Autonomic Nervous System : Outta Your Control

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• Homeostasis will depend on the workings of the ANS
• The effectors of the ANS are cardiac muscle, smooth muscle or glandular
• Where somatic nerves involve a cell body in the anterior horn and its axon which extends to the effector, autonomic nerves have a two neuron chain. The first neuron is the preganglionic neuron, and its cell body is located somewhere in the CNS (brain or lateral horn of the spinal cord). The preganglionic axon forms a synapse with the cell body of the postganglionic neuron, found in an autonomic ganglion outside the CNS. The postganglionic axon extends to the effector.
• Preganglionic fibers are lightly myelinated while postganglionic fibers are unmyelinated.
• Due to the lesser amount of myelination, nerve impulses in the ANS are much slower than those of the somatic system
• The neurotransmitters released by postganglionic fibers include norepinephrine (secreted by sympathetic fibers) and acetylcholine (released by parasympathetic fibers)
• Responses will either be excitatory or inhibitory, depending on the receptors of the effector
• Sympathetic and parasympathetic divisions of the ANS innervate the same organs (for the most part), but perform opposite functions. This is referred to a dual innervation
• Parasympathetic fibers function in non-stressful situations ("resting and digesting"). They keep energy use low and perform "housekeeping" functions
• Blood pressure, heart rate and respiratory rate are regulated at low ("regular") levels. There is active digestion (peristalsis) and the skin is warm. Pupils are constricted
• Sympathetic fibers function in stressful situations ("fight or flight"). It does the opposite of parasympathetic
• Blood pressure rises, increases pulse, increases breathing
• Exercise stimulates sympathetic involvement
• Sympathetic stimulation allows the body to quickly respond to situations that could affect homeostasis
• General anatomy of the ANS

Parasympathetic Division (Craniosacral)

• Preganglionic axons extend almost all the way to the target effector, where they synapse with the postganglionic neurons, which are found very close to or within the target organ
• Cranial nerves III, VII, IX, and X contain preganglionic fibers and so are involved in cranial outflow
• Oculomotor (III) : innervation of the smooth muscles of the eye (iris and ciliary body)
• Facial (VII) : stimulation of facial glands – lacrimal glands, nasal glands, salivary glands
• Glossopharyngeal (IX) : salivary glands
• The postganglionic neurons for III, VII and IX are found in the trigeminal (V) nerve
• Vagus (X) : accounts for ~90% of the preganglionic fibers in the body. The postganglionic fibers are found usually in the target organ. The vagus sends parasympathetic impulses to the heart, lungs, bronchi and esophagus, as well as to the liver, stomach, small intestine, pancreas, kidneys, and the proximal part of the large intestine.
• Sacral outflow arises from gray matter in the spinal cord segments S₂-S₄. These autonomic fibers "piggyback" on the ventral root and ventral ramus, where they then innervate the distal part of the large intestine, the bladder, ureters, and the reproductive organs

Sympathetic Division (Thoracolumbar)

• More complex than its evil twin, mostly due to the fact that it innervates more organs.
• Sweat glands and blood vessels only receive sympathetic effects
• Preganglionic fibers arise from the lateral horns of T₁ through L₂ and leave via the ventral root, then pass through a white ramus communicans which it uses to enter the paravertebral ganglion (part of the sympathetic chain). At this point, one of three things can happen to the preganglionic axon :

1. It can synapse with a postganglion neuron within the same ganglion
2. It can ascend or descend the sympathetic chain, then synapse with a postganglion neuron
3. It can emerge from the paravertebral ganglion without synapsing

• A gray ramus communicans carries a sympathetic impulse from a paravertebral ganglion to the ventral ramus of a spinal nerve, where it then goes to the effector.
• A splanchnic nerve carries the sympathetic impulse from a paravertebral ganglion to the prevertebral ganglion (also called the collateral ganglion ) where it synapses, then goes to a target organ
• There are 23 pairs of sympathetic paravertebral ganglia – 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal
• The autonomic nervous system deals with visceral sensory neurons as well, which are part of an autonomic reflex reaction. Visceral pain and somatic pain do travel along the same afferent pathways, which explains the phenomenon known as referred pain. During this event, visceral pain is taken for somatic.

Physiology of the ANS

• Cholinergic vs. adrenergic fibers
• Cholinergic fibers use acetylcholine (ACh)
• Adrenergic fibers use norepinephrine (NE)
• The effect of ACh or NE – excitatory or inhibitory – will depend on the receptors.
• Cholinergic Receptors : are named according to drugs that bind to them, mimicking the effects of ACh.
• Nicotinic receptors are found in the somatic system on the motor end plates of skeletal muscle cells and in the ANS on all postganglionic neurons and the hormone producing cells of the adrenal medulla. ACh binding to a nicotinic receptor is excitatory.
• Muscarinic receptors (named for muscarine, a mushroom poison) are found in effectors that are stimuled by cholinergic fibers. The effect is inhibitory or excitatory based on the target
• Adrenergic Receptors : are found on the organs that are stimulated by sympathetic fibers (using NE). There are two types :
• Alpha : excitatory
• Beta : inhibitory, except in the cardiac muscle where it is excitatory
• Both types are found in many organs
• Knowledge of where adrenergic and cholinergic receptors are found can help in drug development
• Atropine : an anticholinergic drug that blocks parasympathetic effects, it is used to suppress salivation and respiratory secretions; also used to dilate the pupil
• Neostigmine : inhibits cholinesterase; used to treat skeletal muscle impairment due to lack of ACh
• Tricyclic antidepressants : prolong the activity of NE, which is a "feel good" neurotransmitter
• Over-the-counter treatments: for colds, coughs, allergies and nasal congestion stimulate alpha adrenergic receptors
• Beta blockers : work on the beta-one receptors of the heart to reduce heart rate and prevent arrhythmia
• Blood pressure is controlled almost exclusively by the sympathetic system. Vasomotor tone is the continual state of partial constriction in the blood vessels. When blood needs to be delivered faster, the impulse are sent more quickly and the vessels constrict. Alpha blocker drugs interfere with this vasoconstriction and are used to treat hypertension.
• Parasympathetic tone is exhibited by organs that are controlled primarily by the parasympathetic system – the heart, the digestive tract, and the urinary tract
• Cooperative ANS effects are seen in the external genitalia
• Unique roles of the sympathetic system : sweat glands, blood vessels, adrenal medulla, response to heat, metabolic effects (increased metabolism of body cells, increased blood glucose, readying of fat to be used as fuel, increased mental awareness)
• Localized vs. Diffuse effects : parasympathetic system is local and short-lived, due to the fact that the preganglionic neuron synapses with only one postganglionic neuron (or at most, a few). All of the PNS fibers use ACh, which is very quickly broken down. The sympathetic system has many branches of the preganglionic axons and so works in a diffuse, but interconnected manner. Sympathetic effects are also longer lasting due to NE, which is not broken down quickly.
• Control over the ANS is exercised by the brain stem, spinal cord, hypothalamus, and cerebral cortex (during meditation and biofeedback)
• Major Homeostatic Imbalances : hypertension, Raynaud’s disease, mass reflex reaction and Hirschsprung’s disease