What is Neuron?

• A neuron, also known as a nerve cell, is the fundamental unit of the nervous system and plays a crucial role in transmitting information throughout the body. Neurons are responsible for both receiving and transmitting signals, utilizing both physical and electrical processes. They form the building blocks of the nervous system and enable the coordination of various bodily functions.
• There are different types of neurons that specialize in transmitting specific types of information. Sensory neurons are responsible for carrying information from sensory receptor cells located throughout the body to the brain. These neurons enable us to perceive and respond to external stimuli such as touch, sound, or light. On the other hand, motor neurons transmit signals from the brain to the muscles, allowing us to move and perform physical actions. Finally, interneurons facilitate communication between different neurons within the body, connecting various parts of the nervous system.
• Neurons communicate with other cells through specialized connections called synapses. These synapses use small amounts of chemical neurotransmitters to pass electrical signals from the presynaptic neuron to the target cell across the synaptic gap. This communication is vital for transmitting information and coordinating responses within the nervous system. Neurons are the main components of nervous tissue in animals, excluding sponges and placozoa. Non-animal organisms such as plants and fungi do not possess nerve cells.
• Neurons can be classified into three main types based on their functions. Sensory neurons respond to stimuli detected by sensory organs and send signals to the spinal cord or brain. Motor neurons receive signals from the brain and spinal cord and control muscle contractions and glandular output. Interneurons establish connections between neurons within the same region of the brain or spinal cord, facilitating complex neural circuits.
• A typical neuron consists of three main parts: the cell body or soma, dendrites, and an axon. The soma is a compact structure that contains the nucleus and other organelles necessary for the neuron’s functioning. Dendrites branch extensively from the soma and extend a few hundred micrometers, receiving signals from other neurons. The axon, which leaves the soma at a swelling called the axon hillock, can extend for long distances, reaching up to 1 meter in humans. Axons branch but generally maintain a constant diameter. At the ends of the axon branches, there are axon terminals, responsible for transmitting signals across synapses to other cells. It is important to note that some neurons may lack dendrites or have no axon. The term “neurite” is used to describe either a dendrite or an axon, particularly when the cell is undifferentiated.
• Neurons primarily receive signals through their dendrites and soma and transmit signals along the axon. The majority of synapses connect the axon of one neuron to the dendrite of another, but connections between axons or dendrites are also possible. The signaling process in neurons involves both electrical and chemical components. Neurons maintain voltage gradients across their membranes, making them electrically excitable. When a voltage change occurs rapidly and exceeds a certain threshold, the neuron generates an all-or-nothing electrochemical pulse known as an action potential. This action potential travels rapidly along the axon and activates synaptic connections as it reaches them. Synaptic signals can be either excitatory or inhibitory, either increasing or reducing the net voltage that reaches the soma.
• During brain development and childhood, most neurons are generated by neural stem cells. However, neurogenesis, the process of generating new neurons, largely ceases during adulthood in most areas of the brain.
• In summary, neurons are the fundamental units of the nervous system, responsible for transmitting information through electrical and chemical signals. They play a crucial role in sensory perception, motor control, and overall coordination of bodily functions. Through their intricate connections and communication at synapses, neurons form complex neural circuits, enabling the proper functioning of the nervous system.

Definition of Neuron

A neuron is a specialized cell in the nervous system that receives, processes, and transmits information through electrical and chemical signals.

How Do Neurons Work?

Neurons are the basic functional units of the nervous system, responsible for transmitting nerve impulses and facilitating communication between different parts of the body. The process of how neurons work involves the transmission of electrical signals and the release of chemical messengers called neurotransmitters.

Neurons are not physically connected to each other but are separated by tiny gaps called synapses. When an electrical signal, known as an action potential, travels along the length of a neuron, it reaches the synapse and needs to cross over to the next neuron to continue its journey.

To bridge this gap, neurotransmitters come into play. These chemicals are released from the synaptic vesicles of the pre-synaptic neuron when triggered by the action potential. The neurotransmitters then diffuse across the synaptic gap and bind to specialized receptor sites on the post-synaptic neuron.

Once the neurotransmitters bind to the receptors, they initiate an electrical impulse in the post-synaptic neuron, allowing the signal to be transmitted further. This process is known as synaptic transmission.

Neurons are found throughout the central nervous system, which includes the brain and spinal cord, as well as the peripheral nervous system, which consists of sensory and motor nerve cells. These neurons enable the processing and transmission of information within the nervous system, allowing for the coordination of various bodily functions and responses.

Neuron Structure – Parts of Neuron

The neuron is composed of three main components: the soma or cell body, dendrites, and axon. The soma contains the nucleus and serves as the control center of the neuron. Dendrites are branch-like structures that receive signals from other neurons, while the axon is a long fiber that carries electrical impulses away from the soma.

To enhance the speed of signal transmission, the axon is covered by a myelin sheath, which acts as an insulating layer. This myelin sheath allows nerve impulses to travel more rapidly along the axon.

It’s important to note that neurons do not physically touch each other. Instead, there is a small gap called the synapse between the axon of one neuron and the dendrite of the next. This synapse is where the transmission of signals occurs, as neurotransmitters are released from one neuron and received by the dendrites of the neighboring neuron.

