General & Cellular Physiology GP

Homeostasis is dynamic stability, not a motionless state. A regulated variable is kept within a workable range by sensors, an integrating centre and effectors. Negative feedback opposes deviation and is the dominant stabilizing design; positive feedback amplifies a process and therefore requires a terminating event.

Think in loops: disturbance → sensor → integrator → effector → response. Ask what variable is being defended and what signal closes the loop.

Failure of feedback explains many clinical patterns, including poor temperature regulation, endocrine dysregulation and circulatory instability.

Do not confuse a normal range with a fixed point. Biological variables oscillate around operating ranges and may reset in different physiological states.

The plasma membrane creates selective separation. Lipid-soluble molecules may diffuse through the bilayer; ions and many polar molecules require channels or carriers. Facilitated diffusion follows an electrochemical gradient, whereas active transport requires energy directly or indirectly. Osmosis reflects water movement driven by effective solute concentration.

For every transport problem identify: membrane permeability, concentration gradient, electrical gradient, carrier/channel availability and energy source.

Transport defects can alter fluid balance, neuronal excitability, renal handling and intestinal absorption.

Osmolarity and tonicity are related but not identical: tonicity depends on non-penetrating solutes and predicts sustained cell-volume change.

Unequal ion distribution plus selective membrane permeability generates electrical potential. At rest, potassium permeability is usually dominant, while ion pumps maintain the gradients that make the voltage possible. Threshold activation of voltage-gated channels can create regenerative action potentials.

Separate gradient creation from moment-to-moment voltage generation: pumps maintain gradients; selective conductance converts gradients into membrane voltage.

Disturbances of potassium, sodium, calcium or channel function can alter nerve, skeletal-muscle and cardiac excitability.

The sodium-potassium pump is essential for long-term gradients but is not the sole direct cause of the resting membrane potential.

Cells communicate through chemical messengers acting on receptors. Membrane receptors often use second messengers and phosphorylation cascades; intracellular receptors commonly regulate gene transcription. Signal amplification allows small extracellular signals to produce large cellular responses.

Follow the chain: ligand → receptor → transducer → effector → second messenger/target → response → termination.

Receptor agonists, antagonists and signalling defects are central to pharmacology and endocrine disease.

A receptor does not simply switch a cell on or off; response depends on receptor subtype, tissue context, downstream machinery and desensitization.

Homeostasis is the coordinated maintenance of the internal environment within limits compatible with cellular function. It does not mean that temperature, pressure, glucose, pH or ion concentrations remain perfectly fixed. They fluctuate around regulated operating ranges because metabolism, posture, food intake, exercise, sleep and the external environment continuously disturb the body. A physiological control system therefore has to detect a meaningful change, compare or integrate that information, and alter effector activity so that the regulated variable is defended.

A useful control-loop model is stimulus or disturbance → sensor → afferent information → integrating centre → efferent command → effector → response. In negative feedback, the response reduces the initiating error. Body temperature regulation is a classic example: increased core temperature promotes heat-loss responses, while reduced temperature promotes heat conservation and production. Positive feedback is different: it reinforces the initiating event. Because amplification is potentially unstable, positive-feedback processes such as parturition or parts of haemostasis require a defined endpoint.

The clinically important question is not simply whether a measured value is high or low, but why the control system has allowed it to become so. A fever, for example, differs conceptually from uncontrolled hyperthermia because regulation itself has been reset. Likewise, an endocrine concentration can be abnormal because the gland failed, because its controller failed, or because feedback has been altered.

Common confusion: a reference range is not identical to a physiological set point. Many variables have circadian variation, pulsatile secretion or context-dependent operating ranges. The body also uses feed-forward control and anticipatory responses in addition to feedback.

The plasma membrane is a lipid bilayer containing proteins that create selective permeability. Small lipid-soluble molecules can cross the lipid phase relatively easily, whereas ions and many polar molecules require channels or carriers. Net passive movement is driven by an electrochemical gradient and does not require direct metabolic energy. Simple diffusion depends on the driving gradient and permeability; facilitated diffusion additionally depends on carrier availability and therefore can show saturation and competition.

Primary active transport directly couples transport to an energy-releasing process such as ATP hydrolysis. The Na+/K+-ATPase exports sodium and imports potassium, maintaining gradients that support electrical excitability, cell volume and many secondary transport processes. Secondary active transport does not directly split ATP at the transporter; instead it uses energy stored in an ion gradient created elsewhere. Sodium-glucose cotransport is a classic conceptual example.

Water movement must be analysed using effective osmoles. Osmolarity counts osmotically active particles, whereas tonicity predicts the sustained effect of a solution on cell volume and therefore depends mainly on solutes that cannot freely equilibrate across the membrane. A permeant solute may transiently draw water but will not necessarily sustain the same volume effect.

Clinical reasoning: intravenous-fluid effects, diarrhoeal fluid loss, dysnatraemia and many renal disorders become easier when the student first asks which solutes can cross which membrane and where water will redistribute.

Electrical membrane potentials arise because ions are distributed unequally across cell membranes and the membrane is selectively permeable. At rest, many excitable cells are relatively permeable to potassium, so potassium diffusion strongly influences resting voltage. The Na+/K+ pump is essential for maintaining the sodium and potassium gradients over time, but selective conductance is the immediate mechanism translating those gradients into membrane voltage.

An action potential begins when depolarization reaches a threshold at which regenerative inward current exceeds opposing currents. In a typical neuron, rapid opening of voltage-gated sodium channels produces the upstroke; sodium-channel inactivation together with increased potassium conductance then promotes repolarization. Because recently activated sodium channels pass through an inactivated state, the membrane has an absolute refractory period followed by a relative refractory period.

Myelin increases conduction efficiency by reducing membrane current loss and concentrating regenerative channel activity at nodes of Ranvier. The action potential therefore appears to jump functionally from node to node—saltatory conduction. Larger axonal diameter also reduces internal resistance and increases conduction velocity.

Clinical reasoning: extracellular potassium disturbances can alter resting voltage and excitability; channel disorders and demyelination can disturb impulse generation or conduction. The direction of the clinical effect cannot be predicted merely by saying “more potassium means more activity”; one must consider resting depolarization, channel availability and severity.

A chemical signal changes cell behaviour only if the target cell possesses an appropriate receptor and downstream machinery. Hydrophilic messengers usually bind membrane receptors and activate intracellular signalling pathways. G-protein-coupled receptors can alter cyclic nucleotides, phospholipase pathways or ion channels; enzyme-linked receptors can trigger phosphorylation cascades. Lipid-soluble hormones can cross membranes and bind intracellular receptors that modify gene transcription.

Signal transduction is powerful because it permits amplification, branching, integration and termination. One occupied receptor may activate many downstream molecules, but the same extracellular messenger can produce different effects in different tissues because receptor subtype and intracellular machinery differ. Cells can also reduce responsiveness through receptor desensitization, internalization or downstream adaptation.

Clinical reasoning: agonists mimic or activate receptor signalling, antagonists reduce receptor activation, and disease can occur at the level of ligand production, receptor number, receptor function or post-receptor signalling. Understanding the pathway is more useful than memorizing that a messenger is simply “stimulatory” or “inhibitory.”

Teach it back: explain how adrenaline can produce different physiological responses in different tissues even though the circulating molecule is the same.