What is the difference between the sensory and motor divisions of the peripheral nervous system?

Gestational origin and composition of the mature peripheral nervous system

The PNS is dichotomized into the autonomic and somatic nervous systems, the former being responsible for innervating glands and smooth muscle, and the latter responsible for locomotion and sensation. While the vast majority of functions within the autonomic nervous system (ANS) are involuntary, i.e., activation of the fight-or-flight or rest-and-digest responses, the ANS can act in concert with the somatic nervous systems (SNS) owing to their similarities in cellular and anatomical composition in conjunction with their integration at several neuroeffector connection sites [22, 23]. From gestation, both the ANS and SNS, and thus, the PNS, arise from the neural ectoderm. This is inherently distinct from the adipose tissue and the ASC cell population as they arise from the mesoderm [24]. Notable, however, is that neural crest cells undergo a developmental transdifferentiation into mesodermal cells linking the ASC-PNS cell heritage [25]. It is the neural ectoderm that gives rise to this neural crest as well as the neural tube, which subsequently compartmentalize into the brain, spinal cord, and the network of the peripheral nerves. The neuroepithelial cells differentiate into neurons, oligodendrocytes/Schwann cells, astrocytes/satellite cells, and the neuroglial support cells that surround, myelinate, and sheath the neurons [26], forming the bodily network of peripheral nerve (Fig. 1). Though composed similarly at the cellular level, two distinct nerve types exist that are directionally characterized on the basis of electrical impulse transduction. Afferent nerves signal toward the brain, and efferent nerves signal away from the brain; these nerve types are also commonly referred to as sensory and motor nerves, respectively [27].

Peripheral nervous system

The peripheral nervous system (PNS) provides 2-way communication between the CNS (brain and spinal cord) and the rest of the body. It consists of 12 pairs of cranial nerves, pairs of spinal roots and spinal nerves, autonomic nerve trunks with associated ganglia, nerve plexuses, and nerves. In the PNS, neuronal processes and their myelin sheaths are collectively termed nerve fibers. Each nerve fiber is surrounded by a layer of connective tissue, termed endoneurium. Bundles of nerve fibers form fascicles, and each fascicle is enclosed by connective tissue called the perineurium. A cable of fascicles is collectively termed a nerve and is surrounded by connective tissue called epineurium (Fig. 14.1-2).

Neuropathologic Classification of Peripheral Nerve Trauma

Traumatic nerve lesions are classified into 3 categories according to the structures injured within the nerve fiber. (1) Neuropraxia involves compression without loss of axon or sheath integrity, resulting in temporary loss of function but no morphological change. (2) Axonotmesis is caused by crushing, which injures the axon but preserves the myelin sheath. (3) Neurotmesis is severing of the axon and myelin sheath.

Muscle atrophy occurs with axonotmesis and neurotmesis and becomes clinically prominent in 1–3 weeks. Reinnervation occurs by 2 mechanisms. The first, “collateral sprouting,” depends on the incomplete loss of axons to a muscle with surviving axons “sprouting” terminal branches that establish new connections with denervated muscle units. The second is axonal regrowth at a rate of 1 mm (0.04 in.)/day. Most horses with trauma-induced peripheral neuropathy of the thoracic limb return to athletic soundness following an adequate period of rest.

Nerves contain both afferent (sensory) fibers, which convey information from sensory receptors to the CNS, and efferent (motor) fibers, which send signals from the CNS to the peripheral effector organs. These functional subdivisions of the nervous system have somatic and visceral components. Efferent neurons in the PNS are also referred to as lower motor neurons (LMN). General somatic efferent (GSE) LMN innervate striated voluntary skeletal muscle, while general visceral efferent (GVE) LMN belong to the autonomic nervous system and innervate the smooth muscle associated with blood vessels and visceral structures, glands, and cardiac muscles.

Purpose

The purpose of the PNS is to sense the external environment, conduct this information to the CNS and then respond to the external environment with gross physical movement. Thus, the PNS codes and decodes external information: electromagnetic light, mechanical sound waves, and chemical and electromagnetic odor information packet all into electrical impulses.8 It also senses and responds to aggressions and changes in internal states, thus it has two branches: sensory and motor.8 The sensory aspect detects changes in the temperature, pressure, and pain in order to inform the organism of a potential threat to its structural integrity. It also allows the organism to mechanically respond to these changes in its internal equilibrium or to an external aggression. The PNS determines the quantitative nature of the threat. The CNS determines the qualitative nature of the threat and the ANS affects the intensity of the response.

b. Peripheral nervous system.

