Nervous system
The
nervous system of an
animal coordinates the activity of the
muscles, monitors the
organs, constructs and also stops input from the
senses, and initiates
actions. Prominent participants in a nervous system include
neurons and
nerves, which play roles in such coordination.
In animals without
brains, the nervous system does not generate or conduct
thoughts and
emotions. Thus it is the system that animates "animals" (
sponges are an exception).
Members of infrakingdom
coelenterata, such as
jellyfish and
hydra, have a simple nervous system called a
nerve net. This is a set of neurons, connected by
synapses or
gap junctions, that communicate with one another. The nerve net is centered around the mouth, but there are no anatomical groupings of neurons. Some jellyfish have sensory neurons known as
rhopalia which can perceive
light or
gravity.
Planarian, a type of
flatworm, have dual
nerve cords running along the length of the body and merging at the tail. These nerve cords are connected by transverse nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two large ganglia at the head end function similar to a simple brain.
Photoreceptors on the animal's eyespots provide sensory information on light and darkness.
The nervous system of the
roundworm Caenorhabditis elegans has been mapped out to the cellular level. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is
sexually dimorphic; the nervous systems of the two sexes, males and hermaprodites, have different numbers of neurons and groups of neurons that perform sex-specific functions. In
C. elegans, males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons [
1].
Arthropods, such as
insects and
crustaceans, have a nervous system made up of a series of ganglia, connected by a
ventral nerve cord made up of two parallel connectives running along the length of the belly [
2]. Typically, each body segment has one
ganglion on each side, though some ganglia are fused to form the brain and other large ganglia [
3].
The head segment contains the
brain, also known as the supraesophageal ganglion. In the
insect nervous system, the brain is anatomically divided into the
protocerebrum,
deutocerebrum, and
tritocerebrum. Immediately behind the brain is the
subesophageal ganglion which controls the
mouthparts.
Many arthopods have well-developed
sensory organs, including
compound eyes for vision and
antennae for
olfaction and
pheromone sensation. The sensory information from these organs is processed by the brain.
Most
molluscs, such as
snails and
bivalves, have several groups of intercommunicating neurons called
ganglia. The nervous system of the sea hare
Aplysia has been extensively used in
neuroscience experiments because of its simplicity and ability to learn simple associations.
The
cephalopods, such as
squid and
octopuses, have relatively complex
brains. These animals also have complex
eyes. As in all invertebrates, the
axons in cephalopods lack
myelin, the insulator that allows fast
saltatory conduction of action potentials in vertebrates. To achieve a high enough conduction velocity to control
muscles in distal
tentacles, axons in the cephalopods must have a very wide diameter in the larger species of cephalopods. For this reason, the squid giant axons were used by neuroscientists to work out the basic properties of the
action potential.
The nervous system of
vertebrate animals are often divided into a
central nervous system (CNS) and the
peripheral nervous system (PNS). The CNS consists of the
brain and
spinal cord. The PNS consists of all other nerves and neurons that do not lie within the CNS. The large majority of what are commonly called nerves (which are actually axonal processes of nerve cells) are considered to be PNS. The peripheral nervous system is divided into the
somatic nervous system and the
autonomic nervous system. For example, the human brain is part of the nervous system.
The
somatic nervous system is responsible for coordinating the body's movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control.
The
autonomic nervous system is then split into the
sympathetic division,
parasympathetic division, and
enteric division. The sympathetic nervous system responds to impending danger or stress, and is responsible for the increase of one's heartbeat and blood pressure, among other physiological changes, along with the sense of excitement one feels due to the increase of adrenaline in the system. The parasympathetic nervous system, on the other hand, is evident when a person is resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and
genitourinary systems.
Reference:
*
Kimball's Biology Pages, CNS*
Kimball's Biology Pages, PNS*
neural network*
neuroendocrinology*
neuroscience*
neurotoxin*
neural ensemble *
somatic sensation*
zootomy*
Neuroscience for Kids
