ANS

Autonomic Nervous System Overview

• The Autonomic Nervous System (ANS) is responsible for controlling visceral organs, smooth muscles, and glands.

• It regulates and coordinates functions such as heart rate, blood pressure, respiration, digestion, urination, and reproduction.

• Most activities of the ANS occur independently of consciousness.

• The primary function of the ANS is to maintain homeostasis within the internal environment.

Structure of the Autonomic Nervous System

• The ANS is divided into three major parts:
    - Sympathetic Nervous System (SNS)
    - Parasympathetic Nervous System (PNS)
    - Enteric Nervous System (smaller part that deals specifically with gastrointestinal tract functions).

Neurons in the ANS

• The ANS is characterized by:
    - Preganglionic (presynaptic) nerve fibers: Neurons that originate in the central nervous system (CNS).
    - Postganglionic (postsynaptic) nerve fibers: Neurons that have their cell bodies in peripheral ganglia and make contact with target organs.

Sympathetic Nervous System (SNS)

Anatomical Features

• Originates from the thoracolumbar region, specifically from segments T1 to L2 of the spinal cord.
    - Characteristics:
      - Located nearer the CNS.
      - Short preganglionic fibers and longer postganglionic fibers that often have extensive branching.
      - Preganglionic sympathetic fibers must enter the sympathetic chain via white rami communicantes.

Functions

• The SNS prepares the body for emergency situations, often referred to as "fight or flight" responses. This stimulation leads to the following effects:
    1. Redistribution of blood: Blood is directed away from the skin and intestines to vital organs such as the brain, heart, and skeletal muscles.
    2. Heart rate acceleration: Increases heart rate to pump more blood.
    3. Constriction of peripheral blood vessels: This raises blood pressure.
    4. Inhibition of intestinal peristalsis: Digestive activity is slowed, and sphincters are tightened.
    5. Arrector pili muscle activation: Erection of hairs occurs (goosebumps).
    6. Sweating: Increased perspiration helps cool the body under stress.
    7. Pupil dilation: Enhances vision by allowing more light into the eye.

Parasympathetic Nervous System (PNS)

Anatomical Features

• Originates in the craniosacral region, specifically from cranial nerves (CN) 3, 7, 9, 10 and sacral segments S2, 3, 4 of the spinal cord.
    - Characteristics:
      - Located nearer to the effector organs, sometimes directly within the walls of the organs.
      - Longer preganglionic fibers and shorter postganglionic fibers with fewer branches compared to SNS.

Functions

• The PNS is focused on conserving and restoring energy, often referred to as "rest and digest" functions. This includes:
    1. Pupil constriction: Reduces light intake and enhances focus for near vision.
    2. Slowing of heart rate: Resulting in less stress on the heart.
    3. Stimulation of intestinal peristalsis: Enhances digestion and absorption of nutrients, and relaxes sphincters.
    4. Stimulation of gland secretion: Particularly saliva and digestive enzymes (note: does not include sweat glands).

Differences between Sympathetic and Parasympathetic Systems

Anatomical Differences

1. Location within CNS:
      - Sympathetic: Thoracolumbar outflow (T1-L2).
      - Parasympathetic: Craniosacral outflow (brain and S2,3,4).

2. Location, number & size of ganglia:
      - Sympathetic: Located closer to the CNS.
      - Parasympathetic: Ganglia found near or within the target viscera.

3. Pre- and post-ganglionic fibers:
      - Sympathetic: Short preganglionic fibers and long postganglionic fibers.
      - Parasympathetic: Long preganglionic fibers and short postganglionic fibers.

4. Branching of axons:
      - Sympathetic axons: Highly branched, influencing multiple organs simultaneously.
      - Parasympathetic axons: Few branches, affecting primarily single organ systems.

Neurotransmitters

• Preganglionic axons of both systems release acetylcholine (cholinergic).

• Postganglionic axons:
    - Sympathetic: Mainly release norepinephrine.
    - Parasympathetic: Release acetylcholine.

Visceral Motor System

• The visceral motor system differs from the somatic motor system in several key ways:
    - Effectors:
      - Somatic: Skeletal muscle.
      - Autonomic: Cardiac muscle, smooth muscle, and glands.
    - Type of control:
      - Somatic: Voluntary control.
      - Autonomic: Involuntary control.
    - Neural pathways:
      - Somatic: One motor neuron extends from the CNS to skeletal muscle.
      - Autonomic: Two motor neurons involved (preganglionic and postganglionic).
    - Action on effectors:
      - Somatic: Always excitatory.
      - Autonomic: Can be excitatory or inhibitory.
    - Rate of conduction:
      - Somatic: Rapid due to myelinated axons.
      - Autonomic: Slower due to thinner myelinated or unmyelinated axons.

Summary and Application of ANS

• The ANS operates in conjunction with each division affecting the same structures, maintaining homeostasis with antagonistic control to ensure a stable internal environment:
    - Sympathetic system = stress reaction, fight-or-flight, primes body for intense activity.
    - Parasympathetic system = maintenance functions, rest-and-repair, counterbalancing sympathetic function.