What is nervous tissue?
Nervous tissue is comprised mostly of nerve cells or neurons, which serve as the basis for an extremely rapid communication system in our body. It also provides the basis for thinking. The central nervous system includes the brain and spinal cord and represents the thinking and responsive portion of our nervous tissue. Links of neurons extend from the central nervous system to various organs and tissues in our body thus allowing the central nervous system to regulate their function.
Also, links of neurons extend to our skeletal muscle thereby allowing the central nervous system to initiate and control our movement. Special neurons function as sensory receptors and are located in the skin and sensory organs (i.e., tongue, nose, ears, eyes) as well as deeper in tissue inside our body. These receptors keep the brain informed as to what is going on inside and outside our body. They register pain and sensation (sight, hearing, taste, smell, and touch) and relay that information to the brain where it is interpreted.
How do neurons work?
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Neurons are often referred to as excitable cells. Excitable cells are able to respond to a stimulus by changing the electrical properties of their plasma membrane. Only muscle and nerve cells are excitable and the basis for excitability lies in the electrolytes (ions) that are dissolved into our extracellular and intracellular fluids. As mentioned before, the concentrations of the different electrolytes are not the same across the plasma membrane (see Electrolyte Figure). |
In general the concentrations of sodium, chloride, and calcium are much greater in the extracellular fluid, while the concentration of potassium is greater in the intracellular fluid. This means that these electrolytes have the potential to move across the plasma membrane, down their concentration gradient, when their respective ion channels open up.
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When an excitable cell is stimulated, ion channels open in a specific and timely fashion. This allows electrolytes to move either into or out of the cell depending on the direction of their concentration gradient. The movement of the charged electrolytes changes the electrical nature of the plasma membrane at the site of the stimulus. Furthermore, when the cell is stimulated at one point on its plasma membrane, the excitability or impulse then moves along the plasma membrane like a ripple on a pond. Thus the excitability spreads and is often called a nerve impulse as shown in Nerve Impulse Figure. |
Neurotransmitters released at the end of neuron will interact with receptors on the adjacent cell (muscle or nerve) as shown in Nerve Impulse Figure. This can result in excitability of that cell, which may stimulate muscle contraction or transmitting a nervous impulse.
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