Ascending Tracts of the Spinal Cord
When sensory nerve fibers reach the spinal cord, they are sorted into different bundles depending on their function. They are known as nerve tracts or fasciculi and are found within the white matter of the spine. As the name suggests, the ascending tracts of the spinal cord ascend from the spinal cord and connect it to the brain. These tracts are named based on their origin and termination.
They are found running along the dorsal, lateral, and ventral columns of the white matter. The main role of the ascending tracts of the spinal cord is to transmit somatosensory information. There are two types of afferent information conducted by the tracts: exteroceptive and proprioceptive. Exteroceptive information comes from the body’s exterior, such as pain and touch. In contrast, proprioceptive information comes from inside of the body, such as the muscles.
Group of Neurons Forming Tracts
In terms of basic anatomy, the overall ascending pathway is made up of three different types of neurons. Firstly, there are first order neurons which receive sensory information from the receptors and send them to sensory neurons present in the posterior gray horns of spinal cord. The cell body of these neurons are found within the posterior nerve root ganglion.
Next, there are second order neurons. These sensory neurons are present in the posterior gray horns, their fibers forming the ascending tracts They carry sensory impulses to different subcortical brain areas such as thalamus.
Third order neurons lie in the subcortical areas and their fibers carry impulses to the cerebral cortex of the brain.
Position of the Ascending Tracts
Ascending tracts ascend in the white columns of the spinal cord. However, the precise position of some tracts is still not known with certainty and the territories of some of them overlap.
- Fasciculus Gracilis
- Fasciculus Cuneatus
- Lateral Spinothalamic tract
- Dorsal Spinocerebellar tract
- Ventral Spinocerebellar tract
- Spinotectal tract
- Spinoreticular fibers
- Spino-olivary pathway
- Spinovestibular tract
- Anterior Spinothalamic tract
Fasciculi Gracilis & Cuneatus
Types of Fibers
These two large ascending tracts run on each side of the cord in the dorsal funiculi, separated by the postero-intermediate septum. They contain mostly myelinated fibers which carry proprioception, exteroception and vibratory sensation to higher levels.
The axons from the posterior root ganglion enter the spinal cord through the dorsal root of spinal nerves and split into two types of branches upon reaching the dorsal column: long ascending branches and short descending branches. The descending branches crosse many segments, split into more branches which then synapse with neurons in the posterior horn, internuncial neurons and with anterior horn cells. These fibers are responsible for intersegmental reflexes.
Similarly, the long ascending fibers can also terminate by synapsing with cells at the similar destination. The fibers can extend quite far, covering numerous sections of the spinal cord. However, most of the long ascending fibers ascend upwards through the posterior white column, forming the fasciculus gracilis and the fasciculus cuneatus.
The fasciculus gracilis is found throughout the spinal cord and begins at the caudal end of the spinal cord. It comprises long ascending fibers from different spinal nerves which enter the spinal cord through the ipsilateral dorsal spinal root, including sacral, lumbar and lower six thoracic nerves. The fasciculus gracilis is present medial to fasciculus cuneatus at upper level of spinal cord.
The first order neurons entering through dorsal root of spinal nerves brings tactile, vibratory and proprioceptive information mainly from the lower body and terminate on to the second order neurons in the ipsilateral nucleus gracilis. Axons of second order neuron ascend ventro-medially as internal arcuate fibers. These fibers cross in the midline to form medial lemniscus.
The crossed tract ascends through the medulla, pons and midbrain and finally terminates on to the third order neurons present at the ventroposterolateral nucleus of thalamus. The axons of these third order neurons then enter the posterior limb of the internal capsule and corona radiata, before arriving at the somesthetic area of the postcentral gyrus of the cerebral cortex
The fasciculus cuneatus starts at the mid-thoracic level and runs lateral to gracilis. Its fibers come from the upper thoracic and cervical dorsal roots of the spinal cord. Myelinated fibers from the dorsal root ganglion carrying tactile, vibratory and proprioceptive information from the upper body ascend ipsilaterally and terminate on the second order neurons of the nucleus cuneatus.
Like the fasciculus gracilis, the fibers form internal arcuate fibers and medial lemniscus after crossing the midline and terminate as third order neurons at the ventral posterolateral nucleus of thalamus. The third order axon carry on towards the cerebral cortex and terminate at the lateral aspect of sensorimotor cortex.
