Within the spinal cord, communication lines are organized into bundles of nerve fibers known as tracts, which occupy the white matter surrounding the central gray matter. These tracts are categorized based on the direction in which they carry information relative to the brain. Ascending tracts are responsible for relaying sensory signals from the body up to the brain, while descending tracts transmit motor commands down from the brain to the muscles and glands.
Defining the Spinothalamic Tract
The spinothalamic tract is an ascending pathway, meaning it carries sensory information from the body’s periphery upward toward the brain for conscious processing. The name itself offers a clear indication of its route: “Spino-” refers to its origin in the spinal cord, and “-thalamic” points to its primary destination, the thalamus in the brain. This pathway is responsible for transmitting sensations that relate to the body’s immediate survival and protective responses. The specific types of information relayed by this tract include pain, temperature, and crude touch, which is a non-discriminative sense of contact. The spinothalamic tract is actually composed of two distinct parts: the lateral tract, which primarily conveys pain and temperature, and the anterior tract, which handles crude touch and pressure sensations.
The Three-Step Journey to the Brain
The transmission of sensory information through the spinothalamic tract involves a precise sequence of three neurons. The journey begins with the first-order neuron, which has its receptor ending in the skin or other peripheral tissues. This neuron detects the painful, thermal, or touch stimulus and sends its signal into the spinal cord at the level of the sensory input.
Upon entering the spinal cord, the first neuron synapses with the second-order neuron located in the spinal cord’s gray matter. This second neuron is responsible for a defining characteristic of the spinothalamic pathway: decussation, or crossing over, to the opposite side of the central nervous system. The axon of the second-order neuron crosses the midline immediately, usually within one or two segments of the spinal cord level where it entered, and then ascends toward the brainstem and the thalamus.
This immediate crossover is why an injury to one side of the spinal cord often results in the loss of pain and temperature sensation on the opposite side of the body below the injury level. The second-order neuron finally terminates and synapses in the thalamus, a structure often described as the brain’s main sensory relay center. From the thalamus, the third-order neuron takes the signal and projects it to the somatosensory cortex, where conscious perception and interpretation of the sensation occur.
When the Spinothalamic Tract is Damaged
When the spinothalamic tract is compromised by injury, disease, or stroke, the ability to perceive pain and temperature is immediately affected. A complete inability to feel pain is termed analgesia, and the loss of temperature sensation is referred to as thermoanesthesia. Because of the tract’s early decussation, a unilateral lesion in the spinal cord typically causes the loss of pain and temperature sensation on the contralateral, or opposite, side of the body below the level of the injury. This pattern of sensory loss is a reliable sign used by clinicians to help locate the exact site of a neurological problem, and in severe cases, neurosurgical procedures such as a cordotomy may be performed to intentionally interrupt the tract for pain relief.