Pain & Somatosensory Disorders Flashcards
Overview of somatosensory system
Sensory receptors are classified into five categories. What are these?
- Chemoreceptors; chemicals
- Thermoreceptors; temperature
- Mechanoreceptors; pressure
- Proprioceptors
- Nociceptor; pain
A number of receptors are distributed throughout the skin to respond to various touch-related stimuli. Name these 4 receptors?
N.B. these are all mechanoreceptors
- Pacinian corpuscle
- Ruffini’s corpuscles
- Merkel’s disks
- Free nerve endings
What are the four major types of tactile mechanoreceptors?
Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscles.
How do mechanoreceptors sense stimuli?
Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes.
What is a free nerve ending? What are they sensitive to?
An unencapsulated dendrite of a sensory neuron; they are the most common nerve endings in skin. Free nerve endings are sensitive to painful stimuli, to hot and cold, and to light touch.
Meissner corpuscles;
a) location?
b) what type of skin?
c) what do they respond to?
d) encapsulated or not?
e) speed of adaptation?
- a) Found in the upper dermis, but they project into the epidermis; sit between the dermal pupillae
- b) Glabrous skin only; fingertips and eyelids
- c) respond to fine touch/pressure and low frequency vibratinos
- d) encapsulated
- e) rapidly adapting
Pacinian corpuscles;
a) location?
b) what type of skin?
c) what do they respond to?
d) encapsulated or not?
e) speed of adaptation?
a) Deep receptors located in subcutaneous tissue
b) Glabrous AND hairy skin
c) Respond to deep transient pressure and high frequency vibration
d) encapsulated
e) rapidly adapting
Ruffini endings;
a) location?
b) what type of skin?
c) what do they respond to?
d) encapsulated or not?
e) speed of adaptation?
- a) located deep in dermis, long axis of the corpuscle is orientated parallel to the skin (20% of receptors in skin)
- b) glabrous AND hairy skin
- c) detect skin stretch and deformations within joints (provide valuable feedback for gripping objects and controlling finger position and movement)
- d) encapsulated
- e) slow adapting
Merkel’s discs;
a) location?
b) what type of skin?
c) what do they respond to?
d) encapsulated or not?
e) speed of adaptation?
- a) found just beneath the epidermis; aligned with the pupillae
- b) hairy AND glabrous skin
- c) respond to light pressure/touch
- d) unencapsulated
- e) slow-adapting
Diagram of sensory receptors in skin
How does the diameter of sensory afferents affect speed of conduction?
General principle;
- Large diameter; rapidly conducting afferents (I/II)
- Small diameter; slow conducting afferents (III/IV)
Conduction velocity is positively correlated with axon diameter.
As a general principle, which receptors are sensory afferents with a large diameter associated with?
With low threshold mechanoreceptors.
As a general principle, which receptors are sensory afferents with a small diameter associated with?
Associated with nociceptors and thermoreceptors
The configuration of the different types of receptors working in concert in the human skin results in a very refined sense of touch. Describe this configuration?
- The nociceptive receptors are located near the surface.
- Small, finely-calibrated mechanoreceptors (Merkel’s disks and Meissner’s corpuscles) are located in the upper layers and can precisely localize even gentle touch.
- The large mechanoreceptors (Pacinian corpuscles and Ruffini endings) are located in the lower layers and respond to deeper touch.
There are 2 types of nociceptive fibres. What are these?
- A delta fibres
- C fibres
What is first pain and second pain?
A single painful stimuli yields two successive and distinct sensations referred to as first and second pain sensation. First pain is brief, pricking, and well localised, whereas second pain is longer-lasting, burning, and less well localised.
Describe first pain. What fibres are involved? What receptors are involved?
- Fast A delta fibres
- Sharp or prickling
- Easily localised
- Occurs rapidly
- Short duration
- Mechanical or thermal nociceptors
How is the dorsal horn organised? How does this influence the termination of somatosensory neurons?
The dorsal horn is organised in laminars. Different types of somatosensory neurons terminate at different lamina.
Where do 2nd order nociceptive neurons project to?
The thalamus and then to the somatosensory cortex
There are 2 conscious ascending tracts. What are these?
- Dorsal column-medial lemniscal pathway
- Spinothalamic pathway
What sensory deficits will be seen in a unilateral spinal lesion? (think DCML pathway AND spninothalamic pathway)
- DCML –> A unilateral spinal lesion will produce sensory loss of touch, pressure, vibration and proprioception below the lesion on the same side
- Spinothalamic –> vDiminished sensation of pain below the lesion will be observed on the opposite side.
- vThis is called dissociated sensory loss.
Describe the pathway of 1st, 2nd and 3rd order afferent neurons carrying pain and temperature sensation from the face
Trigeminal System:
(5th Cranial nerve)
- •1st order afferents from face project to pars interpolaris & pars caudalis of medulla/upper cervical cord
- •2nd order neurones ascend contralaterally to thalamus (via trigemino-thalamic tract)
- •3rd order neurones project to cortex
What is the gate control theory of pain?
The gate control theory of pain asserts that non-painful input closes the nerve “gates” to painful input, which prevents pain sensation from traveling to the central nervous system.
e.g. knife wound may not be painful
What is central sensitisation?
Central sensitisation refers to the process through which a state of hyperexcitability is established in the central nervous system, leading to enhanced processing of nociceptive (pain) messages
