Neuroscience Clerkship

UPPER MOTOR NEURON LESIONS

Motor System Overview

The command to contract a muscle voluntarily is initiated in the cerebral cortex and is transmitted through only two synapses. The cortical neuron is called the upper motor neuron (UMN). The first synapse is upon the lower motor neuron (LMN), whose cell body lives in the spinal cord. The second synapse is the neuromuscular junction itself. This direct pathway is collectively called the pyramidal system (after the microscopic appearance of the neuron cell bodies in the cortex).

Other circuitry involved in motor function includes loops through the basal ganglia and thalamus, or to the pons and cerebellum and thalamus. These extrapyramidal circuits are more complex and do not directly activate muscle contraction, but are essential to the proper functioning of the pyramidal output system.

Lesions of the pyramidal system are divided into upper and lower motor neuron types. Lesions of the basal ganglia or cerebellum are neither upper nor lower motor neuron lesions.

• Anatomy. Cell bodies of the primary motor cortex are located in the precentral gyrus (A) of the frontal lobe. Axons travel in the deep white matter (centrum semiovale (B) and corona radiata (C)), internal capsule (D), cerebral peduncle (E) (midbrain), basis pontis (F) (pons), and pyramids (G) (medulla). Most of the fibers cross in the caudal medulla, and proceed down lateral corticospinal tracts (H) in the contralateral spinal cord prior to synapsing at segmental levels on the lower motor neurons.

• UMN Syndrome. Lesion of either the cell body or (more commonly) the axon can produce a characteristic UMN syndrome. To localize to a specific part of the brain or cord, the clinician must use other clues, usually derived from collateral damage sustained by nearby structures serving other functions. The UMN syndrome consists of the following:

Weakness. The weakness may vary from subtle (mild coordination disturbance) to severe. The weakness has a characteristic pattern: in the upper extremities, elbow extension is weaker than flexion; wrist extension is weaker than flexion; and shoulder abduction is weaker than adduction.

In the lower extremities, the hip abduction is weaker than adduction; knee flexion is weaker than extension; ankle dorsiflexion is weaker than plantarflexion; and ankle eversion is weaker than inversion. In addition, most UMN lesions tend to cause more severe weakness distally.

Hyperreflexia. All the muscle stretch reflexes are increased. Abnormal hyperreflexia can be determined by comparison to the contralateral side (for unilateral lesions), or if there is clonus or pathologic spread of reflexes (e.g., tapping on the ankle reflex results in hip adduction as well).  However, it is important to note that severe or acute UMN lesions may result in hyporeflexia initially. This is known as cerebral or spinal shock. Only days or weeks later does hyperreflexia develop.

Spasticity. Spasticity is a type of increased muscle tone (resistance to passive movement). In spasticity, the tone increases as the limb is moved rapidly. If the limb is moved slowly enough, the tone increase is not apparent. The phenomenon of spastic “catch” reflects this behavior. Spasticity also displays a "clasped knife" type of feel - initially there is increased tone which then gives way as the limb is moved (similar to opening a clasped knife). When testing muscle tone, it is usually increased in the flexors of the upper extremity and the extensors of the lower extremity. However, similar to reflexes above, UMN lesions may initially display decreased tone or flaccidity. Again, it is only days or weeks later that spasticity develops in an UMN lesion.

Babinski Response (extensor plantar reflex) is often present in UMN lesions. This response is present in infants prior to complete myelination of motor axons. The Babinski response is elicited by stroking the sole of the foot. In an adult, a normal response is flexion of the toes (A); an abnormal response is extension (moving up) of the big toe and often fanning of the other toes (B and C).