The reflex arc is a neural pathway that is responsible for producing automatic and unconscious movements, known as reflex acts. Unlike most neural pathways, in almost all animal species, these do not pass through the brain. Rather, the responses are created in the spinal cord.
This allows reflex actions to take place much faster than more elaborate responses. Because of this, they are involved in situations in which survival or the absence of damage requires quick action. However, this also has some disadvantages.
The reflex arc, the neural structure that is responsible for carrying out these acts, can be more or less complex depending on which one we are talking about. Thus, some are known as simple reflex arcs, and others as compound ones. On the other hand, they can involve both internal and sensory organs.
The importance of reflex arcs is very high. In fact, some experts consider that they are the basis for the rest of the neural pathways in our organism, and that they were the first to develop evolutionarily speaking. In this article we will see how they work in depth.
Reflex arc components
Due to the importance they have for the survival and health of the organism, our body has a large number of different reflex arcs.
These differ from each other in some key respects. However, they also share certain common characteristics, among which the components that make them stand out.
In general, we can highlight the different parts within the reflex arcs: a sensory receptor, an afferent or sensory neuron, an efferent or motor neuron, an integrating center, and an effector organ. Furthermore, depending on what type of reflex arc we are talking about, it is also possible that there are interneurons.
Next we will see what each of these components consists of.
Sensory receptors are organs or structures that are responsible for transforming environmental information into nerve impulses, which can be interpreted by the Central Nervous System or CNS. There are basically two types: internal and external.
Internal sensory receptors collect information about the state in which the body itself is. Thus, they are in charge of transferring to the CNS data on components of the organism such as the digestive system, the state of the muscles, or the presence of internal pain in any other part.
On the other hand, external sensitive receptors are those involved in interpreting the information we receive from the environment. They are generally found in the sense organs, although they can also be located in other places. Depending on the stimulus they detect, they are given one name or another.
Thus, some of the most common receptor types are chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.
Afferent or sensory neuron
The second component of the reflex arc is the system that is responsible for collecting the information captured by the sensitive receptor and transmitting it to the spinal cord.
In simple reflex arcs, this role is performed by a single neuron; while in compound reflex arcs, there is a chain of neurons that perform this function.
The intermediate neurons that connect the afferent with the efferent and with the integrating centers, two of the components of the reflex arcs, are known as interneurons.
Efferent or motor neuron
The efferent neuron is the part of the reflex arc that is responsible for carrying the orders made in the spinal cord and the integrating centers to the organs that are going to carry out the response.
The integrating centers are the part of the reflex arc in which the afferent neurons connect with the efferent ones, making it possible to transmit information from one to the other and to carry out the automatic response. The neurons that are part of this component are known as interneurons.
The last component of the reflex arcs is the effector organ, that is, the structure that carries out the automatic response designed by the spinal cord. Depending on the type of reflex act we are talking about, the effector organ can be a gland, a smooth or skeletal muscle, or a heart muscle.
Depending on a number of characteristics, there are several types of reflex arcs. The two most important classifications are the division between simple and compound reflex arcs, and the division between autonomous and somatic arcs.
Simple bows vs. Compound bows
The difference between a simple reflex arc and a compound one is very easy to understand. In the first type, only one efferent and one afferent neuron mediate between the sensory organ and the effector organ. On the contrary, a series of interneurons also appear in the compounds, within the integrating centers.
Sometimes the name “monosynaptic” can also be found for simple reflex arcs, and “polysynaptic” for compounds. This nomenclature refers to the number of chemical synapses that exist in each of the groups.
In most cases, reflex arcs are compound or polysynaptic. In fact, only the simplest have only one neuron, such as the patellar reflex or the Achilles reflex.
Compounds have the advantage that they allow the response to be processed or inhibited using the brain if necessary.
Autonomous vs. Somatic arches
There are reflex arcs in both the autonomic and somatic nervous systems . Despite the fact that most of its components are practically the same, there are certain differences in the efferent part between the two. Specifically, in the autonomous system, this component is made up of two types of neurons.
The first neuron of the autonomic efferent arch is located in the medial-lateral nuclei in the gray matter of the spinal cord (specifically in the lateral horns), or in some autonomic nuclei in the brainstem. In any case, it is always located within the CNS.
The second efferent neuron of these reflex arcs is located on the periphery of the prevertebral, paravertebral, intraorganic, or preorganic autonomic ganglia. This means that between the CNS and the effector organ there is always a ganglion, this being the main difference with the other type of reflex arc.
Humans have a large number of different reflex arcs. Most of them take care of functions that are either necessary for our survival right now, or were important in the near evolutionary past, allowing our ancestors to survive and replicate successfully.
Due to this, most reflex arcs are related to dangerous situations, such as exposure to a harmful element or the presence of an uncontrollable situation. On the other hand, they can also have to do with preventing damage to some of our most important organs.
Sometimes, however, some reflex arcs no longer have a positive effect on our modern life. They are, therefore, simple vestiges of our evolutionary past, which no longer serve any specific function in human beings today.
Examples of reflexes in humans
Here are some examples of the most common reflex acts in our species.
The dilation or contraction of the pupils depending on the level of brightness of the environment is a reflex act designed to protect our retina from excessive amounts of light, which could damage it or even render it completely useless.
Involuntary movement when touching a hot or cold object
One of the most common examples of a reflex action is one that involves a rapid movement that forces us to remove any part of the body that comes into contact with a very intense heat source or an excessively cold element. The objective of this reflex arc is to avoid serious burns.
Coughs and sneezes
Coughs and sneezes are also involuntary reflex acts. Its function is to eliminate irritants either from our throat or from the nasal cavities. In addition, in the reflex action of sneezing there is also another involuntary movement, which causes us to close our eyes when doing it.
The grasp reflex belongs to the category of those that made sense in our evolutionary past but no longer serve any function today.
This reflex occurs in babies, and consists of the following: when a small child approaches a cylindrical element to his hands (such as a finger), he unconsciously grasps it with force.
In our past as a species, this reflex had the function of helping children to hold onto their mothers to avoid falling when held. The grasp reflex is shared by virtually every primate species in existence, and is in fact one of the most direct proofs of Darwinian theory.
One of the most studied reflexes in medicine is the movement that occurs in the leg when hitting the kneecap with a blunt object. The presence or absence of this movement can be used to diagnose certain types of neurological or brain damage.
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