Neuron

Neurons, which are also called nerve cells, send and receive signals from your brain. They look and work differently from other types of cells, but neurons have a lot in common.

Axons are special extensions of neurons that allow them to send electrical and chemical signals to other cells. Neurons can also get these signals through root-like extensions called dendride, which allow them to get them.

The human brain contains an estimated 100 billion neurons at birth. Neurons do not replicate or renew like other cells. Even if they are dead, they can’t be replaced.

Structure

The size, shape, and structure of neurons vary based on their function and location. Almost all neurons, however, have three basic components: a cell body, an axon, and dendrites.

Cell Body

The cell body is in charge of carrying genetic information, keeping the neuron's shape, and giving it the energy it needs to move.

A neuron's soma, like other cell bodies, has a nucleus and specialised organelles. A membrane surrounds it, protecting it while also allowing it to interact with its immediate environment.

Axon

An axon is a long, tail-like structure that connects the cell body to the axon hillock, a specialised junction. Myelin is a fatty material that protects many axons. Myelin aids axons in transmitting electrical signals. There is only one main axon in most neurons.

Dendrites

Fibrous roots that branch out from the cell body are known as dendrites. Dendrites, like antennas, receive and analyse messages from other neurons' axons. Multiple sets of dendrites are seen in dendritic trees, which are a form of the neuron.

Purkinje cells, for example, are a type of neuron found in the cerebellum. Dendritic trees in these cells have evolved to the point that they can receive thousands of signals.

Neuron – Function and Classification

Function: Action potentials are used by neurons. An action potential occurs when ions flow into and out of the neural membrane.

They can activate chemical or electrical synapses.

Chemical Synapses: The action potential influences other neurons via a space between two neurons known as a synapse. The action potential travels up the axon to a postsynaptic end, where it triggers the release of neurotransmitters. These neurotransmitters stimulate postsynaptic neurons, causing them to activate.

Electrical Synapses: A gap junction connects two neurons, forming an electrical synapse. The gaps include ion channels, which facilitate the direct passage of a positive electrical signal. Chemical synapses move far more slowly than this.

Classification

Neurons have been classified into 3 types:

Sensory Neurons: Sensory neurons are in charge of translating external stimuli into internal stimuli. Sensory inputs stimulate sensory neurons, which then send sensory information to the brain and spinal cord. Their structure is pseudounipolar.

Motor Neurons: These are multipolar neurons that have axons that extend outside of the central nervous system. This is the most common type of neuron, and it is in charge of sending information from the brain to the muscles of the body.

Interneurons: They have a multipolar structure. Their axons connect only to sensory and motor neurons in the nearby area. They aid in the transmission of messages between two neurons.

More About Neurons

Neurons, which are also called the nerve cell, are the structural and the functional units of the nerve cell in vertebrates and most invertebrates. They are independent, morphological, functional and trophic entities and develop from the neural plate of the ectoderm. They are different from glial cells in their ability to generate action potentials in the release of neurotransmitters which are neuroactive substances. Neurons are also polarised cells which receive information at certain locations on their plasma membrane and release neurotransmitters to other cells. 

Neuron Definition

A neuron could be defined as a greyish granular cell that is the fundamental unit of the nervous system and functions to transmit information to different parts of the body.

Neuron Structure and Function

The neuron cell has all components of normal eukaryotic cells. However, in addition to these components, there are five parts of a neuron which give it its form. A detailed discussion of  the entire neuron structure is given below.

Neuron Parts and their Functions:

1. Dendrites: These are tree-like extensions at the beginning of the neuron and help in increasing the area of the cell body. Most neurons have dendrites going outwards away from the cell body. They receive information from other neurons and transmit electrical stimulation to the soma or the cell body of the neuron. If the electrical signals transmitted inward towards the soma are large enough, they will generate an action potential which results in the signal being transmitted down the axon.​

2. Soma: The soma is where a signal from dendrites are joined and passed on. The soma and the nucleus of the neuron do not play an active role in the transmission of neural signals. The characteristics of the soma are:

It contains numerous cellular components which are involved in various functions of the cell.

It contains a cell nucleus that produces RNA and directs the synthesis of proteins.

It supports and maintains the functions of a neuron.

3. Axon Hillock: It is located at the end of the soma and controls the firing of the neuron. If the total strength of a neural signal exceeds the threshold limit of the axon hillock, the structure will fire a signal called an action potential down the axon.

The axon hillock regulates and keeps an account of the total excitatory and inhibitory signals. If the total number of these signals exceeds a certain threshold, an action potential is triggered and an electrical signal will be transmitted down the axon away from the soma.

In a normal resting state, a neuron possesses an internal polarisation of approximately -70mV. When a signal is received by a cell, it causes sodium ions to enter the cell and reduce the polarisation.

When the axon hillock is depolarised to a certain threshold, an action potential will transmit the electrical signal down the axon to the synapses. It should be noted that the action potential is an all-or-nothing process i.e. the signals are not partially transmitted. 

4. Axon: It is the elongate fibre of the neuron that extends from the soma to the terminal endings and transmits the neural signal. The larger the diameter of the axon, the faster it can transmit the information. Some axons are covered with myelin which is a fatty substance and acts as an insulator. These can be as short as 0.1 mm to as large as 3ft long. The myelin covering is known as the myelin sheath and is broken up at points called the nodes of Ranvier. Neural electrical impulses can jump from one node to the next, which plays a role in increasing the speed of the transmission of the signal.

5. Terminal Buttons and Synapses: Terminal buttons are observed at the end of each neuron and their primary function is to send a neural signal on to other neurons. They are also responsible for reuptake of excess neurotransmitters.

The gap at the end of the terminal button is called a synapse. Neurotransmitters carry the neural or electrical signal across the synapse to other neurons. When an electrical signal reaches the terminal buttons, neurotransmitters are released into the synaptic gap. The terminal buttons convert those electrical impulses into chemical signals. The neurotransmitters then cross the synapse where they are then received by other neurons.

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Neuron Function

Neurons send electrical signals through action potentials which is a shift in neurons electrical potential caused by a flow of ions in and out of the neural membrane.

Action potentials have the ability to  trigger both chemical and electrical synapses:

Chemical Synapses: In this type of synapses, the action potential affects other neurons through a gap present neurons known as the synapse. After the action potential is generated it is carried along the axon to a presynaptic ending which triggers neurotransmitters. These chemical messengers then cross the synaptic cleft and bind to receptors in the post-synaptic ending of a dendrite.

Electrical Synapses: Electrical synapses are observed when two neurons are connected via a gap junction which is much smaller than a synapse and includes ion channels which facilitate the direct transmission of a positive electrical signal. Due to this, electrical synapses are much faster than chemical synapses.

Classification of Neurons

There are three different types of neurons and they are discussed below:

3 Types of Neurons and Functions

1. Sensory Neurons: These neurons convert signals from the external environment into corresponding internal stimuli which activates these neurons. In turn, they carry sensory information to the spinal cord and the brain. 

2. Motor Neurons: Motor neurons are multipolar and they are located in the CNS or the Central Nervous System. They are the most common type of neurons and they transmit information from the brain to the muscles of the human body.

3. Interneurons: These neurons are also multipolar neurons and their axon connects to the nearby motor and sensory neurons. They prevent injury by sending messages to the spinal cord rather than the brain.