Everything about The Nervous System totally explained
The
nervous system is a highly specialized network whose principal components are nerves called
neurons. Neurons are interconnected to each other in complex arrangements and have the property of conducting, using electrochemical signals, and a great variety of stimuli both within the nervous tissue as well as from and towards most of the other tissues. Thus, neurons coordinate multiple functions in organisms.
Nervous systems are found in many multicellular
animals but differ greatly in complexity between species.
Nervous system in humans
The human nervous system can be grouped into both with
gross anatomy, (which describes the parts that are large enough to be seen with the
naked eye,) and
microanatomy, (which describes the system at a
cellular level.) At gross anatomy, the nervous system can be grouped in distinct organs, these being actually stations which the
neural pathways cross through. Thus, with a didactical purpose, these organs, according to their ubication, can be divided in two parts: the
central nervous system (CNS) and the
peripheral nervous system (PNS).
Central nervous system
The central nervous system (CNS) represents the largest part of the nervous system, including the
brain and the
spinal cord. The CNS is contained within the
dorsal cavity, with the brain within the
cranial cavity, and the spinal cord in the
spinal cavity. The CNS is covered by the
meninges. The brain is also protected by the skull, and the spinal cord is also protected by the vertebrae. The nervous system can be connected into many systems that can function together. The two systems are central nervous system (CNS)and the peripheral nervous system(PNS).
Central
nervous
system
| Brain |
Prosencephalon |
Telencephalon |
Rhinencephalon,
Amygdala,
Hippocampus,
Neocortex,
Lateral ventricles
|
| Diencephalon |
Epithalamus,
Thalamus,
Hypothalamus,
Subthalamus,
Pituitary gland,
Pineal gland,
Third ventricle
|
| Brain stem |
Mesencephalon |
Tectum,
Cerebral peduncle,
Pretectum,
Mesencephalic duct
|
| Rhombencephalon |
Metencephalon |
Pons,
Cerebellum,
|
| Myelencephalon |
Medulla oblongata |
| Spinal cord |
Peripheral nervous system
The PNS consists of all the other nervous structures that don't lie in the CNS. The large majority of what are commonly called nerves (which are actually axonal processes of nerve cells) are considered to be PNS.
Microanatomy
The nervous system is, on a small scale, primarily made up of
neurons. However,
glial cells also play a major role.
Neurons
They are the core components of both the
central nervous system &
peripheral nervous system. Neurons are sensors that send electric messages to the Central Nervous System which send the electric messages back to the neurons telling them how to react, where the messages are finally sent back directly to the brain. These messages travel at a usual pace of 100 meters per second.
Glial cells
Glial cells are non-
neuronal
cells that provide support and nutrition, maintain
homeostasis, form
myelin, and participate in signal transmission in the
nervous system. In the
human brain, glia are estimated to outnumber neurons by about 10 to 1.
Glial cells provide support and protection for
neurons. They are thus known as the "glue" of the nervous system. The four main functions of glial cells are to surround neurons and hold them in place, to supply
nutrients and
oxygen to neurons, to insulate one neuron from another, and to destroy
pathogens and remove dead neurons.
Physiological division
A less anatomical but much more functional division of the human nervous system is that classifying it according to the role that the different neural pathways play, regardless whether these cross through the CNS or the PNS:
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.
Of digestion, it regulates from the esophagus to the stomach, small intestine and colon.
In turn, these pathways can be divided according to the direction in which they conduct stimuli:
- Afferent system by sensory neurons, which carry impulses from a receptor to the CNS
- Efferent system by motor neurons, which carry impulses from the CNS to an effector
- Relay system by relay neurons (also called interneurons), which transmit impulses between the sensory and motor neurones.
A useful mnemonic to remember the nature of Afferent vs Efferent is SAME DAVE: Sensory Afferent, Motor Efferent; Dorsal Afferent, Ventral Efferent
However, there are relay neurons in the CNS as well.
The junction between two neurones is called a
synapse. There is a very narrow gap (about 20nm in width) between the neurons - the
synaptic cleft, where an action potential is transmitted from one neuron to a neighboring one. They do this by relaying the message with the use of
neurotransmitters which the next neuron then receives the electrical signal, known as a
nerve impulse. The nerve impulse is determined by the neurotransmitter to then carry the message to its appropriate destination. These nerve impulses are a change in ion balance in the nerve cell, which the central nervous system can then interpret. The fact that the nervous system uses a mixture of electrical and chemical signals makes it incredibly fast, which is necessary to acknowledge the presence of danger. For example, a hand touching a hot stove. If the nervous system was only comprised of chemical signals, the body wouldn't tell the arm to move fast enough to escape dangerous burns. So the speed of the nervous system is a necessity for life.
Development
Some landmarks of
embryonic neural development include the birth and
differentiation of
neurons from
stem cell precursors, the
migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of
axons from neurons and
guidance of the motile
growth cone through the embryo towards postsynaptic partners, the generation of
synapses between these axons and their postsynaptic partners, and finally the lifelong
changes in synapses which are thought to underlie learning and memory.
Importance
Many people have lost basic motor skills and other skills because of spinal cord injuries. If this portion is damaged, the biggest nerve and the most important one gets damaged. This leads to paralysis or other permanent damages.
Abilities
The nervous system is able to make basic motor skills and other skills possible. The basic
5 senses of texture, taste, sight, smell,and hearing are powered by the nervous system. If disabled, basic motor skills may be lost.
Non-humans
Vertebrates
The nervous system of all
vertebrate animals, is often divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal.
Worms
Planaria, a type of
flatworm, have dual
nerve cords running along the length of the body and merging at the tail and the mouth. 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 dark.
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
hermaphrodites, 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
(External Link
)
Arthropoda
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 (External Link). Typically, each body segment has one
ganglion on each side, though some ganglia are fused to form the brain and other large ganglia
(External Link).
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 is composed of three pairs of fused ganglia. It controls the
mouthparts, the
salivary glands and certain
muscles.
Many arthropods 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.
Development
Neural development in most species have many similarities
neural development in humans.
Further Information
Get more info on 'Nervous System'.
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