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ANPS 019
MOTOR SYSTEMS
November 30, 2011
Chapter 13: 436 -443
Chapter 15: 508-513
Lower Motor Neuron (LMN, alpha motor neuron):
-Cell body in spinal cord (spinal nerve) or in brainstem (cranial nerve)
-Axon terminates on muscles
Upper Motor Neuron (UMN):
-Cell body in brainstem or cortex
-Synapses on lower motor neuron
-Strong influence on lower motor neuron
Reflex: 2 neurons and 1 synapse
Multiple Descending Pathways regulate Lower Motor Neuron Activity
Axons from the brain descend via two major pathways
Lateral Pathways – limb innervation for voluntary motor control
Medial Pathways – maintain posture
Motor neurons in the ventral horn are organized medial = postural & lateral = voluntary (limb)
Planning of motor activity occurs in the Premotor Cortex
Primary Motor Cortex executes the Plan (pyramidal neurons)
Pyramidal System
Voluntary movement is initiated by pyramidal neurons (upper motor neurons) in the primary motor
cortex
Axon changes name:
-internal capsule
-cerebral peduncle (midbrain)
-long fibers (pons)
-pyramids (medulla)
-corticospinal tracts in spinal cord
CORTICOSPINAL TRACTS
Anterior corticospinal: remains ipsilateral until cord then innervates bilaterally
-medial muscles (posture muscles)
Because LMNs are innervated bilaterally damage to this tract does not produce a clinical deficit since the
other side compensates
Lateral corticospinal (80% of fibers) cross in medulla
-distal muscles: fine motor control
Damage to Lateral Corticospinal Tract
Loss of RIFMs
Rapid Independent Finger Movements
Clinical Evaluation: Babinski Sign
Why are the cerebral peduncles so much larger than the pyramids?
Any cranial nerve that going to innervate head and neck must get information from the cortex
What other tracts are in the cerebral peduncles?
Corticobulbar Tract
Function:
-innervates cranial nerve nuclei in brainstem
Descending Pathway:
-internal capsule
-cerebral peduncle (midbrain)
-synapses on motor neurons in cranial nerve nuclei: CN III & IV exit at midbrain
CN V, VI & VII exit at pons
CN IX, X, XI & XII exit at medulla
Since upper motor neurons innervate the cranial nerve nuclei bilaterally, the other side provides backup innervation; therefore, damage doesn’t result in clinical deficit (the exception is the facial nucleus)
The Action Comes from:
THE EXTRAPYRAMIDAL SYSTEM
Brainstem control of muscle activity
Rubrospinal –red nucleus
Tectospinal –tectum (midbrain superior and inferior colliculi)
Vestibulospinal – vestibular apparatus in ear
Reticulospinal – reticular information
RUBROSPINAL TRACT
Function:
-innervates arm flexors
Descending Pathway:
-axons exit Red Nucleus and cross
-axons descend in lateral white matter
-synapse on motor neurons in cervical enlargement
Damage to the rubrospinal tract results in a very specific posture-decerebrate
In the medulla, the rubrospinal tracts runs very close to the cardiac and respiratory centers; therefore,
the decerebate posture is a bad clinical sign
Sound: inferior colliculi
Eyes: superior colliculi
Posture is an important Clinical Sign
Decorticate posture: damage above red nucleus
Decerebrate posture – damage below red nucleus = BAD!
TECTOSPINAL TRACT
Function:
-startle response: reflex adjustments to posture in response to auditory (inferior colliculus) or visual
(superior colliculus) stimulus
Descending Pathway:
-axons exit superior and inferior colliculi and cross
-axons descend in medial white matter
-synapse on motor neurons in cervical enlargement bilaterally
THE VESTIBULOSPINAL TRACTS
Function:
Balance (righting reflex)
Descending Pathway:
Axons arise from vestibular nuclei at pons/medulla border (receive info form inner ear0
Exons descend in medial white matter
Synapse on motor neurons in bilaterally
RETICULOSPINAL TRACTS
Biggest brainstem tract involved in involuntary posture muscles
Function:
Modulation of postural muscles
Descending pathway:
Axons descend in medial white matter
Synapse on motor neurons in bilaterally
*damage to the reticulospinal tract results in hyperreflexia and spasticity
DESCENDING MOTOR TRACTS
Rubrospinal
Tectospinal
Vestibulospinal
Reticulospinal