Properties of excitable tissues

From irritability to excitation

The cell is the basic structural and functional unit of life. Absolutely all living cells possess irritability—the universal ability to respond to external influences. Upon irritation, a cell alters its physicochemical parameters: membrane permeability, pH, cytoplasmic viscosity, or metabolic rate. The main rule of biology: if a system responds to a stimulus, it is alive.

In the course of evolution, specialized excitable tissues emerged (nervous, muscle, and glandular). They transformed basic irritability into excitability—the ability to produce a rapid and specific reaction (excitation) in response to a stimulus.

Three main properties of excitable tissues

All excitable tissues share three general physiological properties:

1. Excitability. This is the readiness of tissue to respond to a stimulus. For a nerve, the response is the generation of an action potential; for a muscle, contraction; for a gland, secretion. The measure of excitability is the threshold stimulus strength (the minimum impact capable of triggering an action potential). An inverse relationship applies here: the higher the threshold, the lower the excitability. Nervous tissue leads the ranking, followed by muscle tissue, with glandular tissue at the bottom.
2. Conductivity. The ability to propagate an excitation wave along the entire length of the tissue. The main indicator is conduction velocity. Nerve fibers transmit signals at speeds up to 120 m/s, skeletal muscles at 6–13 m/s, and glandular tissue at only a few centimeters per second.
3. Lability (functional mobility). This is the speed at which elementary excitation cycles occur. It is measured by the maximum number of action potentials that a tissue can generate in one second at the exact rhythm of the stimulus. Nerve fibers can reproduce 500–1000 impulses per second, muscle tissue 200–250, and synapses 100–125.

Features of muscle tissue

Striated muscles of the locomotor apparatus possess an additional specific property—contractility. This is the ability to respond to a stimulus by shortening fibers or increasing their tension.

Skeletal muscle consists of interstitial substance and muscle fibers. Each fiber is covered by a cell membrane (sarcolemma), inside of which lies the cytoplasm (sarcoplasm). Contractile filaments—myofibrils—are located within the sarcoplasm. They consist of two types of myofilaments:

• long and thin actin filaments;
• short and thick myosin filaments, which have cross-bridges.

The muscle work process is divided into two stages. First, an electrical action potential propagates along the sarcolemma decrementlessly (without attenuation). Then, beneath the membrane, a mechanical process is triggered: actin filaments slide between myosin filaments. Ca2+ ions provide the link between the electrical signal and mechanical contraction.

Historical context and automatism

The existence of "animal electricity" in tissues was first proven in the famous experiments of Luigi Galvani (the balcony experiment and the frog sciatic nerve experiment).

It is also important to note that certain excitable cells possess automatism—the ability for spontaneous rhythmic or aperiodic activity. Such cells generate impulses independently, without any visible connection to external triggers.

Class and exam questions

What else to study on this topic

• Mechanisms of changes in membrane permeability during stimulation
• Decrementless propagation of the action potential
• The role of myosin cross-bridges in muscle contraction
• Molecular mechanisms of excitation-contraction coupling by calcium ions
• The nature and types of automatism in various tissues

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