Pain

Question 1

Draw a labelled line diagram to show the neural pathway taken by the pain signal from Ben’s left shoulder to the sensory cortex in the cerebrum [3]

Answer 1

Primary neuron to synapse in dorsal horn. Secondary neuron crosses over and travels up to thalamus. Tertiary neuron runs from thalamus to post central gyrus. Pain is carried in the spinothalamic tract:

• 1st order neuron: between free nerve endings [sensory receptors] in the left shoulder synapse in the spinal chord via the dorsal root with 2nd order neuron
• 2nd order neuron: crosses midline of spinal chord and ascends in the anterolateral system of the spinal chord, becomes part of the spinal lemisicus while in the midbrain, to synapse at ventral posterior nucleus of the thalamus
• 3rd order neuron: from thalamus, ascends in the internal capsule of the cerebrum to synapse in the post-central gyrus of the parietal lobe according to the sensory homunculus. Localises pain very well.

Question 2

35 y/o Dx with Type 1 Diabetes at the age of 4 years. GP notices a she has an ulcer on the underside of her foot - infected and foul-smelling. Patient was unaware of the ulcer and had not experienced any pain.

Outline why the patient is unaware of the pain (1) & Name the type of neuron that detects damage to the skin (½)

Name the parts of the brain that are involved in the perception of pain i.e. its location and unpleasantness (1½)

Answer 2

Nociceptor responds to tissue damage to generate an action potential (1)

which travels along the axon to the cell body in dorsal root ganglion (1)

the action potential enters the CNS via the dorsal horn of the spinal cord where it synapses with the 2nd order neuron (projection neuron) (1)

the signal travels along the axon of the 2nd order neuron, crossing over to the opposite side of the spinal cord, (1)

ascending in the spinothalamic tract (anterolateral system) to the thalamus (1)

where it synapses with the 3rd order neuron which transmits the signal to the cerebral cortex (1)

the somatosensory cortex signals the nature of the stimulus (tissue damage) and the location of the injury by the location of the stimulated neuron in the somatosensory cortex (1)

the insula cortex and cingulate cortex are responsible for the emotional perception of unpleasantness (1)