Download Lec 7 Lab Demo Handout

BPK 422 Lab Handout for Demonstration
PART 1) Eccrine Sweating
A brief synopsis of the physiology and function of eccrine sweat glands is given as background for the first part of
this lab. Eccrine sweat glands are under sympathetic cholinergic control and the density of these eccrine sweat
glands vary over the surface of the body. These glands secrete fluid that varies in their electrolyte and solute
concentrations as a function of training and acclimation state of the individual. Evaporation of eccrine sweat from
the skin surface provides the most powerful cooling system for the human body.
Theory of Eccrine Sweating Measurement: Evaporation of eccrine sweat from the skin is considered quantitatively
to be the most important avenue of heat loss for humans during hyperthermia. Forehead eccrine sweat rate (Esw) in
this lab will be measured using the ventilated capsule method. A capsule (surface area: 5.31 cm2) will be secured to
the forehead using a headband and flushed with a dry compressed air source a rate of 1 L • min -1. The relative
humidity (RH) of the dry air source will be verified by a resistance hygrometer (RH201C, Omega Engineering Inc.,
Stanford, CT, USA) in a sealed container. Next air will be passed through a capsule on the forehead where it will be
humidified after the participant begins to sweat. Finally, the humidified air will be passed from the forehead capsule
through a third sealed container enclosing a capacitance hygrometer (HMT337, Vasaila, Helsinki, Finland) in order
to measure the RH of the air after passing over the forehead. The HMT337 capacitance hygrometer is employed
because it is capable of measuring RH in near condensing conditions. Eccrine sweat rate will be calculated with the
equation in Bullard (1) with the inclusion of the surface area (SA) of the sweat capsule (m 2):
ESW (mg∙m-2∙s-1) = FAIR (Ls-1) • ( RH ÷ 100) • ρair (mgL-1)] • [SA (m2)]-1
(1)
where ESW is the calculated eccrine sweat rate; FAIR is the air flow rate through the forehead capsule;  RH is the
change in RH between the first and second humidity sensor; ρair is the density of the saturated steam at the ambient
dry bulb temperature (°C), use 19.877 mg/L for ρair; and SA is the surface area of skin covered by the sweat capsule.
Human Body Surface Area Calculator: http://www.globalrph.com/bsa2.cgi
Online Conversions: http://www.onlineconversion.com/density_common.htm
PART 2) Cutaneous Blood Velocity:
A brief synopsis of the physiology and function of cutaneous vasodilatation is given as background for this second
part of this lab. Cutaneous vasodilatation is a single-phase response in acral regions and a 2-phase response in nonacral regions of the body. In the acral regions the cutaneous blood vessels are under a sympathetic noradrenergic
control where non-acral regions are under noradrenergic vasoconstrictor control and a non-adrenergic active
vasodilator system. The first phase in the non-acral regions of the body surface includes a sympathetic noradrenergic
control followed by an active cutaneous vasodilatation control that includes acetylcholine and unknown cotransmitter(s).
Cold-induced reflex vasoconstriction in the limbs can substantially reduce peripheral blood flow and allows a
conservation of heat. The CNS mediated mechanism of vasoconstriction is via adrenergic sympathetic neurons
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2) receptors on vascular smooth muscle. Other
details of this reflex vasoconstriction, that appears to involve co-transmitters and local cooling induced
vasoconstriction, are given by Kellogg 2006.
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Theory of Laser Doppler Velocimetry: A beam of laser light, carried by a fibre-optic probe, is widely scattered and
partly absorbed by surface tissues. Light hitting moving blood cells in these tissues undergoes a change in
wavelength known as a Doppler shift. The wavelength of light hitting static objects is unchanged in these tissues.
The magnitude and frequency distribution of these changes in wavelength are directly related to the number and
velocity of blood cells but unrelated to their direction of movement. The information is picked up by a returning
fibre or fibres, is converted into an electronic signal. No currently available laser Doppler instrument can present
absolute perfusion values (e.g. ml/min/100 gram tissue). Measurements are expressed in arbitrary Perfusion Units
(PU) or Arbitrary Units. To enable comparison of results, it is important to calibrate the laser Doppler.
