Somatosensory Systems Flashcards
Definition of proprioception
Limb position and movement of the body
Definition of transduciton
Conversion of energy from the stimulus to electrical
Definition of modality
Specificity of receptors to specific stimuli
Definition of receptive field
Spatial domain where stimulation excites/inhibits neurone
Definition of polymodal
Nerve endings that respond to many stimuli (mechanical, thermal, chemical)
What are the 5 modalities in somatosensation and what are they stimulated by
Touch, -skin deformation and motion Proprioception, -muscle length, force and joint angle Pain -noxious stimuli Itch -histamine Visceral -thermal, chemical, mechanical
What are the 4 steps involved in sensation
When do these steps not happen
Transduction
-Energy from stimulus converted to electrical
Transmission
-AP sent up spinal cord => thalamus => somatosensory cortex
Perception
-The sensation is felt
Modulation
-The intensity of the sensation can be increased/decreased
Perception doesnt happen when under general anaesthesia
What are the 2 types of sensory neurone
Where are they found
What is their function
Dorsal root ganglion neurones
-found in the body
Trigeminal sensory neurones
-found in the head and neck
Transduce stimuli energy
Transmit signals to CNS dorsally
What are the 3 types of primary sensory neurone Describe the -axon diameter -myelination -function -relative speed
Ab
- wide axon
- highly myelinated
- touch, proprioception
- v fast
Ad
- medium axon
- highly myelinated
- cold, nociceptors
- fast
C
- tiny axon in a bundle
- no myelination, bundles surrounded by non myelinating Schwann cells
- warm, itch
- vv slow
What primary sensory neurone is affected in diabetic neuropathy and what are the consequences
C fibres as they are the smallest
Lose pain sensation
What are the 4 main properties of stimuli
What are the effects of these properties on receptors
Modality
- Receptors only respond to specific stimuli
- Depends on presence of specific transduction molecules
Intensity
- Encoded by firing frequency and no of activated receptors
- Different receptors have different thresholds
Duration
-Encoded by duration of firing
Location (sensory field)
-Related to receptive fields
What are the 2 types of receptor
What are the properties of each receptor
Slowly adapting
- APs fire when stimulus applied
- increase stimulus intensity => increased rate of AP firing
Rapidly adapting
- APs fire when intensity of stimulus changes
- Increased change in intensity => increased rate of AP firing
What is the receptive field
How does it relate to the 2 point threshold
Spatial domain where stimulation excites/inhibits neurone
Different concentrations of sensory neurones =>different 2 point perceptual thresholds of different body parts
-the increased [sensory receptors] => lower 2 point threshold
What are the 4 types of mechanoreceptor
- location
- receptive field size
- type of receptor
Meissner corpuscle
- superficial
- small
- rapidly adapting
Merkel cells
- superficial
- small
- slowly adapting
Pacinian corpuscle
- deep
- large
- rapidly adapting
Ruffini endings
- deep
- large
- slowly adapting
What are joint capsule receptors
What do they aim to do and how do they do this
Sensory receptors in joints transduce tension in joint capsule
Perception of angle achieved from afferent signals from muscle spindles and efferent motor commands
Aims to prevent hyperextension
What are the 2 types of proprioceptor in the muscles
Muscle spindles
Golgi tendon organs
Describe the properties of the muscle spindles
What are they sensitive to
Parallel with myofibres
Sensitive to small length changes with range set by special motor input
Regulate motor control of muscle length
Contributes to the sense of limb position
Wraps around intrafusal fibres between extrafusal fibres
Describe the properties of the golgi tendon organs
What are they sensitive to
Series with myofibres. embedded between collagen fibres in tendinomuscular junction
Sensitive to changes in tension
What are the 2 types of thermoreceptors
- what fibres are they
- when do they start and stop responding
Cold
- Ad myelinated
- sensitive to the non damaging cold
- paradoxical response in 45C and above
Hot
- C unmyelinated
- stop responding when temp in damaging range
- some are ad
How are nociceptors involved in temperature detection
How does intensity relate to the APs fired
Fire in damaging temperature range
As you increase temp, thermoreceptors stop firing and nociceptors start firing
Increased intensity of stimuli => increased APs fired
What are the 4 modalities of nociceptor
- types of receptor end
- fibre type
- stimuli
Thermal
- no specialised nerve ending, bare fibres
- Ad
- thermal
Mechanical
- no specialised nerve ending, bare fibres
- Ad
- mechanical
Polymodal
- no specialised nerve ending, bare fibres
- C
- thermal, mechanical, chemical
Silent
- no specialised nerve ending, bare fibres
- C
- mechanical when activated by inflammation
If the nociceptors have no specialised nerve ending, how can they function differently?
