(13) Week 4 Lecture 4 Autonomic Pathways

Overview of the Autonomic Nervous System (ANS)

• The ANS is part of the involuntary motor pathways within the nervous system, regulating smooth muscles, cardiac muscles, and glands.

• It is a component of the broader nervous system, which includes:

  • Central Nervous System (CNS): brain and spinal cord.

  • Peripheral Nervous System (PNS): all other neural elements outside the CNS.

  • The PNS is divided into:

    • Afferent Division: carries sensory information to the CNS.

    • Efferent Division: transmits commands from the CNS to the muscles and glands.

Divisions of the Motor Division

• The motor division is further categorized into:

  • Somatic Nervous System: voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS): involuntary control of visceral functions.

ANS Subdivisions

1. Sympathetic Division

   • Known as the "fight-or-flight" system.

   • Activated during emergencies, physical activity, and stressful situations.

   • Physiological changes include:

     • Increased heart rate.

     • Dry mouth and cold sweats.

     • Dilated pupils.

     • Increased blood flow to muscles.

     • Bronchodilation for improved airflow.

     • Enhanced glucose release from the liver.

   • Referred to as "E division" (Emergency, Excitement, Exercise, Embarrassment).

2. Parasympathetic Division

   • Called the "rest-and-digest" system.

   • Functions to conserve energy and maintain daily bodily operations.

   • Engaged during restful activities like digestion and waste elimination.

   • Implies a contrast to sympathetic action (e.g., relaxation after a meal).

   • Referred to as the "housekeeping division" or "D division" (Defecation, Digestion, Diuresis).

Pathway of the Somatic Nervous System

• Lower Motor Neuron Functionality:

  • The pathway consists of:

    • Upper Motor Neuron: originating in the cerebral cortex, synapses with the lower motor neuron.

    • Lower Motor Neuron (LMN): located in the anterior gray horn of the spinal cord.

      • Sends myelinated axons through the anterior root to synapse with skeletal muscles, releasing acetylcholine (ACh) to induce contraction.

  • Key features:

    • Single neuron pathway.

    • Neurotransmitter: always acetylcholine (ACh), leading to muscle contraction.

Pathway of the Autonomic Nervous System

• Characterized by dual innervation with two motor neurons:

  1. Preganglionic Neuron: cell body in the CNS; myelinated axon extends into the PNS to synapse at autonomic ganglia.

  2. Postganglionic Neuron: located in autonomic ganglia; unmyelinated axon projects to target organs.

• Neurotransmitters:

  • Sympathetic Pathway: Releases norepinephrine (NE) at the effectors.

  • Parasympathetic Pathway: Releases acetylcholine (ACh) at the effectors.

Differences between Sympathetic and Parasympathetic Pathways

• Sympathetic System:

  • Ganglia are near the CNS, far from target tissues.

  • Involves a systemic response via the adrenal medulla releasing norepinephrine and epinephrine as hormones for prolonged effects.

• Parasympathetic System:

  • Ganglia are close to the target tissues; greater localized control.

  • Direct neurotransmitter release to effector tissues results in quick physiological changes.

Functional Dynamics between ANS Divisions

• The ANS operates in a tug-of-war dynamic between sympathetic (emergency response) and parasympathetic (maintenance) systems.

• For example:

  • Upon a perceived threat (axe-wielding maniac), sympathetic responses dominate.

  • Post-threat, bodies remain primed for action due to prolonged effects from sympathetic activation, emphasizing the need for systemic hormones (e.g., norepinephrine).

• The somatic motor pathway differs from ANS, only having one lower motor neuron, solely affecting voluntary skeletal muscles via ACh release.

Summary of ANS and Somatic Responses

• Sympathetic Division:

  • Two neurons: preganglionic in the CNS (myelinated) and postganglionic (unmyelinated, releases NE). Includes systemic response through adrenal medulla.

• Parasympathetic Division:

  • Two neurons: preganglionic (myelinated) releasing ACh to the postganglionic neuron, which also releases ACh to target tissues.

• Somatic Motor Pathway:

  • One lower motor neuron releasing ACh to skeletal muscles, leading to voluntary movement.

Conclusion

• Upcoming lecture will cover anatomical features of the nervous system.