The scientific perspective on infant reflexes has changed over time to suggest

6.4.2 Classification based on response location

Reflex responses produced by a stimulus can be seen in a muscle or muscle group in close proximity to the stimulus site. Such reflexes are typically addressed as autogenic. Reflexes seen in remote muscles are addressed as heterogenic. Since the expression “close proximity” is not well defined, sometimes a reflex can be classified differently depending on the level of analysis. For example, reciprocal inhibition can be called a heterogenic reflex because it involves the antagonist (“remote”) muscle. On the other hand, if one is interested in the distribution of reflex responses within a group of muscles serving a joint versus across joints, or within a limb versus across limbs, reciprocal inhibition can be classified as autogenic (because it causes changes in muscle activation affecting the same joint and the same limb).

Introduction

Reflex is defined as an involuntary motor response, secretory or vascular, elicited shortly after a stimulus, which may be conscious or not. The response to the stimulus is unalterable, it cannot be changed or adapted according to needs or circumstances. It can be concluded, thus, that the response is stereotyped and has a fixed reflex arc, whose response is also fixed. The reflex arc – stimulus reception and motor response to the same stimulus – is a physiological unit of the nervous system (NS).

In its most simple form, the reflex arc comprises: (1) a receptor which corresponds to a special sensory organ, or nerve terminations in the skin or neuromuscular spindle, of which stimulation initiates an impulse; (2) the sensory or afferent neuron, which carries the impulse through a peripheral nerve to the central nervous system (CNS), where it synapses with an internuncial neuron; (3) an internuncial neuron relays the impulse to the efferent neuron; (4) the motor or efferent neuron conducts the impulse through a nerve to the effector organ; and (5) the effector can be a muscle, gland, or blood vessel that manifests the response.

Despite this narrow definition of segmental integration, the polysynaptic involvement of other NS segments is common, constituting intra-, extrasegmental, and contralateral reflexes to the stimulus origin. For the reflex motion to occur, it is necessary to contract the agonist muscles and relax the muscles that perform the opposite motion (antagonist), regarding the latter, instead of causing the muscle to contract, inhibitory synapses will prevent muscle contraction. An example is the knee jerk reflex or patellar reflex: contraction of the quadriceps and extension of the leg when the patellar ligament is tapped (Figures 1 and 2).

However, reflex manifestations are typically diverse after a specific stimulation, as occurs with most primitive reflexes (PRs). Figures 3 and 4 show the complexity of responses to hand-compression stimulus.

The newborn is endowed with a set of reflex and automatic movements, which makes his NS apt to react to the environment where he lives in; the responses necessary to his adaptation and subsistence, such as suction, crying, deglutition, defense, and escape reactions, cannot be simply defined as reflexes in the strict sense of the definition, since these can be subject to alteration or adapted to needs and circumstances, and are therefore alterable, as the responses elicited by a given excitation do not manifest themselves in a clearly predeterminate way, nor are exactly identical over time. These responses express the neurophysiological state upon stimulation, constituting reflex reactions or automatisms; hence, these motor manifestations have been named differently by different authors, such as: PRs, primary reflexes, archaic reflexes, reflex responses, special reflexes, automatic reflexes, neonatal reflexes, primary responses, and developmental reflexes. Without a denomination of their own, some authors have included them among reflexes in general; in this article we call them PRs.

In order to define a reflex, we also need to specifically know its stimulation area, its integration center, and its response. Regarding PRs, it is still necessary to associate a functional concept that accounts for their ontogenetic and phylogenetic purpose. Although it is didactical to study each reflex isolately, we should bear in mind that this is a theoretical abstraction, convenient for the analysis of nervous phenomena, which does not exist in real life, since the PRs constitute a harmonic ensemble and are closely intertwined with one another, depending on the child’s physiological needs and environmental conditions at the moment they are elicited.

Postural Reflexes

Reflexes and reactions help to restore stability before the activation of voluntary systems. As described in Chapter 4 the hierarchical view of motor control attributes certain reflexes and reactions to specific levels of the nervous system. Cervicospinal and vestibulospinal reflexes assist in maintaining postural stability. Vestibular responses are used primarily to stabilize the head in space. Therefore, the brain stem is involved in postural control. It coordinates information from the spinal cord, cerebellum, cerebrum, and special senses. “Brainstem nuclei act reflexively to stimuli and in response to commands issued by other motor centers.15” The reticular formation within the brain stem contributes to postural tone by balancing activation of flexor and extensor muscles to allow us to express these reflexes. Posture and proximal movement emanate from brain stem centers.12

Abstract

Reflexes are involuntary activity arising from an afferent input and a subsequent efferent response. These can be proprioceptive arising from receptors within muscles, tendons, and joints or exteroceptive arising from skin and subcutaneous tissues. Proprioceptive spinal cord reflexes may produce monosynaptic activation (group Ia fibers), disynaptic inhibition (group Ib afferents) of motoneurons, or polysynaptic flexion withdrawal (group II afferents). The blink reflex is the most commonly used exteroceptive reflex – afferent Vth cranial nerve and efferent VIIth. Such reflexes are characterized by a simple short-latency (R1) component followed by a longer latency, more complex second component (R2).

