What Happens in a Transected Spinal Cord Injury?

A transected spinal cord injury severs or destroys the nerve fibers that carry signals between the brain and the body below the injury site, producing immediate and permanent loss of voluntary movement and sensation in those areas. But the initial break is only the beginning. Within minutes, a chain of biological events fans outward from the damage site, killing cells that survived the original trauma, reshaping the immune system, weakening bones, and fundamentally altering how the nervous system below the cut functions on its own. Understanding this cascade helps explain why spinal cord transection is so devastating and why recovery efforts face such steep obstacles.

The Moment of Injury

The primary injury is mechanical. When the spinal column fails through fracture, dislocation, or both, force is transmitted directly into the cord, shearing axons, rupturing blood vessels, and tearing cell membranes apart.¹ In a complete transection, the cord is physically divided or so thoroughly disrupted that no signal can cross the gap. The result is instantaneous loss of all brain-to-body communication below that level.

What follows immediately is a state called spinal shock. Every reflex below the injury goes silent. The muscles do not just become weak; they become completely unresponsive. Bladder and bowel reflexes disappear. Blood pressure can plummet because the sympathetic nervous system, which normally keeps vessels constricted, loses its connection to the brain. Spinal shock typically sets in right away, though in some cases the onset can unfold over hours.² The end of spinal shock is marked not by recovery of voluntary control but by the return of abnormal reflexes, often exaggerated ones. That transition can take days to weeks.

The Secondary Injury Cascade

If the primary injury is the earthquake, the secondary injury is the flood that follows. Starting within minutes and continuing for weeks, a series of biochemical events expands the zone of damage well beyond the original trauma site. This is the phase researchers spend the most effort trying to interrupt, because it kills cells that were initially intact.

One of the earliest problems is the collapse of the blood-spinal cord barrier, the tightly regulated boundary that normally prevents blood-borne substances from leaking into spinal cord tissue. Within the first hour after injury, this barrier breaks down dramatically, allowing proteins, immune cells, and fluid to pour into tissue that was never meant to be exposed to them.³ The result is swelling that compresses surviving tissue and disrupts blood flow even further.

At the same time, damaged and dying cells spill their contents into the surrounding space. One of the most harmful substances released is glutamate, ordinarily a key signaling chemical in the nervous system. In excess, glutamate becomes toxic. It overstimulates receptors on neighboring cells, triggering a process called excitotoxicity that is especially destructive to the support cells of the cord. Laboratory studies have shown that prolonged glutamate exposure damages astrocytes and oligodendrocytes, the cells responsible for maintaining and insulating nerve fibers, while also degrading the myelin sheath that wraps around axons.⁴ When myelin is stripped away, even surviving axons can no longer conduct signals properly.

Mitochondria, the energy producers inside cells, are central to this downward spiral. After transection, mitochondria in the severed axon segments lose their ability to generate energy. Without adequate energy, ion pumps that control calcium and sodium levels fail. Calcium floods into cells, triggering the production of reactive oxygen species and further poisoning cell machinery. This energy crisis and the excitotoxicity it enables are tightly linked to the waves of cell death that continue for days after the initial trauma.⁵ Research in mice has demonstrated that spinal cord injury causes a sharp drop in mitochondrial function near the lesion within hours, though enhancing mitochondrial transport along axons can partially reverse this damage over the following days.⁶

Inflammation That Helps and Hurts

The immune response to a transected cord is intense and prolonged, and it cuts both ways. Within hours, the body sends waves of immune cells to the injury site. Among the most studied are microglia, the brain and spinal cord’s resident immune cells, and macrophages, which arrive from the bloodstream. Both come in two broad flavors. One type, often called M1, promotes inflammation; the other, M2, tends to support tissue repair. After spinal cord injury, the balance tips sharply toward the inflammatory side at the injury site. Peak levels of a key inflammatory signal, TNF-α, appear around four days post-injury at the wound itself, and inflammatory macrophages dominate there through the subacute phase. This persistent inflammatory tilt may be one reason functional recovery stalls.⁷

Meanwhile, the injury also sets off a process of self-digestion within the cord stumps on either side of the cut. Lysosomes, the cellular compartments that break down waste, accumulate massively in both the segment above and the segment below the transection. This lysosomal activity begins within three hours, peaks over the first week, and gradually declines by two weeks. The enzymes these lysosomes release eat away at the cord tissue itself, creating fluid-filled cavities called cysts or syrinxes that can expand over time and damage additional tissue.⁸

The Glial Scar and Why Nerves Cannot Regrow

After the acute damage settles, the body’s attempt at repair creates its own barrier to recovery. Astrocytes, a type of support cell in the spinal cord, become activated and proliferate around the injury, eventually forming a dense boundary known as the glial scar. In one sense this scar is protective: it walls off the damaged area and helps limit the spread of toxic substances. But the scar also produces molecules, especially a family called chondroitin sulfate proteoglycans, that actively block nerve fibers from growing across the injury.⁹ Combined with the fluid-filled cavities that form in the cord stumps, these chemical and physical barriers make the lesion site essentially impassable for regenerating axons.

