Sensory Systems Flashcards
What is the range of sensory receptors?
Mechanoreceptors Chemoreceptors Thermoreceptors Nociceptors Proprioceptors
Each type of sensory information is associated with a specofoc receptor type respnding to a specific sensory modality
How does the structure of sensory receptors differ?
May have free nerve endings:
- Nociceptors,
- Cold receptors
May have complex structure:
- Pacininan Corpuscle
- Meissner’s corpuscle
What is the receptive field?
The area that a sensory receptor will resond to a stimulus
What do Meissner’s corpuscle’s detect?
Light touch
What so Merkle’s corpuscles detect?
touch
What do Pacinian corpuscles detect?
Deep Pressure
What do Ruffini corpuscle’s detect?
Warmth
What do sensory receptors do when they detect a stimulus?
Transduce their adequate stimulus into a depolarisation called the generator potential.
The size of the generator potental encodes the intensity of stimulus.
What happens once a generator potential is evoked?
Receptor potential then evokes firing of action potentals for long distance transmission.
Then frequency of action potentials encodes intensity of stimulus.
What does the receptive field encode?
Receptive field encodes location of stimulus
What determines acuity?
Density of innervation and size of receptive fields
Cutaneous sensation is mediated by 3 types of primary afferent fibres.
What are they?
A-beta fibres
A-delta fibres
C fibres
What are A-beta fibres?
large myelinated
(30-70m/s)
Touch, pressure, vibration
What are A-delta fibres?
small myelinated
(5-30m/s)
cold, “fast” pain, pressure
What are C fibres?
Unmyelinated fibres
(0.5-2m/s)
Warmth, “slow” pain
What 2 types of primary afferent fibres mediate proprioception?
A-alpha and A-beta
e.g. spindle fibres, golgi tendon organs etc
How is mechanoreceptive sensory information transmitted?
A-alpha and A-beta fibres
Project straight up through ipsilateral dorsal columns.
Synapse in cuneate and gracile nuclei.
The 2nd order fibres decussate in the brainstem and project to reticular formation, thalamus and cortex
What is the reticular formation?
A diffuse network of nerve pathways in the brainstem connecting the spinal cord, cerebrum, and cerebellum, and mediating the overall level of consciousness.
How is Thermoreceptive and Nociceptive information transmitted?
A-delta and C fibres
Synapse in the dorsal column
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
Different pathways for transmission of sensory information explains consequences of spinal cord injuries.
What would damage to dorsal columns result in?
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 would damage to anterolateral quadrant result in?
Causes loss of nociceptive and temperature sensation below lesion on contralateral side
Where is the ultimate termination of sensory information?
Somatosensory cortex of the postcentral gyrus.
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.
Will stimulation anywhere along a sensory pathway evoke the same sensation?
Yes
What is adaptation?
Neural adaptation or sensory adaptation is a change over time in the responsiveness of the sensory system to a constant stimulus.
For example, if you rest a hand on a table, you immediately feel the table’s surface on your skin. Within a few seconds, however, you stop feeling the table’s surface.
The sensory neurons stimulated by the table’s surface respond immediately, but then respond less and less until they may not respond at all
What is convergence?
Different sensory neurons synapse onto the same 2nd order neuron.
These may be the same sensory information from different locations (resulting in referred sensation) or different sensory inputs.
-Saves on neurones but reduces acuity
What is lateral inhibition?
Activation of one sensory fibre causes synaptic inhibition of its neighbours.
Gives a better definition of boundaries and cleans up sensory information
Does all sensory information reach the brain?
No
This gives changes in perception
What is the difference between fast and slow pain?
Fast (initial) pain:
- Sharp
- Stabbing
Slow (delayed) pain:
- Diffuse
- Throbbing
What can cause phantom limb pain?
Lesions at the end of axons can spontaneously fire
What may activate signal transduction in Nociceptors?
Low pH, heat (via ASIC, TRPV1 etc)
Local chemical mediators:
- Bradykinin
- Histamine
- Prostaglandins
Describe the segmental control of pain
Activity in A-alpha/beta fibres of same area activates inhibitory interneurones.
Inhibitory interneurones release opoid peptides (endorphins) that inhibit transmitter release from A-delta/C fibres
This explains why rubbing a sore area makes it fell better
Describe how descending controls can limit pain
Descending controls from Peri-aqueductal grey matter (PAG) through Nucleus raphe menus (NRM).
These act on the same inhibitory pathway as segmental control
Common in battlefield wounds (rush suppresses pain)
How do prostaglandins and bradykinin cause pain?
Prostaglandins and bradykinin dont cause pain themselves.
Prostaglandins sensitise nociceptors to bradykinin
How do NSAIDS work?
Inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins
What are NSAIDs great for?
Analgesic, antipiretic and anti-inflammatory so work very well against pain associated with inflammation
How do local anaesthetics work?
Block Na action potential and therefore all axonal transmission
What is Trans-cutaneous Electric Nerve stimulation (TENS)?
Electrical stimulation to area that is painful.
Correct frequency selected for mechanocrepetor fibres so that nociceptor fibres are inhibited.
Sciency version of rubbing your sore leg better
How do opiates work?
Reduce sensitivity of nociceptors.
Block transmitter release in dorsal horn (hense epidural administration)
Activate descending inhibitory pathways
