125:315
BME Measurements and Analysis Laboratory
Fall 2004
Laboratory #13:
Neurophysiology
I. Objectives
1. To examine the relationship between the autonomic and somatic nervous systems.
2. To assess activity of your autonomic nervous system while you are doing the “SAT” test.
3. To test the theory that your performance is affected by cardiovascular “resonance, a state that can be induced by slow deep breathing, generally about 6 breaths per minute.
II. Introduction
The autonomic nervous system
The autonomic nervous system (ANS) consists of a network of nerves and hormonal pathways whose job is homeostasis: maintaining a stable internal environment of the body. The main targets of autonomic nerves and hormones are key internal organs: the pacemaker of the heart, the smooth muscles of vessels and organs, glands and viscera. Sensory and motor nerves of the ANS enter and exit the spinal cord at different neurological levels. The ANS has two major nervous limbs or divisions that operate in parallel to maintain homeostasis: the sympathetic nervous system (SNS) and parasympathetic nervous system (PNS).
The two ANS limbs reciprocate to maintain homeostasis. For example, an increase in SNS activity increases heart rate (HR) and contractility, whereas an increase in PNS decreases them both. Although the sympathetic and parasympathetic systems are largely antagonistic in their action, they share anatomical similarities, since they both leave the CNS as preganglionic neurons and synapse with the postganglionic neurons. While the ANS generally works without voluntary control or consciousness, the brain, such as the hypothalamus and the limbic system, do play important roles in regulating it.
The Sympathetic Nervous System (SNS)
The sympathetic nervous system is a widely distributed motor system with a series of 22 ganglia located along the length of the vertebral column. Preganglionic neurons in the sympathetic system utilize acetylcholine as neurotransmitter. Norepinephrine released by the sympathetic neurons is dissipated by either being taken back into the synaptic nerve endings or action of the enzyme monoamine oxidase. The overall duration of the process can be hours long, so the effect of sympathetic system is both widespread and long lasting.
The Parasympathetic Nervous System (PNS)
The PNS is less widely distributed than the sympathetic and the parasympathetic preganglionic neurons synapse with only a relatively few postganglionic neurons. Therefore, parasympathetic neurons are relatively more precise and localized in their effects. The parasympathetic preganglionic neurons are located either in the brainstem as part of the cranial nerves or sacral spinal cord (S2 – S4).
II. 1 Control of the heart and vascular system
The cardiovascular system consists of the heart, the systemic and pulmonary arterial systems, microcirculatory vessels, and the venous systems. The cardiovascular system adapts the body to different physiological states, such as exercise, stress, and injuries. Modulating the cardiovascular system in accord with need is one of the most important functions of the ANS.
Although the heart pumps periodically, continuous flow to the peripheral vessels is accomplished by distending the aorta and large arteries during ventricular systole and recoiling of the aorta and these arteries to perfuse the peripheral organ vasculature during ventricular diastole.
Regulation of heart rate
A normal human heart beats with the sino-atrial or SA node serving as pacemaker and without conduction defects, which is called a sinus rhythm or a normal rhythm. Most heart rate changes are mediated by autonomic nervous inputs to the SA node, which consists of the sympathetic and parasympathetic nervous systems, along with hormonal influences. Efferent fibers of the SNS and PNS travel to the heart and blood vessels where they modulate activities of these target organs. The preganglionic parasympathetic nerves to the heart originate in the medulla oblongata cells that lie in the dorsal motor nucleus of the vagus. Fibers from these nuclei pass through the neck via the vagus nerves and synapse with postganglionic neurons located directly on portions of the heart. The right branch of the vagus nerve primarily innervates the pacemaker region whereas the left one mainly innervates the atrio-ventricular node. Atrial and to a lesser degree, ventricular muscles are also innervated by vagal nerves.
Sympathetic nerves to the heart originate in the upper five or six thoracic and lower one or two cervical segments of the spinal cord. Sympathetic nerves are present throughout the atria, especially the SA node, and ventricles and the conduction system of the heart. Under normal conditions, sympathetic and parasympathetic systems regulate the heart rate concomitantly. An increase in heart rate is usually achieved by an increase in sympathetic activity and concomitant decrease in parasympathetic activity, while a reduction in heart rate is produced in an opposite manner.
