Neurons are electrically excitable cells. In other words, they have the ability to conduct changes using their membrane potential. Those changes are needed for the transmission of signals from the sensory stimuli within neuronal cells.
The main function of these specialized cells of the central nervous system is to take up, process, and transmit information from the brain to all the parts of the body through electrical and chemical signals.
Highly specialized in their structure, function, and communication links, neurons receive the stimuli from the sensory organs which relay them to the dedicated regulation centers of the brain, in order to inform the corresponding parts of the body of the planned actions, reactions, sensations, or movements.
For this complex process to function impeccably, a neuron needs to change its own charge in relation to the outer part of the neuronal cell. For this reason, the neuron influences the charged ions both inside and outside the neuronal cell membrane.
What Does Depolarization of Neuron Cells Mean?
The neurons receive signals from the sensory stimuli via their dendrites. After that, they transmit that information electrochemically. The outgoing signal continues its way to the other neuronal cells in the brain through the neuron’s axons, with the help of electrical and chemical signals.
When the chemicals in our body have an electrical charge, they are called ions. The most important ions in the nervous system are sodium and potassium.
Furthermore, a cell membrane is a protective barrier that encloses the neuronal cell and allows nutrients and waste to flow back and forth through the structures.
The cell membrane has specialized passageways that allow for the transport of matter from the inside and outside of the cell. It allows some ions to pass through and blocks the passage of other ions.
The inside and outside of a neuronal cell will have different electrical charges (positive and negative) depending on what kinds of nutrients are in and around the cell.
Hence, depolarization is a change in the difference between the electric charge on the inside and the outside of the neuron cell membrane.
When depolarization occurs, the inner surface of the membrane becomes less negative in relation to the outer surface (the neuronal cell becomes more positively charged, thus becoming less negative as a unit).
More specifically put, the electrical charge of the neuronal cell changes due to the molecules entering or exiting the cell, because the molecules can have a positive or a negative charge.
What Is the Action Potential of Neuron Cells?
The transmission of data gained from the sensory stimuli sent to neurons of the corresponding brain structures is mediated by action potentials.
Practically, the information from the sensory organs of the body is conveyed via the neurons throughout the nervous system in the form of tiny waves of electricity labeled as nerve impulses, i.e. action potentials.
The action potential can also be described as a sudden, speedy, transitory nerve signal that triggers a widely spread (propagated) change of the membrane potential while at rest.
As a matter of fact, the action potential occurs when a neuron sends information down an axon, away from the cell body. This property of the neuronal cells is labeled as excitability.
Namely, when nerve cells are stimulated, they undergo chemical changes that produce those nerve impulses, thus creating the action potential of the neuron cell needed so that they can transmit the information from the sensory stimuli.
During their inner or outer movement in regard to the neuron cell, molecules reduce the electric potential across the neuron’s plasma membrane. A stimulus first triggers the sodium channels to open.
But, since there are a lot more sodium ions on the outside part of the neuron, and the inside part of the neuron is negative in relation to the outside part, sodium ions flow into the neuronal cell.
Since sodium has a positive charge, the neuron becomes more positive and becomes depolarized. When the intensity of the stimuli goes beyond the excitatory threshold of the neuron, an action potential is created. Then, a nerve impulse spreads along the stretch of membrane.
Thus, when the stimuli change the neuronal cell membrane potential to the values of threshold potential, an action potential of the neuronal cell membrane is produced with the purpose of transmitting the sensory information to the corresponding centers of regulation and control in the brain.
However, not all stimuli can trigger the electrical change of the neural cell membrane. The corresponding sensory stimuli must possess a sufficient electrical value in order to trigger a reduction in the negative charge of the neuronal nerve cell to the threshold of the action potential.
In this regard, we can distinguish the following degrees of stimuli:
- Subthreshold stimuli (they are not strong enough to trigger the action potential, i.e to create the nerve impulse of the neuronal cell)
- Threshold stimuli (they have just enough energy to trigger the action potential, i.e to create the nerve impulse of the neuronal cell)
- Suprathreshold stimuli (they possess higher strength than the threshold stimuli needed to trigger the action potential, i.e to create the nerve impulse of the neuronal cell).
In addition, after one action potential is generated, neurons become unable to react to the stimuli for a certain period of time, i.e they cannot generate another action potential during that time interval.
