The axons from these cells pass uncrossed to the lateral funiculus and form the dorsal posterior spinocerebellar tract DSCT , which subserve unconscious proprioception from muscle spindles and Golgi tendon organs to the cerebellum, and some of them innervate spinal interneurons.
The dorsal nucleus of Clarke is found only in segments C8 to L3 of the spinal cord and is most prominent in lower thoracic and upper lumbar segments. The homologous dorsal nucleus of Clarke in the medulla is the accessory cuneate nucleus, which is the origin of the cuneocerebellar tract CCT. Intermediolateral nucleus is located in the intermediate zone between the dorsal and the ventral horns in the spinal cord levels.
Extending from C8 to L3, it receives viscerosensory information and contains preganglionic sympathetic neurons, which form the lateral horn.
A large proportion of its cells are root cells which send axons into the ventral spinal roots via the white rami to reach the sympathetic tract as preganglionic fibers. Similarly, cell columns in the intermediolateral nucleus located at the S2 to S4 levels contains preganglionic parasympathetic neurons Figure 3. Lower motor neuron nuclei are located in the ventral horn of the spinal cord. The a motor neurons are the final common pathway of the motor system, and they innervate the visceral and skeletal muscles.
The distribution of cells and fibers within the gray matter of the spinal cord exhibits a pattern of lamination. The cellular pattern of each lamina is composed of various sizes or shapes of neurons cytoarchitecture which led Rexed to propose a new classification based on 10 layers laminae.
This classification is useful since it is related more accurately to function than the previous classification scheme which was based on major nuclear groups Figure 3. Laminae I to IV, in general, are concerned with exteroceptive sensation and comprise the dorsal horn, whereas laminae V and VI are concerned primarily with proprioceptive sensations. Lamina VII is equivalent to the intermediate zone and acts as a relay between muscle spindle to midbrain and cerebellum, and laminae VIII-IX comprise the ventral horn and contain mainly motor neurons.
The axons of these neurons innervate mainly skeletal muscle. Lamina X surrounds the central canal and contains neuroglia. Rexed lamina I — Consists of a thin layer of cells that cap the tip of the dorsal horn with small dendrites and a complex array of nonmyelinated axons. Cells in lamina I respond mainly to noxious and thermal stimuli. Lamina I cell axons join the contralateral spinothalamic tract; this layer corresponds to nucleus posteromarginalis.
Rexed lamina II — Composed of tightly packed interneurons. This layer corresponds to the substantia gelatinosa and responds to noxious stimuli while others respond to non-noxious stimuli.
The majority of neurons in Rexed lamina II axons receive information from sensory dorsal root ganglion cells as well as descending dorsolateral fasciculus DLF fibers. High concentrations of substance P and opiate receptors have been identified in Rexed lamina II. The lamina is believed to be important for the modulation of sensory input, with the effect of determining which pattern of incoming information will produce sensations that will be interpreted by the brain as being painful.
Rexed lamina III — Composed of variable cell size, axons of these neurons bifurcate several times and form a dense plexus. Rexed lamina IV — The thickest of the first four laminae. In addition, dendrites of neurons in lamina IV radiate to lamina II, and respond to stimuli such as light touch. The ill-defined nucleus proprius is located in the head of this layer. Some of the cells project to the thalamus via the contralateral and ipsilateral spinothalamic tract.
Rexed lamina V — Composed neurons with their dendrites in lamina II. This lamina covers a broad zone extending across the neck of the dorsal horn and is divided into medial and lateral parts. Many of the Rexed lamina V cells project to the brain stem and the thalamus via the contralateral and ipsilateral spinothalamic tract. Moreover, descending corticospinal and rubrospinal fibers synapse upon its cells.
Rexed lamina VI — Is a broad layer which is best developed in the cervical and lumbar enlargements. Lamina VI divides also into medial and lateral parts. Group Ia afferent axons from muscle spindles terminate in the medial part at the C8 to L3 segmental levels and are the source of the ipsilateral spinocerebellar pathways. Many of the small neurons are interneurons participating in spinal reflexes, while descending brainstem pathways project to the lateral zone of Rexed layer VI.
