cho 17 cont’d Autonomic Nervous System

Overview of the Nervous System Types

• The human nervous system is divided into two main branches: the sympathetic and parasympathetic nervous systems, both of which are part of the autonomic nervous system (ANS).

Sympathetic Nervous System

• Anatomy:

  • Anatomical name: Thoracolumbar Division

  • Housed in the central nervous system (CNS), specifically between the spinal segments T1 and L2.

  • Contains preganglionic neuron cell bodies located in the lateral gray horn of these regions.

• Postganglionic Fiber Locations:

  1. Chain Ganglia:

  • Runs parallel to the spinal cord and innervates the head, back body wall, arms, legs (front and back), thoracic cavity, and pelvic cavity.

  1. Collateral Ganglia:

  • Found mainly in the abdomen.

  • Innervates the abdominal and pelvic regions.

  1. Suprarenal (Adrenal) Gland:

  • Releases adrenaline into the bloodstream for widespread sympathetic response.

• Physiological Functions:

  • Controls "fight or flight" responses:

  • Increases heart rate and breathing rate, redirects energy away from digestive processes, and prepares the body for immediate physical action.

Parasympathetic Nervous System

• Anatomy:

  • Anatomical name: Craniosacral Division

  • Involves cranial nerves (specifically cranial nerves III, VII, IX, and X) and S2 to S4 sacral spinal segments; cells for preganglionic fibers are found in the lateral gray horns of these areas.

• Postganglionic Fiber Locations:

  • Terminal Ganglia (Collateral Ganglia) in the head (associated with cranial nerves III, VII, IX).

  • Intramural Ganglia within the walls of the organs involved, especially for cranial nerve X (Vagus), which innervates the majority of the body's organs (75-80% of parasympathetic functions).

• Physiological Functions:

  • Controls "rest and digest" responses:

  • Decreases heart rate, stimulates digestive functions, and promotes energy storage in organs.

Contrast between Sympathetic and Parasympathetic Systems

• Dual Innervation:

  • Most organs are innervated by both sympathetic and parasympathetic fibers which can work antagonistically.

  • Example: Sympathetic system increases heart rate while parasympathetic decreases it.

Reflex Classification

• Types of Reflexes:

  1. Born (Innate) vs. Learned (Acquired):

  • Somatic reflexes can be either innate (e.g., blinking) or acquired (e.g., riding a bike).

  1. Cranial vs. Spinal Reflexes.

  2. Monosynaptic vs. Polysynaptic Reflexes:

  • Somatic reflexes can be monosynaptic (single synapse, such as stretch reflex) or polysynaptic (multiple synapses).

  • For visceral reflexes, there are always two motor neurons (preganglionic and postganglionic), making them polysynaptic.

  1. Type of Response: Visceral reflexes are fundamentally innate and involve autonomic functions.

• Short vs. Long Reflexes:

  • Long Reflexes: Involve sensory input traveling to the CNS for processing before response.

  • Short Reflexes: Do not involve the CNS; sensory neurons directly activate postganglionic neurons in the ganglia, often leading to quicker responses.

Enteric Nervous System

• A significant subdivision of the parasympathetic system, this system operates independently for digestive reflexes, with sensory and motor connections occurring entirely within the gastrointestinal tract.

Summary

• Understanding both branches of the autonomic nervous system and their specific functions, anatomical structures, and reflex classifications is crucial for grasping how our body maintains homeostasis through coordinated involuntary responses.