Nerve roots are the connection between the peripheral and central nervous systems. They are located in the spinal canal of the spinal cord, where a spinal nerve carries one anterior and one posterior nerve root. Herniated disc is the best-known condition that can cause nerve root syndrome, with symptoms such as numbness and paralysis.
What is the nerve root?
The central nervous system consists of the nerve tissue in the brain and the spinal nerves of the spinal cord. The extensions of the nerve cells are called axons. They receive communicate with each other, receive stimuli from other nerve cells, and carry these stimuli away from the cell body of the nerve cell, called the soma. The nerve cells of the spinal cord also have nerve roots. These are nerve fibers that exit or enter the spinal cord in segments. The individual fibers of many nerve roots meet in the intervertebral canal in the form of a spinal nerve. Each spinal nerve has two roots: an anterior nerve root and a posterior nerve root. The anterior roots are efferents for transmitting signals to the peripheral nervous system. The posterior roots, in turn, are afferents that carry signals from the central to the peripheral nervous system. In the spinal cord, one nerve cell body counts as the anterior root of each nerve, and in this case is also called a root cell. Charles Bell and François Magendie first recognized the functional separation of the two nerve roots of each spinal nerve and documented it in the Bell-Magendie law. A certain nerve root area near the point of entry into the spinal cord is considered the transition zone of the peripheral and central nervous systems and is referred to as the Redlich-Obersteiner zone.
Anatomy and structure
Nerve roots are located in the spinal canal. Each individual segment of the spine has two nerve roots on the right and left sides. These two roots fuse in the spinal canal to form the spinal nerve and exit the spinal canal through the intervertebral foramina, or intervertebrae. Per segment, the human spine has anterior and posterior nerve roots. Posterior nerve roots each originate with their chamfers from the sulcus lateralis posterior between the spinal cord lateral cord and the spinal cord posterior cord. Anterior spinal nerve roots arise with their fibers from the sulcus lateralis anterior between the spinal cord anterior cord and spinal cord lateral cord. Near the point of entry into the spinal cord, each nerve root bears a so-called Redlich-Obersteiner zone. This zone forms the boundary between the central nervous system and the peripheral nervous system and is located in the area where the posterior root of a spinal nerve enters the posterior horn of the spinal cord. In this area, the afferent nerve fibers appear marbless but bear thinned medullary sheaths. The last Ranvier’s cord ring of each axon marks the transition. The posterior roots do not bear a basement membrane at this point.
Function and tasks
The spinal nerve roots connect the central nervous system to the peripheral nervous system. This connection is critical to every bodily process. Without the nerve roots, commands from the brain would not reach the effectors in the body and thus could not be carried out by muscles, glands, or organs. Thus, the body would not be viable. The central nervous system controls all conscious and unconscious body processes and thus gives the body its ability to survive. The control of body processes by the central nervous system is not only dependent on nerves conducting stimuli to the periphery of the body, but also on nerve pathways from the peripheral nervous system. The former nerve pathways are called efferents. The latter are called afferents. The afferent nerve fibers each enter the spinal cord and thus the central nervous system via the anterior nerve root and thus supply the central nervous system with sensitive information from the periphery, which is transported in the form of excitation. This sensitive information is, for example, notification of the current state of tension in the muscles or the position of joints. The central nervous system needs such information to issue commands in that it can only distribute targeted movement commands to the muscles with this information. The nerve cell bodies of the afferent fibers are located in the spinal ganglion, where efferent nerve fibers also emerge from the spinal cord. Efferent fibers of the nerve roots transmit motor commands to the muscles.The associated nerve cell bodies are located within the anterior horn of the spinal cord in the gray matter. The anterior nerve roots are the roots of the efferent fibers.
Diseases
The best-known nerve root damage is disc herniation. This is a sudden onset or slowly progressive displacement of disc tissue of the nucleus pulposus. Herniated discs can compress the spinal cord and pinch the nerve roots. The clinical picture is also referred to as degenerative spinal disease, since the cause is a degenerative change in the intervertebral disc or adjacent structures. The annulus fibrosus tears and the nucleus pulposus falls forward. Herniated discs usually occur in typical localization and are then located in the lower lumbar spinal segments, where they cause nerve root syndromes. This symptom complex is caused by mechanical irritation of spinal nerve roots. In addition to disc herniation, tumors, infections or fractures of the vertebrae may be involved in nerve root syndrome. The most important symptom of the clinical picture is more or less severe pain, which can radiate from the area of the lumbar spine to all regions of the body. Lumbago, for example, is also a nerve root syndrome. In addition to pain, sensory loss and paresthesias may occur in the area supplied by the affected nerve root, i.e., numbness and other sensations of discomfort. These symptoms are due to damage to sensitive portions of the nerve roots. Since each segment of the spine also carries motor parts in the anterior nerve roots, a nerve root syndrome may additionally be accompanied by paralysis. In this case, the motor disturbances occur in the supply area of the efferent nerve root fibers.