This is fundamentally different from what happens after a nerve injury in your arm or leg. Peripheral nerves have built-in support systems that encourage regrowth. The central nervous system, including the spinal cord, has the opposite problem: its default environment actively discourages it. The proteins in myelin debris, the scar molecules, and the cyst cavities all conspire to make the injury gap a dead end for any nerve fiber attempting to cross it.

What Happens Below the Injury

A transected spinal cord does not simply leave the body below the cut inert. The cord below the lesion is still alive, still contains functional circuits, and still receives sensory information from the skin, muscles, and organs. What it lacks is any instruction from the brain. The consequences of this disconnection play out across nearly every organ system.

Autonomic Dysreflexia

For injuries at or above the sixth thoracic vertebra (T6), one of the most dangerous complications is autonomic dysreflexia. Below the injury, sympathetic nerves that control blood vessel constriction lose their regulation by the brain. When something irritating happens below the injury, such as a full bladder, constipation, or even a tight shoe, the body’s sympathetic system below the lesion fires without restraint. Blood vessels clamp down, and blood pressure spikes, sometimes to dangerous levels. The brain detects this rise and tries to slow the heart through the vagus nerve, producing a characteristic combination of high blood pressure with a slow heart rate. If not treated quickly by removing the irritating stimulus, autonomic dysreflexia can lead to stroke, seizures, or heart failure.¹⁰ The underlying problem is that sympathetic preganglionic neurons below the lesion develop excessive, maladaptive activity in the absence of descending control from the brain.¹¹

Bladder Dysfunction

Normal urination requires precise coordination between the bladder muscle, which contracts to push urine out, and the urethral sphincter, which relaxes to let it pass. After spinal cord transection, these two structures lose their coordination and often work against each other, a condition called detrusor sphincter dyssynergia. The bladder tries to empty, but the sphincter clamps shut at the same time, trapping urine under high pressure. Left unmanaged, this can cause repeated urinary tract infections and, over time, damage to the kidneys.¹²

Immune Suppression

The nervous system plays a larger role in immunity than most people realize, and high-level spinal cord injuries expose this vividly. Patients with transection at upper spinal levels can develop a secondary immune deficiency involving both cellular and antibody-based arms of the immune system.¹³ This phenomenon has been called spinal cord injury immune deficiency syndrome, and it helps explain why pneumonia is the leading cause of death in people with spinal cord injuries, not the injury itself.¹⁴ The higher the injury, the more of the sympathetic chain is disconnected from the brain, and the more pronounced the immune suppression.

Bone and Muscle Loss

Below the level of transection, the body rapidly begins to lose both muscle mass and bone density. Paralyzed muscles shrink because they receive no signals telling them to contract. But the bone loss is not purely from disuse. The disrupted nerve supply itself seems to alter bone metabolism in ways researchers are still working out. Measurements in the first year after injury show bone mineral content dropping at roughly four percent per month in areas rich in spongy bone and about two percent per month in areas of dense bone, while muscle mass in the legs falls dramatically and fat tissue increases to fill the space.¹⁵ Both the speed and the extent of these changes are influenced by whether the injury is complete or incomplete and by how much time has passed.¹⁶ The practical consequence is a sharply elevated fracture risk, particularly in the legs, which becomes a long-term management concern for people living with spinal cord injuries.

Chronic Pain After Transection

It sounds paradoxical: how can you feel pain in a body part you supposedly cannot feel at all? Yet chronic neuropathic pain affects a large proportion of people with spinal cord injuries, including those with complete transections. The pain can be burning, stabbing, or electric in quality, and it often appears weeks to months after the injury.

