The Autonomic Nervous System

Basic Setup of the Nervous System

Basic Setup of the Nervous System

What does the ANS do? Certain fairly slow, bodily functions are so routine, we can have them operate without conscious control Occasionally, things happen that require instantaneous diversion of energy Stop doing the routine and shift resources into escape. Hurry Up and Wait Things

How does the ANS work? The ANS consists of motor neurons that: Innervate smooth and cardiac muscle and glands Operate via subconscious control Have viscera as most of their effectors

Comparison of Somatic and Autonomic Systems Motor Comparison of Somatic and Autonomic Systems Somatic: Control of skeletal muscles Autonomic: Regulates smooth muscle, cardiac muscle and glands

ANS Versus Somatic Nervous System (SNS) The ANS differs from the SNS in the following three areas Effectors Who Efferent pathways How are the neurons arranged Target organ responses What: excitatory or inhibitory

Effectors SNS Skeletal muscles ANS Cardiac muscle Smooth muscle Glands

Efferent Pathways SNS ANS Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector ANS Axons of the ANS are a two-neuron chain The preganglionic (first) neuron has a lightly myelinated axon The ganglionic (second) neuron extends to an effector organ

Target Organ Response SNS ANS All somatic motor neurons release Acetylcholine (ACh), which has an excitatory effect ANS Preganglionic fibers release ACh Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector

Review: Somatic vs. Autonomic efferent neurons Voluntary Involuntary Effectors: Skeletal M. Effectors: Cardiac M. Smooth M Glands Neurons extend from CNS to effectors without synapsing. Two neurons to get from CNS to effectors; therefore one synapse. "Two neuron chain"

Comparison of Somatic and Autonomic Systems

Comparison of Somatic and Autonomic Systems Things this figure points out: Myelination One- vs. two-neuron chain Length of pre- and post-ganglionic neurons Effector organs Neurotransmitters

Adrenal Glands

Divisions of the ANS The two divisions of the ANS are the sympathetic and parasympathetic Basically, the sympathetic mobilizes the body during extreme situations. Basically, the parasympathetic performs maintenance activities and conserves body energy The two divisions counterbalance each other’s activity (for the most part). Parasympathetic: Rest and Digest Sympathetic: Fight or Flight

Role of the Parasympathetic Division Concerned with keeping body energy use low Involves the D activities – digestion, defecation, and diuresis Its activity is illustrated in a person who relaxes after a meal (Thanksgiving)

Role of the Parasympathetic Division Blood pressure is low Heart rate is low Respiratory rates are low Gastrointestinal tract activity is high The skin is warm because the blood is in the skin The pupils are constricted

Role of the Sympathetic Division The sympathetic division is the “fight-or-flight” system (Big Dog) Involves E activities – exercise, excitement, emergency, and embarrassment

Role of the Sympathetic Division Promotes adjustments during exercise – blood flow to organs is reduced, flow to muscles is increased Its activity is illustrated by a person who is threatened Heart rate increases Breathing is rapid and deep The skin is cold and sweaty: the blood is in the muscles, not in the skin. The pupils dilate: “to better see you with”

Anatomy of ANS: Sympathetic vs. Parasympathetic There are two main differences in the anatomy of the sympathetic and parasympathetic systems. Where the ganglia (or second synapses) are. Where they exit the spinal cord

Location of Ganglia Parasympathetic Ganglia (synapses) are on the target organ Sympathetic Ganglia (synapses) are near the spinal cord

Anatomy of ANS Para-sympathetic exits either in: a. cranial nerves b. sacrally Sympathetic exits from T1- L2

Parasympathetic Division Outflow Cranial Outflow Cranial Nerve Ganglion Effector Organ(s) Occulomotor (III) Ciliary Constricts iris Facial (VII) Pterygopalatin Submandibular Salivary, nasal, and lacrimal glands Glossopharyngeal (IX) Otic Parotid salivary glands Vagus (X) Located within the walls of target organs Heart, lungs, and most visceral organs Sacral Outflow S2-S4 Located within the walls of the target organs Large intestine, urinary bladder, ureters, and reproductive organs

Sympathetic Outflow Nerves Sympathetic Chain Ganglia Cardiac and Pulmonary Plexuses Splanchnic Nerves Pre Post Ganglionic

Sympathetic Outflow Arises from spinal cord segments T1 through L2 Sympathetic neurons produce the lateral horns of the spinal cord Postganglionic fibers innervate the numerous organs of the body

Sympathetic Chain Sympathetic neurons go up and down the sympathetic chain, in order to provide reduncancy.

Visceral Reflexes Some people consider the autonomic nervous system to be a purely motor system, despite the presence of the sensory neurons. Clearly, because there are autonomic reflexes, there has to be a sensory component Sweating

Visceral Reflexes

Referred Pain Pain stimuli arising from the viscera are carried in sympatheric nerves. Often visceral pain is perceived as somatic in origin

Referred Pain This may be due to the fact that visceral pain afferents “piggyback” on somatic pain fibers

Interactions of the Autonomic Divisions The parasympathetic and sympathetic nervous systems interact in three ways; Antagonism Tone Cooperatively You don’t want a picture 

Antagonism between the Autonomic Divisions Most visceral organs are innervated by both sympathetic and parasympathetic fibers This results in dynamic antagonisms that precisely control visceral activity

Antagonism between the Autonomic Divisions Heart: Sympathetic increases rate & force Parasympathetic decreases rate & force Lungs Sympathetic dilates air passages Parasynpathetic constricts air passages Digestive System Sympathetic decreases activity Parasympathetic increases activity Urinary System Sympathetic inhibits urination Parasympathetic promotes urination

ANS Tone Tone can be considered to be a system acting on it own. Increased tone means increased activity Decreased tone means decrease activity. It’s a bit like a gas pedal, you control “activity” by how much you press. Pedal to the metal equals a lot of sympathetic activity. Foot off the gas means less sympathetic activity. Both sympathetic tone and parasympathetic tone exist.

