The Autonomic Nervous System

Anatomy and Physiology Chapter 16: The Autonomic Nervous System

Introduction

• Icebreaker:
  • Certain activities of the human body don’t require conscious thought, for example:
  • Regulation of blood pressure.
  • Regulation of heart rate, which increases during exercise automatically.
  • This chapter focuses on the autonomic nervous system's activities that maintain homeostasis.

Overview of the Autonomic Nervous System

• Learning Objectives (Section 16.1):
  • Understand the basic functions and structure of the autonomic nervous system (ANS).

Differences Between Somatic and Autonomic Nervous Systems

• Somatic Nervous System:
  • Controls voluntary movements.
  • Causes the contraction of skeletal muscles.
• Autonomic Nervous System:
  • Controls involuntary responses, such as those of cardiac and smooth muscles and glands.
  • Helps maintain homeostasis in the body.

Characteristics of the Autonomic Nervous System (ANS)

• Primarily innervates internal organs.
• Divisions of the ANS:
  1. Sympathetic Nervous System:
  • Associated with “fight-or-flight” responses, activated in stressful situations.
  1. Parasympathetic Nervous System:
  • Associated with “rest and digest” responses, active during restful states.
• Most organs receive dual innervation from both divisions, allowing for balanced control.

Sympathetic Division of the Autonomic Nervous System

• Function:
  • Prepares the body for stressful or emergency situations by increasing blood flow to skeletal muscles.
  • Increases oxygen delivery to skeletal muscles.
  • Increases sweating.
  • Redirects blood away from the digestive system.
  • Dilation of pupils and heightened alertness.
  • Example: Increased heart rate and blood pressure during exams or presentations.

Sympathetic Pathways

• Preganglionic Neurons:
  • Located in thoracic and lumbar regions of spinal cord (thoracolumbar system).
  • Synapse with postganglionic neurons in sympathetic chain ganglia (23 ganglia adjacent to vertebral column).
  • Can also synapse outside sympathetic chain at prevertebral ganglia or adrenal medulla, with long postganglionic axons projecting to target organs.

Collateral Ganglia

• Include:
  • Celiac ganglion.
  • Superior mesenteric ganglion.
  • Inferior mesenteric ganglion.
• Located anterior to vertebral column, involving the regulation of abdominal organ activity.
• Part of the enteric nervous system and receives input from splanchnic nerves.

Neurotransmitters of the Sympathetic Nervous System

• Acetylcholine (ACh):
  • Released at the synapse between preganglionic and postganglionic neurons.
• Norepinephrine (NE):
  • Released by postganglionic neurons onto target cells, crucial for sympathetic response activation.

Activation of Hormone Release in Sympathetic Nervous System

• Sympathetic activation can induce hormone release from the adrenal medulla:
  • Epinephrine and Norepinephrine are released directly into the bloodstream as these neurons project without synapsing in a ganglion.

Parasympathetic Division of the Autonomic Nervous System

• Function:
  • Active during non-stressful times, promoting "rest and digest" activities such as salivation, urination, digestion, defecation, and sexual arousal.

Parasympathetic Pathways

• Craniosacral System:
  • Cell bodies of preganglionic neurons are situated in the brain stem and sacral spinal cord.
  • Long preganglionic axons extend to ganglia near or within target organs, resulting in short postganglionic axons.
• Major Parasympathetic Ganglia and Pathways:
  1. Oculomotor nerve (CN III) to ciliary ganglion for iris control.
  2. Facial nerve (CN VII) to pterygopalatine and submandibular ganglia controlling salivary, mucus, and tear production.
  3. Glossopharyngeal nerve (CN IX) to parotid gland for saliva production.
  4. Vagus nerve (CN X) innervates organs in thoracic and abdominal cavities.

Neurotransmitters of the Parasympathetic Nervous System

• Acetylcholine (ACh):
  • Both preganglionic and postganglionic neurons in the parasympathetic nervous system release ACh at synapses.

Autonomic Reflexes

• Function:
  • Help maintain internal homeostasis by regulating vital parameters like blood pressure, heart rate, and digestive activity.
• Structure of Reflexes:
  • Consists of an afferent branch (sensory input) and an efferent branch (motor output), involving a two-neuron chain for autonomic reflex arcs.

Discussion Activities

• Activity 1: Describe the sympathetic responses in a stressful scenario, such as walking through a haunted house.
  • Activation results in increased heart rate, blood pressure, and breathing rate, accompanied by pupil dilation.
• Activity 2: Compare autonomic and somatic reflexes:
  • Both are involuntary but target different effectors (smooth/cardiac muscle vs. skeletal muscle).

Maintaining Homeostasis

• Dual Innervation: Most organs are affected by both the sympathetic and parasympathetic systems.
• Autonomic Tone: Indicates a balance between sympathetic and parasympathetic dominance, varying by organ (e.g., sympathetic tone dominates blood vessels, parasympathetic tone dominates heart).

Stress Responses

• Short-term Effects:
  • Lipid breakdown, increased blood glucose, and vascular changes (increased blood flow to skeletal muscles).
• Long-term Effects:
  • Chronic stress can lead to hypertension and diabetes. However, sympathetic activation during exercise can mitigate chronic stress effects.

Summary

• Key topics covered include:
  • Divisions of the autonomic nervous system.
  • Neurotransmitters involved at autonomic synapses.
  • The different receptors associated with the ANS.
  • Definition and implications of autonomic tone and the ANS role in stress response.

Knowledge Check Questions

1. Which neurotransmitter is released by postganglionic neurons of the parasympathetic nervous system?
   • A. Acetylcholine (Correct Answer)
2. The lower heart rate at rest is indicative of:
   • C. Parasympathetic dominance (Correct Answer)