Nervous system
The
nervous system is a highly specialized network whose principal components are cells called neurons. Neurons are interconnected to each other in complex arrangements, and have the property of conducting, using electrochemical signals, 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 system in humans
The human nervous system can be observed both with
gross anatomy, (which describes the parts that are large enough to be seen with the plain 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).
GROSS ANATOMY
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.
Peripheral nervous system
The PNS consists of all the other nervous structures 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.
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.
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.
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. The role of the enteric nervous system is to manage every aspect of digestion, 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 would not 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.