The human spine is the basis of the musculoskeletal system. At the same time, it performs a supporting function, provides the possibility of walking upright, and represents a fairly flexible axis of the body, which is achieved due to the mobility of the vast majority of its parts. In this case, the front part of the spine is involved in forming the walls of the thoracic and abdominal cavities. But one of its most important functions is to ensure the safety of the spinal cord that passes inside it.
At the level of each vertebra, spinal roots depart from the corresponding segments of the spinal cord in pairs. They pass through natural openings formed by the processes of the vertebrae. There are also blood vessels that provide nutrition to the spinal cord. The change in the spine’s position is carried out with the help of muscles attached to the bodies of the vertebrae. Due to their reduction, the body bends, and relaxation leads to the restoration of the normal position of the vertebrae (Leo, 2022). The cervical spine has the greatest mobility. There are two vertebrae in it, the structure of which is very different from the others since they must provide a connection of the vertebral column with the bone structures of the head.
The thoracic region is the least mobile; it has direct connections with the ribs, which provokes the appearance of the corresponding anatomical features of the vertebrae of this department. In general, it provides organ protection and body support. The lumbar spine is distinguished by massive vertebrae that take on the bulk of the body weight. The sacrum formed by the 5th fused vertebrae helps maintain the body’s vertical position and participates in load distribution. The last part of the spine, the coccyx, serves as a place of attachment of ligaments and other anatomical structures.
In terms of the neural properties of the sections, a spinal cord possesses 31 pairs of nerves that stem from their respective roots that are presented along the spine. The cervical section has eight of such pairs between C1 and C7 vertebrae. They are parts of the peripheral nervous system that transmit the signals emitted by the brain into the corresponding parts of the body. In this regard, the cervical nerves are primarily responsible for the functioning of the upper body parts (Mahadevan, 2018). For example, C4 is responsible for shoulder movement, C1-C3 control the head and the neck, whereas C8, the lowest nerve pair of the cervical spine, is connected to the hands and fingers. Therefore, any damage to this section of the spine results in arm and neck being severely affected.
Next, on the level of the ribcage, the thoracic spine enables the normal functioning of the vital organs through 12 nerve roots. For example, nerves T3, T4, and T5 control the muscles that enable the respiratory functions of the body by moving lungs and the chest. Thus, fractures and hernias in this area exhibit themselves by severe pain around the ribcage and various chest organ disfunctions.
Finally, there are four nerve pairs in the lumbosacral area of the spine that stem from the basis of the lumbar plexus. The upper ones are connected to the abdomen, enabling the functions of the organs in the lower part of the torso. For example, L1 and L2 branch into a major nerve that controls the functions of the genitalia (Mahadevan, 2018). The ones at the bottom of the lumbosacral area branch into the legs through thighs, thus controlling the lower limbs of the body. These are some of the largest nerves in a human organism, and any damage dealt to them is likely to result in severe musculoskeletal dysfunctions. One of such conditions is lumbosacral plexopathy that results from damaged nerve bundles in the lumbar or sacral plexus.
The autonomic nervous system (autonomous) is involuntary; for example, it is not controlled by consciousness. The somatic nervous system is arbitrary. The autonomic nervous system innervates internal organs, glands of external and internal secretion, blood and lymphatic vessels, smooth and skeletal muscles, and the central nervous system, maintaining the constancy of the body’s internal environment. The somatic nervous system innervates the striated musculature. The reflex arc of both somatic and vegetative reflex consists of three links: afferent (sensory, sensitive), insertion (associative), and effector (executive) (Choi, 2019). The afferent link may be common to somatic and vegetative reflex arcs. However, in the autonomic nervous system, the effector neuron is located outside the spinal cord or brain in the ganglia.
The fibers of the autonomic nervous system exit the central nervous system only in certain areas of the brain, thoracolumbar and sacral spinal cord. In the intra-organ department, reflex arcs are completely located in the organ and have no exits from the central nervous system. The fibers of the somatic nervous system exit the spinal cord segmentally throughout and overlap with innervation at least three adjacent segments. Vegetative nerve fibers are less excitable than somatic ones and have a longer refractory period, greater chronaxia, and less lability. Therefore, a stronger irritation is necessary for their excitation than for somatic fibers. The axons of somatic neurons are long; they are not interrupted throughout. The autonomic nerve fibers are interrupted in the ganglia. The mediator of the somatic nervous system is only acetylcholine. In the autonomic nervous system, the mediator function is performed by several substances: acetylcholine, norepinephrine, ATP, serotonin.
Overall, the structure of the three levels is similar in nature, as they consist of nerve pairs that derive from the roots above vertebrae. From each section of the spinal cord, major nerves branch into the various parts of the body, ensuring its proper functions. However, there are key differences in terms of the autonomous-somatic system distribution between the segments. More specifically, the thoracic level is mostly responsible for autonomous functions that control organs. It is responsible for the impulses that power the heart and the respiratory system, and they function on autonomous level that does not depend on the conscious intention from the brain. On the other hand, the key functions of the cervical and lumbosacral levels are somatic in the form of limb and head movement, although there is a certain share of autonomous ones, as well.
Choi, D. B., Nam, G., Groh, D. M., Syed, S., Fridley, J. S., & Gokaslan, Z. L. (2019). Spinal cord anatomy. In Spinal Cord Tumors (pp. 43-53). Springer, Cham.
Leo, J. (2022). The Three Long Tracts and Spinal Cord Lesions. In Medical Neuroanatomy for the Boards and the Clinic (pp. 1-14). Springer, Cham.
Mahadevan, V. (2018). Anatomy of the vertebral column. Surgery (Oxford), 36(7), 327-332.