The symmetric tonic neck reflex is an early infant reflex that is physiologic until the third month of life. In the supine position, the examiner flexes the infant’s head, stimulating a reflex movement of the arms and legs. Persistence of the reflex beyond the first three months of life points to neurologic disorders.
What is the symmetric tonic neck reflex?
The symmetric tonic neck reflex is an early infant reflex that is physiologic until the third month of life. Reflexes are automatic and involuntary bodily responses to a specific stimulus. Infants and toddlers possess a set of reflexes that adult humans no longer possess. These reflexive stimulus responses are known as early childhood reflexes. As maturation continues, these reflexes regress. It is not until after birth that the central motor neurons are fully formed, serving as a higher-level control authority and causing many of the early childhood reflexes to disappear. Individual reflexes from the group of early childhood reflexes appear during specific weeks or months of life and regress at an equally specific time. The symmetric tonic neck reflex is a reflex movement from the group of early childhood reflexes. It is present until the third month of life. It must be distinguished from the asymmetric tonic neck reflex, which regresses between the sixth and seventh week of life. In the symmetric-tonic neck reflex, the triggering stimulus corresponds to an extension or flexion of the head, which is responded to by the infant’s body with an automatic flexion or extension of the arms and legs.
Function and task
A large number of all human reflex movements are protective reflexes that serve, not least, survival. Early infant reflexes are also geared toward survival, such as the sucking reflex, in which the infant responds to a touch stimulus near the mouth with sucking movements. Each reflex builds on a so-called reflex arc. The first instance of this reflex arc is always a sensory perception. In the case of the sucking reflex, this sensory perception corresponds to a sensation of the skin sensory cells. Via afferent nerve pathways, the reflex-triggering perception travels to the central nervous system. In the spinal cord, the excitation is switched to efferent nerve pathways that conduct from the central nervous system to the body periphery. In this way, the excitation reaches the efferent system. This system corresponds to the muscle that performs the reflex movement. To trigger the symmetric tonic neck reflex, the child is in the supine position. The examiner moves the child’s head into flexion, or bending. The sensory cells of depth sensitivity report the flexion of the head to the central nervous system via afferent nerve pathways. Thanks to deep sensibility, the central nervous system is permanently informed about body positions and muscle movements. The most important sensory cells of the system are the muscle spindle and the Golgi tendon apparatus. The action potential from nerve excitation is switched to efferent nerve pathways leading to the muscles of the arms and legs during the symmetric tonic neck reflex. Once the excitation reaches the nerves near the muscles, it is transmitted to the muscles themselves via the motor endplate. The muscles of the arms are thus stimulated to contract and flex the arm. At the same time, the muscles of the legs are stimulated to extend, causing the child’s legs to extend. When the examiner moves the child’s head from flexion back into extension, the reverse movement response is elicited. Thus, extension of the head stimulates the arms to extend and the legs to flex. The symmetrical tonic neck reflex is characterized by the symmetrical cooperation of the right and left sides of the body. As soon as the child begins to crawl, the reflex should have regressed. While reflex muscle work in response to a neck movement stimulus still makes sense during the first three months, after this time the reflex prevents crawling and accommodation training.
Diseases and complaints
Early infant reflex testing is an important tool in the assessment of infant development. As part of the preventive examination, the review of reflexes in infancy takes place regularly.If the symmetric-tonic neck reflex is absent or reduced in the first months, this may refer to nerve damage to the nerves of the reflex arc, for example. While the asymmetric presence or absence of the symmetric-tonic neck reflex in the first three months of life points to neurological disorders, after the first three months of life the persistence of the early infantile reflex is considered a neurological disorder indicator. If the reflex persists, sequelae such as poor posture and weak body tension in sitting and standing may result. The child’s attention is disturbed as a result. Sitting positions can hardly be maintained and require a high degree of concentration. In exceptional cases, the symmetric-tonic neck reflex may suddenly and unexpectedly reappear later in a patient’s life. In this context, the reflex is a sign of central nervous system dysfunction. It is possible that the affected person’s superior control of movement is impaired by a pathological process. Such processes may be accidental injuries to the neck. Likewise, tumors, spinal cord infarcts, bacterial or autoimmunologic inflammation, and degenerative diseases of the central nervous system may be responsible for a sudden recurrence of a symmetric tonic neck reflex. Usually, evidence of a persistent symmetric-tonic neck reflex alone is not sufficient to prove, for example, damage to the higher-level controlling motor neurons. Evidence of persistence of several reflexes from the group of early childhood reflexes is more informative in this context. Further workup primarily involves imaging of the spine and brain.
