Physiology Foundations: Control System Components and Information Flow

1) The Standard Control Loop Model: Components and Information Flow

A physiological control system is a structured way to move information about a variable (the regulated variable) from where it is sensed to where decisions are made, and then back out to tissues that can change that variable. The key skill is to track what information is carried at each step and how the output changes the input by altering the regulated variable (and therefore what the sensor detects next).

Labeled Control Loop Diagram (with Vocabulary Callouts)

Disturbance (internal/external) changes regulated variable (X)  ───────────────┐
                                                                              │
                                                                              v
[Sensor / Receptor] --(afferent signal: input information about X)--> [Integrating / Control Center]
       ^                                                                      |
       |                                                                      |
       |                                                                      v
       |                         (efferent signal: output command)       [Effector]
       |                                                                      |
       |                                                                      v
       └────────────── Response changes X (the regulated variable) & feeds back to sensor ──────────

• Regulated variable (X): the quantity being controlled (e.g., core temperature, plasma glucose).
• Sensor/Receptor: detects X (or a proxy for X) and converts it into a signal.
• Afferent signal (input): information traveling to the control center (often neural impulses or hormone levels that reflect X).
• Integrating/Control center: compares sensed information to an internal reference (set point or acceptable range) and decides on an output.
• Efferent signal (output): command traveling from the control center to effectors (neural output, endocrine output, or both).
• Effector: tissue/organ that can change X (muscle, gland, liver, adipose, blood vessels).
• Response: the effector’s action that alters X; this altered X is what the sensor detects next, closing the loop.

Information Flow: What Moves Where?

Key idea: the loop is not “a line,” it is a cycle. Output changes the regulated variable, which changes the sensor input. When you analyze a scenario, always ask: After the response, what does the sensor detect now?

2) Step-by-Step Mapping Exercise: Assign Each Component in Concrete Examples

Use the same six labels every time: Sensor → Afferent → Control center → Efferent → Effector → Response. The goal is to be able to point to a real structure (or signal) for each label.

Exercise A: Thermoregulation (Core Body Temperature)

Scenario: You walk outside on a cold day. Core temperature begins to drift downward.

1. Regulated variable (X): core body temperature (e.g., ~37°C).
2. Sensor/Receptor: thermoreceptors (peripheral in skin; central in hypothalamus) detect temperature.
3. Afferent signal (input): sensory neuron activity from skin + temperature-sensitive signaling within the hypothalamus conveying “current temperature is low.”
4. Integrating/Control center: hypothalamic thermoregulatory centers integrate inputs and compare to the acceptable range.
5. Efferent signal (output): autonomic outputs (sympathetic to cutaneous vessels; somatic motor to skeletal muscle) and endocrine adjustments in some contexts.
6. Effector:
   • Cutaneous arterioles (smooth muscle)
   • Skeletal muscle
   • Sweat glands (primarily for heat loss when hot; included here as an effector class)
7. Response:
   • Vasoconstriction reduces heat loss
   • Shivering increases heat production
   These responses raise core temperature toward the acceptable range, changing what thermoreceptors detect next.

Information-flow checkpoint: The hypothalamus does not “feel cold” directly; it receives encoded information from sensors (afferent) and sends commands (efferent). The effectors change heat transfer/production, which changes the sensor input on the next cycle.

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Exercise B: Glucose Regulation (Plasma Glucose Concentration)

Scenario: You eat a carbohydrate-rich meal. Plasma glucose rises.

1. Regulated variable (X): plasma glucose concentration.
2. Sensor/Receptor: pancreatic islet cells (especially β-cells) sense elevated glucose (they act as both sensor and part of the control center in many simplified models).
3. Afferent signal (input): intracellular metabolic sensing within β-cells (glucose uptake/metabolism) that represents “glucose is high.” In endocrine loops, the “afferent” step is often local sensing rather than a long nerve pathway.
4. Integrating/Control center: pancreatic islets integrate the sensed glucose level and determine hormone output (insulin secretion). (In a broader model, the brain and gut hormones can modulate this, but keep the core loop simple.)
5. Efferent signal (output): insulin released into the bloodstream (a hormonal command signal).
6. Effector:
   • Skeletal muscle and adipose tissue (increase glucose uptake)
   • Liver (increase glycogen synthesis; reduce glucose output)
7. Response: plasma glucose decreases toward the acceptable range; the β-cells then sense a lower glucose level and reduce insulin output accordingly.

Information-flow checkpoint: In endocrine control, the “wire” is the bloodstream. The efferent signal is the hormone concentration reaching target tissues, and the feedback is the new plasma glucose level returning to the pancreatic sensors.

Practice Table: Fill in the Components

Try to complete the blank cells before checking the filled examples above.

3) Quick Checks: Predict the Failure When a Component Is Impaired

For each prompt, identify (a) what information is missing or wrong, (b) whether the control center’s output increases/decreases appropriately, and (c) what happens to the regulated variable over time.

Quick Check Set 1: Thermoregulation

• Sensor loss (skin thermoreceptors damaged): What changes in afferent information? Would the control center detect cold exposure as strongly? Predict the response (shivering/vasoconstriction) compared with normal.
• Afferent pathway impairment (sensory neuropathy): Sensors may detect temperature, but the signal does not reach the hypothalamus effectively. Predict how the system behaves and why the response may be delayed or absent.
• Control center dysfunction (hypothalamic injury): Afferent information arrives, but integration/decision is faulty. Predict whether efferent commands are appropriate, and what happens to core temperature stability.
• Efferent pathway impairment (autonomic failure): The hypothalamus decides correctly, but commands do not reach effectors. Predict cutaneous blood vessel behavior and shivering.
• Effector failure (skeletal muscle cannot shiver): The command is intact, but the effector cannot execute. Predict which compensations might still occur (e.g., vasoconstriction) and the likely direction of core temperature change in cold.

Quick Check Set 2: Glucose Regulation

• Sensor/control center impairment (β-cell failure): What happens to insulin secretion when glucose rises? Predict plasma glucose after a meal and explain the information-flow break.
• Efferent signal problem (insulin not released or rapidly degraded): The “command” does not reach effectors at adequate levels. Predict glucose uptake in muscle/adipose and the resulting plasma glucose trajectory.
• Effector resistance (target tissues respond poorly to insulin): The efferent signal is present, but effectors do not produce the expected response. Predict what happens to insulin levels over time and to plasma glucose after meals.
• Effector organ limitation (liver cannot store glycogen effectively): Identify which part of the response is weakened and predict the effect on post-meal glucose control.

Mini-Drills: Name the Broken Link

Label the primary failure as Sensor, Afferent, Control center, Efferent, or Effector.

• Core temperature is low; hypothalamus output signals are strong; blood vessels and muscles show no change in activity.
• Plasma glucose is high; insulin level is high; muscle glucose uptake remains low.
• Skin is cold; peripheral thermoreceptors fire normally; hypothalamus receives little temperature-related input.
• Plasma glucose is high; insulin remains low; pancreatic tissue is damaged.