Question

Which of the following is not true? A vertebrate skeletal muscle ______ a. contracts in response to signals from a motor neuron b. pushes against and moves a bone c. is a voluntary muscle d. consists of many multinucleated muscle fibers

Short answer

Option B is not true; muscles pull, not push.

Step-by-step solution

Understand the Question

The exercise asks you to identify which statement about vertebrate skeletal muscle is not true. Each option provides a different characteristic of skeletal muscles, so we need to determine which characteristic is incorrect.

Examine Option A

Option A states that a vertebrate skeletal muscle contracts in response to signals from a motor neuron. This is true because skeletal muscles are controlled by motor neurons which transmit signals for contraction.

Examine Option B

Option B claims that a vertebrate skeletal muscle pushes against and moves a bone. This statement is incorrect because skeletal muscles work by pulling on bones through contraction, not by pushing.

Examine Option C

Option C says skeletal muscle is a voluntary muscle. This is true, as skeletal muscles are under voluntary control, allowing conscious movement.

Examine Option D

Option D notes that skeletal muscle consists of many multinucleated muscle fibers. This is correct because skeletal muscle fibers are multinucleated and form part of the muscle's cellular structure.

Identify the Incorrect Statement

From evaluating each option, option B is identified as false. Skeletal muscles pull on bones to facilitate movement, rather than pushing against them.

Key concepts

Motor Neuron

Motor neurons play a crucial role in the functioning of vertebrate skeletal muscles. These neurons are specialized cells responsible for transmitting electrical signals from the brain and spinal cord to muscle fibers.
Once the signal reaches the muscle, it triggers a response leading to muscle contraction.

• Electrical signals, known as action potentials, travel along the neuron until they reach the neuromuscular junction.
• Here, the motor neuron releases a neurotransmitter called acetylcholine.
• This neurotransmitter crosses the synaptic gap, binding to receptors on the muscle fiber.
• As a result, the muscle fiber depolarizes, initiating contraction.

The connection between the nervous system and muscle fibers is vital to voluntary movements and control of muscle actions.

Voluntary Muscle

Skeletal muscles are classified as voluntary muscles because they can be controlled consciously. This means you can decide when and how to move these muscles, allowing for deliberate actions such as walking, running, or lifting objects.
The entire process is an intricate system involving the brain sending signals through motor neurons.

• Decisions to move often originate in the brain's motor cortex, a region responsible for planning and executing movements.
• The signals then travel through the spinal cord to reach motor neurons.
• Motor neurons transmit impulses directly to the skeletal muscles, prompting them to contract.
• This voluntary control makes skeletal muscles unique compared to other muscle types, such as cardiac or smooth muscle, which operate involuntarily.

Understanding this process helps explain how humans perform complex physical activities.

Muscle Fiber

Muscle fibers are the basic building blocks of skeletal muscles. Each fiber is a single elongated cell that groups together to form muscle tissue, facilitating motion and stability.
One defining feature of muscle fibers is that they are multinucleated, meaning they contain multiple nuclei within a single cell.

• The multiple nuclei result from the fusion of individual cells during muscle development.
• This structure supports the extensive protein synthesis needed for muscle repair and growth.
• Muscle fibers are also packed with myofibrils, protein structures made of actin and myosin that allow contraction.
• Each fiber is surrounded by a sarcolemma, which helps conduct electrical impulses to trigger action.

Comprehending the anatomy and function of muscle fibers is essential for studying how skeletal muscles operate and adapt to different demands.

Muscle Contraction

Muscle contraction is a vital process enabling movement in vertebrates. It involves a complex interaction between various proteins, fibers, and the nervous system.
The process of contracting begins with a signal sent by motor neurons and ends with the actual shortening of muscle fibers.

• Once the chemical signal from a motor neuron reaches a muscle fiber, it causes a release of calcium ions within the fiber.
• Calcium then binds to troponin, a regulatory protein, causing the myofibrils to move.
• This movement allows myosin heads to attach to actin filaments, forming cross-bridges, and ultimately pulling the actin filaments closer together.
• The repeated cycles of cross-bridge formations lead to the shortening of the muscle and hence, contraction.
• When the muscle relaxes, calcium ions are reabsorbed, and the cross-bridges detach.

Understanding this process not only highlights how muscle movements occur but also illustrates the coordination needed between the nervous and muscular systems.