A. ASCENDING SPINAL TRACTS
The dorsal column system, which consists of the fasciculus gracilis and the fasciculus cuneatus, transmits highly critical types of sensations to the brain.
The velocity and accuracy of these transmissions are respectively much faster and better than those in the spinothalamic tract. The nerve receptors, which are located in the dermis, and the cell bodies, which are agglomerated in the dorsal root glanglion, are integral to the transmission of information to the dorsal column system. The fasciculus gracilis contains fibers from sacral, lumbar, and the lower six thoracic segments. These axons are located in the medial ipsilateral part of the dorsal column. The fasciculus cuneatus appears at the level of T6 and contains the axons that enter the cord ipsilaterally at the upper six thoracic and cervical levels and that are located in the lateral part of the dorsal column. The fasciculus gracilis and the fasciculus cuneatus convey proprioception, fine touch, and vibratory senses to the medulla oblongata where the axons terminate respectively in the nucleus gracilis and the nucleus cuneatus.
After crossing over in the medulla, the second-order neurons form an ascending bundle, the medial lemniscus, which terminates in the ventral postero lateral (VPLc) nucleus of the thalamus. The third-order neurons reach the post-central gyrus after passing through the internal capsule.
From a clinical point of view, it is essential to understand the fundamental principle of the extreme precision of proprioception. For instance, no practitioner can possibly hope to treat osteoarthritis with any chance of lasting results without a clear understanding of how the sensory system works.
The pathway for proprioception gives a person the ability to localize each segment of his or her body in space. The perception of each segment of the body in its surrounding media is not a gross or general perception but a very refined system capable of detecting very minute movements, positions, or changes in position. If the upper limb is in a prone horizontal position, a slight abduction of the thumb by just a few degrees will activate the receptors in the joints, muscles and tendons, and send the messages to the central nervous system so that the body is aware of the change in positioning.
Fine touch is concerned with stereognosis, which is the ability to identify forms and objects by simple touch. The fasciculus gracilis and cuneatus convey information for two-point discrimination. The tip of the fingers contains a large number of tactile receptors, mostly Meissner's corpuscles. With the help of these receptors, a person is capable of distinguishing two separate points at no more than one mm apart. Vibratory messages are transmitted mostly by Pacinian corpuscles, which can detect vibratory sensations up to 700 cycles per second. Other tactile receptors can detect vibratory sensations up to 200 or 300 cycles per second.
The spinothalamic system transmits crude types of sensations to the brain which are less accurate and slower than those in the dorsal column system. This system is usually described as a system composed of two tracts: the lateral spinothalamic tract concerned with pain and temperature sensations and the ventral spinothalamic tract concerned with pressure and simple touch.
The pathway for pain and temperature originates in the dermis and epidermis with pain receptors (free nerve endings in the skin) and heat and cold receptors in the skin. The sensory neurons enter the spinal cord through the dorsal root and end in the dorsal horn of the gray matter (tract of lissauer). These neurons synapse with the second-order neurons that cross and form the lateral spinothalamic tract. The ascending bundle terminates in the ventral posterolateral nucleus (VPLc) or the thalamus. Tertiary neurons ascend in the internal capsule to reach area 3, 1, 2 in the post-central gyrus.
In the spinal cord itself, the sensory neurons for pain and temperature also have branches in the dorsal horn that synapse with internuncial neurons which relay crossed and uncrossed impulses to motor neurons in the ventral horn of the gray matter. It must be remembered here that internuncial neurons are an extension of the reticular formation in the spinal cord. The axons of those motor neurons terminate in the muscles or group of muscles to form the so-called "arc-reflex." In the pons and medulla, the lateral spinothalamic tract, just like the dorsal column system, sends a large number of fibers to the nuclei of the reticular formation.
Impulses associated with pressure and simple or light touch are conveyed through the ventral spinothalamic tract. The receptors are also located in the dermis, and the nerve fibers enter the spinal cord through the dorsal root, with their cell bodies agglomerated in the dorsal root ganglion. One branch synapses in the dorsal horn gray with the second-order neurons and ascends contra-laterally in the ventral white column. The other branch ascends on the same side for several spinal segments and synapses in the dorsal horn gray with second-order neurons which also cross to the ventral white column. Fibers forming the ventral spinothalamic tract ascend in the ventral white column to terminate in the VPLc of the thalamus. From there, fibers synapse with third-order neurons which carry the impulses to the somatic sensory cortex (area 3, 2, 1) in the postcentral gyrus. At the level of the medulla, fibers from the ventral spinothalamic tract relay impulses to the nuclei of the reticular formation.
The cerebellum is divided into three major parts based on its three major functions: the archicerebellum (equilibrium), the paleocerebellum (proprioceptive information on muscle tone and position in space), and the neocerebellum (integration of somatic motor function from the cerebral motor cortex) (see Figure 1-4). To carry out these functions, the cerebellum is connected to the mesencephalon, the pons, and the medulla oblongata by three pairs of cerebellar peduncles: the superior cerebellar peduncle (brachium conjunctivum), the middle cerebellar peduncle (brachium pontis), and the inferior cerebellar peduncle (restiform body).
