Logan Steele
The refractory period is a critical control mechanism in excitable tissues that prevents hyperactivity and undesirable tetani by inhibiting subsequent stimuli from eliciting action potentials and calcium entry. In the context of muscles, the release of calcium causes muscle contraction, and the refractory period can impact the duration of muscle contraction. While skeletal muscle fibers exhibit a refractory period similar to neurons, there is no functional refractory period observed in muscles due to the time required for calcium to diffuse and initiate contraction.
| Characteristics | Values |
|---|---|
| Refractory period | The time during which a second action potential can be initiated, but with a greater stimulus than before |
| Cause | Inactivation of the Na+ channel |
| Action potential | A wave that travels down the axon |
| Excitable tissues | Have a refractory period to prevent hyperactivity and undesirable tetani |
| Ureteric smooth muscle | Has peristaltic waves that produce long-lasting action potentials and extraordinarily long refractory periods |
| Refractory period manipulation | Can be altered by changing the Ca2+ content or release mechanism, or by inhibiting BK channels |
| Skeletal muscle fibers | Transmit action potentials similarly to neurons |
| Sodium channels | Responsible for the large inward current that depolarizes the membrane |
| Potassium currents | Carry the competing outward current |
| Calcium | Required for muscle contraction |
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What You'll Learn
- Skeletal muscle fibres have an absolute refractory period of 10 milliseconds at 0 °C
- The release of calcium causes muscle contraction, but the time taken for this process means there is no functional refractory period in muscle
- In excitable tissues, the refractory period prevents hyperactivity and undesirable tetani
- The relative refractory period is when a second action potential can be initiated, but with a greater stimulus
- The refractory period can be manipulated by altering the Ca2+ content or release mechanism, or by inhibiting BK channels
Skeletal muscle fibres have an absolute refractory period of 10 milliseconds at 0 °C
Muscle fibres do indeed have refractory periods. In the context of muscle physiology, the refractory period refers to the time during which a second stimulus will not evoke a response. This is functionally different from the refractory period in the nervous system, where an action potential is impossible to elicit during the refractory period.
The refractory period in skeletal muscle is due to the inactivation of voltage-gated sodium channels. These channels are responsible for the large inward current that depolarizes the muscle membrane. However, the competing outward current of potassium ions means that there is no functional refractory period in skeletal muscle. This is because, by the time calcium ions have diffused out of the sarcoplasmic reticulum and bound to troponin C to allow for muscle contraction, the membrane has already repolarized.
In contrast, cardiac muscle has a much longer refractory period of approximately 200 ms, with a relative refractory period of 50 ms, totalling 250 ms. This extended period is critical for proper cardiac function, as it allows the heart muscle to fully contract and pump blood effectively. Without this extended refractory period, premature contractions would occur in the heart, which would not be compatible with life.
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The release of calcium causes muscle contraction, but the time taken for this process means there is no functional refractory period in muscle
Muscle contraction is a complex process involving the coordination of various physiological mechanisms. One of the key players in this process is calcium, which, when released, initiates the contraction of muscle fibres. However, the time required for this calcium release and its subsequent actions means that there is no effective refractory period in muscles.
The process begins with a signal from a motoneuron, which propagates an action potential to the muscle fibre it innervates. This results in the opening of voltage-gated calcium channels in the presynaptic membrane, allowing an inflow of calcium ions. This influx of calcium causes the release of acetylcholine, a neurotransmitter, at the neuromuscular junction. Acetylcholine then diffuses to the postsynaptic membrane of the muscle fibre, leading to further physiological responses.
The release of acetylcholine causes a shift in the resting membrane potential, making it more positive. This change in membrane potential activates voltage-gated channels, resulting in an action potential. The action potential then propagates into the T-tubules of the muscle cell, which are deep invaginations within the muscle fibre. The T-tubules contain proteins called dihydropyridine receptors, which act as "voltage sensors". These sensors transmit the signal to calcium channels in the sarcoplasmic reticulum, also known as the calcium store inside muscle cells.
The calcium channels in the sarcoplasmic reticulum open, releasing calcium ions. This release of calcium is the key step that initiates muscle contraction. The calcium ions bind to troponin, a regulatory protein, forming a complex with tropomyosin and troponin. This complex removes the inhibition on the interaction between actin and myosin, allowing cross-bridge cycling and muscle contraction to occur.
However, the process of calcium release and its subsequent actions takes time. The calcium ions need to diffuse out of the sarcoplasmic reticulum and reach the troponin molecules to initiate contraction. This time lag between the initial signal and the calcium-induced contraction results in a longer time course for muscle contraction compared to the action potential. Consequently, there is no functional refractory period observed in muscles, despite the technical presence of a refractory period due to the inactivation of sodium channels.
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In excitable tissues, the refractory period prevents hyperactivity and undesirable tetani
In excitable tissues, a refractory period is a critical control mechanism that prevents hyperactivity and undesirable tetani. This is achieved by preventing subsequent stimuli from eliciting action potentials and calcium (Ca2+) entry.
