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Nerve Cells
(Neurons)

Structure and Function, Adaptations & Microcopy  


Definition: What are Nerve Cells?


Essentially, nerve cells, also known as a neurons, are the active component of the nervous system. Neurons communicate with each other as well as with other cells through electric signals (nerve impulses), which in turn allows effector organs to respond to the appropriate stimuli.

Nerve cells may be described as receivers and transmitters of information that allow an organism to respond appropriately.  In the human body, the nervous system (which consists of the central and peripheral nervous system) is said to contain about 1020 individual neurons. Each of the neurons is made up of several parts that enable them to perform their functions appropriately.

* In short, a nerve cell/neuron is the basic functional unit of the nervous system.

* The words nerve cell and neuron will be used interchangeably in this article.


The anatomy of neurons consists of:

  • Cell body
  • Dendrites
  • Axon
  • Myelin sheath cells
  • Nodes of Ranvier
  • Axon terminal bundle 



Structure and Function of Nerve Cells



Structure


At the ultrastructure level, a nerve cell, like any other type of animal cell, contains different types of organelles that keep them alive and allow them to remain functional. These include such cell organelles as a nucleus, nucleolus, E.R, golgi apparatus and the mitochondria among others.

The different types of organelles play different roles which contribute to the proper functioning of the neuron. For instance, whereas the DNA contained in the nucleus contains genetic material that controls all characteristics of the cell, the cytoskeleton (which consists of a tubular structure) helps maintain the shape of the neuron as well as the transportation of substances like proteins.

Anatomically, a nerve cell consists of several parts mentioned above. While different types of nerve cells make up the nervous system, they all contain these primary structures:

Cell body (Soma)


The soma is the cell body of the nerve cell that contains the nucleus. Compared to the other sections of the cell, the cell body is larger and may appear spherical under the microscope.

A series of branch-like structures known as dendrites arise from the cell body. Apart from connecting the dendrites and axons, which allows for nerve impulses to be transmitted from one cell to another, the soma is also the site of protein synthesis (proteins are synthesized within Nissl body of the rough E.R in the cell body of the neuron).

* The cell body/soma is also known as the perikaryon.


* The cell body is the metabolic center of the cell consisting of energy producing systems and where macromolecules are synthesized to keep the cell alive, maintain its structure and allow it to function appropriately.


* Cell organelles of the nerve cell contains different types of organelles that are involved in such functions as growth, energy production and the synthesis of proteins among others.

There are different types of cell bodies depending on the neuron.


These include:

· Bipolar - are located in the middle and have a single axon and dendrite on either end


· Pseudounipolar - connected to the axon and dendrite by a tubular projection - as such, it is not directly connected to the two. The axon also splits into two branches at its end


· Unipolar - The cell body here is located on one end and has a single axon. Unlike the other cells, unipolar cells lack dendrites.

·  Multipolar - This is the type of cell body that is commonly depicted in many books. Arising from the cell body are dendrites (branched) while the axon extends from one side of the cell body

Dendrites

Dendrites are the tree-like branched structures that arise from the nerve cell body. Depending on the cell, dendrites may extend significantly resembling a highly branched tree. Apart from the main dendrite branches, dendrites may contain additional protrusions known as dendrite spines.

These small membranous protrusions receive input from the axon of another cell and thus play an important role in the transmission of nerve impulses by increasing the overall surface area.

As expansion of the cell body, dendrites and dendrite spines also contain cytoplasm and different types of organelles. In particular, dendrite spines contain a variety of microtubules and some neurofilaments that contribute to the changes observed in their shape.

* Dendrites receive electrical impulses from axons of other nerve cells which in turn accumulate in the soma before being sent to the axon hillock.

Axon Hillock

The axon hillock is a specialized region from which the axon extends. As such, it is the area at which the axon is attached to the cell body. Unlike the cell body and dendrites, the Axon Hillock lacks many cell organelles. However, it contains various elements of the cytoskeleton as well as a few of the organelles that are transported to the axon from the cell body.

* Initial segment - this is the region between the Axon and Hillock and the front part of the myelin sheath. This region is said to be the area for the initiation of action potential.


* The Axon Hillock is cone-shaped.


The Axon

The axon is a single elongated structure that extends from the Axon Hillock. Compared to dendrites, the axon is straighter in appearance and has a smoother surface. In addition, compared to dendrites which tend to be highly branched, each neuron has a single axon that extends and branches at its end. 

While it lacks many of the organelles found in the cell body, the axon contains microtubules (along the length of the axon) and specialized, insulating substances known as myelin on its surface that boost the transmission of nerve impulses.

* The branched end of the axon is known as the axon collaterales.


* The spaces/gaps between the Schwann cells are known as the nodes of Ranvier and they serve to propagate electrical signals along the axon.


* Myelin sheath is made up of cells (Schwann cells) wrapping themselves around the axon. In the Central Nervous System, this action is performed by the oligodendrocyte cells.

