The Reflex Arc

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Introduction

Our body performs many actions that are not under conscious control. The nervous system of our body detects certain stimuli and performs a response action before we become aware of the stimulus. Reflex action is one example of such responses.

A reflex action is an involuntary response to a stimulus, mostly a pain stimulus, that takes place instantaneously. The action is performed by the components of the peripheral nervous system. Once the action is performed, information is said to the higher areas of the brain so that a person becomes aware of what has happened.

The pathway followed by a nerve impulse during a reflex action is called the reflex arc. In this article, we will discuss the components of the reflex arc, some examples of reflex actions, its importance, and associated clinical conditions.

Components

The reflex arc is a pathway along which the information about the stimulus is carried to the spinal cord and the information regarding the response is carried to the effector organ. A reflex arc consists of the following components.

Receptor

It is the first component of the reflex arc. A receptor serves to detect the stimulus that is any change in the external environment. Certain stimuli require some instantaneous response by the effector organ. Such stimuli are rapidly detected by receptors and an action potential is generated.

Receptors involved in the reflex arc have different structures and properties. Some of the common receptors involved are as follows;

  • Pain receptors: They are the free nerve endings of sensory neurons
  • Thermoreceptors: These include specialized heat or cold receptors.
  • Mechanoreceptors: These include specialized muscle endings like muscle spindle and Golgi tendon organ.

Sensory Neuron

It is the second component of the reflex arc. The action potential generated by the receptor is carried to the spinal cord via the sensory neuron.

These neurons have cell bodies in the posterior root ganglion. The peripheral process of such neurons is long and make connections with the receptor or have a specialized nerve ending that itself acts as a receptor. The central process is small and synapses with the interneurons present in the spinal cord.

Interneuron

It is also called the relay neuron. Interneurons are present in the white matter of the spinal cord. They receive information from the sensory neuron, process this information within a few milliseconds, and decide about the response.

The information regarding the response is fed to the effector neuron to be transmitted to the effector organ.

Motor Neuron

Such neurons are also called effector neurons. They are responsible for carrying the nerve impulses to the effector organ.

The cell bodies of such neurons are located in the white matter of the spinal cord. They have a single large axon that carries the impulses from the interneuron in the spinal cord to the effector organ located at a distance.

Effector Organ

An effector organ is the one that acts in response to the stimulus. In the case of the reflex arc, the effector organ is almost always a muscle fiber or a group of muscle fibers.

It is innervated by a motor neuron carrying information about the response.

In most of the cases, three neurons are involved in a reflex arc. However, sometimes it might have only two neurons i.e. a sensory neuron and an effector neuron.

Types of Reflexes

Based on the number of neurons involved, reflex action can be of the following types.

  • Monosynaptic reflexes: Only one synapse is present in the reflex arc. These reflexes involve a sensory neuron and an effector neuron. Peripheral muscle reflexes like the patellar reflex, brachial reflex are monosynaptic reflexes
  • Polysynaptic reflexes: The include more than one chemical synapse in the reflex arc. They involve at least three neurons in the reflex action. Withdrawal reflexes are examples of polysynaptic reflexes.

Having understood the components and types of reflex actions, let us now discuss some spinal reflexes as examples of reflex actions.

Withdrawal Reflex

The purpose of the withdrawal reflex is to protect the body from harmful stimuli such as heat or pain. It is activated once a pain stimulus is perceived by the receptors. It is also called flexor withdrawal reflex or nociceptive reflex.

It is a polysynaptic reflex involving multiple synapses and neurons. The reflex arc in the case of a withdrawal reflex involves the following.

Receptor

Two types of receptors may be involved;

  • Thermoreceptors
  • Nociceptors

Thermoreceptors detect the changes in the surrounding temperature. They are the specialized nerve cells located in the skin. The convert any change in temperature to the action potential.

Nociceptors are the specialized free nerve endings that can perceive different damaging pain stimuli. These receptors may be stimulated by any thermal, mechanical, or chemical stimulation.

Neurons

Being a polysynaptic reflex,  it involves sensory, association, and motor neurons.

The sensory neuron carries the information regarding the pain stimulus to the association neuron.

The association neuron processes this information and passes it to the motor neurons.

The motor neuron carries this information regarding the response to the effector organ.

Effector Organ

The effector organ is a flexor muscle. When it is stimulated by a motor neuron, the muscle contracts and the body is flexed, moving it away from the stimulus.

The extensor muscles do not take part in this kind of response.

Example

An example of withdrawal reflex is when you accidentally touch a hot object.

  • The high temperature of the object and the pain caused by it is detected by the receptors.
  • The information regarding this pain is fed to the interneurons of the spinal cord.
  • The interneurons process this information and generate impulse for the flexor muscles to contract.
  • This message is carried by a motor neuron to the flexor muscles.
  • The flexor muscles are excited and the arm is pulled away from the damaging stimulus.

This entire process is completed in only a fraction of a second. Meanwhile, some signals are also sent to the higher levels of the brain so that the body becomes aware of what has happened.

Importance

The purpose of this kind of reflex is to save the body from the harms of the damaging stimulus. The painful stimulus is immediately detected and the body is moved away from it.

