Somatosensory system Flashcards
Which 2 tracts are ascending, somatosensory tracts?
- Dorsal column medial lemniscus pathway
- Spinothalamic pathway
What modalities does the dorsal column system carry?
- Fine touch
- 2 point discrimination
- Proprioception
- Vibration
What modalities does the spinothalamic tract carry?
- Pain
- Temperature
- Crude Touch
What are modalities?
Basic units of sensation
What type of signal do first order neurones carry?
Signals are analogue
Converted to digital as information moves along the neurone
How are signals transmited in low stimulation vs high stimulation of receptors on first order neurones?
Low stimulation = low frequency of action potentials
High stimulation = high frequenc of action potentials
What are rapidly adapting receptors? Give an example
Receptors where the initial stimulus is high, sending lots of action potential but then you lose sensation as action potentials become less frequent over time.
e.g. mechanoreceptors in bum cheeks on sitting
What are slowly adapting receptors? Give an example
Receptors where frequency of action potential remains high until you stop/ doing something about the stimulus
e.g. nociceptors
What is a receptive field?
A region of skin that is supplied by a sensory neurone
Explain why there is some overlap between the boundaries of different dermatomes
Receptive fields for different dermatomes can over lap
What is an autonomous region (with regards to receptive fields)?
An area where there is minimal overlap of the receptive fields of different dermatomes
What is the relationship between acuity and receptive fields?
High acuity = small receptive field
What is the relationship between acuity and number of neurones?
High acuity = high number of neurones
What are first order neurones?
Neurones that carry sensory information from the peripheral nerves to the medulla oblongata
Project into the spinal cord ipsilaterally
Where is the cell body for first order neurones?
In the dorsal root ganglion
What are second order neurones?
Fibres that transmit information from first order neurones to third order nuerones
2nd order neurones decussate within the medulla oblongata
What are third order neurones?
Neurones that recieve information from 2nd order neurones, transmitting sensory information from the thalamus to the primary sensory cortex of the brain
What are the 2 different paths, first order neurones can take in the dorsal column medial lamniscus pathway?
From the upper limb (T6 and above): travel in fasciculus cuneatus and synpase in the cuneate nucleus in the medulla oblongata
From the lower limb (T6 and below): travel in the fasciculus gracilis and synpase in the gracile nucleus in the medulla oblongata
How do first order neurones from the upper limb and lower limb in the dorsal column medial lamniscus pathway travel in relation to each other?
Neurones from the lower limb are medial
Neurones from the upper limb are lateral
How do third order neurones travel from the thalmus to the primary sensory cortex?
They travel through the internal capsule
Lower limb neurones project medially
Upper limb neurones project laterally
How does the arrangement of first, second and third order neurones differ in the spinothalamic (anterolateral) pathway compared to the DCML?
- In the spinothalamic pathway, first order neurones enter the spinal cord and can ascend 1-2 levels (Lissaur’s tracts) in an area called substantia gelatinosa
- Secondary sensory neurones decussate within the spinal cord and ascend through the spinothalamic tract
How do secondary neurones from the upper and lower limbs travel in relation to each other in the spinothalamic path?
Upper limb = medial
Lower limb = lateral
What would you see if axons where damaged at the ventral white commisure?
Damage to the ventral white commisure can lead to bilateral deficits
Explain how rubbing a painful area helps alleviate pain
Activation of mechanoreceptors by rubbing excites inhibitory enkephalinergic (endorphins) interneurones in the cord which switches off the C fibres that carry pain
What is Brown-Séquard Syndrome?
If one half of the spinal cord segment is completely destroyed you will see…
- Ipsilateral loss of vibration, proprioception, motor function, 2 point discrimination (modalities mediated by DCML - deccusate in medulla oblongata)
- Contralateral loss of pain, temperature and light touch (modalties mediated by spinothalamic path- decussate at level of spinal cord)