Few second order fibers from the nucleus cuneatus pass on to terminate in the accessory cuneate nucleus. These axons enter the cerebellum through the inferior cerebellar peduncle as the cuneocerebellar tract and the fibers are known as external arcuate fibers. This tract is responsible for carrying muscle joint sense information to the cerebellum.
The spinothalamic tracts carry pain, temperature, non discriminative touch and pressure information to the thalamus. The fibers forming the spinothalamic tracts are different from the fibers of the dorsal white column in two ways:
- the cell bodies of origin are present in the spinal gray matter
- fibers cross within the gray matter before ascending to the thalamus
Neurons forming the spinothalamic tracts are a heterogeneous population of cells having variable size, shape and number in different segments and spinal laminae.
Lateral Spinothalamic Tract
The lateral spinothalamic tract lies in the lateral funiculus, medial to the ventral spinocerebellar tract. It carries pain & temperature sensation.
The fibers enter the spinal cord from the posterior root ganglion and reach the posterior gray column where they divide into ascending and descending branches. These branches cross two-three segments and form the dorsolateral tract of Lissauer. The dorsolateral tract of Lissauer is a small bundle of both myelinated and unmyelinated fibers.
The first order neurons of these tracts terminate by synapsing in the posterior gray column (Rexed laminae I, IV,V & VI) with the cells in substantia gelatinosa (lamina II). The second order neurons cross via the anterior white commissure and ascend contralaterally in the lateral funiculus. The somatotopic organization of the fibers are maintained throughout the extent; the fibers from the sacral region are more laterally placed while those from the upper limb and cervical region are present medially. Similarly, the pain fibers lie anterior to the temperature carrying fibers.
Ascending through the medulla oblongata, it is accompanied by the spinotectal and anterior spinothalamic tracts. Together they form the spinal lemniscus. The spinal lemniscus crosses the brainstem and tegmentum of the midbrain lateral to medial lemniscus.
Many of the lateral spinothalamic tract fibers terminate by synapsing onto the third order neurons in the ventral posterolateral nucleus of the thalamus, which then pass through the posterior limb of the internal capsule and corona radiata to reach the postcentral gyrus of the cerebral cortex.
Anterior Spinothalamic Tract
Fibers of this tract ascend in the anterior and anterolateral funiculi. They are also somatotopically arranged such that the sacral and lumbar fibers are lateral while those from the thoracic and cervical regions are medial. This tract is responsible for carrying light touch and pressure sensation.
The first order neurons of the anterior spinothalamic tract are found in the dorsal root ganglia of the spinal nerves. The axons and dendrites of these neurons penetrate the spinal cord near the posterior horn of the dorsal funiculus. The dorsal funiculus is the white matter on each side of the spinal cord, between the dorsal root and the posterior median sulcus.
After entering the spinal cord, the axons and dendrites then ascend ipsilaterally for seven segments of the cord. The second order neurons are found between laminae IV and VI in the posterior horn, which is where the axons of the first order neurons terminate. The posterior horn is an area of grey matter in the dorsal part of the spine, important in receiving a variety of different types of sensory information, such as vibrations. The axons of the second order neurons then pass through the narrow white commissure. This structure is a thin section of white matter than crosses the midline of the spinal cord. It is found between the posterior grey commissure and the central canal.
After crossing the narrow white commissure, the axons of the second order neurons ascend contralaterally to the anterior funiculus, forming the anterior spinothalamic tract. The anterior funiculus is a bundle of nerve roots found between the anterior median fissure of the spine. The tract then passes through the brainstem to reach third order neurons. These neurons are found in the ventral posterolateral nucleus of the thalamus. Projections of the neurons then enter the posterior limb of the internal capsule and corona radiata, to reach the postcentral gyrus of the brain’s cerebral cortex.
Note that the internal capsule is an area of white matter found within the cerebral hemispheres of the brain. The posterior limb of this white matter is found between the lenticular nucleus and the thalamus in the brain. In addition, the corona radiata is a sheet of white matter that carries information from neurons to and from the cerebral cortex.
