tactile circuits in the mammalian brain

touch receptors on the glaborous skin

• the skin on the palmer surface is called glaborous skin

• touch receptors are known as mechanoreceptors

• glaborous skin contain four distinct types of mechanoreceptor

  • Meissner corpuscle (epidermis)

  • Merkel cell (epidermis)

  • Pacinian corpuscle (deep dermis)

  • Ruffini ending (deep dermis)

• each one is responsive to a particular type of touch stimulus

mechanoreceptor properties

• rapidly adapting (RA) fibres initially respond with a high firing rate and stop firing when the stimulus becomes stationary

  • Meissner corpuscles and Pacinian corpuscles

• slowly adapting (SA) fibres initially fire at a high rate and continue to fire at a lower rate when the stimulus becomes stationary

  • Merkel cells and Ruffini endings

• Meissner corpuscles and Merkel cells have small RFs, Pacinian corpuscles and Ruffini endings have large RFs

tactile sensors on the face

• the whiskers are arranged in a grid-like manner on the snout into rows and arcs

• each whisker is a conical tapering hair whose base is inserted into a whisker follicle

• the follicle has many distinct types of mechanoreceptors, including Merkel cells

• each follicle is innervated by afferents or neurons located in the trigeminal ganglion

organisation of whisker input in the brainstem

• the trigeminal nerve sends ascending excitatory axons to two brainstem regions or trigeminal nuclei called principle trigeminal nucleus (Pr5) and spinal trigeminal nucleus (Sp5)

• these nuclei contain discrete clusters of neurons that respond to a single whisker

• these clusters are called barrelettes. each barrelettes receives input from one whisker

• two pathway from the brainstem- lemniscal and paralemniscal

whisker representation in the thalamus

• neurons in Pr5 send ascending glutamatergic axons to the ventral posterior nucleus (VPM) of the thalamus

  • lemniscal pathway

• neurons in Sp5 send axons to the posterior medial nucleus (POM) of the thalamus

  • paralemniscal pathway

• VPM contains clusters of neurons organised in a somatotopic manner. these are called barreloids

• each barreloid will receive input from the corresponding barrelette

whisker RFs in VPM

VPM barreloid neurons have sharp RFs with strong preference for one whisker (principle whisker)

two distinct thalamo-cortical projections

• anterograde tracing labels the axonal target structures of a particular region

• VPM neurons send axons target discrete structures in primary somatosensory cortex (S1)

• axons from POM neurons are spread out or innervate across S1

barrel map in S1

• S1 contains discrete structures that mirror the layout of the whiskers - barrels

• the barrel containing region of S1 is also called the barrel cortex

• the whisker → barrel pathway is an example of ‘labelled-line’ coding

thalamic inputs to a barrel column

• the cortex in organised into 6 layers

• each cortical column is a functional unit

• in a barrel column, the barrel is located in L4

• VPM sends axons to L4 while POM sends axons to L1 and L5a

whisker RFs in L4 of barrel cortex

• L4 barrel neurons have reasonably sharp RFs with preference for one whisker - principle whisker

• not as sharp as in VPM barreloids, some firing is seen in neighbouring areas

imaging whisker evoked responses in cortex

• upon whisker detection, activity in S1 barrel cortex is restricted to a column for the first 10-12ms

• it then spread and eventually takes over the entire barrel field by ~20ms after the deflection

flow of activity in a barrel column

• L4 axons ascend into L2/3 in a columnar manner

• L2/3 axons spread laterally, innervating other barrel columns

• L2/3 neurons have broader RFs. this helps to integrate signals coming from many whiskers

a ‘simple’ whisker detection task

• head restrained mice can be trained to a single whisker deflection for a small reward

• task rules

  • C2 deflection → lick

  • no deflection → don’t lick

generating touch sensory percepts with optogenetics

• optogenetic activation of S1 can substitute for the C2 whisker stimulus at the periphery

• the sensory percept generated by the optogenetics activation might ‘feel’ similar to that generated by whisker deflection

late responses in S1 correlate with a sensory percept

• the whisker stimulus evokes an early and a late depolarising response in S1 neurons

• the early response is not different between ‘hit’ and ‘miss’ trials

• the late response is larger on ‘hit’ trials and leads to AP firing

hallmarks of conscious perception

late responses might be a common hallmark that signals conscious perception across different primary sensory areas

stick-slip hypothesis of texture coding

• whisking or moving fingers across a texture is not a smooth motion

• it is irregular with brief abrupt accelerations interspaced with stops

• such events are called stick-slip events

• S1 neurons respond to these events with transient increases in AP firing rate