Peripheral Nervous System

Central nervous system and the peripheral nervous system
Parasympathetic and sympathetic nervous system – an anatomical approach
Transmitter substances: Acetylcholine & noradrenaline
Adrenergic and cholinergic receptor’s subtypes and effects

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The central nervous system (CNS) is responsible for controlling most of our functions and it combines most of the information from the other parts of the body. The other component is the peripheral nervous system (PNS) and its primary role is connecting the CNS to receptor receptor signals and relay action signals.

The autonomic nervous system is mainly unconscious and it regulates sensory functions such as bodily functions, such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal. It is also involved in the fight or flight reflex.

The Autonomic NS can be further divided into the sympathetic NS (used in times of stress) and the parasympathetic NS (used to calm the body). Additionally, the enteric nervous system which is composed of the neurons governing the function of the gastrointestinal tract. They coordinate peristaltic contractions which push food through the intestines.

The sympathetic nervous system has information outflow from the thoracic and the lumbar. Therefore, the sympathetic nervous system can be called the thoracolumbar outflow

Whereas, the parasympathetic nervous system has information outflow from the cranial and sacral. Therefore, the information transmitted by the parasympathetic nervous system can be referred to as craniosacral outflow.

The vagus nerve is on the 10th branch, innovating the heart, lung and upper GI.

The intrinsic nerve plexus is found in the gastrointestinal tract and is apart of the enteric nervous system. Whilst it is a complex system, it has local reflex pathways allowing it to function independent of the central nervous system – both the parasympathetic and the sympathetic pathways are closely related in their function.

Both components of the peripheral nervous system generally have a preganglionic, ganglion and a postganglionic nerve cells. The preganglionic nerve fibers are shorter in the sympathetic and postganglionic are shorter in parasympathetic nervous system. This is because the ganglion usually lies closer to or on the tissue being innovated by the parasympathetic nervous tissue.

The heart is an example of an organ which receives information from both the parasympathetic nervous system (bradycardia – slowing the heart down) as well as information from the sympathetic nervous system (tachycardia – speed the heart up). However, the ciliary muscle in the eye only receives information from the parasympathetic nervous system. Likewise the sweat glands/ majority of blood vessels only receive information from the sympathetic nervous system.

Acetylcholine is a neurochemical released in the by sympathetic and parasympathetic NS in the preganglionic/ ganglion synaptic cleft. However, the neurotransmitter noradrenaline is released most of the time in the postganglionic symptomatic nerve fiber. Most of the time in the parasympathetic nerve system, acetylcholine is released.

	Sympathetic nervous system (thoracolumbar outflow)	Parasympathetic nervous system (craniosacral outflow)
Preganglionic neurotransmitter	Acetylcholine	Acetylcholine
Postganglionic neurotransmitter	noradrenaline	Acetycholine

Acetylcholine is also released to innovate muscle’s which are apart of the somatic nervous system, which do not have ganglia.

Both muscarinic and nicotinic receptors are acted on by the acetylcholine.

The nicotinic receptors are ligand gated ion channels and are composed of 5 subunits. They are located in the CNS and the parasympathetic and sympathetic ganglia, the neuromuscular junction of the skeletal muscle and the adrenal medulla.

The muscarinic receptors are G coupled-protein receptors. There are 5 sub types of muscarinic receptors. They can also be be found on the CNS as well as sweat glands, organs innovated by the parasympathetic postganglionic nerve fibers and the autonomic ganglia. The 5 sub types include:

M1: Gq protein results in an increase in Phospholipase C (PLC), this is linked with the autonomic ganglia, salivary and gastric acid secretion
M2: Gi/o protein results in a decrease in PLC causing bradycardia
M3: Gq protein results in increase in PLC, this is linked with pupillary constriction in the eye, smooth muscle contraction in both GI and respiratory tract
M4: Gi/o inhibits the release of ACH in the Striatum
M5: Gq protein results in functions such as adenylate cyclase inhibition

Noradrenaline is formed through the synthesis of tyrosine, an amino acid. The pathway is as follows:

Tyrosine –> DOPA –> Dopamine –> Noradrenaline –> adrenaline

When noreadrenaline is released it targets G protein-coupled receptors. Noripinepherone transporters (NET) then recycle 75% of the noreadrenaline by taking it back into the nerve. It also regulates its own release by acting on alpha2 presynaptic receptors.

Adrenergic receptors

Type	Function	Protein	effect	neurotransmitter
α1	Vasoconstriction	Gα/q	increase	PLC
α2	Transmission release inhibition	Gα/i	decrease	AC
β1	Tachycardia & increase contraction	Gα/s	increase	AC
β2	Smooth muscle relaxation etc bronchiole	Gα/s	increase	AC
β3	smooth muscle bladder relaxation	Gα/s	increase	AC