WEEK 6: The spinal cord and somatosensory pathways Flashcards
What are the 4 categories of sensation, list examples for each category
- superficial: pain, temperature, touch
- deep: proprioception (muscle/joint sense)
- visceral: sensations inside body including hunger/nausea
- special senses: smell, gustatory (taste), vision, hearing, balance
What does somatosensation refer to?
the ability to perceive touch, temperature, pain, and body position through action potentials
What receptors detect pain?
nociceptors
What receptors detect temperature?
thermoreceptors
what receptors detect touch, two point discrimination and proprioception (muscle & joint position sense)
Mechanoreceptors/proprioceptors
What are the types of sensory receptors?
- First-order sensory neuron with free nerve endings eg. cold stimulus
- First order sensory neuron with encapsulated nerve endings eg. pressure stimulus
- Sensory receptor synapses with first-order sensory neuron eg. sugar molecule detected by gustatory taste receptor
What is the difference between nociception and pain?
nociception is the sensory transduction of signals that trigger pain, whereas pain is an unpleasant sensory & emotional experience associated with actual/potential tissue damage
There are different types of axons that perceive pain, what are they?
A delta axons/fibres
C axons/fibres
What type of pain does A delta axons detect?
fast pain
What type of pain does C axons detect?
slow pain
What are the different specialisations in responsiveness to stimuli for axons?
- mechanical
- thermal
-mixed/polymodal (eg. heat & chemical)
What does a skin mechanoreceptor do?
detect vibration, pressure and touch
What are the two types of specialised mechanoreceptors?
Pacinian corpuscle & Meissner’s corpuscle
What does a Pacinian corpuscle detect?
crude touch
What does meissner’s corpuscle detect?
Pressure
What two components of an axon can affect it’s action potential conduction velocity?
- diameter (larger diameter = increased velocity)
- myelination
What is the first somatosensory pathway?
dorsal column medial-lemniscus pathway (DCML)
What is the DCML pathway responsible for
- fine touch & proprioception
two point discrimination, vibration, conscious joint-position sense
How many orders of neurons are in the DCML pathway?
3
What occurs in the 3 orders of neuron in the DCML pathway?
1st. axons descend in dorsal columns & synapse in medulla
2nd. axons cross in medulla to the contralateral side of the brain and head to thalamus
3rd. neurons in thalamus project to somatosensory cortex
What is the second somatosensory pathway?
Spinothalamic Tract (antero-lateral system)
What does the spinothalamic tract consist of?
- Anterior spinothalamic tract (crude touch)
- Lateral spinothalamic tract (pain, temperature)
What is the function of the anterior spinothalamic tract?
Free nerve endings respond to non-noxious mechanical stimuli - eg. a power brush so crude non-discriminative touch
What is the function of the lateral spinothalamic tract ?
Free nerve endings respond to different noxious stimuli eg. thermal, mechanical or chemical
What occurs in the 3 orders of neurons in the Anterior spinothalamic tract (crude touch)
1st order neurons:
- axons synapse in the dorsal horn
- free nerve endings/specialised (meissner’s corpuscle)
2nd order neurons:
- axons IMMEDIATELY cross in the spinal cord and head to thalamus
3rd order neurons:
- neurons in thalamus project to somatosensory cortex
What occurs in the 3 orders of neurons in the lateral spinothalamic tract (pain & temperature)
1st order neurons:
A delta fibres and C fibres indicate fast/slow pain
Axons synapse in dorsal horn
Free nerve endings/specialised receptors
2nd order neurons:
Axons immediately cross in spinal cord and head to thalamus
3rd order neurons:
Neurons in thalamus project to somatosensory cortex
What is the function of the cingulate gyrus?
it can alter the severity of pain based on the emotions that one feels
What are 2 types of pain regulation?
- afferent regulation
- descending regulation
What is the mechanism of afferent regulation?
Pain evoked by nociceptors can be reduced by simultaneous activity in mechanoreceptors
What is the mechanism of descending regulation?
- Periaqueductal gray matter (PAG) neurons
- Influences raphe nucleus in the medulla
- Secretes endogenous opioids (eg. endorphins, serotonin)
- Modulate flow of nociceptive input through dorsal horn
- Exogenous opioids (eg. morphine) affect brain & spinal cord receptors