Dendrites

• Dendrites are branch-like structures that play a crucial role in receiving and transmitting signals within a neuron. They extend away from the cell body and serve as the initial point of contact for receiving messages from other neurons. Dendrites form a complex network of branches, often referred to as a dendritic tree.
• The main function of dendrites is to receive information from neighboring neurons and transmit electrical signals to the cell body. They are covered in synapses, which allow them to receive signals in the form of neurotransmitters from other neurons. Some neurons have shorter dendrites, while others have longer and more elaborate ones.
• In the central nervous system, neurons possess long and intricate dendritic branches that enable them to receive signals from numerous other neurons. For example, specialized cells called Purkinje cells in the cerebellum have highly developed dendrites that receive signals from thousands of other cells.
• Dendrites are equipped with various cellular components, including ribosomes, smooth endoplasmic reticulum, and the Golgi apparatus, which support the synthesis of proteins required for signal transmission. The surface of dendrites is adorned with receptors that become activated upon exposure to neurotransmitters. These dendrites receive synaptic inputs from axons of other neurons. Additionally, dendrites bear specialized structures called dendritic spines, which are small protrusions that further increase the surface area for potential connections with other neurons.
• Overall, dendrites serve as the primary receivers of signals in a neuron, playing a crucial role in the communication and integration of information within the nervous system.

Soma (Cell Body)

• The soma, also known as the cell body, is the central component of a neuron. It encompasses various essential cellular structures and is responsible for maintaining the overall function of the neuron. Enclosed by a protective membrane, the soma interacts with its immediate environment.
• At the core of the soma lies the cell nucleus, which plays a critical role in producing genetic information and overseeing the synthesis of proteins. These proteins are vital for the proper functioning of other parts of the neuron. The size of the nucleus can vary, ranging from 3 to 18 micrometers in diameter.
• Within the soma, several crucial cellular components can be found. These include the Golgi apparatus, mitochondria, pigment granules, neurofibrils, and neurotubules in the cytoplasm. These structures provide support and contribute to the overall function of the neuron. The rough endoplasmic reticulum, often observed in Nissl granules, extends into the dendrites but is absent in the axon hillock.
• Unlike many other cells, neurons lack centrioles, which means they are unable to divide. The plasma membrane of the cell body continues as the axolemma, maintaining the integrity of the neuron. The plasma membrane of the cell body is also adorned with numerous receptor sites.
• The cell body, or soma, is essential for the neuron’s functioning as it houses the nucleus, facilitates protein synthesis, and supports various cellular activities. It serves as the core of the neuron, providing energy and maintaining the genetic structure necessary for neuronal function.

Axon

• The axon, also known as a nerve fiber, is a tube-like structure that serves as the transmitting part of a neuron. It carries electrical impulses away from the cell body and transmits them to the axon terminals, which then pass the signals to other neurons.
• Typically, neurons possess a single axon, although there can be variations. The size of an axon can range from 0.1 millimeters to over 3 feet in length. Some axons are coated with a fatty substance called myelin, which acts as an insulator and enhances the speed of signal transmission.
• Axons are long projections that may branch off to transmit signals to multiple regions before terminating at junctions known as synapses. They play a crucial role in conveying information from the neuron’s soma to other cells or target areas.
• The axon emerges from the cell body at a region called the axon hillock, which not only serves as a junction but also contains a high concentration of voltage-dependent sodium channels. This makes the axon hillock particularly excitable and serves as the spike initiation zone for the axon.
• While the primary function of the axon is to carry signals away from the cell body, it can also receive input from other neurons, contrary to its usual role in information outflow.
• In terms of structure, the axon is a cable-like projection that can extend significantly longer than the diameter of the cell body, sometimes reaching tens of thousands of times its length. It lacks Nissl granules and frequently undergoes extensive branching, enabling communication with numerous target cells.
• To facilitate efficient signal propagation, axons are ensheathed by Schwann cells in the peripheral nervous system, which produce the myelin sheath. This lipid-rich insulation increases the speed of signal conduction. The myelin sheath is periodically interrupted by gaps known as nodes of Ranvier, where the signal is recharged as it travels along the axon.
• In the central nervous system, axons are myelinated by oligodendrocytes, which provide a similar function to Schwann cells.
• In summary, the axon is a tubular structure responsible for transmitting integrated signals to specialized endings called axon terminals. It carries action potentials from the cell body to neighboring neurons, and its length, myelination, and branching patterns contribute to efficient signal conduction throughout the nervous system.

Myelin Sheath

• The myelin sheath is a fatty layer that surrounds the axons of neurons, providing insulation and protection. Its main functions are to insulate one nerve cell from another and to enhance the speed of nerve impulse conduction along the axon.
• The myelin sheath is formed by glial cells, including oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. These cells wrap around the axons, creating the myelin sheath.
• By insulating the axons, the myelin sheath prevents interference between nerve impulses from different neurons, ensuring efficient transmission of signals. It acts as a protective covering, shielding the axons from damage and maintaining their integrity.
• One notable feature of the myelin sheath is the presence of nodes of Ranvier. These are gaps in the myelin sheath along the length of the axon. The electrical signals, known as action potentials, can “jump” or propagate between these nodes, a process called saltatory conduction. This jumping mechanism significantly speeds up the transmission of signals along the axon.
• Overall, the myelin sheath plays a crucial role in optimizing the efficiency and speed of nerve impulse conduction. It facilitates rapid communication between neurons and contributes to the proper functioning of the nervous system.

