What is Neuron?
Neurons Meaning: 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.
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 reuptaking 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 neurones.
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Neuron Diagram and Functions
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 neurones 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
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.
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.
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.
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