PYRAMIDAL SYSTEM

• Definition: motor system with cell bodies in cortex & axons terminating directly on the motor neuron pool.
• Betz cells in motor cortex make up only 3% of this tract.
• Thought to be voluntary motor system while extrapyramidal is involuntary
• Involved in instigating, terminating, and altering movement.
• Functions to relate behavior to environment by use of sensory feedback.

  Motor cortex

  • Precentral gyrus controls individual movements
  • Premotor region (plan & integration of sequences of movements)
  • Motor homunculus -- Organization inverted -- head at bottom
  • Motor cortex area conforms to innervation ratio
  • Muscle vs. movement localization controversy

Cortico-spinal and Cortico-bulbar paths:

• Axons arise from motor areas of neocortex
• Called the Pyramidal tract
• Pass through the Coronal radiata
• Through the Cerebral peduncles
• Decussation (cross) -- in brain stem -- 80%
• Forms Lateral Corticospinal tract
• Uncrossed fibers form the ventral corticospinal tract
• Axons originate - cortex terminate -spinal cord:
• In (motor neuron pool)
• Directly on visceral motor neurons
• Mostly on accessory cells medial to tract
• Connecting neurons -- necessary where fiber must mediate excitor and inhibitor function
• Two synapses
• One bypassed with repeated stimulation
• Corticobulbar tract

EXTRAPYRAMIDAL SYSTEM

• Extremely old system (fish)
• Major motor system of reptiles, birds & lower mammals
• Controls inborn, stereotyped responses
• Maintains rhythmic, phasic behavior (walking); does not initiate movement

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  Anatomy

  • Difficult to separate from pyramidal system (both functionally & structurally).

  Major difference is that the pyramidal is continuous from cortex to spinal cord without synapse, whereas the extrapyramidal represents all other motor components.

  • Includes two groups of cells:
  • Basal ganglia -- putamen, globus pallidus, caudate, claustrum, amygdala
  • Peduncular nuclei -- substantial nigra, subthalamic nucleus, & COEPS

  Generalities of the extrapyramidal

  • Basal ganglia are comparable on the motor side to the thalamus on the sensory side (interaction between the two)
  • Diffuse projections to reticular formation
  • Dual nature of function (excitor-inhibitor)
  • Major motor control in lower animals with highly developed cerebellum
  • Pyramidal bypasses all extrapyramidal stages
  • Both systems cooperate in movements

Neostriatum connections

• Inhibitory
• Caudate nucleus - lateral wall of lateral ventricle, medial to internal capsule
• Putamen -- lateral to internal capsule
• Both formed by many short axon cells
• Afferent input --
  • (1) thalamus, &
  • (2) cortex
• Efferent -- to other segment of globus pallidus, then to lower structures

Paleostriatum (globus pallidus)

• Separated from putamen by thin external medullary lamina
• Two globus pallidus (GP) layers separated by internal medullary lamina
• afferent -- from neostriatum mainly to outer G.P. but some to inner
• inferior thalamic peduncle relays fibers from medial nucleus & centrum medianum to G.P.
• Efferents
• all extrapyramidal influences to the spinal cord are relayed via the
• reticulospinal and cerebrospinal paths
• Thalamic connection considered as part of a feedback circuit to cortex

Affiliates of the Motor System

• Substantia nigra
• Afferents
  • Neostriatum direct or paleostriatum
  • Also feedback from other areas
• Efferents
  • Feedback to basal ganglia
  • RF and red nucleus
  • Waystation for COEPS
  • Discharge to rubo- & reticulospinal tract.
• Subthalamic nuclei
  • Damage -- involuntary movements in limbs to opposite side
  • Releasor phenomena

Summary

• Caudalward "cascade" conduction
• Thalamus and cortex ----> neostriatum -----> paleostriatum midbrain
• Structures (red & reticular nuclei) -
• Reticulo- & rubrospinal tracts ----> motor pool

FUNCTIONAL CORRELATES

• COEPS
• Gap between experimental animals and man
• Neostriatum -- predominantly inhibitory
  • Injury -- uncontrolled movements
• Paleostriatum -- predominantly excitatory; damage relieves above behavior
  • Human -- loss is minor
  • Centrum medianum -- destruction will also relieve over-activity
• Subthalamic and substantia nigra
  • Suppressor function
  • Subthalamic loss
  • Releasing hemiballismus
  • Substantia nigra loss
  • Parkinsonian rigidity

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  CEREBELLUM

  • Cerebellar Cortex
  • Nuclei
  • Input - Receives input from cortex, basal ganglia, spinal cord, & sensory systems

  Has elaborate sensory representation from somatosensory and vestibular systems

  • Function - Receives sensory & motor input and coordinates and integrates

  The result is a smoothing of motor responses

  • Injury - results in an intention tremor, a change in muscle tone & impaired speech.

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  Terms to Know

  Betz cells	Pyramidal tract	Extrapyramidal tract
  Central gray	Globus pallidus	Substantia nigra
  Precentral gyrus	Putamen	Red nucleus
  Corticobulbar tract	Caudate nucleus	Cerebral peduncle
  Insula	Basal ganglia	Fields of Forel
  COEPS	Subthalamic nuclei	Decussation
  Flaccid paralysis	Internal capsule	Corona radiata
  Premotor area	Corticospinal tract	Peduncular nuclei

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  Links to Associated Areas

  LINK Control and Coordination