18. Autonomic Nervous System and Hypothalamus

[Also see Netter’s Atlas of Human Neuroscience p.116-7]

· ANS is entirely efferent (motor)

· involuntary

· has two neurons between CNS and end-organ (preganglionic, postganglionic)

· tissue types involved:

1. cardiac muscle (includes SA and AV nodes),

2. smooth muscle

3. most glands

· organs/systems involved: digestive, respiratory, urogenital, endocrine, spleen, heart, liver, blood vessels, glands

	Sympathetic Nervous System	Parasympathetic Nervous System
Spinal location	thoracolumbar	craniosacral
General function	emergency, catabolic “fight or flight”	homeostatic, anabolic “rest and digest”
Presynaptic cell bodies	ICL: intermediolateral cell column (T1-L2): lateral horns of gray matter T1-T6: head and neck, chest, upper limb, GIàdiaphragm T7-T11: abdomen, wall of trunk, GIàrectosigmoid junction T12-L2: lower limb, pelvic viscera, rectum, anal canal	- gray matter of brainstem (III, VII, IX, X) - S2-S4
Emerge	With thoracic and upper lumbar nerves	Travel with cranial nerves, sacral nerves
Length of fibers	short pre-ganglionic (to sympathetic chain via WRCs), long post-ganglionic	long pre, short post
Presynaptic neurotransmitter	ACh à nicotinic receptors	ACh à nicotinic receptors
Postsynaptic neurotransmitters	NE à adrenergic receptors) ACh (sweat glands only)	ACh à muscarinic receptors
Function	- increase respiratory rate - open airways - increase heart rate and contractility - dilates arteries of heart and skeletal muscle - constricts blood vessels in GI tract - constricts arteries in skin - stimulates glycogen release by liver - decreases peristalsis - arrector pili muscles - sweating - dilates eyes	- decreases heart rate - increases peristalsis - increases digestive functions - constricts pupils (near vision) - decreases respiratory rate
Clinical correlations	Horner’s syndrome

The cervical spinal cord has neither sympathetic nor parasympathetic nuclei

WRC=white ramen communicans

1. carry sympathetic pre-ganglionic fibers to sympathetic chain (paravertebral ganglions)

2. carry non-ANS fibers from viscera to CNS)

Sympathetic Trunk

- presynaptic fibers may terminate at first ganglion encountered, ascend, descend; à exit via gray rami communicans

- presynaptic fibers may go “right through” (terminating to innervate abdominopelvic viscera via splanchnic nerve)

Parasympathetic distribution

1. Oculomotor (III): Edinger Westphal nucleus

a. accommodation (via ciliary muscles à lens)

b. pupillary constriction

c. convergence of eyes (via medial rectus)

2. Facial Nerve (VII)

a. superior salivatory nucleus à pterygopalatine ganglion (à lacrimal gland) and submandibular ganglion (à submandibular/sublingual glands)

3. Glossopharyngeal Nerve (IX)

a. inferior salivatory nucleus à otic ganglion (à parotid gland)

4. Vagus Nerve (X)

a. dorsal motor nucleus (floor of 4^th ventricle) à enteric ganglia (à heart, lungs, abdominal viscera)

5. Sacral Outflow (S₂-S₄)

a. ventral gray matter preganglionic cell bodies à pelvic splanchnic nerves (à distal GI, ureter, genitals)

Hypothalamus (control and integrative center for ANS)

· Cortex influences ANS via hypothalamus through limbic system (visceral manifestations of drives and emotions: eating, sex, fear, rage, aggression)

· regions à function

o satiety (VMN)

o hunger and thirst (LN)

o circadian rhythms (SCN)

o heat/sweating: (AN)

o cold/shivering (PN)

o parasympathetic (PVN)

Relevant anatomy

1. Posterolateral portion (Posterior and lateral nuclei) à sympathetic system

2. Anteromedial portion (Anterior and paraventricular nuclei) à parasympathetic system

3. Medial: satiety (VMN) à aphagia

4. Lateral: hunger and thirst (LN) à hyperphagia

Tracts

1. hypothalamospinal tract (sympathetic)

2. hypothalmomedullary tract (parasympathetic and sympathetic)

Examples of hypothalamic control (via ANS, pituitary, etc.)

1. skeletal muscle contraction during fear response

2. Circadian rhythms + Sleep

3. Reproductive Behavior

4. body temperature control (from lateral spinothalamic tract)

a. input

i. anterior hypothalamus (heat) à sweating

ii. posterior hypothalamus (cold) à shivering

b. output

i. peripheral vasodilation, sweating, increased cardiac output (à lowering body temperature)

ii. vasoconstriction, piloerection, shivering (à elevating body temperature; shivering mediated by hypothalamic dorsomedial nucleus)

Clinical Correlations: Hirschsprung’s Disease (megacolon), Pure Autonomic Failure (PAF), Multiple System Atrophy (MSA), Heatstroke

ANS receptors

1. Adrenergic (NE and epinephrine: sympathetic)

a. alpha 1 receptors (à vasoconstriction of arterioles and veins, mydriasis)

i. agonist: nasal decongestant

ii. antagonist: treats hypertension

b. alpha 2 receptors (à decreased sympathetic outflow (CNS); à increased vasoconstriction (periphery)

i. agonist: treats hypertension

ii. antagonist: treats impotence

c. beta receptors (à increased heart rate and contractility; bronchodilation, vasodilation)

i. agonist: treats asthma

ii. antagonist: “beta blockers” treats hypertension

2. Muscarinic Receptors (ACh: parasympathetic): not so useful clinically