The refractory period is a stage where the muscle is unresponsive to further stimulation, which prevents hyperactivity and undesirable tetani, or muscle spasms. This period follows an initial stimulation, during which the muscle is recovering and unable to respond. This is important as, without this period, the muscle could be susceptible to further stimulation before it has had time to recover, leading to hyperactivity and spasms.
In the case of ureteric smooth muscle, for example, the refractory period can last an extraordinarily long time (more than 10 seconds). This prevents urine reflux and kidney damage. During this time, the muscle is unable to respond to further stimulation, which is critical for preventing damage to the body.
The refractory period is terminated when the Ca2+ load in the sarcoplasmic reticulum (SR) is reduced, allowing electrical inhibition to be released and peristaltic contractions to occur again. The sarcoplasmic reticulum is a cell organelle that plays a key role in this process. On excitation, it takes up calcium and then targets its release to channels at the cell surface, which open and prevent further excitation.
The refractory period in muscle tissues is technically present due to the inactivation of sodium channels, which are responsible for transmitting action potentials. However, some sources argue that there is no functional refractory period in muscle as the release of calcium, which causes muscle contraction, occurs on a longer time course than the action potential.
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The relative refractory period is when a second action potential can be initiated, but with a greater stimulus
Muscle fibres transmit action potentials in a similar way to neurons. Sodium channels are responsible for the large inward current that depolarizes the membrane, while potassium currents carry the competing outward current. These sodium channels are voltage-gated and rapidly inactivating, which causes a refractory period in muscle.
The relative refractory period is the interval of time during which a second action potential can be initiated, but it will require a greater stimulus than the first. This is because the Na+ channel has been inactivated and cannot respond to another stimulus until the gates are reset.
Refractory periods are a critical control mechanism in excitable tissues, preventing hyperactivity and undesirable tetani. They do this by preventing subsequent stimuli from eliciting action potentials and Ca2+ entry. In ureteric smooth muscle, for example, the refractory period can be extraordinarily long (more than 10 seconds) to prevent urine reflux and kidney damage.
The refractory period can be manipulated by altering the Ca2+ content of the sarcoplasmic reticulum or release mechanism, or by inhibiting BK channels. This understanding of the control of excitability provides a focus for therapies directed at pathologies of smooth muscle.
While there is a refractory period in muscle, it is not functionally observed. This is because the release of calcium, which causes muscle contraction, takes time to diffuse out of the sarcoplasmic reticulum and bind to troponin C molecules to allow for cross-bridge cycling and contraction. Therefore, the calcium gradient and muscle contraction have a longer time course than the action potential.
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The refractory period can be manipulated by altering the Ca2+ content or release mechanism, or by inhibiting BK channels
In ureteric smooth muscle, invading pacemaker potentials produce long-lasting action potentials (300-800ms) and extraordinarily long refractory periods (more than 10s). These long refractory periods prevent urine reflux and kidney damage. The mechanisms underlying the refractory period in smooth muscles are not yet fully understood. However, studies have shown that a negative feedback process, depending on Ca2+ loading the sarcoplasmic reticulum (SR) during the action potential, plays a crucial role in determining the refractory period.
During the action potential, Ca2+ is loaded into the SR, and local releases of Ca2+ from the SR, known as sparks, occur. These sparks stimulate plasmalemmal Ca2+-sensitive K+ (BK) channels, leading to the activation of peristaltic contractions. The refractory period ends when the sparks gradually reduce the Ca2+ load in the SR, releasing the electrical inhibition.
The refractory period can be manipulated by altering the Ca2+ content or release mechanism. By adjusting the Ca2+ concentration or modifying the sparks that release Ca2+, the duration of the refractory period can be changed. For example, the refractory period can be extended from 10 seconds to 100 seconds by altering the Ca2+ content. This manipulation provides a way to control the duration of the refractory period and, consequently, the timing of peristaltic contractions.
In addition to altering Ca2+ content, the refractory period can also be manipulated by inhibiting BK channels. BK channels are plasmalemmal Ca2+-sensitive K+ channels that play a crucial role in the negative feedback process determining the refractory period. By inhibiting these channels, the activation of peristaltic contractions can be delayed or prevented, resulting in an extended refractory period. This inhibition of BK channels can be achieved through various pharmacological agents or interventions, allowing for a controlled modification of the refractory period duration.
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Frequently asked questions
Yes, muscles do have refractory periods.
A refractory period is a duration of time after an action potential is initiated during which another action potential cannot be elicited.
An action potential is a change in the voltage of the inside of a cell relative to the extracellular space.
During the absolute refractory period, a new action potential cannot be elicited. During the relative refractory period, a new action potential can be elicited under the correct circumstances.
The absolute refractory period in muscles is approximately 1-3 milliseconds, while the relative refractory period is approximately 2-4 milliseconds.
