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| ORIGINAL ARTICLE |
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| Year : 2019 | Volume
: 8
| Issue : 4 | Page : 106-113 |
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Immediate effect of yogic postures on autonomic neural responses
Anup De, Samiran Mondal
Department of Physical Education and Sport Science, Visva-Bharati University, Santiniketan, West Bengal, India
| Date of Submission | 29-Nov-2019 |
| Date of Decision | 13-Dec-2019 |
| Date of Acceptance | 27-Dec-2019 |
| Date of Web Publication | 30-Jan-2020 |
Correspondence Address:
Anup De
Research Scholar, Department of Physical Education and Sport Science, Visva-Bharati University, Santiniketan, West Bengal
India
 Source of Support: None, Conflict of Interest: None  | 6 |
DOI: 10.4103/rcm.rcm_26_19
Aim: The aim of this study is to identify the autonomic responses after immediate yogasana practices. Materials and Methods: Ten male (n = 10) yoga practitioners having more than 8 years of experience in yogasana practice were selected as subjects. Before and after immediate practices of six specific yoga postures were assessed on three different consecutive days for 15 min, 22.5 min, and 30 min. Heart rate variability (HRV) low frequency, HRV high frequency (HF), HRV amplitude, galvanic skin resistance (GSR), and blood volume pulse were assessed under the condition of autonomic neural activity and measured using NeXus-10 device. Results: Findings of the data generalized increasing of GSR (47.93% and 14.40%) and HF HRV (7.74% and 6.69%) and decreasing of low-frequency HRV (5.43% and 5%) immediately after 15 min and 22.5 min practice of yogasana, which indicates parasympathetic (vagal) activation. However, in the case of 30-min yoga practice, it decreased the GSR (11.03%) and HF HRV (2.59%), increased low-frequency HRV (2.23%) which, in turn, indicates the sympathetic activation. Discussion: The possible mechanism of vagal activation is an increase of baroreceptor sensitivity, tissue oxygenation, nervous system metabolism, and activation of vasodilation. It may be attributed to the activation of the head ganglion of the autonomic nervous system and inhibition of the posterior hypothalamic area. The sympathetic activation depends on the release of epinephrine and norepinephrine hormones, activation of vasomotor center, central neural integration, and peripheral inhibitory/excitatory reflex mechanisms. Conclusions: Immediate yogasana practices may enhance the parasympathetic (vagal) dominance, which increases autonomic flexibility and associates with a calm mental state.
Keywords: Autonomic function, blood volume pulse, galvanic skin resistance, heart rate variability, yoga
How to cite this article:
De A, Mondal S. Immediate effect of yogic postures on autonomic neural responses. Res Cardiovasc Med 2019;8:106-13 |
| Introduction | |  |
Galvanic skin resistance (GSR) refers to electrical resistance current which steadily passes around the skin.[1] A transient change in certain electrical properties of the skin is associated with the sweat gland activity, when elicited by any stimulus, that evokes arousal or orienting response, known as the galvanic skin response. It is a change in the electrical properties of the skin which cause an interaction between environmental events and the individual's psychological state. GSR is a method of capturing the autonomic nerve response as a parameter of the sweat gland activity.[2] Scientists found that stress levels may change the electrical resistance of the skin.[2] Basically, sweat glands are of two types: one is eccrine and the other is apocrine. The eccrine glands do not secrete the contain cytoplasm from granular cells, while apocrine glands are able to perform it.[3] The eccrine sweat glands are solely stimulated through the sympathetic branch of the autonomic nervous system (ANS)[3] and are also regulated by the temperature of the body.[3] The sympathetic nervous system (SNS) activity is responsible for the regulation of the secretory segment of sweat glands. Similarly, electrical properties of the skin change may be due to sweat produced due to the electrolytes present in sweat filling up the ducts. Electrodermal activity (EDA) is a good indicator of the sweat gland output.[3] Sweat gland output is heavily regulated by the SNS activity of an individual and sweating is influenced by psychological phenomena such as fear, agitation, and anxiety or due to physiological phenomena such as tactile stimulation and inspiratory gasps.[3] Electrical activity represents the change in electric current produced by the sum of an electrical potential difference across a specialized tissue, organ, or cell system.[4] In addition, sympathetic stimulation increases sweating, and this improvement is enough to reduce the skin resistance since the sweat contains water and electrolytes, both of which increase the conductivity of the skin.[5] Scientists found that states of relaxation are accompanied by high skin resistance.[6]
