The neuronal synapse is the Transmission area of electrical nerve impulses between two nerve cells (neurons) or between a neuron and a gland or muscle cell. A synaptic connection between a neuron and a muscle cell is called neuromuscular junctionwhile the synaptic transmission It is the process by which nerve cells communicate with each other.
- 1 What are neuronal synapses
- 2 Organization of synaptic connections
- 3 Convergence and divergence
- 4 Types of synapses
What are neuronal synapses?
The term synapse means 'connection' and was introduced by Charles Sherrington in 1897. It had been described by Ramón y Cajal, who visualized them under the optical microscope for the first time.
The synapse is actually a small space that separates the neurons and consists of:
- A presynaptic termination containing neurotransmitters, mitochondria and other cellular organelles
- A postsynaptic termination that contains receptors for neurotransmitters
- A synaptic cleft or space between presynaptic and postsynaptic terminations.
For communication between neurons to occur, an electrical impulse must travel through an axon to the synaptic terminal.
Usually, synapses only let information pass in one direction. Therefore, in any synapse there is a presynaptic neuron and a postsynaptic neuron. The space that remains between the two neurons is called synaptic space.
The presynaptic neuron transmits the electrical impulse to the postsynaptic neuron through the synapse.
Organization of synaptic connections
The human brain contains about 100 billion neurons (or nerve cells) and many more neuroglia (or glial cells) that serve to support and protect neurons. Each neuron can be connected to up to 10,000 neurons, transmitting signals to each other through up to 1,000 billion synaptic connections, which is equivalent, according to some estimates, to a computer with a processor of 1 billion bits per second. It is believed that the memory capacity of the human brain ranges from 1 to 1,000 terabytes.
And is thatthere is more synapse in our brain than stars in the Milky Way.
Convergence and divergence
The divergence and convergence of neural connections are a basic principle of brain organization. For example, the divergence allows the information collected by a single sensory receptor to be distributed in many areas of the brain. Convergence, on the other hand, allows the neurons that are responsible for contracting the muscles to receive the sum of the information of many neurons.
We talk about divergence synaptic when the information of an axon is transmitted to many postsynaptic neurons. In this way, the information is amplified.
We talk about convergence synaptic when several terminal buttons synapse on the same neuron. This convergent information is integrated into a single postsynaptic response.
Schematic representation of the processes of divergence and synaptic convergence.
Types of synapses
The human nervous system uses several different neurotransmitters and neuroreceptors, and not all of them work in the same way. We can group synapses into different types:
According to the form of information transmission
- Electric synapses: represent a small fraction of the total synapse. In these synapses, the membranes of the two cells touch and share proteins. This allows the action potential to pass directly from one membrane to the next. They are very fast, but they are not very abundant and are only found in the heart and eye.
- Chemical synapses: are the most frequent. The synaptic transmission is interceded by the release of chemical substances, by the presynaptic neuron, that interact with specific molecules of the postsynaptic cell (receptors), which causes changes in the postsynaptic membrane potential. The released chemicals are called neurotransmitters.
According to postsynaptic effects
- Exciting synapses: These synapses have neuroreceptors that are sodium channels. When the channels open, positive ions flow inward, causing local depolarization and making an action potential more likely. Typical neurotransmitters are the acetylcholine, glutamate or aspartate.
- Inhibitory synapses: These synapses have neuroreceptors that are chloride channels. When the channels open, the negative ions flow causing local hyperpolarization and making an action potential less likely. With these synapses, an impulse in one neuron can inhibit an impulse in the next. The neurotransmitters Typical are glycine or GABA.
Depending on the type of cells involved
- Neuron neuron: both the presynaptic and postsynaptic cells are neurons. They are the synapses of the central nervous system.
- Neuron-muscle cell: also known as neuromuscular junction. A muscle cell (postsynaptic cell) is innervated by a motor neuron (presynaptic cell).
- Secretory cell neuron: The presynaptic cell is a neuron and the postsynaptic secretes some kind of substance, such as hormones. An example would be the innervation of the cells of the adrenal medulla, which would cause the release of adrenaline in the bloodstream.
According to the contact site
Different types of synapses according to the place of contact.
Any combination between the three regions of the neuron (axon, soma and dendrites) can occur, but the most frequent are the following:
- Axosomatic synapses: they have an axon synapses on the postsynaptic neuron soma. They are often inhibitory.
- Axodendritic synapses: in this case there is an axon that synapses on a postsynaptic dendrite. The synapse can be given to the main branch of the dendrite or in specialized areas of entry, the dendritic spines. They are often exciting.
- Axoaxonic synapses: the axon synapses on a postsynaptic axon. They are usually modulators of the amount of neurotransmitter that the postsynaptic axon will release on a third neuron.
Bradford, H.F. (1988). Fundamentals of neurochemistry. Barcelona: Labor.
Carlson, N.R. (1999). Behavioral physiology. Barcelona: Ariel Psychology.
Carpenter, M.B. (1994). Neuroanatomy Fundamentals Buenos Aires: Panamerican Editorial.
Delgado, J.M .; Ferrús, A .; Mora, F .; Blonde, F.J. (eds) (1998). Neuroscience Manual. Madrid: Synthesis.
Diamond, M.C .; Scheibel, A.B. i Elson, L.M. (nineteen ninety six). The human brain Work book. Barcelona: Ariel.
Guyton, A.C. (1994) Anatomy and physiology of the nervous system. Basic Neuroscience Madrid: Pan American Medical Editorial.
Kandel, E.R .; Shwartz, J.H. and Jessell, T.M. (eds) (1997) Neuroscience and Behavior. Madrid: Prentice Hall.
Martin, J.H. (1998) Neuroanatomy. Madrid: Prentice Hall.
Nolte, J. (1994) The human brain: introduction to functional anatomy. Madrid: Mosby-Doyma.