Rexed lamina VII — This lamina occupies a large heterogeneous region. This region is also known as the zona intermedia or intermediolateral nucleus. Its shape and boundaries vary along the length of the cord. Lamina VII neurons receive information from Rexed lamina II to VI as well as visceral afferent fibers, and they serve as an intermediary relay in transmission of visceral motor neurons impulses.
The dorsal nucleus of Clarke forms a prominent round oval cell column from C8 to L3. The large cells give rise to uncrossed nerve fibers of the dorsal spinocerebellar tract DSCT. Cells in the lateral horn of the cord in segments T1 and L3 give rise to preganglionic sympathetic fibers to innervate postganglionic cells located in the sympathetic ganglia outside the cord.
Lateral horn neurons at segments S2 to S4 give rise to preganglionic neurons of the sacral parasympathetic fibers to innervate postganglionic cells located in peripheral ganglia. Rexed lamina VIII — Includes an area at the base of the ventral horn, but its shape differs at various cord levels. In the cord enlargements, the lamina occupies only the medial part of the ventral horn, where descending vestibulospinal and reticulospinal fibers terminate. The neurons of lamina VIII modulate motor activity, most probably via g motor neurons which innervate the intrafusal muscle fibers.
Its size and shape differ at various cord levels. Rexed lamina X — Neurons in Rexed lamina X surround the central canal and occupy the commissural lateral area of the gray commissure, which also contains decussating axons. In summary, laminae I-IV are concerned with exteroceptive sensations, whereas laminae V and VI are concerned primarily with proprioceptive sensation and act as a relay between the periphery to the midbrain and the cerebellum.
All visceral motor neurons are located in lamina VII and innervate neurons in autonomic ganglia. Surrounding the gray matter is white matter containing myelinated and unmyelinated nerve fibers. These fibers conduct information up ascending or down descending the cord.
The white matter is divided into the dorsal or posterior column or funiculus , lateral column and ventral or anterior column Figure 3. The anterior white commissure resides in the center of the spinal cord, and it contains crossing nerve fibers that belong to the spinothalamic tracts, spinocerebellar tracts, and anterior corticospinal tracts. Three general nerve fiber types can be distinguished in the spinal cord white matter: 1 long ascending nerve fibers originally from the column cells, which make synaptic connections to neurons in various brainstem nuclei, cerebellum and dorsal thalamus, 2 long descending nerve fibers originating from the cerebral cortex and various brainstem nuclei to synapse within the different Rexed layers in the spinal cord gray matter, and 3 shorter nerve fibers interconnecting various spinal cord levels such as the fibers responsible for the coordination of flexor reflexes.
Ascending tracts are found in all columns whereas descending tracts are found only in the lateral and the anterior columns. Four different terms are often used to describe bundles of axons such as those found in the white matter: funiculus, fasciculus, tract, and pathway.
Funiculus is a morphological term to describe a large group of nerve fibers which are located in a given area e. It has been found present in all human DRG neurons of adults Rostock et al. NeuN RBFOX3 is a RNA binding protein found predominantly in the neuronal nuclei, and another common neuronal biomarker found in the vast majority of postmitotic mature neurons Duan et al.
In the limited studies performed to date, NeuN is expressed in all adult human DRG neurons but was not detected at gestational week 10 Schonemann et al. Ion channels are essential for the regulation of neuronal excitability leading to the generation and conduction of action potentials and are therefore critical to sensory neuron function.
Important channel proteins for the generation of inward membrane currents in nociceptors belong to the groups of voltage-gated sodium Na V and calcium Ca V channels as well as transient receptor potential TRP channels Waxman and Zamponi, Voltage-activated sodium channels are key components of action potential generation in DRG neurons.
Out of the nine Na V subtypes, the Na V 1. Studies using multiple labeling immunohistochemistry Coward et al. In contrast to animal studies, in humans the Na V 1. Immunoreactivities for Na V 1. Nevertheless, the strongest immunoreactivity was detected in small neurons Coward et al. A recent study investigating the presence of Na V 1.
