Although the human nervous system is incredibly complex, the nervous systems of other organisms provide insight into how such a complex system could evolve in a long series of gradual transitions.  There is no nervous system in sponges or mesozoans.  Coelenterates lack a nervous system (and all other systems) but they do possess neurons.   Nervous tissue thus appears in the first predatory animals—predators need greater control and faster reflexes than filter feeders.  Bipolar and multipolar neurons are present and some neurons are capable of secretion (Hickman). There are sensory neurons in the epithelia and both motor and associative neurons beneath the epithelia (Beklemishev  vol. 2, p. 75).  Although processes extend from the neuronal cell body, there is no distinction between dendrites and axons (Beklemishev, vol. 2 p. 75).  Both interneuronal and neuromuscular synapses are present. Neural impulses may be conducted in either direction (Hyman, p. 377).

     While most of the neurons (98%) in humans are part of the central nervous system, such centralized structures developed after the first nervous systems evolved.  Nerve cells in coelenterates are not part of a centralized nervous system, they form a diffuse nerve network which may have ganglia (Fretter, p. 68).  Certain regions of the body may have a greater concentration of neurons than others (Fretter).  In cnidarians, there are actually two nerve nets; one is more involved in feeding and the other in moving.  The first nerve net allows local responses while the second activates all muscles simultaneously (Fretter).  These two nets are usually interconnected, but in some species there is little or no connection.  In some species, neurosensory cells of the epithelia synapse with the nerve net (Hickman).   The giant fibers linking the senses to the muscles are similar to the giant fibers of many invertebrates (Fretter, p. 68).   Ganglia can control the regular rhythms of movement in jellyfish and comb jellies (through a structure known as the apical organ; Fretter, 68 & 94). 

     The genes atonal1 and NP1 are expressed only in bilateran neural tissue but are expressed in both neural and muscle tissue in cnidarians, supporting a common origin for neurons and muscle cells (Seipel, 2004; Muller, 2003). 

     Higher acoelomate worms (turbellarians, Rhabdocoela) also retain traces of radial symmetry in their nervous system (Beklemishev, vol. 1, p. 113-4).  In Rhabdocoela, there is a bilobed brain and 2 main long nerves and in Polycladida, there are nerves originating from the brain (2 of which are large).  (Hickman)

     Even in invertebrates, brains became more complex after the most primitive groups.  After the flatworms, nervous systems display a greater cephalization and centralization, a deeper migration of nervous tissue into the body away from epithelia, and a greater complexity of behavior is possible.  Outside the brain, nervous tissue becomes more centralized because axons between ganglion cells are shortened while axons to sensory receptors and muscles are lengthened (Beklemishev, vol. 2, p. 83)  In nemertine worms, the brain is composed of two pairs of ganglia (dorsal and ventral), 2 nerve chords, and smaller nerves.  There are many soma in the nerve chords but typically no ganglionic expansions.  In the most primitive nemertines, nervous tissue is still subepidermal (Hickman).  The two nerve cords of flatworms are homologous to the equal halves of spinal cord (Sarnat, 1985)

     Lophophorates retain a nervous plexus under the epidermis which includes nerve cords and giant fibers (Hickman 288).  Basal deuterostomes still have a diffuse epidermal nervous system with nerve cords and simple eyespots (Hickman, 645-6).  Chaetognaths possess a large cerebral ganglion which sends nerves to the head area (including the eyes, which are inverted as in vertebrates)(Hickman p. 694).  Hemichordates, like echinoderms, have a primitive nerve net (plexus) similar to that of coelenterates (Hickman, p. 699).  This primitive nerve net not only exists in the epidermis, but along the gastrointestinal tract as well.  Some neurons in the plexuses are not polarized (Beklemishev, vol. 3). 

