Introduction

Nerve cells which are also called neurons are the cells of the nervous system. They are present in almost all animals but not present in fungi and plants. Nerve cells are electrically excitable cells. They receive information from the environment and allows the effector glands to respond to the stimuli. Neurons are present abundantly in the body. A single human body contains an average of almost 86 billion nerve cells. Nerve cells are also the longest cells of our body.

During brain development, the nerve cells are differentiated from neural stem cells. This process of differentiation is called neurogenesis. Neurogenesis stops in adulthood in most areas of the brain. The hippocampus and olfactory bulb are, however, the locations where neurogenesis does not stop. Nerve cells or the neurons do not divide by mitosis and sometimes they live throughout the life of the organism. 

Anatomy

A nerve cell consists of three specialized parts, the soma or the cell body, the axon, and the dendrite. 

Soma

Soma or the cell body is the spherical, large, central structure of the nerve cell. It is sometimes called perikaryon because it contains the nucleus of the cell. The nucleus is the source of RNA which is used to make proteins. Soma contains many organelles as well such as Nissl granules, which consist of polyribosomes and rough endoplasmic reticulum. The function of Nissl Granules is the formation and release of proteins and amino acids. 

The cell body is the metabolic center of the nerve cell which produces energy. It is the place where macromolecules are synthesized to keep the cell alive and maintain its function. It contains different types of organelles that are necessary for the growth, energy production, and synthesis of proteins. The shape and the structure of the soma of the nerve cells depend upon the type of nerve cell.

Dendrites 

Dendrites which are also called dendrons are the cytoplasmic extensions of the nerve cells. These extensions propagate the electrochemical stimulation received from other neurons to the cell body. Neurons form new dendritic branches to create new synapses. This process of forming dendritic branches is called dendritic arborization. 

The dendrites have a special morphology such as branch density and grouping patterns which is highly correlated to the function of the nerve cell. Therefore, the malformation of dendrites is strongly correlated to impairment of nervous system function. Malformation of dendrites is associated with disorders such as autism, schizophrenia, depression, Down syndrome, and anxiety.

Dendrites may contain small projections referred to as dendritic spines. These projections increase the receptive properties of dendrites. An increase in neural activity at dendritic spines changes the shape, size, and conduction. This ability is of great importance because it is thought to play an important role in learning and memory. A single cell may contain up to 15,000 spines. 

Axon

Axon is a long, slender cytoplasmic extension of the nerve cell. It conducts electrical impulses away from the soma or the cell body of the neuron. Axons transmit information to other neurons, muscles, and glands. Axons are also called nerve fibers. They are divided into three groups, group A, group B, and Group C nerve fibers. Only the neurons of groups A and B have a covering called the myelin sheath.

Axons differ from dendrites in many aspects. Axons transmit information whereas dendrites receive information. Axons are much longer than dendrites and they do not have a tapering end. Some neurons do not have any axon, but no neuron has more than one axon. A long bundle of many axons makes a tract called the nerve tract in the central nervous system and a fascicle in the peripheral nervous system.

The outer membrane of the axon is called axolemma and the cytoplasm is known as axoplasm. Telodendria, end branches of axons, have a swollen end known as axon terminal. Axon terminals are specialized structures that connect the axon to a dendron or the cell body of another neuron. 

Some nerve cells are covered with an insulating layer called the myelin sheath. In the central nervous system, this sheath is provided by the oligodendrocytes and in the peripheral system, glial cells and Schwann cells myelinate the nerve cells. A myelinated nerve cell has short unmyelinated segments which are known as Nodes of Ranvier. The action potential is generated in these areas. The action potential jumps from one node to another node. This type of conduction is called saltatory conduction. 

Classification

The nerve cells are classified in two ways, according to their morphology and according to their function. First, we will discuss the classification based on the functions of neurons. 

Functional Classification

1. Sensory Neurons

Sensory neurons which are also called afferent nerve fibers are neurons that convert the stimulus into action potential in a process called sensory transduction. They carry information from the sensory receptors to the central nervous system. The stimulus can come either from outside the body or from inside the body. Light, heat, and sound is an example of outside stimulus whereas the blood pressure of the body is an example of an inside stimulus. 

The cell body of afferent neurons is smooth and rounded. Many cell bodies accumulate to make a swelling known as ganglia. Information that comes from the head enters the central nervous system through cranial nerves whereas the information coming from the sensory neurons below the head enters the spinal cord. 

2. Motor Neurons

Motor neurons which are also called motoneurons carry the information from the brain and spinal cord to the effector muscles and glands. The cell body of a motor neuron is located in the brainstem or spinal cord and the axon projects to the spinal cord or outside the spinal cord. Motor neurons are of two types, upper motor neurons, and lower motor neurons. 

Upper motor neurons make connections with interneurons in the spinal cord. Axons of lower motor neurons carry information from the spinal cord to effector muscles or glands. It is important to note that a single motor neuron may innervate more than one muscle fiber. 

Motor neurons and muscle fibers are connected through a specialized synapse called the neuromuscular junction. When motor neurons are stimulated, they release acetylcholine which binds to postsynaptic receptors. Ion channels are opened, and an influx of sodium triggers a muscle action potential and the target muscle fiber contracts. 