Transduction molecules expressed determines nociceptor diversity
How do dorsal root ganglia relate to dermatomes
Each dermatome is innervated by a single dorsal root ganglion
If sensation lost in 1 region of the skin, can pinpoint the location of the spinal lesion
Describe the sensory pathway taken by sensory afferents via the dorsal column/medial leminiscal system
What fibre is associated with medial leminscal system
What happens when the spinal cord is damaged
Sensory afferent from Ab
DRG => Cuneate/gracile fascicle in SC
Cuneate/gracile nucleus in medulla
Decussation to ventral contralateral in medulla oblongata
Medial leminiscal tract => VPL nucleus in thalamus
Somatosensory cortex
Loss of sensory in ipsilateral side
Describe the nociceptor pathway taken via the spinoreticular, spinomesencephalic, spinothalamic system
What happens when the spinal cord is damaged
Nociceptor afferents
DRG => dorsal horn in SC
Decussation to ventral colateral in spinal cord
Spinothalamic tract => VPL nucleus in thalamus
Somatosensory cortex
Loss of pain, temp, visceral sensation in contralateral side
Describe the divisions of the somatosensory cortex
Posterior to the central sulcus
Mainly SI (primary somatosensory cortex)
SII is more lateral (secondary somatosensory cortex
Post central gyrus is between the central sulcus and post central sulcus
Describe the 4 Brodmann areas on the post central gyrus
Where do they receive innervation from
4 cytoarchitectonic areas
From anterior => posterior
-3a deep tissue (muscle stretch receptor)
-3b (skin RA, SA)
-1 (largest area) (skin RA)
-2 (largest on the posterior side) (deep tissue, pressure and joint position)
Describe the somatotopic map of the body surface on the primary cortex
Discontinuous and disturbed
- Face is the most lateral
- lower limbs are the most medial
Areas that need to receive more sensory input get a larger area of the cortex
Describe the 2 methods of organisation on the somatosensory cortex
Different types of modality/sensory info is segregated and processed parallel to each other in columns according to the area innervated and the types of receptor (slow and rapidly adapting fibres)
Projections from the thalamus => info sent to Brodmann’s areas are processed serially
3a => 3b => 1 => 2
Why is it important to process sensory info in parallel and in series
Info segregated in parallel so it can be integrated further in series to get a broader understanding of the sensory info
Describe the relationship between the receptive fields and neurons of SI (primary somatosensory cortex)
As the inputs are processed serially, the receptive fields will converge as you get more modalities
Neurones in Brodmann’s area 1 and 2 have larger receptive fields than 3b
Where is the location of pain processing in the brain
A mystery
Describe the plastic nature of the somatosensory map (learning)
The more you use a body part in sensory innervation, the sensory area on the brain related to that body part increases
Somatosensory evoked potential mapping
Describe the plastic nature of the somatosensory map (phantom pain)
Somatosensory map will change to accommodate for the lack of a limb
Body parts that remain => increased cortical area
Describe the plastic nature of the somatosensory map (chronic neuropathic pain)
Nerve damage can cause areas affected by pain to increase but at the expense of other body parts
Areas of sensory innervation can shift
