Autonomic Nervous System Notes

Autonomic Nervous System (ANS)

  • Fight or Flight vs. Rest & Digest

Introduction to the ANS and Higher-Order Functions

Somatic Nervous System (SNS)
  • Operates under conscious control.
  • Seldom affects long-term survival.
  • Controls skeletal muscles.
Autonomic Nervous System (ANS)
  • Operates without conscious instruction.
  • Controls visceral effectors.
  • Coordinates system functions:
    • Cardiovascular
    • Respiratory
    • Digestive
    • Urinary
    • Reproductive

Organization of the Somatic and Autonomic Nervous Systems

The somatic nervous system involves upper motor neurons in the primary motor cortex, somatic motor nuclei of the brain stem and spinal cord, lower motor neurons, and skeletal muscles.

The autonomic nervous system involves visceral motor nuclei in the hypothalamus and brain stem, autonomic nuclei in the spinal cord, preganglionic neurons, autonomic ganglia, ganglionic neurons, and visceral effectors like smooth muscle, glands, cardiac muscle, and adipocytes.

Divisions of the ANS

The Autonomic Nervous System

  • Operates largely outside our awareness.

  • Has two divisions:

    1. Sympathetic division

      • Increases alertness, metabolic rate, and muscular abilities.
      • Kicks in during exertion, stress, or emergency.
      • "Fight or flight"

    2. Parasympathetic division

      • Reduces metabolic rate and promotes digestion.
      • Conserves energy and promotes sedentary activities.
      • "Rest and digest"
Sympathetic and Parasympathetic Division

  1. Most often, these two divisions have opposing effects.

    • If the sympathetic division causes excitation, the parasympathetic causes inhibition.

  2. The two divisions may also work independently.

    • Only one division innervates some structures.

  3. The two divisions may work together, with each controlling one stage of a complex process.

Responses to Increased Sympathetic Activity
  1. Heightened mental alertness
  2. Increased metabolic rate
  3. Reduced digestive and urinary functions
  4. Energy reserves activated
  5. Increased respiratory rate and respiratory passageways dilate (bronchodilation)
  6. Increased heart rate and blood pressure
  7. Sweat glands activated
  8. Pupil dilation
Responses to Increased Parasympathetic Activity
  1. Decreased metabolic rate
  2. Decreased heart rate and blood pressure
  3. Increased secretion by salivary and digestive glands
  4. Increased motility and blood flow in digestive tract
  5. Urination and defecation stimulation
  6. Constriction of pupils

Overview of the Autonomic Nervous System

Sympathetic Division (Thoracolumbar)
  • "Fight or flight" response
  • Preganglionic neurons are located in the lateral gray horns of spinal segments T1−L2T1-L2.
  • Preganglionic fibers release acetylcholine (ACh), stimulating ganglionic neurons.
  • Ganglia are located near the spinal cord.
  • Most postganglionic fibers release norepinephrine (NE) at neuroeffector junctions.
Parasympathetic Division (Craniosacral)
  • "Rest and digest" response
  • Preganglionic neurons in the brain stem and in the lateral portion of anterior gray horns of S2−S4S2-S4.
  • Preganglionic fibers release acetylcholine (ACh), stimulating ganglionic neurons.
  • Ganglia are in or near the target organ.
  • All postganglionic fibers release ACh at neuroeffector junctions.

