Gray and White Matter of the Spinal Cord

The spinal cord contains two fundamentally different types of tissue, gray matter and white matter, arranged in a pattern that surprises most people who already know a little brain anatomy. In the brain, gray matter forms the outer cortex and white matter fills the interior. The spinal cord flips this: a butterfly-shaped core of gray matter sits in the center, surrounded by columns of white matter. This arrangement reflects the cord’s dual job as both a local processing center and a long-distance communication cable between the brain and the rest of the body, and the distinction between these two tissues turns out to matter enormously for understanding injuries, diseases, and even the limits of medical treatment.

What Gray and White Matter Actually Are

Gray matter gets its color (really more of a pinkish-tan in life, grayish only in preserved tissue) from the dense concentration of nerve cell bodies, their branching extensions called dendrites, and the synapses where signals pass between neurons. In the spinal cord, the gray matter is where actual computation happens: sensory information gets processed, motor commands get relayed to muscles, reflexes get organized, and autonomic signals for things like blood pressure and digestion get routed. If you think of the spinal cord as a highway system, the gray matter is every city along the route, where traffic enters, exits, and gets redirected.

White matter, by contrast, is made up of long nerve fibers (axons) wrapped in a fatty insulating layer called myelin, which gives it its pale appearance. These myelinated fibers are bundled into tracts that carry signals up to the brain or down from it. White matter does not do much local processing; it is the highway itself, carrying information at high speed over long distances. The myelin sheath is what makes that speed possible, allowing electrical signals to jump rapidly along the fiber rather than crawling inch by inch.

The Butterfly Shape and What Each Wing Does

If you slice the spinal cord in cross-section and look at the gray matter, you see a shape often compared to a butterfly or the letter H. The “wings” of this butterfly are divided into horns. The dorsal horns (the ones pointing toward your back) receive incoming sensory information from the body: touch, temperature, pain. The ventral horns (pointing toward your front) contain the large motor neurons whose axons reach out to skeletal muscles to make them contract. In the thoracic and upper lumbar regions, there is also a lateral horn that houses neurons controlling involuntary functions like heart rate and sweating.

The arrangement of motor neurons in the ventral horn follows a spatial logic. Neurons controlling muscles close to the spine (the trunk and back) tend to sit in the medial part of the ventral horn, while neurons controlling the limbs cluster laterally. A study mapping motor neuron locations in rats confirmed this pattern in detail, showing that neurons innervating deep back muscles sat in the ventromedial group, while those controlling abdominal wall muscles were found in the lateral group.

The lateral horn’s autonomic neurons are particularly interesting. In the thoracic segments, a cluster called the intermediolateral nucleus contains sympathetic preganglionic neurons, the cells that relay commands to the sympathetic nervous system. Research on these neurons in cats found that they share overlapping physiological properties regardless of which organ they ultimately control, and that their functional specialization likely depends on the specific inputs they receive rather than intrinsic differences between the cells themselves.¹ These same neurons are regulated by inhibitory chemical signals: both GABA and glycine can suppress their spontaneous activity, providing a braking mechanism for sympathetic output.²

The White Matter Highway System

The white matter surrounding the gray core is organized into three broad columns on each side: dorsal (posterior), lateral, and ventral (anterior). Each column carries specific bundles of nerve fibers running in defined directions.

The dorsal columns carry fine touch and proprioception (your sense of where your limbs are in space) upward toward the brain. The lateral columns contain the corticospinal tract, the main pathway for voluntary movement commands descending from the brain’s motor cortex, along with the spinothalamic tract fibers that carry pain and temperature signals upward. The ventral columns carry additional motor and sensory pathways. Pain and temperature fibers cross from one side to the other through a structure called the anterior white commissure before ascending, which is why damage on one side of the cord can cause sensory loss on the opposite side of the body.³

The corticospinal tract deserves special mention because it was long taught to have a neat, layered arrangement where fibers controlling the arms sat in one zone and fibers controlling the legs sat in another. A critical reappraisal of the evidence found no support for this tidy somatotopic map in humans and instead emphasized that the corticospinal tract is particularly important for fine hand function.⁴ This matters clinically because older models of spinal cord injury syndromes sometimes assumed the layered arrangement was real and used it to predict which limbs would be affected.

How Much Gray and White Matter Is There

The spinal cord is a surprisingly small structure. Its total gray matter volume amounts to only about 3 to 4 milliliters in an adult, roughly the volume of a small marble. White matter is considerably more abundant, making up the bulk of the cord. One detailed measurement study found that a female spinal cord contained about 2.87 mL of gray matter and 11.33 mL of white matter, while a male cord had about 3.55 mL of gray matter and 19.33 mL of white matter.⁵ The sex difference in white matter was substantially larger than the difference in gray matter, which likely reflects the overall size difference in cord length and body height.

