Physiology Notes 1: Stimulus, Response and Homeostasis
Overview of physiology
Anatomy studies the structure of the human body; physiology studies its function.
What medical students study is human physiology, usually shortened to physiology. Taking the human body as its object, it mainly studies the basic laws of the various life phenomena manifested by the normal human body and its cellular, tissue and organ components; it also studies how changes in the internal and external environment affect these functions and how the body regulates them, and it reveals the significance of various physiological functions in whole-body life activities.
The task of physiology is to study the principles, conditions and processes by which life activities arise, as well as the effects of internal and external environmental changes on the body, so as to understand and master the laws of normal human life activities and provide a scientific theoretical basis for preventing and treating disease, promoting health and extending life.
Medicine is the science of disease; physiology is the science of life.
Three levels of physiological study: 1. cellular and molecular level; 2. organ and system level; 3. whole-body level.
Basic characteristics of life: 1. metabolism; 2. excitability (adaptability); 3. reproduction; 4. aging. Metabolism is the most fundamental.
Stimulus and response:
Stimulus: Any change in the internal or external environment that causes a response in the body or a cell is called a stimulus. Stimuli are of many kinds; by nature they are classified as physical (sound, light, electricity, temperature, mechanical force, radiation), biological (bacteria, viruses, antibodies) and social/psychological (changes in social role, emotional fluctuation). Electrical stimulation is most commonly used in physiology experiments because it is easy to control and rarely damages tissue.
Response: A response is the physicochemical and functional change that appears in the body or a cell after receiving a stimulus. There are two forms: when, after stimulation, the body changes from a relatively resting state to an active one, or activity increases in strength, it is called excitation. When, after stimulation, the body changes from an active state to a relatively resting one, or activity weakens, it is called inhibition. Excitation and inhibition presuppose each other, are opposites within a unity, and can transform into each other under certain conditions.
Three elements of a stimulus: A stimulus that produces a response in the body or tissue generally has three basic conditions (the three elements of a stimulus): 1. stimulus intensity; 2. stimulus duration; 3. intensity-time change rate. Only when a stimulus reaches a certain intensity, duration and intensity-time rate can it evoke a response.
Measure of excitability: threshold
Threshold intensity (threshold): The minimum stimulus intensity that can just evoke a tissue response is called the threshold intensity (threshold, or threshold stimulus). A stimulus equal in intensity to the threshold is a threshold stimulus.
Suprathreshold stimulus: A stimulus whose intensity exceeds the threshold.
Subthreshold stimulus: A stimulus whose intensity is below the threshold.
Threshold and tissue excitability are inversely related.
When excitable cells receive a stimulus and become excited, their response forms differ: nerve tissue shows a nerve impulse, muscle tissue shows contraction of muscle fibers, and glands show secretion by glandular cells.
But the common response they all first produce after stimulation is an action potential. Therefore, in modern physiology, excitation is regarded as synonymous with the action potential or the process of generating it, and the ability of an excitable cell to produce an action potential after stimulation is called excitability.
The human body and its environment
The external environment in which the human body lives is called the external environment, including the natural environment and the social environment.
When changes in the external environment exceed the body's adaptive capacity, they adversely affect the organism.
Internal environment and homeostasis:
1. Body fluid: Body fluid is the collective name for all fluids in the body. In a normal adult, body fluid accounts for about 60% of body weight; two-thirds (about 40% of body weight) is distributed inside cells and called intracellular fluid; the remaining one-third (about 20% of body weight) is distributed outside cells and called extracellular fluid, including plasma, tissue fluid, lymph, cerebrospinal fluid, aqueous humor and body-cavity fluids (pleural, synovial and pericardial). Of extracellular fluid, plasma is about one-quarter (about 5% of body weight) and tissue fluid about three-quarters (about 15% of body weight).
2. Internal environment: The vast majority of cells in the body do not exchange substances directly with the external environment but are bathed in the extracellular fluid inside the body. Physiology calls the environment that cells directly contact and depend on for survival—i.e., extracellular fluid—the internal environment of the body.
Homeostasis: Under normal conditions, the various physicochemical factors of the internal environment (temperature, pH, osmotic pressure, ions and nutrients) remain relatively constant. We call this relatively stable state of the internal environment homeostasis. The concept of homeostasis was first proposed by Walter Cannon. Homeostasis is not static; it may vary within a certain range while remaining relatively stable.
Homeostasis is a complex physiological process: on one hand, changes in the external environment and cellular metabolism continually disrupt it; on the other, the body continuously restores balance through various regulatory mechanisms—a dynamic equilibrium. When homeostasis is severely disrupted beyond the body's regulatory capacity, disease results and life may even be threatened.
Experiments for this section
Experiment 1: Stimulus and response
Purpose: To demonstrate that living neuromuscular tissue can receive various effective stimuli and respond; to master the electrical stimulation method of neuromuscular experiments and the method of recording muscle contraction.
Principle: Living neuromuscular tissue has excitability and can change in response to stimuli. Stimuli may be electrical, mechanical, thermal or chemical. For a stimulus to evoke a tissue response, it must have sufficient intensity and duration. Because electrical stimulation is easiest to control and least damaging, it is most commonly used in physiological experiments.
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Experiment 2: Analysis of the reflex arc
Purpose: To analyze the components of the reflex arc, deepen understanding of the concepts of reflex and reflex arc, and demonstrate the relationship between an intact reflex arc and reflex activity.
Principle: With the participation of the central nervous system, the regular response of the body to a stimulus is called a reflex; realization of a reflex requires an intact reflex arc.
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