The Autonomic Nervous System

Chapter 15: The Autonomic Nervous System

Introduction to the ANS

  • Overview: Examination of the structural and functional features of the autonomic nervous system (ANS), comparing its two major components: the sympathetic and parasympathetic divisions.

    • Function:
    • Contributes to homeostasis by responding to subconscious visceral sensations.
    • Regulates smooth muscle, cardiac muscle, and many glands by excitation or inhibition.

  • Structure: The ANS includes:

    • Autonomic sensory neurons
    • Integrating centers in the CNS
    • Autonomic motor neurons

  • Enteric Division:

    • A specialized network of nerves and ganglia in the gastrointestinal (GI) tract.
    • Not discussed further in this chapter, to be covered in Chapter 24.

  • Comparison with the Somatic Nervous System (SNS):

    • Both systems involve sensory and motor neurons, but the ANS operates without conscious control.
    • Conscious Control:
    • SNS: Receives feedback from tactile, thermal, pain, and proprioceptive sensations, consciously perceived. Uses skeletal muscle for reflexive and voluntary movement.
    • ANS: Operates subconsciously to regulate bodily functions without conscious intervention.

  • Effects of Somatic Motor Neurons:

    • If a somatic motor neuron stops stimulating a muscle, it results in a paralyzed, limp muscle with no tone.
    • Example: Respiratory muscles (skeletal muscle controlled by somatic motor neurons) cease to function if their motor neurons become inactive, leading to stopped breathing.

  • Autonomic Regulation:

    • Functions without conscious control but regulated by hypothalamus and brainstem for ANS reflexes.
    • Interoceptors: Sensory receptors monitoring the internal environment located in:
    • Blood vessels
    • Visceral organs
    • Muscles
    • Nervous system
    • Examples:
    • Chemoreceptors: Monitor blood CO₂ levels.
    • Mechanoreceptors: Detect stretch in organ or vessel walls.

ANS Motor Pathways

  • Regulation of Visceral Activities:

    • Autonomic motor neurons can either excite (increase) or inhibit (decrease) activity in effector tissues.
    • Autonomic responses cannot be easily altered voluntarily, thus some responses are used in polygraph tests.
    • Yoga and biofeedback can train individuals to regulate some autonomic activities.

  • Anatomy of Autonomic Pathways:

    • Visualize as a double-barreled neuronal construct:
    • A preganglionic neuron leading to an intermediate ganglion where postganglionic neurons reside.

Comparison of Somatic and Autonomic Nervous Systems

Table 15.1: Comparison Summary

  • Sensory Input:

    • Somatic: From somatic and special senses.
    • Autonomic: Mainly from interoceptors, some from somatic and special senses.

  • Control of Motor Output:

    • Somatic: Voluntary control from cerebral cortex, basal ganglia, cerebellum, brainstem, spinal cord.
    • Autonomic: Involuntary control from hypothalamus, limbic system, brainstem, spinal cord; limited control from cerebral cortex.

  • Motor Neuron Pathway:

    • Somatic: One-neuron pathway; somatic motor neurons synapse directly with effectors.
    • Autonomic: Usually two-neuron pathway, where preganglionic neurons synapse with postganglionic neurons in autonomic ganglion before extending to effectors.

  • Neurotransmitters:

    • Somatic: All somatic motor neurons release acetylcholine (ACh).
    • Autonomic:
    • Sympathetic and parasympathetic preganglionic neurons both release ACh.
    • Most sympathetic postganglionic neurons release norepinephrine (NE), while those to most sweat glands release ACh. All parasympathetic postsynaptic neurons release ACh.
    • Chromaffin cells of adrenal medulla also release epinephrine and norepinephrine.

  • Effectors:

    • Somatic: Skeletal muscle.
    • Autonomic: Smooth muscle, cardiac muscle, and glands.

  • Responses:

    • Somatic: Contraction of skeletal muscle.
    • Autonomic: Contraction or relaxation of smooth muscle; increased or decreased rate and force of cardiac muscle contraction; increased or decreased glandular secretions.

Interactions of the ANS:

Sympathetic and Parasympathetic Divisions
  • Dual Innervation: Most body organs receive impulses from both divisions.
    • One division typically stimulates, and the other inhibits organ activity.

The Sympathetic Division

  • Anatomical Location: Cell bodies of sympathetic neurons are located in the lateral horns of the gray matter in the thoracic (T1-T12) and first two lumbar (L1-L2) segments of the spinal cord.

    • Thoracolumbar Division: The name arises due to the location of sympathetic preganglionic neurons exiting the spinal cord.