The mechanism involves the spinal cord and brain rewiring themselves in unhelpful ways. Neurons that normally process touch and temperature become hypersensitive, amplifying any signal that reaches them. Research comparing people with spinal cord injury who developed neuropathic pain to those who did not found that the pain group showed markedly enhanced temporal summation of pain, a hallmark of the spinal cord’s processing centers becoming overexcitable. Pain that spread over larger body areas correlated with poorer ability of the nervous system to dampen its own responses.¹⁷ The inflammatory environment at and around the injury, where M1-type macrophages persist for weeks, likely contributes to this sensitization.⁷

Circuits That Survive Below the Cut

One of the more surprising aspects of a transected cord is that the spinal circuitry below the injury does not simply die. The spinal cord contains its own networks capable of generating patterned movement, sometimes called central pattern generators. These circuits can produce rhythmic stepping-like activity in the legs even without any input from the brain. They are the reason why, during spinal shock recovery, reflexes return and sometimes become hyperactive.

A class of nerve cells called propriospinal interneurons is especially relevant here. These cells form chains that span multiple spinal segments, connecting different levels of the cord to each other. In incomplete injuries, propriospinal neurons can sometimes bridge around a lesion, rerouting signals through surviving tissue. Even in complete transections, these neurons remain functional below the cut and can be recruited by external stimulation to coordinate movement patterns.¹⁸ This discovery has reshaped how researchers think about rehabilitation, shifting the focus from regrowing long-distance connections from the brain to activating and training the circuits that already exist below the injury.

Epidural Stimulation and Reawakening the Cord

The survival of spinal circuits below a transection has opened the door to one of the most promising areas of spinal cord injury research: epidural electrical stimulation. An electrode array is placed on the surface of the spinal cord’s protective covering in the lower back, and carefully tuned electrical pulses are delivered to the lumbar region. The stimulation does not bypass the injury or replace the brain’s commands. Instead, it raises the excitability of the dormant spinal circuits below the lesion, making them responsive enough to produce useful movement when combined with the person’s intent or with sensory feedback from the legs.¹⁹

The results, while still experimental, have been striking. In a study of four people with chronic complete paralysis, all four achieved full weight-bearing standing with epidural stimulation turned on. Two of the participants, classified as having no motor or sensory function below their injuries, stood without external support other than using their hands on a balance bar.²⁰ Other research groups have since shown stepping movements and even voluntary leg control in some participants. The stimulation works by activating sensory fibers in the spinal cord’s posterior roots, which then trans-synaptically fire the local motor networks.²¹ The technology is not a cure, and it currently requires the stimulator to be on for function to occur, but it has demonstrated that “permanently paralyzed” does not mean the spinal cord below the injury is permanently silent.

Bridging the Gap With Biomaterials

Because the lesion site becomes a combination of fluid-filled cavity and chemical barrier, researchers have increasingly turned to physical scaffolds designed to bridge that gap. Hydrogels, soft materials that mimic the consistency of natural tissue, are leading candidates. They can be injected or implanted into the injury cavity, where they provide a structural framework for cells to grow along. Crucially, hydrogels can also be loaded with growth factors or stem cells, releasing them slowly to encourage nerve fiber extension and support cell survival.²²

In animal models of complete spinal cord transection, combinations of hydrogel scaffolds with stem cells have shown promise. One approach used a hyaluronic acid-based scaffold modified with adhesive peptides, implanted together with mesenchymal stem cells into the transected cord of rats. The two components worked in synergy, with the scaffold providing structural guidance and the stem cells contributing biological repair signals.²³ Another group developed a chitosan-based bridge that delivered neural stem cells along with immobilized growth factors, engineering an environment that guided stem cell organization and development within the injury gap.²⁴ These are still preclinical results, not therapies available to patients, but they represent a shift in strategy: rather than hoping nerves will somehow cross the hostile lesion environment on their own, researchers are building roads for them.

Why Some Animals Can Regenerate and Humans Cannot

Several species, including axolotls, certain marine worms, and flatworms called planarians, can fully regenerate their nervous systems after amputation. The key difference is not that these animals have some exotic biological trick humans entirely lack. They have large populations of pluripotent stem cells, cells capable of becoming any tissue type in the body. Humans have these cells too, but in extremely limited numbers.²⁵ On top of that scarcity, the human central nervous system surrounds any injury with the inhibitory molecules and scar tissue described earlier, creating an environment that actively suppresses regrowth even when stem cells or surviving neurons attempt it.

Studying these regenerative species is more than an academic curiosity. Identifying which molecular pathways allow an axolotl to rebuild its spinal cord could reveal targets for drugs or gene therapies that reactivate dormant regenerative potential in humans. The gap between a salamander regrowing its spinal cord and a person with a transected cord is vast, but narrowing that gap, even slightly, could mean the difference between complete paralysis and meaningful recovery of function. Current research is mapping these pathways with increasing precision, though translating any of it into human therapies remains a long-term project measured in decades rather than years.