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About the Autonomic Nervous System

• Autonomic nervous system is part of the peripheral nervous system
• Remember that the PNS is divided into “somatic” and “autonomic” and then the autonomic nervous system is divided into “parasympathetic and sympathetic”
• Autonomic nervous system regulates cardiac rate, smooth muscle, glands
• Impulses are conducted from the CNS to a second autonomic neuron, and then the second neuron innervates the involuntary effector
  • differs from somatic control (see reflex arc last chapter) where there is one neuron going from the CNS to the effector; somatic cell bodies are in the CNS
• Innervates – synapses with
• Preganglionic and postganglionic
  • 1^st neuron has its cell body in the brain or the spinal cord (midbrain and hindbrain OR the upper thoracic to the fourth sacral levels of the spinal cord)
    • This neuron synapses with a 2^nd neuron at an autonomic ganglion.
    • 2^nd neuron synapses with the effector
• There are also sensory neurons – information enters the spinal cord or the brain (on cranial nerves:
  • EXAMPLES – information about blood pressure, plasma pH, oxygen levels goes into the brain in cranial nerves IX and X.  These are mixed nerves containing both sensory and parasympathetic motor axons.

Visceral Effector Organs

• Visceral effector organs
• The autonomic nervous system regulates all the organs
• Common features of organs that are regulated autonomically:

1. 1. they have a built-in muscle tone – they maintain a resting tone if there is damage to a nerve innervating a visceral organ, it does not result in flaccid paralysis (unlike with damage to a somatic neuron).
   2.   denervation hypersensitivity -  they may actually become MORE sensitive to regulation when nerves are damaged.
      1. EXAMPLE: After a vagotomy (cutting of the Vagus nerve, which could happen if the  person has an ulcer) does not impair acid secretion.
   3. cardiac and smooth muscles contract rhythmically in the absence of nerve stimulation, in response to electrical stimulation (depolarization) initiated by  the muscles themselves.
   4. autonomic innervation increases/decreases their activity, like an accelerator or brake of a car

Sympathetic and Parasympathetic

• There are two divisions of the autonomic nervous system: parasympathetic and sympathetic

Sympathetic

• Sympathetic – (thoracolumbar) – preganglionic fibers exit the spinal cord at spinal nerves T1 through L2
  • they synapse in the paravertebral chain with their postganglionic neurons  (most of them).  (figure 9.2)
  • they can ALSO synapse with postganglionic neurons at different levels of  the chain (fig. 9.3)
    •  if they synapse with postganglionic neurons at different levels of the chain,  this is called divergence
      •  if they receive input from many preganglionic fibers, this is called convergence
      • TOGETHER – divergence and convergence allow for MASS activation.
        • HOWEVER, this does not always happen:
          • EXAMPLE – the sympathetic nervous system may simply increase/decrease the activity of the heart to allow it to maintain homeostasis.
    •  if they do not synapse in the chain, they become splanchnic nerves (which                                    means “inward parts” and synapse in collateral ganglia
      

Collateral ganglia   

• ALSO called prevertebral ganglia
• these include the celiac, superior mesenteric and inferior mesenteric ganglia
• their postganglionic fibers innervate organs of the  digestive, urinary,  reproductive

Dual Innervation

• Remember that both Acetylcholine and N/NE will synapse with the same organ and have different (antagonistic ) effects.
• Effect on the smooth muscle depends on which neuron is the most active.
• Dual innervation – means that both nervous systems go to the same target
• Antagonistic effects means that the two Nervous systems do the opposite things.

Adrenal Glands

• It has no postganglionic fiber. 
• It is a modified sympathetic ganglion.  Except it sends its neurotransmitter into the blood, which is a crossover between the nervous/endocrine system. 
• So your whole body gets exposure to the neurotransmitter. 
• The neurotransmitters are epinephrine (80%) and norepinephrine (20%)
•  they are acting like hormones here.

Sympathoadrenal system:

• Sympathetic nervous system: fight or flight. 
• You need
  1. pupil dilation, to allow in a lot of light so you can see,
  2.  increase heart rate to get more oxygen (tachycardia),
  3. get as much air as possible (bronchiolar smooth muscle regulation,
  4. digestion can wait till later.  Digestive motility ceases. 
  5. adrenal glands release neurotransmitters into the bloodstream,
  6. sweat glands – sweat to reduce body temperature. 
• Blood vessels – affect of sympathetic nervous system determines which blood vessels are contracted or dilated.  What would blood vessels need to do to get you to fight or flee?
• QUESTION: – What would happen to the blood vessels of the digestive tract?
  • Answer: constrict.  What would happen to skeletal muscles?  Answer: they dilate because you use them.  They relax.  What about lungs? Vessels dilate b/c you want blood going to the lungs for air.  Skin?  The blood really does not go to the skin during fight or flight.  This is complicated b/c you are going to heat up, and so your cheeks do heat up, and there are a couple of areas where you will release heat.

