The human nervous system is a complex collection of specialized cells that spans throughout the entire body, encompassing more than 100 billion cells in the brain alone. The purpose of the nervous system is to allow humans to detect stimuli, assess the importance of the stimuli and subsequently react to, or ignore, the stimuli. To be effective, the nervous system must perform all these functions on a millisecond scale. The success of the nervous system, therefore, lies in its use of electrical impulses to carry information quickly throughout the entire body.
Though intertwined, the nervous system is generally divided into two sections, the central and peripheral nervous systems. The central nervous system (CNS) is composed of the brain and spinal cord. Meanwhile, the peripheral nervous system (PNS) comprises the rest of the nervous system, such as the peripheral nerves and autonomic ganglia.
The nervous system is composed of two types of cells, the neurons, or nerve cells and the glia, or supporting cells. Although neurons comprise less than ten percent of the entire nervous system, they are the major workers, conducting information from one area to another in the nervous system. The glia cells, which comprise more than ninety percent of the nervous system, aid the neurons by acting not only as a structural support system, but also by ensuring the path is clear for efficient signal conduction between neurons.
So just how are signals conducted between neurons? It starts with the detection of stimuli, which occurs through the five senses, touch, taste, smell, hearing and sight. Each of the five senses has unique receptors localized throughout the body that convert the sense to an electrical impulse. When enough sensory receptors are activated, the electrical impulse reaches a threshold, triggering what is known as an action potential. Action potentials are large depolarizations in the cell's resting membrane potential. In other words, action potentials are acute changes in the electric current inside a cell. This change in electrical current starts at the location of the sensory receptor and travels through the cell by opening ion channels, allowing charged particles to flow into the cell. When the depolarization (change in electrical current) reaches the end of the neuron a small amount of chemical signal, called a quanta is released into the synapse, the small junction between cells. The chemical signal binds to receptors on nearby cells, causing small changes in the electrical current of that cell. When the conditions are right, an action potential is then triggered on that cell. And so, the sensory information is passed along afferent (sensory) cells in the nervous system until the signal reaches the CNS.
To assess its relevance, once in the CNS, the sensory signal is integrated and compared to other sensory information being received at the same time. This integration occurs in a part of the brain called the thalamus. The thalamus is often compared to a large train station, running at a high speed with several different connecting trains carrying all different types of people. Based on the signals being received, the thalamus sends signals to other parts of the brain, triggering, thoughts, emotions, memories and other reactions to the stimuli received. Some parts of the brain trigger physical responses, which induce muscle movement. The signal to move a muscle is transported down the spinal cord in the same way that stimuli are encoded, except that the signal moves through efferent (motor) cells instead of afferent (sensory) cells until it reaches the appropriate level to control the desired muscle. Thus, the nervous system is an integral part of both human anatomy and physiology.