In autonomic activity, the sympathetic branch increases heart rate (HR), whereas the parasympathetic branch decreases it. In the resting condition, both the sympathetic and the parasympathetic nervous systems (PNSs) are active with parasympathetic dominance.[7] Low-frequency (LF) HR variability (HRV) measures both sympathetic and parasympathetic influences, whereas high frequency (HF) measures only parasympathetic influences and LF/HF ratio is an indicator of sympathovagal balance around the ANS.[8],[9],[10] HRV decreases with age and it is an indicator of dynamic interaction and balance between the sympathetic and the parasympathetic systems which may be affected by meditation.[11] Increased HF component suggests reduced sympathetic activity and increased parasympathetic activity.[12] Whereas, sympathetic activation is of interest and is associated with increased vigilance and hence may be considered essential for performing an attentional task.[13] Attention is generally associated with the increased sympathetic activity which normally decreases when anxiety levels goes down.[14],[15] The reduction of sympathetic arousal is associated with vigilance and performance of a task requiring attention in addition to other cognitive processes.[16] Alternatively, the reduction of HRV has been shown to be an analyzer of mortality after myocardial infarction.[17],[18] HF HRV activity has been found to be decreased under circumstances of acute time pressure and emotional strain,[19] and elevated state anxiety,[20] presumably related to focus attention and motor inhibition.[19] Scientists found that HF HRV spectrum reduces while the LF component elevates.[21] In general, the central nervous system (CNS) sends the efferent signals through parasympathetic and sympathetic neural pathways for controlling HR alteration from a second to second variation.[22],[23] Vagal afferent stimulation leads to enhance excitation of vagal efferent activity[24] and the inhibition of sympathetic efferent activity. Efferent vagal activity is appeared to be under tonic restraint by cardiac afferent sympathetic activity.[22],[23]
Objectives
The objective of this study is to investigate the autonomic neural responses by observing sympathetic and parasympathetic neural activity after immediate yogasana practices.
| Materials and Methods | | |
Participants
Ten healthy male participants (n = 10) from the Department of Yogic Art and Science, Visva-Bharati University, Santiniketan, West Bengal, India, were willing to participate in this study. All the participants were well experienced and participated in Inter-University Yoga Competition. A total of sixty samples were collected in a consecutive 3 days with a three pre-post time variations. All the participants had 8.7 ± 2.49 years' experiences of yogasana practices. The average age of the participants was 20 ± 1.28 years. The height, weight, and BMI of the participants were 164.8 ± 6.21 cm, 55.4 ± 4.97 kg, and 20 ± 1.94 kg/mt2, respectively. Basic physiological statuses, i.e., resting respiratory rate (16 ± 3.4 bpm), resting systolic blood pressure (126 ± 9.54 mm/Hg), resting diastolic pressure (66 ± 5.96 mm/Hg), and resting HR (72.3 ± 6.88 bpm) were also measured.
Assessments
Five autonomic variables were assessed for the measurement of peripheral neural activity, namely HRV LF%, HRV HF%, HRV amplitude, blood volume pulse (BVP), and GSR, which indicates the sympathetic arousal, parasympathetic dominance, and electrical characteristics of the skin.
Study design
The data were collected before and after the immediate yogasana practice for 3 consecutive days at the same time with variation of systematic randomization in different time schedule interventions. The ten participants were included with self as control design with a time series condition, i.e., 15 min, 22.5 min, and 30 min. The details schematic representation of the systematic randomization is shown in [Table 1].
Yogasana intervention
In this experiment, six specific yoga postures that are mainly associated with the nervous system were selected for the intervention.[25] The selected yoga postures were padahastasana (forward bending pose), pranamasana (bowing pose), sarvangasana (shoulder stand pose), chakrasana (wheel pose), vrischikasana (scorpion pose), and sirshasana (head stand). Details of yogasana practice protocol are given in [Table 2].
Autonomic measurements
Nexus-10 Mark II (Mind Media BV, The Netherlands) instrument was assessed for measuring autonomic function. It is a multichannel neurophysiological monitory and biofeedback platform with eight analogs and two digital inputs, supporting a wide range of sensors for ten channels amplifier input. That utilizes bluetooth wireless communication, USB, and flash memory technologies.