Parasympathetic

• Preganglionic fibers exit from brain and sacrum, and the ganglia exist on the target organ.
• There are four cranial nerves that have parasympathetic information.  (Cranial nerve III, VII, IX, and X)
  • Vagus nerve, cranial nerve X, has the most activity going to the organs.
    • Vagus means “wandering” like a vagabond.
• LOWER: pre-ganglionic fibers originating in the sacral levels of the spinal cord provide innervation to the lower half of the large intestine, the rectum, urinary, reproductive
• ACTIONS: rest and digest
  • Pupil constriction
  • Bradycardia (slow the heart rate)
  • Food digestion (smooth muscle contraction for peristalsis, sphincter relaxation, digestion)
  • No effect on sweat glands.  Parasympathetic fibers do not go to sweat glands.

Comparison of Parasympathetic and Sympathetic

• When comparing parasympathetic and sympathetic :
  • SYMPATHETIC ganglia are closer to the spine. They can send signals far and wide, whereas
  • Parasympathetic ganglia are further away; they are only taking care of issues when you have the time and energy to do it.
  • Therefore, you see sympathetic activation of places that are not regulated parasympathetically, such as cutaneous effectors (blood vessels of skin, sweat glands, arrector pilli muscles), and blood vessels that innervate skeletal muscle receive sympathetic but not parasympathetic.

Neurotransmitters and their Receptors in the Autonomic Nervous System

• Epinephrine (adrenalin)  British/American
• Norepinephrine is almost the same thing and is also called noradrenalin. 
• A synapse that uses epinephrine/adrenalin/norepinephrine is called an “adrenergic”synapse
• Epinephrine and Norepinephrine are two neurotransmitters are found in the same synapses.
• Acetylcholine is also found to control these organs as well.  The synapse that uses acetylcholine is called “cholinergic”

Sympathomimetic Drugs

• mimic sympathetic nervous system (epi, norepi, dopamine)
• includes drugs that promote the release of these chemicals
•  includes drugs that block their breakdown or
• includes drugs that block their reuptake
•  includes some abused drugs like amphetamine and cocaine

Which synapse uses what?

• Parasympathetic preganglionic neuron releases ACh, and the postganglionic neuron releases ACh also. 
• Sympathetic preganglionic neuron releases ACh, but the postganglionic neuron releases E/NE.
• The effects of these neurotransmitters on their targets depends on what kinds of receptors you find there.
• Figure 9.9 – Varicosities -  these are swellings along both sympathetic and parasympathetic axons where neurotransmitters are released.

Response to Adrenergic Stimulation

• epi and norepi can have both excitatory and inhibitory effects because there are different receptors!
• Three main kinds of adrenergic receptors, alpha 1, beta 1, beta 2
  • Alpha 1 – stimulates rise in cytoplasmic Ca²⁺, found in smooth muscle, causes (vasoconstriction) contraction when stimulated, in the intestine OR skin [what you want to shut off]  STIMULATES
  • Beta 1 – on the heart; causes faster heart rate [adrenergic]  STIMULATES CONTRACTION
  • Beta 2 – acts opposite to alpha 1, causes smooth muscle relaxation ; skeletal muscle, blood vessels, bronchioles.  (STIMULATES RELAXATION)
• Agonists – mimic the receptor
  • EXAMPLES: Phenylephrine and pseudophedrine (used in cold medicines) stimulate alpha 1 adrenergic receptors in the nasal mucosa, promoting vasoconstriction; relieving nasal congestion, but they raise blood pressure.
  • Antagonists – block the receptor
    •  Examples: propranolol à blocks beta 1 and beta 2 atenolol – blocks beta 1 specifically and is used to lower heart rate specifically (so it will not stimulate bronchodilation)

Response to Cholinergic Stimulation

• the affects of acetylcholine also depend on the nature of the acetylcholine receptor
• ACh is released by all somatic motor neurons, all pre-ganglionic parasympathetic neurons, most post-ganglionic parasympathetic neurons
  • it’s mostly excitatory
  • sometimes inhibitory, ie. It slows the heart!
  • Nicotinic –  blocked by curare
    • 1) nicotinic receptors (found in skeletal muscle and in the autonomic nervous system) or
  • Muscarinic – blocked by atropine or belladonna
    • 2) muscarinic (which can open K+ channels, causing an IPSP; or close K+ channels causing an EPSP).  The effect of ACh depends on the receptor that is found there.

