The conus medullaris is the cone-shaped end of the spinal cord. Paraplegia at the conus medullaris is known as conus syndrome and results in various disorders due to the failure of the nerves supplying the spinal cord. The condition may also present as conus cauda syndrome.
What is conus medullaris?
The conus medullaris forms the lower end of the spinal cord and is located at the level of the first to second lumbar vertebrae. In children and adolescents, however, its position may differ because the spinal cord does not grow at the same rate as the spinal column, inside which the spinal canal (canalis vertebralis) containing the spinal cord runs. In addition to the spinal cord, the spinal canal contains the cauda equina, which consists of spinal nerve roots. Together with the brain, the spinal cord forms the central nervous system and is also known as the medulla spinalis. The name of the conus medullaris means “medullary cone” and alludes to the shape of the anatomical structure.
Anatomy and structure
At the inferior (caudal) end of the spinal cord lies the conus medullaris. Its shape is cone-shaped, with the broader portion pointing upward and its lower portion becoming progressively narrower. In adult humans, the conus medullaris usually extends from the first to second lumbar vertebrae. This section of the spinal cord is part of the lumbar medulla, which extends to the fifth lumbar vertebra. Connected to the lumbar medulla is the sacral medulla or sacrum, which eventually opens into the coccyx. The conus medullaris receives oxygen, glucose, and other nutrients primarily through the anterior spinal artery and the two posterior spinal arteries. Some neonates have a connection between the conus medullaris and the central canal (canalis centralis). This junction is known as the ventriculus terminalis and, like the central canal, contains cerebrospinal fluid and an inner wall lining of ependyma. The ventriculus terminalis represents a rudiment that embodies a remnant from the evolutionary development of humans: It has no function. Caudally, the conus medullaris merges into a 15-20 cm long cord of connective tissue, the filum terminale. The connective tissue originates at the pia mater spinalis, which together with the arachnoid mater spinalis forms the soft skin of the spinal cord. Above it lies the dura mater spinalis or hard skin of the spinal cord.
Function and tasks
The conus medullaris represents a portion of the spinal cord and, as such, plays an important role in the transmission of neuronal signals as well as in the interconnection of nerve cells. Afferent neural pathways rise in the spinal cord and relay information originating from the peripheral nervous system that runs throughout the body. In the context of the conus medullaris, this primarily involves sensory fibers. Conversely, efferent fibers carry signals from the brain to the periphery via descending nerve tracts. Among them is motor information, which serves to control movements. However, the nervous system does not always rely on wiring via the brain; motor reflexes in particular run partly via the spinal cord. For diagnostic purposes, neurologists therefore consult the examination of such reflexes to determine possible disorders in the spinal cord. Nerve tracts passing through the conus medullaris are responsible for anal reflex and the ejaculatory reflex (bulbocavernosus reflex). The nerve cell bodies of the medulla are located in the gray matter, which in cross-section forms a butterfly-shaped structure inside the medulla. The nerve cell bodies continue into the axons, which are surrounded by an insulating layer of myelin that gives the tissue its white coloration. Accordingly, neurophysiology refers to this layer as white matter. Its function is to transmit the action potentials that originate in the nerve cell bodies. Spinal ganglia, located laterally from the spinal cord, switch some of the nerve fibers to other neurons. However, the switching may occur later or may be absent.
Diseases
Cone syndrome is one of the paraplegic syndromes. The area affected is that innervated by the damaged spinal cord nerves. Causes of cone syndrome include external injury, herniated disc, tumors, or shortening of the filum terminale.A neural tube defect called spina bifida can lead to various forms of spinal cord disease during prenatal development, including a shortened filum terminale. Spina bifida is one of the occlusive disorders and can take on different degrees of severity. The conus syndrome typically manifests itself in the form of problems with the release of urine (micturition disorders) and stool (defecation disorders), since the body can no longer control the responsible muscles. Sensory perception in the lower part of the body is also reduced; this symptom manifests as so-called riding pants anesthesia and includes the buttocks, the inner thighs, and the genital area. Sexual functions are also impaired – the leg muscles, however, are not affected in cone syndrome. However, if cone syndrome occurs in combination with cauda syndrome, the leg muscles suffer from flaccid paralysis (paresis). The conus cauda syndrome is characterized by additional damage to the nerve tracts that lie below the conus medullaris. With the help of imaging techniques such as computed tomography, doctors can determine the cause in each individual case and identify individual treatment options. For example, in the case of a tumor, surgical removal, radiation therapy and/or chemotherapy may be considered, while in the case of conus cauda syndrome following a herniated disc, surgery is necessary in many cases to prevent more severe damage. The success of treatment varies depending on the underlying cause and individual factors.
