Autonomic Pharmacology of the Eye

The nervous system together with the endocrine system controls and integrate most body functions among which include; some aspect of ocular physiology. Impulse conduction along the nerve fibers in the nervous system is mainly electrical whereas at the nerve-nerve junction and nerve-effector junction, impulse transmission is chemical. These chemical transmission is mediated through the action of chemical substances called neuro-hormones or neuro-transmitters which are released at the nerve terminals.

The nervous system is usually divided into the central nervous system and the peripheral nervous system, both divisions have what is called an afferent division(sensory) and an efferent division(motor).

The peripheral nervous system is further divided into the somatic (motor nerve) which are concerned with skeletal muscles (voluntary movements) and the autonomic nervous system which controls smooth muscles and involuntary movements and glands. Smooth muscles include the muller muscle found in the eyelid.  Based on the segment of the CNS from which the preganglionic neurons originate, also on the nature of the chemical substance(neurotransmitters) liberated at the various nerve endings. The autonomic system is subdivided into the sympathetic and the parasympathetic nervous system.

The Parasympathetic system

In the parasympathetic system, the preganglionic fibers originate from the cervical and sacral segments of the spinal cord (the cranio-sacral segment) of the autonomic nervous system. Also at the postganglionic nerve endings of the parasympathetic nerve, the neurotransmitter released is acetylcholine. Hence, the parasympathetic nerves are also known as the cholinergic nerves and the system also referred to as the cholinergic system.

The sympathetic system

Here, the preganglionic fiber originates from the thoracic and lumbar segment of the spinal cord (thoraco-lumbar segment) and at the postganglionic nerve terminals of the sympathetic system as well as the adrenal gland, two neurotransmitters are released i.e. adrenaline(epinephrine) and nor-adrenaline(nor-epinephrine). Thus, the sympathetic nerves are referred to as adrenergic system.

  The eye is innervated by both divisions of the autonomic system i.e. Dual innervation. Many tissues in the eye are innervated by both divisions and in many body functions, the effects of this systems are antagonist9physilogic antagonism), examples include; control of pupil size, accommodation, lacrimal gland secretion, aqueous humor flow.

Autonomic receptors are macromolecules substances, of protein nature, located at the cell membrane with which endogenous substances such as neurotransmitters and hormones or some drugs must combine to produce biologic responses or physiological effects. For the cholinergic system, we have muscarinic receptors (M1, M2 and M3) and nicotinic receptors.

Muscarinic receptors are in the iris, ciliary body, glands etc. these receptors are blocked by a drug called atropine (anti-muscarinic agent) (parasympatholytic agent) (anticholinergic agent), homatropine, cyclopentolate, tropicamide etc.

These nicotinic receptors are located at the autonomic ganglion skeletal muscle junction in the eye and adrenal medulla which are blocked by high concentration of nicotine.

Adrenergic receptors are classified thus; alpha receptors (1&2) and beta receptors (1&2) beta blocker (beta adrenergic receptor blocker).