The brain has a large group of cells that are referred to neurons. Neurons are the functional units of the brain which control all the neurological processes, including behavior and thoughts. The nervous system is made up of a group of interconnected neurons. The neurons usually communicate with each other through what are called synapses. The neurons use neurotransmitters to communicate across the synapses. The neurotransmitters are then released by only one neuron into the synapse which then binds to the rest of the neurons. The neurotransmitters can activate only the other neurotransmitters or cause them to be the opposite. A good example is the Dopamine which is a neurotransmitter that can activate neurons in different brain parts. It controls our thoughts and the way we make decisions. High levels of dopamine can create an avenue of thoughts and impulses to flow through the brain. This will lead to hallucinations and delusions.
Synaptic transmission, also known as neurotransmission is defined as an electric movement within synapses that is as a result of propagation of nerve impulses. Each nerve cell receives neurotransmitter from the terminal button to the dendrite. It then sends it back to other neurons which also do the same. This will create an energy wave until the point where the pulse has made its way across an organ. Nerve impulses are very important for the spread of signals. The signals are to be sent to and from the central nervous system through different neurons in order to create smooth, bodily secretions, skeletal and cardiac muscles and organ functions.
Synaptic transmission is also defined as the process where a neuron communicates with others such as muscle cell at a synapse. A neuron consists of dendrites, soma and axon. Electric signals being carried by axons are known to be action potentials. Axons have several terminal branches that end in a bulbous enlargement, the synaptic knob. It is at the synaptic knob that action potential is converted into a chemical message which interacts with the recipient neuron. This process is now what is known as synaptic transmission. Synapses are junctional complexes that are between membranes and postsynaptic membranes. Presynaptic is the side that transmits while postsynaptic is the receiving side. Action potentials that arrive at synaptic knobs trigger the release of neurotransmitter into the synaptic cleft. Released neurotransmitters usually diffuse across the narrow synaptic cleft. It is at the postsynaptic membrane that neurotransmitter molecules bind to the membrane bound receptor molecules with sites of recognition that are specific for that neurotransmitter. Electrical synapses involve presynaptic and postsynaptic membranes being partially fused. This will allow the action potential to cross from the membrane of a neuron to the next. This is without any intervention of a neurotransmitter. This type of synapse may transmit a signal in either direction.
Postsynaptic potentials are the changes that occur in the membrane potential of the postsynaptic terminal of a chemical synapse. They are graded potentials and initiate action potentials. They are as a result of presynaptic neuron which release neurotransmitters. The transmitters then bind to the receptors on the postsynaptic terminal. Postsynaptic receptors are always on the membrane of the postsynaptic cell. Receptors can react to being bound by a neurotransmitter by opening and closing an ion channel. This will allow ions to enter or leave the cell. If the opening of the ion channel will result into a net gain of negative charge, hyper polarization is going to occur. It is an inhibitory postsynaptic potential since it changes the charge across the membrane so as to be further from the firing threshold. However, neurotransmitters are not inherently excitatory or inhibitory.
Postsynaptic potentials will begin to be terminated the moment the neurotransmitter detaches itself from its receptor. The receptor can now return to its previous status. The ion channels opened by the receptor will then close. Then the membranes return to their equilibrium states and in the end the membrane goes back to its equilibrium potential. The neurotransmitter being bound to the receptor will trigger a postsynaptic response specific for that receptor. The responses can either be excitatory or inhibitory; it depends on the receptor properties (Hochschield, 1993). Receptors, if coupled to sodium or calcium channels, are excitatory and will produce a depolarization of the postsynaptic membrane. Receptors that are coupled to chloride or potassium channels are inhibitory and produce a hyperpolarization of the postsynaptic membrane. Such receptors when coupled to ion channels are known as ionotropic receptors. Other receptors are coupled to second messenger system which initiates biochemical reactions in the postsynaptic cell; they are metabotropic receptors.
Studies have shown that specific neuronal circuits and other neurotransmitters and hormones modulate impulsiveness and aggressiveness. Different areas of the brain help in the regulation of emotion and aggression related behavior. The Neuropeptide Y (NPY) and its Y receptor system are responsible for integrating aggressive behavior. It is expressed in the brain, including the cerebral cortex, the hypothalamus, the brainstem, the hippocampus and the septum. Central NPY systems involve in the regulation of feeding, growth, memory, regulation of blood pressure, nociception, gastrointestinal motility, reproductive behavior and physiology. The NPY is located in major brain structures responsible for the regulation of emotionality and aggression (Solomon, 1993). The effect on the postsynaptic cell depends on the properties of receptors. For example, for some neurotransmitters, the most essential receptors have excitatory effects. They increase the probability that the target cell will produce an action potential. However, for other neurotransmitters, the most important receptors have inhibitory effects. Other neurotransmitters have both the excitatory and inhibitory receptors existing. There are other receptors that activate metabolic pathways that are complex in the postsynaptic cell so as to produce effects that cannot be referred to excitatory or inhibitory.
The effects of a neurotransmitter system depend on two things; the connections of the neurons that use the transmitter and the chemical properties of the receptors that the transmitter is bound to. Here are some examples: Glutamate, used at many of the fast excitatory synapses in the brain. It is also used in the synapses that are capable of increasing or decreasing strength. GABA is used of fast inhibitory synapses in every part of the brain. Many sedative drugs have the effects of GABA. Acetylcholine is the transmitter that connects motor nerves to muscles. It operates in many regions of the brain by using different receptors. Serotonin on the other hand is a monoamine transmitter produced mainly in the intestine. It regulates sleep, appetite, memory and learning, behavior, mood, muscle contraction and many more (Ekman, 1993). Substance P is responsible for pain transmission from sensory neurons to the central nervous system. Neurons that express certain types of neurotransmitters form distinct systems. The activation of the system affects a large part of the brain. There are diseases that may affect neurotransmitter systems. An example is the Parkinson’s disease. It is related to the failure of dopaminergic cells in deep brain nuclei.