Week 4 - Somatosensory System - Quick Review

Primary inputs mainly from thalamic relay nuclei (e.g., striate cortex from the lateral geniculate nucleus).
Secondary inputs mainly from primary and secondary cortex within the sensory system.
Association inputs come from more than one sensory system, usually via secondary sensory cortex.

Processing begins in the periphery; becomes more complex and integrated at higher levels.

Distinct functional areas within a level (e.g., vision, audition, somatosensation).
Features such as colour, location, shape processed in specialized areas.

Exteroception: perception of external stimuli.
Proprioception: perception of limb/body position.
Interoception: perception of internal body conditions (e.g., temperature, blood pressure, heart rate).

Pacinian corpuscles: Deep, large receptive fields; fast-adapting; onion-like; detects vibrations.
Ruffini endings: Deep, large receptive fields; slow-adapting; sensitive to gradual skin stretch.
Merkel's disks: Superficial, small receptive fields; slow-adapting; sensitive to gradual indentation, pressure, texture.
Free nerve endings: Temperature and pain.
Meissner's corpuscles: Glabrous skin; superficial, small receptive fields; rapidly adapting; sensitive to stroking/flutters.
Glabrous skin contains papillary ridges (e.g., fingertips).

Receptive field size depends on receptor depth: deeper receptors have larger fields; superficial receptors have smaller fields.

Dermatome: area of skin innervated by the left and right dorsal roots of a given spinal segment.

Dorsal-column–medial lemniscal system: touch and proprioception; first synapse in dorsal column nuclei of the medulla.
Anterolateral system: pain and temperature; synapses upon entering the spinal cord; three tracts: ext{spinothalamic}, ext{spinoreticular}, ext{spinotectal}.
Spinothalamic tract: projects to the ventroposterior thalamus.
Spinoreticular tract: projects to reticular formation, then to thalamus.
Spinotectal tract: projects to the tectum (colliculi).

Primary somatosensory cortex (SI): Postcentral gyrus; somatotopic organization (somatosensory homunculus); more sensitive areas have more cortex; input largely contralateral.
SII: receives input mainly from SI; somatotopic input from both sides.
SI and SII project to association cortex in the posterior parietal lobe.

Highest level of sensory hierarchy resides in association cortex (prefrontal and posterior parietal).
Posterior parietal cortex contains bimodal neurons (respond to two sensory systems) enabling integration of visual and somatosensory input.

Stereognosis: ability to recognise objects by touch alone.
Astereognosia: inability to recognise objects by touch; pure cases are rare; other deficits usually present.

Somatognosis: recognition of parts of one's own body.
Asomatognosia: inability to recognise parts of one’s own body; severe cases may present as somatoparaphrenia (delusions about ownership of body parts).

Touch is the first sensory modality to develop; fetuses respond to maternal touch.
Caregiver touch constitutes a large portion of early interactions and is linked to rewards.
Touch has analgesic effects in neonates and supports growth, neural development, and long-term social outcomes in preterm infants.
Reduced early touch can profoundly affect development (e.g., adverse effects observed in historical cases such as Romanian orphans).

Hairy (non-glabrous) skin contains CT (C-tactile) afferents—free nerve endings, small unmyelinated fibers.
CT afferents are associated with affective/social touch.

Prefer light movement across the skin (light stroking/caress); stimuli near body temperature.
Very sensitive: high firing rate to very light force (≈0.2\text{ g}).
Firing rate follows an \Lambda-shaped pattern with speed; maximal firing at 1-10\,\text{cm/s}.

CT afferents ascend through the \text{anterolateral system} .
System largely comprises unmyelinated, small neurons and free nerve endings that process temperature, pain, and affective touch.
Three tracts in the spinal cord: \text{spinothalamic}, \text{spinoreticular}, \text{spinotectal}.

Insula: cerebral cortex located deep in the lateral sulcus, beneath the opercula; has anterior and posterior regions.
Insula plays a key role in interoception and integration of bodily signals.

Interoception: sensing, interpreting, and integrating signals from within the body, mapping the internal landscape across conscious and nonconscious levels.
Functions include regulation of allostasis and homeostasis: hunger, thirst, thermoregulation, respiration, heart rate, blood pressure.
Involves memory for features of the environment relevant to food/nutrition; contributes to affective and emotional experiences; supports sense of self.
There is a negative association between interoceptive awareness and susceptibility to the rubber hand illusion.

Interoception is linked to mental health; ongoing research outlines pathways and potential clinical relevance.

Three forms of perception in somatosensation: Exteroception, Proprioception, Interoception. 3 forms.
CT firing: maximal at 1-10\,\text{cm/s}; threshold ~0.2\text{ g}.
Three major spinal tracts in the anterolateral system: \text{spinothalamic}, \text{spinoreticular}, \text{spinotectal}.
Receptive fields: deeper receptors = larger fields; superficial = smaller.