Moor Instruments laser Doppler Monitor for assessing skin blood velocity:
Display:
CH1 = Channel 1, CH2 = Channel 2 etc
P1= Probe 1, P2= Probe 2 etc
FLUX = estimate of the red blood cell flux in the cutaneous tissues. It is a quantity proportional to the product of the
average speed of the blood cells and their number concentration (often referred to as blood volume).
CONC = estimate of the red blood cell concentration in the cutaneous tissues
DC = the reflectance value of the tissue. This value gives a measure of the total intensity of the backscattered light
Typical DC values for a standard optical probe (e.g. MP 1 or MP 2) is 30 to 50 although this value varies
FS = Output scale settings for analog output. The default setting is 1000 arbitrary units (AU) for 0 to 10 V. Other
scale settings are 500, 200, 100, 50, 20 and 10.
Calibration: The laser Doppler probes are calibrated with polystyrene microspheres undergoing thermal or
Brownian motion. The average speed of the microspheres is proportional to the square root of the temperature in
Kelvin. Ideally the calibration is at the same temperature, although a change of ±5°C gives only ±1% change in the
mean speed of the microspheres.
Data Output Rate/Frequency: This varies with the number of channels. For 1 to 2 channels the frequency is 40Hz
for 2 to 3 channels the frequency is 20Hz.
Units: Arbitray Units (AU) are employed for FLUX, CONC, SPEED and DC.
Flux Accuracy: ± 10 AU for a Moor Standard DRT4 probe
Flux Precision: ± 3% of measurement value
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PART 3) Shivering Thermogenesis:
Shivering is an involuntary tremor of skeletal muscles not involving voluntary movements or external work. It is a
thermoeffector response giving increased contractile activity of skeletal muscles to increase metabolic heat
production. It is referred to as shivering thermogenesis and has an electromyographically distinct pattern of motor
unit discharges that is quantified as the integrated voltage (V) deflections per unit time (t): Σ(ΔV·Δt)·t –1·[V].
As body temperatures decrease, shivering thermogenesis progresses from
increased thermoregulatory muscle tone, to micro-vibrations, to clonic
contractions of both flexor and extensor muscles. The shivering pathway begins
with signals from the precentral gyrus or the premotor cortex. These impulses
are conducted by upper motor neurons down the corticospinal tract through the
ventral horn of the spinal cord and across synapses to large lower motor neurons
or alpha motor neurons in the brainstem and spinal cord. Upper motor neurons
release acetylcholine (ACh) that is received by sensory receptors of nicotinic receptors on the alpha motor neurons.
Alpha motor neurons send an impulse down their axons via the ventral root of the spinal cord. Subsequently alpha
motor neurons release ACh from their axon terminal knobs at the neuromuscular junctions of skeletal muscles and
this received by postsynaptic nicotinic acetylcholine receptors of skeletal muscles. Shivering thermogenesis is
blocked by curare. Curare, specifically tubocurarine, is a toxic alkaloid that is found as a resinous extract from
tropical plants. It acts as a muscle relaxant and is used in anesthesia. Outside of clinical settings curare was used in
the past in arrow poisons during hunting by people indigenous to South America living in the in the valleys of the
Amazon and the Orinoco rivers. Curare competes with ACh for binding on nicotinic receptors and blocks
transmission of the signal from alpha motor neurons to the skeletal muscle involved in shivering.
Electromyograms (EMG) give one method to quantify shivering thermogenesis. Indirect and direct calorimetry can
also estimate shivering thermogenesis, but the focus here is EMG. Electromyograms give a recording of the ionic
potential associated with the muscle action potentials. With increasing activation of motor units there is an increase
in the electrical activity of the muscle. The EMG method employs surface electrodes that record muscle action
potentials that occur mostly in the frequency range of 30 to 300 Hz. These signals are acquired by a data acquisition
system and then the data is processed to quantify the level of activity in the muscle.
A raw EMG signal typically is given a full wave rectification. This is to invert all signals that are less than the
isoelectric line making all signal values positive. The rectified signal is then typically integrated to give the area
under the curve and this area is proportional to the muscle activity. Another method to quantifying the raw EMG
signal is to calculate a moving average. This moving average method is to allow one to see increases or decreases in
muscle activity as a function of time. The greater number of points chosen for the moving average gives a greater
smoothing of the raw EMG signal.
References:
Bullard WR. Continuous recording of sweating rate by resistance hygrometry. Journal of Applied Physioloy.
1962;17:735-7.
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