The Concept of Lower and Upper Motor Neurons and Their Malfunction

1. The lower motor neuron is classically defined as the alpha (α) motor neuron.

2. Disease of lower motor neurons causes stereotypical clinical signs.

3. Upper motor neurons lie completely in the central nervous system and control lower motor neurons.

4. Signs of upper motor neuron disease differ from signs of lower motor neuron disease.

The majority of veterinary patients with neurological disease display some abnormality of posture and locomotion. The abnormalities range from weakness or paralysis to spasticity, rigidity, and convulsions. The goal of the diagnostic process for such patients is to determine the location, the extent, and the cause of the lesion. Central to diagnostic logic in neurology is deciding whether the patient’s lesion is located in the lower motor neurons or the upper motor neurons. (There are two other possible locations of lesions causing movement disorders: the neuromuscular junction and skeletal muscle.)

This chapter defines lower and upper motor neurons because these concepts are useful in understanding the physiology of posture and locomotion and are essential in locating pathological processes in the nervous system. Malfunctions of these two neuron populations are also described briefly.

The Lower Motor Neuron Is Classically Defined as the Alpha (α) Motor Neuron

The concept of a lower motor neuron is decades old in neurology. The alpha (α) motor neuron is classically defined as a neuron whose cell body and dendrites are located in the central nervous system (CNS) and whose axon extends out through the peripheral nerves to synapse with the extrafusal skeletal muscle fibers (Figure 9-1). The cell bodies of these neurons are located either in the ventral horn of spinal cord gray matter or in cranial nerve nuclei of the brainstem. This is the “final common path” through which CNS commands to a skeletal muscle are channeled to produce movement. This definition predates the discovery of gamma (γ) motor neurons, which innervate muscle spindles. Some authors would include γ motor neurons in the definition of lower motor neurons. Some also consider the pre- and post-ganglionic autonomic neurons to be lower motor neurons (see Chapter 13). The vast majority of clinical signs caused by lower motor neuron disease, however, can currently be explained by the loss or malfunction of the α motor neuron.

FIGURE 9-1 General organization of lower and upper motor neurons. *Blue,* Lower motor neurons typically originate in the ventral horn of the spinal cord *(neuron 1)* or in cranial nerve nuclei *(neuron 2),* and synapse within skeletal muscle. *Green,* Upper motor neurons typically originate in the brain and project to and control lower motor neurons. Upper motor neurons usually belong to the corticobulbar *(neuron 3),* corticospinal *(neuron 4),* or descending brainstem motor (also called bulbospinal, *neuron 5),* pathways. *Arrowheads* indicate that local synapses may be interposed between the upper and lower motor neurons. *Horizontal lines* delimit either spinal cord segments (e.g., L1, L2) or major brain divisions (e.g., medulla, pons).

Disease of Lower Motor Neurons Causes Stereotypical Clinical Signs

Regardless of the pathological basis for disease of lower motor neurons, a stereotypical set of clinical signs results in the skeletal muscles they innervate.

• Paralysis or paresis. Disease of the α motor neurons usually prevents the neurons’ action potentials from reaching the neuromuscular junction. Therefore, despite the brain’s command to the muscle to contract, the message cannot reach the muscle, and paralysis is the result. In fact, such paralysis can be so complete that the adjective flaccid is used to describe the paralysis in which no muscle contraction occurs. Because not all the α motor neuron axons of a peripheral nerve may be affected by an insult, and because muscles can be supplied by axons of more than one spinal nerve, paralysis may be incomplete. This symptom is referred to as paresis.

• Atrophy. Atrophy is the shrinking or wasting of skeletal muscle mass distal to the lower motor neuron lesion (Figure 9-2). This occurs within days of the injury to a nerve. The exact origins of this atrophy are controversial. However, evidence indicates that the reduced frequency of muscle stimulation caused by α motor neuron insult, and the resulting reduced use of the muscle, trigger reductions in muscle protein synthesis and increases in muscle proteolysis. There is evidence that activation of the ubiquitin-proteosome proteolytic pathway underlies this muscle breakdown. The magnitude of this denervation atrophy can be reduced by direct electrical stimulation of the muscle itself. There is also some recent evidence that manually imposed repetitive stretching may reduce denervation atrophy; a molecular signaling pathway suspected to underlie this atrophy reduction has been identified.