Axon hillock

• The axon hillock is a specialized structure located at the junction between the cell body and the axon of a neuron. It serves as a critical site for signal integration, where signals from multiple synapses are summed up.
• After receiving signals from dendrites, the information is transmitted to the cell body. At the cell body, the axon hillock acts as a manager, integrating the various excitatory and inhibitory signals received from different synapses. It calculates the net effect of these signals and determines whether an action potential, an electrical impulse, should be generated and propagated along the axon.
• The axon hillock plays a crucial role in regulating the firing of action potentials. It acts as a threshold detector, monitoring the total input received by the neuron. If the sum of the signals surpasses a certain threshold value, usually around -70mV in a resting state, it triggers the initiation of an action potential. In contrast, if the total input does not reach the threshold, no action potential is generated.
• This region of the neuron is highly sensitive to changes in ion channels. When the threshold is reached, ion channels in the axon hillock undergo specific changes, allowing the electrical signal to propagate down the axon away from the cell body. This electrical signal, known as an action potential, carries information to other neurons or target cells.
• In summary, the axon hillock acts as a crucial control center within the neuron, summing and evaluating incoming signals to determine whether an action potential should be generated. It serves as a critical component in the initiation and regulation of nerve impulses throughout the nervous system.

Axon Terminals

• Axon terminals are specialized structures located at the end of the axon, farthest from the soma (cell body) of a neuron. These terminals play a crucial role in transmitting signals from one neuron to another.
• At the axon terminals, there are synaptic boutons, which are small bulges or swellings. These boutons contain vesicles filled with neurotransmitter chemicals. Neurotransmitters are released from these boutons into the synapse, which is the small gap between the axon terminal and the dendrites or cell body of the target neuron.
• The release of neurotransmitters from the axon terminals allows for communication between neurons. When an electrical signal, known as an action potential, reaches the axon terminal, it triggers the release of neurotransmitters into the synapse. These neurotransmitters then bind to receptors on the postsynaptic membrane of the target neuron, initiating a response in the receiving neuron.
• In addition to synaptic boutons located at the axon terminal, some neurons may also have en passant boutons. These boutons are found along the length of the axon and serve as additional sites for neurotransmitter release and communication with target neurons.
• The axon terminals and their synaptic boutons are essential for the transmission of signals between neurons. Through the release of neurotransmitters, these structures enable the relay of information and facilitate the integration and processing of signals within the nervous system.

Synapse

• A synapse is a specialized junction that facilitates communication between neurons or between a neuron and a target cell, such as a muscle or gland cell. It serves as the site where neurotransmitters are released from the presynaptic neuron into a small gap called the synaptic cleft, which exists between the presynaptic and postsynaptic membranes.
• The synapse is formed at the end of the axon terminal, which is located at the farthest end of the axon away from the cell body. Within the terminal buttons of the axon terminal, there are vesicles that contain neurotransmitters.
• When an electrical signal, known as an action potential, reaches the axon terminal, it triggers the release of neurotransmitters into the synapse. These neurotransmitters are chemical messengers that carry the signal across the synapse to the postsynaptic neuron or target cell. This conversion from electrical signals to chemical signals is a crucial step in neuronal communication.
• Once the neurotransmitters are released into the synapse, they bind to specific receptors on the postsynaptic membrane. This binding can initiate various effects, such as the generation of an action potential in the postsynaptic neuron or the activation of cellular processes in the target cell.
• After neurotransmitter release, it is the responsibility of the terminal buttons to reuptake any excess neurotransmitters that were not passed on to the next neuron. This process helps regulate the concentration of neurotransmitters in the synapse and ensures precise signaling between neurons.
• In addition to the synapses formed at the axon terminal, there may also be en passant boutons along the length of the axon. These specialized structures serve as additional sites for neurotransmitter release and communication with target neurons.
• In summary, a synapse is a junction where neurotransmitters are released from the presynaptic neuron into the synaptic cleft, allowing them to bind to receptors on the postsynaptic membrane and transmit signals to the next neuron or target cell. It is a vital component of neuronal communication and plays a crucial role in information processing in the nervous system.

Part of Neuron	Description
Cell Body (Soma)	The core of the neuron containing the nucleus, Golgi body, endoplasmic reticulum, mitochondria, and other cellular components. It maintains the cell’s functions, synthesizes proteins, and provides energy.
Dendrites	Branch-like structures that receive messages from other neurons and transmit them to the cell body. They are covered in synapses and increase the surface area for connections with other neurons.
Axon	A tube-like structure responsible for carrying electrical impulses away from the cell body to axon terminals. It can be covered by a myelin sheath, which insulates and speeds up the transmission of nerve impulses.
Axon Terminals	Found at the end of the axon, these specialized structures contain synapses and release neurotransmitter chemicals to communicate with other neurons.
Synapse	The junction between the axon terminal of one neuron and the dendrites of another neuron, where neurotransmitters are released and signals are transmitted from one neuron to another.
Myelin Sheath	A layer of fatty material that covers the axons of neurons, providing insulation between nerve cells and speeding up the conduction of nerve impulses.
Axon Hillock	A specialized structure located at the junction between the cell body and the axon. It integrates signals from multiple synapses and acts as a threshold detector, determining whether an action potential should be generated.

