Not a part of nerves
The correct answer is option 'D': Smooth muscles.

Explanation
Nerves are essential components of the nervous system, which is responsible for transmitting signals and coordinating various bodily functions. Nerves are composed of different types of cells and tissues that work together to facilitate the transmission of electrical impulses. Let's explore the components of nerves in detail:

Axons:
- Axons are elongated, thread-like structures that transmit electrical signals from one neuron to another or from neurons to other cells in the body.
- They are considered the main conducting units of nerves and are responsible for carrying information over long distances.
- Axons are wrapped in a protective layer called myelin, which enhances the speed and efficiency of signal transmission.

Connective tissues:
- Connective tissues provide structural support and protection to nerves.
- They surround and encapsulate the nerve fibers, forming a protective sheath.
- These tissues also help in maintaining the structural integrity of the nerves and facilitate their proper functioning.

Schwann cells:
- Schwann cells are a type of glial cells found in the peripheral nervous system.
- They play a crucial role in the formation and maintenance of myelin, which surrounds the axons of peripheral nerves.
- Myelin acts as an insulating layer, allowing for faster transmission of electrical signals along the axons.
- Schwann cells also aid in the regeneration of damaged nerves by providing a supportive environment for nerve regrowth.

Smooth muscles:
- Smooth muscles are not directly associated with nerves.
- Smooth muscles are involuntary muscles found in the walls of various organs, such as the digestive system, blood vessels, and respiratory system.
- They are responsible for the involuntary movements of these organs and are controlled by the autonomic nervous system, which consists of nerves.
- While nerves regulate the activity of smooth muscles, smooth muscles themselves are not considered a part of nerves.

In conclusion, smooth muscles are not a part of nerves. Nerves primarily consist of axons, connective tissues, and Schwann cells, which work together to transmit electrical signals and maintain the structural integrity of the nervous system.