Nerve cells are the longest cells in the body.
Neurons are unique because they are shaped differently from any other cell. There are nerve cells so long that they go from the hips to the balls of the feet. This is very rare for cells, which are generally very short.
Neurons are specialized cells in the nervous system that transmit signals throughout the body.
Neurons can do many different things, from detecting external and internal stimuli to processing information and directing muscle actions.
Neurons have four specialized structures that allow information to be sent and received: the cell body or soma, the dendrites, the axon, and the axon terminals.
- Cell body or soma: The cell body is the portion of the cell that surrounds the nucleus and plays an important role in protein synthesis.
- Dendrites: Dendrites are short, branched structures that extend from the cell body. Dendrites work to receive information, and they do so through numerous receptors located on their membranes that bind to chemicals, called neurotransmitters.
- Axon: An axon is a large structure that extends from the cell body at a point of origin, called the axon mound, and functions to send information. Unlike the shorter dendrites, the axon can extend for more than a meter. Due to this length, the axon contains microtubules and is surrounded by myelin.
- Axon Terminals: Once an axon reaches a target, it ends up in multiple terminations, called axon terminals. The axon terminal is designed to convert the electrical signal into a chemical signal in a process called synaptic transmission.
Most neurons are amitotic or lose their ability to divide.
The exceptions to this rule are found in the olfactory neurons (those associated with smell) and those that are located in the regions of the brain, the hippocampus.
Fortunately, the life expectancy of amitotic neurons is close to 100 years.
Still, if a neuron is damaged or lost, it is not easily replaced.
For this reason, there is usually limited recovery from serious brain or spinal cord injuries.
Perhaps the slow recovery rate or lack of regeneration is ensuring that learned behavior and memories are preserved for a lifetime.
Neurons also have exceptionally high metabolic rates and subsequently require high levels of glucose and oxygen.
The body will do everything possible to ensure that the neurons feed properly.
In fact, if for some reason the brain detects that it is not receiving adequate amounts of nutrition, the body will immediately shut down, that is, it will pass out.
Neurons are capable of transmitting messages to each other using a special form of electrical signal.
There are signals that carry information from outside to the brain such as sensory signals (sight, smell, taste, touch). There are also other internal signals that are instructions for the organs, glands, and muscles.
Neurons are a perfect electrical system. These receive signals from neighboring neurons through dendrites.
The signal travels from the main cell body or soma, and travels to the axon to the synapse.
Myelin covers the axon and works as an insulator to keep the electrical signal within the cell, causing faster movement.
And finally, the signal moves through the synapse to the next nerve cell.
Neurons, like most cells in the body, also have a nucleus that contains DNA.
Classification of neurons
There are many types of neurons in your body. Each type is specialized to do different things.
The structural classification of neurons is based on the number of processes that extend from the cell body.
Three main groups emerge from this classification: multipolar, bipolar, and unipolar neurons.
Multipolar neurons have an axon and several dendritic branches.
They transmit signals from the central nervous system to other parts of your body, such as your muscles and glands.
They are part of more than 99% of neurons in humans, and are the main type of neuron found in the central nervous system.
Unipolar neurons are also known as sensory neurons.
These cells pass signals from outside the body, such as touch, throughout the central nervous system.
Unipolar neurons have a single, short process that extends from the cell body and then branches into two more processes that extend in opposite directions.
The peripherally extending process is known as the peripheral process and is associated with sensory reception.
The process that extends into the central nervous system.
Unipolar neurons are found primarily in the afferent division of the peripheral nervous system.
Bipolar neurons have only two processes that extend in opposite directions from the cell body. One process is called a dendrite and another process is called an axon.
Although rare, they are found in the retina of the eye and the olfactory system.
Neurons are functionally classified according to the direction the signal travels, relative to the central nervous system as:
- Sensory neurons or afferent neurons transmit information from sensory receptors in the skin or internal organs to the central nervous system for processing. Almost all sensory neurons are unipolar.
- Motor or efferent neurons transmit information from the central nervous system to some type of effector. Motor neurons are typically multipolar.