Neuron Membrane

• The plasma membrane surrounds the cell body of a neuron, just like it does in all animal cells. It consists of a bilayer of lipid molecules with various types of embedded protein structures. While a lipid bilayer acts as a strong electrical insulator, neurons possess protein structures within the membrane that are electrically active.
• Among these protein structures are ion channels, which allow the passage of electrically charged ions across the membrane, and ion pumps, which transport ions chemically from one side of the membrane to the other. Different ion channels exhibit selectivity for specific types of ions. Some ion channels are voltage-gated, meaning their open and closed states can be controlled by changing the voltage difference across the membrane. Others are chemically gated, meaning they can be activated or inhibited by chemicals that diffuse through the extracellular fluid.
• The ions involved in these processes include sodium, potassium, chloride, and calcium. The interactions between ion channels and ion pumps generate a voltage difference across the membrane, typically measuring slightly less than 1/10 of a volt under normal conditions. This voltage serves two primary functions. Firstly, it provides a power source for various voltage-dependent protein machinery embedded in the membrane. Secondly, it forms the basis for the transmission of electrical signals between different regions of the membrane.
• The membrane’s electrical properties are essential for the functioning of neurons. The voltage difference across the membrane allows for the generation and propagation of electrical signals, enabling communication within the nervous system. By opening and closing ion channels in response to different stimuli, neurons can regulate the flow of ions and create changes in the voltage, leading to the transmission of signals and the coordination of neuronal activity.
• In summary, the membrane of a neuron consists of a lipid bilayer embedded with electrically active protein structures. Ion channels and pumps within the membrane permit the movement of ions, and the resulting interactions generate a voltage difference across the membrane. This voltage serves as both a power source and a means for electrical signal transmission within the neuron, contributing to its functionality within the nervous system.

Types of Neuron

Neurons can be classified based on their morphology, function, and neurotransmitters they release. Here are different types of neurons:

1. Structural Classification:
   • Unipolar Neuron: Neurons with a single process extending from the cell body. The process acts as both an axon and a dendrite.
   • Bipolar Neuron: Neurons with two processes, one axon, and one dendrite. They are commonly found in sensory organs like the retina and olfactory epithelium.
   • Multipolar Neuron: Neurons with multiple dendrites and a single axon. Most neurons in the central nervous system belong to this type.
   • Pseudounipolar Neuron: Neurons with a single process that splits into two branches, resembling an axon and a dendrite. Commonly found in sensory neurons of the peripheral nervous system.
2. Golgi Classification:
   • Golgi I Neurons: Neurons with long-projecting axons. Examples include pyramidal cells, Purkinje cells, and anterior horn cells.
   • Golgi II Neurons: Neurons with locally projecting axonal processes. The granule cell is an example of this type.
3. Other Unique Neuronal Types:
   • Basket Cells: Interneurons that form a dense plexus of terminals around the soma of target cells. Found in the cortex and cerebellum.
   • Betz Cells: Large motor neurons.
   • Lugaro Cells: Interneurons found in the cerebellum.
   • Medium Spiny Neurons: Most neurons in the corpus striatum.
   • Purkinje Cells: Huge neurons in the cerebellum, belonging to the Golgi I type.
   • Pyramidal Cells: Neurons with a triangular soma, belonging to the Golgi I type.
   • Renshaw Cells: Neurons with both ends linked to alpha motor neurons.
   • Unipolar Brush Cells: Interneurons with a unique dendrite ending in a brush-like tuft.
   • Granule Cells: Neurons belonging to the Golgi II type.
   • Anterior Horn Cells: Motoneurons located in the spinal cord.
   • Spindle Cells: Interneurons that connect widely separated areas of the brain.
4. Functional Classification:
   • Afferent Neurons (Sensory Neurons): Transmit signals from tissues and organs into the central nervous system, enabling sensory perception.
   • Efferent Neurons (Motor Neurons): Transmit signals from the central nervous system to effector cells, such as muscles or glands.
   • Interneurons: Connect neurons within specific regions of the central nervous system, facilitating communication between sensory and motor neurons.

These classifications provide a framework for understanding the diversity and complexity of neurons in the nervous system.

Types of Neuron based on their activities

Neurons can also be classified based on their activities and the roles they play in transmitting information within the nervous system. The three main types of neurons based on their activities are sensory neurons, motor neurons, and interneurons.

1. Sensory Neurons

Sensory neurons are responsible for transmitting sensory information from various parts of the body to the brain. These neurons are pseudo-unipolar, meaning they have a single long process that splits into two branches. One branch receives sensory input from sensory receptor cells located in the skin, organs, and other tissues, while the other branch extends to the central nervous system (CNS).

For instance, when you step on a thorn while walking, the sensory neurons in your foot immediately detect this stimulus. They send signals to the brain or spinal cord, providing information about the pain and allowing you to react accordingly. Sensory neurons play a crucial role in our ability to sense and respond to our environment.

2. Motor Neurons

Motor neurons are responsible for transmitting signals from the brain and spinal cord to muscles and glands throughout the body. These neurons control muscle contractions and glandular secretions, allowing us to perform voluntary movements and regulate bodily functions. Motor neurons are multipolar, meaning they have multiple processes extending from the cell body.

There are two types of motor neurons:

• Lower Motor Neurons: These neurons travel from the spinal cord to the muscles they innervate. They directly control the contraction of skeletal muscles, enabling voluntary movements such as walking, grasping, and talking.
• Upper Motor Neurons: These neurons travel between the brain and the spinal cord. They play a crucial role in coordinating and regulating the activities of lower motor neurons. Upper motor neurons initiate voluntary movements and transmit signals from the brain to the lower motor neurons, which then transmit the signals to the muscles.

3. Interneurons

Interneurons, also known as association neurons, are multipolar neurons that facilitate communication between sensory and motor neurons. They form complex circuits within the nervous system and are responsible for processing and integrating information.

Interneurons transmit signals between various neurons within the CNS. They receive input from sensory neurons and relay it to motor neurons, enabling appropriate responses. Additionally, interneurons communicate with each other, forming intricate networks that contribute to higher-level processing and coordination of neural activity.

In summary, sensory neurons transmit sensory information from the body to the brain, motor neurons transmit signals from the brain to muscles and glands, and interneurons facilitate communication between different neurons, playing a vital role in information processing and coordination within the nervous system.

