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The Neonatal Niche
Neonates face problems not faced by adults Unique to neonates because of their size, anatomy and physiologyNeonates adopt their own strategies to cope with these unique problems
Coping strategies involve both behavior and physiology
Thermoregulation in
Altricial Neonates
Thermoregulation
Homeothermy—the physiological and behavioral maintenance of a relatively constant internal body temperature (homeothermstypically show circadian fluctuations in temperature)
Poikilothermy—the fluctuation of internal body temperatureclosely related to environmental temperature.
Ectothermy—use only behavioral means to regulate temperature.
Endothermy—physiological and behavioral thermoregulation, but body temperatures may fluctuate widely.
Cold blooded Warm bloodedpoikilothermy ectothermy endothermy homeothermy
Two aspects of thermoregulation
1. Heat gainvolume of thermogenic tissuebasal metabolic rateshivering or other thermogenesis
2. Heat losssurface areainsulation—fat + feathers or hairpanting , sweating, etc
Receptors for Temperature
Peripheral thermoreceptors are found in the skin
Central thermoreceptors are found in the anterior hypothalamus
These thermoreceptors are important for behavioral and physiological thermoregulation both in the short term and in the long term.
Short- and Long-term Thermoregulation
Short term thermoregulation is predominantly regulated by autonomic and somatic motor activity
Autonomicshivering when cold, sweating when hotperipheral vasodilaton when hot, constriction
when coldpiloerection and panting in non-human mammals
Somaticseeking warm (e.g. sunny) or cool (e.g. shade
or water) areasminimizing or maximizing surface area
Long term (e.g. seasonal or as adaptation to different climates) thermoregulation is predominantly regulated by hormonal regulation of metabolism.
Thyrotrophin releasing hormone (TRH) is secreted from the hypothalamus to stimulate release of thyroid-stimulating hormone (TSH, also thyrotrophin), which stimulates releaseof thyroxine from the thyroid gland. Tissues respond with an increase in their basal metabolic rate.
Short- and Long-term Thermoregulation
Autonomic and hormonal responses to thermal challenges are mediated primarily by the medial preoptic area.
Behavioral responses to thermal challenges are mediated principally by the lateral hypothalamus.
Maintenance of thermal homeostasis is arguably the greatest factor influencing both energy balance (via calories spent for thermogenesis and the need for fat stores)and water and mineral balance [water lost via evaporation through the skin, metabolic processes necessary for homeothermy and (in many non-humans)], panting.
Thomas, K. (1994) Thermoregulation inneonates. Neonatal Network, 13, 15-22.
Mechanisms of Thermoregulation in
Adult Mammals
1. Sensory and Motor Immaturity2. CNS Immaturity3. Physiological Immaturity
e.g. autonomic control, homeostaticregulation
4. Morphological Immaturitye.g., small size, allometric growth
Special ProblemsFaced by
Altricial Neonates
1. cannot shiver2. has precarious energy balance3. small size4. cannot vasoconstrict5. no hair6. little body fat, thin skin
Thermoregulatory Problems of Altricial Mammals
Linear 1 2 3 4 5Surface 1 4 9 16 25Volume 1 8 27 64 125
The Problem withBeing Small
S:V = linear dimension 2/3
age wt.(g) vol.(cm3) area (cm2 ) s:v s:v, x adult
adult 400 400 328.5 0.8:1 1.060 day 200 200 206 1.1 1.2510 day 30 30 58 2:1 2.5newborn 5 5 17.5 3.5:1 4.4
Effects of Growth onSurface:Volume Ratios, Rat
Brown fat accountsfor 5 to 6 percent ofthe body weight of the newborn rabbit.It is concentrated, asshown in sections,around the neck andbetween the shoulder blades.
Human infant atbirth has a thin sheet of brown adipose tissue between the shoulder blades and around the neck, and small deposits behind the breastbone and along the spine.Dawkins, M.J.R. & Hull, D (1965) The production ofheat by fat. ScientificAmerican, 213, 62-67.
Bignall, K.E., Heggeness, F.W. & Palmer, J.E. (1975) Experimental Neurology, 49, 174-188.
Transection at level 3
Taylor, P.M. (1960) Oxygen consumptionin new-born rats. Journal of Physiology,154, 153-168.
Behavior of rat-shaped robots programmed tomove randomly without sensory controls
37o C
Cold Hot
37o C
Cold Hot
ThermoregulationSummary
Problem: Cannot shiver to produce heatSolution: Brown adipose tissue (BAT)
Problem: Precarious energy balanceSolution: Thermoregulate over narrow range
Shut down to conserve energy Tolerate lowered temperature
Problem: Small size, large surface:volume ratioSolution: Clump to reduce surface:volume ratio
Problem: Poor insulationSolution: Clump, nest insulation, thermotaxis
Also: Ultrasound in response to isolation
ThermoregulationSummary
Non-shivering thermogenesis(BAT thermogenesis)
Neonatal adaptation?
Precursor to adult thermogenesis?
Epiphenomenon?
Not for most mammals, butcould be considered a precursor for hibernators
Is huddling a social behavior?
Factors Contributing to Neonatal Vulnerability to Thermal Stress
Heat Production•The ability to increase metabolic rate in response to cold stress begins around 28—30 weeks postconceptional age. Postconceptionally older infants can increase heat production, but the response is weaker than in the adult. 4’16•Limited stores of metabolic substrates (glucose, glycogen, fat, etc.).•Heat production needs met primarily through non-shivering thermogenesis; however, the amount of brown fat stores are inversely related to gestational age.•Heat production obligates oxygen consumption, challenging the immature cardiovascular and pulmonary systems.•Large surface-to-mass ratio and large surface heat loss relative to heat-producing ability result in high metabolic rate.•Large evaporative loss due to status of skin maturation; evaporative loss itself may exceed heat production abilities.11’23•Shivering response not well developed. Cannot initiate increased tone and shivering to increase heat production.
Insulation•Limited layer of subcutaneous fat, limited development of muscle and other tissues that provide insulation.4•Small body diameter results in thinner layer of still air boundary layer, reducing insulation through this mechanism.
Vasomotor Response•Competent abilities to regulate skin blood flow documented in infants weighing >1 kg; however vasaconstriction abilities are outmatched by propensity for heat loss.4
Sudomotor Response•Sweat production observed in infants of 29 weeks gestational age; maturation of response enhanced by extrauterine development. Response is slower and less efficient than in older child or adult, and occurs at a higher environmental temperature.24
Motor Tone and Activity•Lower postconceptional aged and ill infants prone to decreased motor tone and less activity, resulting in decreased heat production. Infant with poor tone cannot use flexion posture effectively to reduce surface area and hence heat loss.
Behavioral•Limited ability to effectively communicate thermal needs or thermal comfort to caregiver. •Cues are subtle and nonspecific.•Cannot use volitional actions such as altering clothing, increasing ambient temperature, using motor activity to increase heat production, drinking warm or cool beverages to modify temperature.
Thomas, K. (1994) Thermoregulation inneonates. Neonatal Network, 13, 15-22.
Thomas, K. (1994) Thermoregulation inneonates. Neonatal Network, 13, 15-22.
Adult thermoregulation
1. Brown adipose tissue (BAT; brown fat)
brainstem activation
rostral inhibition(NOT hypothalamus)
Feedback
Blood glucose,Thermoreceptors
Infant thermoregulation