The peripheral nervous system is very much less highly organized than the central nervous system in that the peripheral nerves, apart from the larger and medium-sized ones in insects, have a much less developed peripheral glial system. In the nerves of some groups, the peripheral glia are not really perineurial because they do not ensheath the whole of the underlying nervous tissue and are not associated by junctions; only small clusters of inner glial-ensheathed axons, separated by large areas of neural lamella-like connective tissue matrix, are apparent. Although their minute diameter makes the smallest peripheral nerves difficult to impale for electrophysiological recordings, tracer uptake studies on such insect nerves (Lane and Treherne, 1972) show that exogenous substances are allowed access into the nerve interior without restriction, as is also the case for even the largest peripheral nerves of Limulus (Harrison and Lane, 1980), the tick (Binnington and Lane, 1980) and the crayfish (Lane, Swales and Abbott, 1977).

1.7.1.2.1 Symptoms and signs

Exposure to lead may damage the PNS. After severe exposure, the main clinical disorder is peripheral motor neuropathy with paralysis (“wrist drop” and “ankle drop”). In particular, the dominant hand is affected.

At lower exposures, there are motor symptoms in terms of mild distal weakness (decreased pinch and grip strength) of the upper limb, and sensory effects, as tingling or numbness in the arms or legs, muscle pain, affected sensory and pain perception thresholds in the fingers, and decreased vibration thresholds in the hands and toes. The neuropathy is mostly reversible if adequately handled, but sometimes PNS effects remain.

The PNS effects are probably caused by demyelination, axonal degeneration, and possibly also presynaptic block. It seems that the large, fast sensory fibers are particularly sensitive to lead.

Slight sensory symptoms and signs have been noted at a mean B-Pb of approximately 1.5 μmol/L, and higher (Table 19.2).

C Nervous System Development

Cells and tissues of the developing central and peripheral nervous systems rely on integrins for their interactions with the extracellular matrix. Although much of this data has been gathered using cells in vitro, most studies have utilized nonimmortalized embryonic cells, which better reflect the in vivo interactions of these cells and the ECM. Anti-β1 integrin antibodies inhibit retinal neurite outgrowth on fibronectin, laminin, and collagen type IV (Hall et al., 1987), but interestingly require the addition of anti-cadherin antibodies in order to inhibit outgrowth on astrocytes (Neugebauer et al., 1988). These neurons also utilize αVβ1 to mediate outgrowth on vitronectin and thrombospondin (Neugebauer et al., 1992). These data clearly demonstrate the importance of multiple adhesion systems in morphogenetic processes. The TASC antibody, so named for its ability to negate the effects of the function blocking anti-β1 integrin antibody CSAT, promotes retinal ganglia neurite outgrowth on laminin or collagen types I and IV, presumably by holding the receptor in an active conformation (Neugebauer and Reichardt, 1991). Finally, neuroblasts of the optic tectum expressing antisense β1 RNA (using a retroviral expression system) have reduced levels of β1 protein and mRNA in vitro and are deficient in migration in vivo (Galileo et al., 1992). This, plus the localization of β1 integrins in central nervous system growth cones (Cypher and Letourneau, 1991), indicates that β1 integrins are intimately involved in neurite outgrowth and may be involved in neurite guidance. Interestingly, retinal neurons express the α6 integrin subunit (which forms laminin receptors with β1) while they are moving through the laminin-rich optic stalks, but downregulate this integrin upon entering the 1am-inin-poor optic tectum (de Curtis et al., 1991).

