Reflex arc


A receptor, a sensory neuron, a motor neuron, The motor neuron’s innervation of the peripheral nervous system, and at least one association neuron from the central nervous system are the ingredients to a conduction passageway of the reflex arc. The reflex arc is ingeniously designed to help keep the body safe, as it is the mechanism that permits immediate responses to danger related stimulus.

There are individual and specified pathways laid out which are designed to provide special routes for the appropriate impulses to travel through the nervous system. Most nerve pathways originate with the impulses which are conducted to the central nervous system via sensory receptors and sensory neurons that belong to the peripheral nervous system. The central nervous system is then able to send the appropriate motor impulses to the necessary smooth muscles, skeletal muscles, and glands in order to command an instant response. Simultaneous impulses might be sent through the ascending tracts within the spinal cord as the central nervous system designates necessary.


A reflex arc is in fact the simplest and most primitive nerve pathway in the human body. A reflex arc is designed to administer unconscious automatic actions which are determined to be protective in nature to keep a body in homeostasis. There is a very short route for the sensory neurons to travel to reach the motor neurons. To maintain simplicity, only two or three neurons are involved in a reflex arc.

The reflex arc is made up of five basic elements. These include the receptor, the sensory neuron, center, the motor neuron, and the effector. The receptor hosts the dendrite of the sensory neuron as well as the initiation of the entire process, the initiation of nerve impulse. The sensory neuron is responsible for relaying the impulse through the posterior root of the central nervous system, where the center is located.

One or two interneurons, or association neurons are involved in this process. At this point, the arc is made, and the process of synapse permits other parts of the body to receive the necessary impulses. The effector organ, which is almost always a skeletal muscle, then receive the required information through the impulses sent via a motor neuron. The response initiated by the effector organ is known as a reflex action. This is what most people refer to as reflexes.


Autonomic reflex arc
Image: Autonomic Reflex Arc



Visceral reflexes (also known as autonomic reflexes) refer to reflexes such as those which affect the gland secretion or smooth muscle of the cardiac system to contract. The main purpose of the visceral reflexes is to ensure that the involuntary process of the body are in full operating condition and can be tailored to react to a given situation. Heart rate, respiratory rate, blood flow, and digestion are just examples of the types of visceral reflexes that require constant monitoring by the internal body systems. Reflexes like coughing, swallowing, sneezing, or vomiting are considered to autonomic, however, these reflexes require at least some participation of the skeletal muscles.


Somatic reflexes refer to those that include some participation of the skeletal muscles. Somatic reflexes are segregated into three different types, each named after the response that they produce. For instance, the stretch reflex requires only the participation of two neurons and one synapse in the reflexive pathway. Thus, this is referred to as a monosynaptic reflex arc. Neuromuscular spindles initiate a minor stretching within their receptors.

A sensory neuron along the spinal column picks up this stretching, and a synapse occurs within the gray column (anterior) with a motor neuron. The motor unit that corresponds with all this stretching is then activated, and specified muscle fibers automatically contract to cause stretching. The receptors and the effector organs that are involved in this action are situated along the same side of the spinal column, and thus this particular reflex receives the term ipsilateral arc. An additional example of an ipsilateral arc is the knee-jerk reflex.

A withdrawal reflex, also known as a flexor reflex, is a polysynaptic reflex arc. Sensory and motor neurons are part of this reflex, as are association neurons. Pain is the most obvious stimuli that initiates the flexor reflex, as painful stimuli sets the whole reflex into action.

The receptor responds and sends the information via sensory neurons. It reaches the spinal cord, and here, the association neurons take over the situation. Motor neurons receive the information and immediately the muscles involved contract to produce a withdrawing snap action. Additionally and simultaneously, the antagonistic muscles relax, otherwise they may inhibit the body’s ability to snap the muscles into withdraw. It is not uncommon for the body to initiate additional reflexes along with the flexor reflex.


Somatic reflex arc
Image: Somatic Reflex Arc

When numerous spinal cord segments are activated via impulses coming from the receptor organs, and the release of multiple motor neurons is required, this is known as an intersegmental reflex arc. This means that multiple effector organs are initiated and multiple receptor organs sent signals. The required sensory impulses from a receptor organ may very well cross the spinal cord in order to stimulate an effector organ on the opposite side of the body. This is known as a contralateral action. This action is necessary for maintaining balance during a flexor response, and is known as a crossed extensor reflex.

For example, there is ample crossing of information when a person steps on broken glass, the opposite side of the body takes over to maintain the body’s balance while the foot is withdrawn. Additionally, the body will often inhibit other muscles during such reflexes. This is known as reciprocal inhibition, and it is designed to further maintain the body’s balance. Some reflexes are designed for maintaining physiological function, while there are numerous reflexes that are designed to help the body avoid injury, or prevent further injury.
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