To understand the nervous system we'll start with a microscopic description of what composes nervous tissue. We will then talk about the different macroscopic divisions.
Microscopic Description:
There are two types of cells that compose nervous tissue in the body: neurons and glia. Neurons are the information processing cells. They communicate with one another through electrical and chemical means. Electrical communication in the nervous system is accomplished through "action potentials". Action potentials are converted into a chemical message at the "synapse", which is the area where two different neurons communicate with one another.
Glia, on the other hand, are the supporting cells. There are many different types of glia, all of which have different functions. For example, glia known as oligodendrocytes help insulate neuronal axons so that electrical communication in the brain and spinal cord is fast and efficient. Astroctyes, another type of glia, are responsible for ensuring the blood brain barrier. They also maintain the integrity of the chemical environment around neurons. Another type of glia, ependymal cells, line the ventricles, or fluid filled cavities of the brain. It is important to realize that there are other types of glia with important functions.
When we combine many millions of neurons and glia into structural units we arrive at the basis for defining the nervous system at a macroscopic level.
Macroscopic Description:
The nervous system is usually divided into two different divisions depending on anatomical location. These divisions are the central nervous system (CNS) and the peripheral nervous system (PNS).
CNS: Brain and Spinal Cord
The CNS is composed of the brain and spinal cord. The brain is further sub-divided into different parts. For example the cortex, sub-cortex, cerebellum, and brainstem are all considered parts of the brain, and therefore parts of the CNS.
The other part of the CNS is the spinal cord. The cord receives information from the brain and then sends it to the proper body part. It also receives information from the body, which is relayed back to the brain for interpretation. The cord is surrounded by the same protective layers as the brain. It is also protected by bony structures know as vertebrae.
Once neural information leaves the cord and enters the "body proper" it is now in the PNS.
PNS: Somatic and Autonomic Divisions
Like the CNS, the PNS also has several different components. The first major division of the PNS is the somatic system. The somatic division refers to all the nerves that innervate skeletal muscle, as well as the sensory nerves that send information back to the CNS.
The second component of the PNS is the autonomic division. The autonomic nervous system can be further divided into sympathetic and parasympathetic sub-divisions.
The sympathetic nervous system is responsible for "revving-up" the body. When your heart races, pupils dilate, or respiratory rate increases due to a stressor it is because of the sympathetic nervous system. Epinephrine (aka: adrenaline) and norepinephrine (aka: noradrenaline) act on the sympathetic nervous system. The sympathetic system is often humorously summarized as the "4-Fs": fight, flight, fear, and reproduction (I'll let you figure out the best "F" word for the last one).
The parasympathetic division can be thought of as the opposite of the sympathetic system. It is responsible for the "rest and digest" functions of the body. It helps slow the heart rate, decrease the respiratory rate, constrict the pupils, and aids in the control of digestive processes. The parasympathetic division usually dominates the overall state of the body at rest.
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