The somatosensory system Flashcards
What is the function of the 3 types of sensory neurones?
Aβ-fibres: innocuous mechanical stimulation
Aδ-fibres: noxious mechanical and thermal stimulation
C-fibers: noxious mechanical, thermal and chemical stimulation
What is the function of sensory neurones?
They are transducers that convert energy from the environment into neuronal action potentials’’
Describe composition of thermoreceptor?
A delta and C fibres are thermoreceptors. Consist of Transient receptor potential (TRP) ion channels 4 heat activated: TRPV1-4 2 cold activated TRPM8 TRPA1
What is the function of the 4 types of mechanoreceptors?
Meissner’s corpuscle- fine discriminative touch and low frequency vibration.
Merkel cells- light touch and superficial pressure
Pacinian corpscle- deep pressure, high frequency vibrtions and tickling
Ruffini endings- continuous pressure and touch and stretch
Define stimulus threshold?
It is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)’
How do we differentiate on strength of a stimulation?
Stronger stimulus equals more neurotransmitter release.
Describe adaption of tonic receptors?
Detect continuous stimulus strength
Continue to transmit impulses to the brain as long the stimulus is present
Keeps the brain constantly informed of the status of the body
e.g. Merkel cells
Describe adaption of phasic receptors?
Detect a change in stimulus strength
Transmit an impulse at the start and the end of the stimulus….i.e. when a change is occurring
e.g. Pacinian corpuscle
Define receptive field?
The receptive field is the region on the skin which causes activation of a single sensory neuron when activated
Define and explain the basis of two point discrimination?
Minimum distance at which two points are perceived as separate
Related to the size of the receptive field. Smaller receptive field have more densely packed receptors so are able to differentiate between two points much more acutely.
What is the purpose of lateral inhibition?
A receptive field can overlap with another receptive field
Difficult to distinguish between 2 stimulus locations
Lateral inhibition prevents the overlap of receptive fields Facilitates pinpoint accuracy in localisation of the stimulus
How does lateral inhibition occur?
Mediated by inhibitory interneurons within dorsal horn of spinal cord
Which 3 areas is there topographic representation in the sensory pathways?
Gracile nucleus, cuneate nucleus and ventral posterior lateral nucleus (VPL nucleus)
What is special about the somatosensory homunculus?
Size of somatotopic areas is proportional to density of sensory receptors in that body region
What would cause an anterior spinal cord lesion and what would be the effect?
Blocked anterior spinal artery causes ischemic damage to the anterior part of the spinal cord
Spinothalamic tract damage causes pain and temperature loss below the level of the lesion
Retained light touch, vibration and 2-point discrimination due to intact dorsal columns
