How Synapses Work
Neurotransmitter binds to postsynaptic receptors, producing an electric signal that changes the polarity of the membrane. This initiates opening of voltage-gated channels and flow of ions that trigger an action potential.
If multiple EPSPs from different dendrites arrive at the same time, they can sum up to reach threshold and cause a neuron to fire. This is known as spatial summation.
The axons of neurons extend up to 150 meters (500 feet) per second from the cell body or dendrites to synaptic connections with other neurons. At these synapse-forming junctions, neurotransmitters are released to relay an action potential between cells.
The electrical signal from the action potential travels down the axon toward the synapse, where it triggers the terminal buttons to release neurotransmitters into the synapse. These chemicals bind to receptor molecules on the dendrites of the receiving neuron. They can either excite or inhibit the neuron. Different neurotransmitters can be used by different types of neurons to communicate with each other, and these differences are the basis for different neural circuits in the brain.
Once the neurotransmitter has done its job, the synapse must clear away the chemical to allow for new signals. This process is called re-uptake and can be initiated by presynaptic neurons or by glial cells. See animation 8.1 below for an example of this.
A neuron’s dendrites are branching protrusions that receive cellular signals from other neurons and then transfer them to the cell’s soma or body. They often take on a mushroom-shaped, cup-shaped, or tapering form and are referred to as a dendritic tree.
At chemical synapses, there is a delay of several milliseconds between when the action potential from the axon enters the presynaptic terminal and when the neurotransmitter molecules in the vesicles released from the terminal open postsynaptic ion channels. Additionally, these synapses are unidirectional.
In electrical synapses, however, the signal is transmitted almost instantly. This is because at some of the synapses in the brain, ions can flow directly from the presynaptic membrane to the postsynaptic membrane through a connection called a gap junction.
Gap junctions are small pores in the membranes of both the presynaptic and postsynaptic neurons that allow ions to pass through them. Additionally, small molecules like ATP and second messengers can also flow through the gap junction.
Spines are small, specialized dendritic structures that receive and convey signals. They are a key component of the synapse, compartmentalizing biochemical signals within the dendrites and cell body (soma). The size and number of spines on the dendrites and cell body influences signaling strength. Neurons with many spins can transmit more signals than those with few, for example. Some disorders of the brain such as autism and fragile X syndrome are characterized by an imbalance between mature and immature spines.
When a nerve impulse triggers the release of neurotransmitters from presynaptic terminals, these chemicals float across the synaptic cleft and attach to receptors on the postsynaptic neuron. These receptors are able to translate the chemical signals into electrical ones, so the postsynaptic neuron fires an action potential. The electrical signal then travels along the dendrites to other cells. The sum of all dendritic inputs determines whether or not a neuron will fire an action potential.
The Postsynaptic Neuron
In most chemical synapses, the axon terminal on the presynaptic neuron and the dendrites of the postsynaptic neuron come into close contact at a small space called the synaptic cleft. At this spot, the axon terminal releases a packet of neurotransmitter molecules that diffuse across the cleft to bind to specialized receptors on the postsynaptic cell. These neurotransmitters can be excitatory or inhibitory, and they can add together (via spatial summation) to influence whether the cell fires an action potential.
When ACh is released at a chemical synapse, it causes depolarization of the postsynaptic neuron to -70 mV, which is very close to its threshold for firing an action potential. If another synapse fires just a little bit later, it can “add on” to the first depolarization and cause the neuron to fire an action potential. Likewise, if an inhibitory signal arrives from different dendrites at the same time as an EPSP, it can cancel out the effect of the EPSP.