Several ascending and descending tracts are present in the spinal cord. The three most important tracts in the spinal cord are:
The gracile nucleus, along with the cuneate nucleus is a part of the dorsal column medial lemniscus pathway (DCML). The gracile nucleus situates in the midline dorsal medulla at the junction of the brainstem and the spinal cord. The gracile fasciculus which carries sensory input from vertebral level T6 and below ascends into the gracile nucleus to form the gracile tubercle. The cuneate fasciculus carries information from T6 and above, and ascends into the cuneate nucleus to form the cuneate tubercle. These tubercles appear as bumps on the dorsal part of the medulla.
The anatomic location of the gracile nucleus is at the floor of the fourth ventricle. The gracile nucleus gives rise to the second-order neurons, which contain six different types of neurons. It is situated medially to the cuneate nucleus. Together they combine to form the medial lemniscus. The gracile nucleus which forms the part of the dorsal column medial lemniscus pathway is a three order neuron system:
The medial lemniscus and the dorsal column pathway functions to carry sensations from the body, firs to the thalamus and subsequently to the brain. The sensory signals transmitted are:
The dorsal column medial lemniscus pathway is embryologically new. Neuroectoderm gives rise to the neural tube. The dorsal horn of the spinal cord forms from the alar plate of the neural tube, which gives rise to the dorsal column medial lemniscus pathway and is responsible for the sensory input from the body. The ventral horn of the spinal cord forms from the basal plate of the neural tube, which is responsible for the motor control of the body.
The blood supply of every structure in the brain stem depends on the location where that particular structure is situated. The gracile nucleus is at the level of medulla, which is supplied by the posterior spinal artery dorsally.
There are limited surgical options in the dorsal column medial lemniscus pathway. Due to the abundance of nerve fibers and tracts contained in a minimal space, it becomes overwhelming for a surgeon to focus the surgery on the particular area without disturbing the tracts around it. Even a minute error can further affect the tracts around that and lead to severe conditions.
The advances in modern technology have led to the development of:
These modalities help in better visualizing of the specific tracts and pathologies, which would further have an impact on the patients' lives. The nucleus gracilis is thought to be the center where the visceral and cutaneous information integrates.
One such case report by Hong et al. suggested the use of punctate midline myelotomy for the treatment of visceral pain when compared to analgesics in the treatment of cancer-related pain. The patient is a surgical candidate if they maintain a clinically stable medical condition apart from the visceral pain, has an expected life span of more than three months, and the pain is refractory to oral analgesics.
The disease affecting the dorsal column is typically degenerative in nature.
Bedside examination test – This test, called the Romberg test, can help to identify the location of the lesion. The patient is made to stand barefoot, with their feet together and eyes open. Then they are asked to close his eyes. If the patient sways to one side, it suggests a lesion in the dorsal column medial lemniscus. Care is necessary as the test can cause the patient potentially to fall.
Localization - Research has established that the dorsal column medial lemniscus pathway decussates in the closed dorsal medulla and starts as the medial lemniscus from that side. If the patient presents with sensation loss contralaterally, that is, on one side of the face and the other side of the body, we can localize the lesion to the brainstem. Further, if the patient presents with just the sensation loss in the legs and the trunk, this suggests that the lesion is either below the T6 vertebra or is specifically targeting the gracile nucleus medially.
Certain diseases and conditions which disrupt the dorsal column medial lemniscus pathway, all-cause loss of vibration, conscious proprioception, 2- point discrimination, and fine touch. It is difficult to have an isolated lesion of the gracile nucleus and the fasciculus. Any condition that involves the gracile nucleus usually involves the other tracts of the dorsal column medial lemniscus pathway as well. These conditions are:
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