The longest  cell is the nerve cell.
The smallest cell in the human body is the sperm cell.

**Longest Cell in the Human Body:**

The longest cell in the human body is the nerve cell, also known as a neuron. Neurons are specialized cells that transmit electrical signals throughout the body. They are responsible for coordinating and communicating information between different parts of the body, including the brain, spinal cord, and various organs.

**Structure of Neurons:**
Neurons consist of three main parts: the cell body, dendrites, and axon. The cell body contains the nucleus and other organelles necessary for the cell's functioning. Dendrites are branch-like structures that receive signals from other neurons and transmit them to the cell body. The axon is a long, slender projection that carries signals away from the cell body to other neurons, muscles, or glands.

**Length of Neurons:**
Neurons can vary in length, but the longest ones are found in the human body's peripheral nervous system. For instance, the sciatic nerve, which extends from the lower back down to the foot, contains neurons that can reach up to three feet in length. These long nerve fibers allow for the transmission of signals over large distances.

**Function of Neurons:**
Neurons play a crucial role in the functioning of the nervous system. They receive signals from sensory organs or other neurons, process the information, and then transmit signals to other neurons or effector cells (e.g., muscles or glands). This communication between neurons enables us to perceive and respond to stimuli, control bodily movements, and regulate various physiological processes.

**Importance of Neurons:**
Neurons are vital for our overall well-being as they are involved in almost all bodily functions. They facilitate the transmission of information, enabling us to think, feel, and perform various actions. Without neurons, our nervous system would not be able to function properly, leading to a loss of sensation, motor control, and cognitive abilities.

In conclusion, the nerve cell or neuron is the longest cell in the human body. Its specialized structure and ability to transmit electrical signals over long distances make it essential for the proper functioning of the nervous system and overall human health.