MOTOR SYSTEMS This course outline is designed to provide you with a study guide. Use the lecture, book, and other resources to expand on its contents.

Lesson Objectives

1. Trace the pyramidal system and discuss its basic function.

2. Trace the extrapyramidal system and discuss its basic function.

3. Discuss the similarities and differences between the pyramidal and extrapyramidal systems.

4. Explain the basic differences between the neostriatum and paleostriatum.

5. Explain the importance of the substantia nigra in extrapyramidal functions.

6. Explain the function of the COEPS.

General Issues in Movement

• Three types of muscle (smooth, cardiac, & skeletal)
• Skeletal muscles are controlled by alpha motor neurons whose cell bodies are located in the ventral column of the spinal cord.
• They release acetylcholine at the neuromuscluar junction
• This results in muscle contraction or shortening which can produce limb movement
• Frontal lobe controls motor function
  • Prefrontal - plans, obtains feedback, adapts responses to the environment
  • Premotor - area in which control of more complex motor acts are developed (i.e. language)
  • Precentral gyrus - Area of spatial distribution of muscles in the body
• Subcortical system essential for carrying out basic motor functions
• All information decends to the motor neuron pool in spinal cord
• Here information is integrated by spatial and temporal summation and action potentials traverse the motor neuron to reach the muscle fibers.

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

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

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.

Terms to Know

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