Excitatory synapse
An excitatory synapse is a synapse in which an action potential in a presynaptic neuron depolarizes the membrane of the postsynaptic cell, and thus increases the probability of triggering an action potential in that cell. The postsynaptic cella muscle cell, a glandular cell or another neurontypically receives input signals through many excitatory and many inhibitory synapses. If the total of excitatory influences exceeds that of the inhibitory influences and the resulting depolarization exceeds the threshold level, the postsynaptic cell will be activated. If the postsynaptic cell is a neuron it will generate a new action potential at its axon hillock, thus transmitting the information to yet another cell. If it is a muscle cell, it will contract. If it is a gland cell, it will release its product.
In an excitatory synapse, the electric response of the postsynaptic membrane to a single action potential in the presynaptic neuron is known as an excitatory postsynaptic potential. It may occur via direct contact between cells, as in an electrical synapse, but most commonly occurs via the vesicular release of neurotransmitters from the presynaptic axon terminal into the synaptic cleft, as in a chemical synapse.
The excitatory neurotransmitters, the most common of which is glutamate, then migrate via diffusion to the dendritic spine of the postsynaptic neuron and bind a specific transmembrane receptor protein that triggers the depolarization of that cell. Depolarization, a deviation from a neuron's resting membrane potential towards its threshold potential, increases the likelihood of an action potential and normally occurs with the influx of positively charged sodium ions into the postsynaptic cell through ion channels activated by neurotransmitter binding.
Chemical vs electrical synapses
Synaptic transmission
- In neurons that are involved in chemical synaptic transmission, neurotransmitters are synthesized either in the neuronal cell body, or within the presynaptic terminal, depending on the type of neurotransmitter being synthesized and the location of enzymes involved in its synthesis. These neurotransmitters are stored in synaptic vesicles that remain bound near the membrane by calcium-influenced proteins.
- In order to trigger the process of chemical synaptic transmission, upstream activity causes an action potential to invade the presynaptic terminal.
- This depolarizing current reaches the presynaptic terminal, and the membrane depolarization that it causes there initiates the opening of voltage-gated calcium channels present on the presynaptic membrane.
- There is high concentration of calcium in the synaptic cleft between the two participating neurons. This difference in calcium concentration between the synaptic cleft and the inside of the presynaptic terminal establishes a strong concentration gradient that drives the calcium into the presynaptic terminal upon opening of these voltage-gated calcium channels. This influx of calcium into the presynaptic terminal is necessary for neurotransmitter release.
- After entering the presynaptic terminal, the calcium binds a protein called synaptotagmin, which is located on the membrane of the synaptic vesicles. This protein interacts with other proteins called SNAREs in order to induce vesicle fusion with the presynaptic membrane. As a result of this vesicle fusion, the neurotransmitters that had been packaged into the synaptic vesicle are released into the synapse, where they diffuse across the synaptic cleft.
- These neurotransmitters bind to a variety of receptors on the postsynaptic cell membrane. In response to neurotransmitter binding, these postsynaptic receptors can undergo conformational changes that may open a transmembrane channel subunit either directly, or indirectly via a G-Protein signaling pathway. The selective permeability of these channels allow certain ions to move along their electrochemical gradients, inducing a current across the postsynaptic membrane that determines an excitatory or inhibitory response.
Responses of the postsynaptic neuron