Week 6 HUBS - Sensory systems – Proprioception, touch, pain & temperature

Special senses: complex receptor apparatus, head-only locations; include vision, hearing, balance, olfaction, taste
General senses: less specialized receptors, distributed all over body; activated by touch, temperature, pain, movement; may be free nerve endings (e.g., nociceptors)

Exteroceptors: skin; information about outside world (touch, pressure, vibration, temperature, pain)
Proprioceptors: muscles, tendons, joints, subcutaneous tissues; information about position and movement (kinaesthesis), deep pressure, dull aching pain
Interoceptors: internal organs; homeostasis and organ function information

Merkel’s discs, Meissner’s corpuscles, Pacinian corpuscles, Ruffini endings, Krause end bulbs (cold thermoreceptors), free nerve endings
Mechanoreceptors detect light touch, pressure, vibration; receptive field size varies; activation can be tonic (slowly adapting) or phasic (rapidly adapting)

Meissner’s corpuscles: light touch; rapidly adapting; location: dermal papillae of hairless skin (e.g., fingertips, nipples, lips, eyelids)
Pacinian corpuscles: deep pressure, high-frequency vibration; rapidly adapting; location: deep dermis, hypodermis, joint capsules
Ruffini endings: deep pressure, stretch; slowly adapting; location: dermis and hypodermis
Tactile discs (Merkel cells): light touch; slowly adapting; location: basal epidermis
Free nerve endings: various modalities; distributed widely; often nociceptors

Essential for body position in space; usually subconscious
Receptors in muscles, joints, tendons; sense position, stretch, length, and movement
Muscle spindle: length and rate of change of muscle length; intrafusal fibers; afferent endings Ia (primary) and II (secondary); efferent gamma motor neurons control intrafusal fibers
Golgi tendon organ (Ib): muscle tension; senses force transmitted to tendon
Joint receptors: Ruffini endings, Pacinian corpuscles, joint capsule mechanoreceptors (low-threshold mechanoreceptors, LTM)
Alpha motor neurons: innervate extrafusal muscle fibers; Gamma motor neurons: innervate intrafusal fibers

Ascends via dorsal column–medial lemniscal pathway for conscious proprioception
Spinocerebellar tracts carry unconscious proprioception to cerebellum
Dorsal column: gracile nucleus (lower limbs/trunk), cuneate nucleus (upper limbs/trunk); face via trigeminal pathway

Dorsal column–medial lemniscal pathway: precise, discriminative touch and proprioception; decussation occurs in medulla
Spinocerebellar tracts: convey proprioceptive info to cerebellum for coordination; include dorsal and ventral pathways; origins include Clarke’s nucleus (T1–L2/3), spinal border cells (T12–L5)
Cuneocerebellar tract: from upper limbs via lateral cuneate nucleus to cerebellum

Pain: unpleasant sensory and emotional experience tied to tissue damage
Nociception: detection of tissue damage; can occur without conscious pain
Perception vs nociception can be separable (e.g., anesthesia blocks nociception; thalamic pain can occur with little nociception)

Aδ fibers (myelinated): conduction ~ $v_{A heta} \,=\,20\ \,\mathrm{m/s}$ ; respond to intense mechanical or mechanothermal stimuli; rapid sharp pain
C fibers (unmyelinated): conduction ~ $v_{C} \,=\,2\ \,\mathrm{m/s}$ ; respond to thermal, mechanical, and chemical stimuli; polymodal; dull, aching pain
Nociceptors can be mechanical, thermal, or chemical; mediators include ATP, bradykinin, PGE2, NGF, serotonin, etc.; endorphins/opioids act via µ-opioid receptors

Lissauer’s tract: pain and temperature fibers enter and ascend in anterolateral system
Lateral spinothalamic tract: conveys pain and temperature to thalamus
Anterior (crude) spinothalamic tract: crude touch and pressure
Thalamus relays to cortex; affective/emotional components project to limbic structures

Pain has an affective component linked to emotions; pain is a strong motivator for fear, anxiety, and avoidance
Chronic pain can lead to maladaptive states: social withdrawal, depression, anxiety

Visceral pain arises from internal organs; poorly localized
Pain often referred to cutaneous areas served by the same spinal segments (dermatomes)
Examples of referred-pain dermatomes include diaphragm (C3–C4), heart (T1–T4, left), stomach (T6–T9, left), gallbladder (T7–T8, right), duodenum (T9–T10), appendix (T10, right), kidneys/ureters (L1–L2)
Visceral afferents converge on dorsal horn neurons with somatic afferents, then project via spinothalamic tract

Hemicord lesion leads to:
- Ipsilateral loss of dorsal column functions (touch, vibration, proprioception, two-point discrimination)
- Contralateral loss of pain and temperature (anterolateral system) below the lesion
Demonstrates somatotopic organization of dorsal columns and spinothalamic pathways

Phantom limb phenomena; pain perceived in a missing limb; brain map reorganization may contribute

Gracile nucleus = lower limbs and lower trunk; Cuneate nucleus = upper limbs and upper trunk; Trigeminal nucleus = face
Higher receptor density in a region → larger cortical representation (sensory homunculus)

Pain: subjective experience with emotional components
Nociception: detection and signaling of tissue damage
Pain can occur without peripheral nociception and vice versa; e.g., local anesthesia blocks nociception but not necessarily pain, and thalamic pain can occur without peripheral nociception

Superficial pain: skin/subcutaneous tissues; well localized; sharp/pricking/burning
Deep somatic pain: muscles/tendons/joints/bones; dull ache/cramps; less localized
Visceral pain: from internal organs; poorly localized; dull/cramping

Mechanical, thermal, and chemical modalities activate nociceptors
Chemical mediators potentiate nociception; opiates modulate nociception via receptors

Receptors are classified as exteroceptors, proprioceptors, and interoceptors
Skin contains a spectrum of mechanoreceptors with distinct adaptational properties and receptive fields
Proprioception relies on muscle spindles, Golgi tendon organs, and joint receptors; pathways include dorsal column–medial lemniscal and spinocerebellar tracts
Nociception involves Aδ and C fiber nociceptors with distinct conduction velocities and perceptual qualities
Pain is multi-dimensional (sensory and affective); two systems can be dissociated, with visceral pain often referred to somatic dermatomes
Spinal cord injuries illustrate the organization of sensory pathways (Brown-Sequard)
Phantom limb pain reflects cortical reorganization after limb loss