The spinal cord acts as a major communication highway, relaying information between the brain and the rest of the body. Bundles of nerve fibers known as tracts serve as dedicated communication lines within this structure. Ascending tracts specifically carry sensory information, gathered from peripheral receptors, up through the spinal cord toward processing centers in the brain. This allows for the conscious perception of the world and the unconscious coordination of movement, helping the body register sensations like touch, temperature, and pain.
General Organization of Sensory Pathways
Ascending tracts are located within the white matter columns, or funiculi, which surround the gray matter of the spinal cord. These white matter regions are divided into dorsal, lateral, and anterior columns. The placement of a specific tract within these columns indicates the type of information it carries. All major sensory pathways follow a standardized three-neuron relay system to transmit a signal from the body surface to the cerebral cortex for conscious awareness.
The initial sensory signal is picked up by the first-order neuron, whose cell body resides in a dorsal root ganglion outside the spinal cord. This neuron enters the cord and synapses with a second-order neuron, which carries the signal up to the thalamus, a central relay station. The third-order neuron then projects from the thalamus to the primary somatosensory cortex, completing the conscious pathway. A defining feature of most sensory pathways is decussation, where fibers cross over from one side of the central nervous system to the other. This ensures that one side of the brain processes sensory input from the opposite side of the body.
The Dorsal Column Pathway
The Dorsal Column-Medial Lemniscus pathway is a refined sensory system dedicated to transmitting high-fidelity information about the body’s interaction with the environment. It primarily carries discriminative or fine touch, which allows for the precise localization of a stimulus, vibration sense, and conscious proprioception. Proprioception is the awareness of where body parts are in space without relying on visual input. This pathway is characterized by its speed, achieved through heavily myelinated, large-diameter nerve fibers.
Upon entering the spinal cord, the first-order neurons travel immediately upward on the same side of the body (ipsilaterally) within the dorsal white matter column. Signals originating below the mid-thoracic level travel in the fasciculus gracilis, while signals from the upper body travel in the fasciculus cuneatus. These neurons ascend to the brainstem, where they synapse with second-order neurons in the nucleus gracilis and nucleus cuneatus of the medulla. The decussation, or crossing over, for this pathway occurs high up in the caudal medulla, forming the medial lemniscus, which then continues contralaterally to the thalamus.
The Spinothalamic Pathway
In contrast to the fine detail carried by the dorsal column, the Spinothalamic pathway, also known as the Anterolateral system, transmits protective sensory information. This system is responsible for relaying pain, temperature, and crude or non-discriminative touch from the skin and deep tissues. Crude touch is the simple awareness of contact without being able to precisely locate the source. The pathway is subdivided into the lateral spinothalamic tract, which carries pain and temperature, and the anterior spinothalamic tract, which carries crude touch and pressure.
The defining anatomical feature of this pathway is its immediate decussation at the level of the spinal cord. After the first-order neuron enters the dorsal horn, it synapses with the second-order neuron, which crosses the midline almost immediately through the anterior white commissure. These crossed second-order axons then ascend contralaterally in the anterolateral column of the spinal cord, traveling to the thalamus for relay to the cerebral cortex. This low crossing point means the sensory information travels up the side of the spinal cord opposite to where the sensation originated.
Unconscious Proprioception and the Cerebellum
While the dorsal column pathway relays conscious proprioception to the cerebral cortex, the Spinocerebellar tracts transmit unconscious proprioception, bypassing conscious awareness entirely. These tracts carry information about the tension of muscles and the stretch of tendons and joints directly to the cerebellum. This sensory input is needed by the cerebellum to regulate posture, coordinate motor movements, and maintain balance.
The dorsal and ventral spinocerebellar tracts originate primarily from the lower limbs and trunk, while the rostral spinocerebellar and cuneocerebellar tracts serve the upper body. These pathways are unique because they typically involve only two neurons, not the standard three, reflecting their direct, non-conscious destination. They project to the cerebellum, entering via the inferior and superior cerebellar peduncles, providing the coordination center with a constant update on the body’s mechanical state.
How Damage Affects Sensation
The differing decussation levels of the major ascending tracts create predictable patterns of sensory loss when the spinal cord is damaged. A partial injury to one side of the spinal cord, known as a hemisection, illustrates these differences. Since the dorsal column fibers travel up the same side (ipsilaterally) before crossing in the medulla, damage results in a loss of fine touch, vibration, and conscious proprioception on the same side of the body as the injury.
Conversely, spinothalamic fibers cross immediately upon entering the spinal cord, so a hemisection interrupts them after they have already crossed. Therefore, a patient will experience a loss of pain and temperature sensation on the opposite side of the body below the level of the injury. This asymmetrical sensory profile allows neurologists to accurately pinpoint the location of a spinal cord lesion.