1. THE AUTONOMIC NERVOUS SYSTEM (ANS)

2. What is the autonomic nervous system?
The autonomic nervous system (ANS) is the motor division of the peripheral nervous system that controls visceral activities, with the goal of maintaining internal homeostasis.

3. What does the autonomic nervous system do?
It provides motor fibers to smooth and cardiac muscles and glands
It operates subconsciously
It causes excitation and inhibition
It makes adjustments to support body activities
It has an efferent pathway made up of a two-neuron chain: preganglionic and ganglionic

4. Two divisions of the ANS
1) Parasympathetic division
2) Sympathetic
Although they have different roles, they have effects on many of the same organs of the body

5. Organs affected by both divisions

6. Roles of the two different divisions
1. The parasympathetic division…
_conserves body energy and maintains body activities at basal levels
_is involved in digestion, diuresis, and defecation
_causes heart rate and blood pressure to be low, the skin to be warm and the pupils to be constricted

7. 2. The sympathetic division…
_activates the body under conditions of emergency which is why it is often called the “fight or flight system”
_is involved in emergency, exercise, and excitement
_causes blood flow to organs to reduce and blood flow to muscles to increase
_also causes skin to be cold, heart rate to increase, and rapid breathing

8. Parasympathetic division outflow
Neurons of the cranial and sacral outflows of the parasympathetic division are located in the following nerves and create an effect on the organs mentioned in the table below.

9. Table of parasympathetic outflow
Cranial Outflow
Cranial Nerve
Effector Organ(s)
Occulomotor (III)
Eye
Facial (VII)
Salivary, nasal, and lacrimal glands
Glosso-pharyngeal (IX)
Parotid salivary glands
Vagus (X)
Heart, lungs, and most visceral organs
Sacral Outflow
S2-S4
Large intestine, urinary bladder, ureters, and reproductive organs

10. Parasympathetic division outflow

11. Sympathetic division outflow
Sympathetic division outflow is different from parasympathetic division outflow because…
It arises from spinal cord segments T1 through L2
Preganglionic fibers pass through the white rami communicantes and synapse in the chain (paravertebral) ganglia
Fibers from T5-L2 form splanchnic nerves and synapse with collateral ganglia
Postganglionic fibers innervate the numerous organs of the body
Sympathetic neurons produce the lateral horns of the spinal cord

12. Sympathetic division outflow

13. ANS Physiology Neurotransmitters and Receptors
The two major neurotransmitter involved in the ANS are:
Acetylcholine (ACh)
Norepinephrine (NE)
ACh is the same neurotransmitter that is found in the somatic motor neurons and is released in the ANS:
All ANS preganglionic axons
All parasympathetic postganglionic axons at synapses with their effectors
ACh releasing fibers are called Cholinergic Fibers
NE is released by most sympathetic postganglionic axons
NE releasing fibers are called Adrenergic Fibers
The effects of ACh and NE are either excitation or inhibition which is dependant on the receptor type allowing them to exert these different effects at different areas in the body

14. Receptors ACh binds to two types of receptors: Nicotinic Muscarine
Nicotinic Receptors are located on:
Motor end plates of skeletal muscles (somatic targets)
All ganglionic neurons (sypathetic and parasympathetic)
The hormone-producing cells of the adrenal medulla
ACh always produces a stimulatory effect when it binds with nicotinic receptors
Muscarine Receptors occur on all effector cells stimulated by postganglionic cholinergic fibers (parasympathetic targets like the eccrine sweat glands and some blood vessels of skeletal muscle)
ACh binding produces a inhibitory or excitatory depending on the receptor type of the target organ

15. Receptors
There are also receptors know as Adrenergic Receptors (two types):
Alpha with two subtypes (A1,A2)
Beta with three subtypes (B1,B2,B3)
The general effects of NE binding are:
Alpha receptors are mostly stimulatory
Beta receptors are mostly inhibitory
However, an exception to this is when NE binds to the Beta receptors of cardiac muscle and the result is stimulatory

16. Overview of the location and effects of receptor subclasses

17. Drugs and the ANS
Knowing the locations of the cholinergic and adrenergic receptors subtypes allows specific drugs to be prescribed to obtain the desired inhibitory or stimulatory effect on selected organs
An example of this is Atropine (blocks parasympathetic effects):
administered during surgery to prevent salivation and dry up respiratory secretions
Ophthalmologists use it to dilate the pupils for an eye exam
There are several examples of drugs in Table 14.4 that influence the Activity of the ANS on page 544

18. Interactions of the Autonomic Divisions
Most visceral organs receive innervation by both sympathetic and parasympathtic fibers
This dual innervation produces a dynamic antagonism that allows visceral activity to be precisely controlled
Sympathetic fibers increase heart and respiratory rates. They also inhibit digestion and elimination
Parasympathetic fibers allow for digestion and elimination and decrease respiratory and heart rates

19. Parasympathetic & Sympathetic
The parasympathetic division is know as the “resting and digesting” division
The sympathetic division controls blood pressure and keeps the blood vessels in a continual state of partial constriction
Flight or Fright response – the sympathetic can override the parasympathetic in an emergency to increase heart and respiratory rates while inhibiting digestion.
Parasympathetic division restores the heart and respiratory rates back to resting levels when the emergency is over and then returns back to it’s functions of digestion and elimination

20. Sympathetic Tone & Parasympathetic Tone
The Sympathetic Tone:
Constricts blood vessels & controls blood pressure to rise in response to the body’s needs
Prompts vessels to dilate if blood pressure needs to be decreased
The Parasympathetic Tone:
Slows the heart
Controls Digestion and Elimination
ANS control is best seen in the external genitalia:
Parasympathetic fibers cause vasodilation resulting in the erection of the penis and clitoris
Sympathetic fibers cause ejaculation of sperm and reflex peristalsis in females

21. Sympathetic Division
The adrenal medulla, sweat glands and the arrector pili muscles of the skin, the kidneys, and most blood vessels receive only sympathetic fibers.
The Sympathetic Division controls:
Thermoregulatory Responses to heat
Release of Renin from the kidneys
Metabolic Effects

22. Control of Autonomic Functions
Autonomic functions are controlled by several factors:
Reflex Activities mediated by the Spinal Cord and Brain Stem (Medullary Center)
Hypothalamic integration centers interact with both higher and lower centers of autonomic, somatic, and endocrine response
Cortical centers influence autonomic functioning via connections with the limbic system; conscious controls are rare but maybe learned via biofeedback training