Question

Organophosphates (OPs) are toxic chemicals contained in many pesticides used heavily in agriculture. They can have severe, acute effects, including death, beginning within minutes to hours of exposure. Other symptoms can develop over longer time periods. Exposure can occur through inhalation, ingestion, or, to a lesser extent, contact with skin.

The toxicity of OPs stems from their action at cholinergic synapses: these chemicals inhibit acetylcholinesterase, the enzyme that breaks down acetylcholine (ACh) and terminates its action following release into the synapse.

1.   Cholinergic synapses are found in several parts of the autonomic nervous system and somatic motor system. For each of these systems, describe the locations of cholinergic synapses, in terms of which types of neurons release ACh and what types of target cells contain ACh receptors.

2.   Which type of ACh receptor is found in each of the locations you described in question #1? Would you expect OPs to have different effects on the two receptor types? Why or why not?

Answer 1

PART-1 CHOLINERGIC SYNAPSE LOCATION

Cholinergic synapses are present in the following areas in the body:

1. All the autonomic nervous system ganglions. Both sympathetic and parasympathetic ganglions have cholinergic (ACH secreting ) pre ganglionic nerve fibres. (here target cells are the postganglionic fibres of autonomic nervous system )

2. Parasympathetic post ganglionic fibres- all the post ganglionic fibres of the parasympathetic nervous system have cholinergic synapses with the target organ.(these target organs are listed with the type of receptors )

3. postganglionic fibres of SWEAT GLAND.-normally all the postganglionic fibres of sympathetic nervous system have adrenergic synapses . Sweat glands are the only exception where sympathetic postganglionic fibres secrete ACH. (here target cells are the sweat glands )

4. somatic motor system- cholinergic synapses are also present in the neuromuscular junction where the terminal motor nerve endings supply muscle fibres and cause their contraction by releasing ACH. (here target cells are the muscle fibres )

PART-2.TYPES OF CHOLINERGIC RECEPTORS

NICOTINIC RECEPTORS	MUSCARINIC RECEPTORS
1. NM  receptors- these are the Ach receptors present at the neuromuscular junction. Ach from the motor nerve endings reaches the receptors at muscular endplate and causes contraction. 2.NN receptors- these are present in the autonomic ganglia. The Ach from the preganglionic neuron reaches the Postganglionic neuron and causes transmission of signal.	1. M1 receptors- these receptors are present in the stomach and here the Ach cause  secretion of Hcl in the stomach. 2. M2 receptors - these receptors are present in the heart and here via the parasympathetic system, the heart rate and contractility is controlled.(note- parasympathetic nervous system cannot control the force of contraction of heart as there is no parasympathetic supply to the ventricles) 3.M3 receptors -these receptors are present in bronchus- causes bronchoconstriction GIT- causes peristalsis(target-smooth muscles of git) bladder- causes contraction of detrusor and relaxation of sphincter glands- increases secretion of glands like lacrimal, salivary,etc pupil- (target-constrictor Pupillae)causes constriction of pupil or miosis

PART-3 ACTION OF ORGANOPHOSPHATES ON THESE RECEPTORS

Organophosphates act on the enzyme Acetylcholinesterase which functions in both the receptors in a similar fashion. thus there will be no difference in the types of effects in the two receptors. In all the locations, organophosphates will cause increase presence of the Ach in the synapse causing overstimulation of the Ach receptor containing target cells.