Reflex

Reflexes

Mark L. Latash , Vladimir M. Zatsiorsky , in Biomechanics and Motor Control, 2016

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).

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Reflexes

F.S. Pedroso , in Encyclopedia of Infant and Early Childhood Development, 2008

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 ).

Figure 1. Knee jerk reflex.

Figure 2. Spinal reflex arc.

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.

Figure 3. Babkin reflex and other responses to hand compression stimulus.

Figure 4. Babkin reflex and other responses to hand compression stimulus – diagram.

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.

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Reflex activity

J.A. Simpson MD, FRCP, FRCP (Ed), FRCP (Glas), FRS (Ed) , W. Fitch PhD, MB ChB, FFARCS , in Applied Neurophysiology, 1988

Coital reflexes

The feeding and swallowing reflexes are fundamental for preservation of the individual. For preservation of the race, the coital reflexes are of almost equal biological importance. Indeed some of the orofacial and upper limb gripping responses have sexual as well as feeding functions. Like feeding responses, reproductive behaviour is more dependent on higher nervous activity (p. 127 ) (modified by endocrinal status) than on spinal reflexes in its initiation. For completion of coitus reflex activity is complementary, but not essential. Nevertheless the paraplegic male patient is capable of maintained penile erection and ejaculation on manipulation of the genitalia.

It is doubtful whether human subjects display the treading and pelvic elevation behaviour which precedes copulation of quadrupeds. The pelvic thrusting movements of both partners during intercourse are almost entirely voluntary but at orgasm may be reflex as in the spinal animals studied by Sherrington. Autonomic reflexes are, however, important for the tumescence and mucus secretion of the genitalia of both sexes, and for ejaculation in the male, though these components of the sexual response may also be evoked from mid-brain, hypothalamic and rhinencephalic levels of the brain.

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Anatomy and physiology

Jahangir Moini MD, MPH , ... Mohtashem Samsam MD, PhD , in Epidemiology of Brain and Spinal Tumors, 2021

Reflex activity

Reflexes make up many types of control systems in the body. They may be inborn (intrinsic) or learned (acquired ). Inborn reflexes are rapid, predictable motor responses to stimuli. They are unconscious, unlearned, and involuntary. These reflexes help avoid pain, maintain posture, and control visceral activities. Contacting a hot surface causes an instantaneous inborn spinal reflex, and the hand from the surface. Similar reflexes continue without awareness, such as visceral reflexes regulated by subconscious lower CNS regions—mostly the brain and spinal cord.

A learned reflex is developed through practice and repetition, an example of which is the activities required to drive an automobile. The process is mostly automatic, and only develops after a lot of time is used to master the related skills. The difference between inborn and learned reflexes is unclear. Most inborn reflexes are modified by learning and conscious effort. Pain signals identified by the spinal cord's interneurons are quickly transmitted to the brain. The individual becomes aware of pain and its cause. The withdrawal reflex is serial processing regulated by the spinal cord. Pain awareness is based on simultaneous and parallel processes of the sensory stimuli.

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Reflexes, Spinal Cord and Blink

M.A. Fisher , in Encyclopedia of the Neurological Sciences (Second Edition), 2014

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).

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Volume 3

Drew Murray , Thomas Abell , in Neuromodulation (Second Edition), 2018

Gastrointestinal Reflexes

Reflexes occur within the GI tract with the same predictability of response as within the motor system. Stimuli to particular receptors induce a predetermined response. Reflexes are not to be mistaken with local patterns, which are neuron networks responsive for frequent and repetitive patterns for cyclical functions, such as chewing and swallowing (Wood et al., 1999 ). These patterns do not require stimuli for initiation whereas reflex patterns do. Reflex patterns are intrinsic (contained within the ENS) and extrinsic which travel outside of the ENS and are further categorized into long and short. Intrinsic patterns modify secretion, peristalsis, mixing contractions, and local inhibitory effects. Reflexes, such as defecation, vasovagal, vomiting, and pain are considered long reflex circuits as they involve the CNS. Short reflex circuits travel as far as prevertebral sympathetic ganglia and include the gastrocolic, gastroenteric, and ileogastric reflexes.

Possibly the most important long reflex in maintaining adequate digestive function is secondary to the vasovagal reflex circuit. Vagal afferents in conjunction with higher command centers in the CNS allow for fine tuning and rapid adjustments for changing conditions associated with anticipation, ingestion, and digestion (Fig. 114.4) (Wood et al., 1999). Sensory afferents from mechanoreceptors, which measure tension and direct contact, thermoreceptors (temperature) and chemoreceptors, which measure composition, osmolality, glucose concentrations, and pH relay findings to the dorsal motor nucleus of the vagus and the NTS in the CNS. Interneurons of command centers in the CNS integrate information. Efferent signals from the dorsal vagal complex stimulate peripheral tissues directly as well as indirectly through ENS signaling. This circuitry allows coordination of muscular, secretory, and circulatory effects within the GI tract (Wood et al., 1999). Effector responses are obtained through both inhibitory and excitatory pathways. The circuitry involved in the vasovagal reflex relies on in excess of 30 neurotransmitters, with most diversity existing within the CNS and the ENS (Wood et al., 1999).

