Lecture 4- Somatic sensation Flashcards
2 broad categories of sensation
general sensation
special sensation
special sensation
- Hearing
- Vision
- Smell
general sensation is sensation detected through stimulation of
of body walls e.g. skin or mucous membrane
types of general senseation
somatic
visceral
Somatic sensation
- Conscious perception
- Sharp and localised
Visceral sensation
- Unconscious perception
- Dull and poorly localised
- Maintaining homeostasis
- E.g. carotid body
- Only strong stimulation of visceral structure e.g. appendicitis- referred pain in somatic distribution
what are sensory modalities
- Units of the sensory system
- Exist because we have a variety of diff receptors
- Each receptor responsible for detecting a different modality e.g. chemoreceptors, baroreceptors
two main routes that different modalities travel to reach the brain
Spinothalamic pathway
Dorsal column systems
modalities of spinothalamic pathway
- Temperature
- Pain
- Pressure/ crude touch
modalities of dorsal column systems
- Vibration
- Fine touch
-
Proprioception (where our limbs are in space)
- Detected by muscle spindles and receptors in joints
-
2 point discrimination
- Ability to resolve 2 simultaneous stimuli on the skin
- E.g. how many fingers are being pressed on the body e.g. lips can resolve stimuli that are close together e.g. the back cannot resolve stimuli closely together
what are first order sensory neurones
neurones which communicate with receptors in the skin
strong stimulation of receptor
high frequency of APs along axon
Weak stimulation of receptor
low frequency of APs along axon
sensory encoding
types of sneosry neurones
rapidly adapting
slowly adapting
Rapidly adapting
- High frequency of APs which eventually slows down over time
- E.g. mechanoreceptors in skin
- When you put clothes on you are conscious of the clothes touching your skin, but rapidly you are no longer aware of the feeling against your skin
Slowly adapting
- A stimulus may be keep being applied but the frequency of the AP doesn’t change
- E.g. pain transmission (nocipectors)
- If you have a toothache the pain doesn’t go away
- Important because we don’t want to get use to the pain
the receptive field
- The area of skin that a sensory neurone collects information from
- Each sensory neurone only has 1 type of receptor
- Receptive fields of neurones overlap and can be of varying size
the smaller the sensory field (e.g. C5 in this diagram) the
the greater the sensory acuity (proportional to reciprocal (1/number) of the size of the receptive field)
significance of the overlaping nature of receptive fields
- Overlapping of the receptive field implies that dermatomal boundaries can be mixed between 2 spinal segments e.g. C6/C5
- Therefore, when testing dermatomes we test a central region where there is no overlap
the somatosensory system
System which carries conscious sensation from the body wall to the cortex of the brain
e.g. skin, lining of pharynx and oral cavity, mucosa of the anus, parietal pleura and peritoneum
outline the somatosensory system
- First order neurone is a sensory neurone which detects stimuli using receptors (cell body in the dorsal root ganglion)
- It then projects into the CNS and synapses with second order neurone
- Either directly adjacent to where the first order neurone enters the spinal cord- spinothalamic tract and then ascends up the spinal cord (cell body in dorsal horn)
- Or the first order neurone will ascend up to the brainstem and synapse here- dorsal column-medial lemniscus system (cell body in medulla)
- Second order neurones live entirely within the CNS and crosses the midline (decussates) into the opposite side of the CNS then synapses with the third order neurone found in the thalamus (thalamocortical neurones), projecting up to the somatosensory cortex –> perception occurs
cell body of first order neurone found
dorsal root ganglion
cell body of the second order neurone found
spinal cord- dorsal horn
brainstem- medulla
cell body of third order neruone
thalamus
Topographical receptor (somatotopy)
For every part of the body surface there is a corresponding part of the CNS
- Adjacent body parts are represented by adjacent parts of the CNS
why is it important that the trajectory of the somatosensory pathways are as parallel as they are
If the trajectory wasn’t this parallel (i.e. disordered trajectory)- a lot of axon and myelin would be wasted.
- Therefore if the NS keeps structure ordered, it can minimise the amount of building material it uses
somatotopy- dermatomal pattern and homonculus pattern
This trajectory is also involved in the conversion of the dermatomal pattern to the homulculus pattern
At receptor level- dermatomal pattern
At Sensory cortex- homunculus pattern
