Branching involuntary muscle fibers, also known as cardiac muscle fibers, are found in the heart.

Explanation:

Cardiac Muscle
- Cardiac muscle is a specialized type of muscle tissue that is found only in the heart.
- It is involuntary, meaning that it functions without conscious control.
- The main function of cardiac muscle is to pump blood throughout the body, ensuring the delivery of oxygen and nutrients to all tissues.
- Unlike skeletal muscle, which is made up of long, cylindrical fibers, cardiac muscle is composed of shorter, branched fibers.

Branching
- The branching pattern of cardiac muscle fibers allows for efficient communication and coordination between individual fibers.
- This is important for the synchronized contraction of the heart, which ensures that blood is pumped effectively.
- The branching nature of these fibers also increases the surface area available for contraction and force generation.

Involuntary
- Involuntary muscles are those that are not under conscious control.
- They function automatically and are regulated by the autonomic nervous system.
- Cardiac muscle is innervated by the autonomic nervous system, which regulates the rate and force of contraction.

Functions of Cardiac Muscle
- Cardiac muscle fibers contract rhythmically and continuously, allowing the heart to beat and pump blood.
- These contractions are coordinated through specialized structures called intercalated discs, which allow for rapid transmission of electrical signals between adjacent fibers.
- The contraction of cardiac muscle is regulated by the conduction system of the heart, which includes the sinoatrial (SA) node, atrioventricular (AV) node, and bundle of His.
- The SA node acts as the natural pacemaker of the heart, generating electrical impulses that initiate the contraction of the atria.
- The AV node delays the transmission of these impulses to ensure proper coordination between atrial and ventricular contractions.
- The bundle of His and its branches then distribute the electrical impulses throughout the ventricles, causing them to contract.

In summary, branching involuntary muscle fibers are found in the heart. These fibers are specialized for the rhythmic and coordinated contraction of the heart, allowing for efficient blood pumping throughout the body.