Autonomic Nervous System Overview

Overview of the Autonomic Nervous System (ANS)

• The ANS is divided into the Sympathetic and Parasympathetic nervous systems, each serving different functions in the body.
• Sympathetic Nervous System (SNS):
  • Prepares the body for "fight or flight" response.
  • Increases heart rate, blood flow to muscles, and energy release.
  • Key neurotransmitter: Norepinephrine (Adrenaline)
  • Short preganglionic fibers; synapse occurs in ganglia that are often close to the spinal cord.
• Parasympathetic Nervous System (PNS):
  • Responsible for "rest and digest" activities.
  • Decreases heart rate and promotes digestion and energy conservation.
  • Key neurotransmitter: Acetylcholine (ACh)
  • Long preganglionic fibers; synapse generally occurs near target organs.

Neurotransmitters and Receptors

• Acetylcholine (ACh):
  • Released by cholinergic fibers in the PNS.
  • Affects target cells like glands, cardiac muscle, and smooth muscle.
• Norepinephrine:
  • Released by adrenergic fibers in the SNS.
  • Contains two types of receptors:
  • Alpha receptors: Generally excitatory
  • Beta receptors: Generally inhibitory
• Receptors determine whether the response will be excitatory or inhibitory depending on the target tissue.

Control Mechanisms in ANS

• Vasoconstriction and Vasodilation:
  • SNS: Increases blood flow to essential organs and muscles during emergencies.
  • PNS: Promotes blood flow to digestive organs at rest.
• Dual Innervation:
  • Many organs receive innervation from both SNS and PNS, often with antagonistic effects.
  • Example: Heart rate modulation—SNS increases heart rate while PNS decreases it.
• Sympathetic Tone:
  • Constant low-level activation of the SNS to maintain blood pressure and readiness.

Neuropharmacology

• Drugs can mimic or inhibit neurotransmitter actions:
  • Sympathomimetics: Enhance SNS activity (e.g., ephedrine)
  • Parasympathomimetics: Mimic or enhance PNS activity
  • Beta-blockers: Inhibit SNS activity and lower heart rate.

Role of the Brain in ANS Regulation

• Hypothalamus: Central in the regulation of homeostasis and stress response.
• Cerebral Cortex: Influences stress responses based on emotions and experiences.
• Spinal Cord: Plays a role in triggering reflexes and autonomic motor responses.

Additional Concepts

• Excitatory vs. Inhibitory Responses:
  • EPSP (Excitatory PostSynaptic Potential) and IPSP (Inhibitory PostSynaptic Potential) guide the overall response behavior of tissues.
• Receptors: Different receptors respond differently to neurotransmitters, leading to varied physiological effects based on location and type of target cells.
• Pain Perception:
  • Distinction between somatic pain (from skin, muscles) and visceral pain (from internal organs).
  • Referred Pain: Pain perceived in a different location other than its actual source due to overlapping neural pathways.

Taste and Smell Mechanisms

• Taste Buds:
  • Different regions of the tongue detect different tastes: salty, sweet, sour, bitter, umami.
• Chemical processes:
  • Gustation involves detection of chemical particles that stimulate taste cells, leading to interpretations of flavor.
• Odor detection:
  • Chemical molecules enter the nasal cavity and stimulate olfactory receptors which send signals to the brain for smell recognition.

Final Tips for Exam Preparation

• Focus on the key functions and differences between the SNS and PNS.
• Understand neurotransmitter roles and their respective receptors.
• Remember dual innervation and how it affects physiological responses.
• Be familiar with pharmacological terms and concepts related to modulation of the ANS.