Question

Nervous stimulation of muscles a. occurs at a neuromuscular junction. b. results in an impulse that travels down the T system. c. causes calcium to be released from expanded regions of the sarcoplasmic reticulum. d. All of these are correct.

Short answer

Nervous stimulation of muscles occurs at a neuromuscular junction, results in an impulse that travels down the T system, and causes calcium to be released from expanded regions of the sarcoplasmic reticulum. Therefore, the correct answer is (d) All of these are correct.

Step-by-step solution

Understanding the neuromuscular junction

Neuromuscular junction is the point where a motor neuron communicates with a muscle cell by releasing neurotransmitters. The released neurotransmitters then bind to receptors on the muscle cell's membrane, causing electrical signals to be generated in the muscle cell, leading to muscle contraction.

Explaining the T system

The T system (also called the transverse tubule system) is an extensive network of tubules that extend deep into the muscle cell. It serves as a pathway for electrical signals, like the ones generated by nervous stimulation, to travel inside the cell and reach the sarcoplasmic reticulum.

Addressing calcium release

The sarcoplasmic reticulum (SR) is an organelle found within muscle cells and is responsible for the storage and release of calcium ions. Expanded regions of the SR, called the terminal cisternae, release calcium ions into the cytoplasm of the muscle cell when stimulated by electrical signals. The released calcium ions then bind to the protein called troponin, which eventually leads to muscle contraction.

Evaluating the options

Now that we have understood each statement, let's evaluate the exercise options: a. True. Nervous stimulation of muscles occurs at a neuromuscular junction, as explained in Step 1. b. True. The impulse generated by nervous stimulation travels down the T system to reach the sarcoplasmic reticulum, as explained in Step 2. c. True. Calcium is released from expanded regions of the sarcoplasmic reticulum, called terminal cisternae, as explained in Step 3. Since all of the statements are correct, the answer is: d. All of these are correct.

Key concepts

Muscle Contraction Process

Muscle contraction is a fascinating process that starts with a nervous signal. This signal comes from a neuron that forms a connection with the muscle cell at a specific point called the neuromuscular junction. When the signal reaches this junction, neurotransmitters are released to initiate the process. Once the signal is received, it causes an electrical charge to build in the muscle cell's membrane. This charge travels through the muscle cell, ultimately leading to muscle contraction. During this interaction, calcium ions play a significant role by binding to special proteins, enabling the muscle fibers to slide past each other and contract.
The understanding of this detailed process can help unravel the complexities of muscle-related actions in organisms.

Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is a vital structure found within muscle cells. Think of the SR as a storage facility. Its main job is to hold onto calcium ions until they are needed for muscle contraction. This network of tubules can quickly release and reabsorb calcium ions in response to electrical signals that pass through the muscle cell.
The particular areas of the SR responsible for releasing calcium are called the terminal cisternae. They release calcium in response to various stimuli, allowing the contraction process to be swiftly and effectively managed. This management ensures that muscles can contract and relax as required.

T System in Muscle Cells

The T system, or transverse tubule system, is an intricate network of tiny tubes that penetrate into the deep layers of muscle cells. These tubules form pathways that allow electrical signals, derived from nervous impulses, to reach every part of the muscle cell. By facilitating this direct route, the T system ensures that the entire muscle responds promptly to stimulation.
Proper functioning of this system is essential for muscle coordination and effective contraction. It acts as a highly efficient communication network within the muscle cells, enabling quick responses to various bodily demands for movement.

Calcium Ion Release

The release of calcium ions is a crucial trigger in the muscle contraction process. Stored within the sarcoplasmic reticulum, calcium ions are released into the cytoplasm of the muscle cell when stimulated by an electrical signal. Once released, these ions bind to muscle proteins like troponin. This binding shifts other proteins away from actin filaments, which are part of the muscle structure, allowing them to interact with myosin filaments.
This interaction between myosin and actin is what ultimately results in muscle contraction. The regulated release and re-uptake of calcium ions allow muscles to contract and then reset efficiently, ready for another round of activity.

Neurotransmitters and Receptors

At the neuromuscular junction, a key transition from nervous signal to muscle action takes place. It starts when neurotransmitters—chemical messengers—are released from the neuron. These messengers travel across the tiny gap between the neuron and the muscle cell to meet receptors on the muscle cell surface.
When neurotransmitters bind to these receptors, they initiate an electrical impulse in the muscle cell. This impulse is vital for starting the muscle contraction process. Essentially, neurotransmitters and their corresponding receptors act as a switch, turning on the cascade of events that lead to muscle movement.