Nervous Tissue Is Electrical and “Excitable”
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?
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. The concentrations of the different electrolytes are not the same across the plasma membrane.
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.
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.
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.
How do neurons become excited?
Neurons become excited in response to a stimulus. Sensory neurons are sensitive to specific stimuli in their surrounding environment. For example, sensory neurons found in human skin are sensitive to touch, pain, and change in temperature outside of the body. Meanwhile, sensory neurons located inside the body are sensitive to pain and changes in temperature inside the body. Sensory receptors in the ears, eyes, nose, and mouth register sound, light, smell, and taste, respectively. Once these neurons are excited by a stimulus, the excitability or impulse moves along that neuron toward the brain, where it is interpreted. Our brain initiates impulses as well. Some of these impulses travel throughout the brain for thinking processes and memory recall. Or these impulses may travel away from the brain toward destinations outside the central nervous system such as skeletal muscle, the heart, and other organs.
How do neurons communicate?
Although some neurons are very long and may extend several feet or so, the trek of an impulse traveling either from a sensory neuron to the brain or from the brain to other parts of the body requires several neurons linked together. These neurons are lined up end to end, but they do not actually touch. An impulse reaching the end of one neuron is transferred to the next neuron by way of special communicating chemicals called neurotransmitters.
Many different neurotransmitters are employed by our nervous tissue, including serotonin, norepinephrine, dopamine, histamine, and acetylcholine. Many of these will be discussed in later chapters, as either they are derived from nutrients or nutrients play an important role in putting them together. In fact, most neurotransmitters are made of amino acids. Furthermore, some neurotransmitters are very important in regulating how much and what types of foods we eat.
What is the brain?
As an adult, the human brain weighs about three and a half pounds and is protected by the skull. The brain is designed to interpret sensory input and decipher other incoming information, to develop both short- and long-term memory, to originate and coordinate most muscular movement, and to regulate the function of many of our organs. With all that it does it is easy to conclude that our brain is densely packed with neurons. And, with so many neuron operations taking place within the brain the electrical activity can be measured by placing sensors on the skin of the head.
The recorded output of this measurement is called an electroencephalogram or simply EEG.
No other animal on this planet has such a developed brain relative to its body size. In fact, the human brain is so big that during pregnancy the size of the baby’s head is a primary factor dictating the timing of birth. If babies were not born until the tenth or eleventh month of pregnancy, it would be extremely difficult for the head to fit through the mother’s birth canal.
What is the spinal cord?
The spinal cord extends from the brain and serves mostly as a relay station connecting the brain to the rest of the body. For protection, the human spinal cord is encased by bony vertebrae. The region of the spinal cord closest to the brain connects the brain to regions of the body in that proximity. This would include the chest and arms. Moving further down the spinal cord and away from the brain, you begin to find the interconnections between the central nervous system and the lower portions of our body, such as our legs. However, because the nerve links extending from the lower extremities must move through the upper regions of the spinal cord in order to connect with the brain, damage to the upper region of the spinal cord will affect the lower as well as the upper areas of our body. Thus, if damage occurs lower in the spinal cord it may result in temporary or permanent paralysis of only the lower extremities. However, if the spinal cord is damaged higher up, it can result in paralysis of both lower and upper extremities.
When you would like to move a particular body part, the process (idea) originates in the brain in a region called the motor cortex. Motor means movement! Once initiated, the impulse is carried along a linkage of nerve cells to the skeletal muscle responsible for moving the limb or body part that is to move. Incredibly the whole process only requires a couple neurons linked in series connecting the motor cortex of the brain to the muscle and occurs in a fraction of a second
While the motor cortex of our brain is busy sending signals to our skeletal muscle, signaling it to move, another region of our brain is evaluating and refining the movement. This region is called the cerebellum, which is behind and lower than the more recognizable parts of the brain. It is also this region of the brain that is particularly sensitive to the effects of alcohol and explains why movement becomes less refined when we are intoxicated.
Nervous Tissue + Neurons