What Is A Neuron?
The nerve cell, or neuron, is the active communicating body of the nervous system. Using electrical signals otherwise known as nerve impulses, they’re able to communicate with each other, and with other areas of the body.
These nerve impulses can then stimulate a response from an effector organ.
Neurons, alongside glial cells, are a primary component of the nervous system.
The nervous system itself is formed of the central nervous system, including the brain and spinal cord, and the peripheral nervous system, which can be further divided into the autonomic and somatic nervous system.
There’s thought to be roughly 86 billion neurons in the human brain. These cells are often specialized, but feature some key similarities.
To put it very simply, the nerve cell is one of the fundamental building blocks of the nervous system. The terms neuron and nerve cell are interchangeable, and both will be used within this article.
Our guide will cover what the neuron is, what the structure is formed of, and how it functions.
Nerve Cells: Structures And Functions
Nerve Cell Structure
The form of the neuron is similar to that of any type of animal cell at the ultrastructure level. Like other cells, the nerve cell contains a nucleus, golgi apparatus, membrane, mitochondria etc.
These are the organelles, and their function is keeping the cell alive. However, the high specialization of the neuron, which allows for the communication of cellular signals, necessitates for some diversity.
The organelles are a key component of any cell, as their primary function is ensuring the wellbeing of the cell. In the neuron, these organelles aid the processing and transmission of signal, by creating shape, storing information, and assisting electric signaling.
Within the nervous system are different types of nerve cells, each specialized in order to perform a specific function. Regardless of specialization, the structure of the neuron is formed of these crucial elements.
The soma, otherwise known as the cell body, contains the nucleus of the neuron.
The nucleus is typically round when viewed under a microscope, and can be anywhere from 5 micrometers to 10 millimeters in size (depending on the cell, and the invertebrate it’s found in).
Branching out from the soma are the dendrites, and the axon hillock which connects to the axon. Through this, nerve impulses can be communicated between cells.
Contained within the soma are several organelles, including Nissl granules, in which proteins are thought to synthesize and segregate. These organelles within the soma play a crucial role in producing energy, as well as cell growth.
Containing the nucleus, the soma is the site in which most protein synthesis occurs. Here, the loss of old proteins is balanced by the creation of new proteins.
These proteins allow the cell to function correctly, by maintaining the structure and keeping the cell alive. This is the metabolic center of the cell.
The multipolar neuron is the most commonly depicted cell that depends on the neuron. This has a single extending axon, and numerous dendrites.
A unipolar cell is similar, but lacks dendrites. Bipolar neurons have two projections extending from the cell body, while a pseudounipolar cell has a single protruding axon that splits into two.
Pseudounipolar neurons are not directly attached to the axon or dendrite, as they’re connected by a tubular projection.
The soma can also be referred to as the cell body, the perikaryon, or the neurocyton.
Extending outward from the cell body, the dendrites are branching nerve fibers. As they give the appearance of a many limbed tree, the structure is often known as the dendritic tree.
Some dendrites also possess dendritic spines: small protrusions that increase the receptive abilities of the dendrites.
The dendrites play an essential role in the neuron transmitting and receiving information. The membranous dendritic protrusions receive signals from other neurons via the axon.
The increased surface area provided by the dendrite allows for a better input of communications. A dendritic tree might be able to receive over 100,000 inputs from just a single neuron.
Dendrites are thought to be capable of synaptic plasticity, or the ability to change in adult life. In early development, the dendrite is highly influenced by both intrinsic programs and extrinsic signals.
As the cell ages, these extrinsic signals gain greater influence, and can change the structure of the dendritic branch.
Overall, the dendrites are branching nerve fibers that extend from the cell body to receive signals from the axons of other nerve cells. These signals are then able to accumulate in the soma, before traveling to the axon hillock.
Axon Hillock And Axonal Initial Segment
Protruding from the cell body, the axon hillock connects the soma to the axon. Lacking most cell organelles, the cone-shaped axon hillock can be identified both by where it is found in the cell, and the lower distribution of Nissl substance.
It does contain a large number of voltage-dependent sodium channels, making it easily excitable. The action potentials received and summed in the neuron are sent to the axon hillock, and to the axonal initial segment.
The axonal initial segment, sitting between the axon hillock and the axon microdomain, is thought to be the earliest area in which action potentials are initiated.
The axonal initial segment also plays a key role in structure, as it separates the axon from the cell body and the rest of the neuron.
Extending from the axon hillock, the axon is a single nerve fiber, which passes messages from the cell body. It’s often covered and insulated by a myelin sheath.
