Autonomic Nervous System

Csilla Egri, KIN 306 Spring 2012

Pain, fright, and being a mom; all ways of activatingthe sympathetic nervous system

Outline

Function of the autonomic nervous system (ANS)

Organization of the ANS Major neurotransmitters

Classical Nonclassical

Central control of ANS function

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Function of the ANS3

Autonomic Nervous SystemParasympathetic: Rest and

digestSympathetic: Flight or fight

• vasodilates vessels to the GI tracts• constricts the pupil• constrict bronchioles• stimulates salivary gland secretions and peristalsis• sexual arousal

• vasocontricts vessels to GI tract and skin• vasodilates vessels to skeletal muscle and lungs• dilates bronchioles• increases heart rate and contractility• dilates pupils • inhibits peristalsis• constricts intestinal sphincters and urinary sphincter**so why can you pee your pants if you’re really scared?

Organization of the ANS4

ANS efferents all originate in the CNS and innervate tissues via a two-synapse pathway Preganglionic neurons

columns of cells in the brain stem and spinal cord that synapse on:

Postganglionic neurons In peripheral ganglia interposed between CNS and

target cells

Check: What are ganglia?

Organization of the ANS

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SNS preganglionic neurons Spinal cord

between T1-L3 in the _________ horn

Exit thru ventral roots

Synapse on many paravertebral or prevertebral ganglia located far from target organs

B&B Figure 15-4

Organization of the ANS

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PNS preganglionic neurons Nuclei of medulla,

pons, midbrain and segments S2-S4 of spinal cord

Synapse on many distant terminal ganglia close to target organ

B&B Figure 15-4

ANS afferent division7

All visceral organs densely innervated by afferent neurons Monitor nociceptive input and mechanical and

physiological stimuli such as diameter of vessels, blood gas concentrations, pH, glucose, and temperature

Stimuli often don’t reach consciousness, instead initiate ANS reflex arcs

Classical neurotransmitters of the ANS

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Both branches of the ANS release Ach from preganglionic neurons that act on N2 nicotinic receptors

B&B Figure 15-8

Check: To what family of receptors do N2

receptors belong?

Classical neurotransmitters of the ANS

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NTs released by postganglionic neurons of each branch differ

B&B Figure 15-8

Check: To what family of receptors do muscarinic

ACh receptors and adrenergic receptors

belong?

Classical neurotransmitters of the ANS

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Postganglionic neuronsPNS• release ACh onto muscarinic receptors• results in stimulation or inhibition of target cell, depending on downstream signalling cascade• response is slow and prolonged

SNS• most release norepinephrine (NE) that act on α or β adrenergic receptors*• neuroendocrine branch innervating the adrenal medulla has postganglionic chromaffin cells releasing epinephrine (Epi) into the bloodstream

* Except for innervation of sweat glands, where SNS postganglionic neurons release ACh that acts on muscarinic receptors

Adrenergic receptors

Agonists Typical target cell response

α1 NE ≥ Epi Ciliary muscles of iris: pupil dilationBlood vessels: vasoconstriction

α2 NE ≥ Epi Can be located presynaptically as well, modulate NT release

β1 Epi > NE Heart: increased heart rate and contraction

β2 Epi > NE Bronchioles: bronchodilationSkeletal muscle vasculature: vasodilation

β3 Epi > NE Fat cells: lipolysis

Classical neurotransmitters of the ANS

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Acetylcholine receptors

Agonists Typical target cell response

N2 ACh, nicotine Trigger EPSP on postganglionic neuron

M1-M5 ACh, muscarine

Varied, can be excitatory or inhibitory, heterogeneous distribution

*N1 receptors are not part of the ANS, they are part of the somatic nervous system and initiate action potentials at the neuromuscular junction

Nonclassical neurotransmitters of the ANS

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Some postganglionic SNS/PNS neurons contain muscarinic receptors for Ach Results in varied response in target cell to ACh

ANS can release numerous “nonclassical” neurotransmitters Colocalized in ANS axon terminals and can

participate in cotransmission with cholinergic or adrenergic neurotransmitters

Examples: glutamate, glycine, substance P, serotonin, neuropeptide Y, ATP, NO

Central control of ANS function13

ANS activity controlled by reflex pathways in spinal cord and brainstem as well as descending controls originating from higher autonomic brain regions Reflex examples:

Pupillary light reflex Light excites retinal cells which send connections to

PNS ganglia in the brainstem constriction of pupil size

Micturition reflex (overcome by potty training…hopefully)

Distension of bladder sensed by mechanoreceptors which send connections to pontine micturation centre (PMC) in the pons SNS input to bladder decreases, PNS input increases voiding of bladder

Central control of ANS function14

Autonomic brain regions: Hypothalamus Temperature regulation

Hypothalamic thermoreceptors have connections to SNS Cold temp. increased SNS activity shivering,

increased metabolism, cutaneous vasoconstriction Regulation of food intake

Hypothalamic glucoreceptors Blood glucose falls release of opioid peptides and

pancreatic peptides (enteric nervous system connections) stimulate food intake

Emotional behaviour Limbic system sends connections to hypothalamus and other

brain regions (ex. PMC) Influence dietary behaviour Can pee your pants when your scared!

Objectives

After this lecture you should be able to: List the general functions of the ANS Describe ANS neuronal connections Compare and contrast SNS and PNS neurotransmitters

and receptors Describe the role of central control of ANS function

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1. What would happen if you administered a systemic nicotinic receptor antagonist? What about a systemic adrenergic antagonist?

2. Atropine, when applied as eye drops, mimics the effect of the sympathetic nervous system by blocking receptors of the parasympathetic branch. What is the effect on the eye and what specific receptors is atropine an antagonist?

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