Indirect Activation Pathways




Learning Objectives



  1. Describe the indirect motor pathway. How does it differ from the direct pathway?



  2. What is the rubrospinal tract? Describe its function and pathway.



  3. What is the reticulospinal tract? Describe its function and pathway.



  4. What is the vestibulospinal tract? Describe its function and pathway.



  5. What is the tectospinal tract? Describe its function and pathway.



  6. Describe the gross anatomy of the basal ganglia.



  7. What is the function of the basal ganglia? What are the inputs and outputs?



  8. List the symptoms commonly seen in a basal ganglia disorder.



  9. Describe the anatomy of the cerebellum.



  10. What are the typical symptoms seen in individuals with a cerebellar lesion?




Overview of Indirect Influences on Movement


Historically, the corticospinal and corticobulbar tracts were considered to be part of the pyramidal system because their fibers run in the medullary pyramids. More recently, the pyramidal/extrapyramidal nomenclature has been dropped in favor of more descriptive terms such as direct (corticospinal and corticobulbar) and indirect activation pathways in the motor system. Physiologically, the corticospinal and corticobulbar are monosynaptic; an upper motor neuron (UMN) projects and synapses on a lower motor neuron (LMN) which in turn innervates skeletal muscle. Thus, they directly control voluntary motor activity. However, there are numerous other polysynaptic pathways that also influence movement, which were referred to as the extrapyramidal system. For the purposes of this discussion, theses tracts will be identified as indirect pathways. In the indirect system, neuronal activity also begins in the cortex and then project to structures such as the basal ganglia, brainstem nuclei, and cerebellum before synapsing on LMN (). The role of the indirect pathways is to modify or influence neural impulses originating in the cerebral cortex. These pathways include the rubrospinal, reticulospinal, vestibulospinal, and tectospinal tracts as well as the basal ganglia and cerebellum ().



Polysynaptic pathways from brainstem





























Polysynaptic pathways from brainstem

Tract


Origin


Rubrospinal tract


Red nucleus of midbrain


Stimulates flexors and inhibits extensors of the upper limb


Reticulospinal tract


Reticular nuclei of brainstem


Inhibit and facilitates cortical voluntary movement; increase or reduces muscle tone


Vestibulospinal tract


Vestibular nuclei of brainstem


Balance and posture; coordination of head and eye muscles


Tectospinal tract


Superior colliculus of midbrain


Involved in reflex responses of the head to visual stimuli



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Fig. 17.1 Connections of the cortex with the basal ganglia and cerebellum: programming of complex movements. The pyramidal motor system (the primary motor cortex and the pyramidal tract arising from it) is assisted by the basal ganglia and cerebellum in the planning and programming of complex movements. While afferent fibers of the motor nuclei (green) project directly to the basal ganglia (left) without synapsing, the cerebellum is indirectly controlled via pontine nuclei. The motor thalamus provides a feedback loop for both structures. The efferent fibers of the basal nuclei and cerebellum are distributed to lower structures including the spinal cord. The importance of the basal ganglia and cerebellum in voluntary movements can be appreciated by noting the effects of lesions in these structures. Although diseases of the basal ganglia impair the initiation and execution of movements (e.g., in Parkinson’s disease), cerebellar lesions are characterized by uncoordinated movements (e.g., the reeling movements of inebriation caused by a temporary toxic insult to the cerebellum). (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)



Brainstem Nuclei and Tracts


The brainstem houses several nuclei that are important in movement. UMN in the cortex directly influences spinal cord circuits by synapsing on LMN in the ventral horn of the spinal cord or by synapsing on cranial nerve motor nuclei in the brainstem. Indirectly, some cortical neurons influence movement via polysynaptic pathways involving the red nucleus, vestibular nuclei, reticular nuclei, and from neurons located in the superior colliculus of the midbrain. These nuclei are associated with descending motor tracts that influence movement ().



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Fig. 17.2 Descending tracts of the extrapyramidal motor system. The neurons of origin of the descending tracts of the extrapyramidal motor system* arise from a heterogeneous group of nuclei that includes the basal ganglia (putamen, globus pallidus, and caudate nucleus), the red nucleus, the substantia nigra, and even the motor cortical areas. The following descending tracts are part of the extrapyramidal motor system: (A) Rubrospinal tract, (B) Olivospinal tract, (C) Vestibulospinal tract, (D) Reticulospinal tract, (E) Tectospinal tract. These long descending tracts terminate on interneurons which then form synapses onto alpha and gamma motor neurons, which they control. Besides these long descending motor tracts, the motor neurons additionally receive sensory input. All impulses in these pathways are integrated by the alpha motor neuron and modulate its activity, thereby affecting muscular contractions. The functional integrity of the alpha motor neuron is tested clinically by reflex testing. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)



Rubrospinal Tract




  • The rubrospinal tract (RST) originates from the red nucleus (RN) of the midbrain ().



