Somatosensory Neurones And Receptors Notes

Lecture 10

Somatosensory neurones and receptors

• General

  • Somatosensroy NS includes all sensory (or afferent) neural info excluding special senses e.g. from skin, muscle and viscera (organs)

  • System receives info

    • From outside world (sensors beneath skin)= EXTEROreceptors

    • About posture/movement (sensors in muscles/tendons)=PROPRIOreceptors

    • About internal environment/organs (sensors in viscera/tissues)= INTEROreceptors

  • Afferent: proceeding towards a centre (in this case periphery to CNS)

  • Special senses: Vision, olfaction, hearing, vestibial senses

• Sensory Modalities

  • Heat, Cold, Stretch, Acid, Sting, Itch, Press, Stroke, Tickle, Ache

  • Thermal; Mechanical; Chemical

  • Labelled line concept – separate pathways for the transmission of information relating to different modalities. Generally accepted for skin/muscle afferents.

• Primary afferent neurons

  • Pseudo-unipolar

    • Detect sensory stimuli & send message towards the CNS- therefore a type of peripheral neurone (their cell body lies outside the CNS)

  • With cell bodies in the

    • DORSAL ROOT GANGLIA

      • Between each vertebra

      • Peripheral fibres running with segmental nerves (muscle and skin afferents) or with autonomic nerves (visceral afferent fibres)

    • CRNIAL NERVE ROOT GANGLIA

      • Cranial nerve outflow (e.g. swelling on Vth nerve= trigeminal ganglion from head region, or on vagus Xth cranial nerve from visceral afferents)

      • Fibres running with autonomic nerves

• Are there specific pathways related to specific sensory modalities?

  • Separate pathways for transmission of information relating to different modalities (sensations) from sensory receptor cortex

  • Concept accepted for most types of somatic (skin & muscle) afferents, but discussion continues about visceral afferents

• TRANSDUCTION occurs at sensory receptive terminals

  • Energy of stimulus gives rise to RECEPTOR POTENTIAL (depolarisation of membrane)-Stimulus energy transduced into electrical impulse

  • If threshold exceeded an AP results, propagated along fibre

  • Threshold not exceeded, receptor potential remains local to terminal

  • Intensity of applied stimulus is reflected in rate of AP fired (‘all or nothing’)

• Sensory receptive properties of primary afferent neurons

  • Modality –what they respond to

    • PRINCIPLE TYPE of ADEQUATE stimulus transduced into electrical signal by the primary afferent neurone

    • ADEQUATE STIMULUS= stimulus to which a receptor responds effectively & gives rise to a particular sensation

    • Modalities include: touch, temperature, sound, sight, pain etc.

    • Modality depends on the type of channels or membrane structure present in the sensory receptive membrane

    • It is this membrane that is responsible for the transduction into an electrical signal that travels along the nerve

    • Type of adequate stimulus Channels in receptor membrane opened by

Mechanical Understanding channels poor so far

Chemical Some chemoreceptor channels known

Thermal Cool and warm sensitive channels found

• Threshold- low or high “how much” of the modality is required to activate them

  • Unit: Represents single neurone or activity measured from a single neurone

  • LOW THRESHOLD UNITS:

    • Units fire AP to low intensity (non-damaging/non-painful) stimuli

    • Adequate stimulus is non-damaging to the tissues

    • Low threshold mechanoreceptors (LTMs)= sensations of TOUCH, STROKE, SQUEEZE et

    • Low threshold thermoreceptors activated= sensations of WARM, COOL

    • Low threshold chemical stimuli= sensations such as TASTE/SMELL

  • HIGH THRESHOLD UNITS:

    • Stimulus threshold at which APs generated set by fibre/type of neuron

    • Neurones don’t respond to low intensity stimuli, only high intensity

    • High intensity stimuli potentially damaging (NOXIOUS) to the tissues

    • HT neurons therefore called NOCICEPTORS

    • Info interpreted as PAINFUL but ONLY if reaches brain higher centres

    • Very active during child birth but are still active after an epidural

    • Similarly with general anaesthetics

    • Different types

      • HT mechanoreceptors= high intensity mechanical stimuli

      • Thermal receptors= hot/cold

      • Chemical nociceptors= noxious chemical (acid), agents from tissue damage

      • Polymodal nociceptors= respond to at least 2 of above

  • NB: High intensity needed to activate high threshold fibre

• Adaptation rate – how do they react to a maintained activating stimulus

  • Property of primary sensory neurons,...

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