Spinal nerves

Nervi spinales

Definition

The spinal nerves constitute part of the peripheral nervous system. They contain sensory, motor and autonomic neurons which transmit neural information between the spinal cord and the peripheral body structures.

Spinal nerves arise as paired nerves from the spinal cord, with each nerve-pair arising from its respective spinal cord segment. Since, the spinal cord comprises of 31 segments, hence, there exist 31 pairs of spinal nerves in the human body. They are sub-classified into eight cervical (C1-C8), twelve thoracic (T1-T12), five lumbar (L1-L5), five sacral (S1-S5) and one coccygeal (Co) spinal nerves.

Structurally, spinal nerves arise as ventral (motor) and dorsal (sensory) nerve roots, from the anterior (ventral) and posterior (dorsal) horns of the spinal cord. The motor roots emanate from the anterolateral sulci, whereas the sensory roots enter the posterolateral sulci of the spinal cord. The two types of roots combine to form spinal nerves inside the intervertebral foramina. Therefore, the spinal nerves are mixed structures containing both motor and sensory nerve fibers.

While learning the exit pathway of spinal nerves via the intervertebral foramina, it is important to understand the discrepancy between the number of cervical vertebrae and the cervical spinal nerves. In general, each cervical spinal nerve exits the intervertebral foramen, ‘above’ its corresponding vertebrae (for instance, C1 spinal nerve above the C1 vertebra). However, since there are eight cervical spinal nerves but only seven cervical vertebrae, therefore, the eighth cervical spinal nerve (C8) and all others beneath it, exit the intervertebral foramen ‘below’ their corresponding vertebrae. For instance, C8 spinal nerve below the C7 vertebra and T1 spinal nerve below the T1 vertebra, and so on. Knowing this anatomical fact, helps us understand how an L4/L5 posterolateral disc herniation could lead to compression of the L4 spinal nerve exiting the intervertebral foramen beneath the L4 vertebra.

The upper spinal nerves are usually horizontally poised inside the vertebral canal, however, since, the spinal cord falls short of the length of the vertebral canal–ending at L1/L2 vertebral level–therefore, the lower lumbar and sacrococcygeal spinal nerves descend a longer distance, prior to exiting their respective intervertebral foramina. This leads to bunching of spinal nerve roots in the lower vertebral canal, called cauda equina. The region below the L3 vertebrae serves as a safe site to perform lumbar puncture, as there is no spinal cord here, however, the subarachnoid space still exists.

Each spinal nerve, as it exits the vertebral canal via the intervertebral foramen, splits into anterior and posterior rami. The posterior rami are smaller and innervate vessels and glands in the posterolateral body wall. The anterior rami are larger and form somatic plexuses, including the cervical, brachial, lumbosacral and coccygeal plexuses. Both anterior and posterior rami–like the spinal nerves–are mixed structures, containing both sensory and motor neurons.

In addition to the somatic motor and sensory neurons, the spinal nerves also serve as conduits for the autonomic sympathetic neurons. The anterior rami of spinal nerves are connected with the sympathetic trunks via neuronal connections called the gray and white rami communicantes. The white rami communicantes are found only with the first thoracic to the second lumbar (T1-L2) spinal nerves and allow the pre-ganglionic sympathetic neurons to pass from the anterior rami into the sympathetic trunk. These neurons synapse with cell bodies of post-ganglionic sympathetic neurons inside the ganglia of the sympathetic chains. From here, the postganglionic sympathetic neurons re-enter all spinal nerves via their gray rami communicantes. Once back inside the spinal nerves, the post-ganglionic neurons get distributed via the anterior and posterior rami to various body parts.

Text written by Muhammad A. Javaid, MD, PhD © 2022 IMAIOS.

  • Kaiser, J.T. and Lugo-Pico, J.G. Neuroanatomy, Spinal Nerves. [Updated 2022 Aug 22]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK542218/


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