Autonomic setting and recording
Electrocardiography (ECG) signal was recorded using a 10-channel amplifier with a sampling frequency of 256 Hz, and sampling rate of 2048 Hz, providing 12.2-bit-A/D conversion was used for raw ECG data acquisition.[26] Participants were sat comfortably on the chair, while 10 min of ECG data were collected. GSR was measured through electrodes using a low, constant current between the two flat Ag-AgCl electrodes of 10-mm diameter placed on the palm of the nondominant hand.[27] As part of a standard routine, BVP and HRV spectrum were recorded through a BVP sensor which was placed on the middle phalanx of the index finger of the dominant hand.[28]
Electrocardiography data processing and analysis
ECG data were extracted using online Brain Vision Analyzer software (version 2.01, Brain Products GmbH, Munich, Germany, 2009) and filtered with a 0.5–40 Hz bandpass online filter. ECG spectral was calculated through the Short-time Fourier transform in overlapping 2 s epochs in each of the bandwidths.[29],[30]
Statistical analysis
In this study, descriptive statistics, namely mean, standard deviation, standard error of the mean, and percentage changes were measured for generalizing from pre to post assessment outcome. Further one-way analysis of variance was used for the measurements of significant differences from pre to post changes among the three duration yogasana practices. All the data were analyzed using RStudio programming language software, version 3.1.2 (Northern Ave, Boston, MA 02210, USA).
| Results | | |
The outcome of the HRV components showed that there was an increase in HF % (7.74% and 6.69%) band and decrease in LF% (5.43% and 5%) band immediately after 15 min and 22.5 min practice of yoga postures. However, there was a decrease in HF% band with 2.59%, and increase in LF% band with 2.23% after 30 min yoga posture practices. Another component of HRV was HRV amplitude which showed a trend toward a decrease in 15 min (10.5%), 22.5 min (6.97%), and 30 min (18.91%) of yoga postures. Whereas, the GSR result showed that 15 min and 22.5-min yogasana practice increased (47.93% and 14.4%) the skin resistance result, but in case of 30-min yogasana practice, it decreased 11.03%. Increasing trend was observed in BVP result with 3.91%, 6.68%, and 7.53% in respect of yogasana repetition. Detail demographic status is given in [Table 3] and detail results of autonomic components are given in [Table 4] and [Table 5] and [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]. | Table 5: Analysis of variance among different durations of yogasana practice
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 | Figure 1: Changes in galvanic skin resistance level after immediate yogasana practice
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 | Figure 2: Changes in heart rate variability low frequency % after immediate yogasana practice
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 | Figure 3: Changes in heart rate variability high frequency % after immediate yogasana practice
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 | Figure 4: Changes in heart rate variability amplitude after immediate yogasana practice
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 | Figure 5: Changes in blood volume pulse after immediate yogasana practice
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| Discussion | | |
The results indicated that 15–22.5 min yogasana practices decrease the LF% and simultaneously increase the HF% of the HRV spectrum. It may be cause of increase in parasympathetic activity reflected by vagal nerve. However, in the case of 30-min yogasana practice it increases the LF% and decreases the HF% of HRV spectrum. It may be cause of increase in the sympathetic activity reflected by decreases in the parasympathetic tone.[21] Whereas, similar neural activity trend has been observed in the case of EDA; 15–22.5 min yogasana practice activates the parasympathetic neural activity and thereby reduces the EDA as well as sweet gland activity.[31] However, 30-min yogasana practice enhances the arousal of the sympathetic activity of ANS.
High variability (HF) between two heartbeats suggests healthy autonomic adaption to environmental demands and is associated with better aerobic fitness,[32],[33] cognitive, emotional, attentional, and autonomic regulatory ability.[34] Alternatively, low HRV indicates the greater sympathetic activation which suggests poor regulatory capacity, increased cardiovascular morbidity, and mortality with immune dysfunction and inflammation, which are in turn implicated in a wide range of pathological conditions such as cardiovascular disease, diabetes, arthritis, and certain cancers.[35],[36] The measurement of HRV also provides a window into the bidirectional influences that link frontal lobes with modulation of cardiovascular function.[35] For the alteration of GSR, parasympathetic, and sympathetic activity, the present researchers found so many existing scientific mechanisms which may help to generalize the present research outcomes, are explained below.
