Corticospinal tract | Corticobulbar tract | Extrapyramidal Motor System | Vestibular System | Cerebellum

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Pyramidal Motor System Corticospinal tract
Motor system animation

• The pyramidal motor system controls all of our voluntary movements. Pathological processes which damage the pyramidal motor system are extremely important causes of disability and suffering.
  
  
• The pyramidal system is a two neuron system consisting of upper motor neurons in the Primary Motor Cortex and lower motor neurons in the anterior horn of the spinal cord. Each of these neurons have extremely long axons. The upper motor neuron axon extends all the way from the brain down to the spinal cord, a distance 1-3 feet or more, and the lower motor neuron axon extends from the spinal cord to the skeletal muscles of the arms or legs, a distance 4-5 feet in very tall people.
  
  
• The upper motor neurons reside in the precentral gyrus of the frontal lobe also called the "motor strip". These upper motor neurons are arranged in a stereotypical fashion. Neurons which control movements of the face and mouth are located near the Sylvian or lateral fissue and neurons which control the muscles of the thighs and legs are located near the medial longitudinal fissure and within the central sulcus.
  
  
• Many textbooks illustrate this phenomenon graphically using a distorted human figure called Homunculus. The homunculus has very large face and mouth because there are many upper motor which innervate these parts of the body.
  
  
• The axons which extend from these upper motor neurons traverse the deep matter and coalesce to form the Internal Capsule. They then descend through the midbrain in the Cerebral Peduncle.
  
  
• The axons of the corticospinal tract the condense to form the pyramids. The axons are arranged in a very orderly fashion. Axons which will innervate the muscles of the face are located medially. This tract is known as the corticobulbar tract. The axons which form the corticobulbar tract exit at their appropriate levels to synapse with their lower motor neurons in the cranial nerve nuclei. Link to cranial nerves.
  
  
• The axons which will innervate the legs are located laterally within the cerebral peduncle. Thus the name "lateral corticospinal tract". These axons then aggegate to form the pyramids in the medulla. Thence the name "pyramidal tract".
  
  
• 80 - 90 % of corticospinal tract axons cross to the other side in the distal medulla. This is a very distinctive area on cross section known as the Pryamidal Decussation. The pyramidal decussation separates the medulla,above, from the spinal cord, below.
  
  
• Injuries to the upper motor neurons in the cortex or to their axons before they enter the pyramidal decussation result in spastic paralysis of the opposite side of the body. Injuries to the pyramidal tract below the decussation or to the lower motor neurons in the spinal cord cause paralysis on the same side of the body.
  
  
• The Ventral Corticospinal tract contains the axons from the 10-20% of neurons whose axons did not cross to the other side at the pyramidal decussation.
  
  
• The upper motor neuron axons then synapse on lower motor neurons in the Anterior horn of the spinal cord. The axons of these lower motor neurons then exit the spinal cord via the Ventral root. Damage to LMN's causes flaccid paralysis.
  
  
• The ventral root then joins the dorsal root to form the spinal nerve, which finally innervates the skeletal muscle.
  

• Upper motor neurons which innervate the muscles of the face and head are located near the lateral fissure of the brain. Their axons coalesce to form the corticobulbar tract. These axons then descend within the Genu of the internal capsule to the medial part of the cerebral peduncle. The upper motor neuron axons then synapse on Lower motor neurons of the cranial nerve nuclei which are located in midbrain, pons and medulla. Link to cranial nerves.
  

• The extrapyramidal system dampens erratic motions, maintains muscle tone and truncal stability. It is phylogenetically older that the pyramidal system and thus plays a relatively more important role in lower animals. Many of its synaptic connections are extremely complex and even today, poorly understood. Neurodegenerative disorders which affect the extrapyramidal system have yielded much of our knowledge about its normal function.
  
  
• The major parts of the extrapyramidal system are the "subcortical nuclei". This includes the caudate, putamen, and globus pallidus which are also known as the Basal ganglia. The caudate nucleus is especially affected in Huntington's chorea.
  
  
• The Substantia nigra, is located in the midbrain. It is particularly affected in idiopathic Parkinson's disease.
  
  
• The thalamus is a very complex structure with many functions including cognition and pain perception, but parts of the thalamus are also components of the extrapyramidal system.
  
  
• Other nuclei include the Subthalamic nucleus. Unilateral damage to the subthalamic nucleus results in hemiballism.
  
  
• The final major nucleus is the Red Nucleus which is immediately adjacent to the substantia nigra in the midbrain.
  
  
• A rare genetic disorder known as dentatorubropallidoluysian atrophy exhibits degeneative changes in the dentate nucleus of the cerebellum, the red nucleus, the globus pallidus and the subthalamic nucleus which is also known as the nucleus of Luys.
  
  
• To summarize, the extrapymidal nuclei include the substantia nigra, caudate, putamen, globus pallidus, thalamus, red nucleus and subthalamic nucleus. All of these nuclei are synaptically connected to one another, the brainstem, cerebellum and the pyramidal system.

• The vestibular system controls balance. It is synaptically linked to the extrapyramidal system. So that persons with extrapyramidal neurodenerative disorders frequently also have problems with balance and may experience frequent falls.
  
  
• The main sensory organ of the vestibular system is located in the inner ear and consists of the Utricle, the Sacculus and the Semicircular canals. There are three semicircular canals which represent all three spatial planes. The semicircular canals are lined by hair cells and filled with endolymph. The endolymph moves when the head moves and thus stimulates the hair cells. The hair cells then project synaptically to the Vestibular ganglion which is located within the bone of the skull. The ganglion then sends projections to the Superior and lateral vestibular nuclei which are located in the medulla adjacent to the floor of the fourth ventricle. These nuclei in turn send axons via the Inferior cerebellar peduncle to the Flocculonodular lobe of the cerebellum to maintain equilibrium.
  
  
• The major Tracts of the vestibular system include the Lateral vestibulospinal which maintains equilibrium, the Vestibuloocular which controls saccadic eye movements and the Vestibulocortical which causes dizziness when stimulated.
  
  
• The practical implications are that diseases of the inner ear cause loss of equilibrium, dizziness and saccadic eye movements when the head is turned.

• The cerebellum, which located inferior to the tentorium, (link to dura) coordinates muscle activity, equilibrium and tone. It is functionally and anatomically divided into three lobes. The flocculonodular lobe or Archicerebellum maintains equilibrium. The anterior lobe or Paleocerebellum maintains muscle tone. The posterior lateral lobes or Neocerebellum controls coordination and allows us to perform intricate motor tasks like playing the piano. Recent evidence suggests that the neocerebellum also plays a role in memory, especially memory of fine motor skills.
  
  
• Cebebellar motor tracts are uncrossed, so that injuries on one side will cause difficulties on the same side of the body.
  
  
• The major cerebellar tracts are the Spinocerebellar, connecting the spinal cord and the cerebellum, the Vestibulospinal, connecting the vestibular system and the cerebellum, Corticopontocerebellar, connecting the cortex, pons and cerebellum and the Dentatorubrothalamic connecting the dentate nucleus of the cerebellum, the red nucleus and the thalamus.

last update Jan 21, 2002