Beginning Of An Impulse
A nerve cell membrane has special protein molecules that control the opening and closing of its pores. When at rest, the membrane keeps the concentration of sodium ions in the neuron very low. The membrane also keeps the concentration of potassium ions and negative organic ions much higher in the cell than in the surrounding fluids. These differences in ion concentration make the inside of the neuron more negative than the outside, and so the membrane is said to be polarized. The resulting voltage difference across the membrane is called the resting potential. A chemical, electrical, or mechanical stimulus applied to a neuron can affect the membrane's porosity and change the resting potential. The stimulus can cause the membrane's pores to open and allow more sodium ions into the cell. The increase in sodium ions makes the inside of the cell positively charged, and this voltage change is called a depolarization. When a stimulus causes a neuron to depolarize, the neuron is said to fire. The firing of a neuron is the beginning of a nerve impulse. A stimulus must be of a certain intensity, called the threshold voltage, for a neuron to fire. All impulses from a particular neuron have the same size and duration, no matter how large the stimulus that caused the neuron to fire. The fact that neurons fire at maximum strength or not at all is called the all-or-nothing phenomenon. The brain probably detects the intensity of a stimulus by the frequency of impulses generated and the number of nerve fibres stimulated.