Which of the following is characteristic of the chromatophilic material called Nissl substance in neural tissue?
a. Found throughout neurons
b. Site of mRNA translation for proteins of the axolemma
c. Most abundant in unipolar neurons
d. Becomes more abundant as an individual gets older
e. An example of intermediate filament proteins
The Correct Answer and Explanation is:
The correct answer is:
b. Site of mRNA translation for proteins of the axolemma
Explanation:
Nissl substance, also known as chromatophilic substance, refers to large granular bodies found within the cell body and dendrites of neurons. These granules are actually clusters of rough endoplasmic reticulum (RER) and ribosomes. The key role of Nissl substance is in protein synthesis.
- Why answer b is correct:
The primary function of Nissl substance is to provide a site for mRNA translation into proteins, especially those needed for maintaining the neuron’s structure and function. This includes proteins destined for the axolemma (the plasma membrane of the axon). Since axons are quite long and require a continuous supply of membrane and membrane proteins, Nissl bodies synthesize proteins that are transported along the axon to the axolemma and other axonal components. - Why other options are incorrect:
- a. Found throughout neurons:
Nissl substance is not found throughout the entire neuron. It is primarily located in the cell body (soma) and dendrites but absent from the axon. The axon contains microtubules and neurofilaments but lacks rough ER and ribosomes, so no Nissl bodies. - c. Most abundant in unipolar neurons:
Nissl substance is generally most abundant in large, metabolically active neurons, like multipolar motor neurons or pyramidal neurons. It is not specifically most abundant in unipolar neurons, which are usually sensory and have different structural characteristics. - d. Becomes more abundant as an individual gets older:
Nissl substance typically decreases with age or neuronal damage. In fact, a reduction in Nissl substance is often associated with degenerating neurons or neurons under stress. - e. An example of intermediate filament proteins:
Nissl substance is not a filament or protein structure itself; it consists of rough ER and ribosomes. Intermediate filaments in neurons are neurofilaments, a completely different cellular component.
- a. Found throughout neurons:
Summary:
Nissl substance is crucial for the neuron’s ability to produce proteins necessary for its function, especially those required by the axon membrane. It is found in the cell body and dendrites where protein synthesis takes place, and it serves as the site where mRNA is translated to produce these essential proteins. This makes option b the most accurate description.
