Sensory Processing, Touch, and Pain

Sensory Processing: Summary

  • Sensory receptor organs detect energy or substances.
  • Sensory processing begins in receptor cells.
  • Sensory information processing is selective and analytical.

Classification of Sensory Systems

  • Mechanical: Touch, pain, hearing, vestibular, joint, muscle.
  • Thermal: Cold, warmth.
  • Visual: Seeing.
  • Chemical: Smell, taste, common chemical, vomeronasal.
  • Electrical: Electroreception.

Sensory System: Basic Notions

  • Sensory receptor organs detect stimuli.
  • Receptor cells convert stimuli to electrical signals.
  • Adequate stimulus: stimulus sensory organ adapted to (e.g., light for the eye).

Sensory Perception

  • Sensory systems have restricted responsiveness.
  • Detection threshold: the weakest stimulus detectable half the time.

Weber’s Law

  • DI= K \times I (jnd: just noticeable difference)
    • I: stimulus intensity, DI: intensity difference, K: constant.

Weber-Fechner Law

  • R = K \log (I/Io)
    • R: subjective stimulus intensity, Io: threshold, I: perceived stimulus intensity.

Doctrine of Specific Nerve Energies and Labeled Line Hypothesis

  • Receptors and neural channels for different senses are independent.
  • Brain recognizes distinct senses via separate nerve tracts (labeled lines).

Receptors in the Skin

  • Pacinian corpuscles, Meissner’s corpuscles, Merkel’s discs, Ruffini’s endings, and free nerve endings.

Sensory Transduction

  • Conversion of stimulus energy into a change in membrane potential (receptor potential).

(Mechano)receptors

  • Vibration stretches neuronal membrane, enlarging ion channels, allowing Na+ entry, initiating a receptor potential.

Intensity Coding

  • Patterns of action potentials reflect stimulus intensity.
  • Intensity coded by:
    • Frequency of action potentials.
    • Multiple neurons acting in parallel (more neurons recruited as stimulus strengthens).
    • Range fractionation (different cells have different thresholds).

Response Adaptation

  • Adaptation: Progressive loss of response to a maintained stimulus.
    • Tonic receptors: Slow or no decline in action potential frequency.
    • Phasic receptors: Display adaptation and decrease frequency of action potentials.

Nerve Fibers

  • Sensory functions, receptor types, axon types, diameter, and conduction speed.

Control of Sensory Processing

  • Stimulus location determined by activated receptors (bottom-up).
  • Top-down processing: Higher brain centers suppress/amplify inputs.
  • Sensory pathways pass through the thalamus and terminate in the cerebral cortex (except smell).

Levels of Sensory Processing

  • Sensory information enters CNS, reaches thalamus, which transmits to cortex.
  • Cortex directs thalamus to suppress sensations.
  • Primary sensory cortex exchanges information with nonprimary sensory cortex.

Receptive Fields

  • Space in which a stimulus alters a neuron’s firing rate.
  • Differ in size, shape, and response to stimulation.

Lateral Inhibition

  • Capacity of an excited neuron to reduce the activity of its neighbors.
  • Creates contrast in stimulation, aiding perception.

Cortical Receptive Fields

  • Primary sensory cortex (S1) exists for each modality.
  • Secondary sensory cortex (S2) receives input from primary area.

Somatosensory Cortices

  • Primary somatosensory cortex (S1): touch information from opposite side of body.
  • Secondary somatosensory cortex (S2): maps both sides of body.

Organization of the Primary Somatosensory Cortex

  • Topographical organization (somatotopy).
  • Sensory cortical homunculus: distorted representation of the human body.

Columnar Organization

  • Cortical column: Vertically arranged cell constellation.

Higher-Order Cortices

  • Multimodal sensory representation.
  • Attention: Selective awareness; enhances processing.
  • Brain regions important in attention: posterior parietal cortex, cingulate cortex.

Higher-Order Sensory Perception

  • Sensory systems influence each other.
  • Association areas show mixed inputs.
  • Polymodal cells: Intersensory interactions.
  • Synesthesia: Stimulus in one modality creates sensation in another.

Touch Receptors

  • Pacinian corpuscle (vibration), Meissner's corpuscle (touch), Merkel's discs (touch), Ruffini's ending (stretch).

Dermatomes

  • Area of skin supplied by afferent nerve fibers from a single dorsal root of spinal nerve.
  • Adjacent dorsal roots overlap.

Somatosensory Pathways 1: Dorsal Column System

  • Delivers touch information to the brain.
    • Receptors send axons to medulla, cross midline, go to thalamus, then S1.

Definition of Pain

  • Unpleasant sensory and emotional experience, associated with actual or potential tissue damage.
  • Purposes: Withdrawal, energy conservation, social signal.

Sources of Pain

  • Cutaneous, somatic, visceral, neuropathic, phantom (limb).

Nociceptors

  • Peripheral receptors that respond to painful stimuli.

  • Free nerve endings in the dermis respond to temperature, chemicals, and pain.

  • TRP ion channels (CMR1, TRPV1, TRPM3).

    TRP Ion Channels

    • TRP channel temperature range

Neurochemistry of Pain

  • Peripheral fibers use glutamate and substance P to excite spinal cells.
  • Dorsal horn neurons can become hyperexcitable and cause chronic pain.

Peripheral Mediation of Pain

  • Damaged cells release substances that excite free nerve endings.
  • Information enters through dorsal root and synapses on neurons in dorsal horn that send information to thalamus.
  • Pain fibers release glutamate and substance P.

Somatosensory Pathways 2: Anterolateral System

  • Transmits pain and temperature sensations.
    • Free nerve endings synapse on spinal neurons in dorsal horn; information crosses midline, ascends to thalamus, then S1.

Pain

  • Sensory-discriminative, motivational-affective, and cognitive evaluative qualities.

Descending Modulation of Pain: Anatomy

  • Analgesia: Absence or reduction in pain sensation.
  • Opiates reduce pain, opioids are endogenous opiate-like peptides.
  • PAG (periaqueductal gray) involved in pain perception; electrical stimulation produces analgesia.

Descending Modulation of Pain: Neurochemistry

  • PAG, raphe nucleus.

Melzack-Wall’s Gate Control Theory of Pain

  • Pain information can be blocked by a gating action in the spinal cord.

Pain Relief Intervention

  • Psychogenic, Pharmacological, Stimulation, Surgical.

Placebo Effect

  • Inert substance can relieve pain by releasing endogenous opiates.

Key Concepts (Sensory Processing)

  • Transduction of stimuli into neural impulses.
  • Discrimination among stimulus energy forms via specific receptors.
  • Ideal sensory system attributes: reliability, rapid processing, suppression, selectivity.
  • Transduction produces generator potential in receptors.
  • Sensory events coded in patterns of neural impulses.
  • Stimulus quality encoded in labeled lines.
  • Adaptation: Progressive decrease in sensitivity; phasic vs. tonic receptors.
  • Receptive fields: Stimulus area that changes cell responses.
  • Cortical maps display plasticity.
  • Skin contains diverse specialized receptors.
  • Pain involves specialized receptors, fiber projections, substance P, and integration in cingulate cortex.
  • Pain modulated at brain and spinal cord levels.
    The opiate system plays a major role in modulating pain sensation.