The autonomic nervous system (ANS) consists of three main anatomical divisions: sympathetic, parasympathetic and enteric nervous systems. The sympathetic (SNS) and parasympathetic (PNS) systems provide a link between the central nervous system and peripheral organs. The sympathetic nervous system originates in the spinal cord. Specifically, the cell bodies of the first neuron (the preganglionic neuron) are located in the thoracic and lumbar spinal cord. Axons from these neurons project to a chain of ganglia located near the spinal cord.
In most cases, this neuron makes a synapse with another neuron (post-ganglionic neuron) in the ganglion. A few preganglionic neurons go to other ganglia outside of the sympathetic chain and synapse there. The post-ganglionic neuron then projects to the “target” – either a muscle or a gland. The sympathetic nervous system operates through a series of interconnected neurons. The synapse in the sympathetic ganglion uses acetylcholine (Ach) as a neurotransmitter; the synapse of the post-ganglionic neuron with the target organ uses the neurotransmitter called Noradrenaline (NA).
However, there is one exception: the sympathetic post-ganglionic neuron that terminates on the sweat glands uses acetylcholine. The cell bodies of the parasympathetic nervous system are located in the spinal cord (sacral region) and in the medulla. In the medulla, the cranial nerves III, VII, IX and X form the preganglionic parasympathetic fibres. The preganglionic fibre from the medulla or spinal cord projects to ganglia very close to the target organ and makes a synapse. This synapse uses the neurotransmitter called acetylcholine.
From this ganglion, the post-ganglionic neuron projects to the target organ and uses acetylcholine again at its terminal. The two main neurotransmitters that operate in the autonomic system are acetylcholine and noradrenaline. This diagram taken from Rang and Dale Pharmacology shows the type of postsynaptic receptor with which the transmitters interact at the different sites Some general rules apply: All motor nerve fibres leaving the central nervous system release acetylcholine, which acts on nicotinic All postganglionic parasympathetic fibres release acetylcholine, which acts on muscarinic receptors.
All postganglionic sympathetic fibres (with one important exception) release noradrenaline, which may act on either ? – or ? -adrenoceptors . The exception is the sympathetic innervation of sweat glands, where transmission is due to acetylcholine acting on muscarinic receptors. In some species, but not humans, vasodilatation in skeletal muscle is produced by cholinergic sympathetic nerve fibres. Preganglionic neurons are cholinergic, and ganglionic transmission occurs via nicotinic ACh receptors (although excitatory muscarinic ACh receptors are also present on postganglionic cells).
Postganglionic parasympathetic neurons are cholinergic, acting on muscarinic receptors in target organs. Postganglionic sympathetic neurons are mainly noradrenergic, although a few are cholinergic (e. g. sweat glands). Transmitters other than noradrenaline and acetylcholine (NANC transmitters) are also abundant in the autonomic nervous system. The main ones are nitric oxide and vasoactive intestinal peptide (parasympathetic), ATP and neuropeptide Y (sympathetic). Others, such as 5-hydroxytryptamine, GABA and dopamine, also play a role.
The iris is a contractile structure, consisting mainly of smooth muscle, surrounding the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae. When the sphincter pupillae contract, the iris decreases or constricts the size of the pupil. The dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, cause the pupil to dilate when they contract.
These muscles are sometimes referred to as intrinsic eye muscles. The sensory pathway (rod or cone, bipolar, ganglion) is linked with its counterpart in the other eye by a partial crossover of each eye’s fibres. This causes the effect in one eye to carry over to the other. If the drug pilocarpine is administered, the pupils will constrict and accommodation is increased due to the parasympathetic action on the circular muscle fibres, conversely, atropine will cause paralysis of accommodation (cycloplegia) and dilation of the pupil.
When bright light is shone on the eye light sensitive cells in the retina, including rod and cone photoreceptors and melanopsin ganglion cells, will send signals to the oculomotor nerve, specifically the parasympathetic part coming from the Edinger-Westphal nucleus, which terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil. This is the pupillary light reflex, which is an important test of brainstem function. The ciliary ganglion is a parasympathetic ganglion located in the posterior orbit.
It measures 1–2 millimeters in diameter and contains approximately 2,500 neurons. Preganglionic axons from the Edinger-Westphal nucleus travel along the oculomotor nerve and form synapses with these cells. The postganglionic axons run in the short ciliary nerves and innervate two eye muscles: The sphincter pupillae constricts the pupil, a movement known as Miosis. The opposite, Mydriasis, is the dilation of the pupil. The ciliaris muscle contracts, releasing tension on the Zonular Fibers, making the lens more convex, also known as accommodation.
The purpose of this experiment is to understand 1. Pupil diameter control and 2. pharmacology of the autonomic nervous system. In the programme we stimulated the addition of selected pharmacological agents affecting the neurotransmission in the autonomic pathways controlling pupil diameter. The drugs we used were: Atropine: Competitive antagonist of muscarinic receptors. Pilocarpine: Agonist at muscarinic receptors Physostigmine: Inhibitor of Ach Phenylephrine: Agonist at alpha-adrenoreceptors Cocaine: Inhibits neuronal uptake of NA Amphetamine: Releases NA from sympathetic nerves.