What is Neurotransmitter?

Neurotransmitters are chemical messengers that facilitate communication between neurons or between neurons and target cells such as muscle cells or gland cells.

Here are some examples of neurotransmitters and their functions:

• Acetylcholine (ACh): Released by cholinergic neurons, acetylcholine acts as a ligand for both ligand-gated ion channels and metabotropic muscarinic receptors. It is involved in various processes such as muscle contraction, learning, memory, and attention. Acetylcholine is synthesized from choline and acetyl coenzyme A.
• Noradrenaline (norepinephrine): Released by adrenergic neurons, noradrenaline binds to alpha adrenoceptors and beta adrenoceptors. It plays a role in the sympathetic nervous system, regulating functions such as the stress response, heart rate, and blood pressure. Noradrenaline is synthesized from the amino acid tyrosine.
• Gamma-aminobutyric acid (GABA): GABA is an inhibitory neurotransmitter in the central nervous system (CNS). It opens anion channels that allow chloride ions to enter the post-synaptic neuron, leading to hyperpolarization and decreased probability of an action potential firing. GABA helps regulate neuronal excitability and is synthesized from glutamate neurotransmitters.
• Glutamate: Glutamate is the primary excitatory neurotransmitter in the central nervous system. It binds to various receptors, including AMPA, kainate, NMDA, and metabotropic receptors. Glutamate is involved in synaptic transmission, learning, and memory. Excessive release of glutamate can lead to excitotoxicity and neuronal damage.
• Dopamine: Dopamine acts on D1 and D2 type receptors, modulating both pre- and post-synaptic neurotransmission. It plays a crucial role in reward-motivated behavior, mood regulation, and movement coordination. Dysfunction of dopamine neurons is associated with disorders like Parkinson’s disease and schizophrenia.
• Serotonin (5-HT): Serotonin can act as both an excitatory and inhibitory neurotransmitter. It binds to various receptor classes, including GPCRs and ligand-gated ion channels. Serotonin is involved in mood regulation, sleep, appetite, and pain modulation. Altered serotonin levels have been linked to depression and anxiety disorders.
• Adenosine triphosphate (ATP): ATP acts as a neurotransmitter at both ligand-gated ion channels (P2X receptors) and GPCRs (P2Y receptors). It is known as a cotransmitter and plays a role in purinergic signaling, which regulates processes such as synaptic transmission, inflammation, and pain perception.
• Histamine: Histamine functions as a monoamine neurotransmitter and neuromodulator. It is involved in regulating arousal, sleep/wake behaviors, and allergic responses. Histamine-producing neurons are located in the tuberomammillary nucleus of the hypothalamus.

These neurotransmitters play crucial roles in transmitting signals and modulating various physiological and cognitive functions in the nervous system. Imbalances or dysfunctions in neurotransmitter systems can contribute to neurological and psychiatric disorders.

Communication Between Neurons

Communication between neurons is essential for the functioning of the nervous system. Neurons utilize electrical and chemical signals to transmit information to one another. Similar to how humans communicate using language and gestures, neurons rely on these specialized signals to exchange information and carry out various functions.

Neurons receive input from multiple other neurons, and this information is integrated and processed before a decision is made to transmit the message further. Communication between neurons occurs through two main mechanisms: electrical signals and chemical signals.

Electrical signals within a neuron are known as action potentials. An action potential is a brief depolarization, or reduction in the charge across the neuron’s membrane, that travels along the neuron’s axon. This electrical impulse allows for the efficient transmission of signals within a single neuron, from the dendrites (receiving end) to the axon terminal (transmitting end). Action potentials are all-or-nothing events, meaning they either occur fully or not at all, without degrees of magnitude.

Chemical signals, on the other hand, are involved in the communication between adjacent neurons. When an action potential reaches the axon terminal of a neuron, it triggers the release of chemical messengers called neurotransmitters. Neurotransmitters are specialized molecules that can depolarize or hyperpolarize the adjacent neuron, influencing the likelihood of an action potential occurring in the receiving neuron.

The communication between neurons relies on three fundamental phenomena:

1. Resting Potential: The resting potential refers to the electrical charge of a neuron when it is not actively transmitting a signal. It is characterized by a voltage difference across the neuron’s membrane, with a negative charge on the inside and a positive charge on the outside. This resting potential provides the baseline state from which changes in electrical charge can occur.
2. Action Potential: An action potential is a rapid and transient depolarization of the neuron’s membrane. It occurs when the membrane potential reaches a threshold level, triggering a cascade of events that result in the propagation of the action potential along the axon. The action potential allows for the efficient transmission of signals over long distances within a neuron.
3. Neurotransmitters: Neurotransmitters are chemical messengers released from one neuron that bind to receptors on the adjacent neuron. They can either depolarize the receiving neuron, making it more likely to generate an action potential (excitatory neurotransmitters), or hyperpolarize the receiving neuron, making it less likely to generate an action potential (inhibitory neurotransmitters). The specific neurotransmitters released and their interactions determine the overall excitatory or inhibitory effect on the receiving neuron.

Through the coordinated interplay of electrical and chemical signaling, neurons establish complex communication networks within the nervous system. This communication enables the transmission and integration of information, supporting various functions such as sensory perception, motor control, memory formation, and decision-making processes.

