Descending Pathways PDF

Pag. 1 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways #11 - 16.11.21 Neuroanatomy Descending Pathways Prof.Dellavia – 16/11/21 – Au...

Pag. 1 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways #11 - 16.11.21 Neuroanatomy Descending Pathways Prof.Dellavia – 16/11/21 – Author & Reviser: Ekin Su Kalender & Alberto Spolaore 1. General Overview of Descending Pathways The descending pathways can be divided into two parts. The first group has the role to excite the flexors and to inhibit the extensors and the second group has the function to excite the extensors and inhibit the flexors. The first group starts from the motor cortex and goes directly to the common final pathway of motor neurons that are either located in the spinal cord (“corticospinal”) or located at the nuclei of the cranial nerves (“corticonuclear”). This is the direct pathway because it starts from the motor cortex in the telencephalon and goes directly to the motor neurons to exit via the efferent fibers. The other possibility is to use a second group of descending pathways that are originating in nuclei that are located at different levels of the midbrain, pons, medulla, or lamina quadrigemina and connected to the motor neurons of the final common pathway. In the case of the tracts from the first group that acts on the flexors, there is the rubrospinal tract which is associated with the corticospinal. It helps the corticospinal so it is also similarly dedicated to the flexors. But in humans, the rubrospinal is less represented than other animals. Meanwhile in other animals, the corticospinal is less developed so the rubrospinal is working much more. In this group, there is also the reticulospinal tract. The reticulospinal originates from the reticular formation but the reticular formation is a very long column of neurons that can be found in all portions of the brainstem. There are different columns in the reticular formation: some of them are more medial and some of them are more lateral. These columns can contain different types of neurons. So the neurons located in the medullary portion (located in the medulla oblongata) are those that create the medullary reticulospinal tract. The medullary reticulospinal tract is the one that acts with the first group to facilitate the flexors. These belong to the first group of descending tracts which are also referred to as the lateral group of descending pathways since their tracts are located in the lateral funiculus of the white matter. The tracts forming this first group of descending pathways are all originating from the level of the cortex. The remaining descending tracts that form the second group are located more medially. These tracts are called vestibulospinal, tectospinal, and the pontine reticulospinal (in this case the one that is originating from the pons) and these tracts are mainly involved in the maintenance of the posture. They are activating the extensor muscles by acting on the motor neurons of extensors and they are inhibiting the flexors. The vestibulospinal originates from the vestibular nuclei. The tectospinal originates from the tectum specifically at the level of quadrigemina of the superior colliculus and it is the one that is also involved in the visual system. The reticulospinal originates from the neurons located in the pons. This group of tracts is descending at the level of medial funiculus of the white matter and this is why they are also referred to as the medial group of descending fibers.  Pag. 2 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways 2. The Somatotopic Organization of the Spinal Cord and the Brainstem This correspondence between the white and grey matter of the spinal cord is related to the location of the motor neurons in the grey matter. The organization of the ventral horn of the spinal cord grey matter is so organized that the axial muscles are located more medially and the distal muscles more laterally. In the grey matter the motor neurons of extensor and abductor muscles are located more anteriorly (or ventrally) and the flexors and adductors more posteriorly( or dorsally). This organization is reached by the descending tracts from the cortex at the level of the spinal cord. This organization is valid considering the somatic pathway which is the main pathway arising from the cortex. But also other descending pathways which do not arise from the cortex can have this organization in the grey matter. These other pathways which are arising from some nuclei to reach the level of some visceral vegetative pathways are those that have to arrive at the level of either sympathetic or parasympathetic preganglionic motor neurons. But in the brainstem, there are only parasympathetic preganglionic motor neurons while in the spinal cord there are both of them. There is almost a correspondence in the organization of the white matter with the gray matter. In the image, the somatotopic organization of the spinal cord can be seen. The position of the axial and distal muscles and extensor and flexors and also the abductors and adductors can be seen clearly. From medial to lateral there is the proximal to the distal organization of the motor neurons. ,  Pag. 3 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways The organization of the motor nuclei of the brainstem can be seen in the image below. Considering the somatic motor neurons there are the nuclei of the oculomotor in the midbrain and also the trochlear nuclei. These two are somatic nuclei and are very close to the midline. They are in the first position which means located dorsally while the nuclei of the nerves that are of branchial