The archicerebellum consists of the nodulus in the center and a flocculus attached on each side by a peduncular connection. The archicerebellum is mostly concerned with the equilibrium of the body through the vestibular system. Phylogenetically, it is the oldest part of the cerebellum, and fibers from the superior and lateral vestibular nuclei (vestivulocerebellar tract) are ipsilateral and enter the cerebellum through the inferior peduncle (restiform body).
The paleocerebellum is located in the anterior portion or anterior lobe of the cerebellum. It receives unconscious proprioceptive information concerning muscle tone, position of the body and movement of limb muscles. The receptors are located in joints, tendons, and muscles, and the information is transmitted through the spinocerebellar pathways which are made of a posterior spinocerebellar tract and an anterior spinocerebellar tract. Fibers from the former are uncrossed and more numerous than those of the latter. They arise from the dorsal nucleus of Clarke and ascend as a posterolateral tract to enter the cerebellum through the inferior cerebellar peduncle and terminate in the anterior lobe and part of the pyramis.
Since the dorsal nucleus of Clarke is found only between C8 and L2 in the spinal cord, fibers below L3 ascend first to the upper lumbar segments where they end in Clarke's nuclei. Impulses from the upper part of the body are mostly transmitted through the posterior spinocerebellar tract. Fibers from the anterior spinocerebellar tract ascend from the lower cord and are divided into a few ipsilateral (uncrossed) and a majority of contralateral (crossed) fibers. The anterior spinocerebellar tract is mostly concerned with proprioceptive information from the lower limbs and with the control of posture. Fibers enter the cerebellum through the superior cerebellar peduncle, but before doing so, contralateral fibers cross back to the side on which they originated.
The posterior and largest lobe of the cerebellum forms the neocerebellum, which is also the newest part of the cerebellum in the evolutionary process of the central nervous system. Its main function is to integrate impulses emerging from the cerebral cortex and is concerned with somatic motor functions. Information is transmitted to the cerebellum through the cortico-ponto-cerebellar tract. Fibers arising from the cerebral motor cortex synapse with second-order neurons in the pontine nuclei. Axons from second-order neurons cross over to reach the opposite part of the cerebellum by entering through the contralateral middle cerebellar peduncle.
B. DESCENDING SPINAL TRACTS
The most important descending spinal tract originates in the cerebral cortex and is called the corticospinal tract (see Figure 1-5). The other major descending spinal tracts worth mentioning are: the tectospinal tract arising from the superior colliculus, the rubrospinal tract arising from the red nucleus in the mid-brain, the vestibulospinal tract with its nuclei located in the floor of the fourth ventricle, and the reticulospinal tract arising from the reticular formation in the pons and the medulla. The cortico-bulbar tract which is associated with cranial nerves will not be described in this review of neuroanatomy as it is not prominently employed in the treatment of patients.
The corticospinal tract supplies impulses to most of the voluntary muscles. It originates in the precentral gyrus of the cerebral cortex (area 4). The axons pass through the internal capsule and descend to the mid-brain where they form the crus cerebri (basis pedunculi). In the medulla oblongata, 80 to 90 percent of the fibers decussate to the opposite side and descend in the spinal cord where they form the lateral corticospinal tract. In the spinal cord, the axons of the lateral corticospinal tract are located internal to the posterior spinocerebellar tract and posterior to the lateral spinothalamic tract.
The lateral corticospinal tract irradiates branches at all levels of the spinal cord. The fibers enter the gray matter where they synapse in the ventral horn with second-order neurons. The latter emerge from the spinal cord in the ventral spinal roots and supply the voluntary muscles through the peripheral nerves.
The remainder of the corticospinal tract which does not cross over in the medulla oblongata divides into two separate tracts: the anterior corticospinal tract and the anterolateral corticospinal tract. The axons of the anterior corticospinal tract descend uncrossed into the spinal cord. They occupy an antero-medial position in the anterior white commissure and are contiguous to the anterior median fissure. Most of the fibers of the anterior corticospinal tract descend to the upper cervical spine where they cross in the anterior white commissure. The fibers enter the gray matter where they synapse in the ventral horn with second-order neurons.
The anterolateral corticospinal tract is the smallest of the three descending tracts. The fibers descend in the lateral funiculus and remain uncrossed in the entire course of the tract. The axons of the anterolateral corticospinal tract synapse in the ventral horn with second-order neurons. It should be emphasized that the pyramidal or voluntary muscle system is made of a two-neuron system. The neurons of the corticospinal tracts leaving the precentral gyrus and descending in the spinal cord to terminate their course in the ventral horn are called upper motor neurons. The second-order neurons leaving the spinal cord to supply the voluntary muscles are called lower motor neurons. The distinction between upper and lower motor neurons paralysis is important in clinical neurology.
