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

There are different types of reflexes that help us from a very young age to survive 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 components, the functions they perform, as well as the different types of reflexes that exist.

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 are hit hard or a heat source is brought closer to our body. Reflex movements are automatic and involuntary, since, unlike most nerve pathways, sensory neurons transmit nerve impulses to the spinal cord, without them reaching the brain, allowing a faster and more effective motor response.

Reflex arcs can be of two types: simple or compound . If only one sensory neuron and one motor neuron are involved in the reflex arc process, we can speak of a simple reflex arc; on the other hand, if there is another type of neuron involved (e.g. interneurons), we would be looking at a compound reflex arc. Normally, reflex arcs are compound or polysynaptic; that is, their circuitry is composed 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 (the viscera, heart rate, digestion, etc.) and in the somatic nervous system, responsible for sending information from the sensory receptors to the central nervous system, as well as for conducting nerve impulses to the skeletal muscles to produce voluntary movements.

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 controls the automatic and muscular responses); in the latter, between the central nervous system and the efferent organs there is always a ganglion, unlike the somatic efferent arc.

Through the reflex arcs our organism sets in motion numerous nervous mechanisms and their existence seems to have been decisive at the evolutionary level, since it has been suggested that they are the primitive circuits from which the rest of the nervous structures of our body emerged. 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.

Structure and components

A reflex arc is composed of different parts that function in an integrated and coordinated manner: the receptors, the sensory or afferent neurons, the motor or efferent neurons and the efferent organs. Let’s see what each of them consists of.

1. The receivers

The sensory receptors located at the various 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, whether it be visual, olfactory, auditory, gustatory or tactile (by means of 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 sensitive or afferent neurons are in charge of collecting it and transmitting it to the nerve centres (the grey substance) of the spinal cord , the place where the information will be processed in order to elaborate the response that best suits the environmental demands.

3. Motor or efferent neurons

The motor or efferent neurons conduct the nerve impulses of the commands that have been elaborated in the spinal cord and the integrating nerve centers towards the efferent organs that are going to produce the motor response.

Integrating nerve centres fulfil the function of connecting sensitive and motor neurons , thus allowing the transmission of information from one part to another and the consequent automatic response. The neurons that carry out this interconnection task are called interneurons.

4. Effecting organs

The effector organs constitute 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 centres of the spinal cord . There are different types: they can be exocrine glands (e.g. salivary glands or sweat glands) and muscles (e.g. skeletal muscles or heart muscle).

Functions

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

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

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

Classification of reflexes

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

1. Inborn or congenital reflexes

They are common reflexes in all human beings. They are also called unconditional or absolute, and their main characteristic is that it is not necessary to learn how to acquire them , since they are an innate mechanism that protects us from potentially harmful external conditions (e.g. the withdrawal of the hand when feeling a source of heat).

2. Conditioned reflexes

Conditioned reflexes are the opposite of innate reflexes; that is, they are acquired as a result of learning and previous experiences with 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 them.

3. Myotatic reflex

The myotathic or stretching reflex occurs when we stretch a muscle and it causes a contraction reaction opposite to stretching. The best known, perhaps, is the patellar reflex that is usually explored in the doctor’s office and consists of hitting the patellar tendon with a reflex hammer, with the aim of having the person respond with a sudden contraction of the femoral quadriceps muscle.

4. Reflex of medullary automatism

This type of reflex occurs when there is a trauma and the spinal cord is injured . The spinal cord is disconnected from the brain and the lower segment produces the reflex arc response. Some of these reflexes are also involved in bladder or rectal function, the return of muscle tone, or 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. St. Paul’s University Foundation.
  • 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.