Nerve excitation

Excitable tissues and their properties

Any living cell possesses irritability, which is the universal ability to alter its physicochemical parameters (membrane permeability, pH, cytoplasmic viscosity, metabolic rate) in response to external influences. If a cell responds to a stimulus, it is alive.

However, only three types of tissue—nervous, muscular, and glandular—possess excitability. This is the ability to respond to a stimulus not merely with a metabolic change, but with an active, specific reaction.

Excitation itself manifests in two ways. Nonspecific signs include the generation of an action potential, metabolic shifts, and temperature changes. Specific functions depend on the tissue type: nerves conduct signals, muscles contract, and glands secrete products.

The measure of excitability is the threshold stimulus strength—the minimum impact capable of eliciting an action potential. The relationship is strictly inverse: the higher the irritation threshold, the lower the tissue excitability. In the body, nervous tissue has the highest excitability, followed by muscular tissue, and then glandular tissue.

Conduction, lability, and automatism

In addition to excitability, the functional state of tissues is described by three other parameters.

• Conduction is the ability to propagate an action potential along its entire length. The indicator here is velocity: in nerve fibers it reaches 120 m/s, in skeletal muscles it is 6–13 m/s, and in glandular tissue it amounts to a few centimeters per second.
• Lability (or functional mobility) reflects the rate of elementary excitation cycles. Its measure is the maximum number of excitation waves that a tissue can generate in one second in strict accordance with the rhythm of applied stimuli. Nerve fibers lead here as well, conducting 500–1000 impulses per second. Muscular tissue is capable of 200–250 impulses, and a synapse only 100–125.
• Automatism is the ability of certain cells and tissues to undergo spontaneous rhythmic or aperiodic activity without visible external stimuli.

Laws of excitation conduction along a nerve

For a signal to reach its target correctly, the nerve fiber obeys four strict laws:

1. Anatomical and physiological integrity. Conduction is impossible if the nerve is severed (anatomical disruption) or temporarily blocked by freezing, ligation, or pharmacological agents (physiological disruption).
2. Isolated conduction. Nerves and individual fibers within bundles are securely separated from each other by connective tissue sheaths. Normally, an impulse never "jumps" to adjacent pathways.
3. Bilateral conduction. The transmission mechanism is based on local currents. If a section of a fiber is stimulated, currents will spread in both directions from the site of origin. However, the impulse does not return backward: circular currents encounter the previously traversed section in a state of absolute refractoriness. In a living organism, bilateral movement is an abnormal situation encountered either in experiments or in pathology.
4. Relative unfatigability. In 1882, the Russian physiologist N. E. Vvedensky proved that under continuous stimulation for 9–14 hours, a nerve continues to stably conduct excitation. This fact has become axiomatic: fatigue in the nervous system is localized not in the fibers, but in the synapses of the reflex arc (primarily at the input, in the sensory part).

Axon reflexes

The term "axon reflex" was introduced by the English physiologist Langley, the founder of autonomic nervous system studies. Strictly speaking, this phenomenon is not related to true reflexes—it is a reaction associated with the propagation of excitation along the branches of a single axon.

Classical proofs of this phenomenon:

• Babukhin's experiment on the electric organ cells of an electric ray. The electric organ is innervated via a branching axon, where each branch approaches an individual cell. If the axon is severed from the neuron cell body (to prevent the signal from going back) and only a single branch is stimulated, the entire organ discharges. The excitation reaches the bifurcation and spreads along all branches to all cells, despite their isolation from each other.
• Kühne's experiment on a frog nerve-muscle preparation (m. gracilis). The muscle is innervated by a trunk dividing into two branches for the two halves of the muscle. If the muscle and nerve are cut (ruling out feedback effects) and one branch is stimulated, both halves of the muscle contract. The signal travels up the branchlet of the trunk and descends to the second half.

Class and exam questions

What else to study on this topic

• Structure of myelinated and unmyelinated fibers
• Saltatory and continuous mechanisms of impulse conduction
• Factors determining the conduction velocity of an action potential
• Parabiosis and its phases
• Mechanism of action of local anesthetics on the nerve fiber

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.