Neurons and Synapses Overview
The nervous system is the body’s overarching command center composed of billions of neurons that communicate using neurotransmitters. In this post, we will review the anatomy of the neuron, compare the different types of synapses, and discuss the vital neurotransmitters for neurotransmission!
What are neurons?
Neurons are the central building blocks of the nervous system and can communicate with each other through electrical and chemical signals.
Parts of the neuron include:
The Cell Body (Soma) - Houses the nucleus and is where protein synthesis occurs
Dendrites - Branch off the soma and receive information from the axon terminals of other neurons
Axon Hillock - The part of the cell body that connects to the axon and is where the summation of graded potentials occurs.
Axon - The pathway where electrical impulses (called action potentials) travel down to other neurons.
Synapse - The site where the axon terminal (presynaptic terminal) of one neuron and the dendrite (postsynaptic terminal) of another neuron meet. (There are different types of synapses that we will further describe later in this article)
A labeled diagram of a neuron
How many neurons are in the human brain?
The human brain is estimated to be composed of 86 billion neurons.¹ However, as research progresses, this estimate may change.
Computer rendered diagram of neurons in the brain.
What are synapses?
A synapse is the point where two neurons meet and communicate with one another.
Two neurons communicating at the synapse. Source: KhanAcademy.org
Types of Synapses
In the central nervous system, the different types of structural synapses that can be found are:
Axo-dendritic synapses - axon synapsing onto a dendrite
Axo-somatic synapses - axon synapsing onto a cell body
Axo-axonic synapses - axon synapsing onto another axon
Dendo-dendritic synapses - dendrite synapsing onto another dendrite
Types of synapses in the central nervous system
Synapses can also be classified into two general classes by their signal transduction method, these are chemical and electrical synapses.
In chemical synapses, depolarization of the presynaptic membrane (due to an action potential) leads to the opening of voltage-gated calcium channels in the axon terminal. This opening causes calcium influx into the presynaptic membrane and subsequent vesicular fusion that releases neurotransmitters into the synaptic cleft. Chemical synapses can further be classified according to the neurotransmitter they use to communicate with other cells.
A labeled diagram of a chemical synapse. Source: Pereda, A. E. (2014). Electrical synapses and their functional interactions with chemical synapses. Nature Reviews Neuroscience, 15(4), 250-263.
In electrical synapses, the cell membranes of the pre and post-synaptic cells are connected through connexin proteins that form gap junctions (connexion). This direct connection enables the transfer of action potentials from one cell to another.
A labeled diagram of an electrical synapse. Source: Pereda, A. E. (2014). Electrical synapses and their functional interactions with chemical synapses. Nature Reviews Neuroscience, 15(4), 250-263.
How many synapses are there in the human brain?
While the exact number of synapses in the human brain has yet to be defined, some sources estimate that each neuron has an average of 1000 synapses.² That being said, the total number of synapses in the human brain is somewhere around 100 trillion synapses.
Structural Diverseness of Neurons in the Brain
Citations:
Azevedo, F. A., Carvalho, L. R., Grinberg, L. T., Farfel, J. M., Ferretti, R. E., Leite, R. E., ... & Herculano‐Houzel, S. (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled‐up primate brain. Journal of Comparative Neurology, 513(5), 532-541.
https://aiimpacts.org/scale-of-the-human-brain/