3. Interneurons

Interneurons are also called association neurons, relay neurons, or intermediate neurons. They make communication between motor neurons and sensory neurons. Interneurons are known to play an important role in neurogenesis, reflexes, neuronal oscillations. Interneurons are divided into two groups, local interneurons, and relay interneurons. 

Local interneurons have relatively short axons. Their function is to analyze the information that comes from the sensory neurons. On the other side, the relay interneurons have longer axons than local interneurons. They connect the circuits of neurons in different regions of the brains. The interaction between interneurons is necessary for the learning and decision-making process of the brain.

Some interneurons use the neurotransmitter GABA or glycine which are inhibitory neurotransmitters. Some other interneurons are excitatory because they use glutamate and acetylcholine as neurotransmitters. The main function of interneurons is to connect the motor and sensory neurons and conduct a flow of signals between them.  

Structural Classification

Anatomically the nerve cells are divided on the basis of their polarity.

1. Unipolar nerve cells are those who have only one process, usually the axon. 
2. Bipolar nerve cells have one axon and one dendrite. 
3. Multipolar neurons have one axon and more than one dendrite.
4. Pseudo-unipolar axons have a single process which may act as both axon and dendrite.
5. Anaxonic neurons are those in which axons cannot be distinguished from the dendrites. 

The connection between Nerve Cells 

The connection between two nerve cells is called a synapse. The nerve cells communicate and propagate their action potential through synapses. Excitatory synapse increases the activity in the target neurons and inhibitory synapses decrease the activity in the target neuron. A unique type of synapse called autapse connects the dendrites of a nerve cell to its own axon. Some neurons are connected through electrically conductive junctions known as electrical synapses.   

When the action potential reaches the axon terminal of a nerve cell, it opens the voltage-gated calcium channels. This opening of channels allows calcium ions to enter the axon terminal. The influx of calcium causes synaptic vesicles that are filled with neurotransmitter molecules to fuse with the membrane of the nerve cell. These vesicles then release their neurotransmitter into the cavity between two neurons called the synaptic cleft. 

The neurotransmitters released from the vesicles diffuse across the synaptic cleft. This process activates the postsynaptic receptors on the postsynaptic neuron. An increased level of calcium in the axon terminal triggers calcium uptake by mitochondria, which, in turn, triggers mitochondria to produce energy in the form of Adenosine triphosphate (ATP) to support continuous neurotransmission. This is how the transfer of action potential occurs between two neurons. 

Disorders of Nerve Cell

Alzheimer’s disease

Alzheimer’s disease is caused by the degeneration of neurons. Progressive cognitive deterioration occurs in this disease. It is characterized by loss of short-term memory. Patients may forget minor things and daily life events. Older memories may remain preserved but newer memories are lost easily. When the disease progresses, symptoms of aphasia, apraxia, and agnosia appear. Other brain functions such as decision-making and planning may also become impaired.

Parkinson’s disease 

Parkinson’s disease is a neurodegenerative disease in which motor skills and speech are impaired. It is a movement disorder characterized by muscle rigidity, tremor, and slowing of physical movement. In extreme cases, a complete loss of physical movement may occur. Parkinson’s disease is caused by insufficient formation and action of dopamine because the dopaminergic neurons are damaged. It can lead to high-level cognitive dysfunction and language problems.

Myasthenia Gravis 

Myasthenia gravis is a neuromuscular disease caused by inhibition of the stimulative effect of the neurotransmitter acetylcholine. This is caused by circulating antibodies that block the receptors of the neurotransmitter at the post-synaptic neuromuscular junction. Myasthenia gravis is characterized by muscle weakness. It is treated with immunosuppressive drugs and cholinesterase inhibitors. In some cases, thymectomy is required for complete treatment. 

Summary

Nerve cells which are also called neurons are electrically excitable cells present in almost every part of the body. Nerve cells receive information from outside or inside receptors of the body and allow the effector glands and muscles to respond. The process of creation and differentiation of neurons is called neurogenesis. Neurons are non-dividing cells, and they may live throughout the life of an organism. 

The nerve cell has three parts, the soma, the dendrite, and the long cytoplasmic extension known as the axon. The soma or the cell body contains the nucleus and other important organelles. The dendrites receive information from the receptors and carry the information to the cell body. Axons are long, slender extensions of the cytoplasm that carry information from the cell body to another neuron or effector muscles and glands. 

Neurons are divided into three groups. Sensory neurons convert the stimulus to an action potential. This action potential is then carried to the spinal cord and the brain. Motor neurons have long axons, and they carry information from the brain and the spinal cord to effector muscles and glands. There are two types of motor neurons; upper motor neurons, and lower motor neurons. Interneurons connect the sensory and motor neurons in the central nervous system of the body.

The nerve cells may be unipolar, bipolar, multipolar, or pseudo-unipolar depending upon the number of axons and dendrites. The nerve cells are connected through synapses. The action potential is propagated through the release of neurotransmitters in the synaptic cleft. The neurotransmitters activate postsynaptic receptors, and an action potential is generated in the post-synaptic nerve cell. 

Alzheimer’s disease and Parkinson’s disease are neurodegenerative diseases in which neurons are destroyed. Alzheimer’s is characterized by loss of short-term memory whereas Parkinson’s disease is characterized by muscle rigidity, tremor, and slowing down or loss of physical movement.   

References

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