Reflex arc: characteristics, types and functions

This automatic and involuntary mechanism allows us to react quickly to stimuli.

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

There are different types of reflexes that help us from a very early age to survive and protect ourselves 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 close to the body.such as when we receive a strong blow or when a heat source is brought close to our body. Reflex movements are automatic and involuntary, since, unlike what happens with most of the nerve pathways, the sensitive neurons transmit the nerve impulses to the spinal cord, without reaching the brain, which allows 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 other types of neurons are involved (e.g. interneurons), we would be dealing with a compound reflex arc. Normally, reflex arcs are compound or polysynaptic; that is, their circuit is composed of several synaptic connections.

On the other hand, there are reflex arcs in the autonomic nervous system, the part of the organism in charge of controlling the involuntary functions of the body (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 conducting nerve impulses to the skeletal muscles to produce voluntary movements.

There are differences between the neural circuits of the reflex arc of the somatic system and the autonomic system, mainly in the efferent part of the reflex arc.There are differences between the neural 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, a ganglion is always present between the central nervous system and the effector organs, as opposed to 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 an evolutionary level, since it has been suggested that they are the primordial circuits from which the rest of the nervous structures of our body arose. Their value is undeniable, since without them we would not be able to face many dangerous daily 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 sensitive or afferent neurons, the motor or efferent neurons and the effector organs. Let us see what each of them consists of.

1. The receptors

The sensory receptors located in the different nerve endings and distributed throughout the body are responsible for transmitting the information received from the outside into nerve impulses. These receptors are made up of specialized neurons which are responsible for transforming stimuli according to their modality, whether visual, olfactory, auditory, gustatory or tactile (by touch, pain, temperature, etc.).

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

2. Sensitive or afferent neurons

Once the receptors have picked up the information from the outside, the sensitive or afferent neurons The spinal cord is responsible for collecting it and transmitting it to the nerve centers (the gray matter) of the spinal cord, the place where information is processed in order to elaborate the best response to environmental demands.They are the place where the information will be processed in order to elaborate the response that best adapts to environmental demands.

3. Motor or efferent neurons

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

The integrating nerve centers perform the function of connecting the sensory neurons with the motor organs. connecting the sensory neurons with the motor neurons, thus allowing the transmission of information.thus allowing the transmission of information from one part to the other and the consequent automatic response. The neurons in charge of this interconnection work are called interneurons.

4. Effector 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 centers 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 the cardiac muscle).

Functions

Most of the reflex arcs that exist in the human body are intended to prevent us from 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 get burned, through the thermoreceptors.

However, some of the primary reflexes we acquire at birth eventually disappear as we grow up. For example, the sucking reflex, which allows the child to feed itself and disappears at 4 months of age; or the moro reflex, which helps the baby to change position and protect itself against strident sounds, so necessary when we are newborns but dispensable after six months of life.

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 fulfill an essential adaptive function because they fulfill an essential adaptive function for the survival and conservation of the human species itself.

Classification of reflexes

There are several types of reflexes in the human body. Let us review them:

1. Innate or congenital reflexes.

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

2. Conditioned reflexes

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

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

3. Myotatic reflex

The myotatic or stretch reflex is produced when we stretch a muscle and this causes a contraction reaction opposite to the stretch. Perhaps the best known is the patellar reflex. which is usually explored in the doctor's office and consists of percussing the patellar tendon with a reflex hammer, with the aim of making the person respond with a sharp contraction of the quadriceps femoris muscle.

4. Medullary automatism reflex

This type of reflex is produced 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 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. Fundación Univ. San Pablo.
• 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.