The nervous system is divided into the central nervous system and peripheral nervous system.
Central Nervous System: (C.N.S.)
The central nervous system is divided into two parts: the brain and the spinal cord. The brain contains about 100 billion nerve cells (neurons) and trillons of "support cells" called glia. The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. The vertebral column, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. Therefore, the spinal cord is much shorter than the vertebral column. Structure and different regions of brain are to be discussed in coming blogs.
Peripheral Nervous System (P.N.S.)
The peripheral nervous system is divided into two major parts: the somatic nervous system and the autonomic nervous system.
Somatic Nervous System
The somatic nervous system consists of peripheral nerve fibers that send sensory information to the central nervous system AND motor nerve fibers that project to skeletal muscle. The figure shows the somatic motor system. The cell body is located in either the brain or spinal cord and projects directly to a skeletal muscle.
Autonomic Nervous System
The autonomic nervous system is divided into three parts: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. The autonomic nervous system controls smooth muscle of the viscera (internal organs) and glands.
The figure shows the general organization of the autonomic nervous system. The preganglionic neuron is located in either the brain or the spinal cord. This preganglionic neuron projects to an autonomic ganglion. The postganglionic neuron then projects to the target organ. Notice that the somatic nervous system has only one neuron between the central nervous system and the target organ while the autonomic nervous system uses two neurons.
In the peripheral nervous system, neurons can be functionally divided in three ways: