Neurons, or nerve cells, are the smallest living unit of the nervous system responsible for transmitting nerve impulses.
A nerve impulse is electrical activity that travels through the nervous system by stimulating successive neurons.
There are billions of neurons in the human body located in the brain, spinal cord and nerves. There are several types of neurons, but they all have three basic structures: the axon, the cell body and the dendrites.
Dendrites are small branches around the cell body. They carry the nerve impulses to the cell body.
The cell body is the central part of the neuron. It’s bound by the cell membrane and contains the cytoplasm and nucleus.
The axon is the longest part of a neuron. This structure carries nerve impulses out of the cell body. It is insulated by a structure called the myelin sheath. The axon ends with axon terminals.
The 3 Basic Structures in Detail
The axons are unique to each neuron. The length of the axon varies greatly from one neuron to another, to the point that it’s absent in some, while in others it measures almost |1\ \text{m}.| For example, the axon that innervates the foot is connected to its cell body in the lumbar region of the spinal cord. These long axons are called neurofibres or nerve fibres. On one end, an axon is connected to the cell body by an axon hillock. The other end branches out into nerve endings, called axon terminals, that transmit nerve impulses (using neurotransmitters) to one or more neurons or to a motor structure. The nerve impulse travels through the neuron’s membrane. The axons are covered with an insulating sheath of myelin to minimize the loss of nerve impulses.
The cell body is also called perikaryon or soma. It contains the usual organelles found in a cell, except for the centrioles. The cell body produces the materials necessary to create new dendritic and axonal extensions during the embryonic phase. The cell body is usually directly involved in receiving information from other neurons.
The dendrites, of which there can be hundreds, form a highly branched tree of cell body extensions. They are the receptors for nerve impulses, neurotransmitters and stimuli from other neurons. The point of contact between two neurons, which is a very small space where neurotransmitters are released, is called a synapse or a synaptic cleft. The signals received by the dendrites are transmitted to the cell body.
Interestingly, neurons have an incredible lifespan and, when given a constant supply of quality nutrients, can live for over 100 years. However, neurons are unable to undergo cell division, which means that they can’t renew themselves. This means that they’re amitotic (unable to perform mitosis). Neurons also have five important properties:
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They are specialized cells, meaning they only have one function: to transmit the information contained in the nerve impulse.
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They are excitable, because they can react to different environmental stimuli or be stimulated by a neighbouring neuron.
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Neurons are conductors, because they can propagate a one-way electrical current called the nerve impulse.
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Neurons are incapable of dividing, or regenerating.
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Crucially, neurons have tremendous longevity, since they can live as long as the person.
This means that neural losses caused by a stroke, certain diseases or certain drugs are irreversible. However, there are some stem cells that are able to divide into neurons, although this is usually the exception. Because of their especially high metabolism, neurons require a continuous supply of glucose and oxygen. A shortage of either of these two elements for a few minutes can lead to severe damage to the nervous system or even death.
There are three types of neurons classified according to their function.
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Sensory neurons capture information perceived by the sensory organs and transmit it to the brain.
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Motor neurons are responsible for transmitting nerve impulses from the brain to the muscles or glands.
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Interneurons, also known as association neurons, are used to transmit information between neurons of all kinds.
Nerve impulses always circulate in the same direction within a neuron.
Nerve impulses are first received by the dendrites which transmit them to the cell body. From there, the impulses travel from the cell body to the axon and on to the nerve endings of the axon (axon terminals) where small vesicles of neurotransmitters are located.
Neurotransmitters are chemicals that stimulate a nearby neuron, muscle or gland. They are released from the axon terminals in the synapse in order to stimulate the neighbouring cell (neuron, muscle, gland, etc.).
The synapse, also called the synaptic cleft, is the contact point between two neurons or between a neuron and another cell that allows the transfer of information.
The neurotransmitters can stimulate the neighbouring neuron which will, in turn, transmit the nerve impulse. In the case of a muscle, they either make it contract or relax. A gland can secrete a hormone following the reception of neurotransmitters or, conversely, stop the secretion.
