Sensory Systems (Including Physiology of Pain) Flashcards
Each type of sensory information is associated with a specific receptor type responding to a specific sensory modality - name some examples?
mechanoreceptors
chemoreceptors
thermoreceptors
nociceptors
proprioceptors
sensory receptors may have free nerve endings or a complex structure, what are examples of each?
nociceptors, cold receptors
Pacininan corpuscle, Meissner’s corpuscle
What is the response over a specific area called?
receptive field
Image showing the different strucutres of neurons
Some have multiple nerve endings, others have fancy structures attached to the end of them
What are the 2 different sensory receptors in the body?
difference between physiological (sensory) receptors vs pharmacological (protein) receptors
all sensory receptors transduce their adequate stimulus into a depolarisation, what is i that causes this depolarisation?
the receptor (generator) potential
the size of the receptor potential encodes what?
intensity of stimulus
receptor potential then evokes firing of _____________ for long distance transmission
action potentials
frequency of action potentials encodes ________________
intensity of stimulus
What do the receptive fields encode
location of stimulus
What does the receptive field give information on?
gives information on the modality, intensity & location of the stimulus
Image showing graded and action potentials
Generated potentials are local so don’t reach the end but action potentials reach all the way to the end
What determines acuity of a sensory neuron?
density of innervation
size of receptive field
Explain this image
Two pointed both activated by the right sensory field - no 2 point discrimination
Action potentials are transmitted to the CNS by what?
axons
cutaneous sensation is mediated by what 3 types of primary afferent fibres?
Aβ
Aδ
C
Are Aβ, Aδ, C fibres myelinated or not?
Aβ - large myelinated (30-70m/s)
Aδ - small myelinated (5-30m/s)
C - unmyelinated fibres (0.5-2m/s)
What is the function of the Aβ, Aδ, C fibres?
Aβ = touch, pressure, vibration
Aδ = cold, “fast” pain, pressure
C = warmth, “slow” pain
Proprioception is mediated by what 2 types of primary afferent fibres?
Aα & Aβ
eg muscle spindles, golgi tendon organs etc
Where do all primary afferent fibres enter the spinal cord?
all enter spinal cord via the dorsal root ganglia (or cranial nerve ganglia for head)
Transition of sensory information is done through what types of fibres?
mechanoreceptive (Aα & Aβ) fibres
thermoreceptive & nociceptive (Aδ & C) fibres
What type of fibres are mechanoreceptive fibres?
Aα & Aβ
What is the course of mechanoreceptive fibres?
project straight up through ipsilateral dorsal columns
synapse in cuneate & gracile nuclei
the 2nd order fibres cross over midline (decussate) in the brain stem & project to reticular formation, thalamus and cortex
What type of fibres are thermoreceptive & nociceptive fibres?
Aδ & C
What is the course of thermoreceptive & nociceptive fibres?
synapse in the dorsal horn
the 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
Is the left or the right:
mechanoreceptive (Aα & Aβ) fibres
thermoreceptive & nociceptive (Aδ & C) fibres
left - thermoreceptive and nociceptive fibres
right - mechanoreceptive fibres
different pathways for transmission of sensory information explains consequences of spinal cord injuries
What does damage to the dorsal columns lead to?
causes loss of touch, vibration, proprioception below lesion on ipsilateral side
different pathways for transmission of sensory information explains consequences of spinal cord injuries
What does damage to anterolateral quadrant lead to?
causes loss of nociceptive & temperature sensation below lesion on contralateral side
Where does the transmission of sensory information end?
ultimate termination is in the somatosensory cortex (S1) of the postcentral gyrus
How is the sensory information stored and arranged?
endings are grouped according to the location of the receptors
extent of representation is related to the density of receptors in each location
produces the sensory homunculus
Adaptation is one of the processes in the sensory pathway, explain this?
gradual decrease over time in the responsiveness of the sensory system to a constant stimulus
usually experienced as a change in the stimulus e.g. if a hand is rested on a table, the table’s surface is immediately felt against the skin but the sensation of the table surface against the skin gradually diminishes until it is virtually unnoticeable
sensory neurons that initially respond are no longer stimulated to respond; this is an example of neural adaptation
Convergenge is another process of the sensory pathway, what is it and its effects?
saves on neurones
reduces acuity - as if 2 neurons go onto one then you don’t know where the stimulus is coming from
may be the cause of reffered pain
in nonspecific ascending pathways, different stimuli can get merged together
lateral inhibition is another process in the sensory pathway, what is it?
activation of one sensory input causes synaptic inhibition of its neighbours
gives better definition of boundaries, can localize a stimulus on the skin better
cleans up sensory information
What are 3 main processes that happen in the sensory pathway?
adaptation
convergence
lateral inhibition
What are the 2 different types of pain that can be experienced?
sharp, stabbing vs diffuse, throbbing pain
What are the 2 different speeds of pain?
fast (initial) pain vs slow (delayed) pain
What are the different lengths of time that pain can persist?
acute vs chronic pain
What is visceral pain?
occurs when pain receptors in the pelvis, abdomen, chest, or intestines are activated
when our internal organs and tissues are damaged or injured
vague, not localized, and not well understood or clearly defined
often feels like a deep squeeze, pressure, or aching
What is referred pain?
pain percieved in another location form the site of the painful stimulus/origin
What is phantom limb pain?
ongoing painful sensations that seem to be coming from the part of the limb that is no longer there
the limb is gone, but the pain is real
the onset of this pain most often occurs soon after surgery
What is responsible for signal transduction in nociceptors?
low pH, heat (via ASIC, TRPV1 etc)
local chemical mediators (eg bradykinin, histamine, prostaglandins)
How can pain be blocked form nocioceptors?
Nociceptors are the bottom left fibres
Gate control theory – can control information through the mechanoreceptor fibres and has an inhibitory interneuron in red, to block the gate in the pain neurons
Descending controls form the brain, called the PAG and their neurons activate neurons in the NRM and they travel down the spinal cord and also close the gate in the same exact way
Descending control work by inhibiting the same interneurons
Descending controls are activated during things like battlefield injuries
Slide showing the same as before
SG is the (red) interneuron
What is analgesia?
inability to feel pain
medicine that acts to relieve pain
What is a type of analgesia?
Non-steroidal anti-inflammatory drugs (NSAIDs)
How do NSAIDS work?
prostaglandins sensitise nociceptors to bradykinin
work by reducing the production of prostaglandins. Prostaglandins are chemicals that promote inflammation, pain, and fever
NSAIDs are analgesic (and antipiretic & anti-inflammatory) because they inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins
so NSAIDs work well against pain associated with inflammation
How do local anaesthetics work as analgesics?
block Na+ action potential and therefore all axonal transmission
How does trans cutaneous electric nerve stimulation (TENS) work as an analgesic?
electrical stimulus activating the A beta fibres to inhibit the others so electrically rubbing the skin
Rubbing the skin activates interneuron
How do opiates (eg morphine) work as a analgesic?
reduce sensitivity of nociceptors
block transmitter release in dorsal horn (hence epidural administration)
activate descending inhibitory pathways (to close the gate)
Opiate receptors activate potassium channels to hyperpolarize membrane to stop action potentials being fibres in the first place, also block transmitter release in the dorsal horn