The profiles of both tissues along the length of the cord follow a recognizable pattern. There are two enlargements, in the cervical region (serving the arms) and the lumbosacral region (serving the legs), where the gray matter balloons outward to accommodate the extra motor neurons and sensory processing needed for the limbs. Between these enlargements, in the thoracic region, the cord is thinner with relatively less gray matter and proportionally more white matter carrying signals past without stopping.

How the Cord Builds Itself During Development

The gray and white matter distinction starts early in embryonic development. As the neural tube forms, signaling molecules establish a top-to-bottom (dorsal-ventral) axis that determines which neurons will develop where. A key player is Sonic Hedgehog (Shh), a signaling protein released from the floor of the developing cord that helps pattern the ventral side, giving rise to motor neurons and certain classes of interneurons.⁶

On the dorsal side, bone morphogenetic proteins (BMPs) guide the formation of sensory interneurons. Interestingly, research has shown that BMPs do not act as classic concentration gradients (where higher doses create one cell type and lower doses create another). Instead, different BMP family members direct different sensory neuron identities through signal-specific activities, and increasing the concentration of a given BMP simply makes it more efficient at producing its designated cell type rather than switching to a different type.⁷ This is a case where a decades-old assumption turned out to be wrong when tested rigorously.

Gray Matter as a Local Processing Center

One of the most remarkable things the spinal cord’s gray matter does is generate rhythmic movement without any input from the brain. Networks of interneurons in the cord, known as central pattern generators, can produce the alternating left-right, flexor-extensor patterns that underlie walking. Research in neonatal mice pinpointed the rhythm-generating core of this network to the most medial layers of the spinal gray matter, demonstrating that these neurons are both necessary and sufficient to produce locomotor-like activity.⁸ This explains why some animals with complete spinal cord transections can still produce stepping movements when placed on a treadmill: the gray matter circuitry below the injury carries on without the brain’s permission.

The gray matter also plays a critical role in pain processing. Sensory neurons carrying pain signals from the body enter through the dorsal horn, where they synapse onto second-order neurons. But the dorsal horn is not a passive relay station. It can amplify or dampen pain signals through local circuits, and the brain sends descending pathways that modulate this processing from above. This “top-down” modulation profoundly changes the sensory experience of pain, which is why identical injuries can feel very different depending on context, stress, or attention.⁹

Microscopic Differences Between the Two Tissues

Beyond the obvious distinction of cell bodies versus fiber tracts, gray and white matter differ at a finer level. The supporting cells called oligodendrocytes, which produce the myelin that wraps axons, behave differently depending on where they sit. In gray matter, oligodendrocytes form functional connections with each other through gap junctions; about 18% of injected cells in gray matter were coupled to more than three neighboring cells. In white matter, this coupling was essentially absent, with dye restricted to the single injected cell.¹⁰

What Happens When Each Tissue Is Damaged

Injuries to gray and white matter produce very different clinical pictures. Damage to gray matter at a particular spinal level destroys the local motor neurons and interneurons at that level, causing weakness or paralysis of the specific muscles they control, along with loss of reflexes in that segment. Damage to white matter, on the other hand, disrupts the long-distance communication lines, which can affect everything below the level of injury even though the gray matter in those lower segments remains intact. A person with isolated white matter damage in the cervical cord might lose leg function even though the lumbar gray matter controlling the legs is perfectly healthy, simply because the commands from the brain can no longer reach it.

In ischemic injuries (where blood flow to the cord is cut off), both tissues suffer, but not equally. A study in rats found that both gray and white matter were damaged after spinal cord ischemia, and the degree of white matter injury correlated with the severity of gray matter damage.11 Gray matter tends to be more vulnerable to ischemia because nerve cell bodies have higher metabolic demands than the axon fibers of white matter, which is why the central gray matter often shows damage first.

After a spinal cord injury, both tissues continue to deteriorate over time. Longitudinal imaging of patients showed that at two months post-injury, gray matter in the lumbar enlargement had decreased by about 16%, while white matter there had decreased by roughly 9%. The degeneration continued progressively, with white matter losing about 0.34-0.35% per month and gray matter at the cervical level losing about 0.70% per month.¹² This ongoing atrophy, which occurs far from the original injury site, reflects the degeneration of nerve fibers and cell bodies that have lost their connections.

Multiple Sclerosis and the Spinal Cord

Multiple sclerosis provides a particularly informative window into gray-white matter distinctions. MS is classically considered a white matter disease because the immune system attacks the myelin sheaths around axons. But spinal cord lesions in MS rarely confine themselves to white matter alone. A pilot MRI study identified 51 spinal cord lesions and found that 63% involved both white and gray matter simultaneously, 37% were white-matter-only, and none were gray-matter-only.¹³ Lesions most commonly appeared in the lateral columns (59%), followed by the posterior columns (35%).

Higher-resolution imaging using ultra-high-field 7 Tesla MRI confirmed that gray matter has the highest relative lesion load but that no lesions appeared exclusively in gray matter. Mixed gray-white matter lesions tended to be larger and showed more severe tissue damage than white-matter-only lesions.¹⁴ This pattern suggests that the traditional framing of MS as purely a white matter disease understates how extensively the gray matter is involved, at least in the spinal cord.