Enteric Nervous System (ENS)

  • Third division of ANS
  • Extensive network in digestive tract walls
  • Complex visceral reflexes coordinated locally
  • Roughly 100 million neurons
  • All neurotransmitters are found in the brain

The Sympathetic Division

Adrenal Medullae
  • Modified sympathetic ganglion
  • Specialized neuroendocrine cells secrete chemicals into the bloodstream
  • Secrete neurotransmitters epinephrine (E) and norepinephrine (NE)
  • Epinephrine (adrenaline)
    • Is 75–80 percent of secretory output
  • Remaining is norepinephrine (NE) known as noradrenaline
  • Effects last longer

The Parasympathetic Division

Vagus Nerve
  • Provides parasympathetic innervation to structures in:
    • Neck
    • Thoracic and abdominopelvic cavities
  • Provides 75 percent of all parasympathetic outflow
Neurotransmitters of the ANS

  • Cholinergic neurons

    • ACh
    • Excites or inhibits and is short lived due to acetylcholinesterase (AChE)
    • Nicotinic receptors: pre to postganglionic neurons
    • Muscarinic receptors: postganglionic neuron to effector cell

  • Adrenergic neurons

    • Norepinephrine
    • Excites or inhibits and is longer acting
    • Adrenergic receptors: postganglionic neuron to effector cell
      • Alpha (α\alpha) receptor
      • Beta (β\beta) receptor

Dual Innervation

  • Most vital organs receive instructions from both sympathetic and parasympathetic divisions.
  • Two divisions commonly have opposing effects.
Autonomic Tone
  • Is an important aspect of ANS function.
  • If a nerve is inactive under normal conditions, it can only increase activity.
  • If a nerve maintains a background level of activity, it can increase or decrease activity.

Visceral Reflexes Regulate the ANS

Visceral Reflexes
  • Sympathetic nervous system
    • Cardioacceleratory reflex
    • Pupillary reflex
  • Parasympathetic nervous system
    • Gastric & intestinal reflexes
    • Defecation & urination reflexes
    • Direct light reflex (pupil)
  • Enteric nervous system
    • Control digestive functions independent of CNS

Higher-Order Functions

Characteristics
  1. Require the cerebral cortex
  2. Involve conscious and unconscious information processing
  3. Are not part of programmed “wiring” of the brain, and can adjust over time
Memory
  • Fact memories
    • Are specific bits of information
  • Skill memories
    • Learned motor behaviors
    • Incorporated at unconscious level with repetition
Short-term memories
  • Information that can be recalled immediately
  • Contain small bits of information
  • Primary memories
Long-term memories
  • Memory consolidation – conversion from short-term to long-term memory
  • Two types of long-term memory
    1. Secondary memories fade and require effort to recall
    2. Tertiary memories are with you for life
Cellular Mechanisms of Memory Formation and Storage
  • Involves anatomical and physiological changes in neurons and synapses
    • Increased neurotransmitter release
    • Facilitation at synapses
    • Formation of additional synaptic connections
Memory Consolidation
  • Efficient conversion of short-term memory to long-term memory
  • Takes at least 1 hour
  • Repetition crucial
  • Factors of conversion
    • Nature, intensity, and frequency of original stimulus
    • Strong, repeated, and exceedingly pleasant or unpleasant events likely converted to long-term memories
Sleep
  • Has an important impact on the CNS
  • Protein synthesis in neurons increases during sleep
  • Extended periods without sleep lead to disturbances in mental function
  • 25 percent of the U.S. population experiences sleep disorders
Arousal and the Reticular Activating System (RAS)
  • Awakening from sleep
  • Function of the reticular formation
Reticular Activating System (RAS)
  • Important brain stem component
  • Diffuse network in reticular formation
  • Extends from the medulla oblongata to midbrain
  • Output of RAS projects to thalamic nuclei that influence large areas of the cerebral cortex
  • When RAS inactive, so is the cerebral cortex
  • Stimulation of RAS produces widespread activation of the cerebral cortex

Effects of Aging on the Nervous System

  • Anatomical and physiological changes begin after maturity (age 30)
  • Accumulate over time
  • 85 percent of people over age 65 have changes in mental performance and CNS function
Common Age-related Anatomical Changes in the Nervous System
  • Reduction in Brain Size and Weight
  • Reduction in Number of Neurons
  • Decrease in Blood Flow to Brain
  • Changes in Synaptic Organization of Brain
  • Intracellular and Extracellular Changes in CNS Neurons