Unlike the dendrites, the axon is a single structure, which is both straighter and smoother. The length of the axon can be several times the diameter of the soma. In some cases, it’s even tens of thousands times longer than the cell body diameter.
The primary function of the axon is carrying nerve signals away from the soma. Along the length of the axon are microtubules, which overlap and point away from the soma and toward the axon terminals.
These microtubules provide a route for various materials to be transported away from the cell body.
An axon may be myelinated, so it is insulated by a myelin sheath. Consisting of a lipid-rich material, myelin surrounds the axon and increases the rate at which electrical impulses are able to pass from the cell body.
In the peripheral nervous system, Schwann cells provide myelination. In the central nervous system, oligodendrocytes provide this role.
A myelinated axon also contains nodes of Ranvier, which are gaps in the myelin sheath. At a node of Ranvier the axon is reduced in diameter, and action potentials can be generated.
This allows action potentials to jump from gap to gap, quickly traveling the length of the axon.
The axon terminal is the farthest point from the soma, and this is where the synapses of the cell are found. The axon terminal forms contact with other cells, releasing neurotransmitter chemicals and electrochemical signals.
In order to produce the energy necessary for this transmission, the axon terminal contains a dense number of mitochondria.
When two neurons meet, the gap between axon terminals is known as a synapse.
The Function Of Neurons And Their Role In The Body
The primary function of all neurons is to transmit information to gain a response. Nerve cells can be classed into three groups: sensory neurons, motor neurons, and intermediate neurons.
The neuron is also involved in receiving signals, integrating signals, and communicating signals.
Within the body, different types of nerve cells have different functions. The structure of the cell can influence how the neuron behaves in order to complete its unique task. Differing structures are specialized so each function can be effectively carried out.
The receptive function of the neuron allows it to receive information at the synapse, which can then be transmitted to induce a response.
Information that originates from stimuli is passed from cell to neuron via the synapses, allowing the cell to decide whether to induce an action potential. The postsynaptic cell is involved heavily in the receptive function of the neuron.
The dendrites and the cell body are responsible for the integrative function of the neuron. This is where the excitatory or inhibitory responses are summed up, or integrated, and it is determined what information should be passed on.
Not all neurons have the capability to initiate impulse, but for most of them, this is when the membrane potential reaches the threshold necessary to activate an action potential. Information can then be sent and transmitted to a specific target.
Transmission passes information from one neuron to another. This can be either an electrical transmission, or a chemical transmission. Electrical synapses travel via direct conductive junctions that exist between cells.
For chemical transmission, a neurotransmitter is released from one cell, and able to diffuse across the synaptic cleft to bind with another. With the signal transmitted, the next neuron undergoes the same process from receptive function to transmission.
The Neurons Found In The Body
While neurons all follow a similar function of receiving and transmitting information, the exact purpose of the neuron varies.
Sensory neurons respond to outside stimuli. They’re activated by the senses of the body (such as sound, smell, and sight). A physical or chemical stimulus will activate a response.
Sensory neurons may respond to outside stimulus, but that doesn’t just mean things outside the body. Interoceptors detect stimuli that occur within the body, such as blood pressure or temperature.
Even things such as spicy foods. Exteroreceptors are responsible for responding to stimuli that originate outside the body.
Motor neurons can be found in the motor cortex, brain stem, or spinal cord, with an axon that projects to or around the spinal cord. From here, they are able to directly or indirectly control the effector organs such as muscles and glands.
Motor neurons can be divided into two types. Upper motor neurons can be found in the brainstem and cerebral cortex, and they synapse onto the spinal cord or lower motor neurons.
Lower motor neurons carry transmissions from the spinal cord to the effector organs.
By traveling through the upper motor neuron to the lower motor neuron, the central nervous system is able to transmit information and impulses to the effector organs, controlling muscles, organs, and glands throughout the body.
As the name suggests, the interneurons exist between the sensory neurons and the motor neurons, allowing for communication across the central nervous system. They can be divided into two groups.
Local interneurons connect and communicate with nearby nerve cells via a short axon, to analyze small amounts of information. Relay interneurons have longer axons, which they use to connect neurons in one part of the brain with neurons in another.
Nerve Impulse Transmissions
The signals transmitted by the neurons are responsible for the successful function of the body. It’s thanks to these transmissions and communications that a person is able to move and react.
By forming a connection between the brain, spinal cord, and effector organs, the nerve cells effectively communicate across the body, and respond to stimuli.
