“Neurons that Fire Together Wire Together”–But Why? Hebb’s Rule and Synaptic Plasticity

 

I’ve written before about Hebb’s Rule which states that “Neurons that fire together wire together,” and neuroplasticity—the process by which we grow, change, and rewire our brains.  For the interested audience, in this article I explain more about the neurological underpinnings of this process.

In order to understand synaptic transmission, it’s important to first understand the component parts:  neurons, neurotransmitters and synapses. 

The Structure of Neurons:

 Cells in the brain are called “neurons.” Neurons are made up of:

• One Soma—the cell body
• Multiple Dendrites:  Dendrites are branches protruding from the soma that sprout dendritic spines—a popular location for synaptic transmission.  Neurons can have many dendrites, and the dendrites perform an important role:  they receive information. 
• One Axon:  An axon is a tube extending from the soma that transports information from the soma down to the terminal button (see below).  A neuron has only one axon protruding from the soma, but the axon may branch and divide.  Axons can be quite long– the longest axon in the human body extends from the human brain all the way to the foot.
• Terminal Buttons are knobs located at the end of the axon–these structures secrete the cell’s neurotransmitters when they receive the message (which has traveled down the axon) to do so.

What are Neurotransmitters?

Neurotransmitters are the chemical communicators within the brain. 

Neurotransmitters are stored in neurons, in “synaptic vesicles” in the terminal buttons.  When the terminal button receives the message (via the axon) to fire, the synaptic vesicle moves to the “presynaptic membrane” (the edge of the cell) and then squirts the neurotransmitters into the space outside of that neuron called the “synaptic cleft.”

Neurons synthesize neurotransmitters from the chemical substrates of our nutritional intake.  (A compelling reason to eat a healthy and varied diet!)  There are over 100 known neurotransmitters, and most neurons create and transmit more than one type.  Neurons can also “be receptive” to many different neurotransmitters.  Some of the neurotransmitters you may be familiar with are serotonin, dopamine, and norepinephrine.

What is a Synapse?

The synapse is the juncture where two neurons communicate—near the terminal buttons of the presynaptic neuron (the neuron sending the message) and the membrane of the postsynaptic neuron (the neuron receiving the message).

The synapse point on the presynaptic neuron occurs only at the terminal buttons, which are at the end of the axon.  That’s because an “action potential” (see below) which prompts the release of the neurotransmitter, occurs only along the axon of the neuron.

The synapse point on the postsynaptic neuron can occur at many different points on the neuron, but often occurs on the dendrite; “dendritic spines” are studs on the dendrite that facilitate synaptic transmission.  A single neuron can receive messages from thousands of other neurons.  There are likely somewhere between 100 trillion to 1,000 trillion synapses in a normal human brain.

Neurons do not actually touch each other; there is a gap between neurons at the point of synapse–the “synaptic cleft.”  When the presynaptic and postsynaptic neurons are at rest, the synaptic cleft contains only extracellular fluid.  When synaptic transmission occurs (see below), neurotransmitters enter the synaptic cleft and communication begins between the two neurons.

How do Neurons Communicate?

Synaptic Transmission

Neurons communicate via “synaptic transmission.”  Communication takes place between a “presynaptic neuron” (the neuron sending information from the soma down its axon to the terminal button) and the “postsynaptic neuron” (the neuron that receives the information at its membrane—usually the information is received at one of the postsynaptic neuron’s many dendritic spines).

Neurotransmitters are the chemical messengers.  The presynaptic neuron releases neurotransmitter into the synaptic cleft and the postsynaptic neuron receives the neurotransmitter/message.

Neurotransmitters do not linger in the synaptic cleft; the postsynaptic neuron receives some, the presynaptic neuron reuptakes some of the leftovers, and any neurotransmitter remaining in the synaptic cleft is quickly inactivated.  It’s a very efficient system.

Note:  Many antidepressants (such as Prozac, i.e., fluoxetine) are “reuptake inhibitors.”  In other words, they block the ability of the presynaptic neuron to reuptake excess neurotransmitter, which prolongs the effects of the neurotransmitter on the postsynaptic neuron.

