What Is the ALS Tract and Is It Related to ALS Disease?

The term “ALS tract” refers to the Anterolateral System, a bundle of nerve fibers in the spinal cord that transmits specific sensory information to the brain. It functions as an ascending pathway, carrying signals like pain and temperature from the body upwards. The system’s name, however, often leads to confusion with a well-known neurodegenerative condition.

Function of the Anterolateral System

The primary role of the Anterolateral System is to convey sensations of pain, temperature, and what is known as crude or non-discriminative touch. This system is composed of several pathways, with the most prominent being the spinothalamic tract. This tract is divided into the lateral spinothalamic tract for pain and temperature, and the anterior spinothalamic tract for crude touch and pressure. The system is also responsible for protective reflexes and immediate responses to potentially harmful stimuli.

This sensory journey begins at specialized nerve endings in the skin called nociceptors, which detect painful stimuli, and thermoreceptors, which detect temperature changes. From these receptors, first-order neurons carry the signal into the spinal cord. There, they connect with second-order neurons, which cross to the opposite side of the spinal cord and ascend toward the brain as part of the Anterolateral System.

This ascending bundle of fibers travels through the brainstem and terminates in the thalamus, a central relay station for sensory information. In the thalamus, the signals are passed to third-order neurons, which project to the somatosensory cortex. The cortex then processes and interprets these signals, allowing for the conscious perception of the sensation’s location and quality.

Distinguishing from Amyotrophic Lateral Sclerosis

The Anterolateral System (ALS tract) and the disease Amyotrophic Lateral Sclerosis (ALS) are unrelated, though the shared acronym is a point of confusion. Amyotrophic Lateral Sclerosis is a progressive neurodegenerative disease that impacts motor neurons, the nerve cells responsible for controlling voluntary muscle movements. The disease does not primarily target the sensory pathways of the Anterolateral System.

The fundamental difference is the type of nerve cells involved. The Anterolateral System is a sensory pathway, carrying information from the body to the brain. In contrast, ALS disease affects motor pathways, which carry commands from the brain to the muscles, leading to weakness and paralysis as motor neurons degenerate.

The name Amyotrophic Lateral Sclerosis describes its pathology. “Amyotrophic” refers to the atrophy or wasting of muscle tissue. “Lateral sclerosis” refers to the hardening of the lateral (side) portions of the spinal cord where motor nerve fibers are located, which is distinct from the anterolateral (front-side) location of the sensory tract.

Neurological Pathways Affected by ALS Disease

Amyotrophic Lateral Sclerosis specifically targets the motor neuron system, which is composed of upper and lower motor neurons. The disease involves the progressive degeneration and death of both of these neuron types, a defining feature of the condition.

Upper motor neurons originate in the brain’s motor cortex. Their long axons form the corticospinal and corticobulbar tracts, which are the primary conduits for motor commands that descend into the spinal cord. The degeneration of these neurons leads to symptoms like muscle stiffness (spasticity) and exaggerated reflexes.

Lower motor neurons reside in the brainstem and spinal cord. They receive signals from the upper motor neurons and connect directly to muscle fibers, transmitting the final command to contract. When these neurons degenerate, the link to the muscles is broken, resulting in symptoms such as muscle weakness, twitching (fasciculations), and muscle wasting (atrophy).

Sensory System Changes in ALS Disease

While ALS disease is defined by motor system destruction, some patients report sensory changes like numbness, tingling, or pain. This may seem contradictory, as research confirms the main sensory pathways are largely preserved in the disease. For this reason, the perception of pain and temperature remains intact for most individuals.

The reasons for these sensory disturbances are under investigation. Studies show that some patients with ALS exhibit a loss of the large sensory fibers responsible for sensations like vibration and joint position. Pathological studies have also confirmed alterations in sensory nerves, suggesting a subtle, secondary involvement of the peripheral sensory system.

These sensory symptoms are not caused by direct destruction of the Anterolateral System. They are likely secondary effects of the neurodegenerative process, such as changes in sensory nerve conduction. Pain can also be a consequence of immobility, muscle cramps, and joint stiffness caused by motor neuron degeneration.