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Refractory Period: Definition, Ion Channels, Phases And Its Various Applications.

Refractory Period Definition

The refractory period of a neuron is the time frame during which it is incapable of generating an action potential (nerve impulse). There are two kinds of neurons: absolute refractory period and relative refractory period. Once the sodium channels before this impulse are deactivated, the initial indicates an inability to send a fresh impulse. The phrase “relative refractory period” pertains to the time interval immediately after the absolute type during which a second impulse is simply repressed. Even so, this second impulse may be conveyed, but only with a sufficiently powerful stimulation.

What is Refractory Period?

You must grasp how electrical instructions are carried between nerve cells or between nerve cells and other tissue cells in order to comprehend the refractory period.

Action Potentials

The central and peripheral nervous systems both rely on neurons. A soma (cell body), dendrites, and an axon make up a typical neuron. Nerve cells come in a variety of forms, A general neuron gets chemical messages through neurotransmitters that enter at the dendrites and passes them via electrical impulses down the axon to the subsequent cell.

Dendrites function similarly to tree branches, absorbing nutrients and energy from their environment. The axon carries energy (and nutrients) from the branches to the tree’s trunk.

Neurons are cells that may be stimulated electrically. When the cell membrane is triggered, the voltage is changed in the way of the target cell, one segment at a time. The voltage shift that occurs when a neuron is activated travels down the axon. The shift in voltage is referred to as an “action potential.”

When an action potential arrives at an axon’s terminal, the neuron transfers neurotransmitters (chemicals) to the subsequent neuron or goal cell, which is usually a muscle cell. When another neuron is the goal cell, the dendrites absorb signalling neurotransmitters.

The synaptic cleft is the gap between one neuron’s terminal and the next neuron’s dendrite. To send a message, neurotransmitters must float over this gap. The receiving neuron is either stimulated or inhibited by these chemical messengers. When a receiving neuron is stimulated, it generates its own action potential.

Ion Channels

Numerous channels run across an axon’s membrane. Examples include sodium (Na+) and potassium (K+) ion channels. These channels are opened and closed by electrical or chemical signals.

When ion channels open or close, the electrical charge on the inner and outer walls of the neuron membrane fluctuates. This does not happen all at once, but rather in segments.

This is not conceivable in myelinated neurons because the cell membrane is coated with a dense protein coating. If ion channels were put beneath such a thick coating, they would be unable to operate. Ion channels situated in the nodes of Ranvier (unmyelinated regions) continue to convey changes in membrane voltage. Ions are urged to migrate in or out of the cell to achieve balance by adjusting the ion concentrations within and outside the neuronal cytoplasm.

When Na+ channels activate at the start of an action potential, Na+ ions from outside the cell race in, positively charging that portion of the cell. K+ ions from within the cell rush out when K+ channels open, making that region of the membrane of the neuron more negatively charged. The voltage of the cell membrane can be changed by various methods.

Because of the refractory time, an action potential seldom goes backward. Ion channels need time to reopen once they have closed. This implies that the positive charge of the following part attracts the negative charge created at one point on the cell membrane. The action potential travels up the axon after the negative charge generates a response ion channel in the next set.

Action Potential Phases

In the resting state, the inside of the neuron adjacent to the membrane is more negatively charged than the extracellular environment. A resting neuron’s voltage is usually between 60 and 70 millivolts (mV). The intensity of an incoming stimulus causes this voltage to vary. The intracellular region most closely associated with the membrane must, however, attain a threshold voltage of –55 mV before a neuron may send an action potential. Action potential cannot be launched until this is accomplished.

Once the intracellular portion of the neuron membrane hits –55 mV, the Na+ ion channels nearest to the dendrites open. When sodium ions reach a cell, they positively charge the intracellular space surrounding them. The depolarization phase is what it’s termed. Depolarization occurs in a wave-like pattern along the axon. Throughout this phase, the membrane potential becomes more highly positive than it was in the state of rest.

Keep in mind that sodium ions are frequently found outside the membrane, and their positive charges raise the portion of the membrane inside the neuron that they enter; positively charged potassium ions are frequently found inside the neuron, and while they storm out, the inner side of the membrane becomes more negatively charged.

When the intracellular voltage of the neuron reaches around +30 mV, Na+ ion channels inside this portion of the membrane close and K+ ion channels open.

The neuron releases potassium ions into the extracellular space. This is the repolarization period. Once again, repolarization occurs in waves along the axon membrane. The membrane potential becomes highly negative during this phase rather than during depolarization.

