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Lecture 10 – Somatosensation

Raghav RajanBio 354 – Neurobiology 2

February 04th 2015

All lecture material from the following links unless otherwise mentioned:1.http://wws.weizmann.ac.il/neurobiology/labs/ulanovsky/sites/neurobiology.labs.ulanovsky/files/uploads/purves_ch8_kandel_ch23_touching.pdf

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General announcements

● Course presentation – Wednesday 18th February (5:30pm)

– Groups and topics for presentation

– 4 groups – colour, motion, bat echolocation, barrel cortex

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Touch (somatosensation) – processing, function, etc.

● Two systems

– Detection of mechanical stimuli (touch, vibration, pressure, tension in the skin)

– Detection of pain, temperature

● How is touch/pressure sensed?

● Somatosensory coding

– Identity

– Intensity

– Location

– Timing

● Higher order functions

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Mechanosensation – where and how?

● Mechanosensory processing starts with cutaneous and sub-cutaneous mechanosensory receptors

– Mechanoreceptors – mechanical stimulation● All respond to mechanical deformation by closing/opening ion channels

and ultimately firing action potentials

– Thermoreceptors - temperature

– Nociceptors - pain

● Proprioceptors – receptors located in muscles, joints, etc. sense and signal information about forces

– Important for the brain to know where each joint is, where each limb is, etc.

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4 major types of encapsulated mechanoreceptors – All are low-threshold (high sensitivity), fast axons

innervating them – quick transmission

40%smooth, hairless skin – fingertips

30-50Hz vibration

10-15%deeper, rapidly adapting, 250-

350Hz vibrationvibration or tickle

20%unclear function

sensitive to stretch of skin caused by

movements

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Location of receptor types

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Receptive fields, resolution, etc.

● Receptors not distributed uniformly

● Just like fovea of retina, fingertips have much more dense distribution of mechanoreceptors

● Two stimuli can be perceived as distinct

– > 2 mm apart on fingertips

– > 40 mm apart on forearm

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Two-point discrimination thresholds vary through the body

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Receptive fields, resolution, etc.

● Receptors not distributed uniformly

● Just like fovea of retina, fingertips have much more dense distribution of mechanoreceptors

● Two stimuli can be perceived as distinct

– > 2 mm apart on fingertips

– > 40 mm apart on forearm

● Two factors contribute to this difference

– Density of receptors – more dense in certain places (eg: fingertips)

– Receptive field sizes – smaller receptive fields in fingertips

● And top-down feedback influences this too! (not surprisingly – the boss has to say something too!)

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Density of receptors is not uniform

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Receptive fields, sizes, etc.

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Proprioceptors

● Give information about the position of muscles, joints, limbs, etc

● Again not equal, but depends on the muscle, joint, etc

– Muscles that perform fine movements provide more detailed information – eg. ocular muscles

– Muscles that perform coarse movements provide less detailed information

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Active exploration is key for tactile stimulation

● Moving your fingers over an object

● Rats – whisking

● Relative movement between skin and surface – may be most important cue

●

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Somatosensory information path to

brain● Sensory receptors –

spinal cord (dorsal root ganglion cells)

● Cuneate nucleus

● Thalamus

● S1 – primary somatosensory cortex

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Dorsal column – medial lemniscus tract – information

from posterior third of head and rest of body

● Dorsal columns – axons ascending ipsilaterally to dorsal column nuclei, cuneate and gracile nuclei

● Dorsal column lesions – only modest effect on performance on tactile tasks

● Axons from dorsal column nuclei crossover to the other side – medial leminscus

● Medial lemniscus also organised

– dorsal and then medial – axons related to upper limbs

– ventral and then lateral – axons related to lower limbs

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Trigeminal portion carries information

from the face● Cranial nerve V –

trigeminal ganglion

● Trigeminal lemniscus

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Ventral posterior complex of the thalamus gets all somatosensory information

● VPM – from face

● VPL – from medial leminscus – rest of body and posterior third of head

● S1 – Brodmann's area 3a, 3b, 1, and 2

● 3b and 1 – cutaneous stimuli

● 3a – mainly proprioceptive

● 2 – both tactile and proprioceptive

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Somatosensory homunculus

● Somatotopic representation in S1

– All 4 sub-fields have separate maps

● Not equal representation

● Similar motor representation has been shown

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Similar (only more detailed maps obtained form non-human primates

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Mapping receptive fields of cortical cells

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Neurons in higher cortical areas have progressively bigger receptive fields

● Also respond to more complex stimuli

● Shapes, textures, movement across hand

● Bilateral receptive fields too

– seems to come from the corpus callosum

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Columnar architechture – receptive fields of neurons in a column are in the same location on the skin

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How are fine details resolved with such large receptive fields?

● Firing not uniform within a receptive field

● Population code – which neurons are activated strongly and which weakly – together provides information about location

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More organization within columns – each

column receives information primarily

from one receptor type

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Similar principles apply here too

● Convergence

● Surround inhibition

● Lateral inhibition

● All serve to increase resolution

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Touch (somatosensation) – processing, function, etc.

● Two systems

– Detection of mechanical stimuli (touch, vibration, pressure, tension in the skin)

– Detection of pain, temperature

● How is touch/pressure sensed?

● Somatosensory coding

– Identity – different types of receptors sense different things

– Intensity – firing rate

– Location – labelled line and population code for fine discrimination

– Timing – rapidly adpating, slowly adapting

● Higher order functions

–