origin (pharyngeal arches) are in a more ventral position. Starting from the midbrain we have the location of the oculomotor and trochlear very medially and dorsally. Descending at the level of the pons there are the trigeminal nuclei, the trigeminal nuclei are located more dorsally since it is the pharyngeal nerve. Then there are the abducens nuclei. So looking at the image it can be seen that more medially the abducens nuclei, more laterally the trigeminal nuclei, and a little bit more caudally the facial motor nucleus are present in the pons. So the trigeminal nerve is for the muscles of mastication and the facial nerve is for the mimic muscles. If you descend at the level of medulla oblongata it can be observed the somitic (which is not pharyngeal) hypoglossal nuclei which are very close to the midline dorsally. More lateral and ventrally to this hypoglossal nucleus there are the pharyngeal nuclei such as the glossopharyngeal and the accessory nerve nuclei. The arrival to these nuclei occurs with fibers that can arise directly from the cortex so it is the direct pathway. Or in many cases, there are the fibers sent by the cortex to the reticular formation and then from the reticular formation, they reach the nuclei of the cranial nerves. 3. Organization of the tracts in the spinal cord Looking at the organization of the descending tracts in the spinal cord (see the image below) the lateral and medial groups of descending pathways can be seen clearly. The main corticospinal and rubrospinal are located laterally which are the main tracts to excite the flexors and in particular the distal muscles. Moving more medially, the vestibulospinal, the rest of the reticulospinal, and tectospinal could be found. Into the anterior funiculus of the spinal cord, we find also a portion of the corticospinal which is depicted in the image as “ventral corticospinal”. But the main tract is located more laterally, which is also part of the lateral group.  Pag. 4 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways Considering the descending pathways we are either talking about the long tracts that are originating from the cortex or from the nuclei in the brainstem in the craniocaudal direction. There can also be found some long propriospinal neurons in the pathway that connect the motor neurons with the different segments of the spinal cord or brainstem. The medial areas are connected by long propriospinal nerves while the more lateral motor neurons are interconnected by short propriospinal nerves. These propriospinal neurons provide the connection between different areas. 4. Origins of the major descending motor control system The first area of origin of the descending tracts is the motor cortex. Mainly the telencephalon has the primary motor cortex which is the main origin but it is not the only one. There are also other possible places to originate such as the rubrospinal which originates from the red nucleus in the midbrain to the spinal cord. The word “rubro” indicates the red nucleus. But the red nucleus is also under the control of the cortex. So there are fibers that start from the cortex and arrive at the level of the red nucleus, they are called corticorubral fibers and then they descend from the nucleus to the spinal cord. This tract is named the rubrospinal tract. This is not a pathway that goes directly but when there are nuclei on the way -which is, in this case, the red nucleus- there is also the possibility to modulate the signals. There is also the possibility to stop at the level of the tectum which is the superior colliculus and this is the origin of the tectospinal tract. The vestibulospinal originates from the medial and lateral vestibular nuclei. Finally, there is the reticular formation which is a very big complex and inside this formation, the reticulospinal tracts arise carrying somatic and also some visceral information. These are the origins of the major descending systems. They are called “major” because there is also the possibility to have other descending tracts nuclei but the fibers of major descending tracts can be put together into bundles. The other descending pathways other than the major ones may involve the other parts of the reticular formation, mesencephalic periaqueductal gray or Raphe Magnus. 5. Pyramidal System The direct pathway starts from the cortex and goes directly either to the motor neurons located in the ventral horn of the spinal cord or to the cranial nuclei. The pyramidal system consists of corticospinal and corticonuclear tracts. This pathway is dedicated to allowing rapid and skilled movements and this is the reason why this pathway is so developed in humans compared to other animals. It acts mainly on the flexors of the distal muscles. For example, playing the piano is a movement that is mainly under the control of the corticospinal tract.  