Ascending spinal tracts are bundles of nerve fibers within the spinal cord that carry sensory information from the body to the brain. They are fundamental to our perception of touch, temperature, pain, and limb position. These pathways ensure sensory signals reach specific brain areas for processing, enabling both conscious awareness and unconscious regulation of bodily functions.
The Journey of Sensory Signals
Sensory information begins with specialized receptors throughout the body, such as in the skin, muscles, and joints. These receptors detect stimuli like pressure, heat, cold, and muscle changes, converting them into nerve impulses. First-order neurons, with cell bodies in dorsal root ganglia near the spinal cord, transmit these impulses.
Their axons enter the spinal cord through the dorsal root, where they synapse with second-order neurons. These second-order neurons carry the information upwards. Many pathways involve decussation, where nerve fibers cross to the opposite side of the central nervous system. This ensures sensory information from one body side is processed by the contralateral brain side.
From the spinal cord or brainstem, second-order neurons relay signals to third-order neurons, often in the thalamus, a brain relay station. The thalamus projects these signals to specific areas of the cerebral cortex, primarily the somatosensory cortex, for conscious perception and interpretation. Some pathways, however, terminate in other subcortical regions, contributing to unconscious processes.
Key Pathways for Body Sensation
Conscious perception of sensations like touch, vibration, pain, and temperature relies on specific ascending pathways. The dorsal column-medial lemniscus (DCML) pathway and the spinothalamic tract are two major routes. They differ in the sensations they carry and where their nerve fibers cross within the nervous system.
The Dorsal Column-Medial Lemniscus (DCML) pathway
The DCML pathway transmits discriminative touch (e.g., fine touch, two-point discrimination), vibration, and conscious proprioception (awareness of body position). First-order sensory neurons from the periphery enter the spinal cord and ascend in the dorsal columns, located in the posterior white matter. These fibers, the fasciculus gracilis (lower body) and fasciculus cuneatus (upper body), travel upwards without crossing until the medulla oblongata.
In the medulla, these neurons synapse with second-order neurons in the gracile and cuneate nuclei. Their axons then cross the midline, forming the medial lemniscus. This decussation occurs at the caudal medulla.
The medial lemniscus ascends through the brainstem, relaying information to the thalamus’s ventral posterolateral nucleus, where it synapses with third-order neurons. These third-order neurons project to the primary somatosensory cortex for conscious perception and localization of these sensations.
The Spinothalamic Tract
The Spinothalamic Tract conveys pain, temperature, and crude touch. Sensory neurons detecting these stimuli enter the spinal cord and synapse with second-order neurons in the dorsal horn.
These second-order neurons then decussate to the opposite side of the spinal cord within the anterior white commissure. Once crossed, these fibers ascend as the spinothalamic tract, located in the anterolateral spinal cord.
The tract divides into the lateral spinothalamic tract (pain and temperature) and the anterior spinothalamic tract (crude touch and pressure). These fibers reach the thalamus, synapsing with third-order neurons in the ventral posterolateral nucleus. From the thalamus, signals project to the somatosensory cortex for conscious perception.
Pathways for Unconscious Coordination
Beyond conscious sensation, the spinal cord contains ascending tracts that transmit sensory information for unconscious processes, particularly motor coordination and balance. These pathways, known as the spinocerebellar tracts, deliver proprioceptive information directly to the cerebellum. This information is not consciously perceived but is important for the cerebellum to fine-tune movements and maintain equilibrium. The spinocerebellar tracts include the posterior (or dorsal) spinocerebellar tract, the anterior (or ventral) spinocerebellar tract, and the cuneocerebellar tract.
The posterior spinocerebellar tract
The posterior spinocerebellar tract carries unconscious proprioceptive information from the lower limbs and trunk, originating from muscle spindles and Golgi tendon organs. First-order neurons synapse with second-order neurons in Clarke’s nucleus. These second-order neurons ascend ipsilaterally (on the same side) in the posterior spinocerebellar tract, entering the cerebellum via the inferior cerebellar peduncle.
The anterior spinocerebellar tract
The anterior spinocerebellar tract conveys unconscious proprioceptive and some cutaneous information, primarily from the lower limbs and trunk. Its pathway is distinct: first-order neurons synapse with spinal border cells in the anterior horn. Second-order neurons decussate twice, crossing to the opposite side in the spinal cord and then recrossing within the brainstem or cerebellum, reaching the cerebellum via the superior cerebellar peduncle. The cuneocerebellar tract transmits unconscious proprioception from the upper limbs and trunk to the cerebellum, largely remaining ipsilateral and entering via the inferior cerebellar peduncle, similar to the posterior spinocerebellar tract for the lower body. These unconscious pathways provide the cerebellum with continuous feedback about body position and movement, allowing for smooth and coordinated motor actions.
When Ascending Tracts Are Affected
Damage to ascending spinal tracts can lead to specific sensory deficits, providing clues for diagnosing neurological conditions. Such damage can result from spinal cord injury, stroke, degenerative diseases, or vitamin deficiencies. Understanding the functions and decussation points of these tracts helps medical professionals pinpoint lesion locations and predict sensory loss.
When the Dorsal Column-Medial Lemniscus (DCML) pathway is affected
When the DCML pathway is affected, individuals experience a loss of fine touch, vibration, and proprioception. If damage occurs below where fibers cross in the medulla, sensory losses appear on the same side as the injury. If the lesion is above the decussation point in the brainstem, deficits manifest on the opposite side. For instance, a positive Romberg test (difficulty standing with feet together and eyes closed) can indicate a proprioceptive deficit due to DCML pathway damage.
Damage to the Spinothalamic Tract
Damage to the Spinothalamic Tract results in impaired pain, temperature, and crude touch. Because spinothalamic tract fibers decussate immediately upon entering the spinal cord, a lesion within the spinal cord causes loss of pain and temperature sensation on the opposite side of the body. For example, in Brown-Séquard syndrome (hemisection of the spinal cord), there is ipsilateral loss of fine touch and proprioception (due to DCML involvement) and contralateral loss of pain and temperature sensation (due to spinothalamic tract involvement).
If the spinocerebellar tracts are affected
If the spinocerebellar tracts are affected, primary symptoms involve issues with coordination, balance, and gait, known as ataxia. Since these tracts largely remain ipsilateral or decussate twice, coordination problems are observed on the same side of the body as the lesion. This can manifest as an unsteady walk, slurred speech (dysarthria), and difficulties with fine motor skills, without loss of conscious sensation. The varying patterns of sensory loss and motor incoordination provide neurologists with a roadmap to identify the specific ascending tracts involved and the location of neurological damage.