Motor Neuron Versus Lower Motor Neuron Lesions

Motor Neuron Lesions

Overview of Motor Neurons

  • Understanding the distinction between upper motor neurons (UMN) and lower motor neurons (LMN).

Types of Upper Motor Neurons

  • There are two primary types of upper motor neurons:

    • Corticospinal Tract

    • Pathway originating from the cerebral cortex to the spinal cord.

    • Corticobulbar Tract

    • Pathway originating from the cerebral cortex to the brainstem (bulb).

Origins of Upper Motor Neurons

  • UMNs originate in the cerebral cortex.

    • Specific areas involved:

    • Primary motor cortex

    • Premotor cortex

    • Primary somatosensory cortex

    • Supplementary motor area

  • Axons descend via:

    • Corona Radiata

    • Spreading out from the motor areas of the cortex.

    • Internal Capsule

    • Pass through the Midbrain and Pons into the Medulla.

  • Key concept:

    • Decussation occurs as they travel down, leading to the contralateral control of body muscles.

Corticospinal Tract

  • Pathway details:

    • Axons travel from the motor areas, descend through the midbrain, pons, and into the medulla, eventually reaching the spinal cord.

    • Terminates in the anterior gray horn of the spinal cord.

Corticobulbar Tract

  • Pathway details:

    • Originates similarly from motor cortex areas (i.e., premotor, primary motor, supplementary motor).

    • Terminates at cranial nerve nuclei located in the brainstem.

Understanding Lower Motor Neurons

  • Definition: Lower motor neurons are the endpoint of upper motor neuron pathways.

  • Two primary types of LMNs based on UMN pathways:

    • Connect to skeletal muscles through anterior gray horn of the spinal cord (Corticospinal Pathway).

    • Connect to muscles innervated by specific cranial nerves (Corticobulbar Pathway).

Cranial Nerve Nuclei for Corticobulbar Tract

  • Cranial Nerve V: Trigeminal Nerve

    • Function: Innervates muscles of mastication.

  • Cranial Nerve VII: Facial Nerve

    • Function: Innervates muscles of facial expression.

  • Nucleus Ambiguous: Associated with the following cranial nerves:

    • Cranial Nerve IX: Glossopharyngeal Nerve

    • Cranial Nerve X: Vagus Nerve

    • Cranial Nerve XI: Accessory Nerve (cranial component).

    • Functions include those related to the muscles of the pharynx, larynx, soft palate, uvula, contributing to speech and swallowing.

  • Cranial Nerve XII: Hypoglossal Nerve

    • Function: Innervates tongue muscles.

Functional Implications of Motor Neurons

  • UMNs directly control LMNs that ultimately cause muscle contractions.

    • From Nuclei (Corticobulbar): Axons travel from nuclei to target muscles.

    • From Anterior Gray Horn (Corticospinal): Axons project to skeletal muscles throughout the body (upper limbs, lower limbs, trunk).

Lesion Definitions and Locations

  • Upper Motor Neuron Lesion: Damage at any point along the pathway from the cortex to the spinal cord, including:

    • Damage in the cerebral cortex.

    • Damage in the descending pathways (midbrain, pons, medulla).

  • Lower Motor Neuron Lesion: Damage to any of the following:

    • Anterior gray horn (

    • Cranial nerve nuclei.

    • Peripheral nerves (axon terminals).

Causes of Motor Neuron Lesions

  • Discussion to follow regarding the causes of upper motor neuron and lower motor neuron lesions after establishing the anatomy and function of the neurons.