Central Somatosensory System Flashcards
What are the 4 somatosensory modality receptors?
1) Aa fibres (group 1) –> largest, myelinated, 80-120m/s
* proprioception: limb and joint position (or movement which is kinaesthesia). Receptors include
- spindle afferent
- golgi tendon organs
- joint receptors
2) Ab fibres (group 2) –> large, myelinated, 30-70 m/s
* tactile. sub modalities:
- hair afferents
- Pacinian corpuscles
- Merkel discs
3) Ad fibres (Group 3) –> small, myelinated, 5-30 m/s
4) C fibres (Group 4) –> unmyelinated, 0.5-2 m/s
* Thermal (Ad and C fibres) Sub modalities:
- Hot receptors (C fibres)
- Cold receptors (Ad fibres)
* Pain (Ad and C fibres) Sub modalities:
- mechanical noiciceptors
- chemical noiciceptors
- thermal noiciceptors
What is the somatosensory system?
The somatosensory system is a subset of the sensory nervous system that produces the perception of proprioception, touch, pain and temperature.
It is different to other systems in that:
* Broadly distributed receptors - rather than concentrated in a specialised location
* Responds to different kinds of stimuli (at least 4 rather than 1)
What is rate coding?
A method to compare differences in spiking from different stimuli.
a. Rate coding allows receptors to communicate intensity of stimulus
b. Rate of action potential generation depends on the amount of depolarisation
What is the receptive field?
Region of the external environment/spatial location/stimulus space that activates a particular neuron
* Some receptors have very large (Pacinian corpuscles), others have smaller receptive fields (Meissner’s corpuscles)
* Every sensory neuron is best activated by a particular stimulus and a receptive field.
What are the types of pain?
a. Somatic/skin pain:
- Fast, pricking pain, readily localised –> Ad fibres
- Slow, burning pain/itch, poorly localised –> C fibres
b. Deep visceral pain
- Pain with dull or diffuse character –> mainly C fibres
Outline key features of temperature.
a. Temperature is detected by thermoceptors –> hot and cold
b. Temperature appears to be encoded by the rate of firing of action potentials in thermoceptive afferents
Distinguish between projected pain and referred pain.
a. Projected pain: the site at which the noxious stimulus acts is not that at which the pain is sensed (ex. phantom limbs: perception of painful stimulus in body part that is no longer present)
b. Referred pain: the viscera at which the noxious stimulus acts produces sensation of pain not in the affected organ but rather in distant superficial structure (e.g Heart attack –> pain in left shoulder)
What are pain receptors?
a. a.k.a nociceptors - are free nerve endings classified by stimulus sensitivity.
- mechanosensitive
- thermosensitive
- chemosensitive
- polymodal (respond to 2 or more of the previous ones)
What are the two key pathways?
1) Dorsal column-medial lemniscus pathway
2) Spinothalamic pathway
Describe the dorsal column-medial lemniscus pathway.
a. Fine touch, vibration and proprioceptive afferents travel in the ipsilateral dorsal column.
i. Below T6 synapse in the gracile nucleus
ii. Above T6 synapse in the cuneate nucleus
b. They decussate at the medulla (sensory decussation) to form the medial lemniscus
c. Axons from the trigeminal ganglion (CN5) synapse then decussate to join the medial lemniscus
d. Axons in the medial lemniscus synapse in the ventral posterior nucleus of the thalamus –> project to specific regions of the primary somatosensory cortex.
Describe the spinothalamic pathway.
a. Crude touch, pain, temperature fibres synapse in the spinal cord.
b. They then decussate and travel in the contralateral spinothalamic tract.
c. Axons from the trigeminal ganglion synapse then decussate to join the spinothalamic tract
d. Axons synapse in the ventral posterior nucleus of the thalamus –> project to specific regions of the primary somatosensory cortex.
Describe an anterior parietal lobe lesion.
Anterior (areas 1, 2 and 3) - S1
a. Impaired localisation (inability to perceive where on the skin the stimulus is) and astereogenesis (inability to identify objects by touching)
i. Detection may be preserved but loss of cutaneous-kinaesthetic perception is possible
b. Perception of body image and spatial relations may be altered.
Outline posterior parietal lobe (left hemisphere) lesions.
a. Disorders of language
b. Gerstmann’s syndrome - confuse L and R, can’t name finger touched (finger agnosia), dysgraphia, dyscalculia
i. all despite an absence of motor and sensory deficits
ii. dysgraphia: difficulty with spelling and written expression
iii. dyscalculia: math learning disability
Outline posterior parietal (right hemisphere) lesions.
a. Unilateral neglect of left side
i. Patient is often unaware of and may deny hemianopia or hemiplegia
1. hemianopia: loss of sight in one visual field
2. hemiplegia: one sided paralysis
ii. Extends to mental representation of space
iii. 5-10x more common with RHS parietal damage than with LHS
