Synapses are essential to the transmission of nervous impulses from one neuron to another. Neurons are specialized to pass signals to individual target cells, and synapses are the means by which they do so. At a synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target (postsynaptic) cell.
Both the presynaptic and postsynaptic sites contain extensive arrays of molecular machinery that link the two membranes together and carry out the signaling process. In many synapses, the presynaptic part is located on an axon and the postsynaptic part is located on a dendrite or soma. Astrocytes also exchange information with the synaptic neurons, responding to synaptic activity and, in turn, regulating neurotransmission.[3]
Some authors generalize the concept of the synapse to include the communication from a neuron to any other cell type,[5] such as to a motor cell, although such non-neuronal contacts may be referred to as junctions (a historically older term). A landmark study by Sanford Palay demonstrated the existence of synapses.[6]
Silent Synapses
Silent synapses are synapses, which only stably express NMDARs at the postsynaptic site. As discussed above, AMPARs usually accompany NMDARs at the postsynaptic site because NMDARs require an elevated level of depolarization before they are able to function. But in this case, because AMPARs are not stably expressed, when glutamate is released into the synaptic space and on to the NMDARs these specific synapses are nonfunctional – hence the name “silent synapses.”
Silent synapses have shown to induced de novo within the NAc following exposure to cocaine. Evidence shows that silent synapses may be a precursor to a newly generated synapse, existing at a stage during synapse development before AMPARs stably inserted into the membrane to complement the NMDARs.
This line of thought indicates that entirely new circuits and pathways formed within the mesolimbic dopamine system in response to drug experiences. Additionally, synaptic plasticity appears to occur more readily at silent synapses, as these synapses appear primed for LTP-like changes rather than LTD-like changes.
Synaptic Transmission in a Simple Reflex Circuit
One of the simplest behaviors mediated by the central nervous system is knee-jerk or stretch reflex. In response to a neurologist’s hammer to the patella tendon, there is a reflex extension of the leg. Figure 6.1 illustrates the neurocircuitry that controls that reflex response. The stretch to the patella tendon stretches the extensor muscle. More specifically, it stretches a group of specific receptors known as muscle spindle receptors or simply stretch receptors.
The stretch elicits action potentials in the stretch receptors which then propagate over type 1A afferent fibers, the somata of which are located in the dorsal root ganglion. Processes of these sensory neurons then enter the spinal cord and make synaptic connections with two types of cells. First, a synaptic connection formed with the extensor motor neuron located in the ventral horn of the spinal cord.
As the result of synaptic activation of this motor neuron, action potentials elicited in the motor neuron and propagate out the ventral roots, ultimately invading the terminal regions of the motor axon (i.e., the neuromuscular junction), causing release of acetylcholine, depolarization of the muscle cell, formation of an action potential in the muscle cell, and a subsequent contraction of the muscle.
Reflex
The sensory neurons also make synaptic connections with another type of neuron in the spinal cord called an interneuron. Interneurons so named because they interposed between one type of neuron and another. The particular interneuron shown is an inhibitory interneuron. As a result of its activation through the process of synaptic transmission, action potentials elicited in the interneuron.
An action potential in the inhibitory neuron leads to the release of a chemical transmitter substance that inhibits the flexor motor neuron, thereby preventing an improper movement from occurring. This particular reflex known as the monosynaptic stretch reflex because this reflex mediated by a single excitatory synaptic relay in the central nervous system.
In the central nervous system, a synapse is a small gap at the end of a neuron that allows a signal to pass from one neuron to the next. Synapses found where nerve cells connect with other nerve cells.
1897
Synapses are key to the brain’s function, especially when it comes to memory.1
The term synapse first introduced in 1897 by physiologist Michael Foster in his “Textbook of Physiology” and derived from the Greek synapsis, meaning “conjunction.”
What Synapses Do
When a nerve signal reaches the end of the neuron, it cannot simply continue to the next cell. Instead, it must trigger the release of neurotransmitters which can then carry the impulse across the synapse to the next neuron.
Once a nerve impulse has triggered the release of neurotransmitters. These chemical messengers cross the tiny synaptic gap. And taken up by receptors on the surface of the next cell.
These receptors act much like a lock, while the neurotransmitters function much like keys. Neurotransmitters may excite or inhibit the neuron they bind to.