The Neonatal Niche

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.



Adult thermoregulation

1. Brown adipose tissue (BAT; brown fat)

brainstem activation

rostral inhibition(NOT hypothalamus)

Feedback

Blood glucose,Thermoreceptors

Infant thermoregulation

Documents

The Neonatal Niche