Anatomy and Physiology of the Parasympathetic Division and Enteric Nervous System

Anatomy of the parasympathetic division

Its pathways arise from the brain and sacral regions of the

spinal cord (so also called craniosacral division); relatively long preganglionic and short postganglionic

fibers; parasympathetic preganglionic fibers leave brain as components of cranial nerves (III (oculomotor), VII (facial), IX (glossopharyngeal), X (vagus) - innervates structures in neck, thoracic, and abdominopelvic cavities and is 75% of all parasympathetic outflow) which control visceral structures in the head; parasympathetic preganglionic fibers leave spinal cord at sacral level as pelvic nerves

General pathway of the parasympathetic division

Cell bodies of preganglionic neurons in midbrain, pons, and medulla (associated with cranial nerves III, VII, IX, and X), or sacral spinal cord segments S2 to S4; preganglionic fibers travel through cranial or sacral nerves, end in terminal ganglia in or near target organs (intramural ganglion - embedded within organ wall); synapse with ganglionic neurons in ganglia; postganglionic fibers cover rest of the (short) distance to target organ

Neuromuscular and neuroglandular junctions

Small, with narrow synaptic clefts; all release ACh as neurotransmitter (both the preganglionic and ganglionic neurons release acetylcholine (ACh)); effects of stimulation are short lived (few seconds) (inactivated by acetylcholinesterase (AChE) at synapse, ACh is also inactivated by tissue cholinesterase in surrounding tissues)

Five 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 (movement of material in digestive

tract) and blood flow in digestive tract

5. Stimulation of urination and defecation

Enteric plexus (enteric nervous system)

The nervous network of the digestive tract; third division of the ANS; composed of 500 million neurons found in the walls of the digestive tract (5 times more neurons than the spinal cord); unlike the rest of the ANS, it does not arise from the brainstem or spinal cord; like the rest of the ANS, it innervates smooth muscle, glands; has its own ganglia and reflex arcs; regulates motility of esophagus, stomach, and intestines and secretion of digestive enzymes and acid; normal digestive function also requires regulation by sympathetic and parasympathetic systems

Divisions of ANS often have opposite effects on an organ, this is possible from two reasons:

1. Sympathetic and parasympathetic fibers secrete different

neurotransmitters (norepinephrine and acetylcholine)

2. The receptors on target cells vary; target cells respond to the same neurotransmitter differently depending on the type of receptor they have for it; there are two different classes of receptors for acetylcholine and two classes of receptors for norepinephrine

Acetylcholine (ACh)

Is secreted by all preganglionic neurons in both divisions and by ganglionic parasympathetic neurons; neurons that secrete ACh are called cholinergic neurons; any receptor that binds ACh is called a cholinergic receptor; cholinergic (ACh) sympathetic terminals innervate sweat glands of skin and blood vessels of skeletal muscles and brain, and stimulate sweat gland secretion and dilate blood vessels to skeletal muscles and brain

Two categories of cholinergic (ACh) receptors:

Muscarinic receptors and Nicotinic receptors

Muscarinic receptors

Found on cardiac muscle, smooth muscle, and gland cells with cholinergic innervation; act through second-messenger systems; receptor subtypes exist, often providing contrasting effects on organs; example: ACh excites intestinal smooth muscle, but inhibits cardiac muscle

Nicotinic receptors

Found on ganglionic neuron cell bodies in autonomic ganglia, on cells of adrenal medulla, and neuromuscular junction of skeletal muscle fibers; binding of ACh to these receptors is always excitatory; open ligand-gated Na+ ion channels, produce excitatory postsynaptic potential

Dangerous Environmental Toxins

Produce exaggerated, uncontrolled responses; include Nicotine, Muscarine

Nicotine

Binds to nicotinic receptors in sympathetic and parasympathetic ganglion cells ; targets autonomic ganglia and skeletal neuromuscular junctions; 50 mg ingested or absorbed through skin causes poisoning; signs and symptoms: vomiting, diarrhea, high blood pressure, rapid heart rate (tachycardia), sweating, profuse salivation, convulsions, may result in coma or death

Muscarine

Toxin produced from some poisonous mushrooms; binds to muscarinic receptors; targets parasympathetic neuromuscular or neuroglandular junctions; signs and symptoms: salivation, nausea, vomiting, diarrhea, constriction of respiratory passages, low blood pressure, slow heart rate (bradycardia)

Norepinephrine (NE)