The Na V 1. Interestingly, the study directly compared proportions of positive cells in human and mouse DRG and showed significant differences between mouse and humans for Na V 1. The expression of the Na V 1. In contrast, the expression of the Na V 1. These findings are supported by an RNAseq study Ray et al.
In addition to transcriptional and translational data, electrophysiological studies confirm the functional presence of Na V 1. Sensitivity to the puffer fish toxin tetrodotoxin TTX selectively differentiates between channel subtypes, where Na V 1. Human DRG neurons possess TTX-sensitive and TTX-resistant channels, but in contrast to rodents, where TTX-resistant currents are mainly restricted to small diameter neurons, in humans they are present in small and large diameter neurons Han et al.
Nevertheless, the Na V 1. However, in humans, Na V 1. Voltage-gated calcium channels Ca V are essential components of sensory neuron function Park and Luo, as activation of these channels contributes to exocytosis of transmitter-filled vesicles at synaptic endings.
Interestingly, the Ca V 2. Calcium-activated potassium channels K Ca are important contributors to the after hyper-polarization of neurons which can be modulated by NMDA-type glutamate receptor activation and nerve ligation, and therefore contribute to nociceptive signaling Li et al. Immunoreactivities for voltage-independent human K Ca 2. The channels are part of a variety of neuronal signaling pathways including nociception.
Using a carefully tested antiserum, Pan et al. Pan et al. Transient receptor potential cation channel subfamily V member 1 TRPV1 is a channel protein that is activated by the vanilloid capsaicin, an ingredient of hot chili peppers, by low pH and noxious heat. Endogenous agonists are endocannabinoids such as anandamide and N -arachidonoyl-dopamine Suh and Oh, TRPV1 is a non-selective cation channel that has been shown to be an important component of nociceptive signaling Suh and Oh, Capsaicin induces pain in humans Simone et al.
Consequently, topical capsaicin is currently being successfully used in treatment of pain conditions such as postherpetic neuralgia Anand and Bley, Interestingly, most studies describe the presence of immunoreactivity not only in small-sized neurons but also in medium and some studies in large-sized somata Lauria et al. Transient receptor potential cation channel ankyrin 1 TRPA1 is a channel protein activated by mustard oil and cinnamaldehyde, and plays an important role as an irritant sensor of a vast amount of compounds in nociceptive signaling, with its expression confirmed in animal DRG neurons Chen and Hackos, Neuropeptides such as CGRP, SP and galanin are neuromodulators that are co-released with transmitters at the central and peripheral terminals of sensory neurons.
In addition to being important cellular markers used in identifying subpopulations of sensory neurons, they are also fundamental contributors to nociceptor function. Calcitonin-gene-related-peptide CGRP is a neuropeptide composed of 37 amino acids. Although the peptide is a strong arterial vasodilator, it also plays a major role in nociception Marti et al. Substance P SP is a neuropeptide composed of 11 amino acids.
It selectively binds to the neurokinin 1 receptor present on nociceptive projection neurons in the rat spinal cord dorsal horn and causes enhanced synaptic activity Gautam et al. Animal studies demonstrate a clear involvement of SP in nociceptive signaling, however, in humans, the evidence to date is not as convincing Babenko et al.
Similarly, SP immunoreactivity is present early in development in neuronal cell bodies of fetal DRG but reports vary. Marti et al. Despite the apparent similarities between humans and rodents, with both having a subpopulation of nociceptive DRG neurons containing SP, blockade of the action of SP via inhibition of neurokinin 1 receptors is effective in relieving pain in mice Laird et al. Galanin modulates the excitability of dorsal horn neurons and the presynaptic release of glutamate from primary afferents see review, Lang et al.
Somatostatin is a neuropeptide of either 14 or 28 amino acids in length, generated from a precursor peptide and is involved in pain processing via interaction with its cognate receptors producing inhibitory, analgesic effects Mollenholt et al. More recent studies suggest that somatostatin is also involved in the signaling of itch Huang et al.
Dorsal root ganglia neurons with immunoreactivity for somatostatin are present by gestational weeks 9 and 10, and a small population of immunoreactive cells are detectable throughout all fetal stages, with enduring expression within cells present in DRG of 4-month-old infants Charnay et al.
