: cervical, thoracic, lumbar, sacral
  • shingles - virus where you get stripes of sores - single DRG
    • pops out the skin on the dendrite of one DRG
  • peripheral damage won’t give you stripes of pain
  • feeling resolution - depends on density of neurons innervating skin
    • more neurons - small receptive fields
    • two-point discrimination test - poke you at different points and see if you can tell if the points are different
    • higher discrimination is better
    • discrimination is different that sensitivity (like how it hurts when wounded)

4 neuron classes

  • they have certain structures that tune them into certain kinds of vibrations
    1. Proprioception
      1. muscle spindles - on every neuron - fastest
        • measures stretch on every muscles
        • lets you know where your arm is
      2. Golgi tendon organ
        • measures tension on tendon
        • safety switches - numb your body if you’re over-stressing something (make you let go of hanging on cliff)
    2. Ia II - touch neurons
      • superficial - most sensitive
        1. Merkel: hi-res, slow adapt
        2. Meissner: hi-res, fast adapt
      • deeper - sense vibrations, pressure
        1. Ruffini: low-res, slow adapt
        2. Pacinian: low-res, fast adapt
      • these are in order of depth
      • diabetes - tissue loss and pain / numbness are lost
    3. Adelta - fast pain
    4. C fibers - pain, temperature, itch
      • very slow, stay on
      • no myelination - Pruritus - newly discovered set of sensory neurons
      • between pain/touch - itch neurons
      • new in mice: massage neurons
        • can only fire by stimulating in certain pattern
        • goes to emotion center not knowledge - pleasure
  • speed proportional to diameter, myelination
  • adaptation
    • some adapt slowly (you keep feeling something)
    • some adapt quickly (stop feeling)
      • if you move finger slightly, start firing again when changed
      • better if you feel cockroach that starts moving

pathways

  • upper-body
    • S1 cortex - primary somatic sensory cortex - this is the knowledge of where was touched
    • VPL - everything accumulates here in the thalamus then goes to
    • Cuneate nucleus - everything goes into this
  • lower-body (trunk down)
    • everything in the lower body goes to Gracile nucleus - in brain stem
  • special case - sensory for face
    • trigeminal ganglion connects into vpm (thalamus) then goes into S1 cortex
  • proprioceptive pathways
    • starts in lower body
      • axons split - half go up to Clark’s nucleus
        • half go back into muscles
      • Clark’s nucleus goes straight into cerebellum
    • starts in upper body - goes straight into cerebellum
    • thus cerebellum have map of where / how tense muscles are

representation

  • cortex - this is where understanding is
    • dedicates area based on how many neurons coming in
      • lips / hands have more area
    • S1 - primary somatosensory cortex
      • most body parts
      • neurons from functionally distinct columns
      • cortex assigns space based on how much info comes in
        • after amputation and time, map grows into lost space
        • map is different when different stimuli are given to fingers
    • S2 - secondary somatosensory cortex
      • processes and codes information from S1
      • throat, tongue, teeth, jaw, gum

pathway

  • mechanosensory
    1. DRG
    2. Cuneate, Gracile
    3. VPL
    4. S1
  • face mechanosensory
    1. trigeminal ganglion
    2. principal nucleus of trigeminal complex
    3. vpm
    4. S1
  • proprioception
    • lower body
      1. muscle spindles split
      2. half go to motor neurons
      3. other half go to Clark’s nucleus
      4. clark’s nucleus -> cerebellum
    • upper body - straight to the cerebellum

