3.1 somatic sensation and pathways Flashcards
what is the difference between general and special sensation?
general = body wall and viscera
special = senses of vision, hearing, balance, taste and smell
what is a modality of sensation?
a ‘unit’ of sensation, relying on a distinct receptor type
what are the modalities of the spinothalamic system?
temperature (thermoreceptors)
pain (nociceptors)
pressure/crude touch (mechanoreceptors)
what are the modalities of the dorsal column medial lemniscus system?
vibration
proprioception
fine touch
two point discrimination (highest in finger tips and lips, low in elbows)
All by mechanoreceptors
give some examples of primary sensory neurones
dorsal root ganglia
primary afferents
first oder sensory neurones
psuedunipolar neurones
how does the strength of receptor activation affect the frequency of action potentials in the primary sensory neurone
strong receptor activation causes high frequency of action potentials in the primary sensory neurone
what are rapidly adapting receptors? explain with an example.
initially, mechanoreceptors send a high frequency of action potentials e.g when you first sit on a chair. However, eventually the frequency of action potential firing slows so you are unaware of the fact you are sat on a chair. if you change your position in the chair, are AP frequency is higher and you detect the chair
what are slow adapting receptors? give ab example.
e.g nociceptors. Change their frequency of firing very little after the initial stimulus = why pain can be so persistent, and you never reply get use to having pain. e.g tooth ache
what is a receptive field? how do they relate to two point discrimination, and why can they affect dermatome boundaries?
an area of skin that a single sensory neurone innervates
if an area of skin is supplied by sensory neurones with relatively large receptive fields, the area will have low sensory acuity e.g the back
if an area of skin is supplied by sensory neurones with relatively small receptive fields, this area will have high sensory acuity and good two point discrimination e.g fingertips
the overlap of receptive fields of primary sensory neurones from adjacent dermatomes is one of the reasons why dermatomes can have fuzzy boundaries
what are the properties of first order sensory neurones in the somatosensory system?
- Have their cell bodies in the DRG
- Communicate with a receptor
- Their central axon projects ipsilateral to the cell body
- Project onto second order neurones
what are the properties of second order sensory neurones in the somatosensory system?
- Have their cell bodies in the spinal cord dorsal horn or medulla
- Decussate
- Project onto third order neurones
what are the properties of third order sensory neurones in the somatosensory system?
- Have their cell bodies in the thalamus
- Project to the primary sensory cortex (postcentral gyrus)
what are the properties of first order neurones in the dorsal column medial lemniscus system?
- Those from the lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in the medulla)
- Those from the upper half of the body (T6 and above) ascend through the cuneate fasciculus to the cuneate nucleus in the medulla
what are the properties of second order neurones in the dorsal column medial lemniscus system?
- Neurones in the gracile nucleus (lower body) project to the contralateral thalamus
in the medial lemniscus - Neurones in the cuneate nucleus (upper body) project to the contralateral thalamus in the medial lemniscus
what are the properties of third order neurones in the dorsal column medial lemniscus system?
Thalamic neurones receiving information ultimately from the lower half of the body (via gracile nucleus) project to the medial part of the primary sensory cortex
Thalamic neurones receiving information ultimately from the upper half of the body (via cuneate nucleus) project to the lateral part of the primary sensory cortex