     Hemichordates and pognophores possess a dorsal nerve cord which contains giant fibers.  The major aspect of the nervous system in acorn worms is a nerve plexus within the skin.  Middorsal and midventral longitudinal cords form in this plexus (Benito, form Harrison 1997; Hickman, p. 699-700; 706).  In acorn worms (enteropneusts), the visceral and superficial nervous systems are still largely separate. In most enteropneusts the nerve cord is solid but it contains a lumen in some (Beklemishev, vol. 3, p. 134).  Genes which higher vertebrates use to determine position along the anterior/posterior axis of the neural tube (such as Pax6, Pax2/5/8, En, Fgf8/17, Otx, Emx, Dlx1/6, Nkx2.1) are also present in lower chordates (Murakami, 2005).

     Urochordates retain a plexus of neurons around gut. (Burighel, from Harrison, 1997, p. 274).  Nerve nets represent a major (if not the only) aspect of the nervous system in cnidarians, ctenophores, echinoderms, enteropneusts, ascidians, and some mollusks. (Prosser, 1973, p. 644).   Although the central nervous system became increasingly more important in higher animals, there is evidence that the ancestral nerve net was retained around the digestive tract.  All higher invertebrates and vertebrates possess a nerve net (or more than one) which govern the secretion and muscular contractions of the digestive tract. (Hoar, 1983, p. 450).  Even in mammals such as ourselves, the primary innervation of the GI tract is a primitive nerve net which makes most decisions independent of input from the central nervous system.  The human enteric nervous system (the diffuse plexuses of the muscularis and submucosal layers of the gastrointestinal tract) actually possess more neurons than the spinal cord does (Romer p. 547-8).   

The formation of axonal tracts in early embryos is conserved in vertebrates (Barreiro-Iglesias, 2008). 

THE SPINAL CORD

     The human spinal cord is composed of gray matter containing neural cell bodies in the form of a butterfly (or “H”) surrounded by white matter containing neuronal axons.  There is a definite organization to the spinal cord.  The soma in the gray matter are organized into various nuclei (some of which process sensory information, others of which give motor commands, for example) and the axons in the white matter are organized into various tracts (ascending tracts carrying sensory information and descending tracts giving motor commands, for example).

The dorsal roots which enter the spinal cord carry only sensory axons (whose soma are located in dorsal root ganglia) and the ventral roots which leave the spinal cord carry only motor axons.  These dorsal and ventral roots fuse to form spinal nerves which subsequently divide to form dorsal and ventral rami (each of which carry sensory and motor neurons). 

     Although the human spinal cord is very complex, much simpler spinal cords exist in more primitive animals which provide insight into the evolution of the spinal cord.  In the most primitive animals with a central nervous system, the brain simply supplements the activity of the longitudinal nerve cord.  Flatworms without cerebral ganglia still behave almost normally, except that muscle movements are a bit less coordinated. (Hoar, 1983, p. 305). 

     In Amphioxus, there is no distinction between gray and white matter in the spinal cord. Neuronal soma surround the central canal, including some which are very large (Weichert, 1970, p.613).  The soma of motor neurons are located close to this central cavity, unlike the situation in jawless fish in which they are located in horns of gray matter.  There are no dorsal root ganglia since the soma of sensory neurons are located within the spinal cord.  (Weichert, 1970, p.637).  The dorsal roots carry sensory information to the spinal cord and the ventral roots carry motor commands from the spinal cord, as in vertebrates (Ruppert, from Harrison, 1997, p. 474).  The dorsal roots of spinal nerves do not fuse with ventral roots (nor do they leave the spinal cord from the same point) (Weichert, 1970, p.637).  Affarent and efferent neurons innervating the viscera travel through dorsal roots (Weichert, 1970, p.634).

     Jawless fish are the first to possess a spinal cord which is truly tubular. The gray matter is located on the interior of the spinal cord.  (Weichert, 1970, p.613).  The dorsal root is more restricted to sensory information and there are dorsal root ganglia (Hardisty, p. 357).   Unlike lampreys and Amphioxus, the dorsal and ventral roots join in hagfish (Hardisty, p. 357).  In jawless fish almost all the nervous tissue of the spinal cord is involved in local reflexes, rather than the transmission of impulses to and from the brain. Only a few fibers from midbrain and medulla pass through the spinal cord (Ariens, p. 278) but these form the primitive homologs of the spinocerebellar and tectospinal tracts of gnathostomes (Hardisty, p. 328).  Jawless fish lack an intramedullary blood supply to the spinal cord.  (Hoar,Vol. IV, 1970).

