Biology Reference
In-Depth Information
Brain
Subesophageal
ganglion
Abdominal
ganglia
Thoracic
ganglia
Figure 5.13 Generalized diagrammatic representation of the insect nervous system.
Source : Introduction to Insect Anatomy. Available from: http://www.earthlife.net/insects/
anatomy.html .
Insects may be the first group to display clear cephalization. Evolution of a rigid
chitin exoskeleton protects them from predators and drought. The body consists of
the head, thorax, and abdomen, composed of a varying number of segments. They
have simple eyes with a single lens and compound eyes of multiple lenses. As arthro-
pods, they have jointed appendages. Most insects develop via metamorphosis and
undergo ecdysis.
The insect nervous system ( Figure 5.13 ) is composed of a brain that develops
from the fusion of three pairs of anterior ganglia (group of neurons) and the ven-
tral cord, which includes the subesophageal ganglion formed from fusion of three
pairs of ganglia, and pairs of thoracic and abdominal ganglia for each segment linked
together by connective cords that run the length of the body. The ganglia innervate
respective segments. In insects, the ICS displays the hierarchic character. The brain
and the subesophageal ganglion secrete neurohormones that stimulate specialized
endocrine glands such as the prothoracic gland and the corpora allata to respectively
secrete hormones ecdysone and JH. Thus the CNS controls the growth, development,
and reproduction in insects.
Vertebrates
Vertebrate brain consists of CNS and PNS. The hypertrophied CNS consists of
the brain (i.e., forebrain, midbrain, and hindbrain) ( Figure 5.14 ) and spinal cord.
The PNS consists of the visceral (autonomic) and somatic systems consisting
of nerves (axon bundles) innervating respectively the internal organs and mus-
cles, skin, and joints. In vertebrates, an accelerated increase in the size and the
proportion of the CNS and the brain-to-body weight with a strong trend toward
increasing cortex size is observed. The CNS evolved many specialized centers for
new functions and to perfect older functions. The CNS grows and its structure
and function becomes more complex ascending the higher vertebrate taxa ( Figure
5.14 ). Ever increasing elaboration of the structure and function of the brain in
higher vertebrates, especially in humans, made the human brain the most complex
of the known structures.
In vertebrates, the hierarchical character of the ICS becomes more conspicuous as
it is seen in the schematic representation of the flow of epigenetic information from
the brain structures down to particular genes ( Figure 5.15 ).
Search WWH ::




Custom Search