- Interneurons lie between the motor and sensory pathways and are heavily involved in signal integration. The vast majority of interneurons are confined within the central nervous system.
- Pyramidal neurons. Its name comes from the shape of its cell body, which looks like a pyramid. It has an axon and two main branches of dendrites. These cells send signals into the brain and tell the muscles to move.
- Perkinje neurons (named for the man who discovered them) are located in the cerebellum, the part of the brain that controls balance, coordination, and timing of actions. They have an axon and a very complicated dendrite arrangement.
Generally, voltage changes in neurons flow from dendrites, to the soma, and to the axon.
In sensory neurons, however, environmental stimuli (light, chemicals, pain) activate ion channels that produce action potentials that flow from the axon to the soma.
In either case, neurons propagate signals along their axons in the form of action potentials, which is how neurons communicate with other neurons or cells.
The communication that occurs between these cells is called synaptic transmission.
The basic functions of a neuron
The roles of the three classes of neurons are three basic functions:
- Receive signals or receive information.
- Integrate the incoming signals and thus determine if the information should be transmitted.
- Communicate signals to target cells located in other neurons, in muscles, or in glands.
Neurons have dendrites and axons that can extend away from the cell body and transmit signals to and from other cells.
When the dendrite is not transmitting a signal, it is said to be in a resting state.
In this state, the inside of the cell has a net negative charge, and the outside of the cell has a net positive charge.
The membrane is said to be polarized because there are positive and negative charges on opposite sides.
The neuron actively maintains the polarized state of the membrane through the use of sodium and potassium pumps.
These sodium and potassium pumps pump three positively charged sodium ions out of the cell for every two positively charged potassium ions that you pump into the cell.
Each cycle of the pump increases the polarization a little more.
Additionally, potassium ions are filtered through the membrane and out of the cell by diffusion, which, again, creates a more negative charge inside the cell and a more positive charge outside the cell.
When a neuron receives a signal, the sodium channels in the membrane open and allow a localized influx of positive sodium ions into the cell, causing depolarization or a reduction in the charge difference across the membrane.
Localized depolarization also activates nearby sodium channels to open and depolarize the nearby membrane, causing more sodium channels to open and depolarize the membrane there, thus initiating a chain reaction.
Depolarization occurs in a wave through the membrane, starting at the dendrite that received the signal, moving toward the cell body, through the cell body, and then away from the cell down the axon.
The membrane repolarizes by closing the sodium channels and activating the sodium and potassium pumps to restore the charge difference across the membrane, and the neuron is ready to pass another signal.
Structurally, two types of synapses are found in neurons: chemical and electrical.
Chemical synapses occur when neuronal membranes come close together, but remain distinct, leaving a gap.
Electrical synapses occur when membranes join together through specialized proteins that allow the flow of ions from one cell to another.
Electrical synapses develop in the heart muscle.
Chemical synapses use chemicals called neurotransmitters to communicate messages between cells.
The part of the synapse that releases the neurotransmitter at the synapse is called the presynaptic terminal, and the part of the synapse that receives the neurotransmitter is called the postsynaptic terminal.
The narrow space between the two regions is called the synaptic cleft.
The presynaptic terminal contains a large number of vesicles that are packed with neurotransmitters.
When an action potential reaches the presynaptic terminal, voltage-gated Ca ++ channels open, allowing Ca ++ to enter which then activates an array of molecules in the neuronal membrane and the vesicular membrane to become activated.
These newly activated molecules then induce exocytosis of the vesicles, resulting in the release of the neurotransmitter.
The neurotransmitter then binds to receptors located on the postsynaptic membrane and induces a conformational change.
This change in conformation causes the receptor to act like a pore in the membrane for the ions to move.
Depending on the type of ion, the effect on the postsynaptic cell can be depolarizing (excitatory) or hyperpolarizing (inhibitory).
An excitatory response is called “post excitatory synaptic potential,” while an inhibitory response is called “inhibitory postsynaptic potential.”