Pag. 5 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways The corticospinal tract is formed by two tracts which are found in the lateral and anterior funiculus of the spinal cord. Apart from the fact that they descend down in different portions of the white matter, the decussations can also help to distinguish between these two tracts. The main one is the lateral one and it is crossed at the level of the spinal cord for the entire length of the spinal cord. The fibers will cross and decussate and descend in the lateral funiculus of white matter contralaterally. These lateral fibers make up 80-90 % of the fibers. The remaining 10-20 % of fibers descend ipsilaterally. Meaning that they do not decussate but reach the final target of grey matter they can cross the midline to decussate by the anterior commissure of the white matter and so activate the motor neurons on the other side. That is why the ventral corticospinal tract is considered as a bilateral tract while the lateral corticospinal is considered as a contralateral tract. (See the images below) The image below the fibers start at the level of the cortex and the fibers are converged in the internal capsule which is a compulsory area for the ascending and descending tracts. The corticospinal tract is descending into the posterior limb of the internal capsule. While the corticonuclear (the fibers that have to end at the level of cranial nuclei) need to pass at the level of the genu of the internal capsule. Descending from the internal capsule the fibers enter into the diencephalon and they arrive at the level of the cerebral peduncle which is located anteriorly in the midbrain. The pons is crossed at the level of the fascia and some of the fibers synapses at the basilar nuclei of the pons creating the correlation system with the cerebellum. The fibers continuing to descend escape synapsing at these nuclei and descend forward passing the fascia of the pons. They reach medulla oblongata ventrally at the level of pyramids. Pyramid is also the point of decussation. They decussate here by crossing the midline and this decussation occurs at the cranial portion of the medulla oblongata and the progressive movement is more lateral. From there they descend at the lateral funiculus of the white matter and these decussating fibers make up 80-90% of the total fibers. The remaining 10-20% percent does not decussate at the pyramids. They just descend at the anterior funiculus of the white matter till their final target portion of the grey matter is reached. When they arrive at their final neuromer they will synapse collaterally on the grey matter of both sides by passing through the anterior commissure. Since it acts on both of the ventral horns the ventral corticospinal tract is referred to as a bilateral tract. Finally, the lateral corticospinal tract arrives at the level of lower motor neurons Q: When the ventral corticospinal tract synapses, does it always synapses on both sides of the anterior horn?  Pag. 6 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways A: There is a possibility to arrive either directly on the motor neuron which gives rise to the efferent fibers or there is also the possibility to end on the interneurons which can spread the signals on both sides. So the fibers can directly go and divide or the signals are spread by the interneuron which the corticospinal tract ends on. In this way, the motor system has a method for controlling the synchronization. 6. Origin and Course of the Lateral Corticospinal Tract The lateral corticospinal is originating at the level of the cortex. The precise origins of the corticospinal tract are as follows: ⅓ of fibers are originating from the primary motor cortex ⅓ of fibers are originating from the secondary areas of the motor cortex ⅓ of fibers are originating from the primary somatosensory cortex (postcentral gyrus) The ones arising from the motor cortex arrive at the ventral horn but the one arising from the somatosensory cortex arrives at the medial portion of the dorsal horn to synapse at the level of a motor neuron which is in continuation with its efferent fiber or to the interneurons in the area close to the final common way. This arrival of the sensory fibers to the dorsal horn is to coordinate with the ascending system. So the sensory and motor are always concerning each other by communicating. At the level of the spinal cord, they can be modulated. So this system descends from the cortex to the lateral funiculus. 7. Origin and Course of the anterior corticospinal tract The anterior is not following the same pathway as the lateral one. The anterior one is the minor corticospinal tract and it is only originating from the motor cortex. Specifically, it arises from the Brodmann areas 6 and 4 which correspond to the motor areas. In contrast to the lateraşl corticospinal tract, the anterior part does not descend until the last neuromer. It is mainly arriving at the level of the thoracic portion so involved in the neck and trunk muscles. Corticobulbar tract To reach the motor nuclei of the cranial nerves fibers use the corticobulbar tract. All the brainstem nuclei receive directly from the cortex, but the cortex can also end into reticular formation and then reach the nuclei with some colaterals (indirect connection). This is important for the control of some complex functions such as mastication where we need the control of the neurons in the nucleus and also the interneurons. As described for somatosensory cortex, the somatotopism is cranio caudal latero medial, thus also the primary motor cortex can be represented with the homunculus, but it is not superimposable with the sensory one.  