When a neuron isn’t signaling, this is a moment of resting membrane potential. This is a static state, in which there is a different electrical charge between the outside of the membrane and the inside.
The outside has a higher electrical charge, due to an excess of sodium ions. The inside has a lower electrical charge, and an excess of potassium ions.
Also contained within the membrane are large negatively charged proteins, which provide the overall negative charge inside the membrane.
In this state, a cell is considered polarized.
Ions are continuously flowing back and forth, even during resting membrane potential. However, the cell controls the concentration, pumping potassium into the cell, while sending sodium away.
The resting membrane is more permeable to potassium ions, allowing more of these to move about, while few sodium ions are able to diffuse in. This maintains a relatively consistent negative concentration gradient, and the cell remains polarized.
The resting potential of a neuron is roughly -70 millivolts.
When this balance shifts, this is the moment of action potential. Action potential occurs when the cell moves from a negative state to a positive state.
A stimulus triggers the change in state resulting in action potential, and the nerve cell can communicate throughout the body.
Action potential is the result of several events occurring within the nerve cell.
Depolarization is a shift in the negative charge inside a cell. This occurs when a transmitter from another neuron causes a higher concentration of positively charged ions to enter the cell body.
This will shift the membrane potential, increasing the positive charge inside the ion.
The transmitter causes the sodium channels in the cell to open at the trigger zone, near the axon hillock. The strong signal is then able to travel along the cell, opening more sodium channels, and completely shifting the charge of the cell.
Positively charged sodium ions are able to flow into the axon, for complete depolarization. If a signal isn’t strong enough, the channels along the cell will fail to open, and the charge won’t change.
Complete depolarization can shift a cell from -70 millivolts to +30 millivolts.
Repolarization is the state that occurs after depolarization, returning the cell back to the initial negative state. Repolarization typically reverts the cell back to resting membrane potential.
During depolarization, sodium ions are able to flow into the nerve cell. In response, the potassium channels open, causing the potassium ions to exit the cell.
The ion movement reverts the cell back to its original negative charge, only this time, the ions are switched. Excess potassium ions are found outside the membrane, while the excess sodium ions are inside.
Once the potassium channels have opened, the sodium channels will begin to close.
As the action potential continues to move through the cell, the potassium channels remain open while the sodium channels close. Positive potassium ions continue to flow outwards from the cell.
In turn, the charge inside the membrane becomes increasingly negative. During hyperpolarization, the charge inside the cell is more negative than during a state of resting membrane potential.
Hyperpolarization typically shifts the charge to -80 millivolts (compared to a resting membrane potential of -70mV).
The moment following the passing of an action potential is known as the refractory period. At this moment, although the membrane is polarized, the ions are in the wrong places.
The cell pumps will then get to work returning the sodium and potassium ions back to their original positions. When this happens, the axon is unable to receive any new signals, and can’t respond to stimulus.
This brief period in which no action potentials can be generated is the refractory period, and it lasts for roughly two milliseconds.
When the ions are back in the right place, and the membrane is once again polarized, the neuron can receive another signal.
Action Potential And The Myelin Sheath
The myelin sheath is a covering of lipids that insulates the axon of the nerve cell. In many ways, the myelin sheath performs a similar function to the insulation on an electrical wire.
However, the myelin sheath isn’t a single unbroken chain wrapped around an axon. Instead, the myelin sheath is segmented, with the small gaps in between each segment known as the nodes of Ranvier.
Action potential can’t actually travel along the myelin sheath itself. But it’s thanks to the nodes of Ranvier that a myelinated axon can actually pass an impulse faster than an unmyelinated axon.
The action potential can essentially jump from one node to the other, recharging at each gap before reaching the axon terminal. This process is known as saltatory conduction.
Unmyelinated axons pass signals slower, as the action impulse has to travel the full length of the axon. Demyelination is the degradation of myelin, which results in a slower conduction of signal.
The Role Of Neurotransmitters
A neurotransmitter is a chemical messenger released from the neuron to carry messages to other cells. These are a signaling molecule, and they’re sometimes referred to as chemical transmitters.
Neurotransmitters aren’t just released from neuron to neuron. They can also send messages to gland and muscle cells.
The action impulse is able to pass along the axon in the form of an electrical signal. At the end of the axon terminal is the synaptic cleft. To send the signal across the synapse, the neurotransmitter is released as a small vesicle sac.
This is able to diffuse across the synapse, before it comes into contact with another cell. It can then bind to a receptor, carrying the signal. If it binds to another nerve cell, the signal will be transmitted from one neuron to the next.
