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. As mentioned before, the concentrations of the different electrolytes are not the same across the plasma membrane (Figure 2.4). In general the concentrations of sodium (Na+), chloride (Cl-), and calcium (Ca2+) are much greater in the extracellular fluid, while the concentration of potassium (K+) 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

180 160 H 1" 140

120-

S 100

Sodium

Potassium

Chloride

Figure 2.4 Relative difference in the concentrations of sodium, potassium, and chloride dissolved in the fluid inside and outside of our cells. Cells use a lot of energy (ATP) to maintain these concentration differences by pumping sodium and potassium across the plasma membrane.

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 Figure 2.5.

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.

Receptor • Neurotransmitter

Figure 2.5 Neurotransmitters released at the end of the neuron will interact with receptors on the adjacent cell (muscle or nerve). This can result in excitability of that cell, which may stimulate muscle contraction or transmit a nervous impulse.

Receptor • Neurotransmitter

Figure 2.5 Neurotransmitters released at the end of the neuron will interact with receptors on the adjacent cell (muscle or nerve). This can result in excitability of that cell, which may stimulate muscle contraction or transmit a nervous impulse.

0 0

Post a comment