Somatosensory System Flashcards
What is sensation
Conscious/sub-conscious awareness of an external/internal stimulus
What are the types of sensory systems
General:
Somatic (conscious)
Visceral (sub-conscious)
Special
Rules of somatosensory system
- It is a chain of 3 sensory neurones
- First order neurone has its cell body in dorsal root ganglion
- Second order neurone crosses the midline
- Third order neurone projects from Thalamus to cortex
What is modality
Type of stimulus
Each receptor type responds to specific modality
What are the types of somatosensory modalities
Temperature Pain Pressure Vibration Stretch Proprioception 2 point discrimination
What is proprioception
Sense of relative position of ones own body parts and strength being employed in movement
2 receptors involved: muscle spindle, Golgi tendon organ
What is 2 point discrimination
Ability to sense 2 nearby objects as 2 distinct objects
What is Quality of sensation
Subdivision of modality
Depends on receptor subtype
E.g. temperature - hot, cold
Describe the process of sensory transduction
- Stimulus detected by receptor and generator potential produced
- If GP reaches threshold, AP generated
- AP travels down to second order neurone, etc
How does AP encode info about intensity + duration of a stimulus
Frequency coding: stronger stimulus will increase frequency of AP; stimulus intensity is proportional to change in membrane potential
Activation of neighbouring neurones: stronger stimulus will activate neighbouring neurones to greater degree
What are tonic receptors
Receptors that slowly adapt to stimulus and continue to fire AP over presence of stimulus
Frequency of AP is constant
Constant awareness of sensation
E.g. pain receptors
What are phasic receptors
Receptors that rapidly adapt to stimulus
Frequency of AP decreases during maintained stimulus
Loss of sensation over time
E.g. touch receptors
What is acuity
Precision by which a stimulus can be located
What are the factors that determine acuity
2 point discrimination
Lateral inhibition
Convergence + divergence
What is lateral inhibition? Describe its process
Primary afferent fibre whose receptive field is closest to point of stimulation produces more AP than those at periphery
- PAF closest to stimulus produces most AP
- PAF synapses with adjacent secondary AF via inhibitory interneurones
- SAF with receptive field at periphery of stimulus are strongly inhibited
- SAF at periphery produce fewer APs than SAN w receptive field at point of stimulation
What is 2 point discrimination? What does it depend on?
Minimal distance required to distinguish 2 simultaneously applied skin indentations
Density of receptors
Receptive field of PAF: area of skin covered by nerve ending of PAF; larger the receptive field, poorer the 2pd
What is convergence and divergence
Convergence: multiple PAF synapse with one SAF
Poorer acuity
Divergence: one PAF synapse with multiple SAF
Amplification
What brain regions are involved in feeling the sensation
Thalamus
Somatosensory cortex - post-central gyrus
What does the Thalamus do in sensation
Crude localisation
Produces organised projections to somatosensory cortex
What does the cortex do
Localisation of stimulus
Recognise Quality of stimulus
Then relay information to other areas and allow choice to the response to stimulus to be taken
What is somatotopic representation
Point-for-point correspondence of area of body to specific point on somatosensory cortex (post-central gyrus)
