Important elements of synaptic transmission in brain
Four part synapses in brain
It is at synapses that neurons communicate with each other and with other body cells. A synapse is where a neuron’s axon terminal releases a chemical message called a neurotransmitter that diffuses across a small gap to another cell. All six varieties of synapses pictured above occur in the brain. Axodendritic synapses, shown in the illustration as type number 3, is the one most frequently discussed in textbooks.
Studies of the past 10 years discovered that there are four cellular participants at each synapse. In addition to the two neurons involved there are astrocyte cells that pump neurotransmitter out of the small gap between the neurons. The fourth component is another type of brain cell called microglia. Microglia monitors the quality of the events at each synapse. Microglia can remove or repair synapses as the need arises.
The axon terminal that releases neurotransmitter is called a presynaptic compartment, and the area on the next neuron where the neurotransmitter binds to cell surface proteins is called a postsynaptic compartment. The gap between the neurons is called a synaptic cleft.
How presynaptic compartments release neurotransmitter
There are many different types of neurotransmitters used in the brain, but the mechanism for their release from presynaptic compartments is similar. Neurotransmitter is made by cellular metabolic enzymes in the neuron cell body and is packaged there into vesicles. The vesicles are transported along the internal fibrous tracts through the axon to the presynaptic compartment.
When action potentials arrive at the axon terminals, they cause the membrane to depolarize. The depolarization affects voltage-sensitive calcium ion (Ca++) channels in the membrane causing them to open. In adult brain the concentration of Ca++ is higher in the extracellular fluid surrounding neuron terminals than inside the neuron. When Ca++ channels open, Ca++ diffuses into the neuron terminal.
The presence of a high level of Ca++ inside the neuron sets in motion a series of molecular processes. This series of events moves the vesicles from their storage area in the axon terminal to the membrane near the synaptic cleft. The membranes of the vesicles then fuse with that of the axon terminal such that their neurotransmitter is released into the synaptic cleft.
Neurotransmitter quickly wears out its welcome in the synaptic cleft
Neurons usually fire actions potentials as a patterned series. The pattern itself establishes the quality of the message. If neurotransmitter released by each arriving action potential is not removed quickly, the synaptic cleft rapidly becomes saturated making further action potentials ineffective.
Most neurons depend upon neurotransmitter uptake pumps called transporters to remove it from the synaptic cleft. Often most of the neurotransmitter transporters are on the membrane of surrounding astrocyte cells. More than 20 recognized proteins are included in the transporter group. Each type of neurotransmitter uses its own subset of transporter proteins.
Neurons releasing the neurotransmitter acetylcholine are an exception. Acetylcholine is inactivated in the synaptic cleft by the enzyme acetylcholinesterase. Acetylchoinesterase is located on the membrane of the postsynaptic compartment. The enzyme splits acetylcholine into choline and acetate. Choline is then transported back into the presynaptic compartment where it is resynthesized into acetylcholine and repackaged. This approach removes acetylcholine from the synaptic cleft faster than would otherwise be possible.
Elements of postsynaptic compartments
The majority of postsynaptic compartments on brain neurons are located at structures called dendritic spines on neuron dendrites. Neuron dendrites are branched like small trees and each branch may contain thousands of dendritic spines.
The main feature of the postsynaptic compartment is the postsynaptic density. It is a thickening of the postsynaptic membrane that contains a large collection of highly mobile proteins that respond to neurotransmitter. The diversity and size of this protein population is governed by the pace of neurotransmitter release from the presynaptic compartment. Some proteins of the postsynaptic density are adhesion molecules that hold the two compartments together, some are enzymes and many are neurotransmitter receptors.
Some neurotransmitter receptors open ion channels. The ion channels allow the flow of calcium, chloride, sodium or potassium into the postsynaptic compartment. Other neurotransmitter receptors are proteins running through postsynaptic density membrane. The transmembrane receptors alter their shape when neurotransmitter binds. The modification of the receptor’s shape on the inside to the cell activates many intracellular molecular pathways leading to long-term changes in the postsynaptic compartment.
The ion channel receptors for neurotransmitter may depolarize the membrane of the postsynaptic compartment initiating action potentials along the dendrites toward the cell body. Alternately, ion channel receptors may hyperpolarize the membrane and thereby interfere with the production of action potentials along the dendrite. Positioning of brain synapses combined with the various types of receptors for many different neurotransmitters permits robust communication systems within brain tissue.
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