A major part of the spinal cord function is regulated by the brain. Many functions of the spinal cord are also executed independently from the brain, such as a spinal reflex.
The definition of a spinal reflex as well as their components, functions, pathways, and physiology will be described in this article and is a must-know for every student that is passionate about neurosciences. The clinical importance of spinal reflexes is also essential since the examination of them is an inevitable part of daily clinical practice.
|Spinal reflex||Receptor --> sensory fiber --> (interneuron) --> alpha motor neuron --> muscle|
|Monosynaptic reflexes||Biceps brachii, triceps brachii, brachioradialis, quadriceps femoris, triceps surae reflexes|
|Polysynaptic reflexes||Upper abdominal, lower abdominal, cremasteric, plantar, anal reflexes|
The spinal cord is phylogenetically older than most structures of the brain, which means that reflexes are in charge of carrying out functions that the organism needs the most. Basically, it is more important to remove your hand from a heat source in order to avoid getting burnt than to be able to speak. Many of the protective functions necessary for survival are embedded within the spinal reflexes.
By definition, a reflex is an involuntary, stereotypical response of the effector tissue from the stimulation of receptors. These reflexes are executed by the successive activation of a certain number of neurons that are mutually connected. The last neuron generally innervates the effector tissue, which is usually a muscle. These neurons and the effector tissue form relations that are called the reflex arc, which is the basic unit of a reflex. Based on how many neurons participate in one arc, the reflexes can be monosynaptic or polysynaptic.
The monosynaptic reflexes consist of two neurons. The first is located within the spinal ganglion. This is the sensory neuron (afferent) whose peripheral process detects the stimuli from the muscle. Then, the central process of the first neuron conducts this signal to the ventral horn of the spinal cord, where the second neuron is situated. The neuron II is a motor neuron (efferent) that sends the appropriate signal via its axon back to the same muscle in which the sensory neuron had detected the signal. This process occurs after the sensory information is received from neuron I. This means that the entire arc has only one neuronal synapse that is directly between neuron I and neuron II (without the participation of interneurons).
To make it simple, let’s recap this through a specific example.
The perfect example of the monosynaptic reflex is the knee-jerk or the patellar reflex. In this reflex, neuron I has its peripheral ending within the tendons of the quadriceps muscle. Once the tendon is excessively or suddenly stretched, neuron I detects that and informs neuron II that action is needed to avoid any injury to the tendon. Then, neuron II of the patellar reflex, which is in the lumbar segments of the spinal cord, sends the efferent signal through its axon to the quadriceps muscle to contract.
So, the adequate stimulus is the stretching of the muscle, and the adequate response is its contraction. Considering this pattern, we can say that most monosynaptic reflexes are stretching reflexes. When we know this, it’s simple to understand that testing these reflexes actually tests the state of neuron II in the spinal cord and can provide clinically important information about the situation in the spinal cord.
|Biceps brachii||C5, C6|
|Triceps brachii||C6, C7, C8|
|Brachioradialis||C5, C6, C7|
|Quadriceps femoris||L2, L3, L4|
|Triceps surae (Achilles tendon)||S1, S2|
For the complete understanding of stretching reflexes, let’s consider all components of the reflex arc from a physiological aspect. To recap, these components are:
- Afferent nerve fibers
- Spinal motor neuron
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.
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.
Contrary to monosynaptic reflexes, polysynaptic reflexes are accomplished with the participation of one or more interneurons, meaning that the communication between the afferent and efferent neurons is indirect.
The body of the sensory neuron I is also situated within the spinal ganglion; receiving stimuli from muscles and other tissues (I.e. the skin). The afferent neuron sends signals via its central process to interneurons located in the gray matter of the spinal cord. These interneurons then direct these signals to the adequate motor neurons of their specific spinal cord segments, as well as adjacent and distant motor neurons. Because of this, one stimulus transmitted to the interneurons can cause multiple alpha motor neurons to get excited or inhibited, and therefore, can cause more than one muscle to contract or relax. Generally, the different muscle groups susceptible to this way of regulation are the extensors or flexors. Typically, these reflexes cause some muscle groups to contract, while others simultaneously relax. From a physiological point of view, one of the most important polysynaptic reflexes is the inverse stretching reflex.
As previously mentioned, Golgi tendon organs are among deep receptors that take part in spinal reflex arcs.
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.
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:
- The recurrent inhibition
- The reciprocal 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.
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:
- The removal reflex (the reflex of the flexors)
- The reflex of extensors
The 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.
The 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.
Interpretation of a reflex response to stimulation is essential for the assessment of a patient’s entire reflex ability and components of the reflex arcs.
Testing the monosynaptic stretching reflexes is examined with a simple pattern and is explained in the patellar reflex . The patient sits with the specific leg relaxed and hanging slightly above the ground. The physician then strikes the patellar tendon with the reflex hammer. This typically causes excitation of muscle spindles in the quadriceps muscle, which will ultimately end with the involuntary contraction of that muscle group. If this reaction lacks, that means that there may be a pathological process in one or more of the structures that participate in the reflex arc.
When it’s about polysynaptic reflexes, the procedure is described within the stimulation of abdominal skin. The patient must lie on their back and relax the abdominal muscles. Then, the physician uses a blunt object to gently stroke the abdominal skin from the lateral to the medial aspects in all four quadrants. A normal positive response usually involves a contraction of the abdominal muscles at the side of the stimuli and the umbilicus moving towards the source of the stimulation.
The plantar reflex deserves special attention. This reflex happens when the sole of the foot is stimulated with a blunt instrument. Interestingly, the response is different between children and infants. In normal adults, the reflex is presented with the flexion of the hallux, whereas in normal infants up to 12 months of age, it is presented with the extension of the hallux. This is known as the Babinski reflex. This difference originates from the fact that infants do not have a fully myelinated corticospinal pathway at this age, so the reflex is not inhibited in the cerebral cortex. The presence of the Babinski reflex in adults is a burning sign that something is wrong within the central nervous system that affects the corticospinal pathway.