Possible underlying mechanisms of galvanic skin resistance changes
Yoga helps to decrease the sympathetic tone and simultaneously increases the parasympathetic tone[37] by a number of mechanisms. It causes an increase in the sensitivity of the baroreceptor reflex[38] which improves the tissue oxygenation[39] and favorably affects the nervous system metabolism and autonomic functions.[5],[31],[40],[41] Increased baroreceptor sensitivity is attributed to possible mechanisms for the improvement of parasympathetic activity.[42] While scientists reported that regular meditation practices reduce the stress due to changes attributed to head ganglion of ANS.[42] The GSR of the skin depends on a number of factors, the most important is the presence or absence of sweat. Sweat contains water and electrolytes and hence decreases the resistance to the passage of current, thereby decreases the GSR. An increase in the sympathetic tone increases sweating, and thereby decreases the GSR.[31] Autonomic sympathetic changes alter sweating and blood flow which, in turn, affect GSR. If the sympathetic branch of the ANS is highly aroused, sweat gland activity will also increase, which, in turn, decreases the skin resistance.[1] Whereas, increases in skin resistance can be due to the inhibition of the posterior hypothalamic area; or these changes can be due to the effects of the hypothalamus as it acts on the medullary centers through the reticular activating system.[11] Pathways of regulating SNS appear to be less sensitive to excitatory stimuli and more sensitive to inhibitory.[43]
Possible underlying mechanisms of parasympathetic modulation
Higher HRV in experienced yoga practitioners can hence be attributed to its influence on the ANS through the brain stem region.[44] The reduced HR is the cause of decreased sympathetic activity with an alter in the autonomic stability toward parasympathetic dominance,[37] which may cause vasodilation and increase the blood supply to different tissues in the body.[45] During the state of concentration in Yoga Nidra PNS activates and the locus ceruleus stimulation of the paraventricular nucleus of the hypothalamus decreases, which also decreases in baroreceptor stimulation and secondarily releases its inhibition of the supraoptic nucleus, leading to the release of arginine vasopressin.[24],[46],[47] Whereas baroreceptors initiates increased afferent nerve outflow to the solitary tract nucleus and this activation of the vagal nucleus and inhibition of neurons of the vasomotor center, increasing vagal tone, and decreasing sympathetic outflow to the heart and blood vessels.[22] Parasympathetic influence on HRV is mediated through the release of acetylcholine with the vagus nerve.[24] Muscarinic acetylcholine receptors respond to this release mostly by increases in cell membrane K+ conductance. Acetylcholine also inhibits hyperpolarization that activates pacemaker current.[48] Reductions of resting HR appear to be associated with reductions of efferent sympathetic neural outflow to the sinoatrial node, which enhances the higher HRV.[49],[50],[51] Basically, sympathovagal balance is tonically and phasically modulated by at least three main factors, central neural integration, peripheral inhibitory reflex mechanisms, and peripheral excitatory reflex mechanisms.[16],[52],[53] In this way, the vagal and sympathetic activities constantly interact with each other. The sinus node is affluent in acetylcholinesterase, where the effect of vagal impulse is concise because the acetylcholine is swiftly hydrolyzed.[24],[54] Scientists found, a single yoga session can lead to the improvement in sympathovagal balance[55] which indicates to enhance cognitive performance and it may cause better oxygen saturation and reduced chemoreflex sensitivity.[9],[56] Blood pressure is regulated by arterial baroreceptors which alter HR through the CNS through slower sympathetic and the faster vagal action. The baroreceptors provoke a slow sympathetic extraction in the vessels.[57] The interruption in the sympathetic branch of the baroreflex, in turn, results in the synthesis of a new oscillation, which is sensed by the baroreflex and creates a new oscillation in HR.[58] Inhibitory impulses and the hyperpolarization currents can synchronize neural elements leading to modulation of the nervous system and decrease metabolic activity which indicates increase in the parasympathetic state.[10] Researchers found that, yogic breathing influences the stretch receptors in the alveoli, baroreceptors, chemoreceptors, and sensors throughout the respiratory structures and sends the information about the state and activity of the respiratory system through vagal afferents and brainstem relay stations to other CNS structures, where that influences perception, cognition, emotion regulation, somatic expression, and behavior.[59],[60],[61] Furthermore, yoga reduces the risk of cardiovascular diseases through the mechanisms of parasympathetic activation coupled with decreased reactivity of the sympathoadrenal system and hypothalamus–pituitary–adrenal (HPA) axis.[62],[63] Sometimes, low dose scopolamine affects the vagotonic influences which directly increase the HRV,[54] that also indicates the pharmacological modulation of neural activity with increased vagal activity.[54] Atropine and scopolamine of low dose muscarinic receptor blockers may produce a paradoxical vagal efferent activity which increases the HRV.[54],[64]
