A synapse is a junction between which impulses are transmitted from one neuron to another. Transmission of impulses occurs through the release of neurotransmitters like serotonin. The synapses can also be described as the junctions at which neurons communicate to one another.
Those synapses that need the release of some chemical substance to be transmitted are called chemical synapses and these forms almost all forms of synapses in the body (Cowan and Kandel, 2001).
Mechanism of Synaptic Transmission
The following is the mechanism of synapse transmission. An action potential arrives at the synaptic knob causing opening of calcium pathways in the body, allowing a flow of calcium ions into the synaptic knob. The increased saturation of calcium cations causes the acetylcholine neurotransmitter harboring vesicles to proceed towards the presynaptic tissue where they merge and discharge what they contain into the synaptic cleft. Diffusing of the acetylcholine occurs across the opening, where they fit into definite receptor locations on the postsynaptic tissue. This diffusion process leads to a change in permeability of the postsynaptic tissue leading to the movement of sodium ions into the tissue thus creating an Excitatory Postsynaptic Potential charge, (EPSP)
When the EPSP attains its onset, it creates an action potential in the neuron, while at the same time, the acetylcholine is being broken down in the synaptic cleft by enzyme acetylcholinesterase. The synaptic knob reabsorbs the broken down products (Cowan and Kandel,2001).
It is important to note that the resting potential there is a charge across the axon membrane that occurs when ions are unequally distributed across the membrane. The axon membrane is positively charged in the outside while in inside it is negatively charged. Therefore after a stimulus occurs the gates of sodium open causing movement of the sodium ions inward causing a reversal of polarity. After this, the gates close and the resting potential is restored by the pump mechanism. The receipt of an action potential in the synaptic knob promotes the movement of calcium ions into the knob triggering the process of transmission of neurotransmitters (Cowan and Kandel,2001).
Key features of neurotransmitters
According to Ayano, (2016), neurotransmitters are chemical messengers which are endogenous chemicals that aid neurotransmission. The neurotransmitters send signals across a chemical synapse from one neuron to another. The synaptic vesicles release the neurotransmitters in synapses into the synaptic cleft where the target cells receive them.
Another feature of neurotransmitters is that mostly they are synthesized from simple origins like the amino acids that are easily found in diet and their biosynthetic steps for conversion are simple. Indeed, the neurotransmitters shape people's lives and functions since they are brain chemicals that relay information throughout the body and brain.
The neurotransmitters can either be excitatory or inhibitory. The excitatory neurotransmitters are what stimulates the brain while the inhibitory ones are those neurotransmitters that calm the brain and in the process, they can balance someone's mood, and they tend to be depleted when the excitatory neurotransmitters go haywire.
Neurotransmitters are known to be made inactive by the degradation of enzymes. For example, acetylcholine is degraded by an enzyme called acetylcholinesterase.
Mostly, neurotransmitters are stored in vesicles however it has been discovered that gases such as carbon dioxide and nitrogen monoxide can act as neurotransmitters and therefore not all are stored in vesicles. Not all neurotransmitters are synthesized in neurons, for instance, we have Serine that the protoplasmic astrocytes that surround synapses synthesize
How pain is encoded in the brain
Pain is an important function in the body since it has a role of warning the body of potential harm or injury. Description of pain is either by emotions or senses. Encoding of pain by the brain is a complex process which is affected by many factors like cognition, mood or even genetics. Indeed, pain is a biopsychosocial phenomenon that is brought about by many neuroanatomic and neurochemical systems interacting with many cognitive and affective processes. It is a sensory that is unpleasant or an emotional experience that is associated with real or potential tissue damage. It, therefore, means that pain has sensory, effective and cognitive components that help in anticipation of future harm (Garland, 2012).
Pain Encoding
Garland (2012), postulates that when a harmful external or internal stimuli impact upon the body, information regarding the damaging effect of this stimulus on the tissues of the body is relayed through neuro pathways and transmitted to the central and autonomic nervous system through the peripheral nervous system. We call these relaying of information nociception. Therefore nociception is the processes through which a harmful of noxious stimulus is relayed to the brain, and this process is aided by special receptors that are called nociceptors.
The mechanical stimulation of the tissues by either cutting, stretching of pinching or the exposure to harmful chemicals on the body leads to the activation of the nociceptors. The perception of pain occurs when the intensity of nociceptors is enough to activate the Aδ fibers which lead to experiencing sharp pain. If the strength of the pain stimulus increases, the C fibers of the brain are engaged and therefore the person can experience pain even when what is causing the pain has stopped (Apkarian, Hashmi and Baliki, 2011).
Pain encoding happens in two phases whereby the first phase which is not such intense is called the fast pain, and the slow pain that is characterized by unpleasantness is the second phase.
When the nociceptors have been activated, they are relayed along the axons by the nerves that we call the peripheral nerves. The peripheral nerves end at the dorsal horn of the spine. The pain messages are then transduced up the spinal cord via the spinothalamic tract to the thalamus. Consequently, the thalamus relays the message to the cerebral cortex. The nociceptive pathways end at separate subdivisions of thalamic nuclei that are either the posterior lateral nucleus or ventromedial nucleus. The information is then transduced from the nuclei to different cortical and subcortical regions like the hypothalamus of the cerebral cortex (Nestler, Hyman and Malenka, 2001).
References
Apkarian, A. V., Hashmi, J. A., " Baliki, M. N. (2011). Pain and the brain: specificity and plasticity of the brain in chronic clinical pain. Pain, 152(3 Suppl), S49.
Ayano, G. (2016). Common neurotransmitters: Criteria for neurotransmitters, key locations, classifications, and functions. APN, 1(1), 1-5.
Cowan, W. M., " Kandel, E. R. (2001). A brief history of synapses and synaptic transmission. Synapses, 1-87.
Garland, E. L. (2012). Pain processing in the human nervous system: a selective review of nociceptive and biobehavioral pathways. Primary Care: Clinics in Office Practice, 39(3), 561-571.
Nestler, E. J., Hyman, S. E., " Malenka, R. C. (2001). Molecular basis of neuropharmacology: a foundation for clinical neuroscience. East Norwalk, Conn: Appleton " Lange.
