Video: Neurons

Have you ever wondered how the nervous system -- the complex network which orchestrates each of our thoughts and movements -- relays information throughout the body? The answer, of course, are the extraordinary cells known as neurons. But what exactly makes a neuron a neuron and what kind of neurons make up the grand neuronal family that pulse different kinds of electrical signals all around our body?

Well, come along today as we explore the complexities, differences, and intricate details of our neurons.

Neurons receive, generate, conduct, and transmit information in the form of electrical signals called nerve impulses. These functions are reflected in the microanatomy of the neuron and the neuron's basic structure supports its function. On that note, let's take a look at the different parts of a neuron.

Now you may be aware that there are many different types of neurons which we will look at later in the tutorial. However, for now, we'll just be looking at this neuron here, which is known as a multipolar neuron.

Neurons typically consist of four main functional parts, including, the receptive part composed of dendrites; the integrative part, which is usually equated with the cell body or soma; the conductive part called the axon; and the transmissive part composed of axon terminals.

Let's take a closer look at each of these parts, beginning with the cell body, which is also known as the perikaryon or soma.

This is the large spherical or polygonal portion of the neuron and is where all the magic happens. The cell body functions as the trophic center and is responsible for processes that support the survival, growth, and maintenance of the cell. It houses the nucleus, which contains the genetic material, or DNA of the cell, as well as various other cytoplasmic organelles. These organelles include the endoplasmic reticulum which clusters with free ribosomes to form what is known as chromatophilic substance, which we'll often hear referred to as Nissl bodies. This facilitates the synthesis of proteins, such as enzymes, receptors, ion channels, and other structural components.

The Golgi apparatus packages and modifies proteins for transports to their destinations. Mitochondria generate ATP, providing energy for the neuron's activities. Additionally, microtubules form part of the cytoskeleton providing structural support and facilitating the transport of organelles, vesicles, and other materials throughout the neuron.

Then we have the axon hillock, a distinctive cone-shaped region of the cell body, which is continuous with the initial segment of the axon. It lacks large cytoplasmic organelles like chromatophilic substance and Golgi apparatus. It is particularly important as it is the site where incoming excitatory and inhibitory signals are integrated and determines whether or not the sum of all incoming signals warrants the propagation of an action potential. The initial segment of the axon is the actual site where the action potential is then generated, although more recent research states that both the axon hillock and initial segment are capable of generating action potentials.

Now emerging from the cell body of all neurons are elongated arm-like structures known as processes. These are cytoplasmic extensions which facilitate communication with other cells. Typically, neurons possess two types of processes -- two or more dendrites and a single axon.

The receptive part of a neuron is composed of dendrites, which are the tree-like processes extending from the cell body that receive and conduct electrical signals towards the cell body. Primary dendritic branches extend from the cell body which branch again into secondary and tertiary dendritic branches to provide an increased surface area for receiving information from other neurons at specialized areas of contact called synapses.

These branches are lined with numerous tiny protrusions called dendritic spines and these serve a sites for the initial processing of synaptic signals via membrane-embedded neurotransmitter receptors. They then translate the chemical messages received into electrical events which travel down the dendrites.

There are approximately a whopping 10 trillion of these dendritic spines present across all dendrites of neurons in the human cerebral cortex. This therefore greatly increases the area available for synaptic events to occur.

The other type of process emerging from the cell body is the conductive part of the neuron called an axon. The axon, also known as a nerve fiber, is a long slender process which originates from the axon hillock and conducts electrical impulses in the form of action potentials, away from the cell body. The axon's membrane is known as the axolemma and its cytoplasm is referred to as axoplasm.

Unlike dendrites, which form a complex network with many tapering branches, an axon is typically a single long process that can extend a considerable distance before branching and terminating. Axons come in many different lengths and some can even exceed a meter. Take the sciatic nerve, for example. It contains some axons which extend all the way from the spinal cord to the feet.

The proteins and organelles needed for the growth of axons are synthesized in the cell body and then transported to the axon through axonal transport. This is facilitated by microtubules and intermediate filaments known as neurofilaments that provide cytoskeletal tracks for transportation.