10 - nociception

review

  • chronic pain is very import clinically
  • cortex - lets you know if you are sensing something
    1. loss-of-function lesion - piece of cortex is lost - lose awareness
      • can come from stroke, migraine-aura
    2. gain-of-function lesion = excitatory lesion - like epilepsy
      • cortex comes on when it shouldn’t
      • increased awareness
      • can come from stroke / migraine
  • “sixth sense” - measuring stretch of all your muscles in cerebellum
  • nociception = pain
    • has nociceptors - neurons that do nociception
    • thermoceptors - neurons that sense temperature
  • two classes of linking receptors
    1. Adelta fibers - fast pain
    2. C fibers - slow and chronic
  • Trp channels - mechanically or thermally gated
    • let Na+ in
    • trpV heat - binds capsaicin
      • in the class of vanilloids
      • birds not capsaicin sensitive
    • trpM cold - binds menthol
      • adapts in minutes - stop feeling cold after a while
  • synapses of nociceptors go to dorsal horn of drg
    • nociceptor goes contralateral (must cross midline) - if you cut left side of spinal chord, lose - mechanoception (ipsilateral) from left and nociception (contralateral) from right
    • mechanoreceptors, by contrast, send axon up the spinal cord
    • dorsal horn has laminal structure (has layers)
      1. know where pain is
    • somatosensory cortex
      1. care about pain
    • insular cortex - emotional part of brain
      • whether or not you care about pain
      • pairs up with other senses
  • can have both loss-of-function and gain-of-function lesions in both places
  • referred pain map - map that refers to a specific problem (ex. esophagus)
  • visceral pain - don’t know where the pain is
  • hyperalgesia - increased pain sensitivity
    • pain sensing neurons are hyperactive because of inflammation
    • pain sensing neuron releases substance P into Mast cell or neutrophil which releases histamine which strengthens receptor
    • prostaglandins activate nococeptors
  • allodynia - when mechanosensation hurts - not understood
  • turning off pain - add serotonin
    1. exercise
    2. lack of serotonin ~ mood disorders
      • central sensitization: allodynia
  • these mechanisms work through introception
    • senses chemical imbalances
  • phantom limbs and phantom pain - if you lose a limb and still feel pain
  • mechanoreceptors inhibit nociceptors

pathway

  • nociception
    • same as mechanosensory except goes all the way to thalamus
      • doesn’t stop in brainstem
    • crosses the midline after first synapse
  • visceral pain
    • axons mainline straight up, go through vpl, go straight to insular cortex

11 - vision (eye)

  • most of visual system is to read faces
  • eye
    • aqueous humor
    • posterior chamber
    • lens
    • ciliary muscles
    • retina
    • fovea
    • optic disk
    • optic nerve and retinal vessels
  • to see far, stretch lens = accomodation
  • retina - rods and cones are at back
    • cones - color
    • retinal ganglion cells sends down signal
  • 12 days to turnover whole photoreceptor disks into PE (pigment epithelium)
    • PE is what the rods / cones are in
    • PE contains optic disks containing rhodopsin protein that is sensitive to light that break off of rods / cones
  • light leads to inhibition
  • melanopsin - receptor for blue light

circuits

  • accomodation - stretching lens uncrosses lines
  • function photoreceptor
    • usually cGMP is letting in Na/Ca
      • Ca provides negative feedback here
    • when light hits, retinal inside rohodopsin activates phosphodiesterase - breaks down cGMP so channel closes and they aren’t let in
  • light on middle
    • depolarizes cone
    • excites oncenter
    • inhibits offcenter
    • these adjust quickly
  • horizontal cells - takes positive input from photoreceptor and inhibits it back
    • inhibits horizontal cells else around it - creates contrast
    • have these for each color

pathway

    1. rods / cones (2). horizontal cells - regulate gain control, how fast adapts, contrast adaptation
    2. bipolar cells (4). amacrine cells - processing of movements
    3. retinal ganglion cells
    4. go into thalamus then to cortex (6). small amount go into brain stem and control mood / circadian rhythms

12 - central visual system

  • cortex is a pizza box
  • has columns
  • autophagy - process by which cells eat parts of themselves
    • nobel 2016
  • cones - color
  • 12 day cycle for processing optic disks
  • photoreceptors have cyclic G-activated channel
    • light shuts down photoreceptors
    • cell decreases in activity
  • very roughly - each cone connects to cone bipolar cell
    • this gets represented by one column in the cortex
  • 15-30 rods connect to 1 rod bipolar cells
  • cortex has 6 layers
    • each has tons of neurons, mostly pyramidal neurons
    • column is a section through the 6 layers - all does about the same thing
    • orientation columns responds to specific x,y
      • has subregions that respond to specific orientations
  • ocular dominance column - both eyes for same coordinate go to same spot
    • dominated by one eye
  • distance
    • far cells
    • tuned cells
    • near cells
  • V4 in temporal lobe - object recognition

pathways

  • overall
    1. V1
    2. V2
    3. V4 or MT
  • central projections
    • retinal ganglions
    • all go through optic stuff

