Nissl granules are aggregates of free ribosomes they're present in cyton, dendron even schwann cells but are absent in axon.

Nissl bodies, also known as Nissl granules, are specialized structures found in the cytoplasm of neurons. They are composed of rough endoplasmic reticulum and play a crucial role in protein synthesis in the neuron. However, Nissl granules are absent in the axon.

Axon:
- The axon is a long, slender projection of a neuron that carries nerve impulses away from the cell body to other neurons, muscles, or glands.
- Unlike the cyton (cell body) and dendron (dendrites), the axon lacks Nissl granules.

Explanation:
- Nissl granules are found in the cytoplasm of the cell body, dendrites, and initial segment of the axon hillock, but they are absent in the rest of the axon.
- The function of Nissl granules is to synthesize proteins required for the normal functioning of the neuron.
- The absence of Nissl granules in the axon is due to the fact that axons are specialized for transmitting electrical signals rather than synthesizing proteins.
- Protein synthesis primarily occurs in the cell body, where the nucleus and most of the organelles, including the Nissl granules, are located.
- The synthesized proteins are then transported down the axon to the synaptic terminals to support neurotransmission.

Significance:
- The absence of Nissl granules in the axon allows for efficient transmission of electrical signals without the need for protein synthesis.
- The axon is responsible for transmitting nerve impulses over long distances, and the absence of Nissl granules reduces the metabolic demands on the axon, allowing for faster transmission speeds.

In conclusion, Nissl granules are absent in the axon. This is because the axon is specialized for transmitting electrical signals rather than synthesizing proteins. The absence of Nissl granules in the axon allows for efficient transmission of nerve impulses over long distances.