Structure of Neuron

Introduction:

Typically, a neuron consists of a cell body (Soma), dendrites and an axon. The dendrites and axon are generally a protrusion from the soma. The structure of neurons is designed in such a way that dendrites can receive a signal from multiple neurons and the signals are then transmitted forward with the help of an axon. This relay network is absolutely essential for the transmission of electrical and chemical signals from the stimulus to the brain or spinal cord and back again to the motor or effector organs of our body.  This electric potential is developed due to a quick change of voltage gradient, which is maintained across the membrane in a neuron.

 Structure:

The nervous system can be divided into:

•         CNS (Central Nervous System) – It consists of the brain and the spinal cord. It integrates the information that we receive from outside processes and influences the activity of all the parts of the body.
•         PNS (Peripheral Nervous System) – It consists of the nerves and ganglia excluding the brain and Spinal cord. It acts as a connector between the CNS to the limbs and organs.

The structure of a neuron consists of the following:

•         Soma – It contains the nucleus and most of the protein synthesis occurs here.
•         Dendrites – They are cellular extensions that join with various axons from neighbouring neurons. They are responsible for receiving the information from the axons and relaying it further.
•         Axon – It is a single protrusion coming out of the soma. It is responsible for the transmission of the signal away from the soma to other neurons. Even though it is a single protrusion when originating from the soma, at its terminal it undergoes extensive branching that enables it to communicate with multiple cells.

o   Axon Hillock – This is the part of the axon which emerges from the soma. It has a great density of voltage-dependent sodium channels. These channels are responsible for the initiation of the action potential which is then carried forward. This makes it an extremely important site in the structure of neuron.

o   Axon terminal – This is the region of the axon that contains synapses. These are structures that are responsible for the release of neurotransmitters such as acetylcholine. It helps in communicating with target neurons.

The neuron also consists of myelin sheath which is an insulating layer formed around nerves. It is made up of proteins and fatty acids. It allows for the quicker and efficient transmission of electrical impulses through the nerve cells. The gaps in the myelin sheath are called the Nodes of Ranvier. In reflex actions the nerve impulses jump from one node to the other rather than travelling through the axon. This accounts for the quick and immediate response.

Classification of Neurons:

Before understanding the function of a neuron it is first necessary to classify them based on different characteristics. Structurally neurons can be classified as follows:

•         Multipolar – These are the most common type of neurons in the nervous system. They consist of a single axon and multiple dendrites radiating out from the body.
•         Bipolar – They consist of only two extensions, one for the axon and the other for the dendrite. Many bipolar neurons are commonly found in the sensory pathways. Some examples of bipolar neurons are- retina bipolar cell, ganglia of vestibulocochlear nerve, olfactory receptor neurons, spiral ganglia etc.
•         Unipolar – The neural cells of invertebrates are generally of this type. The structure of neuron in this type of cell consists of only one neurite which extends from the cell body. This neurite then branches into an axon and dendrites. Functionally the neurons can be classified as follows:
•         Sensory Neurons – A nerve cell that detects and responds to external stimuli. They are responsible for sensing the stimulus (a sudden change) in the environment and convert that signal into electrical signals which are then transmitted further into the nervous system.
•         Motor Neurons – It is a neuron whose cell body is located into the motor complex or spinal cord and whose axon is connected to the effector organs of our body.
•         Interneurons – They are present in between the sensory and the motor neurons and are responsible for the integration and relay of signals in the nervous system.

Action Potential:

An action potential is generated in the neuron when the homeostatic membrane potential of a cell is disrupted due to the rapid influx and efflux of sodium and potassium ions. In response to a signal the sodium and potassium gated channels are activated. The sodium channel allows for the influx of 3 Na+, this causes the depolarization of the membrane. To balance this sudden influx of 2 K + ions efflux outside the cell, this is known as repolarization. 

 Conclusion:

The Structure of Neuron is integral to the way in which nerve impulses travel throughout the body. All its components are arranged in just the right manner that allows us to process and react to such complex problems which are truly unique and make us humans the most intelligent organism till date. We were able to classify the neurons based on its structure and function.