Lecture # 22: The Autonomic Nervous System (Chapter 15)
Objectives:
2- Define the autonomic nervous system, and compare its anatomy with that of the somatic motor division of the peripheral nervous system.
3- Compare and contrast the sympathetic and parasympathetic divisions of the autonomic nervous system.
1- Distinguish between somatic and autonomic reflexes.
Autonomic neurons in the enteric nervous system of the digestive tract
Thalamus
Postcentral gyrus of cerebrum
Precentral gyrus of cerebrum
Cerebe- llum
Somatic sensory receptors
Visceral sensory receptors
Sensory (afferent) Division
Hypo- thalamus
Autonomic Nervous System
Somatic Nervous System
Autonomic ganglion
It is a rapid involuntary response triggered by the CNS for the purpose of maintaining homeostasis.
Reflex:
1- Somatic Reflex: It is a reflex resulting in the contraction of an skeletal muscles.
2- Autonomic or Visceral Reflex:
Baroreceptors sense increased blood pressure1
Glossopharyngeal nerve transmits signals to medulla oblongata (brain stem)
2
Vagus nerve transmitsInhibitory signals to cardiac pacemaker
3
Heart rate decreases reducing blood pressure4
BP
BP
For example, high blood pressure is controlled by a baroreflex.
It is an unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and the response of visceral effectors (contraction of cardiac muscle, smooth muscle or in the secretion of glands).
The ANS is responsible of the Visceral Reflexes
Upper motor neuron in precentral gyrus of cerebrum
Lower motor neuron in anterior gray horn of spinal cord
Somatic Nervous System:1- The entire distance from the CNS (spinal cord) to the effector is spanned by one neuron.
Somatic effectors(skeletal muscles)
CNS
AcetylcholineMyelinated fiber
2- Only acetylcholine is employed as neurotransmitter.
Autonomic Nervous System:1- The entire distance from the CNS spinal cord) to the effector is spanned by two neurons. 2- Only acetylcholine is employed as neurotransmitter in the preganglionic neuron, but postganglionic neurons can employ either acetylcholine or norepinephrine.
Autonomic ganglion
Preganglionic neuron
Postganglionic neuron
AcetylcholineAcetylcholine or Norepinephrine
Denervation hypersensitivity : It is an exaggerated response of cardiac and smooth muscle if autonomic nerves are severed damaged.
The heart beats at its own intrinsic rate of about 100 beats/min. The parasympathetic tone holds the resting heart rate down to about 70 to 80 beats/min.
Divisions of the Autonomic Nervous SystemThe ANS has two divisions. Both divisions innervate the same target organs
2- The ParasympatheticDivision
1- The SympatheticDivisionIt adapts the body for physical activities: exercise, trauma, arousal, competition, anger, or fear(fight or fly). It increases:
It reduces the activity of:1- Digestive system2- Urinary system
1- Alertness, 2- heart rate, 3- blood pressure, 4- pulmonary airflow, 5- blood glucose concentration, 6- blood flow to cardiac and skeletal muscle.
It has a calming effect on many body functions reducing energy expenditure and assists in bodily maintenance. It reduces:
1- Alertness, 2- heart rate, 3- blood pressure, 4- pulmonary airflow, 5- blood glucose concentration, 6- blood flow to cardiac and skeletal muscle.
It increases the activity of:1- Digestive system2- Urinary system
The two divisions innervate same target organs, and are active simultaneously, producing an autonomic tone.
Autonomic tone
Sympathetic tone
Parasympathetic tone
Heart rate
Heart rate
Parasympathetic SympatheticCraniosacral outflow: Brain-stem nuclei of cranial nerves III, VII, IX and X; and spinal cord S2-S4
Long preganglionic; short postganglionic fibers
Ganglia in (intramural) or close to the visceral organ served
All fibers releases ACh (cholinergic fibers)
Thoracolumbar outflow: Lateral horns of gray matter of spinal cord segments T1- L2
Ganglia within a few centimeters from the CNS: alongside and anterior to the vertebral column
Short preganglionic; long postganglionic fibers
Maintenance functions; conserves and stores energy
Prepares body for activity
All preganglionic fibers release Ach; most postganglionic fibers release Norepinephrine (adrenergic fibers)
Divisions of the ANS
Parasympathetic Sympathetic
Adrenal cortex
Adrenal medulla
Those hormones also function as neurotransmitters of the Sympathetic Division.
