The efferent nerve is also calledmotor nerveIs to transmit nerve impulses from the center to the surroundingsEffector。The efferent nerves of the viscera are called visceral motor nerves, which support the myocardium, smooth muscle and glands, also known asautonomic nerveorAutonomic nerve, they can be divided intosympathetic nerveandparasympathetic。[1]It is the outgoing part of the reflection arc.For example, the motor nerve that distributes in muscle tissue and causes it to contract belongs to the efferent nerve.The fibers of neurons that transmit impulses outward from the nerve center.For example, the motor nerves and efferent nerves that transmit impulses from the brain and spinal cord to the muscles are the fourth component of the reflex arc.
PressNerve endingsThe transmitters secreted are different, and the efferent nerves are mainly divided into two categories:
1、 Cholinergic nerves include all sympathetic and parasympathetic preganglionic fibers;All postganglionic fibers of parasympathetic nerve;Motor nerve;A few sympathetic postganglionic fibers release acetylcholine (the nerve that controls sweat gland secretion and bone and blood vessel relaxation) at their ends when excited.
2、 The noradrenergic nerve includes the postganglionic part of most sympathetic nerves (excluding the sweat glands and vasodilator fibers of skeletal muscle).And some peptidergic fibers.When excited, its terminals release norepinephrine.
In addition, there are dopamine nerves in some effectors.
Receptors and effects of efferent nerve
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The efferent nerve endings mainly release two kinds of neurotransmitters, so according to the receptors receiving these neurotransmitters, they can also be divided into two categories:
1、 Choline receptor and its effect
Receptors that can selectively bind to acetylcholine are called choline receptors.Due to their different reactions to drugs, they can be divided into:
（1） Muscarinic choline receptor (M receptor for short) is a receptor that can selectively bind to muscarine.Mainly distributed inparasympatheticOn the effector organs innervated by postganglionic fibers.
（2） Nicotinic choline receptor (N receptor for short) can selectively bind to nicotine, which can be divided into two subtypes of N1 receptor and N2 receptor.N1 receptor is mainly distributed on the cell membrane of plant ganglion neurons. When it is excited, it can cause ganglion excitation. Impulse is transmitted along the postganglionic fibers, which can release norepinephrine from the sympathetic postganglionic fiber endings, and acetylcholine from the parasympathetic postganglionic fiber endings, resulting in corresponding effects;N2 receptor is mainly distributed in skeletal muscle, and when it is excited, it shows contraction of skeletal muscle.
2、 Adrenaline receptor and its effect
Receptors that can selectively bind to norepinephrine or adrenaline are called adrenaline receptors.
（1） A adrenergic receptor (a receptor for short) can be generally divided into two subtypes, a1 and a2.
（2） β adrenergic receptor (β receptor for short) can be divided into three subtypes: β 1, β 2 and β 3.β receptors are mainly distributed on the cell membrane of effector innervated by sympathetic nerve fibers.
（3） Dopamine receptors (DA receptors for short) can select the receptors that bind to DA.
Selection and Application of Efferent Neuropharmacology Experiment
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（1） General experimental methods and animal selection
When determining the acute toxicity test (LD50) of the new drug, if the animal has bristles, increased activity, excitement, which leads to the development of tonic clonic convulsions, it can be initially considered as sympathomimetic drugs.Then we can observe the reaction of the animal (or cat)'s blood pressure. If the α - receptor is excited, it will have a greater impact on the blood pressure and reflexively slow down the heart rate. If the β receptor is excited, it can be seen that the blood pressure decreases and the heart rate increases significantly.In order to accurately distinguish its effects on α and β receptors, α receptor blocker phentolamine and β receptor blocker propranolol can also be used as tools.In addition to blood pressure test, cat blink film, cat (or dog) intestinal activity in vivo and other experimental methods can also be used.Some in vitro experiments can be used to analyze the action site of sympathomimetic drugs. One of the most sensitive experiments is the gastric fundus strip of rats. In addition, rabbit head muscles, isolated rabbit ears, guinea pig tracheal chains, guinea pig ileum and chicken cecum are prepared.Known α or β - receptor stimulants can be used as a standard to observe their interaction with α or β - receptor blockers and determine their action sites.
