PROCESSES OF NEURONS
The Receiving and Sending Processes of Neurons
Behaviors are controlled by our nervous system. Neurons, a kind of cell in the system, can help us to understand how an action is conducted. A neuron receives information by multi-input via dendrites, makes decisions based on received information in cell body, and releases single-output computational device via its axon. To understand the process of information transmitting, we would explain in terms of EPSPs, IPSPs, temporal summation, spatial summation, and action potentials.
Firstly, we would describe the structure of neurons. The neurons have dendrites, cell baby/soma, axon and presynaptic terminals. Each of them has different functions. The dendrites are branching fibers. They receive information from other neurons mainly. A neuron has more than one dendrite. The soma contains the same basic structures of other animal cells. It is responsible for collecting and processing information. Moreover, each neuron has one axon, which is a thin fiber, to send nerve impulses toward other neurons, organs, or muscles. The presynaptic terminals, at the end points of axon, aim to release chemicals through the junction between one neuron and the next. They can help neurons to communicate with others. Besides, there is a specialized gap between neurons, a synapse. The presynaptic terminals of one neuron release chemicals to the dendrites of a second neuron at the synapse. The chemicals are called neurotransmitters.
Besides, before explaining the processes of receiving information and sending signal, we would outline the state of each structure inside neurons at rest. An electrical gradient is a difference in electrical charge between the inside and outside of a neuron, because different numbers of ions between these two locations. Ions are chemical particles with electrical charges. The whole neuron is covered by a membrane which keeps the electrical gradient. The membrane also can maintain an electrical polarization that is a difference in electrical charge between two locations. Sodium ions are more concentrated outside the cell, and potassium ions concentrated inside. The inside of the membrane has relatively negative electrical potential to the outside. This difference inside the cell body is called resting potential which can be measured by very thin microelectrodes. The value of resting potential which is a state in the absence of outside disturbance is -70 millvolts (mV). However, the ions cannot pass the neuron membrane freely, because the membrane is selective permeability. Only some molecules, such as water, oxygen, and carbon dioxide, can flow freely, while others are restricted. The important ions, sodium and potassium ions, can cross the membrane by potential channels. During the resting potential, sodium channels remain closed, so sodium ions cannot pass through the membrane. On the other hand, potassium channels are nearly but not completely closed, potassium ions are allowed to cross slowly, thus.
Even at rest, the ions may cross the membrane when the sodium-potassium pump exists. This pump is an active transport requiring energy. It transports three sodium ions out of the cell but at the same time it draws two potassium ions into the cell. Because of the pump, the concentration of sodium ions outside the cell is more than ten times to the inside, and that of potassium ions are similar between two locations. Moreover, there are two forces acting on ions to affect their movement inside or outside the cell. One is based on electrical gradient, and the other is concentration gradient. We would use the sodium and potassium ions to explain more. Sodium ions are positive charge, so the charge is negative inside the cell. Because opposite charges attract, the electrical gradient tends to pull them into the cell. Also, the outside sodium ions are more concentrated than inside. The ions are more likely to enter the cell than to leave based on the concentration gradient. Both of two forces tend to push sodium ions into the cell. Furthermore, the forces act on potassium ions in opposite to each other. Potassium ions are also positive. They are in negative charge relative to outside. As opposite charges attract, the ions are pulled into the cell by the electrical gradient. However, the concentration of inside potassium ions is higher than outside, so the ions are more likely to leave the cell.
The resting potential remains stable until the neuron is stimulated. The stimulation may increase or decrease the polarization or the difference between the electrical charges of two places. The process of increasing is known as hyperpolarization. It increases the negative charge inside the axon. The other is depolarization that decreases the negative charge inside the axon. In addition, a rapid depolarization can activate an action potential occurring. Action potential is the release of neuron
a nerve impulse that is an electrical message to transmit down the axon of the neuron.
The neuron remains stable until being stimulated. The stimulation should be higher than a certain level called the threshold of excitation.
Stimuli are transmitted by two methods, temporal summation and spatial summation.
The temporal summation is a phenomenon that repeated stimuli within a brief time. The stimuli would be quickly combined at a single synapse.
The spatial summation has property that synaptic inputs combine their effects from separate locations, several synapses, onto one neuron.
A single stimulus produces a synaptic transmission less than the threshold to activate the action potential.
Kalat, J. W. (2009). Biological psychology (10th Edition; International Student Edition). Belmont, CA: Wadsworth, Cengage Learning.