Synapses are the junctions between nerve cells and sensory, muscle, or glandular cells, or between two or more nerve cells. They serve to transmit signals and stimuli. Stimulus transmission is mostly chemical by means of neurotransmitters. There are also synapses that transmit their action potential directly by electrical means, which makes the transmission of stimuli faster and is therefore advantageous, for example, in muscle reflexes. Electrical synapses, unlike chemical synapses, can transmit stimuli in both directions.
What are synapses?
Synapses enable the transmission of stimuli and signals between nerve cells (neurons) and between nerve cells and sensory, muscle, and glandular cells. The name goes back to the British physiologist Sir Charles Sherrrington and derives from the ancient Greek “syn” for together and “haptein” for grasp or grasp. According to the type of stimulus transmission from the transmitter cell to the receiver cell, a distinction is made between chemical and electrical synapses. In chemical synapses, the electrical potential to be transmitted by the sending cell is converted into a chemical messenger (neurotransmitter) at the synaptic membrane. The narrow gap that exists between the synapses of the sending cell and the receiving cell is overcome by the neurotransmitter and the formerly electrical action potential is translated back into one. If the receiving cell is a muscle or gland cell, it is translated into action or, in the case of another neuron, transmitted as an electrical action potential. This type of signal transmission has the advantage that it is a directed, unidirectional, transfer of information. In contrast, electrical synapses can transmit stimuli in both directions, bidirectionally.
Anatomy and structure
A synapse always consists of a transmitting part or transmitter, the terminal knob of an axon that terminates with what is called the presynaptic membrane. The opposite receiving part of the synapse, the terminal knob of a dendrite, terminates with the postsynaptic membrane. Between the presynaptic and the postsynaptic membrane is the synaptic cleft. It is very narrow and is 10 to 20 nm in chemical synapses. In electrical synapses, the gap only reaches values around 3.5 nm. In humans, the number of synapses is estimated at the unimaginable value of about 100 trillion, corresponding to a 1 with 14 zeros. The presynaptic terminal knobs of the axons hold specific neurotransmitters in so-called vesicles. To ensure energy, the terminal knobs contain numerous mitochondria and still other organelles. When an action potential arrives, the vesicles empty the neurotransmitters into the synaptic cleft in the course of exocytosis. The receptor part of the synapse, the terminal button of a dendrite or action cell (muscle or gland cell), contains special receptors in its membrane to which the released neurotransmitter can dock, resulting in a retranslation into an electrical action potential or muscle contraction or gland secretion.
Function and tasks
Depending on their function, synapses can be divided into effector synapses, sensor synapses, and interneuronal synapses.
- Effector synapses provide the connection between neurons and muscle cells or neurons and glandular cells.
- Excitatory effector synapses serve to command muscle cells to contract or gland cells to secrete.
- Inhibitory effector synapses, on the other hand, transmit the opposite information, namely to relax muscles and stop gland secretion.
- Sensor synapses have the task of receiving sensory signals from sensory cells and receptors such as photoreceptors in the retina, pain receptors (nociceptors), thermal sensors, pressure and voltage sensors and many others, and transmit them to the appropriate switching centers in the brain.
- Interneuronal synapses, which form a cross-connection between two or more neurons, occur in huge quantity in the brain. There are a large number of conceivable interconnections, virtually all of which also occur, each with different tasks.
For example, there are linkages between axons and dendrites,
Axons and cell bodies (soma), between the dendrite plexuses of two neurons, and direct links between the cell bodies of two neurons. Interneuronal synapses are used for complex information processing, e.g., within the autonomic nervous system, but also for processing complex information into an overall picture in the central nervous system.
- Chemical synapses are each specialized for a particular neurotransmitter or retain that particular neurotransmitter in their vesicles. Therefore, chemical synapses can also be differentiated according to “their” neurotransmitters such as adrenergic , cholinergic and dopaminergic synapses, according to the neurotransmitters carried adrenaline, acetylcholine or dopamine.
- Electrical synapsescome into play where the extreme speed of stimulus transmission is important such as in the triggering of muscle reflexes.
Complaints and diseases
In 2014, researchers in Baltimore demonstrated that certain gene mutations lead to impaired synapse formation, which can cause mental illnesses such as schizophrenia and major depression. It is far better known that toxins lead to disruptions in synapse function with sometimes serious effects. Either the substances block the release of neurotransmitters into the synaptic cleft or they are so similar to neurotransmitters that they dock onto the receptors of the postsynaptic membrane in their place. In both cases, synaptic function is significantly to completely disrupted and blocked. An example of blocking exocytosis at the presynaptic membrane is botulinum toxin synthesized by Clostridia bacteria. The neurotoxin, also known as Botox, has a paralyzing effect on the muscles – similar to tetanus toxin – because the effector synapses can no longer transmit a contraction stimulus to the muscle fibers. In severe cases, this can lead to respiratory paralysis resulting in death. Many spider, insect and jellyfish poisons, as well as poisons from various fungi, are synapse poisons. Drugs such as alcohol, nicotine, hallucinogens such as LSD, and also psychotropic drugs are also synapse poisons with varying effects.