The Adrenal Glands
The adrenal medulla secretes a mixture of hormones into bloodstream called catecholamines:
85% epinephrine (adrenaline)15% norepinephrine (noradrenaline).
Effector organs
Precentral gyrus
Hypothalamus
Acetylcholine
Ganglion
AcetylcholineNorepinephrine
Acetylcholine Acetylcholine
Comparison of Somatic and Autonomic Nervous System
Epinephrine & NorepinephrineAcetylcholine
Adrenal medulla
Ganglion
Visceral motor
Somatic motor
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(a) Parasympathetic fiber
ACh
ACh
Targetcell
Postganglionicneuron
Muscarinicreceptor
Preganglionicneuron
Nicotinicreceptor
Ganglion
+ Ach is always excitatory
+ Ach may be excitatory (digestive system) or
inhibitory (heart)
Neurotransmitters and their Receptors
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(b) Sympathetic adrenergic fiber
ACh
Norepinephrine
Adrenergic receptor
Nicotinicreceptor
Postganglionicneuron
Preganglionicneuron
Targetcell
Ganglion
+ Ach is always excitatory
- a adrenergic receptor :
- b adrenergic receptor :
They usually have excitatory effects (labor contractions)
They usually have inhibitory effects (relaxation of bronchioles)
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(c) Sympathetic cholinergic fiber
ACh
ACh
Muscarinic receptor
Nicotinicreceptor
Preganglionicneuron
Postganglionicneuron
Targetcell
Ganglion
+ Ach is always excitatory
Autonomic Nervous System
Parasympathetic Division Sympathetic Division
It keeps the body energy use as low as possible.
Blood pressure and heart rate are regulated at low normal levels.Gastrointestinal tract is active.
The pupils are constricted.
It is called “resting and digesting system”.
Its activity produce a rapidly pounding heart.
Deep breathing.
Dry mouth.
Cold, sweating skin.
Dilated eye pupils.
It is called “fight-or-flight system”.
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display.
Brain
Spinal cord
Iris
Pupil
Pupil dilated Pupil constricted
Parasympathetic fibersof oculomotor nerve (III)
Ciliaryganglion
Superiorcervicalganglion
Cholinergic stimulationof pupillary constrictor
Parasympathetic(cholinergic) effect
Sympathetic(adrenergic) effect
Adrenergicstimulation ofpupillary dilator
Sympatheticfibers
Vagus nerve
Cephalic Phase of Gastric Activity Parasympathetic division
CNS
Regulation of Gastric Activity by Autonomic or Visceral Reflexes
The nervous and endocrine systems collaborate to increase gastric secretion and motility when food is eaten and to suppress them when the stomach empties.
Stimuli:
Vagus nerve (parasympathetic) stimulates gastric secretion even before food is swallowed.
Sight, smell, taste, or thought of food
The Cephalic Phase is directed by the CNS and prepares the stomach to receive food.
Mucous cells
Chief cells
Parietal cells
Mucus
Pepsinogen
HCl
ACh
+
Intestinal Phase of Gastric ActivityIt begins when chyme first enters the duodenum.
Stretch receptors and chemoreceptors in the duodenum trigger the Enterogastric Reflex.
Sympathetic nerve
The medulla oblongata stimulates sympathetic neurons that send inhibitory signals to the stomach.
_
Stimuli:Distention of the duodenum by the chyme.Decrease in the pH of the duodenum by the chyme.
Response:
Mucous cells
Chief cells
Parietal cells
Mucus
Pepsinogen
HCl
XXX
The net result is that immediately after the chyme enters the duodenum, gastric contractions decrease, and further discharge of chyme is prevented, giving the duodenum time to neutralize and digest the acidic chyme.