Pag. 7 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways This tract starts from the lateral areas of the cortex, which can be the primary motor cortex, the other secondary cortices and also the sensory areas can be involved. The tract descends into the genus and reaches the midbrain, where synapses with the 3rd and 4th cranial nerves nuclei. The activation is bilateral, thus it is different from the corticospinal. From the lateral cortex fibers move to the 2 sides at the level of the brainstem. The bilateral control is found for all the brainstem nuclei with some exceptions: The first exception is the facial nucleus, this nucleus is divided into 2 portions, the cranial portion corresponds to the fibers innervating the so-called “upper face” (mainly the upper 1/3 of the face). This portion is bilaterally activated by the corticobulbar tract. The lower part innervates the lower face and this part is activated only contralaterally by the corticobulbar tract. Descending in the medulla, it can be found the ambiguus nucleus, which is shared between the glossopharyngeal and the vagus nerves. The somatic motor neurons of the glossopharyngeal portion of the nucleus control the skeletal muscles of the walls of the pharynx and the vagus part controls the larynx muscles. The nucleus is called ambiguous because we can’t see the division between the two portions. The tract is bilaterally reached on the 2 sides in the upper portion, the lower portion is reached contralaterally by a bundle that descend at the level of the inferior portion of medulla oblongata where we find the corticospinal tract decussation (in the pyramids), there the tract decussate and goes back entering the lower portion of the contralateral ambiguus nucleus. The accessory nucleus is only in part in medulla oblongata, since a major portion is in the first neuromeres of the spinal cord (it’s a descending nucleus as the trigeminal nucleus). This nucleus is activated ipsilaterally only. The accessory nucleus innervates trapezius and SCM muscles, which have a complex embryological origin because the two are made of a group of cells that derive from the pharyngeal arches (annex of vagus), while the remaining portion is innervated by spinal nerves since the rest of the muscles are of somatic origins. Both muscles are important for the posture of the head. The hypoglossal nucleus returns to the general rule of the corticobulbar innervation, thus it is bilaterally innervated.  Pag. 8 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways 8. Cortex organization These are the areas of the cortex where we find the origin of the pyramidal system (corticonuclear and corticospinal), it is found a primary motor cortex located at the level of the precentral gyrus. In this gyrus it’s possible to identify a somatotopism characterized by the homunculus. This cortex is the Brodmann area 4. This area is giving the majority of the fibers for the common way, in particular for the distal muscles, the flexors, especially located mainly in the distal parts such as the hand. In general, this area regulates the rapid skilled movements, so that it is very precise with very fine control. Then, it is possible to find other areas which can help in the control of the motor action since they are supplementary or secondary areas. One of these is area 6, which is involved in the generation of corticospinal tract either lateral and anterior, but also it’s involved in the control of the motor nuclei of the cranial nerves. Area 6 is located anterior to precentral gyrus (in frontal lobe), it has a medial and a lateral subdivision. The medial part is important for the determination of which are the motoneurons needed to be recruited and activated to have the correct synergism. On the other hand, the lateral aspect is involved in the control of movements of girdles, axial muscles and ocular movements. Close to this area (more anterior and lateral) lies the ocular frontal field dedicated to the movement of the eyes. These areas can also be activated only for specific movements. Others are sensitive-motor areas where the integration of the signals of the two systems occurs. In general, this is the organization of the cortex for motor control. The division is conventional: in primary motor cortex we start the signal which is sent to pyramidal tracts, then fibers can descend at the level of basal ganglia which can send back the signal in order to have a reverberation, but fibers can start also in supplementary areas (such as area 6).  Pag. 9 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways We have an additional cortex that is the premotor cortex corresponding to area 8. This area receives fibers back from basal ganglia and then sends fibers to the primary motor cortex. Both areas 6 and 8 can receive fibers from the posterior parietal cortex (associative areas). As said, projections from the cortex can be from primary motor cortex, area 6 and (for cranial nerves) the primary sensory cortex. The cortex has a very extensive area dedicated to the hand. There is a high representation of tongue, while the area of the mouth is more represented in the sensory system, thus the homunculus is similar but not identical. The somatotopism is recognizable at all levels, the fibers to face are medial and progressively the inferior limb fibers are lateral, this means that the fibers turn as what happens in the sensory system using the corona radiata and internal capsule.  Pag. 10 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways 9. Extrapyramidal descending tracts The other major descending systems are the extrapyramidal ones. First, we have to consider the fact that it is possible to find some small bundles included, intersegmental tracts and also it’s important to consider into the motor system the presence of reverberance, so that also other minor structures are present. Some tracts associate with the corticospinal in order to provide the activation of the plexuses (lateral group) and some others for the maintenance of the posture, extension, and so on (medial group). It’s possible to divide the extrapyramidal descending tracts considering that some of them are contralateral and some others are ipsilateral. The contralateral are the rubrospinal and the tectospinal, these decussate immediately after the origin. The rubrospinal has the Forel’s decussation, which is very high, at the level of the red nucleus. Immediately below, dorsally and a bit