Stimulation in physiology

Basic states and universal properties of cells

Any biological system can exist in one of two functional states:

• Physiological rest. In this case, specific external stimuli do not act on the cell. The internal environment remains relatively constant, and the specific functions of the tissue are not manifested in any way.
• Active state. Occurs as a response reaction to changes in the external or internal environment. The quality and properties of the system change accordingly.

The transition between these states is possible due to irritability. This is an absolute universal property inherent in all living cells without exception. If a cell is capable of responding to an external stimulus, it is alive. Irritability manifests as shifts in physicochemical and physiological parameters. For example, a cell may alter its membrane permeability, pH level, cytoplasm viscosity, or the rate of metabolic reactions.

Excitable tissues and their main characteristic

Three types of tissues possess an additional, more complex property — excitability. These include nervous, muscular, and glandular tissues. Excitability allows them to respond to stimulation not merely with a metabolic shift, but with an active, specific reaction — excitation.

Excitation always includes two components:

1. Nonspecific function. It is the same for all excitable tissues. This includes the generation of an action potential (AP), a local temperature change, and shifts in metabolism and other physiological functions.
2. Specific function. This is the primary task of the tissue. Nervous tissue conducts excitation, muscular tissue contracts, and glandular tissue secretes.

To quantify excitability, the concept of threshold stimulus intensity is used. This is the minimum stimulus intensity sufficient to trigger an action potential. It is important to remember the rule: excitability and threshold are inversely related. The lower the threshold, the higher the excitability. When ranking tissues, nervous tissue has the highest excitability, followed by muscular tissue, and glandular tissue rounds out the list.

Conduction, lability, and automatism

In addition to excitability, specialized tissues possess several other crucial physiological characteristics.

Conductivity is the ability of a tissue not only to generate an action potential, but also to propagate it along its entire length. The efficiency of this process is assessed by conduction velocity. The leader here is nervous tissue, capable of transmitting a signal at speeds up to 120 m/s. Skeletal muscles conduct excitation more slowly — from 6 to 13 m/s, and glandular tissue demonstrates a result of only a few centimeters per second.

Lability (functional mobility) reflects the speed at which a single elementary excitation cycle occurs. In practice, it is measured by the maximum number of action potentials a tissue can reproduce in one second while strictly following the rhythm of applied stimuli.

• Nerve fibers: 500–1,000 impulses per second.
• Muscular tissue: 200–250 impulses per second.
• Synapses: 100–125 impulses per second.

Certain cells and organs possess a unique property — automatism. This is the ability for spontaneous rhythmic (or aperiodic) activity. Such structures do not require external driving causes or outside stimuli; they generate impulses independently.

Class and exam questions

What else to study on this topic

• Resting membrane potential and phases of the action potential
• Laws of excitation and conduction
• Classification of stimuli and stages of response development
• Local response and its differences from the action potential
• Chronaximetry and determination of motor chronaxie
• Parabiosis and the experiments of Galvani and Matteucci

Other topics in the "Additional Topics" section

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