Sensory Systems (Including physiology of pain) Flashcards
Examples of sensory receptors with free nerve endings? Examples of ones with complex structure?
- Free endings: nociceptors, cold receptors
- Complex: Pacininan corpuscle (sensitivity to vibration and pressure), Meissner’s corpuscle (sensitivity to light touch)
Describe the signal transduction pathway starting at a sensory receptor
- Sensory receptor transduces stimulus to a graded potential
- If graded potential is large enough, triggers AP at axon
- Long distance transmission then occurs via several nerves
How do sensory receptors encode the intensity of a stimulus?
Frequency of APs generated by a sensory receptor encodes the intensity of a stimulus
What determines the acuity of sensation at different points on the body?
The density of innervation and size of receptive fields
less neurons and bigger fields = lower acuity
What types of neurons carry cutaneous touch, pressure and vibration information? Describe the neuron structure?
A(beta) fibres
- large myelinated, (30-70m/s)
What types of neurons carry cutaneous cold, fast pain and pressure information?
A(delta) fibres
- Small myelinated (5-30m/s)
What types of neurons carry cutaneous warmth and slow pain information?
C fibres
- Unmyelinated (.5-2m/s)
What types of neurons carry proprioceptive information?
A(alpha) and A(beta) fibres
What type of fibres make up the lateral spinothalamic tract?
- Thermoceptive and nociceptive
- A(delta) and C fibres
What type of fibres make up the dorsal column tract?
- Mechanoreceptive fibres
- A(alpha) and A(beta) fibres
What sensory information is lost when damage to the dorsal column of the SC occurs?
Mechanoreceptive information from below the lesion on the ipsilateral side
What sensory information is lost when damage to the lateral spinothalamic tract of the SC occurs?
Nociceptive and thermoceptive info from below the lesion on the contralateral side
What is the difference between rapidly adapting and slowly adapting neurons?
- Rapidly adapting: fires a few APs in response to the stimulus, then adapts to the stimulus and stops firing, only fires again when stimulus is removed
- Slowly adapting: Fires APs at a high rate when stimulus starts, then at a low rate for the duration of the stimulus, stops firing when stimulus is removed
What is convergence?
Where you have lots of neurones synapsing onto one neuron
Advantages and disadvantages of convergence?
- Pros: saves of number of neurones needed
- Cons: reduced acuity
What is referred pain?
Feeling pain at a site other than the site of origin
Can be due to convergence of neurones from different places onto the same neuron on its way up to the brain (eg. MI felt in chest and arm)
What is lateral inhibition?
- Neuron with the receptive field closest to the stimulus inhibits its neighbours with closely associated receptive fields
- Increases acuity
Difference between sensation and perception?
Perception is the limited amount of what you sense that you actually notice
Different types of pain?
- Sharp/stabbing vs. diffuse, throbbing
- Fast (initial) vs slow (delayed)
- Acute vs. chronic
- Visceral pain (hard to localize)
- Referred pain
- Phantom pain
How is signal transduction initiated in nociceptors?
- low pH and heat cause protein channels to open and initiate an AP
- Chemical mediators from damaged tissue cause G-protein coupled receptor cascades that open protein channels
Examples of protein channels involved in initiation of signal transduction in nociceptors?
- ASIC (acid/low pH)
- TRPV1 (heat)
Examples of chemical mediators that initiate signal transduction in nociceptors via G-protein cascade?
- Bradykinin
- Prostaglandins
- Histamine
How does activation of mechanoreceptive neurons in the same area as an injury inhibit pain being felt? (Segmental gate control theory)
Mechanoreceptive fibres in the dorsal column can activate inhibitory interneurons that inhibit presynaptic fibres in the dorsal root (before lateral spinothalamic tract)
- Why rubbing something better works, mechano feedback causes inhibition of nociceptive feedback (closes the gate)
How do nucleii in the peri-aqueductal grey matter inhibit nociception?
- Via the descending control mechanism: nuclei in the PAG activate nuclei in the nucleus raphe magnus (NRM) which travel down the SC to the level where pain is occurring.
- They then activate the same inhibitory interneurones as the mechano receptors do, and close the gate
