Somatosensory System

[blank_start]Sensory Transduction[blank_end] is the transduction of [blank_start]physical[blank_end] energy into neuronal activity patterns. It [blank_start]may involve active[blank_end] processes. [blank_start]Perception[blank_end] results in a conscious [blank_start]percept[blank_end] of stimulus properties after cognitive processing in the brain.

The type of stimulus that requires least energy to procure a sensation is called the [blank_start]adequate stimulus[blank_end] of a sensory receptor. The somatosensory system converts m[blank_start]echanical[blank_end], t[blank_start]hermal[blank_end] and p[blank_start]ainful[blank_end] stimuli.

Sensory receptors and sensory neurons must encode four types of information about a stimulus: - M[blank_start]odality[blank_end] - L[blank_start]ocation[blank_end] - I[blank_start]ntensity[blank_end] - D[blank_start]uration[blank_end]

According to the [blank_start]Theory of labeled lines[blank_end] modality is encoded by a discrete pathway from the sensory cell to a modality specific integrating center in the brain. Exceptions are [blank_start]polymodal[blank_end] receptors. Location can also be encoded in this way or by [blank_start]topographic[blank_end] projections and [blank_start]sensory[blank_end] maps within the brain.

The [blank_start]receptive[blank_end] field is the region of the [blank_start]sensory[blank_end] surface that causes a response when stimulated. In the somatosensory system receptive fields are regions of the [blank_start]skin[blank_end] or of internal [blank_start]organs[blank_end].

Leads to improved stimulus loclization: - [blank_start]Increased sensor density[blank_end] - [blank_start]Lateral inhibition[blank_end] Leads to worse stimulus localization: - [blank_start]Bigger receptive field[blank_end]

The dynamic range of a neuron is bounded by its (a) [blank_start]threshold[blank_end] of detection (b) [blank_start]saturation[blank_end]

A large dynamic range means [blank_start]poor[blank_end] sensory discrimination and a [blank_start]weak[blank_end] onset detection. A narrow dynamic range means [blank_start]high[blank_end] sensory discrimination and a [blank_start]good[blank_end] onset detection.

The trade-off between dynamic range and discrimination of stimuli can be improved by

Phasic and tonic receptors.

Do phasic or tonic receptors adapt to a maintained stimulus?

Mechanoreceptors sense physical deformation of [blank_start]neurites[blank_end]. In mammals these are [blank_start]dendrites[blank_end] of sensory neurons.

Proprioceptors sense [blank_start]internal[blank_end] mechanical signals.

Different types of mechanoreceptors

What kind of receptors?

Different kind of mechanoreceptors

The receptive field of mechanoreceptors are [blank_start]fuzzier[blank_end] for deep layers (Ruffini ending, Pacinian corpuscle). They are [blank_start]very precise[blank_end] for superficial layers (Merkel cell, Meissner corpuscle).

Different proprioceptors: The [blank_start]muscle spindle[blank_end] provides information about changes in muscle length. The [blank_start]Golgi tendon organ[blank_end] provides information about changes in muscle tension.

Feedback inhibition makes a signal more [blank_start]phasic[blank_end]. Together with lateral inhibition it leads to [blank_start]more contrast but also fake contrast[blank_end].

The two point limen is a measure of the [blank_start]tactile acuity[blank_end]. It is determined by the density of [blank_start]receptors[blank_end] and the [blank_start]processing[blank_end] mechanism (e.g. inhibition).

Neurons can be sensitive to the [blank_start]orientation[blank_end] of an object and its [blank_start]direction[blank_end] of movement. In the latter case multiple receptors are needed.

The cell bodies of sensory neurons known as first-order neurons are located in the [blank_start]dorsal root ganglia[blank_end].

A_alpha fibers are [blank_start]thick[blank_end] and myelinated. They have a higher conduction velocity than A_delta fibers which are [blank_start]thin[blank_end] and [blank_start]myelinated[blank_end]. C fibers are [blank_start]not myelinated[blank_end] and thin. Their conduction velocity is very [blank_start]low[blank_end] and they are responsible for the encoding of strong pain. C fibers have a [blank_start]higher[blank_end] activation threshold than A fibers. A fibers: [blank_start]pressure[blank_end] touch, vibration C fibers: pain, [blank_start]temperature[blank_end]

The pathway for pain crosses to the contralateral site [blank_start]directly in the spinal cord[blank_end]. The receptive fields of these sensory neurons is [blank_start]very fuzzy[blank_end]. There is a simple reflex path to motor neurons which goes from the sensory neurons through the [blank_start]dorsal[blank_end] root ([blank_start]afferent[blank_end]) and via the [blank_start]ventral[blank_end] root ([blank_start]efferent[blank_end]) to the motor neuron.

