Drug Receptors N. S.

Drug Action

Drugs of C.N.S
 Elmhurst College
Nervous System Cholinergic Drugs II Adrenergic Drugs II  Chemistry Department
Cholinergic Drugs I Adrenergic Drugs I Adrenergic Drugs III  Virtual ChemBook

Autonomic Nervous System

 Organ Effected

Parasympathetic
Cholinergic
Stimulation Effects

Sympathetic
Adrenergic
Stimulation Effects

 Eye
Pupil contracted Pupil dilated

 Nose
Vasodilation, copious
mucous secretion
Vasoconstriction,
reduced mucous secretion

 Mouth
 Copious saliva secretion Decreased saliva,
dryness in mouth
 Gastrointestinal tract Nausea, vomiting, abdominal cramps, diarrhea, increased peristalsis and tone,
sphincters relaxed
Constipation, intestines
relax, decreased peristalsis and tone

 Lungs
 Bronchi constricted  Bronchi dilated

 Heart
Cardiac rate slowed, arterial contraction force decreased Cardiac rate and contraction force increased, coronary arteries dilated

Nervous System - Overview

Introduction:

The nervous system is composed of all nerve tissues in the body. The functions of nerve tissue are to receive stimuli, transmit stimuli to nervous centers, and to initiate response. The central nervous system consists of the brain and spinal cord and serves as the collection point of nerve impulses. The peripheral nervous system includes all nerves not in the brain or spinal cord and connects all parts of the body to the central nervous system. The peripheral (sensory) nervous system receives stimuli, the central nervous system interprets them, and then the peripheral (motor) nervous system initiates responses.

The somatic nervous system controls functions that are under conscious voluntary control such as skeletal muscles and sensory neurons of the skin.

The autonomic nervous system, mostly motor nerves, controls functions of involuntary smooth muscles, cardiac muscles, and glands. The autonomic nervous system provides almost every organ with a double set of nerves - the sympathetic and parasympathetic. These systems generally but not always work in opposition to each other.

The sympathetic system activates and prepares the body for vigorous muscular activity, stress, and emergencies. While the parasympathetic system lowers activity, operates during normal situations, permits digestion, and conservation of energy.

The two systems generally act in opposition to each other. For example, a stimulation by the sympathetic system on the heart would increase contractions, while a stimulation by the parasympathetic system would decrease heart contractions. Where dual control of an organ exists, both systems operate simultaneously although one may be operating at a higher level of activity than the other. The operation is similar to the operation of a car with both the accelerator and brake pedals depressed.

In the peripheral nervous system, a chemical neurotransmitter carries the nerve impulses from neuron to neuron across a synapse (space between neurons). The neurotransmitters are acetylcholine, norephinephrine, serotonin, and others.

Nerves that release acetylcholine are called cholinergic nerves. Cholinergic nerves are part of the parasympathetic system, somatic motor nerves, preganglionic sympathetic nerves* and central nervous system. (*The nerve that carries the message from the central nervous system to a ganglion - junction for a group of nerve cells - is a preganglionic nerve.)

Nerves that release norepinephrine are called adrenergic nerves. Adrenergic nerves are part of the postganglionic sympathetic nerve system** and parts of the central nervous system. (**A nerve that carries the impulse from the ganglion to the effecter cell is a postganglionic nerve.)


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Neurotransmitters:

The events in a neurotransmission in the synapse are summarized in the graphic on the left.

1) Release of Neurotransmitter: Transmission of nerve impulses is accomplished when a nerve impulse causes the rupture of vesicles containing the chemical transmitter from the nerve ending.

2) Interaction with Receptor: The neurotransmitter crosses the synapse and interacts with receptors located on the membrane of the next neuron. This interaction may produce membrane permeability changes which result in an excitatory response.

3) Degradation of Neurotransmitter: After each impulse it is necessary to inactivate or terminate the neurotransmitter's action. This may be accomplished by degradation as in the hydrolysis of acetylcholine. OR

4) Diffusion from the Receptor: The NT may simply diffuse from the receptor site after a short period of time.

5) Resynthesize or restore NT: The neurotransmitter may be retaken back into the storage site or new NT is synthesized.

As might be anticipated, drugs or poisons act at different steps in the sequence of events to modify the normal neurotransmission. Drugs or poisons may have stimulant or depressant effects by any of the mechanisms listed below.

 
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Excitant or Stimulant Effects:

a) A drug mimics the neurotransmitter and stimulates the receptor on its own as an agonist at 2.

b) There is a decrease in the rate of degradation or diffusion from the receptor site. More impulses are sent by the transmitter if it remains on the receptor site too long at 3 or 4.

c) The rate of release of neurotransmitter from storage is increased at 1.

Depressant or Inhibitory Effects:

a) The receptor site is blocked by an inhibitor or antagonist so that the normal neurotransmitter can not interact to send an impulse at 2.

b) The rate of synthesis and storage of transmitter is decreased at 5.

c) The rate of degradation of transmitter from the receptor site is increased at 3 or 4.

Drug Side Effects:

Since the same neurotransmitter is used in many organ systems, it can be predicted that a drug used for control at one organ may produce side effects in other organs.

For example a cholinergic drug may be given to control gastric spasm by blocking stimulation of the parasympathetic system. The drug side effect is to generally lower all activities in the parasympathetic system with consequent dominance of the sympathetic system. Examination of the table above will reveal that the side effects may be: pupil dilation, dryness of nose and mouth, constipation and increased pulse.

QUES. a: State the difference between an agonist and antagonist.
QUES. 1b: State the difference between cholinergic and adrenergic nerves in causing depressing effects for the organs listed in Table.
QUES. c: Draw a diagram and explain the events in a neurotransmission at the synapse.
QUES. d: List 2 ways to get a stimulant effect and 2 ways to get a depressant effect using a drug interaction in the neurotransmission of a nerve impulse.