1. The Resting Potential

• The resting potential of a neuron refers to its membrane potential when it is not actively transmitting a signal. The lipid bilayer membrane surrounding the neuron is impermeable to charged molecules or ions. In order for ions to enter or exit the neuron, they must pass through specialized proteins called ion channels that span the membrane and regulate the ion concentrations inside and outside the cell.
• The resting potential is established by two main processes: the sodium-potassium pump and the potassium leak channels. The sodium-potassium pump, also known as Na+/K+ ATPase, actively transports sodium ions out of the cell and potassium ions into the cell. This pump utilizes ATP energy to maintain an electrical gradient across the cell membrane. It moves three sodium ions out of the cell and two potassium ions into the cell for each ATP molecule consumed. This pump is particularly abundant in nerve cells and plays a crucial role in maintaining the electrical gradient.
• In addition to the sodium-potassium pump, neurons also possess potassium leak channels and sodium leak channels. These channels allow the diffusion of potassium and sodium ions down their concentration gradients. However, neurons have a greater number of potassium leak channels compared to sodium leak channels. As a result, potassium ions diffuse out of the cell at a faster rate than sodium ions leak in. This causes more positive charges to leave the cell than enter, leading to a net negative charge inside the cell relative to the outside.
• The combined effects of the sodium-potassium pump and potassium leak channels contribute to the resting membrane potential. The inside of the cell is negatively charged, with a resting potential of approximately -70 millivolts (-70 mV). This negative charge is due to the differences in ion concentrations inside and outside the cell.
• It is worth noting that chloride ions (Cl-) tend to accumulate outside the cell as they are repelled by negatively charged proteins within the cytoplasm. The concentrations of ions inside and outside neurons can vary, but typical values include a higher concentration of sodium ions outside the cell (around 145 mM) compared to inside (12 mM), a higher concentration of potassium ions inside the cell (155 mM) compared to outside (4 mM), and a higher concentration of chloride ions outside the cell (120 mM) compared to inside (4 mM).
• The resting potential is crucial for the functioning of neurons as it serves as a baseline electrical state from which changes in electrical charge can occur. It provides the necessary conditions for the generation and propagation of action potentials, which are essential for neuronal communication and information processing within the nervous system.

Ion Concentration Inside and Outside Neurons
Ion	Extracellular concentration (mM)	Intracellular concentration (mM)	Ratio outside/inside
Na+	145	12	12
K+	4	155	0.026
Cl–	120	4	30
Organic anions (A-)	—	100

2. The Action Potential

• The resting potential of a neuron refers to its membrane potential when it is not actively transmitting a signal. Neurons communicate through action potentials, which are brief, positive changes in the membrane potential along the neuron’s axon. When a neuron receives a signal from another neuron in the form of neurotransmitters, it causes a change in the membrane potential. This change is mediated by the opening or closing of voltage-gated ion channels, which are channels that respond to changes in membrane voltage.
• The depolarization of the membrane can lead to an action potential if it reaches a certain threshold potential, typically around -55 mV. If the threshold potential is reached, an action potential is initiated at the axon hillock. The stages of an action potential include depolarization, where voltage-gated sodium channels open, allowing sodium ions to enter the axon and making the inside of the cell more positive. This is followed by repolarization, where sodium channels close and voltage-gated potassium channels open, allowing potassium ions to leave the axon and causing the membrane to repolarize. The process continues with hyperpolarization, where the membrane potential dips below the resting potential, and then the resting potential is reset through the activity of the sodium-potassium pump and potassium leak channels.
• Action potentials travel down the axon as a wave of depolarization, and they always proceed in one direction, from the cell body to the synapse. This is due to the refractory period of voltage-gated ion channels, where the channels cannot reopen immediately after closing. Action potentials are all-or-nothing events, meaning they do not vary in magnitude or speed. However, the frequency of action potentials can vary, representing how many action potentials occur in a given time.
• The speed of action potentials can vary between neurons. In invertebrates, the difference is often due to axon diameter, where larger axons have faster conduction. In vertebrates, myelination of the axon plays a significant role. Myelin acts as an insulator and increases the speed of action potential conduction. The nodes of Ranvier, which are gaps in the myelin sheath, allow the action potential to regenerate along the axon. This saltatory conduction, jumping from one node to the next, saves energy for the neuron and speeds up the conduction of the action potential.
• Overall, the resting potential is an essential aspect of neuronal function, providing the baseline electrical state from which action potentials can be generated and propagated, allowing for communication between neurons in the nervous system.

Initiation of Action potential in axon hillock

The initiation steps of an action potential at the axon hillock involve several stages:

1. Depolarization: When the membrane potential of the neuron reaches the threshold potential of around -55 mV, voltage-gated sodium channels open rapidly. This allows an influx of sodium ions into the axon, causing the interior of the cell to become relatively electrically positive compared to the initial resting potential of approximately -70 mV.
2. Repolarization: Shortly after the depolarization phase, the voltage-gated sodium channels close and remain closed for about 1-2 milliseconds. During this time, voltage-gated potassium channels open, allowing potassium ions to rush out of the axon (efflux). This outflow of potassium ions causes the membrane to repolarize, returning it to a more negative state.
3. Hyperpolarization: The efflux of potassium ions during repolarization can cause the membrane potential to dip below the normal resting potential. This hyperpolarization occurs as potassium continues to leave the axon, making the inside of the cell even more negative than the resting potential. Meanwhile, the sodium channels return to their resting state, ready to be opened again if the membrane potential exceeds the threshold in the future.
4. Resetting the resting potential: Following hyperpolarization, the sodium-potassium pump and potassium leak channels work together to reset the locations of sodium and potassium ions. The sodium-potassium pump actively transports sodium ions out of the cell while bringing potassium ions back in. This process reestablishes the resting potential of the membrane, allowing it to be ready for another action potential to occur.

