Organization of the spinal cord in metameric segments. Relationships with vertebrae and spinal nerves
It has a cylindrical appearance , slightly flattened from front to back. Its average diameter is 1 cm, less than that of the spinal canal, which gives it a relative freedom in the axial plane. It is about 45 cm long and occupies 2/3 of the spinal canal, anchored at its lower end by a fibrous structure, the filum terminale to the coccygeal vertebra and laterally by the denticulate ligament. It comprises two fusiform swellings : the cervical enlargement, corresponding to the nervous structures involved in the innervation of the upper limbs, and the lumbar enlargement, corresponding to the lower limbs.
The spinal cord is connected to the peripheral nervous system by the spinal nerve roots whose regular emission marks a metameric organization mode in superimposed segments. There are 31 pairs of spinal nerves born from the union of ventral motor roots and dorsal sensory roots: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
During fetal life , the spine elongates more than the spinal cord which remains fixed at its cranial extremity so that if the cervical nerves lie in the same plane as the corresponding vertebra, as one goes down, they incline more and more to reach the corresponding intervertebral foramen, resulting in a vertebra-medullary shift of two levels for the thoracic roots and more than three levels in the lumbosacral section, the conus medullaris projecting opposite T12L1. Below, the roots gather to form the cauda equina that occupies the lumbosacral space, thus allowing lumbar puncture without risk of causing spinal cord injury.
The white matter contains the ascending and descending nerve fibers that form tracts that connect the spinal cord to the other sections of the neuraxis. It is organized along an axis of sagittal symmetry which is marked on the surface, anteriorly by the medial anterior sulcus (ventral medial fissure ) and posteriorly by the posterior medial sulcus (dorsal medial fissure ), narrower and in contact with the gray matter. On both sides of this midline plane, the ventrolateral sulcus corresponds to the emergence of the ventral roots of the spinal nerves and the dorsolateral sulcus to that of the dorsal roots. These sulci delineate funiculi or columns of white matter: the dorsal (or posterior) column is located between the posterior medial sulcus and the posterior collateral sulcus. It is subdivided at the level of the cervical segments by an intermediate dorsal sulcus into a lateral tract, inserted as a wedge, the cuneatus fasciculus (Burdach) and an inner tract, the gracilis fasciculus (Goll) . The lateral column is delimited by the ventral and dorsal collateral sulci. The anterior columns are located on both sides of the median fissure and communicate with each other by the ventral white commissure.
The gray matter contains the cells and the nerve centers. It is also arranged along an axis of sagittal symmetry which gives it the shape of an “X” or a “butterfly with spread wings.”
It is described (Figs. 4 and 5) as:
Dorsal or posterior horns do not reach the circumferential edge of the cord where the sensory fibers contained in the dorsal roots of the spinal nerves arrive by the collateral dorsal sulcus. They are separated in the dorsolateral fasciculus of Lissauer that lies in the interval between the surface of the cord and the apex of the dorsal horn. The nucleus proprius occupies most of it, covered by the clearer gelatinous substance (substantia gelatinosa ) and the more distal marginal zone (or substantia spongiosa ) which forms as a hood. The nucleus dorsalis (Clarke’s column ) is recognizable on the posterior edge of the base of the dorsal horns, only at segments C8 to L2.
Ventral or anterior horns are larger. Their jagged edges remain at a distance from the circumferential edge and correspond to the passage of nerve fibers for motor purposes which exit through the ventral collateral sulcus and will constitute the ventral roots of the spinal nerves. They are characterized by the presence of motor neurons grouped in nuclei which are arranged in juxtaposed columns (Fig. 6):
The medial nuclei are present throughout the cord. They correspond to the innervation of the cervical, thoracic, and abdominal axial musculature.
The lateral nuclei are mainly developed at the level of the cervical and lumbar swellings. They correspond to the innervation of the limbs and are arranged according to a double plan of organization: one is functional and corresponds to the ventral nuclei for the extensor muscles and to the dorsal nuclei for the flexor muscles, the other is somatotopic: the medial columns correspond to the axial musculature of the limb roots, the lateral columns to the proximal musculature of the elbow and knees, the retrolateral columns to the distal musculature of the extremities (hand and feet).
