General anesthetics

General anesthetics are medications used to induce an anesthetic state in patients who are about to go under surgery.

The anesthetic state refers to a number of conditions that make surgery tolerable for the patient and more manageable for the surgeon.

The conditions include: unconsciousness, where the person isn’t aware of themselves or their environment; sedation, so they don’t move in response to painful stimulation; analgesia, so they don’t feel pain; and amnesia, so they don’t remember the procedure.

Local anesthetics are different in the fact that they only block pain sensation in a specific part of the body, and don’t affect consciousness.

Okay, to achieve the anesthetic state, general anesthetics depress the central nervous system. In other words, they diminish the total amount of action potentials that are constantly firing in the brain. The generation of these action potentials depends on excitatory and inhibitory synapses.

Excitatory, means that the neurotransmitters released into the synaptic space stimulate the postsynaptic neuron to start an action potential.

The main neurotransmitter involved is glutamate, which binds to postsynaptic NMDA receptors, so some general anesthetics work by blocking these receptors.

Inhibitory synapses, on the other side, do the opposite; they release the inhibitory neurotransmitter called GABA, which binds to the postsynaptic neuron and keep it from firing.

So certain anesthetics work by stimulating these GABA receptors or by increasing their sensitivity to GABA.

Moving on, there are two main phases in anesthesia: induction, which is when the patient enters the anesthetic state; and maintenance, when the anesthetic state is prolonged for as long as required.

Some anesthetics are better for induction, while others are better for maintenance.

Now, depending on how they’re administered, there are two classes of general anesthetics: parenteral and inhalational anesthetics.

Parenteral anesthetics are given by injection into a vein. They’re highly lipophilic agents that are commonly used for anesthetic induction in a single intravenous injection, although some of them, in special situations, can be used for maintenance by continuous intravenous infusion.

Once in the bloodstream, these medications travel through the body to highly lipophilic tissues that receive a lot of blood, like the brain and spinal cord. There, they can induce the anesthetic state.