Studies of the peripheral nervous system have focused on the migratory behavior of neural crest cells and neurite outgrowth from various neuronal ganglia. Neural crest cells migrate through an extracellular matrix rich in fibronectin, laminin, tenascin, and collagens (Newgreen and Theiry, 1980; Krotoski et al., 1986; Duband and Thiery, 1987; Bronner-Fraser, 1988). Both RGD peptides and anti-β1 antibodies perturb mesencephalic neural crest migration in vivo (Boucaut et al., 1984b; Bronner-Fraser, 1985, 1986) and inhibit migration on fibronectin in vitro (Boucaut et al., 1984a; Bronner-Fraser, 1985). Attachment of these cells in vitro to laminin and collagen is also mediated by β1 integrins (Lallier et al., 1992). The α1β1 integrin allows trunk but not mesencephalic neural crest cells to attach to laminin in an EDTA insensitive manner (Lallier and Bronner-Fraser, 1992; Lallier et al., 1992). Furthermore, antisense oligodeoxynucleotides, which reduce integrin α or β1 subunits on the surface of trunk neural crest cells, differentially reduce attachment to ECM molecules in vitro (Lallier and Bronner-Fraser, 1993), and perturb mesencephalic neural crest cell migration in vivo (M. Bronner-Fraser, personal communication). These results indicate that multiple integrins play an important role in neural crest cell migration, and possibly in mesencephalic neural crest cell guidance.

During the later development of the peripheral nervous system, anti-β1 antibodies can be used to inhibit neurite outgrowth from explanted dorsal root ganglia on fibronectin or laminin (Letourneau et al., 1988). Anti-β1 integrin monoclonal antibodies also inhibit ciliary neurite outgrowth on laminin in vitro, but not on cultured astrocytes (Tomaselli et al., 1986, 1988). These antibodies reduce both ciliary and dorsal root ganglion neurite outgrowth on gelatin (Bozyczko and Horwitz, 1986) and laminin, but fail to affect outgrowth on glioma cells in vitro (Letourneau et al., 1988). These studies indicate that although integrins mediate cell-ECM interactions throughout the peripheral nervous system, cell-cell interactions, dominated by cadherins, also are likely to play a role in neurite outgrowth and guidance in vivo.

29.1.1 Galanin/spexin peptide family

Galanin is a neuromodulator in the brain and peripheral nervous system (PNS) of vertebrates. Several structurally related peptides, including galanin-like peptide (GALP, 60 aas) (Ohtaki et al., 1999), Spexin (SPX), and Kisspeptins (KISS 1, KISS2, KISS3, and KISS4), have later been identified, all of which are proposed to be originated from a common ancestral gene and belong to the same peptide family (Kim et al., 2014).

Galanin was isolated from the chicken intestine (Norberg et al., 1991) and quail oviduct (Li et al., 1996). It is a 29-residue peptide which contains an amidated threonine at its C-terminus, which is proteolytically cleaved from a large precursor. Interestingly, GALP gene is likely lost in the avian lineage (Ho et al., 2012).

SPX is a 14-residue peptide encoded by the C12ORF30 gene (Mirabeau et al., 2007; Sonmez et al., 2009) in mammals. In chickens, two ortholog peptides of 14 aas (named SPX1 and SPX2) encoded by separate genes have been identified. Both peptides share high sequence similarity with each other and a comparatively low identity with chicken galanin (Fig. 29.1) (Kim et al., 2014; Lim et al., 2019).

KISS1 has been proposed to be a key regulator of reproduction by activating gonadotropin-releasing hormone (GnRH) neurons, which can initiate the onset of puberty in mammals (Pinilla et al., 2012). However, KISS1 is possibly lost in avian genomes (Um et al., 2010). Interestingly, a KISS2-like gene was identified in ducks (HG328246), zebra finches, and rock pigeons (Pasquier et al., 2014). Nevertheless, avian KISS2 seems to be degenerated considerably in avian lineage, questioning its roles in birds.

In mammals, three GPCRs for GAL, GALP, and SPX, named GALR1, GALR2, and GALR3, have been identified. GALR1 is a high-affinity receptor for GAL. GALR2 and GALR3 are high-affinity receptors for GALP. GALR3 can function as a high-affinity receptor for SPX. In contrast, 5 GPCRs, named GALR1, GALR1-like (GALR1-L), GALR2, GALR2-like, and GALR3, have been identified in birds. GALR1, GALR1-L, and GALR2 can function as the three receptors for chicken GAL, and their activation inhibits cAMP/PKA signaling and stimulates the MAPK/ERK cascade (Ho et al., 2011, 2012). However, it is unknown whether SPX1 and SPX2 can activate the five receptors (Kim et al., 2014).