inferiorly to the position of the red nucleus, at the level of the superior colliculus, the posterior commissure is found, it can let fibers pass on the other side, activating the bilaterally the Edinger-Westphal and oculomotor as seen in the visual pathways, so we have from superior colliculus and the decussation is immediately after this, this decussation is called Meyenert’s. When the rubrospinal tract descends it is part of the lateral group (it descends in the lateral funiculus), very close to the lateral corticospinal tract, while the tectospinal is located medially and it is descend into the anterior funiculus of spinal cord. They descend in different areas because the rubrospinal acts on the flexors, while the tectospinal on the extensors. This is important for adjusting the posture with respect of the visual system information, thus this tract is important for posture. The ipsilateral group is composed of the vestibulospinal and the reticulospinal tract. They descend in the same side, the vestibulospinal lateral is completely independent and the medial descend in the MLF, this remains in the same side even if it can be part bilateral because it is close to the midline. The reticulospinal is completely ipsilateral and fibes can originate at different levels, the 2 groups of neurons either in pons or medulla oblongata have different activities. This is in fact what happens when we originate the fibers into the reticular formation. If we start at the level of the pons we activate directly the motor neurons for the extensors muscles, while medullary reticular formation is controlled by the cortex and its role is to inhibit the extensors and it is considered part of the other group because at the end of the day it inhibits the extensors, allowing the activation of the flexors which are activated by the corticospinal or rubrospinal tracts. So the mechanism is different but in the end the role is similar. Fibers of the autonomic system can descend inside the reticulospinal, these fibers can arise from the hypothalamus, neurohypophysis, etc. Then they arrive at the level of the reticular formation, from the reticular formation the fibers then descend to the beta motoneurons. The reticular formation has 2 tracts which descend mainly in the anterior funiculus but there is a tract which is a bit more lateral and another more medial, in any case they descend anteriorly because they end at the extensor motor neurons, but with opposite roles.  Pag. 11 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways 10. Endings Not all the tracts arrive until the end of the spinal cord, some tracts ends before sacral neuromeres, in particular fibers from the cortex descend through the corticospinal: the lateral tract using the lateral funiculus, the anterior one is descending ipsilaterally and ends before the actual end of the spinal cord. For the extrapyramidal tracts, the tectospinal crosses and ends at the level of the cervical spinal cord, then the rubrospinal, which crossed immediately, arrives contralaterally into the end of the cervical, beginning of the thoracic (it is reductional in the human body). The reticulospinal can start at the level of the pons, in the case of the pons it descend ipsilaterally more medial since it descends in anterior funiculus and it reaches the sacral neuromeres, even if the majority of fibers end in the mid-thoracic portion. The lateral reticulospinal starts at the level of the medulla oblongata and it is more lateral, thus is very lateral, this is the one which has to inhibit the extensors and it is long for the entire length of the spinal cord. The vestibulospinal originates from the medial and lateral vestibular nuclei, the medial fibers enter into the vestibular fasciculus, these fibers can go contralaterally even if the majority remains ipsilaterally, and it is sort ending in the thoracic portion of the spinal cord. The lateral one descends ipsilaterally until the end of the spinal in the lateral funiculus. 11. Modulatory effects These tracts have modulatory effects on the somatic motor system, but they can also have other effects due to their origin, in particular in raphe magnus and PAG, from which are originated some fibers for the modulation of the pain. Inside this reticulospinal are present fibers for the analgesic system, inhibiting the progression of the pain, this is a particular role of neurons in reticular formation descending on the beta preganglionic neurons. In the lateral portion of the medullary we have fibers from raphe magnus, the more  Pag. 12 a 12 International Medical School – NEURO #11 – prof.Dellavia – Descending Pathways anterior are monoaminergic with information coming from the locus coeruleus, so we have different analgesic systems coming from different areas. These are part of reticular formation fibers. Fibers can either go directly on the final motoneurons or interact with interneurons. Interneurons can form circuits, these can act on motoneurons. The Renshaw cells receive from the descending system and inhibit the final common way in order to make the signal remain for a certain time there, then it is possible to have a sort of “opening of the gate” which gives rise to the output. As always at this level interneurons have modulation activity, because we want to organize autonomous systems which can be modulated. For example, the descending system can spread to both motoneurons and Renshaw cells. The Renshaw cells can work on the output and can inhibit another motoneuron which works on another muscle, this is done to synchronize the excitation and the contraction of opposing motoneurons. Thus it is a complex descending system.