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THE NERVOUS SYSTEM AND
SPINAL CORD
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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). |
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Brains had humble beginnings (as evidenced
by the fact that many neuroanatomists would insist
that the term “cerebral ganglion” be used instead of “brain” when referring
to primitive bilateran animals). Acoelomate worms (particularly
a group called Acoela) are the most primitive
bilateral animals (other than some elongated comb jellies) and possess the
most primitive brains. At first this
brain has little connection with or control over the diffuse nerve net throughout
the rest of the body (Beklemishev, vol. 2, p.
50-1; Raikova, 2000). Acoels such as Convoluta stylifera and
Otocoelis gulmariensis
(and even turbellarians such as Xenoturbella) lack longitudinal nerves and their nervous system consists of a
diffuse subepithelial plexus which is more developed
at the anterior end (similar to the situation in coelenterate larvae)(Beklemishev, vol. 2, p. 80). These acoels retain
traces of radial symmetry: there
are four radially arranged brain rudiments and
four roots of ventral and dorsal nerves (Beklemishev,
vol. 1, p. 112-3). |
ACOEL![]() |
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ADVANCED FLATWORM![]() |
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CHORDATE![]() |
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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) |
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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).
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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). |
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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). |
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LAMPREY |
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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). |
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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 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). |
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CAT SPINAL CORD |
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