Ventral root

The dorsal root contains afferent, or sensory, neurons. Impulses in these neurons travel from peripheral tissues toward the spinal cord. The ventral root contains efferent, or motor, neurons. Impulses in these neurons travel away from the spinal cord toward the peripheral tissues. 

Figure 7.1 Cross-sectional view of the spinal cord. In contrast to the brain, the gray matter of the spinal cord is located internally, surrounded by the white matter. The gray matter consists of nerve cell bodies and unmyelinated intemeuron fibers. This component of the spinal cord is divided into three regions the dorsal, lateral, and ventral horns. The white matter consists of bundles of myelinated axons of neurons, or tracts. Each segment of the spinal cord gives rise to a pair of spinal nerves containing afferent and efferent neurons. Afferent neurons enter the spinal cord through the dorsal root and efferent neurons exit it through the ventral root.

The cell bodies of somatic motor neurons are found in the ventral horn. The axons of these neurons exit the CNS through the ventral root of the spinal nerve and innervate skeletal muscles. The two types of motor neurons located in the ventral horn are ... 

The spinal cord is the major route of input and output for the centrai nervous system. It is covered by the meninges and sits within the vertebral column. There are 31 pairs of spinai nerves, exiting at the dorsal and ventral roots of the spinal cord. The spinal cord is divided into four levels (superior to inferior) cervical, thoracic, lumbar, and sacral. A cross section of the spinal cord shows a butterflyshaped core of gray matter (neuronal soma), and an outer surrounding area of white matter (axon tracts). 

Surprisingly, the vanilloid receptor ligand capsaicin supplied the first experimental evidence for the association between SP and nociception (Gasparovic et al., 1964). Capsaicin depletes small primary afferents of at least SP, if not all of their peptide content, and this was accompanied by hypoalgesia. SP depolarizes the ventral root of an isolated rat spinal cord preparation (Konishi and Otsuka, 1974), and has also been shown to excite and/or depolarize neurons in the dorsal root (Urban et al., 1985). Furthermore, in studies on the larger laminae IV and V neurons, the selective agonist, [Sar9,... 

Dendrobine at a dose of 3 X 10-5 M reduced the dorsal root potential and reflex. It provoked a mild hyperpolarization in both dorsal and ventral roots of frog isolated spinal cord. It affected the (3-alanine- and taurine-induced depolarization of primary afferent terminals and reversibly blocked the presynaptic inhibition caused by antidromic conditioning stimulation of the ventral root potential induced by repetitive antidromic stimulation of ventral root and lowered maximum. It would be interesting to learn whether further research of the Dendrobium species discloses any alkaloid interfering with the glycinergic system, an aspect discussed under the following heading. 

Dissect the vertebral column. Note the cell bodies of lumbar motor neurons are more rostral than their exit points, i.e., L4 cells bodies are in upper lumbar or even lower thoracic vertebrae, even though the spinal nerve exits at the fourth lumbar vertebrae. The more caudal vertebral column contains only the cauda equina, the dorsal, and ventral roots of lower lumbar and sacral motor neurons (Fig. 20.2). 

Fig. 20.2. The spinal cord and cauda equina. The caudal spinal cord does not completely fill the caudal vertebral column. As a result, the dorsal and ventral roots of lumbar and sacral neurons are very long, spanning from the more rostral cell bodies to the more caudal exit points. Therefore, the dissected spinal cord resembles a horse s tail. The practical effect of this anatomy is that it becomes very hard to reliably determine the level of the spinal cord that is being studied in any given cross-section.

Cut the ventral root free at the bifurcation point and as close to the spinal cord as possible (NB ventral roots enter cord proximal to spinal nerve entry point). 

Fig. 20.3. Dorsal and ventral root dissection. The anatomy of the ventral and dorsal roots during dissection is shown. Mouse is prone (rostral is up) and the left sciatic nerve (Sci) has been exposed along the femur and to its origin at the lumbar spinal nerves. The spinal nerves are labeled (lumbar 3 through 6). The ischium (Isch) has been removed to follow the sciatic nerve. The inset shows the L4 root separated into its ventral and dorsal roots, note the DRG associated with the dorsal root. The scale bar is 6 mm. (Color figure is available online).

Fig. 20.4. An example of motor neuron loss reflected in ventral root axon number. The L4 ventral root stained for myelin is shown in cross-section in (A) a control mouse, (B) an nmd mouse (Ighmbp2 mutant), (C) an nmd mouse carrying a modifier locus from CAST, and (D) an nmd mouse with transgenic rescue of the Ighmbp2 gene driven in the nervous system. For details, see (39).

In combination with counts of ventral roots, femoral axon counts can be used to distinguish peripheral neuropathy from motor neuron death. The nerve has a primarily motor branch that innervates the quadriceps, and a primarily sensory branch that becomes the saphenous nerve more distally (Fig. 20.5). 

Nerves should be plastic embedded and cross-sectioned as above for the ventral roots. 

Habler et al. (1990, 1992) examined the functional properties of unmyelinated and myelinated primary afferent neurons innervating the pelvic viscera in anesthetized cats. The axons were isolated from the intact dorsal root and the intact or chronically de-efferented ventral root of segment S2. The responses of the neurons were studied with natural stimulation of the urinary bladder using innocuous or noxious increases of intravesical pressure. 

Zhang Y, Goodyer C, LeBlanc A (2000) Selective and protracted apoptosis in human primary neurons microinjected with active caspase-3, -6, -7, and -8. J Neurosci 20 8384-8389 Zhang B, Tu P, Abtahian F, Trojanowski JQ, Lee VM (1997) Neurofilaments and orthograde transport are reduced in ventral root axons of transgenic mice that express human SODl with a G93A mutation. J CeU Biol 139 1307-1315... 

At cross section, the gray matter is the cord from the anterior and posterior columns, which contains the motoneurons and sensory neurons, respectively. An important feature of each spinal cord segment is the presence of a pair of roots (or associated branches or rootlets) called the dorsal and ventral roots. The dorsal roots contain sensory axons whereas the ventral roots contain motor axons. These sensory and motor axons, which are part of the peripheral nervous system, become mixed in the spinal nerves en route to their peripheral targets. The spinal nerves, which are also components of the peripheral nervous system, transmit sensory information to the spinal cord and motor commands to the muscles and viscera. 

Silperisone (32) is a sodium channel blocker acting centrally as a muscle relaxant. The compormd has been in preclinical animal studies [81, 82] and has been proposed recently to have potential as an antispastic drug in demyelinating diseases such as multiple sclerosis. In vivo, silperisone depressed ventral root reflexes and excitability of motoneurons. Whole-cell measurements in dorsal root ganglion cells revealed that silperisone depressed voltage gated sodium channel conductance at concentrations that inhibited spinal reflexes. 

Li L, Xian C J, Zhong J H, et al. (2003). Lumbar 5 ventral root transection-induced upregulation of nerve growth factor in sensory neurons and their target tissues a mechanism in neuropathic pain. Molec. Cell. Neurosci., 23 232-250. 

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