Neurons that carry messages from the sense organs to the brain or spinal cord are called sensory (afferent) neurons. Neurons that carry messages from the brain or spinal cord to the muscles and glands are called motor (efferent) neurons. Interneurons (association neurons) carry messages from one neuron to another. When the neuron is at rest, or at its resting potential, a slightly higher concentration of negative ions exists inside the membrane surrounding the cell body than outside, so there is a negative electrical charge inside relative to outside. At rest, a neuron is in a state of polarization. When an incoming message is strong enough, the electrical charge is changed, an action potential (neural impulse) is generated, and the neuron is depolarized. Incoming messages cause graded potentials, which, when combined, may exceed the minimum threshold of excitation and make the neuron fire. After firing, the neuron goes through the absolute refractory period, when it will not fire again, and then enters the relative refractory period, when firing will only occur if the incoming message is much stronger than usual. However, according to the all-or-none law, the impulse sent by a neuron does not vary in strength.
The neural impulsecommunication within the neuron.
At rest (A), there is an excess of negative ions inside the neuron compared to the outside. When a point on the semipermeable neural membrane is adequately stimulated by an incoming message, the membrane opens at that point, and positively charged ions flow in. (B) This process is repeated along the length of the membrane, creating the neural impulse that travels down the axon, causing the neuron to fire.
Electrical changes during the action potential.
The incoming message must be above a certain threshold to cause a neuron to fire. After it fires, the neuron is returned to its resting state. This process happens very quickly, and within a few thousandths of a second the neuron is ready to fire again.