The nervous system is distributed throughout the body as a way of communication. The nervous system is divided into two major components, the central nervous system (CNS) and the peripheral nervous system (PNS) (Becker Kleinsmith and Hardin, 2002). The central nervous system is comprised of the brain and the spinal cord while the peripheral nervous system consists of nerve fibers and small aggregates of nerve cells outsisde of the CNS (Junqueira and Carneiro, 2005).
Figure 1-1. The general functional organization of the central and peripheral nervous sysems (Junqueira and Carneiro, 2005).Cells that make up the nervous system can be divided into neurons and glial cells (Becker et al, 2002). Neurons usually show numerous long processes and are responsible for the reception, transmission and processing of a stimuli. They trigger certain cell activities, and release neurotransmitters. Glial cells have short processes, they support and protect the neuron, they participate in the activity, nutrition and the defense of the central nervous system (Junqueira and Carneiro, 2005).
A typical neuron is composed of the dendrites, the cell body and the axon (Junqueira and Carneiro, 2005)
Figure 1-2 The structure of a Typical Motor Neuron (a) A diagram of a typical motor neuron. (b) A scanning electron micrograph of the cell body and dendrites of a motor neuron. (Becker et al, 2002).The cell body is the part that contains the nucleus and the surrounding cytoplasm (Junqueira and Carneiro, 2005). It contains extensions or branches, called processes (Becker et al, 2002). There are two types of cell processes, the ones that receive signals and ones that conduct signals, called dendrites and axons respectively (Becker et al, 2002). The cell body of most neurons receives a number of nerve endings that pass on either excitatory or inhibitory stimuli that have been generated in other cells. They usually have a spherical and unusually large pale staining nucleus with a prominent nucleolus. When the appropriate stain is used, the rough ER and free ribosomes appear under the light microscope as basophilic granular areas called Nissel bodies, as seen in figure 1-4. The number of Nissel bodies varies depending on the neuronal type and functional state, being particularly abundant in large nerve cells like motor neurons. Neurofilaments are abundant in the cell body and when viewed under the light microscope and fixed in silver, they form neurofibrils (Junqueira and Carneiro, 2005).
The Dendrites receive the synapses and are the principle signal reception and processing sites of the neurons. Each neuron would have many dendrites which increase the receptive area of the cell, this allows for one neuron to receive many stimuli and integrate them from the surrounding nerve cells. Dendrites become thinner in diameter as the branch out away from the cell body (Junqueira and Carneiro, 2005).
The axon is a cylindrical process which varies in length and diameter that depends on the type of neuron. The axons orginate in a short pyrimad shaped region known as the axon hillock (Junqueira and Carneiro, 2005). The plasma membrane of the within the axon is called the axolemma and its contents the axoplasm (Becker et al, 2002). The initial segment is the part of the axon between the axon hillock and the start of the mylenation, at this point the excitatory and inhibitory stimulations are summed which results in the decision to pass on the stimulus or not (Junqueira and Carneiro, 2005).
Neurons and their processes are variable in their size and in their shape, they can be large enough to be visible to the naked eye, or like the cell bodies of the granule cells of the cerebellum be among the smallest cells in the body. Neurons can be placed into one of three categories multipolar, bipolar and pseudounipolar. Multipolar neurons, have more than two processes, one being the dendrites and the other being the axon. Bipolar neurons have one dendrite and one axon and pseudounipolar neurons have a single process that is close to the cell body and divides into two branches, it then forms a T shape with one branch extending to the peripheral ending and the other to the CNS (Junqueira and Carneiro, 2005).
Figure 1-3. Simplified view of the three main types of neurons, according to their morpholigical characteristics (Junqueira and Carneiro, 2005).In the body, most of the neurons are multipolar, some examples of bipolar neurons are the ones found in the cochlear and vestibular ganglia also the ones in the retina and olfactory mucosa. The ones found in the cranial ganglia and in the spinal ganglia are the pseudounipolar neurons ( Junqueira and Carneiro, 2005).
Neurons can be subdivided into three basic types based on their functions: sensory neurons, motor neurons, and interneurons . Sensory neurons are a diverse group of cells that are specialized for detection of stimuli, they provide a stream of information to the brain about the body and its environment. For example, photoreceptors in the retina, olfactory neurons and various touch neurons located in the skin. Motor neurons transmit signals from the central nervous system to the muscles or glands that they stimulate. Interneurons process signals from other neurons and relay the information to other parts of the nervous system (Becker et al, 2002).
Figure 1-4 Photomicrograph of a motor neuron, a very large cell, from the spinal cord. The cytoplasm contains a great number of nissl bodies. The large cell process is a dendrite. Pararosaniline-toluidine blue (PT) stain. Medium magnification. (Junqueira and Carneiro, 2005).Surrounding the axons of nerve cells are a layer of insulating cells known as the glial cells (Becker et al, 2002). Glial cells are also found in the mammalian brain, they surround the cell body and all their process that occupy the interneuronal spaces. There are five types of glial cells in the nervous system: Oligodendrocytes, Schwann cells, astrocytes, ependymal cells and Microglia. Oligodendrocytes produce the myelin sheath that surrounds the neurons and provides the electrical insulation of the central nervous system, see figures 1-5 and 1-6D. These cells can branch and serve more than one nerve cell and its process (Junqueira and Carneiro, 2005).
Schwann cells have the same function as the oligodendrocytes but they are located around in the axons of the peripheral nervous system. As opposed to the oligodendrocytes, only one schwann cell can wrap around the axon of only one neuron (Junqueira and Carneiro, 2005).
The astrocytes are star shaped cells that have multiple radiating process and they are the most numerous glial cells. The astrocytes function to bind the neurons to the capillaries and to the pia matter. There are two main types of astrocytes called fibrous astrocytes and protoplasmic astrocytes. Fibrous astrocytes are located in the white matter, they have a few long processes, they can be seen in figure 1-5 and figure 1-6A. Protoplasmic astrocytes are located in the gray matter and many short processes, they can be seen in figure 1-5 and figure 1-6B. Astrocytes also control the ionic and chemical environment of the neurons, they influence neuronal survival and activity throughout their ability to regulate the extracellular environment (Junqueira and Carneiro, 2005).
Ependymal cells are columnar cells lining the ventricles of the brain and central canal of the spinal cord (Junqueira and Carneiro, 2005).
Microglia are small cells that have short irregular process, they are derived from precursor cells in the bone marrow. They are involved with inflammation and repair of adult cranial nerves of the central nervous system, they secrete a number of immunoregulatory cytokines and they dispose of unwanted cellular debris caused by CNS legions (Junqueira and Carneiro,2005). They can be seen figure 1-5 and figure 1-6C.
Figure 1-5 Drawings of neuroglial cells as seen in slides stained by metallic impregnation. Note that only astrocytes exhibit vascular end-feet, which cover the walls of blood capillaries.
Figure 1-6 Photomicrographs (prepared with Golgi stain) of glial cells from the cerebral cortes A: Fibrous astrocytes, showing blood vessls B:Protoplasmic astrocyte showing brain surface C: Microglial cell D: Oligodendrocytes