Figure 114.4. Implications of afferent neurons in GI defense.

 Reproduced from Holzer, P., 2007. Role of viceral affarent neurons in mucosal inflammation and defense. Curr. Opin. Pharmacol. 7 (6), 563–569.

Sensory information from irritation and noxious stimuli are the initiators of the vomiting reflex. Afferent fibers relayed by vagal and sympathetic nerves transmit the stimuli to a chemoreceptor trigger zone in the brain known as the area postrema (Gralla, 2003). Numerous triggers including medications can stimulate or inhibit this phenomenon, as the area lies outside of the blood-brain barrier (Aapro, 2007). Receptors for dopamine, serotonin, opioids, ACh, histamine, and substance P are present within this area (Gralla, 2003). Dopamine, histamine, and serotonin are thought to inhibit the response and thus explain the mechanism of action of many antiemetic pharmacologics. Input from the vestibular system explains the role of nausea and vomiting in motion sickness. The chemoreceptor trigger zone provides output via motor, parasympathetic, and sympathetic pathways to provide coordination to protect the body and provide the reflexive response (Fig. 114.5).

Figure 114.5. Schematic diagram of vomiting reflex.

 Reproduced from Aapro, M., 2007. Palonosetron as an antiemetic and antinausea agent in oncology. Ther. Clin. Risk Manag. 3 (6), 1009–1020.

Short reflex patterns exclude the CNS and facilitate movement of ingested food from one area to another. The gastroenteric reflex is stimulated by increased acid presence in the duodenum or very low pH within the stomach. Along with endocrine inhibition, gastric motility, and gastric acid secretion are inhibited. The occurrence where tension and stretch in the stomach results in increased peristalsis in the colon is known as the gastrocolic reflex. The ileogastric reflex functions to open the ileocecal valve and increase movement patterns in the small intestine to make room for digested food to move out of the stomach. The conjunction of these mechanisms maintains motility following ingestion.

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Developmental Anatomy and Physiology of the Respiratory System

Claude Gaultier , André Denjean , in Pediatric Respiratory Medicine (Second Edition), 2008

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

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Respiration

Homayoun Kazemi , Douglas C. Johnson , in Encyclopedia of the Human Brain, 2002

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.

I.A.3.a Hering–Breuer (Stretch) Reflexes

These reflexes arise from the lung parenchyma. There are two primary reflexes: the inhibitoinspiratory reflex and the excitoexpiratory reflex. Afferent stimuli travel through the vagus. When lung is inflated, the inhibitoinspiratory reflex is stimulated and inspiration is terminated. When lung volume is reduced, excitoexpiratory reflex is stimulated, inspiration is initiated, and respiratory frequency is increased. The excitoexpiratory reflex is stimulated not only when lung volume is reduced but also by pulmonary congestion and inflammatory processes in the lung parenchyma—all leading to an increase in respiratory frequency.

I.A.3.b Irritant and Mechanical Reflexes

Receptors are located near the mucosal surface in the tracheobronchial tree and their stimulation by inflammation or irritant gases such as cigarette smoke leads to cough or forceful expiration as well as the sensation of dyspnea.

I.A.3.c Pulmonary Vessels and Great Vessels

Congestion and distention of pulmonary vessels lead to increased ventilation, mostly an increase in respiratory frequency. An increase in transmural pressure of the aorta and carotid sinus can cause a reduction in ventilation or apnea. These reflexes become important in respiratory control primarily in disease states.

I.A.3.d Respiratory Muscles

Muscles of respiration possess a stretch reflex similar to that in all striated muscles. This reflex through the gamma fibers interacts with alpha fibers coming from the anterior motoneurons in the spinal cord. Thus, there is interaction between local muscle reflexes through the gamma efferent system and impulses arriving from higher centers through the alpha system to set the level of muscle contraction and thereby ventilation.

I.A.3.e Other Reflexes

Impulses arising from systemic muscle groups, joints, and possibly other visceral organs can influence the level of ventilation, but their roles and mechanisms of action are not well understood, as is the case with hyperventilation associated with exercise.

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Jaw Movement and Its Control

Greg M. Murray , in Functional Occlusion in Restorative Dentistry and Prosthodontics, 2016

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.

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Vestibular System

J.C. Glover , in Encyclopedia of Neuroscience, 2004

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.

Figure 5. Coordination of vestibulospinal and cervicospinal reflexes. (a) Normal position. Tilting of head and trunk together relative to space (b) stimulates the vestibular apparatus only and elicits limb extension on the same side as the tilt and limb flexion on the opposite side. Tilting the trunk while keeping the head motionless (c) stimulates neck proprioceptors only and elicits limb flexion on the same side as the tilt and limb extension on the opposite side. Both of these responses serve to maintain balance. Tilting the head while keeping the trunk motionless (d) stimulates the vestibular apparatus and neck proprioceptors simultaneously. The reflex responses cancel out. Reproduced from Kandel ER, Schwartz JH, and Jessell TM (2000) Principles of Neural Science, 4th edn. New York: Elsevier, with permission from Elsevier.

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