The on-off switch, like other neural circuits, isn’t perfect; rather than stopping immediately upon reaching resting potential, ions continue to move along their pathways for a short duration. Hyperpolarization occurs in that portion of the membrane—more detrimental than the resting potential—during this time.

The inner surface of the neuron membrane achieves a voltage of roughly 70–75 mV during the hyperpolarization or overshoot phase. Resting-state potassium ion concentrations can be attained only when all potassium ion channels are closed.

Absolute vs Relative Refractory Period

The disparity between the absolute and relative refractory periods may now be understood using the information provided above. Refractory intervals prevent stimuli from overlapping in an action potential.

Every action potential takes about one millisecond to communicate in principle. This suggests that an individual axon should be capable of transmitting at least 1,000 action potentials per second. However, this quantity is far lower in reality. The absolute refractory period is around 1 millisecond long, whereas the relative refractory period is about 2 milliseconds long.

Multiple action potentials do not occur concurrently in a single neuron. This is due to the fact that a neuron is in one of two possible states: impossible or very hard to initiate a subsequent action potential. These two instances illustrate the two distinct types of refractory periods.

No additional stimulus may have any further influence during the period of depolarization once the channels for Na+ ions are open. Ion channels don’t open in stages; they are either closed or open. This is the absolute refractory period of the action potential (ARP). At this moment, a second action possibility is “definitely” not possible. The Na+ ion channels in this portion of the membrane can only respond to subsequent stimulation after they have closed.

The relative refractory period (RRP) occurs during the hyperpolarization phase. The neuron’s membrane is more negatively charged than it is at rest, and K+ ion channels have only just begun to close. Due to the closure of all sodium ion channels, a second action potential might be triggered. Due to the higher negatively charged intracellular region, a stronger stimulus is necessary. A stronger stimulus is required to activate a neuron to the –55 mV threshold level. As a consequence, initiating a second action potential is “relatively” difficult, but not impossible, during the relative refractory period.

In terms of stimulus intensity, the relative refractory time is critical. The importance of a stimulus is determined by the pace at which a neuron sends action potentials. There are no weak or strong action potentials since they all need the same degree of electrical or chemical input to activate. Either the neuron fires when the threshold level is reached, or it does not.

Different effects are caused by the firing rate, not the firing power. Cells in the eye’s retina, for example, send fewer action potentials in low light than they do in strong light. When light levels are high, we see significantly better because the retina transmits more information to the brain in a shorter amount of time.

Effective Refractory Period

Another sort of refractory period occurs in cardiac pacemaker cells that behave similarly to neurons: the effective refractory period, or ERP.

This time period starts concurrently with the ARP and concludes shortly before the RRP. It is frequently overlooked in textbooks, as seen in the illustration above. In the picture above, the absolute refractory time should stop a millimetre or two prior to the refractory period. The effective refractor duration includes both the time spent inside the ARP and the final millimetres.

The sodium ion channels have closed at this moment, and a second action potential may be generated. The effective refractory time, unlike the RRP, does not allow for conduction. The ERP of myocardial cells in this circumstance prevents the heart from constricting early and so disrupts the heartbeat.

Refractory Period in Psychology

The term “refractory” refers to anything that is tenacious or resistant to a procedure. This is self-explanatory in terms of action potentials and neurons. During refractory periods, a neuron is impervious to a subsequent action potential.

The term “refractory period” in psychology refers to a period of time during which a reaction is delayed. This has nothing to do with our intellect, but rather with our response times. Thus, this refractory period has something to do with our bigger brain networks. The psychological refractory period (PRP) defines when the physique and/or the brain are still reacting to a previous stimulus and are unable to respond to a second stimulus.

When we consume alcohol, for example, our responses and reflexes are compromised. When alcohol is consumed while doing another job, our response time is slowed. If you’re driving while inebriated and the automobile in front of you abruptly stops, your reaction time will be slower than if you weren’t drinking. If a passenger in the vehicle in front of you asks a question while the car in front of you stops, the driver may not hear it. Alternatively, the motorist may clearly hear the inquiry but fail to see the automobile in front of him abruptly halt. We can’t handle two tasks at the same time because of their psychological refractory period.

There are other biological applications for this phrase. One example is the gap between male orgasm and a subsequent erection. Numerous sexual enhancers and drugs (like Viagra) are designed to help men reduce their refractory periods.

References

Ropper AH, Samuels MA, Klein J, Prasad S. (2019). Adams and Victor’s Principles of Neurology, Eleventh Edition. New York, McGraw-Hill.

Meriney SD, Faneslow E. (2019). Synaptic Transmission. London, Academic Press.

Wardhan, R, Mudgal P. (2017). Textbook of Membrane Biology. Singapore, Springer

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