Reflexes Originating from the Lung and Chest Wall

Reflexes originating from the tracheobronchial tree and within the lung parenchyma have significant effects in newborns, who differ in this respect from adults. Vagal innervation is of crucial importance in maintaining postnatal breathing and alveolar ventilation.302,303 The Hering-Breuer inflation reflex is an important mechanism for regulating the rate and depth of respiration in newborn mammals. In human infants, the activity of this reflex can be expressed as the relative change in expiratory time after end-expiratory occlusion compared to the resting expiratory time during spontaneous breathing. This parameter has been measured during non-REM sleep in infants younger than 1 year of age. The results showed that the reflex persisted beyond the neonatal period and exhibited no variation in activity during the first 2 months of age.304 Later, activity of the reflex correlated negatively with age.304 The postnatal period characterized by high reflex activity is longer in preterm infants, suggesting delayed maturation.305 The reflex is stronger during REM sleep than during non-REM sleep in newborn infants.306 The Hering-Breuer deflation reflex occurs in newborn infants, including those born prematurely.307 It may play an important role in protecting FRC in the newborn infant. Irritation of the tracheobronchial tree induces apneas in human preterm infants.308 Activation of bronchopulmonary C-fiber afferents induces bronchoconstriction in newborn dogs.309 Activation of C-fiber afferents may play a role in inflammatory lung diseases in infants.

Various reflexes that arise in the rib cage influence the intercostal and phrenic motoneurons. These reflexes are of potential importance in newborns, whose rib cage is compliant and therefore prone to distortion during REM sleep. Rib cage distortion is associated with breathing pattern changes, including decreases in inspiratory time and tidal breathing, prolongation of expiratory time, irregular breathing, and even apnea.310,311

I.A.3 Neural Factors

Neural factors are important in respiratory control but secondary to chemical factors. They become prominent in certain disease states or in special circumstances. Many of the neural stimuli are transmitted to the brain through the vagus nerve.

Reflex Movements

Reflex movements are largely organized at the brainstem or spinal cord level (for review, see Hannam & Sessle 1994). They are stereotyped movements that are involuntary and are little modified by voluntary will.

The classic reflex is the knee-jerk reflex, where a sharp tap to the knee evokes contraction in the thigh muscles and a brief lifting of the lower leg. In the jaw motor system, reflexes include the jaw-closing or jaw-jerk reflex, and the jaw-opening reflex.

The jaw-closing reflex occurs when the jaw-closing muscles are suddenly stretched by a rapid downward tap on the chin. This tap causes stretching of specialized sensory receptors called muscle spindles that are stretch sensitive. They are present within all the jaw-closing muscles. When spindles are stretched, a burst of action potentials travels along the group Ia primary afferent nerve fibers coming from the primary endings within the spindles. The primary afferents synapse directly onto and cause activation of the alpha-motoneurons of the same jaw-closing muscle. Thus a stretch of a jaw-closing muscle leads to a fast contraction of the same jaw-closing muscle. This reflex assists in preventing the jaw from flopping up and down during running.

Reflexes demonstrate a pathway that can be used by the higher motor centers for the generation of more complex movements. They also allow fast feedback that adjusts a movement to overcome small, unpredicted irregularities in the ongoing movement and adds smoothness to a movement. Thus, for example, unexpected changes in food bolus consistency during chewing can modulate muscle spindle afferent discharge, and this altered discharge can change alpha-motoneuron activity to help overcome the change in food bolus consistency.

The jaw-opening reflex can be evoked by a variety of types of orofacial afferents. Activity in orofacial afferents, for example, from mucosal mechanoreceptors, passes along primary afferent nerve fibers to contact inhibitory interneurons that then synapse on jaw-closing alpha-motoneurons. The inhibitory interneurons reduce the activity of the jaw-closing motoneurons. At the same time, primary afferents activate other interneurons that are excitatory to jaw-opening muscles, such as the digastric. The overall effect is an opening of the jaw.

Vestibulospinal Reflexes

Vestibulospinal reflexes involve the activation of spinal motor neurons that innervate neck, trunk, and limb muscles. The function of these reflexes is to maintain balance and body orientation in the face of ongoing perturbations. Like the vestibulo-ocular reflexes, the vestibulospinal reflexes are fast, providing compensatory movements that counteract and protect against the potentially injurious results of tripping or falling. Patients with vestibular disturbances risk bodily harm because these compensatory movements are deficient or absent.

