The automatic and involuntary responses of our body that occur in response to external stimuli (such as beatings or heat) they are produced by a nervous mechanism called a reflex arc.
There are different types of reflexes that help us from an early age to survive and protect us from the dangers of our environment. In this article, we explain what a reflex arc is, what are its main characteristics, structure and components, what 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, Like when we hit ourselves hard or when we bring a heat source closer to our body. Reflex movements are automatic and involuntary because, unlike most nerve pathways, sensory neurons transmit nerve impulses to the spinal cord, without reaching the brain, allowing for a faster and more efficient motor response.
Reflex arcs they can be of two types: simple or compound. If only one sensory neuron and another motor neuron are involved in the reflex arc process, we can speak of a simple reflex arc; instead, if there is another type of neuron involved (eg interneurons), we would be in front of a compound reflex arc. Reflex arcs are generally compound or polysynaptic; that is, its circuit consists 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 (viscera, heart rate, digestion, etc.) and in the somatic nervous system., Responsible for sending information from sensory receptors to the central nervous system, as well as directing nerve impulses to 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 (which is the one that controls the automatic and muscular responses); in the latter, between the central nervous system and the effector organs always mediates the presence of a ganglion, unlike what happens with the somatic efferent arch.
Through the reflex arches, our body sets in motion many nervous mechanisms and their existence seems to have been decisive at the evolutionary level, since it has been suggested that these are the original circuits from which the other nervous structures of our body originated. Their value is undeniable, because without them we would not be able to cope with many dangerous everyday situations that we face in our daily life.
Structure and components
A reflex arc is made up of different parts that work in an integrated and coordinated manner: receptors, sensory or afferent neurons, motor or efferent neurons, and effector organs. Let’s see what each of them consists of.
Sensory receptors located at 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 which are responsible for the transformation of stimuli according to their modality, whether visual, olfactory, auditory, taste or tactile (by grip, pain, temperature, etc.).
Among the most common receptors found in photoreceptors, cells responsible for detecting light intensity; thermoreceptors, responsible for detecting changes in heat and temperature; or mechanoreceptors, neurons that respond to mechanical pressure.
2. Oh afferent sensory neurons
Once the receptors have captured information from the outside, sensory or afferent neurons they are responsible for collecting it and transmitting it to the nerve centers (gray matter) of the spinal cord, The place where the information will be processed in order to be able to develop the response that best adapts to environmental requirements.
3. Motor or efferent neurons
Motor or efferent neurons conduct nerve impulses from orders that have been developed in the spinal cord and integrating nerve centers with effector organs that will produce the motor response.
The integrating nerve centers perform the function of connect sensory neurons to motors, Thus allowing the transmission of information from one party to the other and the consequent automatic response. The neurons responsible for this interconnection task are called interneurons.
4. Effector organs
The effector organs are the last component of the reflex arc. These are the structures responsible for carrying out the automatic and involuntary response that originates from the nerve centers of the spinal cord.. They come in different types: they can be exocrine glands (for example, salivary or sweat glands) and muscles (for example, skeletal muscles or heart muscle).
Most of the reflex arcs in the human body are aimed at preventing us or responding quickly and effectively to potentially dangerous situations. This is why they have been and are so necessary for our survival: they alert us in case of risk of exposure toxic elements, via olfactory receptors; or when we’re about to burn ourselves, through thermoreceptors.
However, some of the primary reflexes we have acquired at birth eventually go away as we get older. For example, the sucking reflex, which allows the child to feed and disappears at 4 months; or the dark reflex, which allows the baby to change posture more easily and to protect himself from high-pitched sounds, both necessary when we are babies and 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 unnecessary over time; and others stay for life because they perform an adaptive function essential to the survival and conservation of the human species itself.
Classification of reflexes
There are different types of reflexes in the human body. Let’s review the:
1. Innate or congenital reflexes
These are reflexes common to all human beings. They are also called unconditioned or absolute, and their main feature is that no prior learning is required to acquire themAs this is an innate mechanism that protects us from potentially harmful external conditions (for example, removing 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 previous learning and experiences with certain external situations and stimuli.
The best known is classical or Pavlovian conditioning, A type of learning where a stimulus of neutral value, which initially does not elicit a response, ends up producing automatic responses by association with another stimulus which normally elicits them.
3. Myotatic reflex
The myotatic or stretch reflex occurs when we stretch a muscle and causes a contraction reaction opposite to stretching. The best known, perhaps, is the patellar reflex which is usually explored during the medical consultation and consists of striking the patellar tendon with a reflex hammer, in order to respond with a sudden contraction of the femoral quadriceps.
4. Spinal cord automation reflex
This kind of thinking it happens when there is trauma and the spinal cord is injured. This disconnects 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.
- Castell, GD and de Jordi, JLV (2015). Anatomy and physiology of the central nervous system. Univ. St. Paul.
- Dewey, J. (1896). The concept of reflex arc in psychology. Psychological review, 3 (4), 357.
- Guyton, AC, Hall, JE, Zocchi, L. and Aicardi, G. (2006). Medical physiology (vol. 11). Madrid: Elsevier.