W1 L6 ANS anatomy

Lecture Overview

Date: 4th March 2025Presenter: Dr. Ian Mullaney, Senior Lecturer in Medical Sciences at QMUL Malta

Learning Objectives

• Understand the anatomy and physiology of the sympathetic nervous system (SNS) and its role in preparing the body for stress-induced responses.

• Comprehend the anatomy and physiology of the parasympathetic nervous system (PNS) and its function in promoting relaxation and energy conservation.

• Explore higher-level control of the autonomic nervous system (ANS) through its relationship with the endocrine system.

• Recognize the critical role of the ANS in the acute stress response and its implications on overall homeostasis and health.

Lecture Content

• Overview of the peripheral and autonomic nervous system, illustrating the distinct yet interconnected roles.

• Detailed functions and mechanisms of the parasympathetic and sympathetic nervous systems, emphasizing their opposite but complementary roles in maintaining bodily functions.

• Discussion of pharmacological agents affecting the ANS:

  • Cholinergic agonists and antagonists, which manipulate acetylcholine pathways.

  • Adrenergic agonists and antagonists that affect noradrenaline responses, altering physiological reactions in stress and relaxation scenarios.

The Autonomic Nervous System (ANS)

• Definition: The ANS works in conjunction with the endocrine system to regulate and harmonize bodily functions, managing both involuntary and subconscious processes that maintain homeostasis.

  • Endocrine system: Relies on slower hormonal signals that can last longer in the bloodstream.

  • ANS: Utilizes rapid electrical impulses that quickly reach effector cells, ensuring immediate responses to changes in the internal and external environment.

• Autonomic Drugs: These are used clinically to modify ANS functions, either by stimulating or blocking autonomic nerves, impacting various body systems such as cardiovascular, respiratory, and digestive systems.

Organization of the Nervous System

• Divisions: The nervous system is divided into:

  • CNS (Central Nervous System): Comprising the brain and spinal cord, responsible for integrating sensory information and coordinating activity.

  • PNS (Peripheral Nervous System): Encompasses nerves outside the CNS and is further divided into:

    • Efferent Division: Transmits signals from the CNS to peripheral tissues and organs.

    • Afferent Division: Carries sensory signals from the body to the CNS.

Efferent Division Subdivisions

• Somatic Division: Controls voluntary movements by innervating skeletal muscles, allowing conscious control and motor function.

• Autonomic Division: Governs involuntary functions that are critical for survival, including:

  • Innervation of smooth and cardiac muscles, which operate independently of conscious control.

  • Glandular secretions, impacting metabolic rates and hormonal balance.

  • Comprised of three subdivisions:

    • Enteric Nervous System (ENS): Manages complex digestive processes independently, often referred to as the "second brain" due to its autonomy and complexity.

Divisions of the ANS

• Sympathetic Nervous System (SNS):

  • Functions to prepare the body for stressful situations (fight-or-flight response) by increasing alertness and physical performance through:

    • Increased heart rate and blood pressure to deliver oxygen and nutrients more efficiently.

    • Bronchodilation, expanding air passages to increase oxygen intake during exertion.

    • Reductions in digestive activity to redirect energy towards muscles.

• Parasympathetic Nervous System (PNS):

  • Promotes relaxation and energy conservation through:

    • Decreasing heart rate and lowering blood pressure.

    • Stimulating digestion and enhancing metabolic activities, facilitating nutrient absorption.

    • Controlling pupil constriction and regulating sexual arousal.

• Enteric Nervous System (ENS): Involuntary control of digestive processes, coordinating actions such as peristalsis without direct CNS input.

Structure of the ANS

• The ANS utilizes a two-neuron system for signal transmission:

  • Preganglionic Neuron: Originates in the CNS and projects to ganglia in the PNS.

  • Postganglionic Neuron: Found in peripheral ganglia, it transmits signals to effector organs.

• Sympathetic Outflow: Initiates from thoracic and lumbar spinal regions (T1-L2,3).

• Cranial Nerve Outflow in PNS: Involves cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), X (vagus), along with sacral spinal regions.

Functions of the ANS

• Manages essential body functions unconsciously, ensuring vital processes such as heart rate, digestion, and blood pressure regulation operate effectively.

• Sympathetic vs. Parasympathetic Responses:

  • Sympathetic: Increases heart rate, facilitates bronchodilation, and decreases digestive activity to prioritize energy allocation.

  • Parasympathetic: Decreases heart rate, enhances digestive processes, and promotes relaxation of body systems.

Neurotransmitters in the ANS

• Main Neurotransmitters:

  • Acetylcholine: Released at all motor nerves leaving the CNS, primarily acts on muscarinic receptors within the PNS to induce rest-and-digest responses.

  • Noradrenaline: Secreted by postganglionic sympathetic fibers, primarily acting on adrenoceptors to mediate fight-or-flight responses.

  • Adrenaline: Produced in the adrenal medulla, playing a crucial role in the stress response and facilitating functions such as circulation.

Effects of the Sympathetic Nervous System

• Activation of the SNS triggers several physiological changes:

  • Increase in heart rate and elevated blood pressure to enhance oxygen distribution.

  • Bronchodilation to facilitate increased oxygen uptake during stress or physical activity.

  • Diminished digestive function to prioritize energy use for immediate physical demands.

  • Stimulation of energy release from the liver, preparing muscles for swift action.

Functions of the Parasympathetic Nervous System

• Encourages processes that conserve energy and promote relaxation:

  • Slowing of heart rate and reduction in blood pressure for a state of calm.

  • Stimulation of digestive activities by augmenting gastric and salivary secretions, enhancing nutrient absorption.

  • Regulation of pupil constriction and sexual arousal responses, vital for reproductive health.

Higher-Level Control of the ANS

• The Hypothalamus plays a critical role in integrating sensory information and coordinating both autonomic and hormonal responses, maintaining homeostasis:

  • Regulates various functions including body temperature, food intake, reproductive activities, and emotional responses.

  • Influences stress responses through adrenal gland activation, leading to increased release of adrenaline and cortisol.

Summary of Neurotransmission in the ANS

• Steps of Neurotransmission (Adrenergic Neurons):

  1. Synthesis of noradrenaline from the amino acid tyrosine in nerve terminals.

  2. Storage of noradrenaline within synaptic vesicles until nerve impulse triggers release.

  3. Release of noradrenaline into the synaptic cleft upon action potential arrival, initiating the signal in target cells.

  4. Binding to specific receptors on target cells, leading to physiological responses.

  5. Removal of neurotransmitter from the synapse via various mechanisms, which may involve reuptake into the presynaptic neuron or enzymatic degradation.

  6. Recycled or metabolized for subsequent use in neurotransmission.

Conclusion

Understanding the intricacies of the ANS's structure and functioning is essential for grasping critical concepts pertaining to stress responses, homeostasis, and the profound effects of pharmacological agents on bodily functions. This knowledge is pivotal for fields such as medicine, psychology, and health sciences as it highlights the interplay between nervous and endocrine systems in maintaining overall bodily health and response to external stimuli.