LAMPREY

     The spinal cord of gnathostomes is round in cross section instead of more primitive flattened condition known in jawless fish (Ariens).   The gray matter of the spinal cord forms distinct dorsal and ventral horns. Dorsal and ventral roots can still leave the spinal cord separately (Ariens).   Ventral columns exist in the gray matter.  (Weichert, 1970, p.613).  The white matter of the spinal cord contains tracts homologous to the spinotectal, dorsal spinocerebellar, spino-bulbar, spinocerebebellar, reticulo-spinal and tracts (Ariens, p. 166).  Myelination exists in some of these tracts (Ariens, p. 278).  Gnathostome fish possess a dorsal fissure of the spinal cord, but not a ventral one (Weichert, 1970, p.613).  No spinal plexuses exist in jawless fish.   Plexuses are first known in cartilaginous fish although they are variable. For example, 25 spinal nerves may form cervicobrachial plexus in rays (Weichert, 1970, p.638).

     The amount of gray matter and the complexity of the synapses within the spinal cord increase from the jawless fish through the bony fish (Hoar,Vol. IV, 1970, p. 69).  Dorsal and ventral roots of the spinal cord are always fused, although the dorsal and ventral roots may not arise at exactly same level.  Although these roots exit the spinal cord at different points, they are nevertheless closer than found in lower fish (Ariens).   The soma of motor neurons are located more ventrally (Ariens, p. 280).  Bony fish and tetrapods possess ventrospinocerebellar, reticulospinal, and vestibulospinal tracts (Ariens).  In some teleosts, the spinal cord is shortened, reaching its most extreme form in marine sunfish which can measure 8  foot long but whose spinal cord is less than an inch long.  (Weichert, 1970, p.614).  Bony fish and tetrapods possess a cervico-brachial plexus (Ariens).

     Tetrapods possess cervical and lumbar enlargements of the spinal cord to process the additional information pertaining to the limbs.  (Ariens).  The spinal cord as a whole is expanded in tetrapods (MacLarnon, 1996) and there is an increase in the volume of the dorsal column (Ariens, p. 188).  Amphibians are the first vertebrates to possess a ventral sulcus and the gray matter begins to appear in the form of an “H” as in amniotes.  The axonal tracts in the white matter are more clearly defined (Weichert, 1970, p.614). 

      Tetrapods have rubrospinal, reticulospinal, and vetstibulospinal tracts.  (Webster, 1974, p. 291)  Amphibians possess a cervicobrachial plexus and a lumbosacral plexus, each of which form two separate plexuses in mammals (Weichert, 1970, p.636).  The origins of dorsal and ventral roots may still have the primitive condition of being offset.  The last spinal nerves form a cauda equina  (Weichert, 1970).  Giant Rohon Beard cells are present in developmental stages of amphibians but not in adults (Ariens).

     In amniotes, there is no alternation in the origins of dorsal and ventral roots.  Amniotes possess a dorsal column divided into the fasciculus gracilis and fasciculus cuneatus (although some subdivision of the dorsal column is also known in apmphibians), a nucleus gracilis and a nucleus gracilis in the medulla and a medial lemniscus (Ariens).  The substantia gelatinosa exists over the dorsal horn (Ariens, p. 198).  Some snakes and limbless lizards possess an underdeveloped lumbosacral plexus (Weichert, 1970, p.638).

     The overall size of the spinal cord is increased in mammals.  In mammals, the spinal cord doesn’t extend the length of vertebral column.  After monotremes, the spinal cord does not extend into sacral region (Weichert, 1970).  Chimps and humans have the shortest spinal cords (relatively) in the order primates (MacLarnon, 1996).  Human embryos begin with coccygeal ganglia which degenerate although a coccygeal medullary vestige is retained until later in fetal life (Ariens, p. 221). 