Endothelin-1 ET1 is one of three peptide isoforms, 21 amino acids in length, which act as vasoconstrictors but also induce pruritus and pain Smith et al. The ET1 peptide is elevated in patients suffering from sickle cell disease, which is associated with episodes of severe pain and animal studies showed that absence of the ET A receptor subtype blocked sickle cells disease-related pain behavior Lutz et al.
The eight amino acids long peptide angiotensin II is part of the renin—angiotensin—aldosterone system RAAS that controls water and electrolyte balance and therefore blood pressure. This peptide also contributes to the regulation of nociception. Animal studies show intrathecally applied angiotensin II elicits nociceptive behavioral responses Cridland and Henry, ; Nemoto et al.
Direct involvement of angiotensin II in pain signaling pathways, was supported by experiments where angiotensin II treatment of cultured human DRG neurons increased their response to capsaicin, whereas treatment with an AT 2 R antagonist reduced capsaicin responses Anand et al. Whether this occurs via a direct action on neurons, or via effects on immune cell-neuron interactions remains to be determined.
Despite the uncertainty surrounding their mechanism of action, AT 2 R antagonists are being used as effective analgesics in humans and laboratory animals Rice et al. In mice, these neurons have been shown represent the group of GDNF-dependent, non-peptidergic nociceptors Bogen et al.
Although Davidson et al. However, it is known that control for the specificity of lectin binding in human sections is difficult, and this is further highlighted in these studies reporting varying detection between membrane and cytosolic I-B4 staining identified by different research groups. Receptors for these factors include the tropomyosin receptor kinases Trk A, B, and C, and the low affinity receptor p75 Lewin and Nykjaer, NGF and the activation of its cognate receptor TrkA, is a key factor in the development of DRG neurons, but also critical for the induction of hyperalgesia and pain via modulation of signaling events in adult DRG neurons.
Neurotrophin receptors are present in DRG from early in development through to adulthood, however, the dynamics of receptor expression patterns from development to adulthood remain to be studied.
Glial-derived neurotrophic factor is another neurotrophic factor which, after interaction with its receptors RET proto-oncogene tyrosine kinase RET and co-receptor GFRalpha1, modulates a subpopulation of nociceptive, I-B4 binding neurons. The effect of GDNF is complex. Nerve growth factor and its receptors have been described in human DRG Vega et al. Therefore, it is of fundamental interest to determine factors that drive neurotrophin receptor mRNA expression in development.
Nitric oxide synthase NOS isoforms 1—3 are present in DRG and its product nitric oxide NO is involved in nociceptive signaling with evidence supporting analgesic and algesic actions.
Gamma amino butyric acid GABA and its receptors are the main inhibitors in the nervous system. Both are expressed in DRG neurons. Additional evidence for the presence and involvement of GABA B receptors in the excitability of human DRG neurons has been provided from experiments investigating the inhibitory action of a cone-snail venom V C 1.
In summary, only a small population of molecules that have been described to be involved in the function of DRG neurons in laboratory animals have so far been investigated in humans. It is evident from existing studies that expression patterns and functions of molecules in DRG do not perfectly match between human and laboratory animal.
Important differences exist between human DRG compared to laboratory animals and careful conducted future studies will be essential to reconcile and validate these to appropriately translate animal data into human context. At the physiological level, the longer peripheral processes and associated soma size of human DRG is likely to account for some of these differences, such as immunoreactivity for neurofilament in all human DRG neurons including those classified as large.
Similarly, many molecules characteristic of nociceptors including TRPV1, CGRP and P 2 X 3 and voltage-gated sodium channels are restricted to small and medium sized neurons in mice but in not in humans where nociception-related proteins immunohistochemistry and mRNAs in situ hybridization are present in neurons of all sizes.
By all means, these discrepancies do not completely invalidate results from animal studies in a human translational context. Indeed, most human DRG neurons show remarkably similar patterns in respect to immunoreactivities for pain-related molecules being detected in smaller sized neurons characteristic for nociceptors. But the diversity across sizes combined with differences in electrophysiological properties Zhang et al.
