**3. Physiology of nerve conduction**

Reviewing the physiology of nerve conduction is important before any discussion of local anesthetics. Nerves transmit sensation as a result of the propagation of electrical impulses; this propagation is accomplished by alternating the ion gradient across the nerve cell wall, or axolemma.

In the normal resting state, the nerve has a negative membrane potential of -70 mV. This resting potential is determined by the concentration gradients of 2 major ions, Na+ and K+, and the relative membrane permeability to these ions (also known as leak currents).

The concentration gradients are maintained by the sodium/potassium ATP pump (in an energy-dependent process) that transports sodium ions out of the cell and potassium ions into the cell. This active transport creates a concentration gradient that favors the extracellular diffusion of potassium ions.

In addition, because the nerve membrane is permeable to potassium ions and impermeable to sodium ions, 95% of the ionic leak in excitable cells is caused by K+ ions in the form of an outward flux, accounting for the negative resting potential. The recently identified 2-pore domain potassium (K2P) channels are believed to be responsible for leak K+ currents.

When a nerve is stimulated, depolarization of the nerve occurs, and impulse propagation progresses. Initially, sodium ions gradually enter the cell through the nerve cell membrane. The entry of sodium ions causes the transmembrane electric potential to increase from the resting potential. Once the potential reaches a threshold level of approximately -55 mV, a rapid influx of sodium ions ensues. Sodium channels in the membrane become activated, and sodium ion permeability increases; the nerve membrane is depolarized to a level of +35 mV or more.

Local Anesthesia for Husbandry Procedures and Experimental Purposes in Farm Animals 237

Equilibration is crucial because, although the ionized form is injectable, the nonionized base has the lipophilic properties responsible for its diffusion into the nerve cell membrane. The duration of action of an anesthetic or the period during which it remains effective is determined by its protein-binding activity, because the anesthetic receptors along the nerve

The intermediate chain, which connects the aromatic and amine portions, is composed of either an ester or an amide linkage. This intermediate chain can be used in classifying local

Lidocaine is the most common local anaesthetic drug in worldwide veterinary use. It is used to alleviate the acute pain experienced by animals during and for 1–2 hours after a number of painful procedures, including some husbandry practices in farm animals (Mellor &

It is a short-acting local anaesthetic that is usually cleared from the site of injection quickly

Lidocaine block of the corneal nerve prior to dehorning of calves virtually eliminates the plasma cortisol response indicative of pain for about 2 hours (Stafford and Mellor, 2005a).

There are other local anaesthetics such as bupivacaine and mepivacaine, with different characteristics, that may be useful in husbandry procedures. For instance, bupivacaine blockade of the cornual nerve of calves virtually eliminates the cortisol response to dehorning for about 4 hours (Stafford & Mellor, 2005a). However, most such local

Epidural nerve block is produced by injecting local anaesthetic into the epidural space of the

Effective epidural analgesia can also be achieved by injecting alpha-2 agonists such as xylazine epidurally and this is used when castrating adult cattle because it has the added

A lidocaine-xylazine mixture produces effective epidural analgesia in cattle castrated using

Accidental intravenous injection of local anesthetics is the most common cause of adverse reaction associated with local anesthetic administration. In severe cases it can cause cardiac

When the plasma concentration of local anesthetics is excessive, sufficient cardiac sodium channels become blocked so that conduction and automaticity become adversely depressed. For example, excessive plasma concentration of lidocaine may slow conduction of cardiac impulses through the heart, manifesting as increased PR interval and widened QRS complex

**5. Nerve block for husbandry procedures in farm animals** 

cell membrane are proteins.

Stafford, 2000; Stafford & Mellor, 2005a, b).

enough for its effects to last for about 60–120 minutes.

anaesthetics are not licensed for livestock in many countries.

a castration clamp (Burdizzo®) and extends the duration of analgesia.

**6. Systemic and toxic effects of local anesthetics** 

spinal cord (Flecknell & Waterman-Pearson, 2000).

advantage of being accompanied by sedation.

anesthetics.

arrest.

on the ECG.

Once membrane depolarization is complete, the membrane becomes impermeable to sodium ions again, and the conductance of potassium ions into the cell increases. The process restores the excess of intracellular potassium and extracellular sodium and reinstates the negative resting membrane potential.

Alterations in the nerve cell membrane potential are termed the action potential. Leak currents are present through all the phases of the action potential, including setting of the resting membrane potential and repolarization.
