Touch

Somatosensory system

the sensory network that monitors the surface of the body and its movements.

Exteroception

sensation from the skin providing information about external world.

Interception

sensation from internal organs.

Proprioception

sensation from muscles and tendons to convey body position.

Layers of skin

• Glabrous (hairless skin)

• Epidermis

• Dermis

• Subcutis

Free nerve endings

any skin area responding to pain and temp.

Hair-follicile receptors

hair-covered skin, movement of hairs, skin and stroke.

Low-threshold mechanoreceptors

low-threshold because they register stimulation less than pain, includes:

• Pacinian corpuscle, meissner corpuscle, ruffini ending, merkel disk.

Pacinian corpuscle

• any skin area responding to high-frequency vibration or sudden touch/pressure changes.

  • Onion-like outser structure providing mechanical support so that only a sudden stimulus can bend the neuron membrane.

  • glabrous skin (hairless/palms) + hairy skin

  • rapidly adapting type 2

Meissner corpuscle

• Hairless areas, mainly in fingertips. Discriminative for changes in pressure andd low frequency vibration.

  • Contain lamellar schwann cells, alphabeta axon fibres, and collagen fibres.

  • Sandwiched in epidermal cells.

  • glabrous skin.

  • rapidly adapting type 1.

Ruffini ending

• Any skin area (scarce in humans) responds to continues pressure, skin stretch and roughness.

  • glabrous skin (hairless/palms) + hairy skin

  • slowly adapting type 2

Merkel disk

• Any skin area, responds to light touch and very low frequency vibrations.

  • glabrous skin (hairless/palms) + hairy skin

  • slowly adapting type 1

High-threshold mechanoreceptors (HTMR)

free nerve endings below or within the epidermis which vary in speed.

Longitudinal lanceolate endings

attached to zigzag hair with 2 fibres (C slow and Alpha-Delta fast) that reflect the sense of intimate touch and sensitivity.

Temperature receptors

containing cold and heat-sensitive neurons in the spinal cord to respond to absolute temperature.

Alphabeta fibres

very fast axons that connect ot the epidermis.

Simultaneous Two-Point Discrimination Task

taking a very thin filament, and presenting either one or another to the skin.

• Most receptors in fingertips and least in calf.

Mechanotransduction

ways: stretch activation, activation by tethers, indirect activation.

Stretch activation

pulling apart mechanically lets ions flow into cell

Activation by tethers

part of ion channel attached to extracellular matrix by tether which is attached to cytoskeleton, pulling tether opens ion channel.

Indirect activation

tether attached to a separate structure beside ion channel (microtubule) and indirectly opens the ion channel.

TRPV 1 ion channel

• when heat is applied it folds differently to open a channel in the protein.

  • Ion channels are polymodal; respond both temperature and a ligand.

  • Capsaicin can attach to the protein and open the ligand activated channels.

  • TRPV 2 used for higher heat

Cold transduction

• TRPM8 channels or CMR 1.

• Ions are polymodal responding to both temperature and ligand.

  • Menthol can attach and open the ligand channel.

Somatosensation in the CNS

• Touch receptors leads to cranial nerves then to the 31 spinal nerves.

• Spinal nerves innervate (connect to) a dermatome (area of body connected to particular spinal nerve).

Groups of spinal nerves

• top-down; Cranial nerves, cervical nerves (8 pairs), first thoracic vertebra +thoracic nerves (12 pairs), lumbar nerves (5 pairs), sacral nerves (5 pairs), coccygeal nerves (1 pair).

Primary somatosensory cortex (s1)

essential for touch experiences, activity in this cortex matches what is experienced.

• Damage to this area impairs body perceptions.

• Concious perception of touch depends on myelined axons.

• 4 areas:

  • 2 - codes for size and shape of objects.

  • 1 - codes for texture of objects and pain.

  • 3b - codes for size, shape and texture + pain.

  • 3a - responds to movement of joints, tendons, muscles and pain.

Numbsense

• when the primary sensory cortex (s1) is inactive, some people correctly “guess” the location of a touch while insisting that they did not consciously feel it.

Emotion and pain

• Feelings, memories and cues associated with pain activate the somatosensory cortex via the ventral posterior nuclei of thalamus.

• Emotional evaluation of pain activates the intralaminar nuclei of the thalamus, the amygdala, hippocamous, prefrontal cortex and cingulate cortex.

Pain in the CNS

• Pain begins with a bare nerve ending

• Little or no myelin, the thicker and faster ones convey sharp pain and thinner ones convey duller pain.

• Mild pain releases glutamate, stronger pain releases glutamate and neuropeptides, substance P and CGRP.

Pain / temp pathway

• Pass through dorsal root ganglia

• Cross to contralateral side in the dorsal column of spinal cord c8

• Travel up spinothalamic tract.

• Go to VPN of thalamus.

Touch pathway

• Carried in alpha-beta fibres from mechanoreceptors.

• Passes through dorsal root ganglia.

• Stay on ipsilateral side of dorsal column of spinal cord c8

• Travel up dorsal-column medial leminscus

• Cross to contralateral side in medulla

• Go to VPN on thalamus.

Opioid mechanisms

ystems that respond to opiate drugs and similar chemicals.

Endorphins

transmitters that attach to the same receptors as morphine, the brain produces several types to relieve different types of pain.

Gate theory

the idea that stimulation of certain axons can close the “gates” for pain messages.

• Spinal cord neurons that can receive messages from pain receptors also receive input from other inputs that can close the gates for the pain messages.

• Cannibinoids can treat neuropathic pain + block paiin in the periphery of the body.

• Capsaicin can also help release pain.

Nav1.7

The 7th type of voltage-gated sodium channel, important for axons conveying pain and olfactory sensations; psychologists are developing drugs to block Nav1.7 for chronic pain.

Nocebo effect

opposite of placebo, the unpleasant reaction to a drug is increased by the nervous system.

Placebo effect

releiving emotional pain, increasing activity in the prefrontal cortex.

Periaqueductal gray area

the area of the brainstem that is rich in endorphin synapses.

Itch

• 2 types; something crawling on skin and tissue damage (histamines dilating blood vessels producing the itching sensation).

• Inhibitory relationship between pain and itch; decreasing itch increases pain and decreasing pain increases itch.