Enrique Roche and Jonathan Jones. Bioengineering Institute, Univ. Miguel Hernández (Elche, Spain)
The nervous transmission is a type of communication between tissues, being faster than the hormonal system, which travels using either the bloodstream (endocrine effect) or through the intercellular space (paracrine and autocrine effects). The nervous transmission can be considered to be instant, as it transforms chemical energy (slow) into electric (fast). The nervous system is divided into 2 parts, both interconnected: the central and peripheral nervous systems. The central nervous system is composed of the brain and spinal cord, whereas the peripheral nervous system comprises the nerves that connect the central nervous system to their target organs. Both systems contain specialized cells that transmit the nervous impulse, called neurons. The messages that they transport require depolarization processes of the cell membrane and neurotransmitter release in the small interconnected space between neurons called synapse. The neurotransmitter is synthesized by the presynaptic neuron and stored in secretion vesicles. When a depolarization signal reaches the neuron, it releases the vesicles that travel to the postsynaptic neuron, thus producing the stimulus. The main neurotransmitters are the following:
|Biogenic amines||– Chatecolamines: dopamine, noradrenaline
|Aminoacids||– Excitatory: glutamate, aspartate.
– Inhibitory: GABA, glycine.
|Neuropeptides||– Endorphins: proencephalin, β-endorphin, dynorphin.
– Substance P
|Miscellaneous||– Gases: NO
– Purines: adenosine, ATP.
Therefore, it is a special intercellular communication system that greatly influences nutrition. In this manner, it controls the digestive system, the senses, appetite, hunger/satiety, and in general the individual´s behaviour towards food. Also, this control is not performed separately; all the functions are interconnected, modulating the nutritional state in an integrated fashion.
The nervous stimulation through the senses of taste and smell
The nervous transmission is performed using electric action potentials that begin with the presence of certain stimuli. The afferent neurons (those that detect the stimulus) initiate the stimulation by releasing a potential that originates in the sensorial receptors. The receptors connect the organism´s interior with the exterior. Here, the taste buds and olfactory epithelium are involved. The stimulus in the membrane receptor must reach a certain level in order to activate an action potential or nervous transmission. In this regard, the most determinant and instinctive stimulus in nutrition is through the intake of food, which activates taste and smell senses. The receptors in these sensorial systems are called chemoreceptors, since they respond to chemical agents that reach the mouth and nose. In fact, both cavities are connected, making the taste and smell of food to mix in an infinite amount of sensorial possibilities. It is for this reason that the scientific literature have defined the term “flavour”, referring to the combined effect of stimulating taste and smell. The chemoreceptors of these senses do not only affect the appetite, but also the secretion of saliva or gastric juices, as well as acceptance or rejection of certain substances that could be toxic for the individual. Therefore, the smell and taste of food play a vital role in the evolution of the human species, allowing for the selection of safer and tastier foods.