What is the difference between cholinergic and adrenergic receptors?

NE preferentially binds α1-adrenoceptors to cause smooth muscle contraction and vasoconstriction. Similar responses occur when NE binds to postjunctional α2-adrenoceptors located on some blood vessels. NE also binds weakly to postjunctional β2-adrenoceptors, which causes vasodilation (this can be observed during alpha adrenoceptor blockade), although this vasodilator effect of NE is relatively minor and overwhelmed by alpha adrenoceptor-mediated vasoconstriction. Circulating epinephrine (EPI) binds with high affinity to smooth muscle β2-adrenoceptors to cause vasodilation in some organs; however, the effect EPI is very concentration dependent. While EPI has a higher affinity for β2 than postjunctional α1 or α2-adrenoceptors, at high concentrations it does bind to the postjunctional α1 and α2-adrenoceptors, which can override the vasodilatory effects of β2-adrenoceptor stimulation and produce vasoconstriction.

Some blood vessels in the body are innervated by parasympathetic cholinergic fibers (e.g., coronary vessels). These nerves release ACh, which binds to muscarinic receptors on the smooth muscle and/or endothelium. It has been shown in many arterial vessels that M3 receptors located on the vascular endothelium are coupled to the formation of nitric oxide (NO), which causes vasodilation; however, ACh causes smooth muscle contraction through a smooth muscle M3 receptors (coupled to Gq-proteins and increased IP3) and M2 receptors (coupled to Gi-proteins and decreased cAMP) when formation of NO is blocked. This latter finding has been used to assess coronary vascular dysfunction in humans in which NO production is diminished in diseased coronary arteries. In contrast to other arteries, cerebral arteries appear to have M5 muscarinic receptors that produce vasodilation in response to ACh.

Some arterial blood vessels in skeletal muscle of dogs and cats (but not humans) are innervated by sympathetic cholinergic nerves that release ACh and cause vasodilation. In species having sympathetic cholinergic nerves, their activation may contribute to active hyperemia in skeletal muscle, particularly at the onset of exercise. Sympathetic cholinergic nerves controlled by hypothalamic thermoregulatory nuclei play an important role in producing cutaneous active vasodilation when skin and body core temperature increase in warm environments.

Drugs are available for blocking vascular adrenergic receptors. Alpha-blockers, for example, are used in treating hypertension. Some of the alpha-blockers are relatively selective for a specific receptor subtype, whereas other as non-selective.  Drugs such as atropine block muscarinic receptors.

Revised 11/10/18

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.

Sympathetic adrenergic nerves innervate the SA and AV nodes, conduction pathways, and myocytes in the heart.  These adrenergic nerves release the neurotransmitter norepinephrine (NE), which binds to specific receptors in the target tissue to produce their physiological responses. Neurotransmitter binding to receptors activates signal transduction pathways that cause the observed changes in cardiac function.

There are also β2-adrenoceptors in the heart and stimulation by β2-agonists has similar cardiac effects as β1-adrenoceptor stimulation. The β2-adrenoceptors become functionally more important in heart failure because β1-adrenoceptors become down regulated.

NE can also bind to α1-adrenoceptors found on myocytes to produce small increases in inotropy. Circulating catecholamines (epinephrine) released by the adrenal medulla also bind to these same alpha and beta adrenoceptors in the heart.

In addition to sympathetic adrenergic nerves, the heart is innervated by parasympathetic cholinergic nerves derived from the vagus nerves. Acetylcholine (ACh) released by these fibers binds to muscarinic receptors in the cardiac muscle, especially at the SA and AV nodes that have a large amount of vagal innervation. ACh released by vagus nerve binds to M2 muscarinic receptors, a subclass of cholinergic receptors. This produces negative chronotropy and dromotropy in the heart, as well as negative inotropy and lusitropy in the atria (the negative inotropic and lusitropic effects of vagal stimulation are relatively weak in the ventricles).

The autonomic nerve terminals also possess adrenergic and cholinergic receptors (prejunctional receptors) that function to regulate the release of NE (not shown in figure). Prejunctional α2-adrenoceptors inhibit NE release, whereas prejunctional β2-adrenoceptors facilitate NE release. Prejunctional M2 receptors inhibit NE release, which is one mechanism by which vagal stimulation overrides sympathetic stimulation in the heart.

Drugs are available for blocking adrenergic and cholinergic receptors. For example, beta-blockers are used in the treatment of angina, hypertension, arrhythmias, and heart failure. Muscarinic receptor blockers such as atropine are used to treat electrical disturbances (e.g., bradycardia and conduction blocks) associated with excessive vagal stimulation of the heart. Many of these adrenergic and cholinergic blockers are relatively selective for a specific receptor subtype.

Revised 4/23/2014

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.

Adrenergic and cholinergic are two pathways of the autonomous nervous system of our body that controls all the involuntary work of the body. Their differences are part of the body’s balancing system.

Adrenergic vs Cholinergic

The main difference between adrenergic and cholinergic pathway is that adrenergic pathway involves the use of neurotransmitters adrenaline and noradrenaline while cholinergic pathway involves the use of acetylcholine neurotransmitter.