Imaging Gray and White Matter in Living Patients

Telling gray from white matter in the spinal cord has historically been difficult with standard MRI because the cord is so small and moves with every heartbeat and breath. Advanced techniques like diffusion tensor imaging (DTI) have changed this. DTI measures how water molecules move through tissue; in healthy white matter tracts, water flows preferentially along the length of the axons, creating a strong directional signal. In gray matter, water moves more freely in all directions because there are no long parallel fibers to channel it.

DTI provides a quantitative way to assess neural damage in various spinal cord conditions, and studies in both animal models and humans have shown that DTI reliably detects microstructural changes in the cord.¹⁵ In animal experiments, DTI-based fiber tracking could identify which specific white matter tracts were disrupted after injury. It could even detect the orientation of the glial scar that forms in gray matter adjacent to the injury site, where reactive astrocytes aligned themselves perpendicular to the damaged white matter tracts.¹⁶ Clinically, this kind of information helps surgeons and neurologists understand not just where damage has occurred, but what type of tissue is affected and how the cord is remodeling itself.

Why the Spinal Cord Cannot Repair Its White Matter

One of the most frustrating features of spinal cord injury is that damaged white matter tracts in the central nervous system do not regrow. Peripheral nerves, which are also wrapped in myelin, can regenerate slowly after being cut. But the spinal cord’s white matter actively discourages regrowth through a collection of molecules embedded in its myelin. These myelin-associated inhibitors, including proteins called Nogo-A, MAG, and OMgp, bind to receptors on damaged axon tips and trigger a biochemical cascade that collapses the growth machinery, effectively telling the axon to stop trying.¹⁷

Nogo-A has received particular attention as a therapeutic target. It is expressed by oligodendrocytes in the cord and restricts both long-distance axon regeneration and shorter-range compensatory sprouting, where surviving nerve fibers try to form new connections around the injury site. Anti-Nogo antibodies have been tested in clinical interventions starting within four weeks of injury.¹⁸ However, the overall picture remains complex. Experiments targeting these inhibitory molecules have produced conflicting results: some showed improved axonal growth, others did not, and a cohesive explanation for how to reliably promote repair by blocking these molecules has not yet emerged.¹⁹ The problem is likely that multiple inhibitory systems act in parallel, so blocking one is not enough to open the door.

How the Cord Compares Across Species

Researchers studying the spinal cord in animal models need to know how well their findings translate to humans, and the gray-white matter proportions vary considerably across species. A comparative study of the lumbosacral spinal cord in rats, cats, pigs, monkeys, and humans found that the peak gray matter cross-sectional area was largest in humans (about 22 mm²) and smallest in rats (about 4.3 mm²), with pigs, cats, and monkeys falling in between.²⁰

But when adjusted for body mass, the relationship inverts: rats have the largest lumbosacral cord volume relative to body weight (about 321 mm³ per kilogram), while humans have the smallest (about 42 mm³ per kilogram). This reflects a general principle in neuroscience: as animals get larger, the nervous system does not scale up proportionally. The cord grows enough to handle the limb muscles and sensory territory, but it becomes a smaller fraction of the total body. For researchers, these differences matter because the ratio of gray to white matter, the distances signals must travel, and the density of neurons all influence how experimental treatments behave. A drug that promotes axonal growth across a 2 mm rat lesion faces a very different challenge in a human cord where the distances are many times greater.

The Glial Scar and Gray Matter’s Role in Injury Response

After a spinal cord injury, the boundary between gray and white matter becomes a dynamic zone. Astrocytes, the most abundant glial cells in the cord, react aggressively to injury by proliferating and forming a dense scar. This glial scar serves a dual purpose: it walls off the damage site to prevent it from spreading, but it also creates a physical and chemical barrier that axons cannot cross. The DTI studies mentioned earlier revealed that these reactive astrocytes in gray matter adjacent to injured white matter organize themselves perpendicular to the damaged tracts, creating a barrier that is oriented specifically to block regrowth along the natural path of the fibers.¹⁶

Meanwhile, the gray matter surrounding the injury site becomes a zone where surviving neurons attempt to rewire. Compensatory sprouting, where intact nerve fibers send out new branches to take over functions lost by damaged neighbors, occurs primarily in the gray matter. But this is precisely where Nogo-A is upregulated after injury, creating a paradox: the tissue where plasticity is most needed is also where the brakes on plasticity are applied most forcefully.¹⁸ Understanding and resolving this contradiction is one of the central challenges in spinal cord injury research, and it depends entirely on appreciating the different roles that gray and white matter play, both in normal function and in the cord’s frustrated attempts at self-repair.

1
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PubMed. Localization of Rhythm Generating Components of the Mammalian Locomotor Central Pattern Generator
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