Responding to stimuli and homeostasis

Homeostasis – Keeping the Internal Environment Stable

• **Homeostasis** is the maintenance of a stable internal environment despite external changes.
• It ensures optimal conditions for enzyme activity and cell function.
• Key variables regulated include **body temperature**, **blood glucose**, **pH**, and **fluid balance**.
• Homeostatic mechanisms involve **receptors**, **control centres**, and **effectors**.
• Most homeostatic control uses **negative feedback** to reverse a change.

Components of a Homeostatic Control System

• **Receptor** – detects a change (stimulus) in the environment (e.g., thermoreceptors detect temperature change).
• **Control centre** – processes the information and determines the response (e.g., hypothalamus in the brain).
• **Effector** – carries out the response to restore balance (e.g., muscles, glands, or organs).
• The response feeds back to the receptor to reduce or stop the stimulus (**negative feedback**).

Negative Feedback – The Core Mechanism

• **Negative feedback** counteracts deviations from the set point, bringing the variable back to normal.
• Example: Body temperature rises → receptors send signal to brain → effectors (sweat glands) cool the body → temperature drops.
• Example: Blood glucose rises after eating → pancreas releases **insulin** → cells take up glucose → glucose level falls.
• Most homeostatic systems rely on negative feedback to maintain stability.

The Nervous System – Rapid Responses

• The **nervous system** detects stimuli and sends electrical signals (**nerve impulses**) for rapid, short-lived responses.
• **Sensory neurones** carry impulses from receptors to the central nervous system (CNS – brain and spinal cord).
• **Motor neurones** carry impulses from the CNS to effectors (muscles or glands).
• **Reflex arcs** are automatic, rapid responses that bypass the brain for speed (e.g., pulling hand from a hot object).

The Endocrine System – Slower, Longer-Lasting Responses

• The **endocrine system** uses **hormones** (chemical messengers) released into the blood to target cells.
• Hormones act more slowly than nerve impulses but have longer-lasting effects.
• Example: **Adrenaline** prepares the body for 'fight or flight' – increases heart rate, breathing rate, and blood glucose.
• Example: **Insulin** and **glucagon** regulate blood glucose levels via negative feedback.

Coordination of Nervous and Endocrine Systems

• The **hypothalamus** links the nervous and endocrine systems; it controls the pituitary gland.
• The **pituitary gland** releases hormones that regulate other endocrine glands (e.g., thyroid, adrenal).
• Together, the two systems coordinate responses to internal and external stimuli to maintain homeostasis.

Examples of Homeostatic Regulation

• **Thermoregulation**: Body temperature kept around 37°C via sweating, shivering, and blood vessel dilation/constriction.
• **Osmoregulation**: Water and salt balance maintained by the kidneys; antidiuretic hormone (ADH) controls water reabsorption.
• **Blood glucose regulation**: Insulin lowers glucose; glucagon raises glucose; both are produced by the pancreas.
• **pH regulation**: Buffers in blood and respiration control CO₂ levels to keep pH near 7.4.

Stimuli and Receptors

• A **stimulus** is any change in the internal or external environment that triggers a response.
• **Receptors** are specialised to detect specific stimuli: **thermoreceptors** (temperature), **mechanoreceptors** (pressure), **chemoreceptors** (chemicals), **photoreceptors** (light).
• Receptors convert stimulus energy into electrical signals (action potentials) for the nervous system.

A spinal reflex arc showing the pathway: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector (muscle). This is an example of a rapid, involuntary response.

Structure of a motor neurone: dendrites, cell body, axon, myelin sheath, nodes of Ranvier, and axon terminals. Neurones transmit electrical impulses for rapid responses.

Negative feedback loop in thermoregulation: a rise in body temperature is detected by receptors, the hypothalamus coordinates a response (sweating), which cools the body and reduces the initial stimulus.