Now if you've ever looked at electrical wires, you may have noticed that some may have a protective coating wrapped around them. Well, axons have something similar called the myelin sheath. This is a protective layer of lipids and proteins that acts as an insulator while it also enhances the speed of impulse transmission. Oligodendrocytes in the central nervous system and neurolemmocytes, also known as Schwan cells, in the peripheral system are responsible for forming myelin sheath.

Now not all axons are equally myelinated so nerve fibers can actually be classified into groups based on their myelination. Group A nerve fibers are heavily myelinated, group B are moderately myelinated, and group C are nonmyelinated. Along myelinated axons, evenly distributed gaps known as myelin sheath gaps, commonly referred to as nodes of Ranvier, allow electrical impulses to jump from node to node. This propagation pattern is referred to as saltatory conduction.

The final functional part of the neuron is the transmissive part, which is composed of axon terminals. These terminals are where axons communicate with other neurons or effectors which are the target structures that respond to nerve impulses. Axons typically terminate as fine branches called terminal arborizations. Each of these terminal arborizations is capped with a specialized structure called a terminal bouton, also known as an axon terminal. These boutons contain synaptic vesicles that store neurotransmitters to be released into the synaptic cleft when an action potential reaches the axon terminal.

There are three different locations at which synapses between neurons can occur. An axodendritic synapse, the most prominent type, is where the presynaptic neuron synapses with the dendrites of the postsynaptic neuron. Axosomatic synapses occur where the presynaptic neuron synapses on the cell body of the postsynaptic neuron. And finally, axoaxonal synapses are formed between the axon of the presynaptic neuron and the axon of the postsynaptic neuron. We will revisit this process in more detail later in this tutorial.

Now as mentioned, the neurons that we have been looking at so far have all been multipolar neurons. However, there are several different types of neurons that vary widely in their shape and exhibit various arrangements. Not only can we classify neurons based on their morphology, but we can also classify them functionally based on the information they carry, by the effects they have on other neurons, as well as by the neurotransmitters that they release.

Morphologically, neurons can be classified into five different groups, according to their shape and structural organization. There are multipolar, bipolar, pseudounipolar, unipolar, and anaxonic neurons.

Unipolar neurons are actually named as the prefix uni- means one and unipolar neurons feature a single process extending from the cell body which then branches into dendrites or an axon. These neurons are mostly found in invertebrates, however, some neurons in the human central nervous system may have a unipolar morphology such as the unipolar brush cells in the cerebellum and posterior cochlear nucleus. Some recent sources also consider rod and cone cells in the retina as unipolar neurons, however, historically, these have typically been classified as modified bipolar neurons.

We also have bipolar neurons which are oval-shaped neurons possessing -- you guessed it -- two processes extending from opposite poles of the cell body -- one dendrite and one axon. In humans, these neurons serve as sensory neurons and are primarily found in sensory organs such as the olfactory epithelium, retina, and vestibulocochlear apparatus.

Then we have multipolar neurons, which by now we're very familiar with. These are the most predominant type of neurons in the human body. The prefix multi- means many, which we can see is represented by the many processes including a single axon and multiple dendrites. Multipolar neurons are known for having extensive diversity and come in a wide range of sizes, shapes, and complexity within their dendritic tree.

Some common subtypes with distinctive morphologies include pyramidal neurons found in the cerebral cortex and subcortical structures with their characteristic pyramid-shaped cell body; stellate neurons, also found in the cerebral cortex, which have a star-like appearance formed by dendrites of equal lengths, radiating uniformly in all directions from the cell body; Purkinje cells, found in the cerebellum, which have large pear-shaped cell bodies and characteristic fan-shaped dendritic trees; and granule cells, these small oval-shaped neurons found in the cerebral cortex, cerebellum, the olfactory bulb, and the dentate gyrus.