As its name suggests, a post excitatory synaptic potential elicits an excitatory response, or membrane depolarization, while an inhibitory postsynaptic potential results in an inhibitory response or membrane hyperpolarization.
A cell body will have many synapses on it and on its surrounding dendrites.
Some of the synapses will cause the cell body’s membrane potential to approach the threshold.
Other synapses result in the cell body’s membrane potential moving away from the threshold (hyperpolarization).
Any synapse that moves the potential closer to the threshold is called an excitatory postsynaptic potential, and any synapse that moves the potential away from the threshold is called an inhibitory postsynaptic potential.
The net effect of the excitatory postsynaptic potential and the inhibitory postsynaptic potential is experienced in the axon mound.
If the threshold is reached, then an action potential will continue down the axon.
The ultimate goal of an excitatory postsynaptic potential is to cause a sufficient change in the membrane to initiate an action potential.
The goal of the inhibitory postsynaptic potential is to cause a change in the membrane to avoid an action potential.
Each excitatory postsynaptic potential or inhibitory postsynaptic potential lasts for a few milliseconds and then the membrane returns to the original resting membrane potentials.
In many cases, a single excitatory postsynaptic potential is not sufficient to cause an action potential.
Therefore, many excitatory postsynaptic potentials from multiple synapses can combine in the soma, resulting in a much larger voltage shift that can exceed the threshold and the cause and action potential. This phenomenon is called spatial summation.
Excitatory postsynaptic potentials from the same synapse can also combine if they arrive in rapid succession; this phenomenon is called time summation.
Requiring multiple excitatory postsynaptic potentials to fire an action potential are ways that neurons increase sensitivity and precision.
A response such as an excitatory postsynaptic potential or an inhibitory postsynaptic potential will depend on the type of neurotransmitter or receptor combination present at the synapse.
There are more than 100 known neurotransmitters, and many of them have unique receptors.
Receptors can be divided into two large groups: ion channels with chemical gates and second messenger systems.
When chemically closed ion channels are activated, certain ions are allowed to flow through the membrane.
The ion type will determine whether the result is an excitatory postsynaptic potential or an inhibitory postsynaptic potential.
When a second messenger system is activated, a cascade of molecular interactions occurs within the target or postsynaptic cell.
The type of cascade that is obtained will cause the response to be either excitatory or inhibitory.
Most excitatory synapses in the brain use glutamate or aspartate as the neurotransmitter.
These neurotransmitters bind to non-selective cation channels that allow the passage of Na + and K +.
As mentioned above, it takes many EPSPs from this type of synapse to depolarize a postsynaptic neuron enough to reach the threshold and trigger an action potential.
A very important subset of synapses in the brain includes a group capable of forming memories by increasing the activity and strength of the synapse.
This process is called long-term empowerment.
Long-term potentiation operates at the synapse, using the neurotransmitter glutamate and the receptor known as the N-methyl D-Aspartate receptor.
The N-methyl D-Aspartate receptor is unique in that it is regulated by both ligands and voltage.
When activated by ligands, it becomes permeable to Na +, but if the charge difference is sufficient, the channel also becomes permeable to Ca ++.
Ca ++ can initiate a second messenger cascade that results in an increase in the number of glutamate receptors, increasing the strength of the synapse.
The change in strength can last for weeks, months, or even years, depending on whether the synapse is used continuously or not.
It may seem like a paradox to have inhibitory synapses, but the excitability of neurons is essentially governed by a balance between excitation and inhibition.
The main inhibitory neurotransmitters are gamma-aminobutyric acid and glycine.
Both neurotransmitters bind to receptors that result in increased Cl- conductance.
Due to the negative charge of Cl- and the fact that it generally moves into the cell, the effect is to oppose depolarization and cause the membrane to move away from the threshold.
Modulatory synapses are those that can be “primed” by neuromodulators so that they can respond more powerfully to other inputs.
An example of a primer neuromodulator is norepinephrine.
By itself, norepinephrine has little effect on synaptic transmission, but when a cell is first exposed to norepinephrine, it will react more powerfully to glutamate.