Regardless of species, DRG contain multiple types of neurons and multiple types of other cells including satellite cells and cells associated with immune and vascular function. Dissociation of ganglia and culture of primary sensory neurons is useful to identify neuronal characteristics, but inferences from these studies must recognize that some types of neurons, specifically those with larger size, will likely not survive mechanical isolation and subsequent culture conditions.
More importantly, critical issues related to antibody specificity highlight challenges relevant to data collections from both human and animal tissues, including the ability to compare neuronal subpopulations across species. An emerging and clinically significant area for further investigation is the interaction of neuronal and non-neuronal cells within DRG, and certain neuron-immune cell interactions involved in pain sensitivity have been shown to be consistent in humans and in laboratory animals.
Sensory neuron-immune cell interactions are increasingly recognized as important mechanisms that contribute to chronic pain, yet there is surprisingly sparse investigative reporting of cells such as macrophages and satellite cells in human DRG. In the next few decades, researchers will hopefully find increasing opportunities to investigate and validate molecular and cellular characteristics of human DRG tissues.
The failure of swathes of clinical trials based on animal model data in the past few decades reinforces the importance of human studies in clinical translation and therapeutic development, especially in very complex conditions such as chronic pain. Early insights from a handful of comparative studies suggest fundamental differences in molecular characteristics of rodent and human DRG nociceptive neurons, as well as other cell types in the DRG, and may provide key pieces of information to select optimal targets and aid more effective drug design strategies.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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FEBS J. Borbely, E. Role of tachykinin 1 and 4 gene-derived neuropeptides and the neurokinin 1 receptor in adjuvant-induced chronic arthritis of the mouse. PLoS One 8:e Boucher, T. Potent analgesic effects of GDNF in neuropathic pain states. Science , — Castro, J. Each of these funiculi are made up of a variety of ascending and descending tracts, but the funiculi are frequently associated with a small number of important, well-defined tracts whose fibers are carried within them. For example, the posterior funiculi contain the posterior columns , important fibers that carry information regarding tactile i.
At the level of the medulla , these fibers form the medial lemniscus , another tract that continues to carry the information on to the thalamus and somatosensory cortex. The whole pathway from the spinal cord to the somatosensory cortex is often referred to as the posterior or dorsal columns-medial lemniscus system.
The lateral funiculi contain important fibers that carry pain and temperature sensations to the brain. These fibers some of which enter the lateral funiculi from the substantia gelatinosa are part of what is called the anterolateral system , which consists of several pathways that carry information regarding painful sensations to various sites in the brain and brainstem. The tracts that are part of the anterolateral system include: the spinothalamic tract , which is important for creating awareness of and identifying the location of painful stimuli; the spinomesencephalic tract , which is involved in inhibiting painful sensations; and the spinoreticular tract , which is involved in directing attention to painful stimuli.
The lateral funiculi also contain an important motor pathway: the corticospinal tract. The corticospinal tract fibers originate in the cerebral cortex e. These alpha motor neurons then travel to skeletal muscle to initiate movement, and therefore the corticospinal tract plays an important role in voluntary movement. The anterior funiculi aren't defined by a specific tract that travels through them. They contain a variety of ascending and descending tracts, including some fibers from the corticospinal tract.
Thus, the spinal cord acts as the intermediary between the brain and the body, and all sensory and motor signals pass through it before reaching their final destination.
This is why a healthy spinal cord is crucial and damage to the spinal cord can be debilitating or life threatening. Nolte J. Philadelphia, PA. Viruses that remain dormant in nerve ganglia, such as the varicella zoster virus that causes both chickenpox and shingles, often cause either pain, rash, or both in a pattern defined by a dermatome. Shingles rash : The shingles rash appears across a dermatome. In this patient, one of the dermatomes in the arm is affected, restricting the rash to the length of the back of the arm.
Shingles is one of the only diseases that causes a rash in a dermatomal pattern, and as such, this is its defining symptom. The rash of shingles is almost always restricted to a specific dermatome, such as on the chest, leg, or arm caused by the residual varicella zoster virus infection of the nerve that supplies that area of skin.
