Stimulus-Response, Reflexes and Homeostasis — Study Notes Summary & Study Notes

🏠 Homeostasis

Homeostasis is the maintenance of a relatively constant internal environment inside the body. The body uses sensors, processing centers and effectors to keep variables (like temperature and water balance) near optimal set points.

🌡️ Temperature regulation

Body temperature is normally around 37°C. If core temperature rises, the body activates cooling responses such as sweating and vasodilation (capillaries widen so blood is closer to the skin). An evolutionary example is the large, blood-rich ears of some elephants which help dissipate heat.

If the body gets too cold the hypothalamus (acts like a thermostat) triggers warming responses such as shivering. Shivering is repeated muscle contractions that generate heat to raise body temperature.

💧 Water regulation

Water balance is controlled to prevent dehydration. The hypothalamus detects dehydration and signals the pituitary gland to release antidiuretic hormone (ADH). ADH travels in the bloodstream to the kidneys, making them conserve water and produce smaller volumes of more concentrated (darker, yellower) urine.

🔁 Stimulus-response model

A stimulus is any change that elicits a response. Example: a ball thrown at you is the stimulus; raising your hands to catch it is the response. Information flows from receptors to the brain, which interprets the signal and issues commands to effectors.

Stimulus-response sequences can be simple or involve many steps. For example, feeling thirsty (stimulus) can trigger a multi-step response: decide to get water, walk to the kitchen, fill a glass and drink.

🔗 Sequential and chained responses

One stimulus-response event can trigger another. Example: something touches your shoulder (mechanoreceptors) and you turn to look (response). Seeing a spider becomes a new stimulus that may trigger fear and screaming. Complex behavior often arises from linked stimulus-response events.

⚡ Reflexes (automatic responses)

Reflexes are rapid, automatic responses that often bypass the brain for speed. In a withdrawal reflex (e.g., touching a hot pan) nociceptors (pain receptors) in the skin send signals along sensory neurons to the spinal cord, where a relay (interneuron) connects directly to motor neurons. Motor neurons activate muscles to pull the limb away before the brain has fully processed the pain.

Reflex arcs protect the body by producing very fast reactions; conscious awareness usually follows the reflex.

🧠 Key receptors and pathways

• Thermoreceptors: detect temperature changes in the skin.
• Mechanoreceptors: detect pressure and touch.
• Nociceptors: detect painful stimuli.
• Sensory pathway: carries receptor signals toward the central nervous system.
• Relay (inter) neuron: in the spinal cord can link sensory input directly to motor output for reflexes.
• Motor pathway: carries commands from the CNS to muscles.

✅ Summary

Homeostasis keeps internal conditions stable using sensors (receptors), controllers (hypothalamus/pituitary) and effectors (sweat glands, muscles, kidneys). The stimulus-response model explains how inputs produce actions, while reflexes are special, fast stimulus-response circuits that prioritize rapid protection over conscious processing.