The automatic and involuntary responses of our body that arise as a reaction to external stimuli (such as shock or heat) They 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 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 reflections that exist.
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 bring a heat source close to our body. Reflex movements are automatic and involuntary, since, unlike what happens with a large part of the nervous pathways, sensory neurons transmit nervous impulses to the spinal cord, without them reaching the brain, which allows a more rapid motor response. fast and effective.
The 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; On the other hand, if there are other types of neurons involved (e.g. interneurons) we would be faced with 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 (the 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 conducting 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 what controls 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 the reflex arcs, our body 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 emerged. Their value is undeniable, since without them we would not be able to face many dangerous everyday situations that we face in our daily lives.
Structure and components
A reflex arc is made up of different parts that function 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.
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 nervous impulses. These receptors are made up of specialized neurons which are responsible for transforming stimuli depending on their modality, whether visual, olfactory, auditory, gustatory or tactile (due to grasping, 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. Sensory or afferent neurons
Once the receptors have captured the information from the outside, the sensory or afferent neurons They are responsible for collecting it and transmitting it to the nervous centers (the gray matter) of the spinal cord the place where the information will be processed in order to develop the response that best adapts to environmental demands.
The motor or efferent neurons conduct the nervous impulses of the orders that have been prepared in the spinal cord and the integrating nerve centers to the effector organs that will produce the motor response.
The integrative nervous centers fulfill the function of connect sensory neurons to 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 constitute the last component of the reflex arc. They are the structures responsible for executing the automatic and involuntary response that comes from the nervous 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 cardiac muscle).
Features
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. For this reason 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 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. For example, the sucking reflex, which allows the child to feed itself and disappears at 4 months; or the Moro reflex, which makes it easier for the baby to change position and protect itself from loud sounds, as necessary when we are newborns as it is dispensable after 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 essential adaptive function for the survival and conservation of the human species itself.
There are various types of reflexes in the human body. Let’s review them:
1. Innate or congenital reflexes
They are common reflexes in all human beings. They are also called unconditioned 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 (e.g. withdrawing 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 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 myotatic or stretch reflex occurs when we stretch a muscle and it causes a contraction reaction opposite to stretching. The best known, perhaps, is the knee jerk reflex which is usually explored in the medical consultation and consists of striking the patellar tendon with a reflex hammer, with the aim that the person responds with a sudden contraction of the quadriceps femoris muscle.
4. Spinal automatism reflex
This type of reflection It 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 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.