Shingles typically appears years or decades after recovery from chickenpox. A myotome is the group of muscles that a single spinal nerve root innervates. The myotome is the motor equivalent of a dermatome. The testing of myotomes provides the clinician with information about the level in the spine where a lesion may be present. During testing, the clinician looks for muscle weakness of a particular group of muscles. Results may indicate lesions to the spinal cord nerve root, or intervertebral disc herniation that presses on the spinal nerve roots.
The peripheral nervous system PNS consists of the nerves and ganglia outside of the brain and spinal cord. Unlike the CNS, the PNS is not protected by the bones of the spine and skull, or by the blood —brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. Spinal nerve : This diagram indicates the formation of a typical spinal nerve from the dorsal and ventral roots.
Numbers indicate the types of nerve fibers: 1 somatic efferent, 2 somatic afferent, 3—5 sympathetic efferent, 6—7 sympathetic afferent. The peripheral nervous system includes 12 cranial nerves and 31 pairs of spinal nerves that provide communication from the CNS to the rest of the body by nerve impulses to regulate the functions of the human body.
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. Each pair of spinal nerves roughly correspond to a segment of the vertebral column: 8 cervical spinal nerve pairs C1—C8 , 12 thoracic pairs T1—T12 , 5 lumbar pairs L1—L5 , 5 sacral pairs S1—S5 , and 1 coccygeal pair.
The anterior divisions of the lumbar, sacral, and coccygeal nerves form the lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic. For descriptive purposes, this plexus is usually divided into three parts: lumbar plexus, sacral plexus, and pudendal plexus.
The sympathetic division typically functions in actions that need quick responses. The parasympathetic division functions with actions that do not require immediate reaction. The sympathetic system is often considered the fight or flight system, while the parasympathetic system is often considered the rest and digest or feed and breed system.
Autonomic nervous sytem : The functions of the parasympathetic and sympathetic nervous systems are detailed. The somatic nervous system consists of afferent and efferent nerves and is associated with the voluntary control of skeletal muscle movements. The afferent nerves are responsible for relaying sensations from the body to the central nervous system CNS , while the efferent nerves are responsible for sending out commands from the CNS to the body to stimulate muscle contraction. Upper motor neurons release acetylcholine.
Acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors nicotinic acetylcholine receptors of muscles, thereby relaying the stimulus to contract muscle fibers.
Privacy Policy. Skip to main content. Peripheral Nervous System. Search for:. Spinal Nerves. Overview of the Spinal Nerves Spinal nerves, a part of the peripheral nervous system PNS , are mixed nerves that send motor, sensory, and autonomic signals between the CNS and the body. Learning Objectives Describe spinal nerves of the peripheral nervous system. Key Takeaways Key Points Afferent sensory axons bring sensory information from the body to the spinal cord and brain; they travel through the dorsal roots of the spinal cord.
Efferent motor axons bring motor information from the brain to the body; they travel through the ventral roots of the spinal cord. All spinal nerves—except the first pair—emerge from the spinal column through an opening between vertebrae, called an intervertebral foramen.
The spinal nerves are typically labeled by their location in the body: thoracic, lumbar, or sacral. Key Terms ventral root : Also called the anterior root, it is the efferent motor root of a spinal nerve. Branches of Spinal Nerves The spinal nerves branch into the dorsal ramus, ventral ramus, the meningeal branches, and the rami communicantes.
Learning Objectives Describe branches of the peripheral nervous system. Key Takeaways Key Points The dorsal and ventral rami contain nerves that provide visceral motor, somatic motor, and sensory information, with the dorsal ramus feeding the dorsal trunk skin and muscles of the back , and the ventral ramus feeding the ventral trunk and limbs through the ventrolateral surface.
The meningeal branches supply nerve function to the vertebrae themselves, including the ligaments, dura, blood vessels, intervertebral discs, facet joints, and periosteum. The rami communicantes contain autonomic nerves that carry visceral motor and sensory information to and from the visceral organs. Key Terms nerve plexus : A branching network of intersecting nerves.
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