L4 Somatosensory System Flashcards
Types of sensation
General sensation - pain and touch etc.
Special sensory - sight, smell, hearing, taste, balance
2 types of general sensation
Somatic:
- conscious
- from body surface
Visceral
- unconscious
- most sensation does not reach consciousness
Spinothalamic modalities
Temperature
Pain
Pressure - crude touch
Spinothalamic pathway
- Responsible for modalities of sensation that are crucial for survival
- Located anterolaterally
Dorsal column modalities
- vibration
- fine touch
- proprioception
- 2 point discrimination
Dorsal column
- Responsible for more sophisticated modalities
- located posteriorly
Modalities
- Units of sensation that cannot be subdivided
- mediated by a single type of receptor
- stickiness for example is made up of multiple modalities
Intensity of a stimulus
Analogue signal - can vary continuously
Analogue signal converted to digital signal (0 and 1s) into frequency of action potentials
Rapidly adapting receptors
- initial high frequency of action potential firing but then becomes less frequent
- in e.g. cutaneous mechanoreceptors
E.g when sitting down, you are initially are of it but then you can’t consciously fell it anymore
Slowly adapting receptors
E.g nociceptors - pain
- action potential frequency remains high until the stimulus is removed
- frequency doesn’t change with time
Receptive field
- Region of skin that a sensory neurone supplies
- can overlap between sensory neurones therefore loss of sensation is less than expected
Size of receptive field
Large receptive field - decreased sensory acuity as receptors are more spread out
Small receptive field - high sensory acuity as receptors are more concentrated
Sensory acuity
Acuity is inversely proportional to the size of the receptive field
Acuity is directly proportional to the number of sensory neurones present e.g high in tongue
Primary sensory neurone
- communicates with receptors of the same type
- has multiple dendrites
- cell body is in the dorsal root ganglion
- projects ipsilaterally into spinal cord (same side as cell body)
- synapses on to the second order sensory neurone
Secondary sensory neurone
- cell body in the dorsal horn or the medulla
- DECUSSATES - crosses the midline
- synapses on to the third order sensory neurone in the thalamus
Tertiary sensory neurone
- cell body in the thalamus
- projects to the primary sensory cortex in the post central gyrus via the internal capsule
Somatotopy
1 to 1 correspondence between the surface of the body (dermatome) and an area of the primary motor cortex (homunculus)
Conversion and convergence occurs in the thalamus in an organised system
Dorsal column medial lemniscus pathway
- Primary neurone - projects into cord on the ipsilateral side
- Ascends in the gracililis fasciculus CLOSE TO THE MIDLINE
- Synapses in the gracile nucleus (lower limb) in the medulla onto the 2nd order neurones
- The second order neurones decussate and ascends via the medial lemniscus tract
- In the thalamus it synapses onto the third order neurones
- Third order neurones project to the associated sensory cortex via the internal capsule
Upper limb
- Joins ascending neurones laterally
- the first order sensory neurone synapses in the cuneate nucleus in the medulla
- the first order sensory neurones ascend via the cuneate fasciculus
Gracile fasciculus
Region of the dorsal column
Below T6
Contains the gracile nucleus
Cuneate fasciculus
Region of the dorsal column
Above T6
Contains the cuneate nucleus
Injury to medial dorsal column
Affects lower limbs
Injury to lateral dorsal column
Affects upper body
Spinothalamic system
- Primary sensory neurone enters the cord ipsilaterally
- Synapses onto the secondary sensory neurones in the dorsal horn and DECUSSATES EARLY IN THE DORSAL HORN
- Secondary sensory neurones ascend via the spinothalamic tract LATERALLY to the thalamus
- Synapses on to the third order sensory neurone in thalamus and projects on to the primary sensory cortex
Where do second order sensory neurones in the spinothalamic tract decussate? The
Ventral white commissure
Lissauer’s tract
The first order sensory neurone can ascend a few segments and synapse to a higher secondary neurone via the Lissauer’s tract
E.g the C7 primary sensory neurone can ascend and synapse on to the C5 secondary neurone
Therefore the spinothalamic loss may be less than the dorsal column loss
A fibres
Carry impulses from mechanoreceptors in the skin
C fibres
carry pain impulses
Rubbing wound
- Activation of mechanoreceptors stimulates A fibres.
- Stimulates inhibitory enkephalinergic interneurones which releases enkephalins (endorphins)
- Inhibits secondary spinothalamic sensory neurones therefore pain decreases
Huge trauma and pain
Higher relay station in the periaqueductal region of grey matter inhibits secondary spinothalamic sensory neurones therefore cannot feel pain
Opiates
E.g morphine
Inhibit secondary spinothalamic sensory neurones in the dorsal horn
Why is the hand particularly vulnerable to damage with brain tumours
- Represents a large area of the homunculus - very tactile and large sensory innovation
- hand is represented laterally
Which modalities are affected in a brain tumour of the primary sensory cortex
All modalities as both the dorsal column and spinothalamic tracts are affected
Glove and stocking paraesthesia
Only affects hands and feet
- Due to peripheral neuropathy e.g diabetes or vitamin B12 deficiency
- 2 lesions in the primary sensory cortex
Affect of destroying then right half of the C5 segment
Spinothalamic tract:
- sensory loss of both left and right side at C5
- contralateral sensory loss from C5 below (left)
- affects lower spinal levels as lower spinal level primary sensory neurones can ascend via Lissauer’s tracts
- loss of pain, temperature and crude touch sensations
Dorsal column:
- ipsilateral sensory loss from C5 below
- loss of vibration, fine touch and proprioception
