Neurotransmitters within the body include excitatory and inhibitory, noradrenalin, dopamine, serotonin; acetylcholine and glutamate are examples of excitatory neurotransmitters. GABA and glycine are examples of inhibitory neurotransmitters.
General anaesthetics act mainly on the central nervous system to stop information processing; these may be given by inhalation or intravenously.
Nitrous oxide or laughing gas is used in maintain anaesthesia with oxygen and is often used in combination with other drugs. It is a potent analgesic or pain reliever. Other inhaled anaesthetics include isoflurane, desflurane and sevoflurane.
Commonly used intravenous anaesthetics include propofol, a rapid action and rapid recovery without a patient hangover. Thiopental is used in very brief procedures and has a sedative like affect. Ketamine is often used for patients in shock, it increases heart rate and blood pressure, it can lead to bad dreams and hallucinations during recovery and is also a known abused drug,
Anaesthetics work on the central nervous system to promote insensitivity to the feeling of pain. They are usually fat soluble and can cross the blood brain barrier. They can also work on the cell membrane receptors to reduce the excitability of neurones.
Typical side effects include nausea, vomiting, shallow breathing, reduced blood pressure and cardiac output, loss of temperature control, decrease kidney profusion and can cross the placenta during pregnancy.
Anxiolytics and hypnotics (Benzodiazepines)
These drugs potentiate the inhibitory effects of the neurotransmitter GABA they may act in the spinal cord and the supraspinal sites to produce muscle relaxation. these drugs also reduce the excitability of specific neurones. These drugs are part of the benzodiazepine group and include midazolam, diazepam (vallium) and loroazepam.
Further important points about this group of drugs is that they do not causes excessive sedation in low doses and they have no analgesic effect
Adverse effects of these drugs may include dependence of the drug, drowsiness, confusion, muscle weakness and they also interact with alcohol.
Non de-polarising neuromuscular junction blockers
These drugs are similar in structure to acetylcholine and occupy the neuromuscular cholinergic receptor site, which prevents acetylcholine reacting with the nicotinic receptor. These drugs do not cause activation of the muscle cells and therefore muscle contraction does not occur.
These drugs are used when clinical situations require muscular paralysis during the inserting of an endotracheal tube during assisted ventilation. These drugs can be further classified by their duration of action, short acting mivacurium, intermediate acting atracurium and long acting pancuronium. The action of these drugs can be reversed using neostigmine.
Adverse effects of neuromuscular blockers include prolonged paralysis, inability to breathe due to the histamine release associated with many of the neuromuscular blockers which causes respiratory obstruction, wheezing and bronchospasm. They also included weakness, muscle pain and soreness and effects of immobility which are related to the functions of muscle blocking.
Depolarising neuromuscular blockers
These drugs mimic or copy the effect of acetylcholine but produce sustained depolarisation of motor end-plate resulting in paralysis. They are short acting unless administered via drip and example of a drug is suxamethonium.
Narcotic (opioid) analgesics
These drugs act on the central nervous system (opioid receptors) to relieve pain; they also have a euphoric effect and allow the patient to escape from reality. Examples of these groups of drugs are morphine and fentanyl which are used as local anaesthetics and act on specific nerve pathways.
Local anaesthetics block the transmission of nerve impulses carrying pain signals from receptors to the brain and are used mainly in minor surgery. They also inhibit sodium entry through sensitive sodium channels and block transmission of nerve impulses. Most common method of administration is via injection into the epidural space which directly affects the nerve roots, intradermal/topical administration can be used superficially.
Lidocaine or lignocaine is a very widely used local anaesthetic; it is often used in dental surgery, peripheral, sympathetic and central nerve blocks and topical use on skin.
Lidocaine blocks the generation and conduction of action potentials in sensory nerves by reducing sodium permeability, therefore reducing the height and rate of rise of action potential, increasing excitation threshold and slowing conduction velocity.
Adverse effects of Lidocaine are confusion, respiratory depression, hypotension, abnormal heartbeat, and convulsions.
Other drugs which work on the nervous system
Antidepressants increase levels of excitatory neurotransmitters in the brain.
Antiparkinsonian drugs increase levels of the neurotransmitter dopamine.
Anticonvulsants treat epilepsy by reducing the conduction of excitatory nerve impulses and/or increasing GABA activity.
Nervous system drugs act either centrally like general anaesthetics, antidepressants or peripherally like local anaesthetics Lidocaine. They may cause excitation or depression of brain activity. These drugs tend to open or close ion channels in the body. Muscle blockers are used to relax muscles during surgery.