Autonomic Nervous System

• The autonomic nervous system (ANS) is a complex system of nerves that govern involuntary actions.

• The ANS works constantly with the somatic nervous system (SNS) to regulate body organs and maintain normal internal functions.

  • Somatic Nervous System

    • Type of Control: Voluntary Control.

    • Number of Neurons in Pathway: One neuron in pathway.

    • Ganglia Associated with Motor Neurons: None.

    • Sensory Input: General somatic senses, proprioceptoes and special senses.

    • Ganglia Associated with Sensory Input: Posterior root ganglia, sensory ganglia of cranial nerves.

    • Effector Organs: Skeletal muscle fibers.

    • Response of Effector: Excitation Only.

    • Neurotransmitter Released: Acetylcholine (ACh).

    • Axon Properties: Myelinated, thick and fast conduction.

  • Autonomic Nervous System

    • Type of Control: Involuntary Control.

    • Number of Neurons in Pathway: Two neurons in pathway, preganglionic neuron in CNS projets preganglionic axon to ganglionic neuron, ganglionic neuron projets postganglionic axon to effector.

    • Ganglia Associated with Motor Neurons:  Autonomic ganglia, sympathetic trunk ganglia, prevertebral ganglia, terminal or intramural ganglia.

    • Sensory Input: Some somatic and visceral senses.

    • Ganglia Associated with Sensory Input: Posterior root ganglia, sensory ganglia of cranial nerves.

    • Effector Organs: Cardiac muscle fibers, smooth muscle fibers and glands.

    • Response of Effector: Either excitation or inhibition of effectors.

    • Neurotransmitter Released: ACh from all preganglionic axons and parasympathetic postganglionic axons, and a few sympathetic postganglionic axons; norepinephrine (NE) from most sympathetic postganglionic axons.

    • Axon Properties: Preganglionic axons are thin, myelinated; postganglionic axons are thinner, unmyelinated, have slow conduction.

• The ANS is subdivided into the parasympathetic division and the sympathetic division.

• The parasympathetic division is primarily concerned with conserving energy and replenishing nutrient stores. The parasympathetic division also helps maintain homeostasis, a constant internal environment.

• The sympathetic division is primarily concerned with preparing the body for emergencies. It is often referred to as the “fight-or-flight” division because increased sympathetic activity results in the increased alertness and metabolic activity needed in stressful or frightening situations. During these fight or flight events, the sympathetic division exhibits a mass activation response, whereby all components receiving sympathetic innervation get stimulated. 

• The parasympathetic and sympathetic divisions are similar in that their preganglionic axons are myelinated, while the postganglionic axons are unmyelinated. These two divisions are also distinguished by several anatomic differences. The major difference is that their preganglionic neuron cell bodies are housed in different regions of the CNS. Parasympathetic preganglionic neurons originate in either the brainstem or the lateral gray matter of the S2–S4 spinal cord segments, while sympathetic preganglionic neurons originate in the lateral horns of the T1–L2 spinal cord segments.

• The parasympathetic preganglionic neurons are housed either within the brainstem or within the sacral region of the spinal cord.

• The ganglionic neurons in the parasympathetic division are located within either terminal ganglia or intramural ganglia.

• Cranial Nerves 

  • Parasympathetic preganglionic axons extend from cell bodies in brainstem nuclei through the oculomotor, facial, glossopharyngeal, and vagus cranial nerves.

• Sacral Spinal Nerves 

  • The remaining preganglionic parasympathetic cell bodies are housed within the S2–S4 segments of the spinal cord and form pelvic splanchnic nerves.

• Effects and General Functions of the Parasympathetic Division 

  • The parasympathetic division of the ANS alters activities of effector organs to manage and control food processing, energy absorption, and relaxation activities.

• The sympathetic division outflow is from the T1–L2 lateral horn segments.

• Organization and Anatomy of the Sympathetic Division 

  • Preganglionic neuron cell bodies are housed within the lateral gray horn of the spinal gray matter.

  • Myelinated, preganglionic sympathetic axons exit the spinal cord through the anterior root of a spinal nerve and travel through the white rami communicantes to the sympathetic trunk ganglia.

• Sympathetic Pathways 

  • In the spinal nerve pathway, the postganglionic axon enters the spinal nerve through the gray ramus and travels to the blood vessels and glands distributed throughout the limbs and body wall of the trunk.

  • In the postganglionic sympathetic nerve pathway, the postganglionic axon leaves the sympathetic trunk and extends directly to the target organ.

  • In the splanchnic nerve pathway, the preganglionic axon passes through the sympathetic trunk without synapsing and travels to the prevertebral ganglia.

  • In the adrenal medulla pathway, the preganglionic axons extend through the autonomic ganglia without synapsing. They synapse on secretory cells in the adrenal medulla that release epinephrine and norepinephrine.

• Effects and General Functions of the Sympathetic Division 

  • Sympathetic division pathways prepare the body for fight or flight.

• Both divisions of the autonomic nervous system innervate organs through specific axon bundles.

• Autonomic Plexuses 

  • Autonomic plexuses are meshworks of postganglionic sympathetic axons, preganglionic parasympathetic axons, and visceral sensory neuron axons in the anterior body cavities that merge and intermingle but do not synapse with each other.

• Neurotransmitters and Receptors 

  • Two neurotransmitters are used in the ANS: acetylcholine (ACh) and norepinephrine (NE).

  • Both the preganglionic and postganglionic axons in the parasympathetic division release acetylcholine; the preganglionic axon and a few postganglionic axons in the sympathetic division release acetylcholine; however, most of the postganglionic axons of the sympathetic division release norepinephrine.

• Dual Innervation 

  • Many visceral effectors have dual innervation, meaning they are innervated by axons from both ANS divisions.

  • The actions of the divisions often oppose each other, and thus they exert antagonistic effects on the same organ.

• Autonomic Reflexes 

  • Homeostasis in the human body is maintained through the activity of autonomic reflexes. These reflexes result in smooth muscle contractions, cardiac muscle contractions, or secretion by glands

■ Autonomic function is influenced by four CNS regions: cerebrum, hypothalamus, brainstem, and spinal cord.