Somatosensory Physiology Notes

Lecture 11

Somatosensory Physiology- Peripheral terminations of non nociceptive DRG neurons

• Somatosensory nervous system

  • Transmits all sensory (or afferent) neural info, excluding the special senses

  • Includes nerves sending info from

    • Skin; Sensors in muscles and tendons; Sensors in visceral organs and tissues

• Non-nocicpetive cutaneous afferent fibre types

  • Glaborous skin

    • Pacinian corpuscle

      • Non hairy and hairy skin

      • 1mm in length

      • Fingers, toes, genitals

      • 20-60 concentric lamellae of fibrous connective tissue/fibroblasts

      • The lamellae are very thin, flat, modified Schwann cells

      • Fluid-filled cavity with a singleafferent unmyelinated nerve ending

      • Detect tap and (particularly) vibration

      • Very rapidly adapting low threshold mechanosensitive Abeta fibres

    • Meissner’s corpuscle

      • Responsible for “light touch” and low frequency vibration

      • Just below epidermis

      • Rapidly adapting low threshold mechanoreceptor, Abeta fibres

      • Not in hairy skin

      • Unmyelinated endings enclosed in capsule

      • Corpuscle=30-140micrometres length; 40-60micrometers diameter

Composed of elastin attached to epidermis

• Merkel’s cells

  • Oval receptor ~10micrometres cells in skin and oral/rectal mucosa and mammary glands

  • Synaptic contact with disc like terminal of SA1 low threshold mechanosensitive AB fibres (branch innervating up to 90 cells)

  • Stratum basale of epidermis clustered in touch domes

  • Light touch (Braille)

  • Small receptive field

  • Transmitter may be glutamate

• Ruffini end organ

  • Slowly adapting mechanoreceptors in deep sites skin (hairy, glabous) & also in joints

  • Monitor tissue stretch

  • May also sense heat

  • Innervated by single Abeta fibres which branch in corpuscle ending

• End-bulbs of Krause

  • Encapsulated nerve ending in conjunctiva, lips, tongue, penis, clitoris

  • Not in hairy skin

  • May be a cold receptor not well understood

• Unencapsulated nerve endings

  • Respond to temperate, pain and pressure

  • Project to spinal cord via ‘A-delta’ and C sensory afferents (slow)

• Hairy skin

  • Hairs receive sensory innervation which provides important means of mechanosensation in hairy skin

  • G-hair unit

    • Fulfils role played by Meissner’s in glabrous skin

    • Activated by hair movement- particularly rapid movements

    • Signal through rapidly adapting Abeta fibres

  • D-hair unit

    • On finer down hairs

    • Rapidly adapting signal through Adelta fibres

    • Very sensitive to slow movements “flea detectors”

• Thermoceptors (in skin)

  • Different units respond to different temperatures

• Proprioceptors

  • Proprioceptors provide information about the position and movement of the body

  • Signal muscle stretch, tension, & joint position

  • Receptors involved are in muscle, tendons and joints

• Afferents projecting to muscle

  • General

    • Send sensory information about muscle dynamics

    • Located in “belly” of muscle

    • ‘Intrafusal’ muscle fibres contain ‘muscle spindles’, which signal passive, static and dynamic muscle strength

  • Types

    • Ia (primary fibres)

      • Annulospiral endings

      • Firing pattern: dynamic and static

      • Dynamic rapidly adapting

      • Detect change in stretch

    • II (secondary)

      • Flowerspray endings (spread over the muscle)

      • Firing pattern: static

      • Static slowly adapting

      • Detect tonic stretch

  • Tendon and joint receptors

    • Nerve type fibre= Ib

    • Ending on golgi tendon organs

    • Static firing pattern

    • Slowly adapting

    • Detect stretch

• Ascending pathway for information from low threshold mechanoreceptors=DORSAL COLUMN/MEDIAL LEMNISCAL PATHWAY

  • Sensory pathway (overview)

    • Primary afferent neuroneNeurones in dorsal columni of medullaThalamic nucleiSomatosensory cortex

  • Pathway

    1. Mainly A-alpha and A-beta LTMs and types I & II LTMs into dorsal part of spine

       • FIRST ORDER NEURON (primary afferent)

       • Branches in dorsal horn contribute to

         • Local spinal cord reflexes

         ...

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