Question

Which of the following statements about the \(\mathrm{Ca}^{2+}\) release channels in the sarcoplasmic reticulum is false? a. They are also called ryanodine receptors. b. They are one-tenth the size of the voltage-gated \(\mathrm{Ca}^{2+}\) channek. c. They are opened by \(\mathrm{Ca}^{2+}\) release channels in the transverse tubules. d. They permit \(\mathrm{Ca}^{2+}\) to diffuse into the sarcoplasm from the sarcoplasmic reticulum.

Short answer

The false statement about the Ca²⁺ release channels in the sarcoplasmic reticulum is option (b). They are not one-tenth the size of the voltage-gated Ca²⁺ channels but are in fact larger.

Step-by-step solution

Statement a: Ryanodine receptors

This statement is true. The calcium release channels in the sarcoplasmic reticulum are indeed called ryanodine receptors. They play a key role in excitation-contraction coupling in muscle cells by releasing Ca²⁺ ions from the sarcoplasmic reticulum into the sarcoplasm.

Statement b: Size comparison to voltage-gated Ca²⁺ channels

This statement is false. Ryanodine receptors (Ca²⁺ release channels in the sarcoplasmic reticulum) are significantly larger than the voltage-gated Ca²⁺ channels, not smaller. The false statement is an incorrect portrayal of the relative size of these channel types.

Statement c: Opening mechanism

This statement is true. The ryanodine receptors (Ca²⁺ release channels) are opened by the voltage-gated Ca²⁺ channels located in the transverse (T) tubules. The influx of Ca²⁺ ions through these T-tubule channels triggers the release of more Ca²⁺ ions from the sarcoplasmic reticulum through the ryanodine receptors.

Statement d: Ca²⁺ diffusion

This statement is true. The function of the ryanodine receptors (Ca²⁺ release channels) is to permit the diffusion of Ca²⁺ ions from the sarcoplasmic reticulum into the sarcoplasm. This increase in sarcoplasmic Ca²⁺ concentration enables muscle contraction to occur. In conclusion, the false statement about the Ca²⁺ release channels in the sarcoplasmic reticulum is option (b). They are not one-tenth the size of the voltage-gated Ca²⁺ channels but are in fact larger.

Key concepts

Ryanodine Receptors

Understanding the role of ryanodine receptors is fundamental to grasp how muscles contract. These receptors are specialized proteins that act as channels in the membrane of the sarcoplasmic reticulum, a storage site for calcium ions (Ca²⁺). When a muscle cell is stimulated, these channels open and release Ca²⁺ ions into the sarcoplasm. The key takeaways:

• Ryanodine receptors are crucial for initiating muscle contraction.
• They are named after the plant alkaloid ryanodine, which binds to these receptors.
• Their primary function is to release stored Ca²⁺ into the cytoplasm to facilitate contraction.

It is this surge of calcium that activates the muscle fibers to contract, thus highlighting the importance of ryanodine receptors in the physiology of muscle function.

Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is a form of the endoplasmic reticulum found in muscle cells and plays a starring role in muscle contraction. Its main job is to store Ca²⁺ ions which are essential for muscle fibers to contract.

• The SR has a large surface area to maximize storage of Ca²⁺ ions.
• In response to stimulation, it releases these ions to begin the process of contraction.
• Post-contraction, it reabsorbs Ca²⁺ to help the muscle relax and prevent continuous contraction.

This cycling of calcium in and out of the sarcoplasmic reticulum is a finely tuned process that is essential for controlled muscle movement.

Excitation-Contraction Coupling

Excitation-contraction coupling is the physiological process that converts an electrical stimulus (excitation) into a mechanical response (contraction) in muscle cells. This is a complex mechanism that involves several steps:

• An action potential travels down a motor neuron to a muscle cell.
• The electrical signal passes through the cell membrane and along the T-tubules.
• Voltage-gated Ca²⁺ channels in the T-tubules are activated.
• Ryanodine receptors in the SR release Ca²⁺ into the sarcoplasm.
• Ca²⁺ binds to troponin, causing the muscle to contract.
• Relaxation occurs when Ca²⁺ is pumped back into the SR.

The importance of each step cannot be overstated as each one must occur correctly for efficient muscle contraction and relaxation.

Muscle Contraction

Muscle contraction is the end result of a cascade of events beginning with an electrical signal and ending with a mechanical force. To understand muscle contraction, you must understand its building blocks:

• Myosin and actin are proteins in muscle fibers that interact to cause contraction.
• Myosin 'heads' pull on actin filaments, resulting in muscle shortening or force generation.
• ATP (adenosine triphosphate) provides the energy necessary for muscle contractions.
• Ca²⁺ ions released from the SR bind to troponin, allowing actin and myosin to interact.

This interaction is highly dependent on the availability of Ca²⁺ ions, reinforcing the essential roles of the ryanodine receptors and the sarcoplasmic reticulum in muscle physiology.