 Before discussing the details of the nervous system, it is helpful to have a basic understanding of the cellular building blocks of the nervous system. The business cell in the nervous system is the neuron. In addition to neurons, there are also numerous support cells that will be covered in detail in other courses. Here, we will focus on neurons. A neuron's job is basically to pass messages from one part of the body to another. There are several types of neurons based on their structure and function, but all have certain common characteristics. First, they have a cell body with nucleus and organelles to provide for general metabolism and cellular maintenance. Second, they have processes specialized to receive(dendrites) and transmit (axons) signals. They have one to many dendrites, some of which have special endings that receive specific types of stimuli, such as pressure or temperature. Other dendrites may receive signals from other neurons. Neurons have only one axon process, which has one to several terminal endings that release neurotransmitters. Only the axon is capable of transmitting a signal to another neuron or to a cell of an effector organ through its neurotransmitter. In the nervous system axon terminals usually end on other neurons, most commonly on the dendrites of another neuron, but also on the cell body and rarely on an axon. The contact point between the neurons is called a synapse. A neuron's axon may be very long, such as an axon that runs all the way from the spinal cord to the big toe, or it may be very short, staying within a spinal cord segment. Signals that travel along axons are essentially electrical and consequently processes and neurons have to be insulated from one another like electrical wires. This is accomplished by supporting cells that encapsulate the neurons (except at synapses and axon terminals) or form sheaths around the processes. The sheath cells around some processes produce a lipid and protein material that is whitish in appearance called myelin. The myelin insulates the processes and greatly increases the speed of signal transmission. Neurons or processes that have myelin sheaths are said to be myelinated. Some neurons have processes that have a cellular sheath but lack myelin and they are said to be unmyelinated. The vast majority of axons in the nervous system are myelinated and accumulations of them appear whitish. Cell bodies never have myelin and accumulations of them have a grayish appearance. The nervous system is composed of billions of neurons that make billions of connections. This complexity gives us the ability to do everything from walk, to feel emotion, to learn, to constrict blood vessels, make us breathe and all of the other things that make us human. |