At the level of the cervical cord, the central nuclei correspond to the nucleus of the phrenic nerve which extends from C3 to C7. Its fibers borrow the anterior roots of C4 to form the phrenic nerve that descends to innervate the diaphragm. The fibers of the spinal accessory nerve (nerve XI) occupy segments C1–C6. Its fibers, after having emerged along the lateral cord, go up towards the foramen magnum to penetrate into the posterior cerebral fossa and to join the fibers of the medullary root of accessory nerve (XI) to leave posterior fossa by the jugular foramen to innervate the trapezius and sternocleidomastoid muscles.
An intermediate central area between the ventral and dorsal horns, centered by the ependymal canal which is usually obstructed. It is characterized by its reticular organization forming rich networks of interneurons articulating between them and also includes the intermediate columns which belong to the autonomic nervous system involved in vegetative life: the intermediomedial cell column is located along the ependymal canal, is present along all of the spinal cord and the intermediolateral cell column which occupies only the thoracic segments from T1 to L2, where it corresponds to the lateral horn. They both belong to the sympathetic system. The intermedioventral column corresponds to the pelvic parasympathetic system. It is present only at the level of the sacral segments of S2–S4 and specifically intended for the innervation of the viscera of the pelvic cavity.
Anatomofunctional Organization of the Neural Centers of the Gray Matter
A central formation or fundamental zone is the oldest in the phylogenetic scale and corresponds to the intermediate zone and to laminae VII and VIII. It is characterized by a network organization of interneurons in charge of the basic motor and vegetative programs essential to the survival of the individual.
The dorsal horns correspond to laminae I–VI. Their radial organization corresponds to the complex systems in charge of the specific treatment of pain and temperature messages essential for survival by alerting for the environment danger by the information brought to each metameric level by the dorsal roots of the spinal nerves.
The ventral horns correspond to the IX layer and the motor functions from which the orders coming from the spinal and the supraspinal structures converge to form the common final motor pathway. The axons of the motoneurons which constitute them correspond to the ventral roots of the spinal nerves intended for the somatic musculature.
The Central Formation Area
The small interneurons are largely anastomosed to each other and also articulate with equivalent cells of the contralateral cord and with those on the neighboring levels by the intersegmental neurons of the proprioceptive spinal system . They form networks of varying complexity that reproduce on the spinal level what has been described for the reticular substance of the brainstem and where elementary somatic programs are developed. These are present from birth but intended to be refined when they are then invested by supraspinal afferents which will widen their functional potentialities.
Locomotion is an example of such an autonomous mode of activity, just like swimming or birds flying .
Its reality is easily attested by the example of the chicken where its head has been cut off and who is able to continue to run for a few moments, that is, to realize a rhythmic and alternating activation of the flexor and extensor muscles of the two legs.
It has been studied experimentally in lamprey , the spinal cord of which can be easily isolated. The recording of its swimming by a ripple shows a rostrocaudal sequential activation in the form of rhythmic pulses, responsible for contractions of the longitudinal paravertebral muscles. This activity is not reflexive but reflects the intervention of central generators , CPG (central pattern generators), right and left segmentaries connected by commissural interneurons responsible for rhythmicity and alternation (Diagram 1). The functional plan is that of oscillatory systems based on the interaction of commissural inhibitory interneurons.
In mammals, the pattern is identical according to a cycle of increasing complexity to ensure the synchronization of the different limb segments. The stance phase is when the limb touches the ground. It solicits the extensor muscles and it is from there that depend on the variations of the duration of the locomotive cycle. The swing phase , conversely, varies little and involves the flexor muscles.
In newborns, these patterns can be effectively observed in the form of automatic swimming or the cyclical motion of the legs akin to walking. They then disappear with myelination of supraspinal tracts. However, they are likely to reappear clinically, in the form of automatisms (mass reflex, triple withdrawal, crossed synkinesis, and Babinski’s sign) when cord lesions prohibit supraspinal downward influences.
