Autonomic Nervous System

Overview

• Chapter 15 in the book.
• Comparison with the somatic nervous system.
• Focus on structural features and physiological responses.

Somatic Nervous System

• Receives sensory information (pain, temperature, proprioception, and other senses like sight, hearing, taste, smell, equilibrium).
• Effector: Skeletal muscles.
• Reflexes involve skeletal muscle contraction.

Autonomic Nervous System (ANS)

• Receives sensory information from organs or effectors we can't control consciously (e.g., smooth muscle, cardiac muscle).
• Regulates reflexes for physiological control of unconsciously perceived sensory input.

Structural Differences

• Somatic Nervous System:

  • One motor neuron from CNS to effector.
  • Motor neurons are myelinated, release acetylcholine (always excitatory).

• Autonomic Nervous System:

  • Two neurons in series (preganglionic and postganglionic).
  • Preganglionic neuron (myelinated) originates in CNS.
  • Postganglionic neuron (unmyelinated) is in peripheral ganglion.
  • Neurotransmitter: Acetylcholine (can be excitatory or inhibitory based on receptor).

ANS Divisions

• Sympathetic Nervous System (SNS): Fight-or-flight; thoracolumbar division.

  • Preganglionic neurons in lateral gray horns of thoracic-lumbar spinal cord.
  • Involves sympathetic chain ganglia and prevertebral ganglia.
  • Can communicate via multiple pathways including adrenal medulla.
  • Releases norepinephrine or acetylcholine.

• Parasympathetic Nervous System (PNS): Rest and digest; craniosacral division.

  • Preganglionic neurons in brain (cranial nerves 3, 7, 9, 10) or sacral spinal cord.
  • Involves terminal ganglia near or on organs.
  • Releases acetylcholine consistently.

Neurotransmitters and Receptors

• Cholinergic Neurons: Release acetylcholine.
  • Receptors:
    • Nicotinic: Excitatory; found on postganglionic neurons.
    • Muscarinic: Can be excitatory or inhibitory; found on effectors.
• Adrenergic Neurons: Release norepinephrine.
  • Receptors:
    • Alpha (α): α1 typically excitatory, α2 inhibitory.
    • Beta (β): β1 excitatory, β2 inhibitory, β3 for brown fat (thermogenesis).

Physiological Responses

• Sympathetic (Fight-or-Flight):

  • Increases heart rate, force of contraction, blood pressure.
  • Dilates pupils, bronchioles.
  • Reduces blood flow to non-essential organs (digestive, reproductive).
  • Increases blood flow to essential organs (heart, brain, skeletal muscles).

• Parasympathetic (Rest-and-Digest):

  • Promotes digestion, salivation, lacrimation, urination, defecation.
  • Decreases heart rate, constricts pupils, bronchioles.

Exceptions and Special Cases

• Sympathetic exception: Direct pathway to adrenal medulla.
• Paradoxical fear: Extreme fear can trigger parasympathetic responses causing urination, defecation.

Study Tips

• Review tables comparing somatic and autonomic systems and their divisions.
• Focus on neurotransmitter types and receptor functions.
• Understand pathways and effects of sympathetic vs parasympathetic activation.