Is secreted by nearly all sympathetic ganglionic neurons; nerve fibers that secrete NE are called adrenergic fibers; receptors for NE are called adrenergic receptors; two categories of adrenic (NE/E) receptors: Alpha-adrenergic receptors and Beta-adrenergic receptors

Alpha-adrenergic receptors

Usually excitatory; two subclasses use different second messengers (α1 and α2)

Beta-adrenergic receptors

Usually inhibitory; two subclasses with different effects, but both act through cAMP as a second messenger (β1 and β2)

Three Main Types of Beta Receptors

Beta-1: Increases metabolic activity, increase activity of skeletal and cardiac muscle

2. Beta-2: Triggers relaxation of smooth muscles along respiratory tract, increases diameter of air passages

3. Beta-3: Leads to lipolysis, the breakdown of

triglycerides in adipocytes, releases nutrients into

bloodstream

Effects

Autonomic effects on glandular secretion are often an

indirect result of their effect on blood vessels (increased blood flow, increased secretion; increased blood flow, decreased secretion); sympathetic effects tend to last longer than parasympathetic effects (NE released by sympathetic fibers can be reabsorbed by axon terminal, diffuse to adjacent tissues, or pass into bloodstream; ACh from parasympathetic fibers broken down quickly at synapse); Some ANS fibers release other neurotransmitters in addition to ACh or NE (Examples: enkephalin, substance P, neuropeptide Y, somatostatin,

neurotensin, gonadotropin-releasing hormone, nitric oxide (NO))

Dual innervation

Most viscera receive nerve fibers from both parasympathetic and sympathetic divisions; even when both division innervate a single organ, innervation is not always equal or exert equal influence; not always necessary for ANS to produce opposite effects in on an organ, some effectors receive only sympathetic fibers (adrenal medulla, arrector muscles, sweat glands, many blood vessels) (Examples: regulation of blood pressure and routes of blood flow; baseline sympathetic tone maintains vasomotor tone - vessels in a continual state of partial constriction; increase in firing frequency - vasoconstriction, decrease in firing frequency - vasodilation; can also shift blood flow from one organ to another as needed; during stress, blood vessels to skeletal muscles and heart dilate while blood vessels to skin constrict)

Antagonistic effects

Oppose each other; can be exerted through dual innervation of same effector cells (Example: heart rate); can be exerted because each division innervates different cells (Example: muscles of iris)

Cooperative effects

Two divisions act on different effectors to produce a unified overall effect (Example: saliva production - parasympathetic stimulates serous cell secretion and sympathetic stimulates mucous cell secretion)

Central Control of Autonomic Function

The ANS is regulated by several levels of the CNS: Cerebral cortex and limbic system; Hypothalamus; Midbrain, pons, and medulla oblongata; Spinal cord

Cerebral cortex and limbic system

Emotions influence the ANS because of the connections between our limbic system and the hypothalamus

Hypothalamus

Contains nuclei for primitive functions - hunger,

thirst, sex

Midbrain, pons, and medulla oblongata

Contain nuclei for cardiac and vasomotor control, salivation, swallowing, sweating, bladder control, and pupillary changes

Spinal cord

Reflexes for defecation and micturition; we control these functions because of our control over skeletal muscle

sphincters; if the spinal cord is damaged, the smooth muscle of bowel and bladder is controlled by autonomic reflexes built into the spinal cord

Neuropharmacology

Study of effects of drugs on the nervous system

Sympathomimetics enhance sympathetic activity

Stimulate receptors or increase norepinephrine release; cold medicines that dilate the bronchioles or constrict nasal blood vessels

Sympatholytics suppress sympathetic activity

Block receptors or inhibit norepinephrine release; beta-blockers reduce high BP interfering with effects of epinephrine and norepinephrine on heart and blood vessels

Drugs and the Nervous System

Parasympathomimetics enhance parasympathetic effects,

while parasympatholytics suppress them; many drugs also act on neurotransmitters or their receptors in the CNS (Prozac; MAO inhibitors; Caffeine)

Prozac

Is a selective serotonin reuptake inhibitor (SSRI); blocks

reuptake of serotonin to prolong its mood-elevating effect

MAO inhibitors

Block enzyme monoamine oxidase (MAO) to slow the breakdown of monoamine neurotransmitters; aid in treating depression

Caffeine

Competes with adenosine (the presence of which causes

sleepiness) by binding to its receptors