Posterior Spinocerebellar Tract
This uncrossed tract ascends in the posterolateral aspect of the spinal cord and begins in the L2-3 segment. The tract carries information from the muscle spindles, Golgi tendon organs and pressure receptors. The carried sensations are proprioception, cutaneous touch and pressure information from the lower limb and trunk.
The first order neurons entering from the posterior root ganglion terminate by synapsing with second order neurons in the posterior grey column of the spinal cord. This group of neurons is called the nucleus dorsalis (Clark’s column) located in lamina VII in cord segments C8-T1 through L2-3.
The fibers ascend in the posterolateral part of the lateral funiculus, lateral to the corticospinal tract. On reaching the medulla it is incorporated in the inferior cerebellar peduncle and enters the cerebellum. It terminates in the rostral and caudal part of vermis. In the rostral part fibers mainly end in the Larsell’s lobules I-IV, while posteriorly termination is at the paramedian lobule. The information related to muscle tendons and muscle & joint movement is used by the cerebellum to coordinate the limb movements and posture maintenance.
Anterior Spinocerebellar Tract
This tract, like the posterior spinocerebellar is involved in carrying unconscious proprioception from the lower limb. The second order neurons are the spinal border cells (nucleus pericornualis anterior) located in the periphery of the ventral horn and neurons located in laminae V through VII in cord segments L1 and below.
The large number of tract fibers cross in the ventral white commissure and ascend in the lateral white column just ventral to the posterior spinocerebellar tract. After passing through the medulla and pons, it exits the brainstem via the superior cerebellar peduncle and terminates in the vermal and paravermal regions of the cerebellum. The tract fibers become ipsilateral by decussating again within the white matter of the cerebellum and terminate in the cerebellar hemisphere. Fibers of the anterior spinocerebellar tract are less numerous than those of the posterior spinocerebellar one and are also larger in size.
The Cuneocerebellar Tract
It is the upper limb equivalent tract of the posterior spinocerebellar tract. The first-order fibers ascend in the fasciculus cuneatus and synapse in the lateral or accessory cuneate nucleus in the caudal medulla. Axons arising from the lateral cuneate nucleus are called posterior external arcuate fibers and they form the cuneocerebellar tract. They course with the posterior spinocerebellar tract. It leaves the brainstem through the inferior cerebellar peduncle and terminate in the vermal and paravermal regions of the cerebellum.
The fibers of this crossed tract ascend in the anterolateral part of the spinal cord closely to spinothalamic tract. The functional significance is not very well known but evidence suggests that they are involved in transmitting nociceptive information and facilitation of reflexive head movement towards noxious stimuli. The second order neurons are present in the laminae I & V of the posterior horn. The fibers cross and ascend in the anterolateral white column. On reaching the level of the midbrain, they project medially into intermediate and deep layers of the superior colliculus and the lateral region of periaqueductal gray matter. In the contralateral superior colliculus they terminate somatotopically.
The fibers of this tract ascend in the anterolateral part of the spinal cord and reach the widespread region of the brainstem reticular formation. It runs parallel to the spinothalamic tract up to the brainstem. These fibers are concerned with behavioural awareness and modification of motor and sensory activities. The cell bodies of second order neurons lie in the intermediate gray laminae.
The fibers ascend mostly uncrossed and terminate chiefly upon cells of the reticular formation in medulla oblongata, pons and midbrain. The lateral reticular nucleus of medulla project some of the fibers to specific portions of the cerebellum, showing that some fibers have their involvement in the transmission of exteroceptive impulses to cerebellum. Spinoreticular fibers passing to pontine levels are distributed bilaterally but are less numerous than those terminating in the medulla. A small number of fibers are found in the region of midbrain.
Fibers of this tract arise from all levels of the spinal cord and are somatotopically organized. These fibers convey information from cutaneous and proprioceptive organs. The second order neurons are present in the posterior gray column.
The axons cross the midline and ascend in the posterior white column. They end by synapsing with the inferior olivary nuclei in medulla oblongata. The axons of the third order neurons cross the midline and enter the cerebellum via inferior cerebellar peduncle.
Other Ascending Pathways
In addition to the above mentioned tracts there are few other ascending pathways but relatively little is known of their functional significance. Among those are the spino-vestibular fibers and the spino-pontine fiber pathways.