Acetylcholine has muscarinic and nicotine like effects. The former can be blocked by atropine, while the latter can be blocked by ganglion blockers and striatum relaxants.The drugs that excite the parasympathetic effect points through direct or indirect action can cause the symptoms of tearing, salivation, urination and defecation.Therefore, we can get a preliminary impression in the LD50 experiment of mice, and then observe their reactions to blood pressure, saliva, pupil and gastrointestinal tract respectively.The cholinergic drugs and anticholinergic drugs can be tested and observed on the cat blood pressure test, frog heart, frog rectus abdominis muscle, leech dorsal muscle and other samples, and can also be observed by the methods of overall experiment, such as inhibiting gastric ulcer in rats, inhibiting the downward movement of activated carbon in rats' intestines and so on.
（2） Cardiovascular Experiment
Blood pressure test is an extremely sensitive method to test efferent nerve drugs. Acute blood pressure test is generally used, and dogs, cats, rabbits and rats are commonly used in animals.Rabbits are not suitable for hypotension test because they are prone to death.Anesthetized or brain destroying animals can be used in the experiment, because the blood pressure of anesthetized animals often has three levels of fluctuations (the first level of fluctuations, also known as pulse fluctuations, is caused by the impact of each heart beat on blood pressure, the second level of fluctuations, also known as respiratory fluctuations, that is, when inhaled, the blood pressure slightly rises, when exhaled, the blood pressure slightly drops; the third level of fluctuations, refers to the periodic changes of excitability in the vascular motor center at a slightly long interval),Make blood pressure rise and fall unsteadily.If an animal destroys its brain, the influence of the central nervous system above the spinal cord on blood pressure can be excluded, and only the first level fluctuation occurs, and the blood pressure curve is extremely unbalanced.
In vitro rabbit aorta strip experiment: rabbit aorta contains α - receptor, which is a good sample for determining drugs acting on α - receptor, and has been widely used to identify and analyze the effects of sympathomimetics and their antagonists.Rabbit aorta has been prepared in various forms, such as aortic rings, slices and strips, but spiral strips of rabbit aorta are one of the most suitable methods.This specimen has many advantages. For example, 3~4 specimens can be made from one aorta for paired test. It is very sensitive to low concentration sympathomimetics, and has good tissue stability, which can be maintained for a long time.
（3） Gastrointestinal smooth muscle experiment
The isolated intestines of many animals can be used to test efferent nerve drugs, and the isolated intestines of guinea pigs and rabbits are generally used.The spontaneous activity of guinea pig ileum is less, and there is a stable baseline when tracing, which is suitable for drug identification.The rabbit intestine (especially the jejunum) has regular swinging movement, which is suitable for observing the effects of drugs on animals.The guinea pig ileum specimen has completely relaxed after loading, so the addition of sympathomimetics will not make it more relaxed.
The isolated guinea pig ileum can be used to observe the dose-response relationship between acetylcholine (Ach) and cholinergic drugs;The content of Ach and cholinergic drugs can be determined.The isolated rabbit jejunum has rhythmic contractile activity. The effects of epinephrine (4 μ g), norstigmine (2 μ g) and other drugs on the jejunum wiggle movement can be observed.The gastric fundus strips of rats are the most sensitive samples for the detection of catecholamines and 5-hydroxytryptamine (5-HT).The main observation was the effect of the drug on the longitudinal muscle of the stomach, because the circular muscle in the specimen had been cut off.After verification, the sensitivity of catecholamine to it is 10~100 times greater than that of rat uterus specimen.The chicken esophagus is innervated by parasympathetic nerve, so the isolated chicken esophagus specimen is suitable for testing parasympathetic drugs.Because its action cannot be completely blocked by ganglion blockers, it is not suitable for testing drugs acting on ganglia.
