Reflex arc: characteristics, types and functions

The automatic and involuntary responses of our body that arise as a reaction to external stimuli (such as blows or heat) are produced thanks to a nervous mechanism called reflex arc.

There are different types of reflexes that help us survive from a very young age and protect us from the dangers of our environment. In this article we explain what a reflex arc is, what its main characteristics are, its structure and its components, the functions they perform, as well as the different types of reflections that exist.

• Related article: "Peripheral nervous system (autonomic and somatic): parts and functions"

Reflex arc: definition and characteristics

The reflex arc is a neurophysiological mechanism of the nervous system that is activated in response to an external stimulus, such as when we hit ourselves hard or a source of heat is brought close to the body. Reflex movements are automatic and involuntary, since, unlike what happens with most nerve pathways, sensory neurons transmit nerve impulses to the

spinal cord, without reaching the brain, allowing a faster and more effective motor response.

The reflex arcs They can be of two types: simple or compound. If only one sensory neuron and another motor neuron intervene in the reflex arc process, we can speak of a simple reflex arc; on the other hand, if there are other types of neurons involved (p. ex. interneurons) we would be facing a compound reflex arc. Typically, reflex arcs are compound or polysynaptic; that is, its circuit is made up of several synaptic connections.

On the other hand, there are reflex arcs in the autonomic nervous system, the part of the body responsible for controlling the involuntary functions of the body. body (the viscera, heart rate, digestion, etc.) and in the somatic nervous system, responsible for sending the information from the receptors sensory to the central nervous system, as well as conducting nerve impulses to skeletal muscles to produce movements volunteers.

There are differences between the neuronal circuits of the reflex arc of the somatic system and the autonomic system, mainly in the efferent part (which is the one that controls the automatic and muscular responses); In the latter, the presence of a ganglion always mediates between the central nervous system and the effector organs, contrary to what occurs with the somatic efferent arc.

Through reflex arcs, our body sets in motion numerous nervous mechanisms and their existence seems to have been a determining factor in evolutionary level, since it has been suggested that they are the original circuits from which the rest of the nervous structures of our Body. Their value is undeniable, since without them we would not be able to face many dangerous everyday situations that we face in our day to day life.

• You may be interested: "Types of neurons: characteristics and functions"

Structure and components

A reflex arc is made up of different parts that work in an integrated and coordinated way: the receptors, sensory or afferent neurons, motor or efferent neurons, and organs effectors. Let's see what each of them consist of.

1. The receivers

The sensory receptors located in the different nerve endings and distributed throughout the body are responsible for transmitting the information they receive from the outside in nerve impulses. These receptors are made up of specialized neurons that are in charge of transforming the stimuli according to their modality, be it visual, olfactory, auditory, gustatory or tactile (by grip, pain, temperature, etc.)

Among the most common receptors we can find photoreceptors, the cells in charge of detecting light intensity; thermoreceptors, responsible for detecting heat and temperature changes; or mechanoreceptors, neurons that react to mechanical pressure.

2. Sensory or afferent neurons

Once the receptors have captured the information from the outside, the sensory or afferent neurons are responsible for collecting it and transmitting it to the nerve centers (the gray matter) of the spinal cord, the place where the information is going to be processed in order to elaborate the answer that best adapts to environmental demands.

3. Motor or efferent neurons

The motor or efferent neurons conduct the nerve impulses of the orders that have been elaborated in the spinal cord and integrating nerve centers to the effector organs that will produce the response motorboat.

The integrating nerve centers fulfill the function of connect sensory neurons with motor neurons, thus allowing the transmission of information from one party to another and the consequent automatic response. The neurons that are responsible for this interconnection work are called interneurons.

4. Effector organs

The effector organs are the last component of the reflex arc. They are the structures in charge of executing the automatic and involuntary response that comes from the nerve centers of the spinal cord.. There are different types: they can be exocrine glands (p. ex. salivary glands or sweat glands) and muscles (p. ex. skeletal muscles or heart muscle).

Features

Most of the reflex arcs that exist in the human body aim to prevent us or respond quickly and effectively to potentially dangerous situations. For this reason they have been and are so necessary for our survival: alert us when there is a risk of exposure to toxic elements, through smell receptors; or when we are about to burn, through the thermoreceptors.

However, some of the primary reflexes that we acquire at birth end up disappearing as we grow older. For example, the sucking reflex, which allows the child to feed and disappears at 4 months; or the Moorish reflex, which makes it easier for the baby to change position and protect himself against sounds strident, so necessary when we are newborns as dispensable from six months of life.

In short, there are different types of reflections with different functions; some are necessary from birth and become dispensable over time; and others remain for life because they fulfill an adaptive function essential for the survival and conservation of the human species itself.

Reflex classification

In the human body there are various types of reflexes. Let's review them:

1. Innate or congenital reflexes

They are common reflexes in all human beings. They are also called unconditional or absolute, and their main characteristic is that no prior learning is necessary to acquire them, since they are an innate mechanism that protects us from potentially harmful external conditions (p. ex. the withdrawal of the hand when feeling a source of heat).

2. Conditioned reflexes

Conditioned reflexes are the opposite of innate ones; that is, they are acquired as a result of learning and previous experiences in certain situations and external stimuli.

The best known is classical or Pavlovian conditioning, a type of learning according to which a stimulus with a neutral value, which initially does not provoke any response, ends up producing automatic responses by association with another stimulus that normally provokes.

3. Myotatic reflex

The stretch reflex occurs when we stretch a muscle and the muscle causes a contraction reaction opposite to stretching. The best known, perhaps, is the knee jerk reflex. that is usually explored in the medical consultation and consists of percussion of the patellar tendon with a hammer reflexes, with the aim that the person responds with a sudden contraction of the quadriceps muscle femoral.

4. Reflex of spinal automatism

This kind of reflection occurs when there is trauma and the spinal cord is injured. This is disconnected from the brain and the lower segment produces the reflex arc response. Some of these reflexes also intervene in the functioning of the bladder or rectum, in the reappearance of muscle tone or in the performance of certain involuntary movements.

Bibliographic references:

• Castillo, G. D., & de Jorge, J. L. V. (2015). Anatomy and Physiology of the central nervous system. Univ Foundation. Saint Paul.
• Dewey, J. (1896). The reflex arc concept in psychology. Psychological review, 3 (4), 357.
• Guyton, A. C., Hall, J. E., Zocchi, L., & Aicardi, G. (2006). Medical Physiology (Vol. 11). Madrid: Elsevier.