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.

Grey matter is distinguished from white matter by the:

Four healthy people in their twenties faced injuries resulting in damage and death of a few cells given below. Which of the cells are least likely to be replaced by new cells?

Myelin sheath is a layer covering

Which of the following structures distinguishes a neuron from other cells?

Mark the following statements as true/false and choose the correct option from the codes given below:
$I.$ Neuroglial cells are the packing and supporting cells found in the brain and spinal cord.
$II.$ Oligodendrocytes are a category of glial cells that form myelin sheaths around the axon.
$III.$ Microglia provide mechanical support to the neurons.
$IV.$ Astrocytes communicate with one another through potassium channels.