The afferents are segmental, of a proprio- and exteroceptive sensory (external stimuli) nature, adapting to the conditions of the environment. A cat with transected spinal cord is thus able to experimentally adapt its speed of unwinding of the carpet from the proprioceptive information provided by the spinal sensory endings. Similarly, electrical or mechanical stimulation of the cutaneous receptors of the cat’s foot, applied during the swing phase, leads to a considerable increase in the activity of flexor motoneurons and a greater elevation of the limb as to avoid a hypothetical obstacle whereas if it intervenes in the stance phase, it causes on the contrary a reinforcement of the activity of the extensors of the limb so as to compensate a possible unexpected load, or during a misstep creating the sudden failure of the limb (Diagram 1).
The supraspinal afferents intervene to trigger and modulate these motor patterns (Diagram 2)
The corticospinal tracts come from the cerebral cortex investing these interneuronal networks so as to widen the range of possibilities and place them under voluntary control. Babinski’s sign, which is physiological in the newborn, disappears thus when the pyramidal tract imposes its regulating influence so that clinically, when it is found, it is a pathognomonic sign of a pyramidal lesion.
At the level of the brainstem, the nuclei of the mesencephalic reticular formation and the pedunculopontine nucleus have been identified as a locomotor region. Its stimulation in the cat triggers perfectly organized locomotor sequences whose speed follows the intensity of the stimulation. It intervenes as a trigger for spinal patterns. It receives the median forebrain bundle (FMB) which places it in relation with the ventral pallidum, the nucleus accumbens, and the hippocampus. It projects on the pontine reticular formation that activates central pattern generators by reticulospinal tracts. Thus, starting from an initial impulse and because of this central connectivity, the rhythmic generators of the cord enter into activity, with longitudinal and transverse logical coordination and by a harmonious combination of the upper and lower limbs more complex locomotive modules can afterward be acquired such as trotting, galloping, jumping, etc.
The cerebellum intervenes to achieve the necessary locomotor adjustments that occur through the spinocerebellar pathways organized into feedback loops where rapid conduction velocity (120 m/s) allows almost an instantaneous response:
The ventral spinocerebellar tract originates in the pericornual zone near the ventral motor horns and therefore informs the cerebellum directly of the activity of the central pattern generators (CPG) as soon as they come into play and as a reference copy.
The dorsal spinocerebellar tract originates in the Clarke dorsal nucleus, which is based on the dorsal horn that receives proprioceptive information from muscle and the neuromuscular spindle during movement and relates to the motor function of the lower limbs. Its equivalent for the upper limbs is represented by the cuneocerebellar tract . These bundles are projected on the somatotopically organized spinocerebellum (paleocerebellum) and on the corresponding intermediate nuclei (globulus and emboliform nuclei).
This double cerebellar device thus allows the cerebellum to compare the copy of the schedulated spinal pattern and its effective realization at the level of the muscular effectors and to make the necessary adjustments by acting on the spinal networks. They are explored clinically for the lower limb by the “sensibilisated” Romberg test and the Stewart-Holmes maneuver for the upper extremity.
The spinocerebellospinal feedback loops involve brainstem structures:
The rubrospinal tract originates from the mesencephalic red nucleus, which receives cross afferents from the paleo (or spino) cerebellum. It controls the activity of the flexor muscles of the limb thus involved during the swing phase in locomotion.
The ventromedial reticulospinal tract that originates from the pontine reticular nuclei alternatively intervenes during the stance phase during locomotion being a facilitator of the extensor muscles.
The vestibulospinal tract provides postural balance essential for locomotion that requires constant adjustments due to the imbalance created by bipodal and alternating walking in humans. Its facilitating action is bilateral and is exerted on the extensors of the anti-gravity axial muscle chains on which the verticalization depends.
The central formation also contains the intermediate columns that belong to the autonomic system .
