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Introduction

  • Without our sensory system, we would be cortices without interaction with the environment
  • The cortex doesn’t have pain receptors
    • In neuro operations, only need a local anaesthetic to remove the skull
  • All receptors just take a stimulus and change it into a change in the membrane potential of the receptor neuron

Receptors of the somatosensory system

Transduction

  • Receptors undergo a change due to stimulus and transducer this change into a membrane potential change
    • Ie, they take energy from the environment and transducer a membrane potential change (cell membrane permeability)
    • The change in membrane potential is the receptor potential
  • Sequence of events:
    • Stimulus
      • Change in ionic permeability of receptor cell
      • Change in membrane potential ie, a change in the receptor potential
        • Generation of action potentials in afferent nerve terminal
        • Propagation of action potentials to CNS
  • How do we differentiate between different stimuli?
    • Answer 3 questions:
      • What is the stimulus – type/modality
      • Where is the stimulus – location/receptor field + somatotopy
      • How strong is the stimulus – rate code (number of action potentials in a certain time. Faster, stronger stimuli cause action potentials at a higher rate)
        • Extension in time and space (population code – number of receptors) completes the description

Rate code

  • The number of ion channels opened, and ultimately the number of action potentials is proportional to the strength of the stimulus



Location

  • The receptor field defines the location
    • Stimuli stimulates different regions that are covered by specific receptors
    • Eg: muscle spindle field is stretch of muscles, in the skin there are dermatomes; similar for ears/eyes, etc
  • Pacinian corpuscles tell you gross localisation (e.g. on that finger, not sure where); Meissner's corpuscle localises more tightly, responds to light touch

Types – somatosensory modalities

Note that you might not have all the receptors on every single part of the skin on the body (some places on the skin might just not have receptors)

  • Four somatosensory modalities
    • Receptors are distributed throughout the body
  • Group I and II afferent fibres (A-beta) – large myelinated fibres
    • Tactile
      • Includes hair afferents, Pacinian corpuscles, Merkel discs (all have slightly different jobs related to sense of touch)
    • Proprioception – limb and joint position + movement (kinaesthesia)
      • Includes spindle afferents, Golgi tendon organs, joint receptors
  • Group III and IV afferent fibres (Ad and C) – small (Ad = myelinated, C = unmyelinated)
    • Thermal
      • Includes hot and cold receptors
    • Include nociceptors that respond to mechanical, chemical or thermal stimulus (pain submodalities)
      • Pain – travel speed of <1m/s which is very slow, however if there is a small pain, it is still felt
        • There are distinct pathways for pain vs tactile/thermal: if you crush someone's hand, the tactile receptor can detect very intense pressure, but no pain until pain receptors turn on (pain itself can become more intense, but you won't feel pain based on intense tactile stimulation)
  • Groups define size, A-->C etc defines speed
  • The skin contains all tactile, thermal and pain receptors
  • Observations
    • Difference between tactile and thermal – tactile only works while in contract, thermal can work while not
    • Difference between tactile and pain – touch travels faster, thus we have time to think ‘OMG’ before we feel the pain
  • Locations in the skin
    • Free nerve endings – the general nerve terminals control temperature and pain
    • Meissner’s corpuscle, Pacinian corpuscle – some deep, some shallow, are responsible for different qualities of touch
  • Two types of skin: hairy skin and glabrous skin
    • Free nerve endings occur in both locations (i.e. the afferent itself functions as the receptor: checks for pain and temperature - Ad and C)
    • Touch is different: Meissner's (detailed touch on fingertips) and Pacinian corpuscles
      • Hairy skin does not have Meissner's corpuscles (so no high resolution touch)
    • Glabrous skin also has difference in how Merkel's discs are arranged
    • Deep receptors e.g. Ruffini (stretch) and Pacinian (contact/vibration) are not very good with resolution and localisation (e.g. bad for Braille, unlike Meissner's)

Responses of thermoreceptor fibres

  • Cold receptors are small myelinated fibres
    • Warmth receptors are unmyelinated
    • Therefore, cold receptors are faster
  • We need two types of thermoreceptor otherwise we can’t tell the difference between separate temperatures
    • Ie with cold, can’t tell between 20 and 30 degrees (rate coding would be nondiscriminate if you had only cold receptors above 33 degrees: the rate of activity would be the same no matter how far above the 40 degrees). Ie all receptors have an optimum point they respond at, and would fall off at either side
      • Putting warm and cold receptors together, you can break ambiguity
    • Similar situation with colourblindness and colours; particularly colourblindness at night

