Which of the following statements is incorrect regarding Sliding Filament Theory?

Options

A. Contraction of muscle is initiated by a signal from motor neuron
B. in Ca2+ ion uptake by muscle fibre from ECF, leading to increase in intracellular Ca2+ level
C. Ca2+ causes change in shape of troponin tropomyosin complex, thus exposing myosin binding sites on actin filaments
D. Length of A-band remains unchanged during muscle contraction

Answer

To determine which statement is incorrect regarding the Sliding Filament Theory, we will analyze each statement based on our understanding of muscle contraction and the theory itself.

### Step-by-Step Solution:

1. **Understanding the Sliding Filament Theory**:
- The Sliding Filament Theory explains how muscles contract at the molecular level. It involves the interaction between two types of filaments: actin (thin filaments) and myosin (thick filaments).
- During contraction, the actin filaments slide over the myosin filaments, leading to the shortening of the sarcomere, which is the functional unit of muscle fibers.

2. **Analyzing the Statements**:
- **Statement A**: "Contraction of muscle is initiated by a signal from motor neurons."
- This statement is correct. Muscle contraction is indeed initiated by signals from motor neurons that release neurotransmitters, leading to muscle fiber activation.

- **Statement B**: "In calcium ion uptake by muscle fiber from ECF, leading to increase in intracellular calcium level."
- This statement is incorrect. During muscle contraction, calcium ions are not taken up from the extracellular fluid (ECF) but are released from the sarcoplasmic reticulum into the sarcoplasm. The calcium ions that are necessary for muscle contraction are already present in the muscle fiber and are released when the muscle is stimulated.

- **Statement C**: "Calcium ion causes change in shape of troponin-tropomyosin complex."
- This statement is correct. Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin, allowing contraction to occur.

- **Statement D**: "Length of A-band remains unchanged during muscle contraction."
- This statement is also correct. The A-band, which corresponds to the length of the thick filaments (myosin), does not change during contraction; only the I-bands shorten as the actin filaments slide over the myosin filaments.

3. **Conclusion**:
- The incorrect statement regarding the Sliding Filament Theory is **Statement B**: "In calcium ion uptake by muscle fiber from ECF, leading to increase in intracellular calcium level."