Somatosensory System Flashcards
What three broad categories is the somatosensory system divided into?
Exteroceptive - cutaneous senses
Proprioceptive - posture and movement
Enteroceptive - internal state of the body
Where are first order neurones cell bodies located?
Dorsal root ganglia
Cranial ganglia - innovates anterior head
Where are second order neurones cell bodies located?
Dorsal horn or brainstem nuclei
Where are third order neurones cell bodies located?
Thalamic Nuclei
What does threshold mean?
The intensity of stimuli required to excite the sensory unit.
Low threshold receptors
Respond to low intensity generally non damaging stimuli
Mediate fine discriminatory touch
Thermoreception - cold to warm
Low threshold receptors - increasing stimuli strength
Increased rate of firing which denotes an increased intensity. However qualitative nature doesn’t change so long as only other receptors aren’t activated
e.g pressure doesn’t become pain
High threshold receptors
Respond only to high intensity potentially damaging stimuli Mechanoreceptors Thermal >45 Chemical - Substances Polymodal - at least two of the above
Adaption
Allows alterations to the firing rate in response to altered intensity or fire at a constant rate regardless of changing intensity.
Slow adapting neurones
e.g. stretch receptors
Fires continuously whilst deformed
Provides continual information
Amplitude of AP is still dependant on frequency
Fast adapting neurones
Muscle Spindle
Number of impulses is proportional to the rate of change
Detects change in stimuli strength
Very fast adapting
Respond to only very fast movement
e.g. Pacinian corpuscles and vibration
Conduction Velocity -Group 1
Thick myelination and large diameter
V.fast conduction
Proprioceptors of skeletal muscles
Conduction Velocity -Group 2
Moderate myelination and moderate diameter
Fast conduction
Mechanoreceptors of the skin
Conduction Velocity -Group 3
Thin myelination and small diameter
Moderate conduction
Pain and Temperature
Conduction Velocity -Group 4
No myelination and thin diameter
Slow conduction
Temp pain and itch
What is a receptive field?
Territory in which a sensory unit can be excited
What correlates inversely with receptive field?
The larger the receptive field the smaller sensory acuity, so less sensitive to touch.
What is sensory acuity?
The ability to differentiate between two separate points, in this case two points of pressure pain etc
Why is sensory acuity inversely proportional to the receptive field?
As increasing the receptive field decreases the innervation density.
A patch of skin may contain overlapping receptive fields with different modalities? T/F
True
Where are meissners corpuscles present?
Abundant in areas of very high activity, not present in hairy skin
Meissners Corpuscles
Sensitive to light touch
Where are merkels discs present?
Same distribution but also found in hairy skin.
Free nerve ending distribution.
Ubiquitous
List the hair end organs
Krause end bulbs
Ruffins endings
Paccinian Corpuscles
Krause End bulb distribution
Borders of dry skin and mucous membrane
Ruffins endings distribution
Within dermis and joint capsules
Paccinian corpuscles distribution
Dermis and fascia
Ruffins Endings
Very little adaption and respond to sustained pressure
Pacinian corpuscle
Vibration
What three forms of pain are there?
Nociceptive
Inflammatory
Pathological
Nociceptive
Adaptive
Immediate short lived protective
Inflammatory
Adaptive
Assists in healing
Persistent
Pathological pain
Maladaptive
No physiological purpose
V.Persistant
Aδ fibres
Mechanical or thermal
Thinly myelinated
Aδ describe the pain they relay.
First or fast pain
C fibres
Unmyelinated nociceptors respond to all noxious stimuli
C Fibres the pain they relay
Slow or second pain
e.g. throbbing aching etc
Which receptors are activated by thermal stimuli?
TRP receptors
e.g. TRPA1
Which receptors are activated by H+ stimuli?
ASICs
Which receptors are activated by ATP
P2x
P2Y
Which receptors are activated by bradykinin?
B2 receptors
How can some sensory C fibres have efferent action?
They release pro inflammatory mediators upon activation.
What Pro inflammatory mediators released by C fibres cause neurogenic inflammation?
CGRP
Substance P
Substance P
Vasodilation
Promotes bradykinin and histamine release
Prostaglandin formation
CGRP
Induces vasodilation
What is the affect of pro inflammatory mediators being released by sensory neurones?
Induces Hyperalgesia (increased pain sensitivity )
What is the main neurotransmitter in the dorsal horn?
Glutamate
What is pain?
The relative and subjective perception of nociception.
Highly variable between patients.
Visceral Pain
Poorly localised dull throbbing
Inflammation and ischaemia of internal organs
Viscerosomatic Pain
Diffuse pain progressing to sharp well localised.
Inflammatory exudate contacts somatic structure.
What is the gate control theory?
If non nociceptive sensory afferents are firing at such a greater rate than the nociceptive afferents. Then the synaptic transmission within the dorsal horn to the ascending tracts of the nociceptive axons does not occur.
List the major nociceptive tracts
Spinothalamic
Spinoreticular
Where do the spinothalamic nociceptive tract project too?
Thalamus
Where do the spinoreticular nociceptive tract project too?
Reticular nucleus
What is the function of the spinoreticular tract?
Autonomic response to pain
