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.