Clinical meaning
Primitive reflexes are mediated by brainstem and spinal cord circuits that develop in utero between 25-38 weeks gestation. The Moro reflex arc involves vestibular nuclei sensing head position change, with efferent signals via the reticulospinal tract producing bilateral upper extremity abduction-extension followed by adduction-flexion. Cortical maturation progressively inhibits these subcortical pathways through descending corticospinal tract myelination, which occurs in a cephalocaudal pattern explaining the sequential disappearance of reflexes. The corticospinal tract achieves functional myelination between 12-24 months, coinciding with disappearance of Babinski sign. Persistent primitive reflexes beyond expected age indicate failure of cortical inhibition - the hallmark of upper motor neuron dysfunction. This is a key diagnostic feature of cerebral palsy, where periventricular leukomalacia or cortical injury prevents normal corticospinal tract development. The tonic neck reflex is mediated by cervical proprioceptors activating ipsilateral extensor motor neurons and contralateral flexor motor neurons through the vestibulospinal tract. Obligatory tonic neck response (inability to turn out of position) is always pathological and suggests severe cortical dysfunction. Postural reflexes (Landau, parachute, lateral propping) are cortically mediated protective responses that EMERGE as primitive reflexes disappear - their absence beyond expected age also indicates developmental delay.