3 autonomic nervous system

1 DAWN V TO MY M.Pharm., Asst. Professor, Dept. of Pharmacology, ST.JOSEPH’S COLLEGE O F PHARMACY, CHERTHALA.

AUTONOMIC NERVOUS SYSTEM

Autonomic/involuntary part of the nervous system.

Control vital functions of organs. E.g. an increase and decrease in heartbeat.

Impulses originate below the cerebrum.

Essential for regulation of homeostasis.

Effector organ includes:

1. Smooth muscle.

2. Cardiac muscle and

3. Glands.

Autonomic stimulation can lead to:

1. Changes in rate (chronotropic effect) and force of the heartbeat (ionotropic effect).

2. Stimulation or depression of secretion of glands.

3. Vasoconstriction/vasodilation.

4. Bronchoconstriction/bronchodilation.

5. Changes in the size of the pupils of the eyes.

Efferent (motor) nerves arise from nerve cell in the brain.

Afferent (sensory) nerves arise from the glands and internal organs.

The autonomic nervous system is divided in to 2:

1. Sympathetic (Thoracolumbar outflow) and

2. Parasympathetic (Craniosacral outflow)

• Both differ structurally and functionally.

• Normally work in an opposing fashion, enabling/restoring balance of involuntary

functions, maintaining homeostasis.

• Sympathetic activity predominates in stressful situations.

• Parasympathetic activity predominates during rest.

Both divisions have 2 efferent neurons in its peripheral pathways between CNS and effector

organs. They are:

1. The preganglionic neurone and

2. The Postganglionic neurone.


• The cell body of the preganglionic neurone is in the brain or in the spinal cord.

• Its axon terminals synapse with the cell body of the postganglionic neurone in an

autonomic ganglion outside the CNS.

• The postganglionic neurone conducts impulses to the effector organ.


The sympathetic nervous system:

• The motor impulses originate in the hypothalamus, limbic system, reticular formation

and medulla oblongata and conveyed to effector organs/tissue supplied.

The preganglionic neurone:

• Cell body originates in the lateral column of grey matter in the spinal cord between

the 1st thoracic and 2nd or 3rd lumbar vertebrae.

• It leaves the spinal cord by anterior root and terminates in the lateral chains of

sympathetic ganglia or passes through it to one of the prevertebral ganglia.

• Acetylcholine is the neurotransmitter.

The postganglionic neurone:

• Cell body originates in the ganglion and terminates in the organ/tissue supplied.

• Noradrenaline is the neurotransmitter.

Sympathetic ganglia (the lateral chains of sympathetic ganglia):

• It extends from the upper cervical level to the sacrum.

• One chain lying on each side of the bodies of the vertebrae.

• Each ganglion is interconnected by nerve fibres.

• Preganglionic neurone originating from spinal cord synapse with the cell body of the

postganglionic neurons at the same level or pass up/down the chain through one or

more level of the ganglia before synapsing.

• The sweat glands, the skin and the blood vessels of skeletal muscles are supplied only

by sympathetic fibres.

• Some have neurotransmitter acetylcholine, some adrenaline and noradrenaline. Hence

these neurotransmitters have effects on both sympathetic and parasympathetic nerve

supply.

• Preganglionic sympathetic fibres pass through lateral chain to reach the prevertebral

ganglia.

Prevertebral ganglia:

There are 3 prevertebral ganglia situated in the abdominal cavity.

1. Coeliac ganglion.

2. Superior mesenteric ganglion and

3. Inferior mesenteric ganglion.

• Each ganglion consists of nerve cell bodies diffusely distributed among a network

of nerve fibres forming plexuses.


The Parasympathetic nervous system:

Preganglionic and postganglionic neurons are involved in the transmission of impulses from

their source to the effector organs. Acetylcholine is the neurotransmitter at both synapses.

The preganglionic neurone:

Cell body originates either in the brain or in spinal cord; in the brain it originates from the

cranial nerves III, VII, IX and X (from nuclei in the midbrain and brain stem) and end in the

terminal/intramural ganglia; in the sacrum from the grey matter (at the distal end segments

S2, 3 and 4.) of the spinal cord. It synapses with the postganglionic neurones in the walls of

pelvic organs.


The postganglionic neurone:

Cell body originates either in a ganglion or in the wall of the organ supplied.

Functions of the autonomic nervous system:

The autonomic nervous system is involved in reflex activities and depends on sensory input

to the brain/spinal cord and on motor output. The reflex action is rapid contraction/inhibition

of involuntary (smooth and cardiac) muscle or glandular secretion. The organs of the body

are supplied by both sympathetic and parasympathetic nerves in order to maintain

homeostasis.


