Muscles can be classified into three main types based on their structure and function: striated muscles, smooth muscles, and cardiac muscles. Out of these three types, smooth muscles and cardiac muscles are the ones that act involuntarily.

Smooth Muscles:
Smooth muscles are found in the walls of internal organs, such as the stomach, intestines, blood vessels, and bladder. They are also present in the respiratory system, reproductive system, and the walls of blood vessels. Smooth muscles are not under conscious control and therefore act involuntarily. They contract and relax to regulate functions like digestion, blood flow, and breathing. For example, the smooth muscles in the intestines contract to propel food through the digestive tract.

Cardiac Muscles:
Cardiac muscles are specific to the heart. They are responsible for the rhythmic contractions of the heart, which pump blood throughout the body. Similar to smooth muscles, cardiac muscles are also involuntary and not under conscious control. The contraction and relaxation of these muscles are controlled by electrical signals from specialized cells in the heart. This ensures that the heart beats regularly and maintains a constant blood flow.

Striated Muscles:
Striated muscles, also known as skeletal muscles, are the muscles that are attached to the skeleton and enable voluntary movements of the body. These muscles are under conscious control, meaning we can decide to contract or relax them. For example, when we want to move our arms, legs, or any other part of the body, we consciously send signals from our brain to these muscles, causing them to contract and generate movement.

Skeletal Muscles:
Skeletal muscles work in pairs, with one muscle contracting while the other relaxes to produce movement. They are responsible for all types of body movements, including walking, running, jumping, and even facial expressions. These muscles are striated in appearance due to the arrangement of protein fibers within them.

In conclusion, smooth muscles and cardiac muscles are the two types of muscles that act involuntarily. While smooth muscles are found in the walls of internal organs and regulate various functions, cardiac muscles are specific to the heart and ensure its rhythmic contractions. On the other hand, striated muscles, also known as skeletal muscles, are under conscious control and enable voluntary movements of the body.