These initiation steps of the action potential at the axon hillock are crucial for the transmission of electrical signals along the neuron’s axon, enabling communication between neurons in the nervous system.

Features of action potentials

Action potentials, or nerve impulses, possess several distinct features:

1. Unidirectional propagation: Action potentials travel along the axon in a one-way direction, from the cell body (soma) to the synapse(s) located at the end of the axon. They do not reverse or travel backward. This unidirectional flow is due to the refractory period of voltage-gated ion channels. After an action potential is generated, these channels enter a state where they cannot be reopened for a brief period of 1-2 milliseconds. This refractory period ensures that the action potential proceeds in a forward direction.
2. All-or-nothing principle: Action potentials follow the all-or-nothing principle, meaning that they either occur fully or do not occur at all. Once the membrane potential of a neuron reaches the threshold level, an action potential is triggered with a consistent magnitude and speed. The magnitude of an action potential remains the same from initiation to termination, and it always depolarizes the membrane to the same extent. Furthermore, the speed at which the action potential travels along the axon is constant and does not vary.
3. Frequency modulation: While the magnitude and speed of an individual action potential remain constant, the frequency of action potentials can vary. Neurons can generate action potentials at different rates, determining the frequency of their firing. The frequency of action potentials encodes information, and the varying frequency can convey different levels or types of signals.

In summary, action potentials are characterized by their unidirectional propagation, all-or-nothing nature, and the ability to modulate their frequency. These features allow for the efficient and reliable transmission of electrical signals along the axon, facilitating communication between neurons in the nervous system.

3. The Chemical Synapse and Neurotransmitters

The chemical synapse is a crucial component in the transmission of signals between neurons. Neurons are not physically connected to each other but instead communicate at specialized structures known as synapses. Within a synapse, there are two key players: the presynaptic neuron, responsible for sending the signal, and the postsynaptic neuron, which receives the signal.

The synaptic cleft, a small gap between the presynaptic and postsynaptic neurons, plays a vital role in the transmission process. It is within this space that neurotransmitters, chemical messengers, are released by the presynaptic neuron to transmit the signal to the postsynaptic neuron.

So, how does synaptic transmission occur? When an action potential, an electrical signal, reaches the end of the presynaptic neuron’s axon, a series of events take place:

1. The action potential depolarizes the presynaptic membrane by opening voltage-gated Na+ channels. Sodium ions (Na+) flow into the cell, further depolarizing the membrane.
2. The depolarization of the presynaptic membrane triggers the opening of voltage-gated Ca2+ (calcium) channels. Calcium ions (Ca2+) enter the presynaptic neuron at the synapse.
3. The entry of calcium ions initiates a signaling cascade that leads to the fusion of synaptic vesicles, small membrane-bound vesicles containing neurotransmitter molecules, with the presynaptic membrane.
4. Fusion of a vesicle with the presynaptic membrane releases neurotransmitters into the synaptic cleft, the extracellular space between the presynaptic and postsynaptic membranes. The neurotransmitters then diffuse across the synaptic cleft and bind to receptor proteins on the postsynaptic membrane.

The binding of neurotransmitters to receptors on the postsynaptic membrane can have different effects on the postsynaptic neuron. Excitatory postsynaptic potentials (EPSPs) occur when the binding of neurotransmitters makes the postsynaptic neuron more likely to generate an action potential. For example, acetylcholine released at the neuromuscular junction between a nerve and muscle causes the opening of Na+ channels, leading to depolarization of the postsynaptic membrane.

In contrast, inhibitory postsynaptic potentials (IPSPs) make the postsynaptic neuron less likely to generate an action potential. When the neurotransmitter GABA is released from a presynaptic neuron, it binds to and opens Cl– channels, allowing chloride ions (Cl–) to enter the cell and hyperpolarize the membrane.

After neurotransmission has occurred, the neurotransmitter must be cleared from the synaptic cleft to allow the postsynaptic membrane to reset and be prepared for another signal. This removal can happen in three ways: diffusion of the neurotransmitter away from the synaptic cleft, degradation of the neurotransmitter by enzymes present in the synaptic cleft, or recycling (reuptake) of the neurotransmitter by the presynaptic neuron.

It is important to note that while action potentials are “all-or-nothing,” EPSPs and IPSPs are graded in nature, meaning they can vary in the magnitude of depolarization or hyperpolarization they induce. Often, a single EPSP may not be sufficient to trigger an action potential in the postsynaptic neuron, and multiple presynaptic inputs must produce EPSPs around the same time to achieve sufficient depolarization for firing an action potential. This phenomenon is known as summation and takes place at the axon hillock, the region where the axon connects to the cell body. Additionally, since a neuron receives inputs from multiple presynaptic neurons, some excitatory and some inhibitory, IPSPs can counteract EPSPs, and vice versa. The net change in postsynaptic membrane voltage determines whether the threshold for excitation is reached, ultimately deciding if the postsynaptic cell will generate an action potential.

Together, synaptic summation and the threshold for excitation act as a filter, ensuring that only relevant and important information is transmitted while preventing random “noise” in the system from affecting the overall output of the neuron.

What is Reflex Arc?

A reflex arc is a rapid and involuntary response to an external stimulus that occurs through the spinal cord rather than the brain. It is a mechanism that allows the body to respond quickly to potentially harmful or dangerous situations without the need for conscious thought. A classic example of a reflex arc is the withdrawal of a hand from a hot object.

