In the spinal canal of the spine, the cauda equina forms a bundle of spinal nerve roots below the spinal cord. It lies within the spinal cord skin and supplies the lower half of the body with nerve signals from the central nervous system and receives sensory information from the periphery. Damage to the cauda equina can lead to cauda syndrome, which is characterized by flaccid paralysis of the legs.
What is the cauda equina?
The cauda equina is located within the spinal column and runs along the lower portion of it. It consists of spinal nerve roots, which are the junction between the spinal cord and spinal nerves. The spinal nerve roots are also known as the radix spinalis. The motor anterior root, which is responsible for transmitting brain signals when controlling movement, consists of eight to twelve such spinal nerve roots. Sensory information, on the other hand, travels via the posterior root, which also leads to the spinal cord. The name “cauda equina” literally means “horse’s tail” and refers to its attached location in the spinal column. In addition, the nerve processes form long fibers that at first glance resemble horsehair.
Anatomy and structure
In the lower part of the spine, the cauda equina extends through the spinal canal (canalis vertebralis). Its fibers consist of spinal nerve roots and begin at the level of the conus medullaris, which is the cone-shaped extension of the spinal cord. From the conus medullaris, the filum terminale – a thread of connective tissue – extends to the second sacral vertebra. There, the filum terminale attaches to the pia mater. The fibers of the cauda equina do not lie at the same level as the other spinal nerve roots because the spinal cord shifts within the spinal column during physical development. As this occurs, the spinal cord moves upward (ascensus), forcing the lower fibers to develop into an elongated tail. The cauda equina runs within the tubular sheath, which consists of the pia mater and dura mater. Both represent skins that surround the spinal cord. Here, the dura mater forms the outer and thicker layer. It is more robust than the pia mater, which lies beneath it and also surrounds the spinal nerve roots. For this purpose, the pia mater even spreads into the fissura mediana anterior. This forms a narrow gap in the spinal cord. The brain has an analogous skin consisting of several layers, which can be divided into hard meninges (pachymeninx encephali) and soft meninges (leptomeninx encephali).
Function and tasks
The cauda equina is composed of nerve fibers from the spinal nerves, whose function does not differ significantly from the task of the incidental spinal nerves. The spinal nerves enter the spinal cord through spinal canals. The motor fibers of the anterior root are efferent pathways that carry nerve signals from the brain through the spinal cord. From there, they continue to travel via the spinal nerves and, after any further wiring, reach the muscles they are responsible for controlling. In contrast to the motor anterior root, the posterior root carries sensitive fibers. Their myelinated axons represent afferent nerve pathways that carry sensory information. The nerve signals originate from the viscera, skin, and musculoskeletal system, which includes the muscles. Before the sensory fibers enter the spinal canal, they pass through a ganglion. This is a collection of nerve cell bodies. The ganglion interconnects nerve cells and partially computes their information with each other. This function plays an important role in the execution of some reflexes because, unlike voluntary responses, reflexes can be interconnected via the spinal cord. In this case, sensory action potentials originating in relevant receptors and entering the spinal cord via the anterior root trigger an automatic response from the associated motor fibers. The motor fibers then send their signal via the anterior root to the organ that performs the reflex. Neurology refers to such simple wiring across the central nervous system as a reflex arc.
Diseases
Damage to the cauda equina can lead to interruption of information transmission. As a result, characteristic motor and sensory deficits develop. Both the extent and location of these disturbances depend on which fibers of the cauda equina are affected.Kauda syndrome is the medical term for a flaccid paralysis (paresis) affecting the legs. Sensory disturbances in the legs accompany the motor disorder. Acute Kauda syndrome requires rapid diagnosis by qualified physicians to enable immediate intervention. Depending on the individual cause of the syndrome and other conditions, surgical pressure relief may be possible – for example, if the cauda equina is pinched off due to a herniated disc. In addition to paresis and sensory dysfunction, other symptoms may include problems passing urine (micturition dysfunction) and stool (defecation dysfunction) and sexual dysfunction. In addition, many men with Kauda syndrome have impaired fertility. The neurological clinical picture can accompany damage to the nerve fibers at the conus medullaris as conus cauda syndrome. However, in stand-alone conus syndrome, there is no paresis. Potential causes of cauda syndrome include herniated discs, malformations, and traumatic injuries in this area of the spine. Various types of tumors can also cause the neurological disorder – including lipomas, which arise from fatty tissue. The filum terminale, which begins as a 15-20 cm long thread of connective tissue at the conus medullaris, can become fatty or too short in the context of spina bifida, causing Kauda syndrome in this way.