GPR54 can function as a receptor for KISS1 peptide in mammals. However, GPR54 is likely lost in birds. The failure in identifying KISS1 and GPR54 does not support the role of KISS–GPR54 signaling in avian reproduction (Pasquier et al., 2014).

Galanin peptide is widely expressed in avian CNS, PNS, and other tissues, such as the intestine and oviduct (Norberg et al., 1991). In chickens, galanin is distributed in the CNS including the hypothalamus and brainstem (Jozsa and Mess, 1993; Klein et al., 2006) and is important in its regulation, such as stimulating food intake of layer and broiler chicks (Tachibana et al., 2008). In quail oviduct, galanin is expressed in the lumbosacral sympathetic ganglion neurons innervating the avian uterine muscle and thus can induce oviposition in quails (Li et al., 1996; Sakamoto et al., 2000; Ubuka et al., 2001). To date, the roles of SPXs in birds remain unclear.

1.1 Anatomical organization

The nervous system is anatomically divided into the central and peripheral nervous systems (CNS and PNS, respectively). The CNS is comprised of the spinal cord, brainstem, and forebrain and the PNS is comprised of the nerves and ganglia outside the brain and spinal cord (Table 20.1). Structurally, the spinal cord is a bundle of nervous tissue including neuronal cell bodies, axons, glia, and structural proteins within the vertebral column stretching from the base of the skull to approximately the first lumbar vertebra (L1). Functionally, the spinal cord receives, sends, and processes sensory and motor information between the PNS and the brain. The brain stem is made up of the medulla, pons, and midbrain, which are critical for the regulation of vital blood pressure and respiration functions. It is also the origin of cranial nerves involved in special senses such as taste and hearing, as well as conveying sensory and motor information relating to the head and neck. The cerebellum is also considered to be part of the midbrain; it receives many sensory and motor inputs, and functions in part to regulate balance, posture, and fine motor skills. The forebrain consists of the diencephalon (thalamus and hypothalamus) and the cerebral hemispheres, including the cerebral cortex, white matter, and subcortical structures, such as the basal ganglia and hippocampus. The thalamus relays sensory and motor signals to and from the cortex and acts to regulate sleep and wakefulness. Underneath the thalamus is a small set of distinct nuclei called the hypothalamus, which links the nervous system with the endocrine system to regulate sex hormones, hunger and satiety signals, and circadian rhythms. The remaining cortical and subcortical structures of the forebrain function as higher processing systems that control the sensory and motor cortices, the limbic system, the basal ganglia, and the hippocampus.

Table 20.1. Anatomical organization of the nervous system.

CNS, central nervous system; CSF, cerebral spinal fluid; PNS, peripheral nervous system.

The PNS can also be divided into several distinct anatomical and physiological components (Table 20.1). The somatic nervous system is composed of sensory neurons in the skin, muscles, and joints, as well as motor neuron axons that control skeletal muscles, while the autonomic nervous system contains neurons and axons involved in visceral sensation and the control of smooth muscles and exocrine glands. The autonomic system is further divided into the sympathetic nervous system, which underlies the “fight or flight” response, the parasympathetic nervous system, which restores homeostasis, and the enteric nervous system, which is relatively autonomous and regulates smooth muscle in the gut.

The CNS is uniquely sequestered from the PNS and the rest of the body by several anatomical barriers—the blood–brain barrier (BBB), the blood–cerebral spinal fluid (CSF) barrier, and the blood–retina barrier (Table 20.1). The function of these barriers is to isolate the CNS, including the retina, from toxins and other damaging substances in the blood, such as bacteria, viruses, and inflammatory agents, and to regulate the nutritional and ionic composition of the CSF.

12.1.4 Nervous Tissue

Nervous tissue makes up the autonomic and central nervous and the peripheral nervous systems of animals. This includes the brain, the spinal cord, and the peripheral nerves that regulate and control all body functions. Neurons, the basic currency of nervous tissue, are responsible for detecting stimuli and transmitting these signals between the different parts of the animal's body. However, nervous tissue as a source of food is very limited. In beef, for instance, because of the human variant of mad cow disease, it has become important to keep central nervous system tissues (especially of beef) out of the food supply. This requires strict regulation at the point of slaughter, where tissues are at risk of contaminating the rest of the meat. However, central nervous tissue is important to the quality of the meat in another way. That is in the function and treatment of the animal both immediately before and during the slaughter process, which can, as is described later, greatly influence the final quality of the meat—especially if the preslaughtered animal is overly stressed (Section 12.3.2) (Ockerman, 1996; Aberle, 2012; Zhang, 1999; Farley et al., 2012).