The parasympathetic system has a trophotropic function and predominates under physiological conditions. Its action is local and relies on short postganglionic cholinergic neurons, located near and in the wall of the viscera. At the level of the digestive tract, it captures the intrinsic enteric system consisting of networks of interneurons located in the submucosal and myenteric plexuses that already reflexively provide intestinal peristalsis. This function is largely controlled by the vagus nerve (X). Only the viscera contained in the pelvic cavity receive parasympathetic innervation of spinal origin. It corresponds to the parasympathetic sacral or pelvic contingent that comes from the intermedioventral column present on the sacral segments S2–S4 (Fig. 8). Preganglionic fibers are the pelvic splanchnic nerves that accompany the spinal nerves and directly enter the external genitalia, bladder, distal portion of the colon and rectum, as well as the internal vesical and anal sphincters. The relay ganglia are located in contact with or even in the wall of these viscera making the postganglionic effector neurons very short.
The sympathetic system , on contrary, has an ergotropic purpose. It is involved in emergency or warning situations where energy reserves must be mobilized to respond to threats from the external environment. The relay ganglia are therefore remote from the effector so as to cover a large functional area. Its nerve centers are all located at the level of the spinal cord and correspond to two modes of organization, one visceral, the other somatic 
The visceral sympathetic contingent (Fig. 9) is intended for the innervation of the viscera itself, and its action is most often in opposition to that of the parasympathetic system. The nerve centers are contained in the intermediolateral columns that occupy the lateral horns of segments T8 to L2. The preganglionic fibers accompany the ventral roots of the spinal nerves for a short distance, then follow the white communicating branches and pass directly through the ganglia of the paravertebral chains, without any synaptic articulation. They constitute the splanchnic nerves that join the prevertebral and preaortic relay ganglias, celiac ganglia, and superior and inferior mesenteric ganglias. The postganglionic fibers are noradrenergic and are distributed to the viscera of the abdominal and pelvic cavities, forming with the visceral arteries neurovascular pedicles. The adrenal gland, which is a chromaffin organ and secretes adrenaline, directly receives preganglionic fibers. For the organs contained in the thoracic cavity (heart, lungs, trachea, esophagus) and those of the cephalic extremity (head and neck), the relay ganglia are represented by the three cervical ganglias and the first four ganglias of the paravertebral chain. Postganglionic fibers join their target by forming periarterial plexuses surrounding the carotid and subclavian vessels and cardiac plexuses (Fig. 8).
Vegetative reflexes are produced by various sensory afferents stimuli coming from the viscera or the skin. Thus, the thermal information conveyed by the Aδ and C sensory fibers from their exteroceptive origin allows local thermoregulation so that the application of cold causes arterial vasoconstriction with paleness of the limb and piloerection whereas, conversely, an increase in temperature leads to vasodilation and sweating to create heat loss.
This mode of reflex functioning is particular to the parasympathetic pelvic system and responsible for the physiological control of urination, defecation, and sexual reflexes.
The supraspinal afferents come from the nuclei of the medullopontine lateral reticular zone (superficial reticular area and nuclei of the pontine tegmentum) where the pneumotaxic, inspiratory and expiratory respiratory centers, the cardioregulatory and vasomotor centers, and the micturition center have been located. The noradrenergic fibers visible in histofluorescence (Fig. 26) descend into the lateral spinal cord and terminate on the intermediolateral and intermedioventral nuclei and on the phrenic and intercostal nuclei. They correspond to the control exerted by the hypothalamus where the ergotropic and trophotropic regulatory centers are located in relation to the limbic system. They integrate them into behavioral patterns of an emotional nature, for example, but also dietary, sexual, or thermoregulatory factors involved in homeostasis.
The Aδ fibers terminate on the I and II layers and on the deep V layer. The amyelinic C fibers terminate on II layer only. Many substances are involved in the transmission of the nociceptive transmission: substance P in the superficial layers I and II but also other peptides: somatostatin, vasoactive intestinal peptide, cholecystokinin, CGRP (calcitonin gene related peptide), and excitatory amino acids (glutamate).