Sympathetic Tone The sympathetic division controls blood pressure and keeps the blood vessels in a continual state of partial constriction This sympathetic tone (vasomotor tone): Constricts blood vessels and causes blood pressure to rise as needed Prompts vessels to dilate if blood pressure is to be decreased

Sympathetic Tone Increased Tone = Vasoconstriction Decreased Tone = Vasodilation “Tone up the muscle”= constriction

Parasympathetic Tone Parasympathetic tone: Slows the heart Dictates normal activity levels of the digestive and urinary systems The sympathetic division can override these effects during times of stress

Parasympathetic Tone Increase Tone = Decrease HR Decreased Tone = Increase HR

Cooperation between the Autonomic Divisions ANS cooperation is best seen in control of the external genitalia Parasympathetic fibers cause vasodilation and are responsible for erection of the penis and clitoris Sympathetic fibers cause ejaculation of semen in males and reflex peristalsis in females

Neurotransmitters and Receptors Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the ANS Cholinergic fibers – ACh-releasing fibers Adrenergic fibers – NE-releasing fibers

Neurotransmitters and Receptors Cholinergic fibers – All preganglionic axons All parasympathetic postganglionic axons May bind to two different receptors Nicotinic Muscarinic Adrenergic fibers – Sympathetic postganglionic axons

Neurotransmitter Comparison of Somatic and Autonomic Systems Cholinergic fibers All preganglionic axons All parasympathetic postganglionic axons May bind to two different receptors Nicotinic Muscarinic nAChR mAChR NE Adrenergic fibers – Sympathetic postganglionic axons

Neurotransmitter Comparison of Somatic and Autonomic Systems nAChR NE mAChR The cell bodies of postganglionic autonomic fibers are located in:

ANS Neurotransmitters Neurotransmitter effects can have different effects on different targets. Different effects are due to different receptors (α1, α2 ,β1, β2) NTs can be excitatory or inhibitory depending upon the receptor type. In adrenergic receptors, α1 is stimulatory and α2 is inhibitory Different organs carry different receptors. For example, there are α NE receptors in blood vessels and β NE receptors in cardiac muscle. This allows pharmaceutical targeting.

Noradrenergic Receptors Two main types ( and ) and subdivisions: Allows stimulation of some things (bronchodilation) without affecting heart rate.  and  ADRENERGIC RECEPTORS 1 – stimulation 2 – inhibition 1 – stimulation 2 – inhibition

NE Pharmaceuticals 1 stimulants would constrict blood vessels and dilate the eyes. 2 stimulants promote blood clotting 1 stimulants increase HR and beat strength. 2 stimulants dilate bronchials and coronary vessels. More air, more blood. You are inhibiting sympathetic tone, thus causing dilation.

Cholinergic Receptors The two types of receptors that bind ACh are nicotinic and muscarinic These are named after drugs that bind to them and mimic ACh effects

Cholinergic Receptors

Nicotinic Receptors Nicotinic receptors are found on: Motor end plates (somatic targets) All ganglionic neurons of both sympathetic and parasympathetic divisions The hormone-producing cells of the adrenal medulla The effect of ACh binding to nicotinic receptors is always stimulatory

Comparison of Somatic and Autonomic Systems nAChR mAChR NE

Muscarinic Receptors Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic fibers The effect of ACh binding: Can be either inhibitory or excitatory Depends on the receptor type of the target organ Slows heart rate and strength of muscle contractions Increases digestive activity Constriction of the iris

Effects of Drugs Atropine – blocks parasympathetic effects Bella donna (and antidote to nerve gas) Neostigmine – inhibits acetylcholinesterase and is used to treat myasthenia gravis Tricyclic antidepressants – prolong the activity of NE on postsynaptic membranes Over-the-counter drugs for colds, allergies, and nasal congestion – stimulate -adrenergic receptors Beta-blockers – attach mainly to 1 adrenergic receptors and reduce heart rate and prevent arrhythmias

Cholinergic blockers Muscarinic blockers block parasympathetic effects on target organs. Atropine used topically during eye exams to dilate pupils May be used to reduce salivation and respiratory secretions.

Drugs that Influence the ANS

Drugs that Influence the ANS

Localized Versus Diffuse Effects The parasympathetic division exerts short-lived, highly localized control The sympathetic division exerts long-lasting, diffuse effects

Effects of Sympathetic Activation Sympathetic activation is long-lasting because NE: Is inactivated more slowly than ACh Is an indirectly acting neurotransmitter, using a second-messenger system Epinephrine is released into the blood and remains there until destroyed by the liver

Levels of ANS Control The hypothalamus is the main integration center of ANS activity Subconscious cerebral input via limbic lobe connections influences hypothalamic function Other controls come from the cerebral cortex, the reticular formation, and the spinal cord

Levels of ANS Control

Hypothalamic Control Centers of the hypothalamus control: Heart activity and blood pressure Body temperature, water balance, and endocrine activity Emotional stages (rage, pleasure) and biological drives (hunger, thirst, sex) Reactions to fear and the “fight-or-flight” system