Spinal Cord Cross-Section Anatomy: Tracts, Laminae, and Clinical Lesion Patterns

A spinal cord cross-section shows a butterfly-shaped central gray matter surrounded by white matter organized into columns (funiculi) containing specific tracts. The three tracts you absolutely must know are the dorsal columns (fine touch, proprioception, vibration — ascend ipsilaterally), the lateral corticospinal tract (voluntary motor — descends contralaterally after crossing in the medullary pyramids), and the anterolateral system including the spinothalamic tract (pain and temperature — crosses within 1-2 segments of entry). Knowing where each tract sits in the cross-section lets you predict exactly which functions are lost based on the lesion location.

The white matter of the spinal cord is divided into three columns (funiculi) on each side: dorsal (posterior), lateral, and ventral (anterior). Each column contains bundles of axons — tracts — running either up to the brain (ascending/sensory) or down from the brain (descending/motor). The dorsal column sits between the dorsal median septum and the dorsal horn. It carries fine touch, proprioception, vibration, and two-point discrimination. Below T6, it is a single fasciculus (gracilis, carrying lower limb information). Above T6, it splits into fasciculus gracilis (medial, lower limb) and fasciculus cuneatus (lateral, upper limb). These fibers ascend ipsilaterally — they do not cross until they reach the medulla, which is why a right-sided dorsal column lesion causes right-sided sensory loss. The lateral column contains the lateral corticospinal tract (the main voluntary motor pathway — already crossed in the medulla, so it controls the ipsilateral side) and the lateral spinothalamic tract (pain and temperature — already crossed within the cord, so it carries contralateral information). The lateral column also houses the rubrospinal, spinocerebellar, and other tracts, but for exam purposes the corticospinal and spinothalamic are by far the most important. The ventral column contains the anterior corticospinal tract (a small, uncrossed motor pathway that crosses at the segmental level) and the anterior spinothalamic tract (crude touch and pressure). These are lower-yield for most exams but show up occasionally.

The gray matter is organized into 10 layers called Rexed laminae, numbered I through X from dorsal to ventral. You do not need to memorize all 10 for most anatomy courses, but understanding the general layout is essential. Laminae I-VI form the dorsal horn and process incoming sensory information. Lamina II is the substantia gelatinosa — it is where pain fibers synapse and where pain modulation happens. This is the target of opioid receptors and the gate control theory of pain. If you remember one lamina, make it this one. Lamina VII is the intermediate zone and contains the intermediolateral cell column (IML) at levels T1-L2. This is where sympathetic preganglionic neurons live. A lesion here disrupts sympathetic output — which is why a T1 lesion can cause ipsilateral Horner syndrome (ptosis, miosis, anhidrosis). Laminae VIII and IX form the ventral horn, where lower motor neurons reside. Lamina IX specifically contains the motor neuron pools organized somatotopically: medial motor neurons control axial (trunk) muscles, lateral motor neurons control limb muscles. This medial-to-lateral organization means a small central lesion might spare the limbs while affecting the trunk, and vice versa. Lamina X surrounds the central canal and is relatively small. The central canal itself is relevant because syringomyelia — a fluid-filled cavity expanding from the central canal — preferentially destroys the anterior white commissure where spinothalamic fibers cross, producing the classic bilateral cape-like loss of pain and temperature with preserved fine touch.

If there is one concept that makes spinal cord lesion localization click, it is understanding where each major tract crosses the midline. Get this right and you can predict the deficit pattern from any lesion. Rule 1: Dorsal columns do NOT cross in the cord. They ascend ipsilaterally and cross in the lower medulla (the internal arcuate fibers forming the medial lemniscus). A right spinal cord lesion causes right-sided loss of fine touch, proprioception, and vibration below the lesion. Rule 2: The lateral corticospinal tract has ALREADY crossed. It crossed in the pyramidal decussation at the junction of the medulla and spinal cord. So a right spinal cord lesion causes right-sided (ipsilateral) motor weakness below the lesion. This feels counterintuitive until you remember the crossing already happened above. Rule 3: The spinothalamic tract crosses WITHIN the cord. Pain and temperature fibers enter the dorsal horn, synapse, and then cross through the anterior white commissure to ascend in the contralateral anterolateral system. They cross within 1-2 segments of entry. So a right spinal cord lesion causes left-sided (contralateral) loss of pain and temperature starting 1-2 segments below the lesion level. These three rules are the entire foundation of spinal cord localization. Every named syndrome is just a specific combination of these rules applied to a specific lesion pattern.

Brown-Sequard syndrome is a hemisection of the cord — one entire side is damaged. Apply the three rules: ipsilateral motor loss (corticospinal, already crossed), ipsilateral fine touch and proprioception loss (dorsal columns, not yet crossed), and contralateral pain and temperature loss (spinothalamic, already crossed in the cord). Classic cause: penetrating trauma like a stab wound. The exam question gives you a patient with right leg weakness, loss of vibration sense on the right, and loss of pain sensation on the left — that is Brown-Sequard on the right. Syringomyelia is a cavity expanding from the central canal, typically in the cervical cord. It first destroys the anterior white commissure — the crossing point for spinothalamic fibers. This produces bilateral loss of pain and temperature in a cape-like distribution across the shoulders and upper limbs, with preserved fine touch (dorsal columns are far posterior and spared). Patients classically present with painless burns on their hands because they cannot feel temperature. As the syrinx expands, it can eventually compress the ventral horns (causing lower motor neuron signs in the upper limbs) and the lateral corticospinal tracts (causing upper motor neuron signs in the lower limbs). Anterior cord syndrome results from anterior spinal artery occlusion. The anterior two-thirds of the cord loses blood supply, damaging the corticospinal tracts (bilateral motor loss), the spinothalamic tracts (bilateral pain and temperature loss), and the ventral horns. The dorsal columns are spared because they receive blood from the posterior spinal arteries. So the patient has paralysis and cannot feel pain, but can still feel vibration and proprioception — a devastating but diagnostically distinctive pattern. AnatomyIQ has interactive cross-section diagrams where you can tap a region and see which tracts are affected and what the resulting clinical syndrome would be.

Draw the cross-section from memory. Seriously — get a blank piece of paper and draw the butterfly, label the dorsal columns, the lateral corticospinal tract, and the spinothalamic tract. Color-code them: blue for sensory ascending tracts, red for motor descending tracts. Mark the crossing point for each one. Do this five times and you will never forget the layout. Then practice lesion scenarios. Cover half the cross-section with your finger (Brown-Sequard). Circle the center (syringomyelia). Block out the anterior two-thirds (anterior cord). For each one, walk through the three crossing rules and predict the deficits. When you can do this without looking at notes, you own this topic. The most common exam mistakes are mixing up which tract has already crossed and which has not. If you get confused, just ask: where does this tract cross? If the answer is the medulla (dorsal columns) or the pyramidal decussation (corticospinal), then a spinal cord lesion affects the ipsilateral side. If the answer is within the cord (spinothalamic), the lesion affects the contralateral side.