     In mammals, the dorsal horns are divided into the zona marginalis, substanctia gelatinosa of Rolando, and a main mass.  Mammals possess proprioceptive collaterals to the dorsal column and from there to the nucleus gracilis and cuneatus.   More of these connections are present in higher mammals (Ariens, p. 288).  Mammals increase the size of the dorsal column and placental mammals increase the size of the nuclei cuneatus and gracilis (Ariens, p. 263).  The mammalian spinothalamic tracts are homologous to the spino-mesencephalic and spinobulbar tracts of more primitive tetrapods (Ariens, p. 289).  In mammals the red nucleus is larger, as is the rubrospinal tract (Ariens, p. 270).

     Reptiles lack any direct connections between the forebrain and the spinal cord.  Mammals may be the only group with a corticospinal tract, although its position in the spinal cord varies in many mammal groups.  In monotremes and marsupials it only reaches the cervical region of the cord; in insectivores and bats, the few cortico-spinal fibers also end in the cervical region (Ariens, p. 271).  Skilled forelimb movements are characteristic of a number of mammalian groups and some lower tetrapods as well.  Frogs, for example, can manipulate their prey in a number of ways including grasping, grasping with rotation, prey stretching, wiping, and scooping.  Since frogs lack a corticospinal tract, skilled limb movements must be possible without it.  A number of pathways (corticospinal, rubrospinal, tectospinal, and reticulospinal tracts) may contribute to skilled forelimb movements but lesions of any one of them will not ablate these movements, suggesting that multiple tracts interact synergistically to establish these movements (Iwaniuk, 2000).  In most placental mammals, the lateral cortico-spinal tract is no longer located in the dorsal column (as in monotremes, marsupials, ungulates, and rodents) (Ariens, p. 291

     Of the five fossil mammals with a foramen magnum relatively smaller than any modern placental mammal, 4 are of fossil primates (3 adapids and a lorisid; the fifth was an Eocene carnivore) (MacLarnon, 1996).  The cervical and thoracic regions of the vertebral canal in the fossil primates Notharctus and Smilodectes were smaller than in any modern primate while the size of the vertebral canal in the lumbar region was comparable to those primates alive today (MacLarnon, 1996).

     The ability to move the hand with fine, skilled movements evolved before primates and is present in a number of mammalian lineages, such as mice.  Primates brought the control of skilled movements under the control of the visual system (Whishaw, 2003).  Primates increased the extension of corticospinal tract in spinal cord and its size (Heffner, 1983; Ariens).  In primates, more ventral spinal cord gray matter receives corticospinal input than in other mammals.  Digital dexterity and the size of the corticospinal tract increase gradually throughout the primates (Heffner, 1983)

     Human bipedal movement seems to be modified from ancestral quadrupedal motion given some regulatory mechanisms still possessed in humans such as neuronal coupling of the upper and lower limb muscles and propriospinal tracts which link the cervical and lumbar enlargements of the spinal cord (Dietz, 2002).  Primates possess a larger cervical region of the spinal cord (Ariens).

     Mammals possess separate cervical, brachial, lumbar, and sacral plexuses (Weichert, 1970, p.639).  It appears that a cranial shift in the expression of some Hox A genes is responsible for a forward migration of the brachial plexus of birds compared to mammals (Gaunt, ?).  In prosimians and New World monkeys, the cervical and brachial plexuses are separated by a greater distance than they are in Old World monkeys and apes due to a cranial migration of the brachial plexus in the latter group.  (Hartman, 1933).  The contribution of the second thoracic nerve to the brachial plexus in Old World monkeys occasionally occurs in humans and apes (Hartman, 1933).

     Gorillas, chimpanzees, and humans share a number of neural modifications.  The median nerve usually innervates 3 ½ digits, the femoral nerve innervates the psoas minor, and the flexor digitorum longus is innervated by muscular branches of tibial nerve.  In chimps and humans, the axillary nerve supplies the subscapularis and the medial digit II is innervated by superficial peroneal nerve (Gibbs, 2002).

CAT SPINAL CORD