Posterior column - Medial lemniscus pathwayThe spinal cord and cerebrum are interconnected by several white matter tracts. Some of these tracts or pathways are ascending while the others are descending. Of the ascending tracts is a group called medial lemniscus pathway and this group originates from within the posterior column of the spinal cord.
The posterior column-medial lemniscus system conveys sensory impulses to different regions of the cerebrum and modulates impulses such as light or fine touch (tactile sensation), vibration, proprioception, and pressure. The pathway also mediates tactile discrimination, conscious proprioception, vibration, and joint and muscle sensation, as well as receives input from lamellar (Pacinian) and tactile (Meissner’s) corpuscles. Apart from the medial lemniscus pathway, other somatosensory and ascending tracts from the spinal cord include the ventral and dorsal spinocerebellar tract, spinothalamic tract, cuneocerebellar tract etc.
Description and course
The posterior funiculus of the spinal cord is occupied by fasciculus gracilis and fasciculus cuneatus. Because of these tracts (fasciculus gracilis and cuneatus), the posterior funiculus is also referred to as the posterior column tracts, and the fibres of these tracts extend upwards as far as the lower part of the medulla oblongata where they terminate by synapsing with neurons in the nucleus gracilis and nucleus cuneatus respectively. The gracilis and cuneatus fasciculi are formed predominantly by first order sensory neurons. First order neurons have axons which ascend in the dorsal columns and terminate in the gracile and cuneate nuclei of the medulla, and the dorsal columns are somatotopically organized so that lower levels (gracilis tract) are more medial and higher levels (cuneate tract) are more lateral. These neurons carry sensory information from touch or proprioceptive receptors to the medulla oblongata.
Signals from the upper limb and trunk travel in the fasciculus cuneatus (the lateral part of the dorsal column) to synapse in the cuneate nucleus of the medulla oblongata, while impulses from the lower limb and trunk travel in the fasciculus gracilis (the medial part of the dorsal column) and then synapse in the gracile nucleus of the medulla oblongata. However, the medial lemniscus proper is formed after the axons of the gracile and cuneate nuclei run forwards, medially and cross the midline. Prior to crossing the midline, while running upwards, they are referred to as internal arcuate fibres. The neurons of the gracile and cuneate nuclei of the medulla oblongata are second order sensory neurons, thus the medial lemniscus is predominantly composed of second order sensory neurons and their fibres run upwards through the medulla, pons and midbrain (the entire brainstem) to terminate in the ventral posterolateral nucleus of the thalamus. The medial lemniscus receives the sensory signals from the preceding neurones, and delivers it to the third order sensory neurons in the thalamus.
In the ventral posterolateral nucleus of the thalamus, the third order sensory neurons give off axons that course through the posterior one-third of the posterior limb of the internal capsule and the corona radiata to terminate in the postcentral gyrus, the somatosensory areas of the cerebral cortex (i.e., Brodman’s areas 3, 1, and 2). Thus the third order sensory neurons convey sensory signals from the thalamus to the primary sensory cortex of the brain. They also have fibre connections with the posterior external arcuate fibres (dorsal external arcuate fibers which take origin in the accessory cuneate nucleus.
Furthermore, as a general rule, nerve fibres carrying proprioceptive impulses through the fasciculus gracilis predominantly terminate in the dorsal nuclei situated in the posterior gray column of thoracic segment of the spinal cord and therefore do not reach the cervical levels of the spinal cord. Hence most of the fibres coursing through the fasciculus in the upper cervical segment are from the cutaneous receptors. The fasciculus cuneatus carries numerous proprioceptive fibres (as well as those for cutaneous sensations) throughout its extent. Both of the fasciculi are made up predominantly of myelinated fibres that are grouped in terms of the dermatomes they receive sensory impulses from.
Injury to the sensory pathways may lead to various types of sensory disorders. Interruption of ascending tracts carrying various sensations results in loss of sensory perception, also called anaesthesia, over parts of the body concerned. In the case of spinal cord lesions, the level of injury can be inferred from the level of sensory loss. In this sense it must be remembered that the finer modalities of touch are carried by the posterior column tract which are uncrossed. Severe touch, pain and temperature are conveyed by the spinothalamic tract which are crossed, and thus a unilateral lesion in the spinal cord can cause loss of the power of tactile localization, tactile discrimination and of stereognosis on the side of lesion; with loss of crude touch, pain and temperature on the opposite side. Thus lesions of the posterior column may lead to lack of sensation to fine touch, vibration, two-point discrimination, and proprioception (position sense) from the skin and joints.
The medial lemniscus is most vulnerable to cerebrovascular accidents (CVA) in the internal capsule, and that damage to the somatosensory "association cortex" produces an inability to interpret a sensory signal, even though it can be detected. This condition is referred to as somatosensory agnosia. In thalamic syndrome, the threshold for appreciation of touch, pain or temperature is lowered, and sensations that are normal may appear to be exaggerated or unpleasant. Also, there are symptoms of abnormal emotions, as well as spontaneous pain.
In a disease called syringomyelia, the region of the spinal cord close to the central canal undergoes a degenerative phase, with formation of cavities. Fibres of the first order neurons crossing in this region are interrupted leading to loss of sensation of pain and temperature over the concerned regions of the skin. Position and vibration sense of the feet are also lost. However, in this condition, sensation of touch is retained as there is an additional tract for it in the posterior column pathways. This occurrence is referred to as “dissociated anesthesia”.