Nitric Oxide

Nitric oxide –

•  is this just a  paracrine regulator?
• it’s produced by nitric oxide synthase from L-arginine and it diffuses into smooth muscles, causing relaxation
• Viagra – blocks the breakdown of cGMP, which stimulated continued relaxation                            of blood vessels, stimulating erection
• BUT its formation seems to be triggered by the influx of Ca²⁺ into axon terminals.
• EXAMPLE : Nitric oxide keeps blood flowing, it is thought to inhibit plaque formation. 

Antagonistic Effects

• Each organ is regulated by both sympathetic and parasympathetic fibers
• EXAMPLE – pacemaker of the heart
  • sympathetic and parasympathetic fibers innervate the same cells
  • adrenergic stimulation increases heart rate; whereas the release of ACh from parasympathetic fibers decreases heart rate
  • Note: heart rate will increase when
    • 1) parasympathetic activity decreases. but, sympathetic stimulation remains constant. OR 2) sympathetic activity increases, such as during intense exercise
• EXAMPLE – pupil constriction/dilation  (two sets of muscles make up the iris)  like the reciprocal contraction/flexion of opposing muscle groups
  • sympathetic nerves --> radial muscles of iris contract --> pupils dilate
  • parasympathetic nerves --> circular muscles contract --> pupils contract

Complementary Versus Cooperative Effects

• Complementary
  sympathetic and parasympathetic stimulation produces similar effects
  • EXAMPLE:
    • parasympathetic fibers increase saliva secretion and other exocrine digestive secretions
    •  sympathetic fibers cause digestive blood vessel constriction, which causes saliva to be more mucus-y, thicker, viscous
• Cooperative
  • EXAMPLE: MALE:  parasympathetic fibers stimulate erection; sympathetic fibers stimulate orgasm
  • EXAMPLE: FEMALE: parasympathetic fibers stimulate clitoral erection and vaginal secretions; sympathetic fibers stimulate orgasm
  • EXAMPLE: MICTURITION REFLEX
    • Sympathetic division inhibits the internal sphincter (must relax to allow bladder emptying). 
    • At the same time the parasympathetic division activates (ACh) the detrusor muscle, which stimulates bladder contraction (urination)
      • Drugs that block ACh (muscarinic receptor) are used to treat “overactive bladder.”

Organs Without Dual Innervation

• Sweat glands, arrector pili muscles, adrenal medula, liver, adipocytes, lacrymal glands, radial muscle of the iris, juxtaglomerular apparatus, uterus and most vascular smooth muscles have only sympathetic innervation.
• Regulation is achieved by increases/decreases in the firing rate of sympathetic axons:
• EXAMPLE
  • Sympathetic fibers stimulate alpha 1 receptors leading to constriction of cutaneous blood vessels. 
    • a decrease in firing rate causes vasodilation
• EXAMPLE
  • Nonshivering thermogenesis – animals deprived of their sympathetic system and adrenals cannot tolerate cold stress
    • Hot room inhibits sympathetic fibers and blood vessels dilate
  • Exercise activates sympathetic fibers and blood vessels of the skin constrict, stimulation of sweat glands (which secrete bradykinin, dilating surface blood vessels near sweat glands, results in cooling; note: other cutaneous blood vessels are constricted)
  • During exercise, only the sympathetic nervous system is involved w/o direct involvement of the parasympathetic nervous system

CNS of the Autonomic Nervous System

• Medulla oblongata is responsible for almost all autonomic responses;
  • they can be elicited by experimental stimulation of the medulla oblongata
• Many scientists consider the Hypothalamus to be the control center for autonomic functions because it controls the medulla, in terms of body temp., hunger, thirst
• Also the limbic system is involved, which includes the cingulate gyrus, they hypothalamus, the amygdaloid nucleus)
• Cerebellum – when motor tracts of the cerebellum are cut, the following activities are eliminated: motion sickness, nausea, sweating, cardiovascular changes.
  • this says that impulses from the cerebellum --> medulla oblongata regulate autonomic activity

Video Made for High School on the Autonomic Nervous System

Videos Made for this class on the Autonomic Nervous System
ANS Part I

ANS Part II

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