Nervous system

The nervous system of fish drives the integration with the (external) environment and the control of organs and systems. The perception or understanding of the surrounding environment allows the perfect coordination during displacement (i.e., swimming and migration), feeding, reproduction, and ordinary behavior. The integration between systems allows controlling all bodily functions and processes necessary for the survival and steadiness of the species. Although the nervous and the endocrine systems may be considered as working independently, they often work synergistically in the regulation of the organic responses to changes in the external and internal environments.

The nervous system of fish can be parted into the central and peripheral nervous system. The central nervous system comprises the brain and spinal cord while the peripheral nervous system is made of nerves and ganglia. The functions of the structures of the central nervous system are defined in Table 2.1 (see also Fig. 2.24) (Bernstein, 1970; Genten et al., 2009; Butler, 2011; McLean and Dougherty, 2015; D'Elia and Dasen, 2018). The brain cavity is not clearly separated by the meninges to isolate the cerebrospinal fluid, but by connective structures with plenty of adipose tissue between the blood vessels and rich membranes covering the brain. The olfactory bulbs are positioned rostrally to the telencephalic hemispheres; some Characins such as the white cachama Piaractus brachypomus (Fig. 2.24) and the cardinal tetra P. axelrodi (Obando et al., 2013) do not present an olfactory tract. This characteristic could be related to a higher velocity of olfactory stimuli to reach the olfactory bulbs in Characins in comparison to Siluriform, which exhibit large olfactory tracts (Londoño and Hurtado, 2010). Nocturnal Siluriformes such as Pseudopimelodus spp. and Spectracanthicus javae have a large cerebellum and a relatively small tectum opticum compared to Characins (Chamon et al., 2018).

Table 2.1. Divisions of the central nervous system

The autonomic nervous system (ANS) is hold part of the peripheral nervous system. The classic division of the ANS in tetrapods comprises the sympathetic nervous system, the parasympathetic nervous system (cranial and sacral nerves), and the enteric nervous system (autonomic nerves intrinsic to the intestine). However, this division is not coherent for fish, so the terminology proposed by Nilsson (2011a) is herein adopted:

Cranial autonomic system: parasympathetic pathways that follow the cranial nerves.

Autonomic spinal system: sympathetic pathways parallel to the spinal cord and parasympathetic sacral pathways. In the elasmobranchs, the paravertebral ganglia are segmentally distributed parallel to the length of the spinal cord but not completely connected longitudinally, as in Teleosts.

Enteric system: autonomic nerves intrinsic to the intestine (that is, maintain the definition used for mammals).

Fish have 10 pairs of cranial nerves (Nilsson, 2011a,b; Taylor et al., 2010):

I Olfactory: innervates the olfactory bulb, responsible for the transmission of olfactory impulses.

II Optical: innervates retina, transmits impulses related to vision.

III Oculomotor: innervates most of the muscles of the eye.

IV Trochlear: innervates the upper oblique eye muscle.

V Trigeminal: innervates the anterior portion of the head and the mandible and maxilla, transmitting motor and sensorial signals (thermal, tactile, and proprioceptive).

VI Abducens: innervates the posterior rectus muscle.

VII Facial and VIII auditory: can be considered as a facial auditory set, transmitting motor signals for some muscles of the head and sensorial (visceral, lateral, auditory, gravity, tactile, gustatory, proprioceptive).

IX Glossopharyngeal and X vagus: sometimes fused, lead sensory signals (lateral line, gustatory), innervate muscles related to breathing. The vagus nerve also transmits motor signals to the viscera.

Because of the absence of salivary and lacrimal glands in fish, the autonomic cranial pathways usually occur only in the cranial nerves III and X. The facial and glossopharyngeal nerves of elasmobranchs may also be autonomic; the autonomic cranial pathways of the South American lungfish L. paradoxa are restricted to the vagus nerve. Finally, a very particular feature is that the fish ANS also innervates chromaffin cells, which are inserted into the kidney.