Autonomic Nervous System & Higher Order Functions

chapter 16

Two divisions of ANS

Sympathetic (fight or flight) and parasympathetic (rest and digest)

Responses from increased sympathetic activity

1. heightened mental awareness

2. increased metabolic rate

3. reduced digestive and urinary functions \

4. activation of energy reserves

5. increased respiratory rate and dilation of respiratory passageways

6. increased heart rate and blood pressure

7. activation of sweat glands

Responses from 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. Stimulation of urination and defecation

Preganglionic fibers

short in thoracic and lumbar segments of spinal cord. Preganglionic neurons located between segments T1 and L2

Sympathetic chain ganglia

On either side of vertebral column

One preganglionic fiber synapses on many ganglionic neurons

Fibers interconnect sympathetic chain ganglia, making the chain look like a string of pearls

Postganglionic fibers 

long expect at adrenal medullae 

Sympathetic activation

occurs during crisis where entire division responds

controlled by sympathetic centers in hypothalamus

caused increased alertness, energy, blood pressure, heart rate, breathing

Stimulation of preganglionic neurons

releases acetylcholine (ACh) at synapses with ganglionic neurons 

ganglionic neurons release neurotransmitters at target organs 

each swollen segment is called a varicosity 

Alpha 1 receptors

more common, found mainly in smooth muscle

Alpha 2 receptors

found on preganglionic sympathetic neurons, stimulation lowers cAMP levels in cytoplasm and has inhibitory effect

Beta receptors 

located on membranes of cells in skeletal muscles, lung, heart, liver etc. 

stimulation increases intracellular cAMP levels and triggers metabolic changes

Types of beta receptors

beta 1 - increases metabolic activity

beta 2 - triggers relaxation of smooth muscles

beta 3 - leads to lipolysis, breakdown of triglycerides in adipocytes

Majority of postganglionic fibers release…..

NE (adrenergic), some release ACh (cholinergic) which stimulates sweat glands and dilates blood vessels of skeletal muscles and brain

Major effects of parasympathetic division

1. Constriction of pupils and focusing on near objects

2. Secretion by digestive glands

3. Absorption and use of nutrients by peripheral cells

4. Changes associated with sexual arousal

5. Increased smooth muscle activity in digestive tract

6. Stimulation and coordination of defecation

7. Contraction of urinary bladder during urination

8. Constriction of respiratory passageways

9. Reduction in heart rate

Effects of parasympathetic neurons releasing ACh

different effect on postsynaptic cells

localized and short lived because most of it is inactivated at synapse and AChE

Nicotinic Receptors

• type of cholinergic receptor

• on ganglion cells of sympathetic and parasympathetic divisions

• occur at neuromuscular junctions of SNS

Muscarinic receptor 

• type of cholinergic receptor 

• at cholinergic neuromuscular or neuroglandular junctions in parasympathetic division 

• at cholinergic junctions in sympathetic division 

• longer lasting effects than nicotinic receptors

Dual Innervation

vital organs innervated by both divisions of ANS, both with opposing effects

• parasympathetic postganglionic fibers travel by cranial nerves to peripheral destinations

• sympathetic innervation reaches same structures, superior cervical ganglia of sympathetic chain

Autonomic plexuses

nerve networks in thoracic and abdominopelvic cavities formed by mingled sympathetic postganglionic fibers and parasympathetic preganglionic fibers

cardiac and pulmonary plexus of dual innervation

intersecting autonomic fibers in thoracic cavity 

contain sympathetic and parasympathetic fibers for heart and lungs and parasympathetic ganglia that affect those organs 

esophageal plexus of dual innervation

contains descending branches of vagus nerves and splanchnic nerves leaving sympathetic chain

parasympathetic preganglionic fibers of vagus nerv3es enter abdominal cavity with esophagus

celiac plexus of dual innervation

associated with smaller plexuses such as inferior mesenteric plexus

innervates viscera within abdominal cavity

hypogastric plexus of dual innervation

innervates digestive, urinary, and reproductive organs of pelvic cavity 

contains parasympathetic outflow of pelvic nerves and sympathetic postganglionic fibers form inferior mesenteric ganglion 

autonomic tone of dual innervation

autonomic motor neurons have resting level activity even without stimulation, since nerves maintain background level of activity they ca increase or decrease activity

the heart of dual innervation

acetylcholine released by parasympathetic postganglionic fibers slows heart rate

NE released by varicosities of sympathetic division accelerates heart rate

Centers involved with somatic motor control

Found in all portions of CNS - lower motor neurons of cranial and spinal reflex arcs and pyramidal motor neurons of primary motor cortex 

simple reflexes based in spinal cord respond rapidly and autonomically to stimuli 

centers in brainstem control activity of sympathetic and parasympathetic divisions

Visceral reflexes

autonomic polysynaptic reflexes initiated in viscera and provides automatic motor responses

Visceral reflex arc consists of…

receptor, sensory neuron, processing center and two visceral motor neurons

Long reflexes of visceral reflexes 

autonomic equivalents of polysynaptic reflexes 

visceral sensory neurons deliver info to CNS along posterior roots of spinal nerves

Short reflexes of visceral reflexes

bypass CNS entirely and involve sensory neurons and interneurons whose cell bodies lie in autonomic ganglia

interneurons synapse on postganglionic neurons and postganglionic fibers distribute motor commands

Higher levels of autonomic control are controlled…

in processing centers in medulla oblongata that coordinate complex reflexes - centers and nuclei involved in salivation, swallowing, digestive secretions etc.

regulated by hypothalamus