  • The red nucleus receives input from the motor cortex and the cerebellum.




    • Its function is to facilitate control of motor neurons supplying flexors and inhibiting motor neurons supplying extensors of the upper extremity.



  • The rubrospinal tract represents an alternative to the corticospinal tract by which motor commands can be sent to the spinal cord.




    • The rubrospinal tract decussates immediately after leaving the RN and descends in the spinal cord.




      • It can be found running lateral to the corticospinal tract.



  • The rubrospinal tract is more prominent in species that use limbs for locomotion. In humans, the tract is primarily directed to cervical levels in the spinal cord and its size is somewhat diminished. In lower species, it continues into the lumbar area.



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Fig. 17.3 The rubrospinal tract originates from the red nucleus of the midbrain. It receives input from the motor cortex and the cerebellum. It facilitates control of motor neurons supplying flexors of the upper extremity (among other functions). (Reproduced with permission from Alberstone CD, Benzel EC, Najm IM, et al. Anatomic Basis of Neurologic Diagnosis. © Thieme 2009.)



Reticulospinal Tract


The reticular formation forms the core of the brainstem. It extends from the caudal medulla and continues rostrally to include the midbrain. There are two reticulospinal tracts originating from the reticular formation: the lateral, which arises from the medulla, and the medial, which originates from the pons ().



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Fig. 17.4 The reticulospinal tract originates from the reticular formation of the midbrain. Functionally, this tract is involved in preparatory and movement related activities, postural control, and some autonomic responses. (Reproduced with permission from Alberstone CD, Benzel EC, Najm IM, et al. Anatomic Basis of Neurologic Diagnosis. © Thieme 2009.)




  • Functionally, these tracts are involved in preparatory and movement related activities, postural control, and some autonomic responses.



  • The reticulospinal tract is a major alternative to the corticospinal tract. It allows cortical neurons to control motor function by projecting to the reticular formation.



  • It receives input from the cortex, cerebellum, vestibular system, auditory system, somatosensory system, and other brainstem nuclei.



  • Pathways are generally bilateral and have large numbers of fibers projecting to axial and proximal muscles.



  • Collectively, the reticulospinal tract regulates the sensitivity of the flexor response so that only noxious stimuli will elicit a response. Damage will result in innocuous stimuli such as a gentle touch to cause a reflex response.



  • Individually, the medial and lateral tracts have opposing actions.




    • The medial reticulospinal tract facilitates cortical and voluntary movement and increases muscle tone.



    • The lateral reticulospinal tract inhibits cortical and voluntary movement and reduces muscle tone.



    • Both tracts exit at all spinal levels.



Vestibulospinal Tract


The vestibulospinal tract originates in two vestibular nuclei located beneath the floor of the fourth ventricle in the brainstem. Both tracts convey neural impulses from the labyrinth of the inner ear to the spinal cord ().



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Fig. 17.5 The vestibulospinal tract originates from vestibular nuclei in the floor of the fourth ventricle. This tract conveys neural impulses from the inner ear to the spinal cord. It is involved in the maintenance of balance and posture. (Reproduced with permission from Alberstone CD, Benzel EC, Najm IM, et al. Anatomic Basis of Neurologic Diagnosis. © Thieme 2009.)




  • The medial vestibulospinal tract originates from the medial vestibular nucleus.




    • Projects as the descending medial longitudinal fasciculus.



    • Primarily reaches cervical levels (T6 and above).



    • Stabilizes head muscles.



    • Important for coordination of head and eye muscles.



    • Plays an important role in the maintenance of posture and balance.



  • The lateral vestibulospinal tract originates from the lateral vestibular nucleus.




    • Projects as ventrolateral funiculus of the spinal cord.



    • Projects to all levels of the spinal cord.



    • Activates extensor motor neurons.



    • Plays an important role in maintenance of posture and balance.




      • Facilitates extensors of the neck, back, and extremities. It is inhibitory to flexors.



Tectospinal Tract


The tectospinal tract originates in the deep layers of the superior colliculus of the midbrain ().



