16.19: Overview of Synapses
A synapse is a specialized structure where two neurons connect, allowing them to pass an electrical or chemical signal to another neuron. It is the point of communication between neurons. The term "synapse" is derived from the Greek word "synapsis," which means "conjunction." The entire process of neural communication revolves around the synapse. When activated, a neuron releases chemicals known as neurotransmitters into the synapse. These neurotransmitters cross the synapse and bind to receptors on the receiving neuron, triggering a response. This is how neurons 'talk' to each other.
The significance of synapses in neurobiology is immense. They play a crucial role in the formation of memories and learning. This is due to a phenomenon called synaptic plasticity, which refers to the ability of synapses to strengthen or weaken over time, depending on the amount of activity they experience. This adaptability enables one to learn new things and form new memories.
There are two main types of synapses: electrical and chemical synapses. Electrical synapses allow direct, rapid communication between cells through structures called gap junctions. They are often found in systems that require fast, synchronized activity, for example, in the heart muscle. Chemical synapses, on the other hand, are slower but allow for more complex communication because different neurotransmitters can elicit different responses. They are the most common type of synapse in the human brain.
There are distinct variations between chemical and electrical synapses. In chemical synapses, the transmission of signals relies on the release of neurotransmitter molecules, resulting in a delay of approximately one millisecond between when the axon potential reaches the presynaptic terminal and when the neurotransmitter prompts the opening of postsynaptic ion channels. It is noteworthy that this communication is unidirectional.
Contrarily, electrical synapses facilitate near-instantaneous signaling, which is crucial for synapses involved in reflexes. Some electrical synapses even allow bidirectional signaling. Furthermore, electrical synapses exhibit greater reliability as they are less prone to blockage. These synapses play a crucial role in synchronizing the electrical activity of a group of neurons. For instance, electrical synapses in the thalamus are believed to regulate slow-wave sleep, and their disruption can lead to seizures.
In conclusion, electrical synapses are essential for the proper functioning of many neural circuits. They enable faster information transmission and greater reliability compared to chemical synapses, making them a key component in the processing of signals in the nervous system.
The gap junction is another type of specialized contact between neurons that allows ions and small molecules to pass directly from one neuron to another.