Nerve ending/ Axon terminal

This is the distal part of the axon that comes in contact with other cells. Because this part of the axon is largely involved in the release of the  neurotransmitter, it contains a large number of mitochondria that produce the energy required to facilitate the process. 


Types of Neurons Based on the Location of the Cell Body



Function


Although nerves are functionally classified into three main groups (sensory, motor and intermediate neurons) they are all involved in the transmission of information which in turn ensures the appropriate response.

They are involved in the signal reception, integration of the incoming signal as well as the communication of the signal.


Here, the different parts of the cells (cell body, dendrites, axons etc) play different roles which in turn allow the cell as a whole to effectively carry out its functions:

Receptive functions of a neuron - Neurons come into contact with other cells at sites known as synapses. This is the site at which the nerve endings of the cells come in contact allowing for successful communication.

In this case, neurons play a receptive function by receiving information that originated from the stimuli. It is this receptive function of the neurons that ensures the effective transmission of information and consequently the appropriate response to stimuli.

* The postsynaptic cell is involved in the receptive function (This will be discussed in detail in the next section).

Integrative function of a neuron - The integrative function occurs in the dendrites (receptive components) as well as the cell body of the neuron. For the most part, it involves the summing up of excitatory and inhibitory responses (this being integration of incoming signals) in order to determine whether certain information should be transmitted.

Impulse initiation - For a majority of the neurons, nerve impulses are initiated when the membrane potential of the neuron is sufficiently depolarized and reach a certain threshold. This allows some of the neurons to initiate impulses and thus information to specific targets.

* Not all neurons are capable of impulse initiation.

Transmission - Transmission from one neuron to another is either electrical or chemical.

* In electrical transmission, a neuron is influenced by another through passive electrical means.


* In chemical transmission, it's the potential change in one of the neurons that results in the release of a chemical neurotransmitter which in turn diffuses another neuron.


 A brief summary of the three main types of neurons in the body:

Sensory neurons - These are the type of neurons that are activated by external physical or chemical stimuli. This, therefore, involves sensory activation of any of the five senses (feel, smell, sound, sight, hear).

* The stimuli may be physical or chemical.


* A majority of the sensory neurons have been shown to be pseudounipolar (described above) - As such, their axons split into two at the end.

Motor Neurons - Motor neurons are the type of neurons in the spinal cord that connects the organs, muscles and different types of glands in the body. As such, they function to transmit impulses from the Central Nervous System to the organs, glands, and muscles. This, in turn, controls the movement of different types of muscles as well as the activity of organs and glands in the body. Motor neurons are composed of multipolar neurons.

* There are two types of motor neurons. These include the lower motor neurons (from the spinal cord to the muscle) and the upper motor neurons that travel between the spinal cord and the brain. 

 

Intermediate neurons - These are the type of neurons that connect the motor neurons to the sensory neurons thus allowing for signals to be transmitted between the two. Like motor neurons, this system is composed of multipolar neurons. 



Transmission of Nerve Impulses




Neurons are some of the most important cells in the body. This is because they are involved in cell communication that, in turn, allows an organism to function as it should in its environment.

By sending signals through the nerve cells in the nervous system, the brain makes it possible for an individual to move their hand, legs or other parts of the body through its action on the muscle. This process, however, involves several processes that will be discussed in this section.

Most of the time, a neuron is at resting membrane potential (negative concentration gradient). In this state, the concentration of positively charged ions is higher outside the cell than inside. This is characterized by higher sodium ion concentration outside the cell than inside and higher potassium ion concentration inside the cell than outside.

While ions still flow in and out of the cell during this state, the cell continually controls their concentrations in order to maintain a relatively consistent negative concentration gradient. This involves actively pumping potassium into the cell while pumping sodium out.

* The resting potential (resting membrane potential) is about -70mV.

* While potassium ions, like sodium, are positively charged, they are mixed with large negatively charged proteins in the neuron which causes the inside of the cell to be negatively charged as compared to the outside.

* During the resting potential, the neuron is polarized.

* For every two potassium that is pumped into the cell by the sodium-potassium pump, three sodium ions are pumped out which maintains the state of the resting potential.

Unlike the negative resting membrane potential, the action potential is a shift from the negative to the positive state. As such, it's the state in which signals are sent around the body through the neurons.

During action potential triggered by a stimulus, a number of events take place in the neuron.


These include:

Depolarization - When a signal (neurotransmitters) from other cells reaches another neuron, it results in positively charged ions flowing into the cell body through specific channels. The incoming ions cause the membrane potential to fall resulting in depolarization.

The voltage-gated sodium channels near the axon hillock are also activated (due to the depolarization of the cell body) thus allowing the positively charged ions (sodium ions) to flow into the axon (which is negatively charged). This action results in the depolarization of the axon along its length as more channels are opened.

* As the action potential passes through, the neuron becomes positively charged.

Voltage-gated channels include gate h and gate m (voltage-gated sodium channels) and gate n (potassium channel).


Repolarization - As the sodium ion gates become inactivated, they start closing, which in turn stops the positive ions from flowing into the cell. Potassium channels also start to open resulting in more potassium ions moving outside the cell thus causing the cell to become more negative as it reverts to the resting state.


