AUTONOMIC NERVOUS SYSTEM
I. GENERAL
A. Primary function is regulation of: cardiovascular, respiratory, digestive, urinary and reproductive systems, maintain body temp.
B. Innervates smooth and cardiac muscle and glandular epithelium
C. Effectors of homeostasis – nervous system control
1. Humoral (endocrine) – develops slowly, effects prolonged
2. Neural (autonomic) – immediate, short term
D. Under control of CNS, integrated from many sources
1. emotional state
2. sensory signals
E. Major subdivisions
1. Parasympathetic
2. Sympathetic
3. Enteric
II. ORGANIZATION OF VISCERAL MOTOR SYSTEM
A. Neural control of:
1. Smooth muscle
2. Cardiac muscle
3. Glandular secretory cells
B. Parasympathetic and sympathetic systems overlapping and antagonistic influences
C. General features
1. Consists of 2 neuron chain
a. preganglionic – in CNS, to ganglion
b. postganglionic – in ganglion, to effector
c. release of neurotransmitter via varicosities along axon
III. DEVELOPMENT
A. Preganglionic motor neuron
1. Derived from GVE cell column
2. Arises from neuroblasts in basal plate of neural tube
B. Postganglionic motor neuron
1. Derived from neural crest cells that migrate to appropriate location
2. Congenital megacolon (Hirschsprung disease) results from failure of enteric neurons to populate wall of lower gut
3. Nerve growth factor (NGF) is essential for survival and development
IV. SYMPATHETIC DIVISION
A. Preganglionic neurons – "thoracolumbar"
1. Found only in spinal cord segments T1-L2, in intermediolateral nucleus (cell column)
2. Axons exit the spinal cord in the ventral roots and enter the sympathetic trunk via white rami communicans
3. Once in the sympathetic chain, preganglionic fibers may:
a. synpase at that level
b. ascend or descend in chain to synapse at another level
c. pass through chain as a splanchnic nerve
4. Crude viscertopic organization (i.e. Head supplied by upper thoracic preganglionic neurons
5. Target adrenal chromaffin cells that release catecholamines (mostly epinephrine)
B. Sympathetic ganglia
1. Postganglionic neurons grouped into discrete ganglia making up sympathetic chain (paravertebral) or prevertebral ganglia
2. Sympathetic chain extends the length of the vertebral column but the # of ganglia do not match the # of spinal nerves (i.e. 3C, 10-11T, 3-5L, 1C)
3. Inferior cervical ganglion and T1 fuse to form stellate ganglion
4. Sympathetic chain connected to spinal nerves by white and gray rami communicans
a. Only spinal nerves T1-L2 have WRC
b. Every spinal nerve has GRC
5. Postganglionic neurons send axons in 2 general directions
a. Postganglionic fibers join spinal nerves via GRC to blood vessels, sweat glands and erector pill muscles
b. Fibers from cervical and thoracic chain ganglia form cervical and thoracic cardiac and pulmonary nerves – innervate esophagus, lungs and heart – these fibers mix with those of the vagus nerve (CN X) to form various plexuses in the thorax and abdomen
6. Superior cervical ganglion is largest chain ganglion
a. Postganglionic fibers innervate blood vessels and cutaneous targets of face, scalp and neck
i. Territories of C1-4
ii. Salivary, nasal and lacrimal glands
iii. Pupillary dilator muscles
iv. Superior tarsal muscle
v. Sweat glands of face
vi. Damage produces Horner’s syndrome (mitosis, ptosis, anhidrosis) –
7. Prevertebral ganglia found in plexuses associated with abdominal aorta
a. Project via splanchnic nerves to visceral targets of corresponding branches of the aorta (i.e. celiac to foregut)
8. Internal organization of sympathetic ganglia
a. Convergence within ganglion
b. Divergent output
c. High degree if integration from various sources (i.e visceral afferents)
9. Functional and chemical coding
a. "en masse" activation: increase heart rate, blood pressure, blood to skeletal muscle, increased blood glucose, sweating and pupillary dilation
