Clusters of rer and free ribosomes in neurons are called

Nissl bodies.

Explanation:

1. A Nissl body is also known by the name Nissl substance. It is a large granular body which is found in neurons (nerve cells). These granules are of rough endoplasmic reticulum with free ribosomes, and their role is that, they are the site of protein synthesis in neurons.

2. Nissl bodies may dissolve and disappear, and show changes under various physiological and in pathological conditions.

3. The functions of Nissl bodies are : the manufacture and release of proteins and amino acids.