What Determines the Speed of the Action Potential?
The speed required to transmit signals via the nerve cells of the brain to the corresponding parts of the body, i.e. the action potential, depends on several factors:
- The diameter of the nerve fiber – The fastest transmission of signals occurs in the longest nerve fibers.
- The temperature of the nerve fiber – The speed of the signal transmission increases at high temperatures, and decreases at low temperatures.
- Myelination of the nerve fiber – Since the ions cannot cross the lipid content of the myelin cover, they flow passively down the nerve fiber until reaching the unmyelinated nodes of Ranvier (a gap in the myelin sheath of a nerve).
What Is the Difference Between Action Potential and Membrane Potential?
At this point, it is important to distinguish that the term membrane potential refers to the difference in electric charge between the inside and outside part of a neuron, while an action potential refers to the electrical signals that occur within the neurons in order to transmit the information from the sensory organs of the body.
What Is Depolarization in Action Potential?
In the process of depolarization, the negative internal charge of the neuronal cell temporarily becomes more positive (less negative). This change from a negative to a more positive membrane potential of the neuronal cell occurs during the course of several complex chemical processes, including the action potential.
In order to change the electrical chargeback to a non-agitated state, the neuron sends another electrical signal. The entire process, again, occurs when the cell allows specific ions to flow into and out of the neuronal cell.
In this occurrence, the positively charged potassium ions flow in the opposite direction, restoring the balance of the neuronal cell charge. Positive ions rush in through the ion channels.
The neuronal cell membrane is first depolarized, then its polarity is reversed, resulting in an action potential of +30 millivolts inside. This stimulates an adjacent area of the membrane so that the impulse flows through the brain structures like a wave.
What Does the Resting Potential of Neuron Cells Denote?
A nerve impulse is mainly based upon the flow of the positively charged sodium and potassium ions through the neuronal cell membrane.
When neurons are not excited, i.e when they are not transmitting any signals, they are at rest. In this way, the resting potential of neuron cells informs us about what happens when a neuron is not active.
At this phase, the neurons don’t transmit or receive the nerve impulse, and this cell potential occurs due to the presence of negative charges inside the neuronal membrane.
In other words, when a neuron is at rest, the inside of its cell is negative in relation to the outside.
What Is Hyperpolarization of the Neuron Cells?
On the other hand, hyperpolarization is the opposite process of depolarization. It occurs when the neuronal cell becomes more negatively charged at a particular spot on the membrane.
This process prevents any stimulus which has already been sent up an axon from triggering another action potential in the opposite direction. Hyperpolarization can also raise the threshold for any new stimulus.
To Sum Up
Depolarization occurs when the neuronal cell membrane is stimulated and sodium ions flow into the cell. If the intensity of the stimuli exceeds the threshold of the neuron, an action potential is created and a nerve impulse is triggered.
Nerve cells at rest are positively charged on the outside and negatively charged on the inside. When the nerve cell reverses these charges, depolarization occurs.
This process can occur in the neurons as a result of some sensory stimuli, such as heat, chemicals, light, electrical or physical stimuli. This sensory information received by the neurons generates a positive potential for further transmission inside them.
Fun Facts
Did You Know?
- The nerve impulses travel at speeds between 3–400 ft/s (1–120 m/s), depending on the type of nerve. Movement is faster in myelin-coated axons.
- For any given neuron, the size of the action potential is always the same.
- The neuron either does not reach the threshold, or a full action potential is activated (fired) – this is called an ‘all-or-none principle’, because there are no gradations between threshold potential and fully activated potential.
- It takes longer for potassium channels to open. When they do open, potassium ions rush out of the cell, thus reversing the depolarization.
- Because of the fact that nerve impulses are not graded in amplitude, the size of the action potential is not the most important in processing information within the central nervous system. Instead, it is the number and frequency with which the impulses are fired.
- Both depolarization and hyperpolarization occur when ion channels in the membrane open or close. In that way, they change the ability of particular types of ions to enter or exit the cell.
- Hyperpolarization is the state which a neuron enters right after depolarization.
- An action potential has several phases: hypo-polarization, depolarization, overshoot, repolarization, and hyperpolarization.
- The phase of extreme positivity of the cell is called an overshoot phase.
- This phase of restoring the resting membrane potential is called a repolarization phase.