13 - auditory system

  • ear parts
    • outer
    • middle
      • tympanic membrane
    • inner
      • cochlea - senses the sound
      • oval window
      • round window - not understood
  • conductive hearing loss - in the outer/middle ear
  • sensorineural hearing loss - in the cochlea
    • can’t be fixed with hearing aids
  • humans
    • 2-5kHz ~= human speech (can sometimes hear more)
    • 30-100x boost for tympanic membrane
      • this differs between people
    • 200x focus onto oval window
  • cochlea
    • 4 layers
      • inner hair cells - what you hear with
      • outer hair cells - generate sound
        • generates noise at every frequency except one you want to hear
    • otoacoustical emmision - low buzz that is produced
    • tenitis - ringing in the ears
      • can be internal
      • can be peripheral - generated by otoacoustical emmision
    • high frequencies right next to cochlea
    • low frequencies on distal tip
    • human high frequency cells die with age
  • hair cells
    • bundle of cilia
    • have an orientation
    • kinocilium is the tallest
    • tall ones are in the back
    • dying hair cells - can’t be replaced
      1. loud sounds
      2. certain antibiotics
  • auditory pathwayz
    • MSO - medial superior olive - decides where the sounds is coming from
      • takes input from right / left ear, decides which came in first
    • medial geniculate complex of the thalamus
  • brain shape
    • folds are pretty random
    • phrenology - shape of skull was based on brain
      • thought it could determine personality
      • false
      • Hsechl’s Gyrus folding pattern is not random
        • argument that if you have one, you are more musical
  • any sounds is made up of a bunch of frequencies

circuits

  • K depolarizes hair cells, lets in Ca, releases vesicles

14 - vestibular system

  • very related to cochlea
    • same hair cells
  • differences
      1. vestibular system doesn’t use cortex (you don’t think about it)
      • goes right into spinal chord 2. controls eye movements
      • one of the fastest circuits in the brain
    • clinically important
      • you have to be able to have your balance
        1. each column is computational unit of the cortex
        2. ocular dominance column
    • one for each eye
  • labyrinth and its innervation
    • semicircular canals
      • can only measure one axis of rotation
      • remember horizontal canal - measures turning head left to right
        • this measures acceleration
        • like a hoola hoop filled with glitter
        • has ampulla at one place in the hoop
        • cupula - sits over the ampulla’s hair cells
        • if the “glitter” hits the cupula, it will bend the hair cells
        • if you keep spinning, fluid starts moving and you stop detecting movement
          • this means the canals adapt mechanically
        • if you stop spinning, fluid keeps moving and system thinks you’re spinning the other way
        • right horizontal canal activated by turn to the right
          • same for left
    • scarpa’s ganglion - has hair cells inside
      • sends axons into vestibular nuclei
    • lots of fluid (high in K+)
    • macula - place where all the hair cells are
      • Ampullae - at base of canals
        • hair cells all in the same direction
      • utricle and saccule - measure head tilt
        • hair cells in multiple orientations
        • these contain otoconia
          • these are little crystals that move with gravity
          • measure acceleration and tilt
  • tilts do not adapt - they keep firing while you’re leaned back
    • they basically report tilt / position at all times
  • tiplink - connect cilia together for hair cells
    • when they move, tiplink move, pull on ion channels
    • motor on connected hair cell moves up and down to generate correct amount of tension
      • motor uses myosin and actin
      • harming these proteins can cause deafness
  • both eyes must always be looking in the same direction
    • also must be sitting over image for a while
  • ipsilateral - same side
  • contralateral - different side
  • vestibular ocular reflex VOR - turn your head to the right, eyes move left
    • doesn’t require cortex
    • only have to learn excitatory

15 - chemical senses

  • cAMP is used by GPCR