Ascending and Descending Tracts of the Spinal Cord
What are the Spinal Tracts?
These are stimulatory nerve pathways that transmit information via action potentials. The direction of travel of the ascending tracts is from the site of the stimulus (outside of the central nervous system, in the periphery on one of the limbs) to the designated area of the brain. Since the brain is situated in the skull at the top of the body (when erect), the direction is said to be ascending via the nervous tracts of the spinal cord. The direction of the descending tracts is of course the opposite.
This article will highlight first the main ascending tracts of the spinal cord, otherwise known as the somatosensory and muscle joint sense pathways to consciousness, which sensation triggers them, which receptor receives the stimulus, the order of the neurons and where they synapse and lastly, the destination and termination of the tract. Then in the same way the descending tracts will also be discussed starting with the name of the tract, its origin, where it crosses over, the branches it gives, its destination and its function within the periphery.
It should be noted that the level of the medulla oblongata, between the inferior olivary and trigeminal nerve nuclei, the lateral and anterior and spinotectal tracts run together, in a bundle known as the spinal lemniscus. Also, all of the ascending pathways mentioned below send branches to the reticular activating system.
The Ascending Tracts of The Spinal Cord
The Lateral Spinothalamic Tract
The primary sensations related to this tract are pain and temperature. The sensory receptors that are sensitive to the previously mentioned stimuli are the free nerve endings. The first synapse is in the posterior root ganglion, the second synapse is in the substantia gelatinosa and the third synapse is in the ventral posterolateral nucleus of the thalamus (VPL). The information's final destination is the posterior central gyrus.
The Anterior Spinothalamic Tract
The stimulus or sensation is either light touch or pressure and the receptors that collect that information are the free nerve endings, similar to the lateral spinothalamic tract. From this point on the anterior and lateral spinothalamic tracts run the same course of neurons and end up in the same final destination within the brain.
The Posterior White Column: Fasciculi Gracilis and Cuneatus
This tract is stimulated by discriminative touch, vibration and muscle joint consciousness. It’s receptors include the Meissner’s Corpuscles, Pacinian Corpuscles, Muscle Spindles and Tendon Organs. Its only difference from the previous tracts is its second synapse, which deviates from the tract to the nuclei Gracilis and cuneatus.
The Posterior Spinocerebellar Tract
The last of the ascending tracts is separate from the others, because its sensation stems from a different anatomical origin. The other tracts pick up external stimuli from receptors on the skin’s surface, while the posterior spinocerebellar tract picks them up from unconscious muscle joint sense, which means that they occur from interior structures within the body. The receptors include the muscle spindles, tendon organs and joint receptors of the trunk and lower limbs. The first synapse occurs on the posterior root ganglion, not unlike the other tracts, however this is the only part of the pathway that mimics the other tracts. The second synapse occurs in the nucleus dorsalis. The third synapse happens upon the superior or inferior cerebellar peduncles and the final synapse of the whole tract is in the cerebellar cortex.
The Descending Tracts of The Spinal Cord
The Corticospinal Tract
It originates in the primary and secondary motor cortices as well as the parietal lobe which coincide with the brodmann areas 4 to 6 and 3,1 and 2 respectively. Most of the fibers cross over to the contralateral side at the level of the decussation of the pyramids and become lateral corticospinal tracts, while the rest become anterior corticospinal tracts, crossing at the level of the pathways final destination. Branches are given to the cerebral cortex, the basal nuclei, the red nucleus, the olivary nuclei and the reticular formation. At the pathways destination it synapses upon the internuncial neurons, otherwise known as the alpha motor neurons and pass along the information from the central nervous system to the periphery which in turn creates rapid, skilled and voluntary movements, especially at the distal ends of the limbs.
The Reticulospinal Tract
Originating in the reticular formation, this tract crosses at various levels and branches continuously as the fibers descend. They finally synapse on the alpha and gamma motor neurons to produce the initiation or inhibition of voluntary movements, where the hypothalamus has control of the sympathetic and parasympathetic outflows.
The Tectospinal Tract
The superior colliculus contributes fibers to this tract which quickly cross over once they are released and it is unknown if they give branches or not. They synapse just as the fibers of the reticulospinal tract and reflex postural movements concerning sight.
The Rubrospinal Tract
This tract starts off in the red nucleus and immediately crosses over before descending to reach the alpha and gamma motor neurons. The adult human rubrospinal tract consists of only a small number of large myelinated fibers. Its clinical importance is uncertain, but it may participate in taking over functions after corticospinal injury. It may also play a role in flexor posturing of the upper extremities, which is typically seen with lesions above the level of the red nuclei.
The Vestibulospinal Tract
The vestibular nuclei house the origin of this pathway and allow it to descend uncross until it reaches the same motor neurons mentioned in the previous two tracts. It also has a similar branching situation as the tectospinal and the rubrospinal tracts. Its function is the exact opposite of that of the rubrospinal tract.
The Olivospinal Tract
This tract is peculiar because it is the tract for which the least is known about. It is not known what function this pathway has or if it branches at all during its descension, or if it even synapses on the alpha and gamma motor neurons. All that is known is that is originates in the inferior olivary nuclei and it crosses somewhere in the brainstem.
Descending Autonomic Fibers
These fibers have several origins including the cerebral cortex, the hypothalamus, the amygdaloid complex and the reticular formation. It remains uncrossed and synapses without branches on the pathways that control sympathetic and parasympathetic outflows, which lead to its control over the sympathetic and parasympathetic systems.