Neurons specialize in transmitting signals from the brain, throughout the body. These nerve cells are the fundamental units of the nervous system.
Neurons: An overview
The primary role of neurons is the transmission of chemical and electrical signals across the body. The structure of nerve cells is such that it facilitates its signalling functions. The human body is home to more than eighty-six billion nerve cells, a majority of which, are developed during the embryonic stage of human development. However, studies have shown that the creation of new nerve cells occurs even throughout adult life. The process of nerve cell development and maturation is known as neurogenesis. This process is yet to be fully understood.
Anatomy of a nerve cell
The structure of nerve cells is in accordance with their specialized functions. Neurons are made up primarily of the following major parts-
- Soma or the cell body is where all the cell organelles of the nerve cells are housed. This is also where the nucleus is present. The soma of a nerve cell possesses all the relative genetic information required for the proper functioning of the cell.
- Dendrites are branch-like structures that originate from the main cell body or the soma. These branches are responsible for receiving signals in the form of chemicals from other adjacent neurons. Neurons can have multiple dendrites in the form of dendritic trees.
- The axon is a tail-like structure that is covered by a lipid membrane, also known as a myelin sheath. The main function of the axon is to process and conduct electrical signals. The site of joining between the cell body and the axon is called the axon hillock.
Types of neurons
Based on the number and arrangement of the dendrites, neurons can be classified into the following types-
- Multipolar neurons are neurons that possess a single axon with multiple, symmetrical dendrites extending from it. This is the most commonly observed nerve cell in the human body.
- Unipolar neurons possess a single axon and are usually found only in animals of lower organisation i.e., invertebrates.
- Bipolar neurons possess an axon towards one polar end of the cell body and dendrites towards the other polar end of the cell body. These nerve cells are commonly found in the retina.
- Pyramidal neurons are called so due to the pyramid shape formed by the dendritic arrangement of these nerve cells. They possess one axon and multiple dendrites that are arranged in such a way that they form a pyramid shape. These neurons are primarily found in the cortex of the brain.
- Purkinje neurons, also known as inhibitory neurons, are characterised by the presence of multiple dendrites that spread out directly from the cell body. These nerve cells transmit inhibitory neurotransmitters that regulate the functioning of other neuronal cells.
How do neurons function?
Neurons transmit signals both in the form of chemicals and electrical impulses. This signal transmission works on the basis of action potentials. Due to the flow of charges across the nerve cell, a change in potential energy occurs leading to the production of an action potential. An action potential moves along the axon and towards the presynaptic ending. As mentioned before, action potentials can be in the form of a chemical signal or an electrical impulse. The location where signal transmission takes place is known as a synapse.
A synapse is characterised by the presence of a presynaptic region, a synaptic cleft and a postsynaptic region. Based on the nature of the signal, synapses can be categorised as follows-
- A chemical synapse refers to neurotransmitters. Neurotransmitters are chemicals that produce an action potential in a postsynaptic neuron after it crosses the synaptic cleft and binds to the receptors of the postsynaptic neuron.
- An electrical synapse is characterised by the presence of a small space between two neurons, known as a gap junction. A gap junction is composed of ion channels that enable the transmission of positive signals across neurons.
It is important to note that electrical signals travel much faster than chemical signals. However, chemical signals persist in the body for a long period of time as compared to electrical signals. Electrical signals are also known to diminish in intensity as they travel across each electrical synapse.
The term ‘neurodegenerative disorders’ refers to a set of diseases that are primarily caused by the degeneration of the neurons in the brain. These diseases cause a lot of damage to the body and are currently incurable. They are mostly experienced by elderly individuals. Some of the major symptoms of neurodegenerative disorders are-
- Loss of motor control and movement (ataxias).
- Loss of cognitive function (dementias).
- Loss of ability to speak and breathe properly.
Some examples of the most commonly observed neurodegenerative disorders are as follows-
- Parkinson’s disease
- Alzheimer’s disease
- Muscular atrophy
- Prion disease
- Huntington’s disease
Neurodegenerative disorders can be attributed to both nature and nurture. In this case, both environmental and genetic factors come into play while determining how susceptible an individual is to falling prey to neurodegenerative disease.
Nerve cells are the building blocks of the brain and the nervous system. These cells have unique structures that are built in accordance with their specific functions within the body. The basic structure of a nerve cell is characterised by a soma or main cell body, tail-like axon and branch-like dendrites. Neurons transmit signals across the body in the form of chemicals or neurotransmitters and electrical impulses. The region where these signals are transmitted is known as a synapse. The degeneration of nerve cells can lead to the occurrence of neurodegenerative diseases. It is important to note that nerve cells cannot repair or replicate themselves.