b. Selective activation
i. pathways to skin influenced by temperature
ii. pathways to skeletal muscle influenced by blood pressure (via baroreceptors)
c. Heterogeneity of preganglionic neurons
i. neuropeptide expression (substance P, enkephalin)
ii. location in spinal cord
iii. dendritic morphology
iv. target cell-type in ganglion
d. Most postganglionic neurons use norepinephrine (NE), some are cholinergic (to sweat glands and dilator innervation of skeletal muscle)
e. Some postganglionic neurons co-release other peptides (Neuropeptide Y)
f. Noradrenergic receptors-
i. Alpha receptor - pupil dilation and vasoconstriction
ii. Beta receptor - cardiac pacemaker, bronchodilation skeletal muscle vessels, enteric neurons
10. Receptor types in sympathetic targets
a. Effect of neurotransmitter depends on target cell receptor
b. Clinical administration dependent on target specificity
c. Diffuse effects
d. Divergence
V. PARASYMPATHETIC DIVISION
A. Preganglionic and postganglionic neurons – "craniosacral"
1. More restrictive distribution
2. Located in CN’s III, VII, IX, X and spinal cord segments S2-4
a. Postganglionics of cranial structures generally travel with CN V
b. Postganglionic of viscera scattered with nerve plexuses in wall of gut (intermingle with enteric neurons)
B. Parasympathetic outflow pathways
1. Edinger Westphal nucleus > ciliary ganglion > sphincter of iris and ciliary muscle
2. Superior salivatory nucleus > pterygopalatine ganglion > lacrimal and nasal glands
3. Superior salivatory nucleus > submandibular ganglion > salivary glands
4. Inferior salivatory nucleus > otic ganglion > parotid gland
5. Dorsal vagal nucleus > ganglion in wall of viscera > viscera(pulmonary and enteric)
6. Sacral parasympathetic nucleus > pelvic nerves > pelvic viscera
7. Nucleus ambiguus and cardiac ganglia
C. Functional and chemical coding
1. Preganglionic and postganglionic are cholinergic
2. Some co-release neuropeptides (VIP)
3. acts in localized and discrete regions
D. Receptor types
1. Nicotinic ACh receptors in skeletal muscles and ganglia
2. Muscarinic ACh receptors in smooth and cardiac muscle and glandular cells
3. Response depends on receptor subtype
4. Atropine blocks cholinergic responses
VI. ENTERIC NERVOUS SYSTEM
A. Contains as many neurons as spinal cord(108) – mainly in myenteric and submucosal plexuses
B. Influenced by sympathetic and parasympathetic inputs
C. Largely autonomous – intrinsic neural circuits
D. Include mechanoreceptors (stretch) control peristalsis, blood flow and secretions
E. Express many neuropeptides (VIP, neuropeptide Y, CCK)
F. Afferents are Psy and Sy
VII. REGULATION OF VISCERAL MOTOR OUTFLOW
A. Sensor inputs integrated by Central Autonomic Network (CAN) which coordinates signals to visceral motor, endocrine and somatic motor outflow
B. Emotion and mental activity integrated via CNS centers involved in cognition and behavior
C. Hypothalamus is highest integration center for autonomic and endocrine function
1. Hypothalamic nuclei project directly to preganglionic neurons
2. Large involvement by brainstem reticular formation to autonomic centers involved in respiration, micturition and vomiting
D. Cardiovascular system
1. Baroreflex buffers changes in posture- Orthostatic hypotension (i.e. lowered blood pressure that occurs when one is in a standing position)
2. Chemoreceptor reflex maintains blood gas composition
a. pCO2 and pO2 levels detected by carotid and aortic bodies
b. Coordinated with respiration and heart rate
E. Urinary bladder and micturition
1. Parasympathetic input to detrusor muscle is excitatory
2. Sympathetic input inhibits parasympathetic ganglion cells and detrusor muscle
3. External urethral sphincter under reflex and voluntary control