To begin the journey into neuroscience, we must first understand the fundamental unit of the nervous system: the neuron. There are multiple types of neurons and their helper glial cells. This week we will explore the structure of a basic neuron and the different types of neurons.
Structure of a Neuron:
There are three parts to a neuron: dendrite, cell body, and axon. The best way to see the function of each part is by looking at the transmission of an action potential.
The action potential begins at the dendrites. The dendrites are long branches that extend from the cell body. They receive the signals transmitted by other neurons at the synapse. The synapse is the area between the axon of one cell and the dendrite of another, where the signals are transmitted between cells. These signals are then passed down the length of the dendrite to the cell body.
All the signals “combine” at the axon hillock. If the combined potential is strong enough, the signal will propagate itself down the axon. The axon is covered by myelin, a fatty substance that insulates the nerve fibers. This insulation has gaps, which are known as nodes of Ranvier, which provide for saltatory conduction (which speeds up transmission time).
The signal ends at the axon terminals where the action potential causes the release of neurotransmitters across the synapse. The released neurotransmitters propagate the signal down the next neuron’s dendrite.
Types of Neurons:
There are two ways of classifying neurons: based on the structure or function of the cell.
Classification: Structure
A cell which only has an axon and no dendrite is known as unipolar.
A neuron with both an axon and a dendrite is known as bipolar.
A cell which has one axon split into two branches is known as pseudo unipolar.
A neuron which has multiple dendrites and one axon is known as multipolar. They are the most common type seen.
See picture for more details.
Classification: Function
Sensory Neurons:
nerve cells that are activated by input from the environment
(ex. The neuron that is activated when you perceive sensation with your fingers)
Sensory neurons are usually pseudo-unipolar and include physical and chemical
signals.
Interneurons:
nerve cells that relay signals from sensory neurons to motor neurons
Interneurons are multipolar.
Motor Neurons:
Nerve cells that transmit signals to skeletal and smooth muscles in response to impulses transmitted through interneurons
Types:
Upper motor neurons: transmit signals between brain and spinal cord
Lower motor neurons: travel between spinal cord and muscles
Motor Neurons are usually multipolar.
These are just two ways of classifying neurons but there are many characteristics with which to group nerve cells. An obstacle to making a “perfect neuron classification” is the complexity of the brain and the number of specialized neurons it contains.
Glial cells:
Oligodendrocytes: create the myelin sheath around axons in the CNS
Schwann cells: create the myelin sheath around axons in the PNS (peripheral nervous system)
Microglia: are the brain’s immune cells and protect it from damage, can be hyper activated and worsen diseases such as Alzheimer’s.
Astrocytes: star shaped cells, they clean up the environment around the neurons (such as controlling the level of ions), are a part of the blood - brain barrier
Ependymal cells: line the spinal cord and ventricles of the brain
Satellite cells: surround neurons in PNS and clean up the environment around the nerve cells, they’re suspected to play a role in chronic pain
Fun Facts:
According to Brain Facts, “Each person has as many as 100 billion nerve cells called neurons, and the communication between these cells forms the basis of all brain function."
Neurons can stretch from a mere centimeter to over three feet long.
Works Cited:
“Chapter 1.” Brain Facts: a Primer on the Brain and Nervous System, 7th ed., Society for
Neuroscience, 2012, pp. 6–13.
“Types of Neurons.” Queensland Brain Institute, Queensland Brain Institute, 26 Mar. 2018,
Blog by : Lasya Kambhampati
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