Just as the vestibulo-ocular reflexes are coordinated with other reflexes and vision, the vestibulospinal reflexes are coordinated with other spinal reflexes and the vestibulo-ocular reflexes. This coordination is an essential element of our daily activities. An example is the coordination of the vestibulospinal reflex and the cervicospinal reflex (a proprioceptive reflex that generates limb movements in response to activation of neck proprioceptors). Each of these reflexes initiates unilateral limb extension when the head is tilted appropriately, but on opposite sides of the body (Figure 5). Tilting the neck while keeping the trunk motionless activates both reflexes, which therefore cancel out. In patients with vestibular pathology, a similar neck tilt will cause the patient to fall immediately to the same side because the cervicospinal reflex is activated alone. This uncomfortable (and potentially harmful) result can be experienced by healthy people following the vestibular imbalance produced by spinning in one direction for several seconds.

SPINAL REFLEXES

Reflexes can be divided into two categories: spinal reflexes and supraspinal reflexes. Spinal reflexes are segmental and monosynaptic. For example, tapping a patella tendon with a reflex hammer causes the tendon to stretch rapidly, exciting muscle spindles within the quadratus muscle (Fig. 1.5). The signal from the muscle spindle is carried to the spinal cord, where it is relayed through interneurons to the α motor neurons of the ventral horn. The α motor neurons signal extrafusal muscle fibers that cause the quadratus muscle to contract. This is a spinal or stretch reflex.

The mature stretch reflex can be broken down into two components: a dynamic stretch reflex, which responds quickly to rapid changes in muscle length, and a weaker static stretch reflex, which continues to maintain contraction of the muscle as long as the stretch force persists. The entire circuit is contained within the spinal cord. The interneuron may also send a signal to the brain to let it know what has happened, but the reflex is not dependent on input from the brain. In fact, input from the brain actually dampens the reflex. As the nervous system matures, myelination in the corticospinal and pyramidal tracts increases, and the spinal reflexes are down-modulated. This process is important for motor control. The ability to execute smooth gross and fine motor activity necessitates modulation of the stretch reflex. Imagine what would happen if you suddenly turned rapidly stretching your patella tendon. Without cortical modulation, the quadratus muscle would quickly contract, destabilizing your posture and balance. Damage to cortical structures involved with motor activity will interfere with the brain's ability to modulate these reflexes. This occurs in spastic cerebral palsy. These children develop increased muscle tone (spasticity) because they cannot properly modulate the stretch reflex. This affects their ability to smoothly execute voluntary movement.

Sacral Reflexes

Sacral reflexes refer to electrophysiologically recordable responses of perineal or pelvic floor muscles to electrical stimulation in the urinary-genital-anal region. Two reflexes, the anal and the bulbocavernosus reflex, are commonly clinically elicited in the lower sacral segments. Both have the afferent and efferent limb of their reflex arc in the pudendal nerve, and both are centrally integrated at the S2 to S4 cord levels.49,53 In women, the bulbocavernosus reflex is clinically elicited by squeezing or taping of the clitoris and observing movement of the perineum or anal sphincter. It is, however, much less reliable than in men,53,54 and in our opinion, is not useful. The anal reflex is elicited by a pinprick of the perianal skin, producing an anal wink.

Electrophysiologic correlates of these reflexes have been described using electrical, mechanical, and magnetic stimulation. Whereas the latter two modalities have been applied only to the clitoris, electrical stimulation can be applied to other sites, such as the dorsal clitoral nerve and perianal area. Responses are usually detected by needle electrode inserted into the EAS or bulbocavernosus muscle. The bulbocavernosus detection site is preferred because traces do not contain continuously firing, low-threshold MUPs.

The bladder neck or proximal urethra can be stimulated using a catheter-mounted ring electrode, and reflex responses can be obtained from perineal muscles. With visceral denervation, such as after radical hysterectomy, these reflexes may be lost while the sacral reflex mediated by pudendal nerve is preserved. Loss of vesicourethral reflex with preservation of vesicoanal reflex has been described for patients with urethral afferent injury after recurrent urethral operations.

Reports of sacral reflexes obtained after electrical stimulation of the clitoral nerve give consistent mean latencies of between 31 and 39 ms (see Fig. 10-3). Sacral reflex responses obtained on perianal, bladder neck, or proximal urethra stimulation have latencies between 50 and 65 ms.49 This more prolonged response is thought to be caused by the afferent limb of the reflex being conveyed by thinner myelinated pelvic nerves with slower conduction velocities than the thicker myelinated pudendal afferents. The longer-latency anal reflex, the contraction of the EAS on stimulation of the perianal region, may also have thinner myelinated fibers in its afferent limb because it is produced by a nociceptive stimulus.49