Final answer:
The medial lemniscal pathway is one of the principal somatosensory pathways responsible for transmitting fine touch, vibration, and proprioception. It begins with first-order neurons entering the spinal cord and ascending to synapse with second-order neurons in the medulla; these then cross over and ascend to the thalamus before reaching the cortex.
Step-by-step explanation:
The medial lemniscal pathway, also known as the dorsal column-medial lemniscus pathway, is one of the two primary somatosensory pathways that carry sensory information to the brain. The dorsal column system is crucial for transmitting sensations of fine touch, vibration, and proprioception from the peripheral sensory receptors to the brain. It starts with the first-order neurons, which are the sensory neurons located in the dorsal root ganglion.
These neurons enter the spinal cord and ascend ipsilaterally in the dorsal columns, specifically the fasciculus gracilis for the lower body and fasciculus cuneatus for the upper body, until they synapse in the medulla oblongata. At the medullary nuclei - nucleus gracilis and nucleus cuneatus – second-order neurons of the dorsal column system are found. These second-order neurons decussate (cross over) at the medulla, forming the medial lemniscus, and continue to ascend to synapse in the thalamus. Finally, third-order neurons project from the thalamus to the primary somatosensory cortex, where sensory perception occurs.