7.5. sensory input¶

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

7.5.1. 9- somatosensory¶

7.5.1.1. 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

7.5.1.2. 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)

7.5.1.3. 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

7.5.1.4. 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

7.5.1.5. 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

7.5.1.6. 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

7.5.2. 10 - nociception¶

7.5.2.1. 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

  2. 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

7.5.2.2. 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

7.5.3. 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

7.5.3.1. 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

7.5.3.2. 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

7.5.4. 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

7.5.4.1. pathways¶

  • overall

    1. V1

    2. V2

    3. V4 or MT

  • central projections

    • retinal ganglions

    • all go through optic stuff

7.5.5. 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. ​

      2. loud sounds

      3. 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

7.5.5.1. circuits¶

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

7.5.6. 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

7.5.7. 15 - chemical senses¶

  • cAMP is used by GPCR