Somatosensory System Flashcards
What are the different types of sensation?
General sensation
Special sensation
What is general sensation?
Sensory information from all over the body
What is special sensation? What are some examples of special sensation?
Sensory information from special sense organs e.g. vision from eyes, hearing from ears, smell from nose
What are the types of general sensation?
Somatic
Visceral
What is somatic sensation?
Refers to sensory information from skin, muscles, bones and joints that a person is conscious of
What is visceral sensation?
Refers to sensory information from internal organs, glands and blood vessels that a person is unaware of
What are modalities?
Refers to the different types of sensation that a person can experience
What are some examples of modalities?
Temperature
Pain
Pressure
Vibration
Proprioception
Light touch
How are different modalities detected?
By different types of receptors
One type of receptor for each modality
How are receptors in the somatosensory system classified?
Based on the modality they detect
Based on how they adapt to stimuli
What are the different types of receptors based on how they adapt to stimuli?
Rapidly adapting
Slowly adapting
What are rapidly adapting receptors?
Initially generate high frequency action potentials
But then the frequency decreases
What are rapidly adapting receptors good at detecting?
Changes in stimuli rather than absolute levels
What is an example of a rapidly adapting receptor?
Cutaneous mechanoreceptors detecting pressure
What are slowly adapting receptors?
Generate action potentials with the same frequency
What is an example of a slowly adapting receptor?
Pain receptors
What is a primary sensory neurone?
Cell body located in the dorsal root ganglion
What do the axons of primary sensory neurones do?
One axon enters the dorsal horn of the grey matter of the spinal cord
One axon travels through the spinal nerve into the PNS
What is located at the end of the primary sensory neurone’s axon in the PNS?
Receptors
All of same type, detect same modality
What is a receptive field?
Refers to the region of body surface that a sensory neurone supplies
What carries sensory information from a receptive field?
The primary sensory neurone that supplies that receptive field
How do different receptive fields relate to each other?
There is often overlap between the receptive fields of different sensory neurones
What is an autonomous region?
Regions where there is no overlap between receptive fields
What is acuity?
Refers to how precise sensation is
How are receptive fields and acuity related to each other?
Acuity is inversely proportional to the size of the receptive field
If a sensory neurone has a large receptive field, what is its relative acuity?
Low acuity
If a sensory neurone has a small receptive field, what is its acuity?
High acuity
What is a secondary sensory neurone?
Its cell body is located in the dorsal horn or the medulla
It decussates, meaning it crosses the midline to the contralateral side of the body
What is a tertiary sensory neurone?
Its cell body is located in the thalamus
Projects to primary somatosensory cortex in the post-central gyrus of the parietal lobe
What is somatotopy?
Refers to how adjacent parts of the body correspond to adjacent parts of the cortex
How are sensory neurones organised in the spinal cord?
By dermatomes
How are sensory neurones organised in the cortex?
Sensory homuncular pattern
By dermatomes converging together
What are the two main sensory pathways?
Dorsal column-medial lemniscus pathway
Spinothalamic pathway
Where is the dorsal column-medial lemniscus pathway located?
Dorsal funiculus
What modalities does the dorsal column-medial lemniscus pathway carry?
Vibration
Proprioception
Light touch
What forms the dorsal column part of dorsal column-medial lemniscus pathway?
Primary sensory neurone axons ascending up to the medulla
How are primary sensory neurones in the dorsal column organised?
Primary sensory neurones from lower half of body are medial
Primary sensory neurones from upper half of body are lateral
Where do primary sensory neurones synapse with secondary sensory neurones in the dorsal column-medial lemniscus pathway?
In the medulla
Primary sensory neurones from the lower half of the body in the gracile nucleus
Primary sensory neurones from the upper half of the body in the cuneate nucleus
What forms the medial lemniscus part of the dorsal column-medial lemniscus pathway?
Secondary sensory neurones decussating and ascending up to the thalamus
Where do secondary sensory neurones synapse with tertiary sensory neurones in the dorsal column-medial lemniscus pathway?
Thalamus
How are tertiary sensory neurones organised in the dorsal column-medial lemniscus pathway?
Tertiary sensory neurones for the lower half of the body project medially
Tertiary sensory neurones for the upper half of the body project laterally
Where is the spinothalamic pathway located?
In the lateral ventral funiculus
This overlies the ventral horn of the grey matter of the spinal cord
What modalities does the spinothalamic pathway carry?
Temperature
Pain
Pressure
Where does the primary sensory neurone synapse with the secondary sensory neurone in the spinothalamic pathway?
Dorsal horn of grey matter of spinal cord
What does the secondary sensory neurone do in the spinothalamic pathway?
Decussates through ventral white commissure
Ascends up spinal cord to thalamus
How are secondary sensory neurones organised in the spinothalamic pathway?
Secondary sensory neurones for the upper half of the body are medial
Secondary sensory neurones for the lower half of the body are lateral
Where do secondary sensory neurones synapse with tertiary sensory neurones in the spinothalamic pathway?
Thalamus
How are tertiary sensory neurones organised in the spinothalamic pathway?
Tertiary sensory neurones for upper half of body project laterally
Tertiary sensory neurones for lower half of body project medially
Cross over each other as they do so
What is Brown-Sequard syndrome?
Destruction of one half of spinal cord segment
What causes Brown-Sequard syndrome?
Trauma
Ischaemia
What are the sensory abnormalities in Brown-Sequard syndrome? E.g. T10 level
Loss of all sensory modalities in ipsilateral T10 dermatome
Loss of vibration, proprioception and light touch in ipsilateral T11-S5 dermatomes
Loss of temperature, pain, pressure in contralateral T11-S5 dermatomes
What are primary sensory neurones carrying pain stimuli called?
C fibres
What are primary sensory neurones carrying pressure stimuli called?
A fibres
How are A fibres and C fibres related to each other?
A fibres activate interneurones in the grey matter of the spinal cord
These interneurones inhibit secondary sensory neurones that synapse with C fibres
What else can activate inhibitory interneurones in the grey matter of the spinal cord apart from A fibres?
Descending nerve fibres from the brain
What can inhibit a painful sensation?
Pressure on the painful site, activates mechanoreceptors and A fibres supplying that site
Psychological inhibition