Sympathetic stimulation prepares the body for stressful situations. The adrenal glands are

stimulated to secrete the hormones adrenaline and noradrenaline into the bloodstream. These

hormones potentiate and sustain the effects of sympathetic stimulation. Sympathetic

stimulation prepares the body for fight or flight responses.

Parasympathetic stimulation slows down body processes except digestion, absorption of food

and the functions of the genitourinary systems. The two systems function together

maintaining a regular heartbeat, normal temperature and an internal environment.

EFFECTS OF AUTONOMIC STIMULATION

Cardiovascular system

Sympathetic stimulation:

Increases the rate and force of the heartbeat.

Causes dilatation of the coronary arteries, increasing the blood supply to

cardiac muscle.


Causes dilatation of the blood vessels supplying skeletal muscle, increasing

the supply of oxygen and nutritional materials and the removal of metabolic

waste products, thus increasing the capacity of the muscle to work.

Raises peripheral resistance and blood pressure by constricting the small

arteries and arterioles in the skin permitting increased blood supply to highly

active tissues, such as skeletal muscle, heart, brain.

Constricts the blood vessels in the secretory glands of the digestive system,

reducing the flow of digestive juices. This raises the volume of blood available

for circulation in dilated blood vessels.

Blood coagulation occurs more quickly because of vasoconstriction.

Parasympathetic stimulation:

Decreases the rate and force of the heartbeat.

Causes constriction of the coronary arteries reducing the blood supply to

cardiac muscle.

No effect on blood vessels except the coronary arteries.

Respiratory system


It causes dilatation of the airways, especially the bronchioles and increases the

respiratory rate in conjunction with the increased heart rate, the oxygen intake

and carbon dioxide output of the body.


It produces constriction of the bronchi.

Digestive and urinary systems


The liver converts an increased amount of glycogen to glucose, making more

carbohydrate immediately available to provide energy.

The adrenal (suprarenal) glands are stimulated to secrete adrenaline and

noradrenaline which potentiate and sustain the effects of sympathetic

stimulation.

The stomach and small intestine. Smooth muscle contraction and secretion of

digestive juices are inhibited, delaying digestion, onward movement and

absorption of food and the tone of sphincter muscles is increased.

Urethral and anal sphincters. The muscle tone of the sphincters is increased,

inhibiting micturition and defecation.

The bladder wall relaxes.


The metabolic rate is greatly increased.


The stomach and small intestine. The rate of digestion and absorption of food

is increased.

The pancreas. There is an increase in the secretion of pancreatic juice and the

hormone insulin.

Urethral and anal sphincters. Relaxation of the internal urethral sphincter is

accompanied by contraction of the muscle of the bladder wall and micturition

occurs. Similar relaxation of the internal anal sphincter is accompanied by

contraction of the muscle of the rectum and defecation occurs. In both cases

there is voluntary relaxation of the external sphincters.

Eye


This causes contraction of the radial muscle fibres of the iris, dilating the

pupil. Retraction of the levator palpebral muscles occurs, opening the eyes

wide and giving the appearance of alertness and excitement. The ciliary

muscle that adjusts the thickness of the lens is slightly relaxed.


This causes contraction of the circular muscle fibres of the iris, constricting

the pupil. The eyelids tend to close, giving the appearance of sleepiness.

Skin


Causes increased secretion of sweat, leading to increased heat loss from the

body.

Produces contraction of the arrectores pilorum (the muscles in the hair follicles

of the skin), giving the appearance of 'goose flesh'.

Causes constriction of the blood vessels preventing heat loss.

No parasympathetic nerve supply to the skin. Some sympathetic fibres are adrenergic,

causing vasoconstriction, and some are cholinergic, causing vasodilatation.

Afferent impulses from viscera

Sensory fibres from the viscera travel with autonomic fibres and are sometimes called

autonomic afferents. The impulses are associated with:


Visceral reflexes e.g. cough, blood pressure etc.

Sensation of hunger, thirst, nausea, sexual sensation, rectal and bladder

distension.

Visceral pain.

Visceral pain:

Normally the viscera are insensitive to cutting, burning and crushing. However, a sensation of

dull, poorly located pain is experienced when:

Visceral nerves are stretched.

A large number of fibres are stimulated.

There is ischemia and local accumulation of metabolites.

The sensitivity of nerve endings to painful stimuli is increased, e.g. during

inflammation.

Referred pain:

Pain perceived to originate from the tissues supplied by the damaged nerve (not in the point

of origin but in other places supplied by the same nerve) is known as referred pain e.g. the

angina pain felt over the left shoulder.

3 autonomic nervous system

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