Pain

  • An ancient sense
    • Unmyelinated because during evolution, there has never been enough time for the pain sense to be taken offline and upgraded to myelinated fibres; similar for the lack of localisation
  • Somatic/skin pain (small myelinated fibres)
    • Fast prickling pain using Aδ (group III) fibres
      • Readily localised (large unmyelinated)
    • Slow burning pain/itch uses C (group IV) fibres
      • Poorly localised
      • Large receptive fields e.g. when mosquito bites, chemoreceptors detect chemical changes to the bitten area, causing irritation and itching; not only to the bitten area but the surrounding areas too
  • Deep/visceral pain
    • Pain with a dull or diffuse character, uses C fibres
  • Experiment with fingers, note that proprioception is pretty bad at localising

Pathways

  • Tactile and proprioception
    • Travel in the ipsilateral dorsal column
    • Decussate at the medulla and form the medial lemniscus
    • Medial lemniscus heads up to the contralateral thalamus to the original stimulus, then corticothalamic fibres take it to cortical regions
  • Pain and temperature and crude touch
    • Synapse in the spinal cord
    • Decussate at their vertebra and travel in the contralateral spinothalamic tract
    • So reaches the same target from the same origin, but takes quite a different pathway
  • Lesions on one side of the spinal cord can affect both sides since different tracts decussate at different levels
    • Damage left side of spinal cord at level of spinal nerve for left leg --> no touch but can still feel pain on that side (but on other leg, can feel touch only but no pain).

Spinal nerves

  • A pair of nerves exit between each vertebra
    • Each spinal nerve innervates a dermatome
      • Viruses can live in a dorsal root ganglion and cause a characteristic rash in a dermatome (e.g. VZV)
    • Grey matter is enlarged in regions of upper limb due to nuclei for the nerves for the upper limb muscles
  • Cauda equina = no grey matter, only nerves coming out to leave within the right vertebral space (here is where you do lumbar puncture/spinal block)
  • A spinal nerve innervates a dermatome

Topographic organisation, somatosensory and motor maps

  • Somatosensory and motor maps match
  • Note that there are association areas for each sense surrounding the primary somatosensory areas
  • Homunculus
    • Shows body with proportions of sensory input
    • Little finger is bigger because it is on the outside and receives more sensory stimulation
  • Perception of 2 points:
    • Requires a silent receptor field between two fields to discriminate 2 points
      • Depending on the size of the receptive field, you may feel one or two points when you use a 2 point stimulus (you need a region of silence in between the two active areas as well)
    • You need an area of high tactile acuity
    • Somatotopy facilitates discrimination of a 2 point stimulus
  • Genitals also have a disproportionately big genital representation

The homunculus represents the cortical area devoted to each body region

  • Body space is represented topographically in M1 and S1

Parietal lesions

  • Anterior areas
    • Impaired localisation and astereognosis (can’t recognise objects by touch)
    • Detection may be preserved, but cutaneous-kinaesthetic perception may be lost
    • Perception of body image and spatial relations may be lost (normally body map adjusts with growth, but if you lose part of cortex, you lose part of body map, changing your image of your own body)
  • Posterior parietal – left hemisphere
    • Aphasia
    • Gerstmann’s syndrome:
      • Confuse left and right, can’t name finger touched
      • Dysgraphia (difficulty writing), dyscalculia (difficulty in arithmetic)
  • Learn to count on fingers, thus same area used for calculations
  • Posterior parietal – right
    • Unilateral neglect of left side
      • Ie, patient may be unaware and deny hemianopia or hemiplegia
    • More common (~5-10 fold) with right hemisphere damage than with left.
    • It's not that they can't see the other side (they can), it's just that their spatial perception of that side is lost
  • This extends to their mental image of space
    • If various functions are lost, patient can lose the knowledge that they ever had that function
    • E.g. an artist that loses their perception of colour, not only do they not see colour, but they also lose the understanding that they ever painted in colour or even knowing what colour is