Which of the following structures are the specialty of nerve cells?

Four healthy people in their twenties were involved in injuries resulting in damage and death of a few cells of the following types. Which of these cells are least likely to be replaced by new cells?

Nodes of Ranvier are found in

Nissl granules in a neuron are composed of...

Choose the correct statements about Nissl's granules from the codes given below:
$I.$ There are regular masses of ribosomes
$II.$ There are irregular masses of ribosomes and $ER$
$III.$ There are granular bodies
$IV.$ They synthesise proteins in the cell

Which of the following statements are correct for a nerve cell?
$(I)$. Each neuron has a cell body
$(II)$. Each neuron has a single axon
$(III)$. Neurons are the functional units of the nervous system
Select the correct option.