Final answer:
A lesion in the lateral spinothalamic tract leads to the loss of pain and temperature sensations on the side of the body opposite the lesion due to the pathway's fibers crossing over into the spinal cord. Fine touch and proprioception, conveyed by the dorsal column-medial lemniscus pathway, would be lost on the paralyzed leg on the same side as the lesion.
Step-by-step explanation:
If there is a lesion in the lateral spinothalamic tract, it would affect the body's ability to transmit pain and temperature sensations. This tract is responsible for carrying these sensory signals to the thalamus and then to the cerebral cortex. When damaged, it can lead to a condition known as spinothalamic tract syndrome or anterolateral cordotomy syndrome, which is characterized by a loss of pain and temperature sensation on the side of the body opposite to the lesion, due to the fact that the fibers of the spinothalamic tract decussate (cross over) at the level of the spinal cord and then ascend to the opposite side.
The spinothalamic pathway decussates immediately upon entering the spinal cord, which means that if one side of the pathway is damaged, the patient will still be able to feel pain stimuli on the paralyzed leg due to the crossing-over of the fibers. However, sensations such as fine touch and proprioception conveyed by the dorsal column-medial lemniscus pathway, which ascends ipsilaterally, would not be perceived on the paralyzed leg, as this pathway would be damaged in the same way as the lateral corticospinal tract.
The ascending tracts in the spinal cord's spinothalamic pathways carry sensory information to the brain, crucial for processing touch and pain sensations. If the dorsal column pathway is damaged on one side, the patient may not recognize fine touch or proprioception on the leg with motor function loss, but will still recognize pain due to the intact spinothalamic tract on that side.