Sensory system - including physiology of pain Flashcards
Each sensory information is associated with a specific receptor type, name 5 receptors
Mechanoreceptor - mechanical stimuli
Chemoreceptor - chemical stimuli
Thermoreceptor - sensory temp
Nociceceptor - pain
Proprioceptors - muscle spindles
Define receptve field
Response to a stimulus over a specific area
What is the different structures of sensory receptors
Pacinian corpuscle (deep pressure) Messiners corpuscle (light touch) Free nerve ending (pain) Ruffin corpuscle (warmth) Merkels corpuscles (touch)
How do sensory (physiological) receptors work in signal tranductions
transduce their adequate stimulus into depolarisation, creating a receptor (generator) potential
What encodes the intensity of stimulus
The size of the receptor potential
affecting the frequency of action potentials
What does the receptor potential evoke
Firing of action potential for long distance transmission
What is the function of the receptive field
Encode location of stimulus
giving info on intensity and modality
What determines acuity
Density of innervation, and size of receptive fields,
Cutaneous sensation is mediated by what 3 types of primary afferent fibres
Aβ = large myelinated (30-70m/s)
Aδ = small myelinated (5-30m/s)
C = unmyelinated fibres (0.5-2m/s)
What are A alpha fibres responsible for what cutaneous sensation
Transmission of touch, pressure, vibration sees
What are A delta fibres responsible for what cutaneous sensation
Transmission of cold, “fast” pain, pressure senses
What are C fibres responsible for what cutaneous sensation
Transmission of warmth, “slow” pain senses
What two primary afferent fibres are responsible for proprioception
A alpha
A beta
Define acuity
Ability to locate a stimulus & to differentiate it from one nearby
Where do all primary afferent fibres enter the spinal cord
Via the dorsal root ganglia
What is the transmission pathway of mechanoreceptive fibres (Aα & Aβ) to first synapse
Project straight up through ipsilateral dorsal column pathway and synapse in cuneate and gracile nuclei of the medulla
Where do second order mechanoreceptive fibres decussate
Brain stem
Where does second order mechanoreceptive fibres project and synapse
Via the contralateral medial lemniscus
synapsing in the Reticular formation, thalamus
Where does first order thermoreceptive and nociceptive fibres (Aδ & C) synapse
In the dorsal horn
Where does second order thermoreceptive and nociceptive fibres (Aδ & C) cross over
The midline in the spinal cord
What tract do the second order thermoreceptive and nociceptive fibres project up
contralateral spinothalamic (anterolateral) tract
Where do the second order thermoreceptive and nociceptive fibres synapse
reticular formation, thalamus
What is the pathway of third order neurone of the mechanoreceptive, thermoreceptive and nociceptive fibres and where do they synapse
Ascend from the ventral posterolateral nucleus of the thalamus, travel through the internal capsule and terminate at the sensory cortex
What is the clinical consequence of damage to dorsal columns
causes loss of touch, vibration, proprioception below lesion on ipsilateral side
What is the clinical consequences of damed to anterolateral quadrant
causes loss of nociceptive & temperature sensation below lesion on contralateral side
Where does ultimate termination of sensory information occur
somatosensory cortex (S1) of the postcentral gyrus
How does the somatosensory cortex produce a sensory homunculus
Sensory transmission endings are grouped according to the location of the receptors
extent of sensory representation is related to the density of receptors in each location
What allows processing of sensory pathways
Adaptation
Convergence
Lateral inhibition
Not all info reaches the brain - filtration
Perception
When does adaptation of receptors occurs
When receptors are continually stimulated by a maintained stimuli
What does adaption result in
Receptors become less sensitive, or switch off
What is the difference between rapid and slow adapting cutaneous receptors?
Rapidly adapting receptors are best at detecting rapidly changing signals, while slowly adapting receptors are capable of detecting a long, continuous signal
What stimuli does not trigger adaptation
Painful stimuli
What is the features of convergence
Saves on neurones
Reduces acuity
and can underlie referred pain
What occurs in lateral inhibition
activation of one sensory input causes synaptic inhibition of its neighbours
What is the benefit of lateral inhibition
Gives better definition of boundaries - enhances perception
Cleans up sensory information
How does perception affect the processing in sensory pathway
Importance of inputs varies according to your state of mind
What activates signal transduction in nociceptors
Low pH
Heat
Local chemical mediators
What is the usually cause of referred pain
Convergence of visceral pain fibres and skin pain fibres
What is the structure of nociceptors
Free nerve ending
What nociceptors directly respond to damaged stimuli
ASIS
VR1
What direct nociceptor responds to low pH
ASIS
What direct nociceptor responds to heat
Vr1 receptors
What are the indirect hormones released that stimulate noceiceptor
Bradykinin, Histamine, Prostaglandins
all released with tissue damage
What does stimulation of nociceptors result in
Transduction as depolarisation of cells occurs due to the opening of sodium and potassium channels and an AP is fired
What nerve fibres does the nociceptive AP travel in
C fibres (slow pain) A delta fibres (fast pain)
Where does C/ A delta fibres synapse
In the dorsal route of the spinal cord
How is the gate closed in controlling pain in the spinal cord
By an Inhibitory interneurone release opioid peptidase and inhibits the release from A delta/C fibres
What two controls allow the closing of the gate
Segmental
Descending
How does segmental control close the gate
An active A beta activates inhibitory interneurone
How does deadening control close the gate
Cell bodies from Periaqueductal Grey Matter (PAG) and Nucleus Raphe Magnus (NRM) - sends axons down the spinal cord and activates inhibitory interneurons
How does prostaglandins cause analgesia
Sensitises nociceptors to bradkyinin
Activates voltage gates sodium channel
How does bradykinin cause analgesia
Activates VR1 receptor
How does NSAIDS work in treating analgesia
because they inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins
What is the affect of prostaglandin have on the body
Inflammation
Pain
Fever
How does local aesthetics treat analgesia
block Na+ action potential and therefore all axonal transmission
What is transcutaneous electrical nerve stimulation
Low-voltage electrical current for pain relief
The stimulating pulses across the skin help prevent pain signals from reaching the brain
How does opiates work in treating analgesia
reduce sensitivity of nociceptors
block transmitter release in dorsal horn (hence epidural administration)
activate descending inhibitory pathways
What does the Nucleus Raphe Magnus release when stimulated
5-HT (serotonin) released from the raphe nuclei descends to the dorsal horn of the spinal cord
What stimulates the nucleus raphe magnus
Stimulation of the periaqueductal gray matter of the midbrain activates enkephalin-releasing neurons that project to the raphe nuclei in the brainstem
Where is the the periaqueductal gray matter
The periaqueductal gray is the gray matter located around the cerebral aqueduct within the tegmentum of the midbrain