The role of the neurotransmitter is to activate a receptor, and, depending on the receptor it binds to, this can provoke different responses.
Neurotransmitters are stored in the synaptic vesicles. Their release is regulated by a voltage-dependent chemical channel, which is activated when the cell becomes depolarized.
As the level of calcium ions within the cell changes, the channels are opened, and the neurotransmitters are released.
The role of neurotransmitters is vital to the function of the human body. So far, over 100 types of neurotransmitters have been identified in the human body. However, we’re not yet sure of how many neurotransmitters might potentially exist.
A Quick Overview Of Types Of Neurotransmitters
Types of neurotransmitters are still being discovered within the nervous system. Some types include:
- Dopamine (DA). Dopamine is a neurotransmitter synthesized in the brain that plays a role across various regions of the body. It’s sometimes referred to as the ‘pleasure chemical’, because one of the dopamine pathways in the brain is crucial to the ‘reward’ part of the reward-motivated behavior system. The role of dopamine goes beyond just reward systems. It plays a part in memory, decision-making, motivation, and even learning.
- Serotonin (5HT). A monoamine neurotransmitter, serotonin is commonly associated with a calming effect. Although many of us associate serotonin with improving mood, the overall biological function of this neurotransmitter is complex. Roughly 90% of serotonin in the human body is found in the digestive tract, where it can regulate intestinal movement. Serotonin is thought to play a role in memory, sleep, and decision-making, while a lack of serotonin has been linked to depression.
- Glutamate (GLU). Glutamate is the most common neurotransmitter in the vertebrate nervous system. It’s an excitatory neurotransmitter, used in every major excitatory function within the brain, including both learning and memory. However, too much glutamate has a negative effect. Strokes, Alzheimer’s disease, motor neuron disease, and lathyrism are all associated with excessive glutamate.
- Acetylcholine (ACh). Acetylcholine is a chemical released by motor neurons to activate muscles, and is found at the neuromuscular junction. Found in both the central nervous system and the peripheral nervous system, Acetylcholine also plays a role in learning and memory, as well as awakening and paying attention.
- gamma-Aminobutyric Acid (GABA). GABA neurotransmitters are inhibitors, correlating with the excitatory GLU neurotransmitters. GABA also plays a key role in the early development of the brain, and is known as the ‘learning chemical’ as the level of GABA within the brain is linked to successful learning.
Neurons can be observed using a light microscope, however a super-resolution microscope is necessary to visualize the complex morphology of the nerve cell.
But using Luxol Fast Blue Dye and a light microscope (otherwise known as a compound microscope) can reveal parts of the neuron.
What You Need
- 95% Alcohol
- 70% alcohol
- Distilled water
- Section of your sample
- 10% Formalin solution
- Luxol Fast Blue Dye solution
- Lithium carbonate solution
- Eosin Y solution
- Cresyl Violet
- 100% Ethanol
- Light (compound) Microscope
- Microscope coverslip
How To Observe Neurons Using A Compound Microscope
- Use the 95% alcohol, the 70% alcohol, and the distilled water to deparaffinize and then hydrate the sample.
- Immerse your sample in the Luxol Fast Blue Dye solution for 24 hours at room temperature, or for 2 hours at 60 degrees Celsius.
- Rinse the sample in alcohol, and then in water.
- Gently place the sample in a lithium carbonate solution for roughly 5 seconds.
- Remove, and then rinse the sample in 70% alcohol for 10 seconds. Rinse again in fresh alcohol.
- Wash using distilled water.
- The sample should start to show the contrast between blue, white, and gray. Repeat steps 4 to 6 until the contrast is sharp, and you can clearly observe the gray matter. When you feel confident in the contrast, rinse in 70% alcohol.
- Gently place the sample in the eosin solution. Remove after one minute, and rinse in distilled water.
- Gently place the sample in the Cresyl violet. Remove after one minute, and rinse in distilled water.
- Use the 95% alcohol to dehydrate the sample, and then dehydrate again using the 100% ethanol.
- Clear the sample with the xylene, and cover with your microscope coverslip.
- Place under the microscope, and view.
Under the microscope, the myelin sheath should be visible in blue, and contrasting from the purple of the rest of the nerve cell.
By passing and transmitting signals across the body, the nerve cells play a key role in correct function. Action potentials travel along the cell structure, before converting to chemical signals, and transmitting to other neurons, or binding to effector organs.
There are various types of nerve cells found within the body, with specializations designed to better communicate certain signals. However, they all follow a similar function, acting as an effective communicator across the nervous system.
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