Hyperpolarization - While the action potential is passing through, the potassium channels remain open a little longer, which allows positive ions to continue flowing out of the cell. This, in turn, causes the cell to become increasingly negative (more than it usually is during the membrane potential).

This is only temporary given that these channels close allowing the sodium-potassium pump to start working to revert to the normal resting state.

* A neuron only has one signal it can send at a time which is only transmitted at a uniform strength and speed.

* The frequency of the signal sent, however, can vary - Number of pulses sent.

* Nerve impulse is the action potential.

* As the axon is involved in the current activity, it cannot respond to any other stimulus. This period is known as the refractory period. 


Representation of Action Potential


As the nerve impulse moves along the axon as represented in the image above, it's possible to see the change in ion movement in and out of the cell. However, once the impulse passes, the part behind the impulse on the axon starts reverting back to the resting membrane potential.

Although the image above gives a general representation of action potential, it does not show the myelin sheath and nodes of Ranvier. In a normal nerve cell, these structures are present and enhance the propagation of action potential.

The areas covered with the myelin sheath prevent the exchange of ions along the axon. However, at the nodes of Ranvier, which are the uncovered gaps, ion exchange takes place which allows for faster propagation.

This is due to the fact that the process jumps from one node to the next rather than the transmission occurring along the entire length of the axon.

Transmission that occurs due to the presence of the myelin sheath cells (with discrete jumps) is known as saltatory conduction


Image with Myelin Sheath cells:



Neurotransmitters


Also referred to as chemical messengers, neurotransmitters are molecules of the nervous system that transmit messages from one neuron to another or from a neuron to other cells.

As described above, nerve impulses are transmitted along the neuron axon in the form of electrical signals. However, once these signals reach the synapse, the signals are converted to chemical signals.

Here, the neurotransmitter is released from the axon terminal at the synapse, moves through the synaptic cleft (the gap between the chemical synapse of two neurons) to reach the other cell. The neurotransmitter is released in the form of small vesicle sacs.

Once they come in contact with the other cell, the neurotransmitter binds to the receptors on the other cell which in turn causes a change on the cell.


In doing so, the neurotransmitter may cause any of the following events:

  • Promote action potential - The action of excitatory transmitters
  • Regulate neurons - Neuromodulators


Types of Neurotransmitters


There are several types of neurotransmitters that include:

  • Acetylcholine
  • Glutamate
  • γ-aminobutyric acid
  • Glycine
  • Dopamine
  • Noradrenaline
  • Serotonin
  • Histamine


Nerve Cells Microscopy


While super-resolution microscopy is required to visualize the morphology of nerve cells, the use of Luxol Fst Blue dye (in the modified Kluver-Barrera's technique) has been used to view parts of the neuron (the myelin and the axon) under the light microscope.

Requirements:

  • 10 percent formalin
  • Sections of the sample (10u)
  • Luxol fast blue solution
  • Eosin Y solution
  • Compound microscope
  • Xylene
  • Alcohol
  • Cresyl violet
  • Lithium carbonate
  • Distilled water
  • Microscope coverslip

Procedure:

· Using 95 percent alcohol, deparaffinize and hydrate the sections (sample)

· Place the section in Luxol fast blue solution overnight at 60 degrees Celsius

· Rince the sample in alcohol

· Rinse the sample in water

· Place the sample in a solution of lithium carbonate for about 5 seconds

· Place the sample in 70 percent alcohol (repeat this after 10 seconds in fresh 70 percent alcohol)

· Wash the sample using distilled water

· Repeat steps 5 to 7 until a sharp contrast between the blue part of the white matter and the colorless gray matter is observed

· Rinse the sample in 70 percent of alcohol

· Place the sample in eosin solution for about 60 seconds

· Rinse the sample in distilled water

· Place the sample in Cresyl violet for about 60 seconds

· Rinse the sample in distilled water

· Dehydrate the sample using 95 percent alcohol

· Dehydrate the sample for the second time in 100 percent ethanol

· Clear using xylene and cover using a coverslip

· View under the microscope

Observation

When viewed under the microscope, myelinated fibers appear blue in color while the other parts of the nerve cell appear purple in color.



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References


Akash Gautam. (2017). Nerve Cells. Encyclopedia of Animal Cognition and Behavior. 

Alan G. Brown. (1991). Nerve Cells and Nervous Systems: An Introduction to Neuroscience

Jack C. Waymire. (1997). Chapter 8: Organization of Cell Types. Department of Neurobiology and Anatomy, McGovern Medical School. 

Jennifer Kenny. (2010). The Nerve Cells. 

Sinauer Associates, Inc. (2001). Neuroscience. 2nd edition. 

Silvia Helena Cardoso. (2002). Parts of the Nerve Cell and Their Functions. 

Links


https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/13%3A_Human_Biology/13._16%3A_Nerve_Cells

https://webpath.med.utah.edu/HISTHTML/MANUALS/LFB.PDF


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