03 Jan Synapses and Neurotransmitters
Electrical impulses are transmitted between neurons either electrically or chemically, with chemical synapses being the most numerous by far. The synapse is where electrical transduction between neurons occurs, facilitating perception, thought and action. The gap between a synapse and its target is called the synaptic cleft (tagged in the center right of the illustration and characterized with neurotransmitter molecules indicating a transmission event). Membrane on the sending side of the cleft is presynaptic; the receiving membrane is postsynaptic. The membranes have channels like the pores in epidermis. Just as pores open to permit secretion of perspiration, synaptic membranes open to permit the passage of chemicals that induce action potential.
While heat opens pores, the agents that open synaptic channels are chemical neurotransmitters. The liberation of chemicals through synaptic transmission changes the balance of internal and external chemicals. To counteract this, the membrane contains pump devices that reclaim chemicals and restore the balance. This brings the system to a near loss-less state, and proper nutrition replenishes the small quantities of attritted chemicals. Enzymes between neurons quickly neutralize the neurotransmitters.
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The catalysts of action potential are neurotransmitter chemicals such as Acetylcholine (ACh) whose structure is shown at right. The primary chemicals in the vicinity of are sodium, potassium, and calcium. The negative resting potential of cells is maintained by a chemical disequilibrium in which higher concentrations of potassium reside within the membrane, and higher concentrations of sodium reside without. The agents of action potential are molecules present in the environment of synaptic junctions. Positive and negative impulses, though triggered by electromotive potential traveling along the axon, are actually mechano-chemically propagated at the synaptic junction through the opening of the “pores” stimulated by the change in electrical potential in the axon.
Synapses contain synaptic vesicles and other organelles such as the mitochondria shown here. Chemical neurotransmitters are manufactured by neurons in the soma, then stored in little bundles called synaptic vesicles and transferred to the synapses. Synaptic vesicles are synthesized in the endoplasmic reticulum and transferred along cytoskeletal “tracks” in axons to the synaptic junctions at the termini of axonal processes. Manufacture of synaptic vesicles is continuous throughout the life of the cell. When the vesicle’s neurotransmitter is spent, its empty mass is moved back to the soma to be recycled by the cell.
When the electrical potential across the synaptic cleft is positive, it is called excitation. Cholinergic neurons use the chemical acetylcholine to alter synaptic-membrane permeability. Adrenergic neurons use norepinephrine or noradrenaline. Dopamine is also an excitatory neurotransmitter. Dopamine and noradrenaline are prominent in “alerting” processes; acetylcholine is more prominent in most other inter-neural communication. As described earlier and shown in the illustrations, the excitatory chemical is sodium. Neurotransmitters with inhibitory influence are gamma-aminobutyric acid (GABA – shown in illustration) and glycine. Both of these increase the permeability of postsynaptic membranes to potassium and chloride, thus holding the net potential below the threshold. Inhibitory potentials are less common than excitatory.
Some synapses facilitate direct electrical transmission from presynaptic to postsynaptic membranes. The low resistance of these synapses is due to the minute spacing of the gap between the presynaptic transmitter and the postsynaptic receptor membranes. Not unlike an automotive spark plug, the gap is essential for proper functioning because the neurotransmitter chemical needs an open space to navigate from the point of emission to the point of binding. This can allow a strong enough excitatory action potential in the presynaptic membrane to cause an arc, like the spark of an automotive spark plug. Action potential can be transduced across this arc. Electrical transmission occurs when the channels in the receptor membrane are shocked open by the intensity of the excitatory impulse in the presynaptic membrane.
Even an electrically induced transmission of excitation is actually electrochemical in origin. Despite the absence of neurotransmitter chemicals, the charge is propagated by an influx of sodium ions. Furthermore, it is possible that chemical transmission of E/I occurs simultaneously with the electrical arc, increasing or prolonging the impulse. This phenomenon shows the complexity of spreading activation in biological neural networks, particularly when impulses occur in rapid succession. As we evaluate models for intelligent system design, we’ll look for ways to model some of these complex capabilities of neurons, synapses and neurotransmitter in the human brain. The model will include both structures and functions. A structure is analogous to the components and their arrangement in a spark plug. The function is analogous to the spark in the spark plug. And, perhaps the most interesting part, is the way structure and function interact to elegantly deliver the desired outcome: a spark at the right time in the right place (the gap).
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