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Spinal Nerve Supply

Spinal cord

There are 31 pair of spinal nerves. With the exception of C1 all nerves have a ventral and dorsal nerve root exiting through the intervertebral foramen. This division somewhat resembles the tongue of a snake as it is bifid.  The ventral nerve root enters the anterior horn while the dorsal nerve root enters the posterior horn. C1 exits between the cranial occipital bone and 1st cervical vertebra, and frequently does not have a dorsal root.  C2 exits between the axis and atlas (or between C1 & C2). C8 exits between the 7th cervical vertebra and the 1st thoracic vertebra.  The first thoracic nerve exits between T1 & T2. Therefore in the cervical region the respective nerves exit above the referenced segment with the exception of C8, where there is not a corresponding vertebra. In the thoracic, lumbar and sacral regions the reference segmental nerve supply exits beneath the referenced vertebral segment.

The upper cervical nerves associate with cranial nerves as referenced in our cranial nerve section. Additionally they form the cervical plexus. Plexi are a network of nerves. The spine has five plexi which frequently overlap. The names of these plexi are: the cervical, brachial, lumbar, sacral and coccygeal. The thoracic spine for the most part is not involved in the plexi.

All spinal nerves consist of mixed fibers both sensory & motor. This allows for neurotransmission from the sensory receptors to the spinal cord or afferent transmission. The dorsal root is responsible for the conducting of sensory transmission.    Efferent transmission is conducted from the spinal cord to the muscles for motor response as well as for the conducting of the sympathetic & parasympathetic pathways.  This efferent transmission is predominantly conducted through the ventral root. The dorsal root merges with the posterior horn and the ventral root merges with the anterior horn. The spinal roots traverse laterally and merge slightly distal to the dorsal root ganglion to form a spinal nerve. At which time they traverse through the intervertebral foramen of their respective segment and form the rami. The rami can either divide or return back towards the spine to provide nerve supply to the meninges, vertebra and ligaments or move into the periphery to supply the skin, muscles and viscera. 

The sympathetic chain consisting of 22 ganglia emerge from the ventral root and involve three cervical segments, eleven thoracic segments, four lumbar segments, and four sacral segments. And are responsible for elevated heart rate, increased electrical activity of the brain, deep and rapid breathing, and dilation of blood vessels, eyes and galvanic skin response. Or in other words, it is the system which conducts the “fight or flight” mechanism.

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The parasympathetic or craniosacral division is more simplistic anatomically than the sympathetic division of the autonomic nervous system, due to its preganglionic neurons being located in the brain stem and the sacral region.  Its postganglionic neurons are located in close approximation to the organ to be supplied. While the parasympathetic division is considered to be a supportive system for the sympathetic, frequently they have an antagonistic relationship.   This antagonistic relationship must be carefully balanced and regulated. The parasympathetic division is active during rest. This is when it provides digestion and the conservation of energy, however, should you eat a large meal and immediately jog, you have thrown these two divisions into direct opposition. 

Hilton’s Law states: “a nerve trunk which supplies the muscles of any given joint also supplies the muscles which move the joint and the skin over the insertions of such muscles.”

Based upon this law and supporting EMG studies, we can assume that underlying dermatomes are residing myotomes and sclerotomes with resulting sensory and motor dysfunction.  Should there be an organic or biomechanical encroachment or compression affecting the ventral nerve root you would anticipate autonomic impairment and subsequent viscerotomes. The most obvious evidence of a dermatome pattern is the lesions produced by herpes zoster. As this infection predominantly affects the dorsal root ganglia of the thoracic segments, dermatomal patterns are outlined by defined pain, hyperesthesia and pustules. However, it can occur at any level of the spine and following the active state of the lesions post-herpetic neuralgia may be experienced periodically for years, and is usually predicated by stress and a compromised autoimmune system. Radiculopathies also follow these pathways with resulting dysesthesia.

The spinal nerves have overlapping supply thus serving as a protective mechanism against injury as it pertains to nerve innervation of a given structure or organ.  Segmental supply as referenced below is derived from consistencies from Gray’s Anatomy, Correlative Neuroanatomy by Waxman & deGroot, Human Anatomy and Physiology by Dr. Marieb and various EMG studies. The reader should note that innervation can occur within a segment or two above or below our specific reference depending upon the individual.

Below are links to data tables containing information about spinal nerve segment, plexus, innervation of muscle and visceria, and dermatome pattern for each spinal region.

C1 through C8 - the cervical region

T1 through T12 - the thoracic region

L1 through L5 - the lumbar region

S1 through S5 + coccyx - the sacro-coccygeal region

 

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