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UPSC MainsANI-HUSB-VETER-SCIENCE-PAPER-I202510 Marks150 Words
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Q1.

Write short notes on/Answer the following in about 150 words each : (a) Nervous control of respiration in animals.

How to Approach

The question asks for short notes on the nervous control of respiration in animals. The approach should involve defining respiration's nervous control and then detailing the key components: respiratory centers in the brainstem (medulla and pons), chemoreceptors (central and peripheral), and reflexes (e.g., Hering-Breuer). Structure the answer with an introduction, separate sections for each component, and a concise conclusion. Emphasize how these elements work together to regulate breathing.

Model Answer

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Introduction

The nervous control of respiration in animals is a complex and finely tuned physiological mechanism that ensures adequate oxygen uptake and carbon dioxide elimination to maintain metabolic homeostasis. Unlike the heart, which has intrinsic rhythmicity, breathing is primarily controlled by neural centers located in the brainstem, which generate and modulate the rhythmic pattern of inspiration and expiration. This involuntary control is continuously adjusted based on the body's metabolic demands, blood gas levels, and various reflexes.

Central Respiratory Centers

The primary control centers for respiration are located in the brainstem, specifically the medulla oblongata and the pons. These centers are responsible for generating the basic rhythm of breathing.
  • Medulla Oblongata:
    • Dorsal Respiratory Group (DRG): Primarily controls inspiration, receiving sensory input from peripheral chemoreceptors and mechanoreceptors. It sends signals via the phrenic nerves to the diaphragm and intercostal nerves to the external intercostal muscles, initiating inspiration.
    • Ventral Respiratory Group (VRG): Contains both inspiratory and expiratory neurons. It remains inactive during quiet breathing but becomes active during forceful breathing, sending signals to accessory muscles for forced inspiration and expiration. The pre-Bötzinger complex within the VRG is crucial for generating the respiratory rhythm.
  • Pons:
    • Pneumotaxic Center (Pontine Respiratory Group): Located in the upper pons, it regulates the rate and pattern of breathing by inhibiting inspiration, thus limiting the duration of inspiration and allowing for a smooth respiratory cycle.
    • Apneustic Center: Located in the lower pons, it stimulates the inspiratory neurons of the DRG, promoting deep and prolonged inspiration. Its activity is inhibited by the pneumotaxic center and stretch receptors.

Chemoreceptors

Chemoreceptors monitor blood gas levels and pH, providing feedback to the respiratory centers.
  • Central Chemoreceptors: Located in the medulla oblongata, these are highly sensitive to changes in the partial pressure of carbon dioxide (PCO2) and hydrogen ion (H+) concentration in the cerebrospinal fluid. An increase in PCO2 (leading to a decrease in pH) strongly stimulates these receptors, increasing ventilation.
  • Peripheral Chemoreceptors: Found in the carotid bodies (at the bifurcation of the common carotid arteries) and aortic bodies (in the aortic arch). They primarily respond to significant decreases in arterial partial pressure of oxygen (PO2), but also to increases in PCO2 and H+ concentration. They send signals to the DRG via the glossopharyngeal and vagus nerves, respectively.

Pulmonary Reflexes

Various reflexes originating in the lungs and airways also modify respiratory patterns.
  • Hering-Breuer Reflex: Triggered by stretch receptors in the walls of the bronchi and bronchioles, this reflex prevents over-inflation of the lungs. When the lungs are excessively stretched, these receptors send inhibitory signals via the vagus nerve to the inspiratory centers, shortening inspiration and promoting expiration.
  • Irritant Reflexes: Receptors in the airway mucosa respond to irritants, leading to coughing, sneezing, or bronchoconstriction to clear the airways.

The coordinated activity of these neural components ensures that animals maintain efficient gas exchange, adapting to varying physiological conditions and metabolic demands.

Conclusion

The nervous control of respiration in animals is a sophisticated interplay of central pacemakers, chemosensitive feedback loops, and protective reflexes. The respiratory centers in the brainstem establish the fundamental breathing rhythm, while central and peripheral chemoreceptors continuously monitor blood gas levels, ensuring optimal oxygenation and carbon dioxide removal. Furthermore, pulmonary reflexes like the Hering-Breuer reflex safeguard lung integrity. This intricate neural network allows animals to precisely adjust their breathing patterns to meet changing metabolic requirements, from rest to intense physical activity, thereby maintaining vital physiological balance.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Pre-Bötzinger Complex
A small, bilaterally located region within the ventral respiratory group of the medulla oblongata that is considered crucial for generating the basic respiratory rhythm in mammals, acting as an inspiratory pacemaker.
Hering-Breuer Reflex
A protective reflex initiated by pulmonary stretch receptors in the lungs that inhibits inspiration and promotes expiration when the lungs are over-inflated, preventing lung damage.

Key Statistics

A rise in brain PCO2 of approximately 2 mmHg (0.5% change from normal values) can increase resting ventilation by around 50% in a conscious goat, highlighting the high sensitivity of central chemoreceptors. (Source: Pappenheimer et al., 1965, as cited in relevant physiological literature)

In humans at rest, ventilation approximately doubles for a 1.5 mmHg rise in arterial PCO2. (Source: Haldane and Priestley, 1905, as cited in relevant physiological literature)

Examples

Diving Reflex in Aquatic Animals

Many aquatic mammals and birds exhibit a pronounced diving reflex, where submersion in water causes a reflexive inhibition of breathing (apnea), along with bradycardia (slowed heart rate) and peripheral vasoconstriction. This reflex is neurally mediated and helps conserve oxygen during prolonged dives.

Respiratory Response to Exercise

During strenuous exercise, mechanoreceptors in the exercising muscles and joints send signals to the respiratory centers, along with increased CO2 production and oxygen demand. This neural input, combined with chemoreceptor stimulation, causes a significant increase in both the rate and depth of breathing to meet the elevated metabolic needs.

Frequently Asked Questions

▶What is "Ondine's Curse" and how is it related to nervous control of respiration?

Ondine's Curse, or Congenital Central Hypoventilation Syndrome (CCHS), is a rare disorder where the automatic nervous control of breathing is impaired. Individuals with CCHS must consciously remember to breathe, especially during sleep, as their brainstem respiratory centers fail to function automatically. It highlights the critical role of the autonomic nervous system in respiration.

▶Do higher brain centers influence breathing?

Yes, while the basic rhythm is involuntary, higher brain centers like the cerebral cortex can exert voluntary control over breathing, allowing actions like holding breath, speaking, or singing. The hypothalamus and limbic system also modulate respiration in response to emotions, pain, and changes in body temperature.

Topics Covered

Animal PhysiologyZoologyRespirationNervous SystemBreathing Mechanisms
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