Possible underlying mechanisms of sympathetic modulation
The decreases in sympathetic tone may be speculated to reduce the magnitude of reflex augmentation in sympathetic activity occurring as a reflex postural readjustment.[12] Decreased LF component of HRV indicates increases in baroreflex gain, which decreases sympathetic outflow.[58] Reduction of LF band spectrum may be accredited to inhibition of the sympathetic nerve of the hypothalamus which also optimizes the body's sympathetic responses to any stressful stimuli. This helps the body to restore autonomic regulatory reflex mechanisms associated with stress.[24],[65] The sympathetic dominance on HRV is interposed by secretions of epinephrine and norepinephrine hormones.[24],[54],[66] While parasympathetic dominance exceeds sympathetic activation probably through two independent mechanisms: A cholinergically instigating reduction of norepinephrine secretion in response to sympathetic neural activity and a cholinergic attenuation of the response to adrenergic impulses.[24],[54] Efferent sympathetic and vagal activities intend to the sinus node which is characterized by discharge largely synchronous with both cardiac cycles which can be modulated by the central and peripheral oscillators.[54],[67] Tachycardia is generally associated with a marked diminution in the total spectrum of HRV during sympathetic activation, whereas the reverse result occurs during the vagal activation.[54] Yoga reduces autonomic reactivity as a result of downregulation of the HPA axis and SNS responses.[35] Scientifically, sympathetic and vagal outputs from the brain mainly occur from the limbic hypothalamic medullary axis.[24] Vasomotor center and vagal nuclei in the medulla of the brain are the sympathetic and parasympathetic outflow nuclei, respectively, which are controlled by the anterior and posterior nuclei of the hypothalamus.[10] The neural connections convey information from the vagus nerves to the thalamus, basolateral nucleus of the amygdala, hypothalamus, anterior insula, and prefrontal cortex that mediate interoceptions, threat perceptions, and affective states. Through this network, vagal activity influences emotional states and thought processes as well as their somatic expression.[61],[68] The myelinated vagal efferent fibers enable rapid control of the HR by increasing vagal tone to reduce HR and blood pressure or decreasing vagal tone to accelerate HR which promotes calm states consistence with the metabolic demands of growth, repair, and restoration.[61],[69] The myelinated vagal brake failure of SNS is recruited to regulate metabolic output in response to stress with negative health consequences such as hypertension, hyperarousal, and over reactivity.[61],[70] The role of the vagus in social interactions, according to the polyvagal theory, is extended in the neurovisceral integration model of affect regulation. It proposes that dysfunctional psychological states are rooted in an impaired vagal inhibitory mechanism which is associated with low HRV.[71],[72] Basically, the polyvagal theory suggests that periods of stress functionally remove the potent vagal break from the heart and thereby facilitate increased metabolic output to fuel the sympathetic fight and flight reaction.[9],[24] A overactive autonomic imbalance may be the cause of dysregulated SNS and HPA axis,[50],[73],[74] while sympathetic tone decreases with increasing inhibitory neural discharge.[75],[76],[77] Reciprocal changes in parasympathetic and sympathetic nerve activity do not always occur even during the activation of the baroreceptor reflex.[78],[79] The neurovisceral assimilation model describes how the prefrontal cortex regulates in limbic action which helps to restrain parasympathetic activity and activate sympathetic circuits.[80] Another scientific experiment suggests that the practice of yogic asana helps to reduce the SNS activity by increasing catecholamine, improved thyroid functioning, and microcirculation of the body. This leads to increasing levels of glucose to the brain and other parts of the body, which helps to enhance mental and physical efficiency.[81]
| Conclusions | | |
Single bout-specific yogic posture (asana) practices for a stipulated period may enhance immediately the parasympathetic (vagal) dominance, which increases the autonomic flexibility and associates with a calm mental state. More than 22.5-min yogasana (posture) practice influences the sympathetic arouseness which may exceed the yogic nature and transplant to physical exercise form.
Limitations
The physiological baseline of the subjects before starting the experiments was beyond the control of the researcher which might influence the autonomic responses. The small sample size was the major limitation because very few subjects were able to fulfill the purpose of the yogasana intervention. Control group was another limitation where this study was implemented self as control design and subjects acted themselves as control and practiced with consecutive three interventions. We used a 10 channel NeXus instrument which was relatively lower than the current source estimate standards of 256 channels.
Acknowledgments
The researchers are thankful to the University Grant Commission, Ministry of Human Resource Development, Government of India, for their support.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]
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