Neurotransmitters released from sympathetic and parasympathetic nerves affect the heart rate by altering currents across the pacemaker cell membranes. Norepinephrine released by sympathetic stimulus increases the heart rate. Through the release of acetylcholine, parasympathetic stimuli diminish heart rate. Because the cardiac tissues are rich in the enzyme acetylcholinesterase, which breaks down acetylcholine rapidly, the effects of parasympathetic stimulation decay quickly. Due to the brief latency and rapid decay of response, the parasympathetic system can exert a beat-to-beat control of heart rate and predominate over sympathetic effects.
Heart rate is also regulated by higher centers in the cerebral cortex, the thalamus and hypothalamus. Cortical and diencephalic centers are responsible for the variations in heart rate during emotional states, and the hypothalamic centers are involved in the alteration due to environmental temperature. In addition to the nervous control of heart rate, there are hormonal and respiratory influences. The heart rate variation of cardiac transplant patients provides an indication of these influences, since they possess no direct nervous control.
The heart generates a sequence of electrical activity during each beat. Since the body tissues surrounding the heart are conductive to electricity, a fraction of the cardiac electrical activity can be recorded on the body surface. The electrocardiogram (ECG) consists of several waves, the largest of which, the R wave, represents systole.
Regulation of Blood Pressure
Arterial pressure is determined by several physical factors, including arterial blood volume, peripheral resistance and the arterial compliance (Li, 2000). The mean arterial pressure can be changed by modifying the peripheral resistance, which affects capillary flow. Generalized arteriolar constriction increases the peripheral resistance, and thus the outflow from the arterial reservoir is reduced. If the cardiac output remains constant, the imbalance between the input and output of the arterial system will result in a rise in mean arterial pressure. A reverse alteration, dilation of the resistance vessels, will drop the mean pressure.
The ANS is pivotal to the maintenance of adequate arterial pressure. Under physiologic conditions, systemic arterioles receive a continuous barrage of sympathetic vasoconstrictor impulses, which originates from a group of cells called the vasomotor center in medulla. Consequently, the smooth muscle in the wall of its arterioles constricts constantly, and blocking the sympathetic supply to the arteriole permits the vessel relax. Resistance to the vascular flow is then decreased. The vasomotor center can be divided into vasoconstrictor and vasodilator areas. The vasodilator area does not exert its effect directly on the vascular system but rather acts by inhibiting the action of vasoconstrictor area (Little, 1977). The discharge rate of the vasoconstrictor area can be modified by a large number of afferent impulses, among which the negative feedback from the aortic and carotid baroreceptors is a critical one. Under resting conditions, the arterial pressure is determined mainly by the peripheral resistance. As a result, the blood pressure will be controlled within the normal range by the acute negative feedback reflex depending on the baroreceptors.
Origins of Heart Rate Variability and Blood Pressure Variability
Cardiovascular fluctuations, most commonly referring to heart rate and blood pressure variability, are integral features of health ; they also provide a rich source of insight information about cardiovascular control. HRV occurs in several frequency bands, as listed in Table 1. The most prominent rhythmic heart rate and blood pressure oscillations in humans are related to respiratory activity. This rhythm is called “respiratory sinus arrhythmia” (RSA) and operates in synchrony with breathing frequency, generally ranging from 0.15 to 0.4Hz. RSA is generally believed to be mediated predominately by fluctuation of vagal activity, because the slow and latent effect of the sympathetic nerve is damped out.
Heart rate and blood pressure also oscillate at lower frequencies than the respiratory frequency. Oscillations with a frequency at about 0.1Hz (from 0.05 to 0.15Hz) are defined as low frequency (LF). LF components reflect both sympathetic and vagal activities. The oscillations with frequency between 0.003 to 0.05Hz are termed as very low frequency (VLF) and may reflect thermoregulatory cycle or fluctuations related to plasma renin activity. The frequencies of heart rate variability (HRV) and blood pressure variability (BPV) lower than 0.003Hz is ultra low frequency (ULF). The ULF component is affected by a wide range of factors including posture, autonomic pathways, and behavioral variables. (Xiao et al., 1999).
Even though BPV shares the same frequency components with HRV, the changes in BPV can vary from those in HRV, especially under some pathologic conditions. For example, HRV may be diminished in hypertensive patients, whereas blood pressure variation increases as the baroreflex regulation is reduced. Therefore investigation in both HRV and BPV can disclose more comprehensive view about the cardiovascular regulation.
Table 1: Frequency components of heart rate variability and blood pressure variability.
