|Memory > Neuron and Nerve Impulse|
The nervous system is made up of neurons and glial cells (glia). Neurons are specialized cells that can receive and transmit chemical or electrical signals. Glia provide support functions for the neurons.
The Structure of a Neuron
Figure 1.1. The structure of a myelinated neuron.
A neuron is composed of three parts: cell body (soma), dendrites, and an axon. The cell body is similar to other types of cells, containing a nucleus and other typical organelles. The dendrites project from the cell body to receive signals from other neurons. The axon extends from the cell body to transmit signals to other neurons. The endings of an axon is called axon terminals. Some axons are covered with myelin sheath, which is produced by glial cells. The signal transmission speed along myelinated axon is faster than unmyelinated axon.
Like other types of cells, a neuron also contains a membrane separating the extracellular fluid from the cytoplasm. The Na+ concentration on the extracellular side is much higher than inside, but the K+ concentration is just the opposite: inside is much higher. The membrane potential (also called "membrane voltage") is defined as the inside electric potential minus the outside electric potential. When a neuron is at rest, the membrane potential is about -70 mV, that is, inside is more negative than outside.
Ion channels are a special class of proteins embedded in the cell membrane. A channel which preferentially conducts Na+ ions while excluding other ions is called Na+ channels. Similarly, a K+ channel preferentially conducts K+ ions. At the resting potential, most Na+ and K+ channels are closed. Although there is dramatic difference in ion concentration which tends to drive Na+ ions inward and K+ ions outward, the Na+ influx and K+ outflux passing through the channels are quite small, which can be balanced by the Na+/K+ pump (Na+/K+-ATPase) - an enzyme that pumps Na+ ions outward and simultaneously K+ ions inward.
A membrane is said to be depolarized if the new membrane potential Vm is more positive than the resting potential Vm0, and hyperpolarized if Vm < Vm0.
Figure 1-3. A typical nerve impulse (action potential).
The nerve impulse is a sharp change of the membrane potential. Therefore, it is also known as action potential (Figure 1-3). In a neuron, the action potential is elicited when the membrane potential is depolarized to a critical value (the threshold). In most neurons, the threshold is about 15 mV above the resting potential. Before the threshold is reached, the nerve membrane behaves like an ordinary inactive substance characterized by certain resistance and capacitance.
The active properties of a nerve membrane is due to the voltage-dependence of Na+ and K+ channels whose open probability increases with increasing depolarization. The opening of Na+ channels increases Na+ influx, while the opening of K+ channels increases K+ outflux. Since the Na+ ions carry positive charges, more Na+ ions in the cytoplasm will increase the inside electric potential, making the membrane more depolarized. The K+ outflux will make the membrane potential more hyperpolarized.
The response of Na+ channels to depolarization is faster than K+ channels. Therefore, the rising phase of the action potential is mainly due to the opening of Na+ channels. After the Na+ channel is opened, it may become inactivated due to the closing of an inactivation gate. This phenomenon is called sodium inactivation. The falling phase arises from both sodium inactivation and the opening of K+ channels.
Author: Frank Lee