Which of the following sequenes is correct to intite inspiration ?
(i) the contraction of external intercostal muscles raises the ribs and sternum
(ii) Volume of thorax increases in the dorso-ventral axis
(iii) intrapulmonary pressure decreases
(iv) Diaphragm contraction
(v) Air rushes into lungs
(vi) Volume of thorax increases in the anterior-posterior axis

Options

A. `(i), (ii), (iv),(v),(iii), (vi)`
B. `(i), (ii), (iii), (iv), (vi), (v)`
C. `(i), (ii), (iv), (vi), (iii), (v)`
D. `(vi), (v), (i), (ii), (iii), (iv)`

Answer

To determine the correct sequence of events that initiate inspiration, we can break down the process step by step:

### Step-by-Step Solution:

1. **Contraction of External Intercostal Muscles**:
- The process begins with the contraction of the external intercostal muscles. These muscles are located between the ribs and when they contract, they raise the ribs and sternum.
- **Sequence**: (i) The contraction of external intercostal muscles raises the ribs and sternum.

2. **Increase in Thoracic Volume (Dorso-Ventral Axis)**:
- As the ribs and sternum are raised, the volume of the thoracic cavity increases in the dorso-ventral axis (front to back).
- **Sequence**: (ii) Volume of thorax increases in the dorso-ventral axis.

3. **Contraction of the Diaphragm**:
- Simultaneously, the diaphragm, which is a dome-shaped muscle at the base of the thoracic cavity, contracts and flattens. This further increases the volume of the thoracic cavity.
- **Sequence**: (iv) Diaphragm contraction.

4. **Increase in Thoracic Volume (Anterior-Posterior Axis)**:
- The contraction of the diaphragm also contributes to an increase in thoracic volume, specifically in the anterior-posterior axis (front to back).
- **Sequence**: (vi) Volume of thorax increases in the anterior-posterior axis.

5. **Decrease in Intrapulmonary Pressure**:
- As the volume of the thoracic cavity increases, the intrapulmonary pressure (the pressure within the lungs) decreases, creating a pressure gradient.
- **Sequence**: (iii) Intrapulmonary pressure decreases.

6. **Air Rushes into Lungs**:
- Due to the decreased pressure in the lungs compared to the atmospheric pressure, air rushes into the lungs.
- **Sequence**: (v) Air rushes into lungs.

### Final Sequence:
The correct order of events to initiate inspiration is:
1. (i) The contraction of external intercostal muscles raises the ribs and sternum.
2. (ii) Volume of thorax increases in the dorso-ventral axis.
3. (iv) Diaphragm contraction.
4. (vi) Volume of thorax increases in the anterior-posterior axis.
5. (iii) Intrapulmonary pressure decreases.
6. (v) Air rushes into lungs.