The pathway for touch, proprioception, vibration is called [blank_start]dorsal[blank_end] [blank_start]column[blank_end] [blank_start]medial[blank_end] [blank_start]lemniscal[blank_end] pathway. The pathway for pain, temperature is called [blank_start]spinothalamic[blank_end] tract.

The topographic projection of the whisker pad is maintained throughout all processing stages: - [blank_start]Brain stem[blank_end] nuclei - [blank_start]Thalamic[blank_end] nuclei - Primary [blank_start]somatosensory cortex[blank_end]

In the [blank_start]thalamus[blank_end] there are topographic AND non-topographic projections of the whisker field.

In the vibrissal pathway there is a [blank_start]non-topographic[blank_end] [blank_start]inhibition[blank_end] from the [blank_start]cortex[blank_end] to the [blank_start]thalamus[blank_end]. This leads to an unspecific wake-up call upon the first stimulus and very specific interpretation afterwards.

Each whisker is represented by a barrel-like structure in the [blank_start]trigeminal[blank_end] nucleus of the brain stem.

What are the thalamic relay nuclei of the somatosensory system? - [blank_start]Ventral posteromedial[blank_end] nucleus (VPM) - [blank_start]Ventral posterolateral[blank_end] nucleus (VPL) - [blank_start]Posteromedial[blank_end] complex (PoM)

In area 3b columns of the postcentral gyrus rapidly adapting receptors of the skin surface (Meissner corpuscle) are located next to

The topographic projection of braille letters for example can be found

The receptive fields of different neurons

The projection fields of sensory neurons go further than the specialized region. This leads to [blank_start]plasticity[blank_end] of brain regions. If a finger is lost for example, in the corresponding cortical region there is no [blank_start]lateral[blank_end] inhibition anymore towards neurons that correspond to other fingers. These will then grow more [blank_start]synapses[blank_end].

In which layer of their somatosensory cortex have rats their whisker-barrels?

Rats sense actively by moving their whiskers in an ellipsoidal pathway with a frequency of [blank_start]7 - 12[blank_end] Hz.

[blank_start]Trigeminal[blank_end] sensory neurons are sensitive to the timing of whisker deflections with sub-millisecond precision. They can also be phasic or tonic.

Sensing with whiskers has an active and a passive component. It is cyclical and repetitive with an active [blank_start]forward[blank_end] motion. It involves an active motor aspect and a [blank_start]forward[blank_end] model to interact with the sensation.

For the same whisker

Receptive fields in the somatosensory cortex of the rat are defined based on: - [blank_start]Primary[blank_end] whiskers (PW): the one eliciting the [blank_start]largest[blank_end] response with the shortest latency - [blank_start]Adjacent[blank_end] whiskers (AW): smaller responses with longer latencies

The receptive field in the somatosensory cortex of rats

The direction sensitive neurons in the somatosensory cortex of rats are located in layer [blank_start]2/3[blank_end]. They sit [blank_start]between[blank_end] the barrels of the whiskers that are compared.

The term [blank_start]divergence[blank_end] describes the phenomenon that even a precise stimulus does not excite a single mechanoreceptor but several adjacent mechanoreceptors as well. [blank_start]Lateral inhibition[blank_end] is need for a precise stimulus loclization.

A [blank_start]dermatome[blank_end] is an area of skin that is mainly supplied by a single spinal nerve.

Topological representations of the whisker field in brain stem nuclei: [blank_start]barrelettes[blank_end] thalamic nuclei: [blank_start]barreloids[blank_end] primary somatosensory cortex: [blank_start]barrels[blank_end]

In rats the nontopographic pathway to the thalamus is called the [blank_start]paralemniscal[blank_end] pathway.

The primary somatosensory cortex consists of Brodman Areas [blank_start]1[blank_end], [blank_start]2[blank_end] and [blank_start]3b[blank_end]. Brodman Area [blank_start]5[blank_end] is also involved in the somatosensory system. In total there are [blank_start]four[blank_end] (write out) somatosensory fields for the whole body that code different information (superficial or deep touch, complex touch, active touch).

In rodents, the [blank_start]trigeminal ganglion[blank_end] is the first part of the pathway from the whiskers to the brain. These sensory neurons are sensitive to the timing of whisker deflections with sub-millisecond precision.