What is meant by functional unit of a system?

Like when we say that the neuron is the basic unit of neural system, do we mean that all those things that are performed by neural system can be performed by an isolated neuron?

If yes, how? And if not, then why do we call it as functional unit of neural system?

Like we say hepatic lobule functional unit of the liver is not hepatocytes, because we say that liver has complex functions, and single hepatocytes cannot perform it. So the functional unit of liver is the hepatic lobule.

Basically I am asking can a single neuron perform the most complex functions of neural system: like memory storage, control and coordination (to a smaller extent) regulation of a cell.

How can a single neuron control or regulate another cell, because wouldn't it need other neurons for processing of stimulus?

Can an isolated neuron process any stimulus and make other cell to respond in some way? If not so why we call it the functional unit of neural system?


2 Answers 2


In biology, functional unit of a system refers to the smallest structural element that is capable of performing the tasks typical for that system (MP Hlastala, Physiology of Respiration).

In the nervous system, a neuron is usually considered a functional unit, because it is capable of performing the basic task of this system, that is transmitting signals. But to perform more complex tasks, more neurons need to be organized into structures, which are usually not called functional units but, for example, pathways (visual pathway, somatosensory pathway) or centers (respiratory center, cardiovascular center, etc.).

Functional unit smaller than a cell

In the muscle cells, the proteins myosin and actin form myofilaments, which are organized into sarcomeres, which are considered functional units of the skeletal muscles, because the main muscle function (contraction) occurs on the level of a sarcomere.

Functional unit as a single cell

A single fat cell (adipocyte) is a functional unit of the adipose tissue, because it can perform its main task, that is to store fat.

Functional unit as a structural element

A structural element can consist of various cells, ducts, small blood vessels, etc.:

  • A nephron in the kidneys
  • A hepatic acinus in the liver (not a hepatic lobule)
  • A pulmonary acinus in the lungs (a respiratory bronchiole and its alveolar ducts and alveolar sacs, not a pulmonary lobule)
  • An acinus in the exocrine and Langerhan's islets in the endocrine part of the pancreas (Access Medicine)
  • A lymhoid lobule in the lymph nodes
  • An osteon in bones

"Functional unit" doesn't have a very specific meaning. It's mostly a term used to help biology students imagine organs as collections of parts.

The entities named functional units in a textbook don't typically perform every single function of a whole organ. A cardiomyocyte, for example, is a functional unit of the heart. It can perform really important heart-related functions like spontaneous rhythmic contraction. However, it can't pump any blood.

Similarly, neurons do important nervous system functions by themselves, like firing action potentials, and ultimately things like memory occur due to changes within individual neurons. They can't, however, do anything useful to the whole organism alone.


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