Sensory Systems (and Physiology of Pain) Flashcards
Describe the range of sensory receptors
Each type of sensory information is associated with a specific receptor type responding to a specific sensory modality
i.e. mechanoreceptors, chemoreceptors, thermoreceptors, nociceptors, proprioceptors
How do sensory receptors initiated?
Respond to stimulus over a specific area, called the receptive field
What is signal transduction?
All sensory receptors transduce their adequate stimulus into a depolarisation, the receptor (generator) potential.
What does the size of the receptor potential reflect?
Intensity of stimulus
What is the function of the receptor (generator) potential?
Evokes firing of action potentials for long distance transmission
- The frequency of AP encodes intensity of stimulus
- Receptive field encodes location of stimulus
What do receptors give information on?
Modality, intensity and location of the stimulus
What determines acuity?
Density of innervation and size of receptive fields
What are the three different types of primary afferent fibres which mediate cutaneous sensation?
- Aβ
- Aδ
- C
Describe Aβ primary afferent fibres
Large myelinated (30-70m/s) touch, pressure, vibration
Describe Aδ primary afferent fibres
Small myelinated (5-30m/s) cold, ‘fast’ pain, pressure
Describe C primary afferent fibres
Unmyelinated fibres (0.5-2m/s) warmth, ‘slow’ pain
What are the two primary afferent fibres responsible for proprioception?
Aβ and Aα (i.e. muscle spindles, golgi tendon organs)
Describe the pathway of mechanoreceptive (Aβ and Aα) fibres
- Project straight up through ipsilateral dorsal columns
- Synapse in cuneate and gracile nuclei of the medulla
- The 2nd order fibres cross over at the decussation in the brainstem and project to reticular formation, thalamus and cortex
Describe the pathway of thermoreceptive and nociceptive (Aδ & C) fibres
- Synapse in the dorsal horn
- 2nd order fibres cross over the midline in the spinal cord
- Project up through the contralateral spinothalamic (anterolateral) tract to reticular formation, thalamus and cortex
What are the effects of damage to dorsal columns?
Causes loss of touch, vibration, proprioception below lesion on ipsilateral side
What are the effects of damage to anterior columns?
Causes loss of nociceptive and temperature sensation below lesion of contralateral side
Where do the ascending tracts terminates?
In the somatosensory cortex (S1) of the post central gyrus
What produces the sensory homunculus?
Endings are grouped according to the location of the receptors
Extent of representation is related to the density of receptors in each location
What are the three different types of processing in sensory pathways
Adaptive, convergence and lateral inhibition
Describe adaptive processing in sensory pathways
Change in sensitivity of your perception of a stimulus – change over time in the responsiveness of the sensory receptor to a constant stimulus.
Describe convergence processing in sensory pathways
Saves on neurones, but reduces acuity and may underlie referred pain
Describe lateral inhibition processing in sensory pathways
- Activation of one sensory input causes synaptic inhibition of its neighbours
- Gives better definition of boundaries
- Cleans up sensory information
List seven types of pain
- Sharp stabbing vs diffuse, throbbing pain
- Fast (initial) pain vs slow (delayed) pain
- Acute vs chronic pain
- Visceral pain
- Referred pain
- Phantom limb pain
How in signal transduction in nociceptors are activated?
- Low pH, heat (via ASIC, TRPV1 etc)
* Local chemical mediators (i.e. bradykinin, histamine, prostaglandins)
How do reflexes begin?
Activation of neurone free nerve endings. Ion channels responding to a variety of chemical, mechanical and thermal stimuli creates a graded potential that trigger AP if the stimulus is strong enough
Describe how reflexes continue after AP is initiated?
The primary sensory neurones from nociceptors (Aδ & C) terminate in the dorsal horn of the spinal cord. Ascending pathways enter the thalamus and sensory areas of the cortex.
What is Gate Control Theory?
Aβ fibres carrying sensory info abut mechanical stimuli help block pain transmission. They synapse on inhibitory interneurone and enhance the interneuron’s inhibitory activity. If simultaneous stimuli reach inhib. Interneurons from the Aβ and C fibres, the integrated response is partial inhibition of the ascending pain pathway so that pain perceived by the brain is lessened. The inhib. Interneuron release opiod peptide (endorphins) which inhibit transmitter release from Aδ & C fibres
What are four analgesics?
- NSAIDs
- Local anaesthetics
- Trans-cutaneous electric nerve stimulation (TENS)
- Opiates (i.e. morphine)
Why are NSAIDs analgesics?
- Prostaglandin sensitise nociceptors to bradykinin
* They inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins
What are the actions of local anaesthetics?
Block Na action potential and therefore all axonal transmission
What are the actions of opiates?
- Reduce sensitivity of nociceptos
- Blocks transmitter release in dorsal horn (hence epidural administration)
- Activate descending inhibitory pathways