Even though neurotransmitters don’t linger in the synaptic cleft, increasing amounts of neurotransmitter can be released into the synaptic cleft by rapid firing of the presynaptic neuron.  The faster the presynaptic neuron fires (via action potential—see below), the more neurotransmitter is released into the synaptic cleft, and consequently, the stronger the message received by the postsynaptic neuron.

How Synaptic Transmission is Initiated:

Triggering an Action Potential

Synaptic transmission is initiated when a neuron is stimulated.  Stimulation causes the neuron to want to send information—a message—to neighboring neurons. Neuronal stimulation triggers an “action potential.”  An action potential begins at the juncture between the soma and axon and involves rapid depolarization followed by hyperpolarization, which sets in motion a wave of electrical energy down the axon, which triggers the terminal buttons to release neurotransmitters into the synaptic cleft.

An action potential is referred to as neuronal “firing.” 

An action potential travels in one direction only and is “all or nothing”—once triggered, it travels down the axon to its end; it doesn’t strengthen, nor does it diminish—it stays exactly the same.  If the axon branches, the action potential splits–but the energy stays exactly the same.

Although action potentials are “all or nothing” and stay exactly the same, the rate at which action potentials fire can increase or decrease, thus the rate of firing controls the strength of the message.  Slower neuronal firing (fewer action potentials) minimizes the release of neurotransmitter.  As a result, the postsynaptic cell is less activated, and the connection between those two neurons is weaker.

In contrast, rapid action potentials (rapid firing) facilitates more neurotransmitter release, which leads to increased activation of the postsynaptic neuron, which creates a stronger connection between those two neurons.   This is called “temporal summation.”

Just like a muscular contraction can be weak or strong, and repetitive use of a muscle leads to a stronger contraction, repeated action potentials create stronger connections between neurons.  In fact, the more a postsynaptic cell is activated by a presynaptic cell, the more sensitive the postsynaptic cell becomes to the messages of the presynaptic cell.

In the postsynaptic neuron, what happens after it receives its message (neurotransmitter)? 

The received neurotransmitter causes excitatory postsynaptic potentials (EPSP’s) in the dendrites of the postsynaptic neuron.  The EPSP’s travel up the dendrites to the soma, then to the juncture of the soma and axon.  If the EPSP is strong enough, then an action potential occurs in the postsynaptic (receiving) neuron—and that neuron fires.  Thus, the neurons “fire together.”

Note:  Neurotransmitters can also cause inhibitory postsynaptic potentials (IPSP’s), which inhibit firing of the postsynaptic neuron—but IPSP’s do not pertain to our topic of neurons firing together.

Also–while the neurons are not technically firing at precisely the exact same time, it’s the temporal proximity of the firing that is important, and causes the neurons to like each other.

Hebb’s Rule—“Neurons that Fire Together Wire Together”

“When an axon of cell A is near enough to excite cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased.”                                   

Donald Hebb, The Organization of Behavior:  A Neuropsychological Theory

In other words, the pre-and post-synaptic neurons “wire together.”

What Occurs when Neurons Wire Together?

Synaptic Plasticity: 

The Shaping of Communication between Neurons

When neurons fire together, the synaptic strength between the two neurons increases.  The most important changes occur in the postsynaptic (receiving) neuron.

Changes in the postsynaptic neuron include:

• Growth of new dendrites and more dendritic spines (which allow for more synapses)
• Altered shape of dendritic spines which allows for a stronger effect of the EPSP
• More receptors at the location of synapse
• More sensitive receptors

Changes in the presynaptic neuron include:

• Prolonged action potential which allows more neurotransmitter to be released
• Priming of enzymes that increase absorption of neurotransmitter in the postsynaptic neuron

In conclusion, Hebb’s Rule explains the underlying mechanism of neural and synaptic plasticity.   

You can shape your neuronal architecture by choosing what you focus on.