They occupy the spaces between the ribs know as intercostal spaces.
Every intercostal space is made up of three types of respiratory muscles which are;
- external intercostal muscles,
- internal intercostal muscles
- and the innermost intercostal muscle.
On the inside of each intercostal space is parietal pleura which lines the endothoracic fascia . The external side of the space is covered by facial and skin. Intercostal blood vessels and nerves are located between the innermost layer and the middle layer i.e. between internal intercostal muscles and the innermost intercostal muscles.
The 3 Types of Intercostal Muscles are;
External intercostal muscles– These muscles are responsible for quiet and forced inhalation. Its fibers are positioned downwards and forward. When they are in use, the ribs are raised which consequently expands the chest cavity. They are found originating from rib 1 all the way to rib 11 and their insertions running from rib 2 to 12.
Internal intercostal muscles– these muscles are used during forced expiration. Their fibers are positioned downwards and backwards. They will be found originating from the ribs 2-12 and their insertions will be found on ribs 1-11. When the muscles are being used, they result in the lowering of ribs which reduces the space in the thoracic cavity.
Innermost intercostal muscles– they form the innermost part in intercostal space. On the outer surface of these muscles are intercostal nerves as well as blood vessels and on the inside we shall find parietal pleural and endothoracic fascia. Innermost intercostal muscles are divided into 3 parts which are;Subcostalis- found on the posterior end,Intercostalis intimus- it's the intermediate part,Sternocostalis- forms the anterior part.
Intercostal muscles are usually attached on the rib bones. When these muscles contract, the ribs are pulled towards each other. This means that if rib no.1 is fixed, when the muscles contract, it will result in the upwards pulling of the rib cage. In the same note, if rib no.12 is fixed, it will mean that any contraction in intercostal muscles will force the rib cage downwards. During inhalation, rib no. 1 is fixed due to the action of root neck muscles (scalene muscles) and therefore the rib cage will be raised.
This results in the increase in space of the thoracic cavity.
During exhalation, the last rib i.e. rib no.12 is held in position by obliques muscles and quadratus lumborum (which are all located on the abdominal region). The fixing of the last rib ensures that the rib cage is lowered thus resulting in the decrease in volume of the thoracic cavity.
How are the Muscles Fed
They are well supplied with blood vessels and nerves. Intercostal nerves form the anterior part of thoracic spinal nerves running from T1 to T11. These nerves are distributed to the abdominal peritoneum and thoracic pleural. In the intercostal space, these muscles are found in the neurovascular bundles
These muscles are nourished by blood coming through intercostal arteries. Blood usually leave these muscles via intercostal veins. The arrangement of these arteries, veins and nerves from the outermost of intercostal space to the innermost is; intercostal veins followed by intercostal arteries and finally intercostal nerves. Thus, that is how intercostal muscles are fed.