Which of the following is not a structural or functional classification of neurons?

Although the nervous system is very complex, there are only two main types of cells in nerve tissue. The actual nerve cell is the neuron. It is the "conducting" cell that transmits impulses and the structural unit of the nervous system. The other type of cell is neuroglia, or glial, cell. The word "neuroglia" means "nerve glue." These cells are nonconductive and provide a support system for the neurons. They are a special type of "connective tissue" for the nervous system.

Neurons

Neurons, or nerve cells, carry out the functions of the nervous system by conducting nerve impulses. They are highly specialized and amitotic. This means that if a neuron is destroyed, it cannot be replaced because neurons do not go through mitosis. The image below illustrates the structure of a typical neuron.

Each neuron has three basic parts: cell body (soma), one or more dendrites, and a single axon.

Cell Body

In many ways, the cell body is similar to other types of cells. It has a nucleus with at least one nucleolus and contains many of the typical cytoplasmic organelles. It lacks centrioles, however. Because centrioles function in cell division, the fact that neurons lack these organelles is consistent with the amitotic nature of the cell.

Dendrites

Dendrites and axons are cytoplasmic extensions, or processes, that project from the cell body. They are sometimes referred to as fibers. Dendrites are usually, but not always, short and branching, which increases their surface area to receive signals from other neurons. The number of dendrites on a neuron varies. They are called afferent processes because they transmit impulses to the neuron cell body. There is only one axon that projects from each cell body. It is usually elongated and because it carries impulses away from the cell body, it is called an efferent process.

Axon

An axon may have infrequent branches called axon collaterals. Axons and axon collaterals terminate in many short branches or telodendria. The distal ends of the telodendria are slightly enlarged to form synaptic bulbs. Many axons are surrounded by a segmented, white, fatty substance called myelin or the myelin sheath. Myelinated fibers make up the white matter in the CNS, while cell bodies and unmyelinated fibers make the gray matter. The unmyelinated regions between the myelin segments are called the nodes of Ranvier.

In the peripheral nervous system, the myelin is produced by Schwann cells. The cytoplasm, nucleus, and outer cell membrane of the Schwann cell form a tight covering around the myelin and around the axon itself at the nodes of Ranvier. This covering is the neurilemma, which plays an important role in the regeneration of nerve fibers. In the CNS, oligodendrocytes produce myelin, but there is no neurilemma, which is why fibers within the CNS do not regenerate.

Functionally, neurons are classified as afferent, efferent, or interneurons (association neurons) according to the direction in which they transmit impulses relative to the central nervous system. Afferent, or sensory, neurons carry impulses from peripheral sense receptors to the CNS. They usually have long dendrites and relatively short axons. Efferent, or motor, neurons transmit impulses from the CNS to effector organs such as muscles and glands. Efferent neurons usually have short dendrites and long axons. Interneurons, or association neurons, are located entirely within the CNS in which they form the connecting link between the afferent and efferent neurons. They have short dendrites and may have either a short or long axon.

Neuroglia

Neuroglia cells do not conduct nerve impulses, but instead, they support, nourish, and protect the neurons. They are far more numerous than neurons and, unlike neurons, are capable of mitosis.

Tumors

Schwannomas are benign tumors of the peripheral nervous system which commonly occur in their sporadic, solitary form in otherwise normal individuals. Rarely, individuals develop multiple schwannomas arising from one or many elements of the peripheral nervous system.

Commonly called a Morton's Neuroma, this problem is a fairly common benign nerve growth and begins when the outer coating of a nerve in your foot thickens. This thickening is caused by irritation of branches of the medial and lateral plantar nerves that results when two bones repeatedly rub together.

Image credit: iStockphoto

Neurons are the cells that make up the brain and the nervous system. They are the fundamental units that send and receive signals which allow us to move our muscles, feel the external world, think, form memories and much more.

Just from looking down a microscope, however, it becomes very clear that not all neurons are the same. So just how many types of neurons are there? And how do scientists decide on the categories? For neurons in the brain, at least, this isn’t an easy question to answer. For the spinal cord though, we can say that there are three types of neurons: sensory, motor, and interneurons.

Sensory neurons

Sensory neurons are the nerve cells that are activated by sensory input from the environment - for example, when you touch a hot surface with your fingertips, the sensory neurons will be the ones firing and sending off signals to the rest of the nervous system about the information they have received.

The inputs that activate sensory neurons can be physical or chemical, corresponding to all five of our senses. Thus, a physical input can be things like sound, touch, heat, or light. A chemical input comes from taste or smell, which neurons then send to the brain.

Most sensory neurons are pseudounipolar, which means they only have one axon which is split into two branches.

Motor neurons

Motor neurons of the spinal cord are part of the central nervous system (CNS) and connect to muscles, glands and organs throughout the body. These neurons transmit impulses from the spinal cord to skeletal and smooth muscles (such as those in your stomach), and so directly control all of our muscle movements. There are in fact two types of motor neurons: those that travel from spinal cord to muscle are called lower motor neurons, whereas those that travel between the brain and spinal cord are called upper motor neurons.

Motor neurons have the most common type of ‘body plan’ for a nerve cell - they are multipolar, each with one axon and several dendrites.

Interneurons

As the name suggests, interneurons are the ones in between - they connect spinal motor and sensory neurons. As well as transferring signals between sensory and motor neurons,  interneurons can also communicate with each other, forming circuits of various complexity. They are multipolar, just like motor neurons.

Neurons in the brain

In the brain, the distinction between types of neurons is much more complex. Whereas in the spinal cord we could easily distinguish neurons based on their function, that isn’t the case in the brain. Certainly, there are brain neurons involved in sensory processing – like those in visual or auditory cortex – and others involved in motor processing – like those in the cerebellum or motor cortex.

However, within any of these sensory or motor regions, there are tens or even hundreds of different types of neurons. In fact, researchers are still trying to devise a way to neatly classify the huge variety of neurons that exist in the brain.

Looking at which neurotransmitter a neuron uses is one way that could be a useful for classifying neurons.

However, within categories we can find further distinctions. Some GABA neurons, for example, send their axon mostly to the cell bodies of other neurons; others prefer to target the dendrites. Furthermore, these different neurons have different electrical properties, different shapes, different genes expressed, different projection patterns and receive different inputs. In other words, a particular combination of features is one way of defining a neuron type.

The thought is that a single neuron type should perform the same function, or suite of functions, within the brain. Scientists would consider where the neuron projects to, what it connects with and what input it receives.

This is really the purpose of trying to classify neurons: in the same way as we can say that spinal cord sensory neurons bring sensory input from the periphery to the central nervous system, we would like to be able to say that the role of ‘neuron X’ in the hippocampus is to (for example) let you distinguish between similar but slightly different memories.

So the answer to the question ‘What types of neurons are there?’ isn’t something we can fully answer yet. In the spinal cord, it is pretty simple. But part of what gives the brain its complexity is the huge number of specialised neuron types. Researchers are still trying to agree on what these are, and how they should be classified. Once we can do that, we’ll be in a good position to delve even deeper into how the brain operates.

What are the 4 structural classifications of neurons?

What are the structural and functional classifications of neurons?

What is not a structural classification of neurons?

What are the 3 classifications of neurons?