The reflex arc involves several key components:

1. Receptor: The receptor is a specialized sensory structure that detects the stimulus. It could be a sensory nerve ending in the skin, for example, that senses heat from the hot object.
2. Afferent pathway (sensory neuron): The sensory neuron carries the nerve impulses from the receptor to the central nervous system (CNS), which consists of the brain and spinal cord. In the case of the reflex arc, the sensory neuron transmits the information about the hot object to the spinal cord.
3. Interneuron: The interneuron, located within the spinal cord, serves as a connector between the sensory neuron and the motor neuron. It receives the sensory input and relays it to the appropriate motor neuron, which then initiates the response.
4. Efferent pathway (motor neuron): The motor neuron carries the nerve impulses from the CNS to the effector, which is usually a muscle or gland. In the case of the reflex arc, the motor neuron transmits the signal from the spinal cord to the muscle fibers responsible for the withdrawal reflex.
5. Effector: The effector is the target organ that carries out the response. In the context of the reflex arc, the effector is typically a muscle fiber. When the motor neuron sends the signal, the muscle fibers contract, causing the hand to be pulled away from the hot object.

It’s important to note that spinal reflexes, like the reflex arc, can occur without the involvement of higher brain centers. The entire reflex response is coordinated within the spinal cord itself, allowing for a rapid and automatic reaction to the stimulus.

Reflex arcs play a vital role in protecting the body from harm. They allow for swift responses to potential dangers without the need for conscious thought or decision-making processes, ensuring the safety and survival of an individual.

Glia

Glia, often referred to as the supporting cells of the nervous system, play a crucial role in its proper functioning. In fact, the number of glial cells in the brain is approximately ten times greater than the number of neurons. These glial cells perform essential functions that enable neurons to function effectively. They contribute to neuronal development, protect neurons from harmful substances, provide insulation for axons, and regulate communication between nerve cells. When glia malfunction, it can have severe consequences, as many brain tumors are caused by glial mutations.

There are several distinct types of glia, each with its own specific functions:

1. Astrocytes: Astrocytes provide essential support to neurons by supplying them with nutrients and other substances. They also help regulate the concentration of ions and chemicals in the extracellular fluid surrounding neurons. Additionally, astrocytes play a crucial role in synapses by providing structural support. They are responsible for forming the blood-brain barrier, a protective mechanism that prevents the entry of toxic substances into the brain.
2. Satellite glia: Satellite glia primarily exist in the peripheral nervous system (PNS). They provide support and nutrients to neurons in the PNS, ensuring their proper functioning.
3. Microglia: Microglia are the immune cells of the central nervous system (CNS). Their primary role is to scavenge and degrade dead cells and protect the brain from invading microorganisms. They act as the first line of defense against pathogens or damage within the CNS.
4. Oligodendrocytes: Oligodendrocytes are responsible for the formation of myelin sheaths around axons in the CNS. A single oligodendrocyte can provide myelin for multiple neurons, and one axon can be myelinated by several oligodendrocytes. The myelin sheath serves as insulation for the axon, allowing for faster and more efficient transmission of electrical signals.
5. Schwann cells: Schwann cells, like oligodendrocytes, play a role in myelination. However, they are found in the peripheral nervous system (PNS). Unlike oligodendrocytes, a single Schwann cell provides myelin for only one axon. The entire Schwann cell wraps around the axon, creating the myelin sheath.
6. Ependymal cells: Ependymal cells line the fluid-filled ventricles of the brain and the central canal of the spinal cord. They play a role in the circulation of cerebrospinal fluid, which acts as a cushion for the brain and spinal cord. Ependymal cells help maintain the appropriate balance and flow of cerebrospinal fluid within the central nervous system.

Overall, glia are essential for the proper functioning and protection of neurons in the nervous system. Their diverse functions contribute to the development, maintenance, and overall health of the nervous system, highlighting their critical role in supporting neural processes.

Functions of Neuron

• Transmission of Signals: Neurons are responsible for transmitting electrical and chemical signals throughout the nervous system. They generate and propagate action potentials, which are electrical impulses that allow for the transmission of information within and between neurons.
• Integration of Information: Neurons receive inputs from other neurons through synapses, where they integrate and process this information. They sum up the excitatory and inhibitory signals they receive and determine whether to generate an action potential based on the net input.
• Chemical Synapses: In chemical synapses, the action potential triggers the release of neurotransmitters from the presynaptic neuron into the synaptic cleft. These neurotransmitters bind to receptors on the postsynaptic neuron, leading to the generation of a new electrical signal in the postsynaptic neuron. This process allows for the transmission of signals between neurons.
• Electrical Synapses: In electrical synapses, neurons are directly connected by gap junctions, which allow for the flow of ions and electrical current between the neurons. This type of synapse enables rapid and synchronized communication between neurons.
• Signal Processing: Neurons process and modify incoming signals through various mechanisms, such as spatial and temporal summation. Spatial summation occurs when multiple inputs from different neurons are integrated at the same time, while temporal summation involves the integration of inputs that occur in close succession. These processes help in determining whether an action potential will be generated.
• Connection and Circuit Formation: Neurons form intricate networks and circuits within the nervous system. They establish connections with specific target cells, such as other neurons, muscle cells, or gland cells, to coordinate and regulate various physiological processes.
• Plasticity and Learning: Neurons possess the ability to modify their connections and adapt to changes in the environment through synaptic plasticity. This plasticity is the basis for learning, memory formation, and the ability of the nervous system to adapt and respond to new experiences.

Overall, neurons play a fundamental role in the communication and functioning of the nervous system, enabling the transmission, integration, and processing of information necessary for the control of bodily functions, behavior, and cognition.

FAQ

References

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