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Fig. 17.6 The tectospinal tract originates in the superior colliculus of the midbrain. Although not entirely certain, it is thought to be involved in visual responses. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy. Third edition, Vol 3. © Thieme 2020. Illustrations by Markus Voll and Karl Wesker.)




  • It terminates in the cervical levels of the spinal cord.



  • The function of the tract is not well known. However, due to the fact that the superior colliculus is known to be involved in visual responses, it is likely that this tract facilitates movement of the head in reflex responses to visual stimuli.



Basal Ganglia


The basal ganglia (BG) consist of several subcortical nuclei, subthalamic nucleus, and the substantia nigra (a, b). Damage to these structures or the pathways connecting them with the cortex and spinal cord results in distinct movement disorders. In addition to movement disorders, basal ganglia disease can impact cognitive function and emotions (Clinical Correlation Box 17.1 and 17.2).



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Fig. 17.7 (a) Basal ganglia. Transverse section through the cerebrum at the level of the corpus striatum, superior view. The basal ganglia consist of the caudate nucleus, putamen, and globus pallidus and are an essential component of the extrapyramidal motor system, which controls involuntary movement and reflexes and coordinates complex movements (see p.108). The caudate nucleus and putamen, which are separated from each other by the fibrous white matter of the internal capsule, together constitute the corpus striatum. Deficiency of dopamine in the basal ganglia is responsible for Parkinson’s disease. (b) Basal ganglia nuclei and their relationship to other cortical structures. (Fig. 17.7a: Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker. Fig. 17.7B: Reproduced with permission from Alberstone CD, Benzel EC, Najm IM, et al. Anatomic Basis of Neurologic Diagnosis. © Thieme 2009.)




  • One of the main functions of the basal ganglia is to provide feedback to the cerebral cortex in order to initiate and control movement.




    • Much of the control is inhibitory; thus it modulates output.



    • It is involved in the facilitation of practiced motor activity.



    • It does not initiate voluntary motor movements.



  • A significant amount of the output from the basal ganglia is facilitated by the thalamus ().



  • The components of the basal ganglia encompass the telencephalon, diencephalon, and mesencephalon () ().




    • Forebrain structures make up the corpus striatum.




      • Putamen




        • Separated from the caudate nucleus by the anterior limb of the internal capsule.



      • Caudate nucleus




        • C-shaped



        • Associated with the lateral ventricles



      • Globus pallidus



      • Nucleus accumbens



    • Striatum/neostriatum




      • Caudate and putamen



    • Lentiform/lenticular nucleus




      • Putamen and globus pallidus



    • The subthalamic nucleus is part of the diencephalon.




      • Located inferior to the thalamus.



    • The substantia nigra is found in the midbrain.




      • Plays a significant role in dopaminergic pathways to the striatum.



      • Loss of dopamine secreting neurons in this area results in Parkinson’s disease.



  • The largest source of afferent fibers to the basal ganglia comes from the cerebral cortex (). Afferents from the thalamus also project to the basal ganglia.




    • Cortical input includes motor, sensory, association, and limbic regions.



    • Striatum receives most of the afferents.



    • Projections from the cortex are directed toward specific regions of the basal ganglia.




      • The different regions of basal ganglia are arranged somatotopically, much like the motor homunculus found in the primary motor cortex.



  • The majority of the output from the basal ganglia is from the globus pallidus and the substantia nigra.




    • The efferents from the basal ganglia and the substantia nigra primarily project to relay nuclei of the thalamus. The thalamic nuclei project to the cerebral cortex ().



  • There are a several intrinsic connections between the different components of the basal ganglia.



  • There are two distinct pathways that are responsible for processing signals through the basal ganglia. These circuits have an opposite effect on the thalamic target structures.




    • Direct pathway: Excites thalamic neurons resulting in the excitation of cortical motor neurons (a).



    • Indirect pathway: Excitations results in inhibition of thalamic neurons resulting in the inability to excite cortical motor neurons (b).



    • Normal function of the basal ganglia involves a balance of these two pathways. Disruption of the balance results in motor dysfunction, which is characterized by movement disorders.



  • Damage to the basal ganglia typically causes problems with speech, movement, and posture. Collectively, these symptoms are referred to as parkinsonism.




    • Symptoms can vary but may include:




      • Movement changes.



      • Increased muscle tone.



      • Muscle spasms and rigidity.



      • Tremors.



      • Difficulty walking.



      • Vocal tics.


Sep 13, 2022 | Posted by in NEUROLOGY | Comments Off on Indirect Activation Pathways
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