Difference from Lower Motor Neurons: Upper motor neurons control lower motor neurons
Lower Motor Neurons Recap
Somas Location: Brain stem or spinal cord
Axons Path: Peripheral nervous system → synapse on skeletal muscle cells
Function: Tell skeletal muscle cells when to contract
Control:
Spinal nerves → muscles of limbs and trunk
Cranial nerves → muscles of head and neck
Upper Motor Neurons Overview
Function: Control lower motor neurons
Somas Location: Cerebral cortex (outer surface of cerebrum)
Axons Path: Descend to synapse on lower motor neurons in brain stem or spinal cord
Information Flow: Cerebral cortex → axons → lower motor neuron somas → skeletal muscle cells
Upper Motor Neurons Pathways
Corticospinal Tract
Path: Cerebral cortex → spinal cord
Axons Travel: Down cerebral cortex → deep white matter → brain stem (midbrain, pons, medulla) → cross over → down opposite side of spinal cord → synapse on lower motor neurons
Function: Control skeletal muscle contraction
Dysfunction Effects:
Spinal cord dysfunction → same side muscle weakness
Brain dysfunction → opposite side muscle weakness
Corticobulbar Tract
Path: Cerebral cortex → brain stem
Axons Travel: Similar to corticospinal tract but terminate in brain stem
Function: Control lower motor neurons in brain stem (muscles of head and neck)
Wiring Complexity: One cerebral cortex side sends neurons to both sides of brain stem
Dysfunction Effects: Vary due to complex wiring
Dysfunction and Weakness
Causes: Both upper and lower motor neuron issues can cause weakness
Diagnosis: Determine if problem lies in upper or lower motor neurons
Upper Motor Neuron Signs
Hyperreflexia
Definition: Increase in muscle stretch reflexes (MSR)
Opposite of Hyporeflexia: Lower motor neuron sign
Example: Exaggerated knee-jerk reaction
Possible Cause: Lower motor neurons become super sensitive without upper motor neuron stimulation
Clonus
Definition: Rhythmic contraction of antagonist muscles
Examples: Rapid up-and-down foot movement when pulled up
Possible Cause: Hyperreflexia triggering antagonist muscle responses
Hypertonia
Definition: Increased tone of skeletal muscles
Opposite of Hypotonia: Lower motor neuron sign
Possible Cause: Related to hyperreflexia, but not fully understood
Extensor Plantar Response (Babinski Sign)
Definition: Abnormal extension of toes when bottom of foot is scraped
Normal Response: Flexion of toes (toes curl down)
Possible Cause: Unknown
Significance: Indicates upper motor neuron dysfunction
Conclusion
Summary: Understanding upper motor neuron pathways and signs helps diagnose the location of neurological issues and contribute to identifying underlying dysfunctions.