There are also pseudounipolar neurons, which share characteristics with both unipolar and bipolar neurons. They have a single process extending from the cell body, similar to unipolar neurons. However, this process is quite short and splits into two other processes -- a central and a peripheral process -- giving it a similar appearance to bipolar neurons.

In contrast to multipolar neurons which have numerous dendrites branching from the cell body, the cell bodies of pseudounipolar neurons contain no dendrites. They serve as sensory neurons, and along with bipolar neurons, constitute the entirety of the primary sensory neurons within the human peripheral nervous system.

Finally, there are anaxonic neurons, which are small neurons with many processes but they either lack an axon or it cannot be distinguished from its many dendrites. These neurons can be found in the brain and the retina; for example, the amacrine and horizontal retinal cells.

Now that we know about the structure of neurons, we can explore how they can also be classified functionally based on the type of information they carry into sensory neurons, interneurons, or motor neurons. We can think of this like a relay race between the neurons. Each neuron has to ensure the baton, in the form of information, is passed onto the next neuron and to its final destination.

So when you touch something, the sensory receptors in your skin send information to your sensory or afferent neurons. These neurons have the task of relaying that information towards the central nervous system. So they pass the information onto their cell body that lies within the peripheral nervous system and then down their axon to the brain or spinal cord. These are usually pseudounipolar or bipolar neurons because they only receive information from one area.

Next, this information is received by interneurons, which act as middlemen and create a link between sensory neurons and motor neurons. They are the most abundant of the three functional types and can be further characterized as either short axon, also known as local circuit interneurons, which have short axons and form circuits with nearby neurons, or long axon interneurons, such as projection, association, or commissural interneurons which have long axons and connect circuits of neurons in different regions of the central nervous system. Most interneurons are usually multipolar in structure.

The final step in this relay race is to pass the information onto the motor or efferent neurons. These neurons receive the information from the local circuit neurons and carry this information away from their cell bodies that lie in the central nervous system, primarily to muscular and glandular tissue. Most motor neurons are multipolar.

We can also go one step further and classify these neurons based on the effect they have on other neurons such that they are classified into excitatory, inhibitory, and modulatory neurons.

Excitatory neurons facilitate the transmission of signals which promote the generation of action potentials in neighboring neurons, subsequently activating them and promoting activity in the affected target. Inhibitory neurons, on the other hand, have an opposite effect. They form intricate circuits that provide inhibition for a wide array of stimuli while also regulating the activity of excitatory neurons. And finally, we have modulatory neurons, which do not directly stimulate action potentials but they do influence the activity of other neurons by modifying their sensitivity or responsiveness to other signals.

The final classification is based on the type of neurotransmitters that the neurons release. Neurotransmitters are the chemicals inside the body that allow neurons to communicate with each other. They are made inside the cell body of the neuron or in the axon terminal and are packed into synaptic vesicles and then released into a space called the synaptic cleft.

This is a small gap at a synapse between neurons where nerve impulses are transmitted by a neurotransmitter. The neurotransmitters are released when an action potential reaches the axon terminal. The neuron that they are released from is called the presynaptic neuron and the one that receives the neurotransmitter is called the postsynaptic neuron.

Now that we have a basic understanding of how neurotransmitters work, let's look at the different types of neurons according to the neurotransmitters that they release.

Firstly, there are glutamatergic neurons that produce and secrete the neurotransmitter called glutamate. Glutamate is the main excitatory neurotransmitter of the brain. Then we have cholinergic neurons. These secrete acetylcholine. Acetylcholine is an excitatory neurotransmitter that is located in both the central and peripheral nervous systems.

Next, we have neurons that secrete GABA such as Purkinje cells and many interneurons. GABA is the main inhibitory neurotransmitter of the central nervous system. Lastly, we have dopaminergic neurons that produce and release the monoamine neurotransmitter, dopamine, and are mainly located in the midbrain, hypothalamus, and olfactory bulb. Dopamine can be both excitatory and inhibitory, depending on the type of receptor that it binds to.

And that's all we have for today's tutorial as we close the curtain on the fascinating world of neurons.

See you next time!