lecture 10

Somatosensation: Touch

Touch Receptors and Pathways

Sensory Receptors

• Sensory receptors are specialized to respond to changes in the environment (stimuli).
• Activation of these receptors results in graded potentials, which can trigger nerve impulses.
• Sensation is the awareness of a stimulus, while perception is the interpretation of the meaning of that stimulus; both occur in the brain.

Classification of Receptors

• Receptors can be classified based on:
  • Type of stimulus they detect.
  • Location in the body.
  • Structural complexity.

Classification by Stimulus Type

• Mechanoreceptors: Respond to touch, pressure, vibration, and stretch.
• Thermoreceptors: Sensitive to changes in temperature.
• Photoreceptors: Respond to light energy (e.g., retina).
• Chemoreceptors: Respond to chemicals (e.g., smell, taste, changes in blood chemistry).
• Nociceptors: Sensitive to pain-causing stimuli (e.g., extreme heat or cold, excessive pressure, inflammatory chemicals).

Classification by Receptor Structure

• Simple receptors for general senses:
  • Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense.
  • Modified dendritic endings of sensory neurons.
• Receptors for special senses:
  • Vision, hearing, equilibrium, smell, and taste.

From Sensation to Perception

• Survival depends on sensation and perception.
• Sensation is the awareness of changes in the internal and external environment.
• Perception is the conscious interpretation of those stimuli.

Sensory Integration

• Levels of neural integration in sensory systems:

  1. Receptor level: Sensory receptors.
  2. Circuit level: Processing in ascending pathways.
  3. Perceptual level: Processing in cortical sensory areas.
  • This involves the spinal cord, medulla, pons, cerebellum, reticular formation, thalamus, somatosensory cortex, and motor cortex.

Touch: Somatosensation - Active and Passive Touch

• Touch involves direct contact between two physical bodies.
• In neuroscience, touch is the special sense by which contact with the body is perceived consciously.
• Active Touch: Top-down process (e.g., wanting to know what something is or how to use it).
• Passive Touch: Bottom-up process (e.g.,sensing something touching you).

Active Touch Processes

• Involves seeing, identifying, deciding, making a plan, and executing it.

Processing at the Receptor Level

• To produce a sensation:
  • Receptors have specificity for stimulus energy.
  • The stimulus must be applied within the receptive field.
  • Transduction occurs where the stimulus is changed to a graded potential.
    • This can be a generator potential or receptor potential.
    • Graded potentials must reach the threshold to trigger an action potential (AP).

Receptor Level Details

• In general sense receptors, the graded potential is called a generator potential.

  $Stimulus \rightarrow Generator\ potential\ in\ afferent\ neuron \rightarrow Action\ potential$

TRP Channels

• TRP channels are involved in touch processing at the receptor level.
• They are ion channels that respond to specific changes and allow charged ions to pass through.
• Various modulators affect TRP channels, including PGS, BK, H+, ATP, NGF, CGRP, SP, ROS/RNS, CNP, 5-HT, Histamine, ILs, CCs, C5a, LTB4, LPA, TNF-$\alpha$, and PTHrP.
• Examples of TRP channels:
  • TRPV1: Activated by mechanical & heat. Associated with inflammatory pain, visceral pain, migraine, neuropathic pain.
    *Activated by heat, H+, LPA, LMS, Mechano
  • TRPM8: Activated by cold (≤ $25°C$) and menthol. Associated with migraine and dental pain.
    *Cooling and Testosterone
  • TRPA1: Activated by pH, temperature, and mechanical force. Associated with Visceral & inflammartory signals.
  • TRPV4: Activated by mechanical force, osmotic changes, temperature, H+, PG metabolites, ROS. Associated with visceral (internal) pain.

Piezo Channels

• Piezo channels are also involved in touch processing at the receptor level.
• These channels are anchored to the cell membrane.
• Activation occurs when the membrane bends upon touching due to physical pulling apart.
• Diezo 1: Autonomic visceral, walking, touch, walking, urination.

Processing at the Circuit Level

• Pathways of three neurons conduct sensory impulses upward to appropriate cortical regions.

  • First-order sensory neurons: Conduct impulses from the receptor level to spinal reflexes or second-order neurons in the CNS.
  • Second-order sensory neurons: Transmit impulses to third-order sensory neurons.
  • Third-order sensory neurons: Conduct impulses from the thalamus to the somatosensory cortex (perceptual level).

Touch: Somatosensation - Mechanoreceptors of the Hand

Four Types of Mechanoreceptors

1. Meissner corpuscles (RA1): Rapidly adaptive.
2. Merkel cells (SA1): Slowly adaptive.
3. Pacinian corpuscles (RA2): Rapidly adaptive.
4. Ruffini endings (SA2): Slowly adaptive.

• Rapidly adaptive receptors fire only upon change, while slowly adapting receptors fire continuously but slow down over time.
• Meissner corpuscles are near the junction between the dermis & epidermis, allowing for very specific sensations.

Mechanoreceptor Types & Functions

• All mechanoreceptors are myelinated and thin.
• Best Stimulus Summary
  • SA1: Edges & points; beginning & end
  • RA1: Lateral Motion
  • SA2: Stretch
  • RA2: Vibration

Size & Location

• Meissner corpuscles: Specific
• Merkel disks: Generalized
• Pacinian corpuscles: Deepest & largest; coiled membranes
• Ruffini endings: Eye shape

Fingerprint Structure

• Fingerprint structure enhances touch sensitivity in the hand.
• This involves limiting ridges, papillary ridges, sweat ducts, Meissner corpuscles, and Merkel cells in glabrous skin.

Cell's Receptive Field

• A cell's receptive field defines its zone of tactile sensitivity.
• Superficial layers have huge receptive fields compared to deeper layers

Somatosensation: Tactile Acuity

• Tactile acuity is a function of location, age, and sex.
• Higher acuity in the finger than the trunk.
• Merkel nerve endings degrade over time, leading to less acuity and more proneness to pain.
  • They get damaged and can't be replaced.
• Women generally have higher tactile acuity than men.

Slowly Adapting vs. Rapidly Adapting Fibers

• Slowly adapting fibers detect object pressure and form.
• Rapidly adapting fibers detect motion and vibration.

Load Force & Grip Force

• Lifting Task Actions:
  1. Hand contact
  2. Start of lift
  3. Table contact
  4. Release
• Neural Responses:
  1. Grip & release only (2 sets) - Meissner (RA1)
  2. Whole time -Merkel (SA1)
  3. Grip, lift, hold & release (Any pressure change) - Pacinian (RA2)
  4. All about stretch - Ruffini (SA2)

Pathways & Cortices

Somatosensory Ascending Pathways

• Involve the dorsal column medial lemniscal system (touch) and the anterolateral system (pain).

Cortical Areas

• Somatosensory cortex (S1): Located in the postcentral gyrus.
• Somatosensory cortex (S-II):
• Include areas 3a, 3b, 1, 2, 5, 7, 39, and 40.
• The body is mapped onto the somatosensory cortex (somatotopic map).