Spinal Reflex

Introduction

Simple behaviours induced by the central nervous system (CNS) circuits that are completely contained inside the spinal cord are known as spinal cord reflexes. The stretch reflex, the golgi tendon reflex, the crossing extensor reflex and the withdrawal reflex are all spinal reflexes. The human brain controls the body’s entire physiology and functions. The spinal cord is a vital part of our body. It is required for transmitting nerve impulses from the brain to the body. A reflex action is controlled by the spinal cord that allows us to operate normally. 

Spinal reflexes are in charge of performing the most important functions for the body, for example being able to remove your hand from a heat source to avoid being burned. 

A spinal reflex is an automatic, stereotyped effector tissue reaction to receptor activation. These reflexes are triggered by the activation of a group of neurons that are linked to each other. The spinal reflexes contain many important defensive mechanisms for survival. While a major part of the spinal cord functioning is controlled by the brain, many functions of the spinal cord are also controlled by spinal reflex. 

Spinal reflexes – Neural plasticity

For a long time, spinal reflexes were thought to be hard-wired with basically constant input-output correlations. The spinal reflex circuits have been demonstrated to undergo functional and maybe structural changes over the last 25–30 years, which may be essential in nociception (pain) processing, locomotor patterns, sensory information regulation and other processes. 

Non-associative plasticity refers to a set of processes that range from temporary to essentially permanent changes in reflex excitability. Classically conditioned, instrumentally conditioned and operantly conditioned changes are examples of associative plasticity.

Nociceptive input is the driving factor underlying many of these changes, but non-nociceptive inputs are equally crucial. These discoveries have implications for human functions and spinal injury rehabilitation.

Types of Spinal Reflexes

The basic unit of a reflex is a spinal reflex arc, which is the result of the relationship between the neurons and the effector tissue. Depending upon the number of neurons participating in one spinal reflex arc, the reflexes can be classified into two types: monosynaptic and polysynaptic.

Monosynaptic reflex

Monosynaptic reflexes are made up of two neurons. The first is found within the ganglion of the spinal cord. This is the sensory neuron (afferent) that perceives muscular stimulation through its peripheral process. The first neuron’s central process then sends this signal to the second neuron, which is located in the ventral horn of the spinal cord. Neuron II is an efferent motor neuron that transmits the proper signal back to the same muscle where the sensory neuron detected the signal via its axon.

Let us understand this concept with an example of knee jerk or patellar reflex. Neuron I has its peripheral terminal within the quadriceps muscle tendons in this spinal reflex. Neuron I detects excessive or sudden stretching of the tendon and signals neuron II that action is required to avert tendon injury. The efferent signal is then sent to the quadriceps muscle by its axon by neuron II of the patellar reflex, which is located in the lumbar regions of the spinal cord. As a result, the appropriate stimulus is muscular stretching and the appropriate response is muscle contraction. Through this example, we can say that monosynaptic reflexes are mostly stretching reflexes.

Let us learn more about the receptors that provoke these types of spinal reflexes.

Receptors – Proprioceptors are found in muscles, tendons and ligaments, among other places. These specialised receptors detect information such as muscle length, ligament tension and tendon stretching level, among other things. The receptors in skeletal muscles are known as muscle spindles, whereas the ruffini corpuscles and free nerve endings are found in joints and the Golgi tendon organs are found in ligaments and tendons.

The position of the limbs, the strength and pace of muscular contractions, and the feedback information necessary for movement control are communicated to the central nervous system through the receptors. The sensation that the somatosensory cortex has after getting all this information is truly kinesthesia or the awareness of one’s body’s position in space.

Polysynaptic reflex

Polysynaptic spinal reflexes, unlike monosynaptic reflexes, require the involvement of one or more interneurons. The sensory neuron I’s body is also located within the spinal ganglion and it receives impulses from muscles and other tissues (i.e. the skin). The afferent neuron delivers signals to the interneurons in the grey matter of the spinal cord via its central process, implying that communication between the afferent and efferent neurons is indirect. The appropriate motor neurons in their specific spinal cord segments as well as the adjacent and distant motor neurons, receive these signals from these interneurons.

Extensors and flexors are the muscle groups that are most responsive to this type of control. These reflexes usually cause some muscle groups to contract while others relax at the same time. The inverse stretching reflex is one of the most important polysynaptic responses from a physiological standpoint.

Clinical Relations in Spinal Reflexes

The capacity to interpret a reflex response to stimulation is critical for determining a patient’s overall reflex aptitude as well as the components of the reflex spinal reflex arcs.

A basic pattern is used to test monosynaptic stretching responses, which is detailed in the patellar reflex. The patient sits with one leg relaxed and slightly elevated from the ground. The reflex hammer is then used to strike the patellar tendon.

This usually causes the muscle spindles in the quadriceps muscle to get excited, resulting in an involuntary contraction of that muscle group. If this reaction is absent, it indicates that a pathogenic process is occurring in one or more of the reflex arc’s structures.

When it comes to polysynaptic reflexes, the procedure is described as abdominal skin stimulation. The patient lies down flat on their back and relaxes their abdominal muscles. The physician then softly strokes the abdomen skin in all four quadrants from the lateral to the medial sides with a blunt tool.

The abdominal muscles at the side of the stimulus are normally contracted, and the umbilicus moves towards the source of the stimulation in a normal positive reaction. When the sole of the foot is stimulated with a blunt instrument, the plantar reflex occurs. Surprisingly, children and newborns have varied reactions.

Conclusion

The spinal reflex is a vital part of human physiology. It controls and permits your body to react without thinking about it. The neural pathway involved in a reflex action is known as the reflex spinal reflex arc. Most sensory neurons in your vertebrae synapse in the spinal cord rather than going straight to the brain.

Our bodies use reflexes to defend themselves by reacting quickly and employing automatic processing. The presence and degree of spinal reflex control are important indicators of the development and function of your neurological system.