Spinal reflex

Receptors that provoke the reflex

The physiology offers an answer as to why these reflexes happen. Well, proprioceptors are located in the muscles, tendons, and ligaments. These specific receptors detect information about the length of the muscle, the tension of the ligaments, the level of the tendon stretching, etc. Specifically, within the skeletal muscles, the receptors are known as muscle spindles, within the joints are the Ruffini corpuscles and free nerve ends, whereas in the ligaments and tendons are the Golgi tendon organs.

All receptors mentioned above inform the central nervous system about the position of the limbs, the strength and the speed of the muscle contraction, and feedback information necessary for the control of movements. The impression that the somatosensory cortex gets after receiving all of this information is actually the awareness of the body’s position in space, which is called kinesthesia. 

Muscle spindle

This is a special type of sensory receptor that is located in the muscle. These receptors are mechanoreceptors and detect a change in muscle length. To make that possible, they are placed parallel to the muscle fibers, so when the fibers change in length, the spindle does as well.
 
Muscle spindles consist of functionally different central and peripheral parts. The central part is sensory where it is sensitive to stretching. On the other hand, the peripheral part is contractile and is innervated by the gamma-motor neuron.
 
The spindles are constantly exposed to the muscle’s environment and with the afferent projections of the sensory neurons which innervate the spindles, they communicate with the alpha-motor neurons of the spinal cord that innervate the muscle fibers. Simply speaking, the muscle fiber is a teenager and the muscle spindle is a friend that constantly witnesses what the fiber is doing and is informing the parent. The parent in this analogy is the alpha motor neuron where it is informed about what the muscle fiber experiences in order for the alpha motor neuron to take action if needed. 

Gamma motor neuron

As previously stated, the peripheral portions of the muscle spindles are innervated with the efferent gamma motor neurons. When these neurons activate, the muscle spindles contract. The relationship between the gamma motor neuron and the muscle spindle is analogous to the relationship between the alpha motor neuron and the skeletal muscle fiber.

So, how do all of these structures function grossly? Well, when the muscle is stretched, the muscle spindles stretch too. Since it is innervated with the afferent neuron (neuron I of the reflex arc), that information goes to the ventral horn of the spinal cord to inform the proper alpha motor neuron. Then, the alpha motor neuron sends the signal that causes the stretched muscle to contract.

Golgi tendon organ

The Golgi tendon organ is located at the junction of the muscle fiber and tendon where it registers the tension of the muscle during contraction. This receptor stretches and excites during muscle contraction since the organ is serially connected with the muscle fibers. When this occurs, the afferent fiber that innervates the receptor sends signals to the inhibitory interneurons of the spinal cord and informs them that tension in the muscle is too high. These interneurons synapse with the alpha motor neurons that innervate the contracted muscle and block them so that they can no longer send excitatory signals.

After this, the muscle relaxes. Because this reflex is opposite to the monosynaptic stretching reflex, it is most literally the inverse stretching reflex. This reflex is protective because it prevents the potential tearing of the muscle and the tendons during excessive muscle contractions.

Regulatory mechanisms

Since polysynaptic reflexes involve entire muscle groups, it is necessary for the organism to know when to stop the execution of movement and how to isolate the muscle group that needs to be activated from other groups. That is made possible through interneuron connections that take part in polysynaptic reflexes and their ability to provoke two processes:

• Recurrent inhibition
• Reciprocal inhibition

Recurrent inhibition

Let's quickly recall how the spinal reflex arc looks like: receptor – sensory fiber – (interneuron) – alpha motor neuron – muscle. With polysynaptic reflexes, there are some additional details since interneurons are involved. The complexity happens at the spinal cord level and involves alpha motor neurons and interneurons.
 
Axons of alpha motor neurons that leave the spinal cord and course towards the muscle give rise to a collateral branch that goes back to the ventral horns of the spinal cord and synapses with the inhibitory interneurons called the Renshaw cells. This cell then synapses with the same alpha motor neuron that initially sent the collateral branch and inhibits it. In this way, negative feedback is realized as a mechanism of control.

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Reciprocal inhibition

The monosynaptic stretching reflex and polysynaptic inverse stretching reflex are the two basic units of the spinal motor actions. These reflexes do not generate isolated patterns, but instead,  are executed to coordinate specific muscle groups. This coordination is realized through the third component of the spinal motor process that is called the reciprocal inhibition.
 
For execution of planned movement, it is necessary for the synergistic muscles to activate while simultaneously inhibiting antagonistic muscles. Activation of synergists is a part of voluntary movement control, but interest remains with the involuntary antagonists' inhibition realized through the spinal reflex arc.
 
Reciprocal inhibition is realized via the inhibitory interneurons of the spinal cord. After these neurons get excited, they send their elongations to both the synergistic alpha motor neurons and to the antagonist neurons. They simultaneously perform the excitation of alpha motor neurons that innervate the synergistic muscles, and inhibition of alpha motor neurons that innervate the antagonistic group of muscles.
 
This pattern is used while performing the protective polysynaptic reflexes:

• Removal reflex (reflex of the flexors)
• Reflex of extensors

Reflex of the flexors

This reflex has the purpose of removing a body part from painful stimuli. The receptors for this reflex are located within the skin and known as nociceptors or simply pain receptors. The effector organs are skeletal muscles that remove the affected body part.
 
In this way, the reflex arc is polysynaptic. Afferent fibers from the receptors synapse with the interneurons of the dorsal horns of the spinal cord. These interneurons excite the ipsilateral motor neurons of the flexors, and at the same time, they inhibit the motor neurons that innervate the ipsilateral extensors. This occurs when extensors do not oppose the contraction of flexors. For example, when you touch a hot stove, the biceps brachii contracts and flexes the arm, whereas the triceps brachii remains relaxed.

Reflex of the extensors

The reflex of flexors can be followed by the contralateral reflex of extensors. While the affected limb has its flexors contracted and extensors relaxed as a response to the injuring stimuli, the other limb may undergo the opposite process: relaxation of flexors with contraction of extensors.
 
But, how? Well, the same interneurons that excite the ipsilateral motor neurons of the flexors and inhibit the ipsilateral motor neurons of the extensors, send collateral branches to the contralateral side of the spinal cord where they excite the contralateral extensors and at the same time, inhibit the contralateral flexors.
 
But, why? Well, if you accidentally step onto a hedgehog while walking down a beach, you need the injured leg to flex and prevent additional pain as you also need the opposite leg to extend to provide you with support. This prevents falling after flexing the injured leg.

When we talk about clinically detectable and examined polysynaptic reflexes, they are mostly provoked by stimulating the skin. This stimulation causes muscles of the examined region to contract in specific ways because it simulates an attack on the organism. For this reason, when you fight with your younger brother and he tries to hit you in the abdomen, your abdominal muscles will involuntarily contract.