The nervous system is anatomically divided into central and peripheral parts, which are the CNS and PNS respectively. The CNS is made up of the brain and spinal cord, while the PNS is made up of peripheral nerves, cranial nerves, and spinal nerves. The nervous system is functionally classified into two components which are the somatic nervous system and autonomic nervous system (ANS). The autonomic component of the nervous system is basically further subdivided into sympathetic, parasympathetic, and enteric division. The nervous system primarily has many nerve cells. The two main principal types of cells are the neurons (also called nerve cells) and glial cells which are cells that support the neurons.
Neurons (nerve cells) are the cells that form both of the primary structural and functional unit of the nervous system. Neurons consist of different structural parts which are the cell body, dendrites, axon, and axon terminals (Figure 1). The cell body of the neuron is where the nucleus is located. Dendrites obtain stimuli from other neurons. Axon is a process that extends and carries information from the cell body of the neuron.
The human nervous system includes over 10 billion neurons. Thus neurons show the greatest variation in the shape and size of every group of body cells. Neurons can be categorized functionally into sensory, motor, and interneurons (Figure 2). Neurons can also be categorized as per the number of extending cell body processes into unipolar (pseudo-unipolar), bipolar, and multipolar neurons (Figure 3).
Peripheral Nervous System
As a peripheral part of the nervous system, the PNS is located beyond the brain and spinal cord. It consists of a motor, sensory neurons (in which their cell bodies are normally situated inside the spinal cord), and axons, which transmit impulses to the body organs. Histologically, individual axons are covered by an inner layer of connective tissue which is the endoneurium. A group of axons called nerve fascicles are surrounded by a mid-layer of connective tissue known as perineurium, whereas a group of nerve fascicles within a peripheral nerve is completely enclosed by an external layer of connective tissue termed as the epineurium (Figure 4)
The principal responsibility of the PNS is mainly to join CNS to other sections of the body as well as the outside environment. The PNS has cranial nerves which are twelve pairs in number. The cranial nerves provide sensory, motor, and autonomic innervation to the structures of the neck and head.
Nerve fibers are the smallest functional units in the peripheral nerves. Nerve fibers can be myelinated consisting of single axon surrounded by a single Schwann cell (SC) or unmyelinated containing few axons enclosed by just one SC. Schwann cells (SCs) cover the nerves as a layer of the myelin sheath (Figure 5). Myelin increases the velocity of nerve conduction by promoting its action faster. The PNS comprises of three main cell types: nerve cells, supporting cells, and stromal cells. Peripheral nerves transmit impulses between the spinal cord of the CNS and the other parts of the human body. The main function of the PNS is provided mostly by body cells.
Development of Nervous System
The nervous system begins to develop directly from the outer germ layer known as the embryonic ectoderm, starting in the 21 days of the developing human. During the third week, the first signs of the developing nervous system appear as the neural plate and neural groove grow on the posterior part of the trilaminar embryo (Figure 2.6). It is the notochord and paraxial mesenchyme that provokes the overlying ectoderm to develop into the neural plate. Signals from the notochord thicken the ectoderm to build the structure of the neural plate. The new lateral edges of the formed neural plate curl to the upper side, curve, and expand medially towards one other to approach one another and fuse to form the neural tube in a few days. The cells of the neural tube offer to the whole CNS. When the folds merge and the neural tube detaches from the surface ectoderm, the neural crest emerges from the neuroepithelium and turns into mesenchyme. Neural crest cells move widely to offer to cells that make up most of the PNS and ANS.