Another key difference between them is that adrenergic pathway is associated with Sympathetic Nervous System while cholinergic pathway is associated with parasympathetic pathway.  

Furthermore, sympathetic nervous system related to the adrenergic pathway stimulates the heart by increasing its activity while parasympathetic nerves related to cholinergic pathway down regulates the heart’s activity.

There are basically two types of adrenergic nerve receptor, they are alpha and beta receptors while cholinergic nerve receptors are of two types, nicotinic and muscarinic. Adrenergic receptors are G-protein bound receptors while cholinergic receptors are inotropic and metabotropic.

Adrenergic pathway is responsible for the fight or flight response by releasing the catecholamines adrenalin from the adrenal gland whereas cholinergic pathway is in charge of the digest and rest response.

Adrenergic nerves are a part of the Sympathetic nervous system that houses the adrenergic receptors. These are G-protein coupled receptors that bind with a number of catecholamines that are released from the adrenal gland.

The two main neurotransmitters associated with adrenergic receptor binding are epinephrine or adrenaline and norepinephrine or noradrenaline. These are also responsible for the bodies fight or flight response.

When the sympathetic nervous endings in the heart bind with these neurotransmitters, they increase the heart’s activity by increasing heart rate and myocardial contractibility along with the conduction velocity.

Apart from their effect on heart, they also improve temporary performance of the body by directing blood from unimportant organs to skeletal muscles. Other effects include dilation of pupils, increasing blood pressure, expanding the lung cavity, etc. 

Adrenergic receptors are of two type’s alpha and beta, which are further divided based on their function and effect on the body. Adrenaline when binds to these receptors cause vasoconstriction with alpha and vasodilation with beta receptor.

Cholinergic pathway is related to the parasympathetic nervous system that involves the functions of cholinergic receptors. These receptors are regarded as intropic and metabotropic and are activated by the neurotransmitter Acetylcholine (ACh).

Acetylcholine neurotransmitters bind to the muscarinic and nicotinic receptors, the two main cholinergic receptors to carry out its functions. The cholinergic system drives the digest and rest response of the body.

The vagus or the parasympathetic nervous ending in the heart are responsible for the cholinergic effects on it. When Acetylcholine binds to these receptors in the heart, they are responsible for down regulating the heart’s functions.

This effect of lowering heart beat and blood pressure by acetylcholine binding is a balancing act of the body. Other effects of acetylcholine include dilating blood vessels, increasing bodily secretions and smooth muscle contraction.

These receptors are found all over the body but mainly in the target organs such as sensory glands, respiratory tracts, heart and eyes and gastro-intestinal tract. They are also found in the somatic efferent nerves in the skeletal muscles.

Cholinergic receptors are part of both Somatic and Autonomic nervous system and are further divided into Nicotinic and Muscarinic receptors. Nicotinic receptors bind to nicotine and muscarinic receptors binds to muscarine.  

1. The main difference between adrenergic and cholinergic is that adrenergic involves the use of neurotransmitter adrenaline and noradrenalin whereas cholinergic involves the use of neurotransmitter Acetylcholine.
2. Another key difference is that adrenergic receptors are present in sympathetic nervous system while cholinergic receptors are present in parasympathetic nervous system.
3. Adrenergic receptor binding induces improved activity of the heart and overall body performance while cholinergic receptor binding is responsible for down regulating this effect.
4. Adrenergic receptors are of two types i.e. alpha and beta receptors while the two types of cholinergic receptors are nicotinic and muscarinic receptor.
5. Adrenergic receptor works by G-protein coupling while Cholinergic are intropic-ligand gated and metabotropic receptors.

Adrenergic and Cholinergic receptors are part of the Autonomous nervous system of our body. They are governed by the neurotransmitters binding to them for regulating their specific functions.

The main difference between adrenergic and cholinergic is that adrenergic receptors bind to the neurotransmitter adrenaline or epinephrine and noradrenalin or norepinephrine and that of cholinergic bind to acetylcholine.

The next key difference is that adrenergic receptors are associated with the sympathetic nervous systems while the cholinergic receptors are associated with the parasympathetic nervous system.

The adrenergic nerve endings in the heart and the specific receptors present are responsible for the increased heart activity when they bind to the adrenaline neurotransmitters while that of cholinergic receptors in the heart down regulate heart’s activity when stimulated.

Binding with adrenergic receptors also include effects such as dilation of pupils, increasing blood pressure, expanding the lung cavity, and redirecting blood flow into skeletal muscles to improve glucose availability.

Binding with cholinergic receptors are associated with dilating blood vessels, increasing bodily secretions and smooth muscle contraction apart from down regulating heart’s activity.

Adrenergic receptors are of two types alpha and beta which are further divided into Alpha1 and alpha2 and beta1, beta2and beta3 depending upon their functions. Cholinergic receptors are also divided into nicotinic and muscarinic which binds to nicotine and muscarine respectively. Another difference is that the adrenergic receptors work by a G-protein coupling method whereas cholinergic receptors bind by ligand-gated intropic and metabotropic mechanisms.  

References

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