Components / Frequency (Hz) / Likely OriginsHF / 0.15 - 0.40 / Parasympathetic outflow
LF / 0.05 - 0.15 / Both sympathetic and parasympathetic outflow or sympathetic outflow
VLF / 0.003 - 0.05 / Possibly thermoregulatory or plasma rennin activity
ULF / <0.003 / Wide range of determinants like posture, autonomic pathways, behavioral variables
Cardiovascular Resonance
Oscillations of the ANS can be put into ‘resonance’ by certain behaviours. One method is to breathe deeply at the correct resonant frequency (Leher & Vaschillo, 2003). For most individuals, this frequency is about 6 breaths per minute. The technique involves abdominal breathing through pursed lips with a prolonged exhalation.
The main function of the heart is to pump blood through two circuits:
II.3 Methodology
Heart Rate Variability (HRV) Measurements
The subject is fitted for heartbeat monitoring with sensors, such as Lead II ECG electrodes or finger plethysmograph for pulse. Breathing can also be recorded using a chest band containing pneumatic sensors. While being recorded, the subject can either be stationary of perform daily activities, including ambulation and moderate exercise.
Obtaining valid HRV measurements depends upon careful data collection, strictly objective procedures for data preparation, and proper statistical and mathematical analyses. The methods used in this project are designed with these considerations, and have been validated in several publications [1-8]. Procedure sequence is as follows:
1. Firstly, the Tachogram, consisting of the sequence of accurate R-R intervals, is collected. This requires accurate detection of R waves, and storage of each interval with sufficient temporal resolution. R waves can be detected from the ECG using a template matching algorithm or a simple amplitude thresholding method. Intervals are stored digitally.
2. Once obtained, the tachogram must be examined and edited if necessary to ensure its suitability for analysis. Suitable tachograms must be (1) free of artifacts, (2) sufficiently long in duration, and (3) stationary. Objective criteria will be used to ensure such suitability of tachogram epochs for HRV analysis. Epochs with excessive artifacts, i.e > 8 per minute, will be rejected. Intermittent artifacts such as non-sinus rhythm beats, ie. premature or aberrant beats, and for spurious noise artifacts or missed beats must be removed and replaced. To prepare epochs containing an acceptable number of artifacts for further analysis, they are replaced with "corrected" beats. Tachogram editing is a multi-level procedure that is objective and applies fixed criteria to all subjects:
a. The first editing level is an automatic algorithm that recognizes and replaces each aberrant interval with a corrected one. An example of the automatic beat classification, using sample parameters is shown in the appendix below: The software program illustrated operates by color coding the abnormal beat intervals. Correction of the intervals then can be done by an algorithm that uses linear predictive coding (LPC) to insert a beat at the most statistically appropriate point. Alternative methods, such as simply inserting a duplicate interval from the preceding one, are also done, but are less accurate than the LPC method, since the latter predicts an interval based on the pattern from several preceding ones.
b. The second editing level, if required, involves manual identification and correction of those artifacts not eliminated automatically. This may be required when multiple artifacts occur in rapid succession.
c. A third editing level is sometimes required, that involves 'cutting and splicing' of the tachogram. This may be done by cutting out epochs (of a few minutes duration) with excessive artifacts, and joining the record with the subsequent record. The spliced interval is determined by LPC. The process selects only stationary epochs.
3. The edited and stationary tachogram, once free of artifacts and of sufficient duration, now can be further analyzed using time domain HRV methods. For this study the minimum data record will be 600 heart beat intervals. Time domain statistics will include all those listed in the Task force on Heart rate variability (Journal of the American College of Cardiology). These include MinN, MaxN, MeanNN, SDNN, RMSSD, PNN50, HRVidx and TINN (as defined by the task force).
4. Spectral analysis measures periodic variation in the heart rate by reducing the heart rate signal into its constituent frequency components and quantifying the relative power. The spectrum is then obtained using the standard periodogram method. This involves applying a Hanning window to the epoch followed by the Fast Fourier Transform of overlapping segments of at least 512 points. Frequency domain measures will include total spectral energy, and energy in selected frequency bands, including ultra low, very low, low, and high, expressed in (beats/min)2. The frequency band limits will be similar to those previously published
5. Spectra of the breathing records will be done using methods similar to the above, and additionally the cross spectrum between respiratory and IHR signals can be estimated for each segment. The complex transfer function will then be estimated using the cross-spectral method and separated into its magnitude and phase components. Distinct patterns for these two components correlate with distinct states of sympathetic and vagal modulation of heart rate. Inference with respect to autonomic states, with respect to sympathetic or vagal modulation will be possible from this analysis.