  • Major Groups of Sympathetic Ganglia:

    • Sympathetic trunk (vertebral chain) ganglia
    • Prevertebral ganglia
    • Celiac, superior mesenteric, inferior mesenteric, aorticorenal, and renal ganglia

  • Axon Pathways:

    • Four ways sympathetic axons leave the sympathetic trunk:
    1. Enter and travel with spinal nerves.
    2. Form networks around arteries and travel to synapse in cervical ganglia.
    3. Form sympathetic nerves to the heart and lungs.
    4. Leave sympathetic trunk without synapsing to form splanchnic nerves.

  • Diverging Circuit: A single sympathetic preganglionic fiber can synapse with many postganglionic branches (20+), leading to widespread effects.

    • Example: The diffuse response to anger, making it hard to control.

The Parasympathetic Division

  • Anatomical Location: Cell bodies of preganglionic neurons reside in nuclei of cranial nerves III, VII, IX, and X, and in the lateral gray matter of the sacral spinal cord (S2-S4).

    • Vagus Nerve (CN X): Carries 80% of parasympathetic flow to thoracic and upper abdominal organs.
    • Sacral Output: Innervates lower abdominal and pelvic organs.

  • Parasympathetic Ganglia: Called terminal ganglia because they are located near the target organs.

    • Four pairs of cranial ganglia innervate structures in the head: Ciliary, Pterygopalatine, Submandibular, and Otic ganglia.

  • Functional Characteristics:

    • Compared to sympathetic responses, parasympathetic responses focus on specific organs due to synapsing typically only with 4-5 postganglionic neurons.
    • SLUDD Responses: Parasympathetic actions summarized by the acronym:
    • Salivation (increased)
    • Lacrimation (increased)
    • Urination (increased)
    • Digestion (increased)
    • Defecation (increased)
    • Three decreases: heart rate, airway size, breathing rate, pupil size.

ANS Neurotransmitters

  • Neurotransmitter Variety: The exact number of neurotransmitters in the nervous system is not known but exceeds 100.

    • However, predominantly only two are significantly influential in ANS functioning:
    1. Acetylcholine (ACh)
    2. Norepinephrine (NE)

  • Definitions:

    • Cholinergic Synapses: Synapses that utilize ACh.
    • Adrenergic Synapses: Synapses using norepinephrine or epinephrine.

  • Ganglionic Synapses:

    • All preganglionic to postganglionic synapses (both sympathetic and parasympathetic) utilize ACh.
    • Cholinergic receptors include:
    • Nicotinic Receptors: Located in ganglia.
    • Muscarinic Receptors: Found at synapses with effector organs.

  • Postganglionic Synapses:

    • Most sympathetic postganglionic synapses utilize norepinephrine; sympathetic to sweat glands use ACh.
    • Parasympathetic postganglionic synapses exclusively utilize ACh, acting on muscarinic receptors.

Physiology of the ANS

  • Sympathetic Stimulation Effects:
    • Secretion of norepinephrine from adrenal glands
    • Increased rate and strength of heartbeat
    • Constriction of blood vessels in non-essential organs
    • Dilation of blood vessels in essential organs (skeletal muscle, cerebral cortex)
    • Increased breathing rate and depth
    • Conversion of glycogen to glucose in the liver
    • Decreased gastrointestinal activity.
Table 15.4: Effects of Sympathetic and Parasympathetic Divisions

  • Glands:

    • Sympathetic: Adrenal medullae release epinephrine, slight secretion of tears, inhibition of digestive enzyme secretion, promotes glucagon secretion from pancreas.
    • Parasympathetic: No effect on adrenal medullae, secretion of tears, secretion of digestive enzymes, secretion of insulin.

  • Cardiac Muscle:

    • Sympathetic: Increased heart rate and strength of contractions.
    • Parasympathetic: Decreased heart rate, decreased force of heart contraction.

  • Smooth Muscle Responses:

    • Sympathetic Effects: Dilation of pupils, relaxation of lung muscles, decreased gastrointestinal motility, and more.
    • Parasympathetic Effects: Constriction of pupils, constriction of bronchial muscles, increased gastrointestinal motility, and more.

  • Vascular Smooth Muscle:

    • Sympathetic Effects: Vasoconstriction and decreased secretion in various glands.
    • Parasympathetic Effects: Vasodilation increasing saliva secretion and more.

Regulation of ANS Balance

  • Regulatory Mechanisms: The balance of autonomic sympathetic and parasympathetic tone is managed through feedback loops between the spinal cord and brainstem, involving input from the limbic system and regulation by the hypothalamus.

End of Chapter 15: Summary

  • Discusses the intricate balance and interplay between sympathetic and parasympathetic systems in maintaining homeostasis and responding to internal changes in the body.