All muscle types will respond to an electrical stimulus because all muscle cells are _________.

• A. involuntary
• B. contractile
• C. elastic
• D. excitable

Answer: (D)

Verified step by step guidance

1. Understand the key concept: Muscle cells respond to electrical stimuli because they possess a property called excitability. Excitability refers to the ability of a cell to respond to a stimulus, such as an electrical signal, by generating an action potential.
2. Review the definitions of the given options: 'Involuntary' refers to actions controlled by the autonomic nervous system, such as smooth and cardiac muscle movements. 'Contractile' refers to the ability of muscle cells to shorten and generate force. 'Elastic' refers to the ability of muscle cells to return to their original shape after being stretched. 'Excitable' refers to the ability to respond to stimuli by generating electrical signals.
3. Eliminate incorrect options: While muscle cells are contractile and elastic, these properties do not directly explain their ability to respond to electrical stimuli. Similarly, 'involuntary' applies only to certain muscle types (smooth and cardiac) and does not describe the universal property of all muscle cells.
4. Focus on the correct property: Excitability is the universal characteristic of all muscle cells that allows them to respond to electrical stimuli. This property is essential for muscle function, as it enables the generation of action potentials that lead to contraction.
5. Conclude the reasoning: Based on the definitions and elimination of incorrect options, the correct answer is 'excitable,' as it directly explains the ability of all muscle cells to respond to electrical stimuli.