Sympathetic and Parasympathetic System

Sympathetic and Parasympathetic System

Sympathetic and Parasympathetic Nervous System

In parasympathetic nervous system (PNS) acetylcholine (ACh) binds to Colinergic recerptors (can be classified further into: Muscarinic , Nicotinic): - M1-M5, Nicotinic (eg; GABA).

In sympathetic nervous system (SNS) norepinephrine (NE) binds to adrenergic recerptors - α₁, α₂, β₁, β₂. Abundance of the receptors:

• α₁ - smooth muscle cell.

• α₂ - inhibitory auto receptor.

• β₁ - Heart, Kidney.

• β₂- Smooth muscle, Liver. 

Binding of ACh to colinergic receptors activate PNS and deactivate SNS. 
Alternatively, binding of NE to colinergic receptors activate SNS and deactivate PNS. 

In Adrenergic receptor

1. Binding of NE to α₁ adrenergic receptor (G_qα) (present mostly in smooth muscles) activates Phospholipase C (PLC). PLC cleaves PIP2 (naturally attached in smooth muscle's inner cell membrane) to IP3 & DAG. IP3 travels to cytoplasm. IP3 activates L-type Ca2+ channels and Ca2+ channels. Increased Ca2+ results in contraction of smooth muscles (intestine, respiratory system). (Note: no SR in smooth muscle).
2. Binding of NE to α₂ adrenergic receptor (G_iα) will provide negative feedback to stop release of NE from presynaptic neuron. It also reduce cAMP concentration. NE normally binds to α₂ when there is a higher concentration of NE in synapse for a while or in other words when reuptake mechanism is saturated.
3. Binding of NE to β₁ (G_sα) (present mostly in heart and kidneys) activates Adenyly Cyclase (AC) pathway. AC produce the second messenger cAMP by converting ATP to cAMP. This higher concentration of cAMP in the cytoplasm activates the cAMP-dependent protein kinase (also called Protein Kinase A or PKA). Cellular effects of G_sα acting through PKA are described here(link not yet created). PKA opens up L-type Ca2+ channels, phosphorylates RyR2 of SR (sarcoplasmic reticulum) to release more Ca2+, phosphorylates SERCA and inhibit taking up Ca2+ back to SR. All these result in higher contraction of heart muscles. 
4. Binding of NE to β₂ (G_sα) have the same effect. So intestine contractility is decreased.