In the case of drunk or drugged people, the withdrawal reflex is not active. This is the reason why such people don’t exhibit any response to a painful stimulus.

The withdrawal reflex might be intentionally inhibited during some medical procedures.

Stretch Reflex

This reflex is activated once the skeletal muscle is stretched. The purpose of this reflex is to maintain the length of the skeletal muscle by causing it to contract.

It is a monosynaptic reflex and involves a sensory neuron and a motor neuron. The reflex arc in this case includes the following.

Muscle Spindle

Stretch reflex involves muscle spindles as receptors to detect any stretch force applied to the skeletal muscle. Each muscle spindle consists of 3 to 12 intrafusal fibers in the center of the muscle. These fibers have pointed ends that are connected via glycocalyx to extrafusal muscle fibers.

The intrafusal fibers are short skeletal fibers. However, the central area of these fibers has no contractile proteins. It does not contract with the ends of the fibers. It serves as the sensory receptor.

The muscle spindle can be activated in two ways;

  1. If the whole muscle is stretched, it stretches the central portion of the spindle and the spindle is excited.
  2. If only the ends of the intrafusal fibers are stretched, the central part gets stimulated.

Neurons

Only two neurons are involved in the stretch reflex.

The sensory neurons have a cell body located in the posterior root ganglion. The peripheral fiber surrounds the receptor portion of the muscle spindle. The central fiber enters the spinal cord and synapses with the alpha motor neuron.

The axon of alpha motor neurons leaves the spinal cord and excites the skeletal muscle fibers.

Neural Circuit

The reflex arc of this reflex action is simple. The muscle spindle gets excited when the muscle is stretched. The nerve impulse thus generated is carried by the sensory neuron to the spinal cord. Here, the alpha motor neuron is excited by the sensory neuron. The axon of motor neuron excites the muscle and causes it to contract.

The synergistic muscles are also stimulated and contract along with the muscle being stretched.

Sensory neuron also sends signals to the brain regarding the contraction of muscle.          

Importance

The main purpose of the stretch reflex is to prevent the tearing of muscle due to stretching. The stretch force is detected and the muscle is stimulated to undergo contraction so that excessive stretching does not occur.

Stretch reflex also serves to provide important feedback information during movements.

Stretch reflex has clinical importance too. It is used to assess the background tone sent by the brain to the spinal cord. Different stretch reflexes are performed during the normal medical examination of a person such as knee jerk, etc.

Golgi Tendon Reflex

Another example of reflex action and the reflex arc is the Golgi tendon reflex. It detects the changes in muscle tension. The muscle contraction is inhibited if the tension in the muscle increases a certain limit.

Golgi tendon reflex is an inhibitory reflex involving two synapses. It is a polysynaptic reflex. The important components of the reflex arc are the Golgi tendon organ and the neurons.

Golgi Tendon Organ

These are the encapsulated receptors present in the muscle tendons. The fibers of skeletal muscle tendons pass through these receptors. Each receptor is linked with 10 to 15 tendon fibers.

The Golgi tendon organ gets excited when tension develops in the tendon fibers.

Neurons

The sensory neuron attached to the Golgi tendon organ synapses has its cell body in the posterior root ganglion.

The central process of the sensory neuron synapses with the inhibitory neurons in the spinal cord.

The inhibitory neurons synapse with the alpha motor neuron and inhibit them.

Neural Circuit

When tension in muscle increases, the Golgi tendon organ sends negative feedback to the interneurons. These inhibitory interneurons inhibit the alpha motor neurons to cause muscle relaxation.

If the tension in the muscle becomes extreme, the inhibitory effect of this reflex becomes so potent that the muscle becomes relaxed instantaneously.

During this process, information is also sent to the higher centers of the brain.  

Importance

The Golgi tendon reflexes have two main functions.

  1. They prevent the development of excesisve tension in the muscle and the possible tearing by inhibiting muscle contraction.
  2. They also serve to equalize the contractile forces among the muscle fibers. the fibers exerting excess force become inhibited due to increased tension whereas the fibers exerting little force become more excited because of the absence of the reflex inhibition.

Summary

Reflex action is an involuntary response to a stimulus that is manifested at the level of the spinal cord. The pathway followed by a nerve impulse during this action is called the reflex arc.

The components of a reflex arc are;

  • A receptor to detect the stimulus
  • A sensory neuron to carry the information towards the spinal cord
  • An interneuron to process the information (not present in all reflexes)
  • An effector neuron to carry signals to the effector organ
  • An effector organ that performs the response action

Reflexes that involve one synapse are monosynaptic while those that involve two or more synapses are polysynaptic.

Withdrawal reflex is an example of polysynaptic reflex action.

  • The pain stimulus is detected and the body is moved away from it
  • Receptors include nociceptors and thermoreceptors
  • It is not active when a person is drunk or is on drugs

Stretch reflex is a monosynaptic reflex.

  • The change in muscle length is detected and the muscle is con6tracted to prevent any damage
  • The muscle spindles are the receptors
  • It is performed during a clinical examination to assess the background contractile tone

Golgi tendon reflex is a polysynaptic reflex.

  • The tension in the muscle tendons is detected and the inhibitory signals are sent to the muscle to relax
  • Golgi tendon organs are the receptors
  • The interneurons are inhibitory

References

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