Sensory Systems Flashcards
What are examples of sensory receptors?
Mechanoreceptors
Chemoreceptors
Thermoreceptors
Nociceptors
Proprioceptors
How does the nerve endings of different sensory receptors differ?
May have free nerve endings or complex structures
What are examples of sensory receptors that have free nerve endings?
Nociceptors
Cold receptors
What are examples of complex structures at the end of sensory receptors?
Pacininan corpuscle
Meissner’s corpuscle
What is the specific area that sensory receptors respond to stimulus over called?
Receptive field
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How do sensory receptors tranduce their adequate stimulus?
Into depolarisation, the receptor (generator) potential
This evokes firing of action potentials for long distance transmission
Gives information on modality, intensity and location of the stimulus
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What does the size of receptor potential encode?
Intensity of stimulus
What does the frequency of action potentials encode?
Intensity of stimulus
What does the receptive field encode?
Location of stimulus
What determines acuity?
Density of innervation and size of receptive fields
What are action potentials from sensory receptors transmitted to the CNS by?
Axons
What are the 3 types of primary afferent fibres that mediated cutaneous sensation?
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What are Aß fibres also known as?
Large myelinated
What is the speed of Aß fibres?
30-70m/s
What kinds of sensory information is transmitted by Aß fibres?
Touch, pressure, vibration
What are Aσ fibres also known as?
Small myelinated fibres
What is the speed of Aσ fibres?
5-30m/s
What kind of information is transmitted by Aσ fibres?
Cold, “fast” pain, pressure
What are C fibres also known as?
Unmyelinated fibres
What is the speed of C fibres?
0.5-2m/s
What kind of information is transmitted by C fibres?
Warmth, “slow” pain
What are the 2 types of primary afferent fibres that mediate proprioception?
Aa and Aß (such as muscles spindles, golgi tendon organs etc)
What do proprioceptors give information about?
Position and movement
What do all primary afferent fibre types enter the spinal cord through?
Dorsal root ganglia (or cranial nerve ganglia for the head)
What fibres transmit information from mechanoreceptors?
Aa and Aß fibres
Explain the pathway of mechanoreceptor (Aa and Aß fibres)?
1) Project straight up through ipsilateral dorsal columns
2) Synapse in cuneate and gracile nuclei
3) The 2nd order fibres cross over midline in the brain stem and project to reticular formation, thalamus and cortex
Where do mechanoreceptor fibres cross the midline?
Brainstem
What do mechanoreceptor fibres project up the spinal cord through?
Ipsilateral dorsal columns
What fibres do thermoreceptors use?
Aσ and C fibres
What fibres do nociceptive receptors use?
Aσ and C fibres
Explain the pathway of thermoreceptive and nociceptive fibres?
1) Synapse in the dorsal horn
2) The 2nd order fibres cross over the midline in the spinal cord
3) Project up through the contralateral spinothalamic (anteriolateral) tract to reticular formation, thalamus and cortex
What do mechanoreceptors detect?
Touch, pressure, stretching and motion
What do nociceptive receptors detect?
Pain
Where do thermoreceptive fibres cross the midline?
Spinal cord
Where do nociceptive fibres cross the midline?
Spinal cord
How are thermoreceptive fibres transmitted up the spinal cord?
Contralateral spinothalamic (anteriolateral) tract to reticular formation, thalamus, and cortex
How are nociceptive fibres transmitted up the spinal cord?
Project up through the contralateral spinothalamic (anteriolateral) tract to reticular formation, thalamus, and cortex
Different transmission for sensory information explains consequences of spinal cord injuries, what does damage to dorsal columns cause?
Loss of touch, vibration, proprioception below lesion on ipsilateral side
Different transmission for sensory information explains consequences of spinal cord injuries, what does damage to anteriolateral quadrant cause?
Loss of nociceptive and temperature sensation below lesion on contralateral side
Where is the ultimate terminate of all sensory receptor fibres?
Somatosensory cortex (S1) of the postcentral gyrus
What is S1?
Somatosenosry cortex
What are the endings of sensory fibres grouped according to?
Location of the receptors, forming sensory homunculus
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What are the steps of processing in sensory pathways?
Adaptation
Convergence
Lateral inhibition
What is the adaptation step of processing in sensory pathways?
Decline in the electrical response of a receptor neuron over time in spite of the continued presence of an appropriated stimulus of constant strength
Is apparent as a gradual decrease in frequency of spikes generated within the receptor neuron
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What are the 2 kinds of adapting that sensory receptors can display?
Rapidly adapting
Slowly adapting
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What are consequences of convergence occuring when processing sensory information?
Saves on neurons
But reduced acuity
May underlie referred pain
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What is the lateral inhibition step of processing in sensory pathways?
Activation of one sensory input causes synaptic inhibition of its neighbours
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What are consequences of lateral inhibition in sensory receptors?
Gives better definition of boundaries
Cleans up sensory information
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What is perception?
The ability to see, hear or become aware of something through the sense, or the way something is interpreted
What are examples of the different kinds of pain?
Sharp, stabbing vs diffuse, throbbing
Fast (initial pain) vs slow (delayed) pain
Acute vs chronic pain
Visceral pain
Referred pain
Phantom limb pain
What is signal transduction?
Transmission of molecular signals from a cell’s exterior to its interior
What is signal transduction in nociceptors activated by?
Low pH, heat (via ASIC, TRPV1 etc)
Local chemical mediators (such as bradykinin, histamine, prostaglandins)
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What are examples of local chemical mediators that activates signal transduction in nociceptors?
Bradykinin, histamine, prostaglandins
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Explain the processing of nociceptive pathways?
1) Segmental controls (such as gate theory control of pain where activity in Aa/ß fibres activates inhibitory interneurons)
2) Inhibitory interneurons release opiod peptides (endorphins) that inhibit transmitter release from Aσ/C fibres, “closing the gate”
3) Descending controls, the same inhibitory interneurons are also activated by descending pathways from PAG and NRM, hence also “closing the gate”
4) Transmitted to thalamus
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What does PAG stand for?
Peri-aqueductal grey matter
What does NRM stand for?
Nucleus raphe magnus
What is the medical term for the inability to feel pain?
Analgesia
What is the “gate control theory of pain”?
Non-painful stimulus input closes the nerve “gates” to painful input, which prevents pain sensation form travelling to the central nervous system
What can analgesia be achieved with?
Non-steroidal anti-inflammatory drugs (NSAIDs)
Local anaesthetics
Transcutaneous electrical nerve stimulation (TENS)
Opiates
What does NSAIDs stand for?
Non-steroidal anti-inflammatory drugs
How do NSAIDs achieve analgesia?
1) Prostaglandins sensitive nociceptors to bradykinin
2) NSAIDs inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins
3) So NSAIDs work well against pain associated with inflammation
How do local anaesthetics achieve analgesia?
Block Na+ action potential and therefore all axonal transmission
What does TENS stand for?
Transcutaneous electrical nerve stimulation
What is an example of an opiate?
Morphine
How do opiates achieve analgesia?
Reduce sensitivty of nociceptors
Block transmitter release in dorsal horn (hence epidural administration)
Activate descending inhibitory pathways