Homeostasis, negative feedback, the nervous system and the action potential

Homeostasis is the body holding a variable near a set point, and the machinery is negative feedback: a receptor senses the change, a control centre compares it with the set point, and an effector pushes the variable back the other way. The nervous system is the fast wiring of those loops, and its one message is the action potential, a reversal of a neuron's membrane voltage from about −70 mV to a positive peak and back in a couple of milliseconds, carried by sodium rushing in and potassium rushing out. Below, one loop traced on body temperature, one action potential traced with the millivolts, and the two swaps that lose the marks.

What is homeostasis in biology: a set point and a loop with five parts

Homeostasis is the maintenance of a stable internal environment. Not a fixed one: every variable drifts a little above and below its set point, and the loop keeps pulling it back. The loop has five parts, and a test asks for all five.

1. Stimulus. The variable moves away from the set point.
2. Receptor. A sensor detects the change (thermoreceptors for temperature, osmoreceptors for water concentration, cells that sense glucose).
3. Control centre. It compares the reading with the set point and sends the instruction. For many loops in mammals this is the hypothalamus.
4. Effector. A muscle or a gland that acts.
5. Response. The action changes the variable, back toward the set point, which removes the stimulus and switches the response off.

The loop runs continuously, so the variable oscillates around the set point rather than sitting on it. The examples the course uses: body temperature (traced below), blood glucose and water balance (both hormonal, on the endocrine and kidney page), and blood pH.

Negative feedback traced: body temperature after a run

Trace the five parts on the loop every student has felt.

• Stimulus: core temperature rises above the set point of about 37 °C during a run.
• Receptor: thermoreceptors in the skin and in the hypothalamus itself detect the rise.
• Control centre: the hypothalamus compares the reading with the set point and signals the effectors.
• Effectors: sweat glands secrete sweat, which cools the skin as it evaporates; the arterioles in the skin dilate, so more warm blood flows near the surface and loses heat; and you stop and drink, which is a behavioural effector.
• Response: heat is lost, core temperature falls toward 37 °C, the stimulus is removed, and sweating and dilation ease off.

The cold half runs the same loop the other way: temperature falls, the hypothalamus signals shivering (muscle contraction generates heat) and vasoconstriction (less blood near the skin), and the temperature rises back.

The test sentence: negative feedback reverses the change. The response opposes the stimulus. If your answer describes the five parts and ends with "which brings the variable back toward the set point", the mark is yours.

Negative vs positive feedback: the loop that amplifies, and why it is rare

In positive feedback the response increases the stimulus, so the loop runs faster and faster until an end event stops it. The classic example is labour: the baby's head stretches the cervix, the stretch signals oxytocin release, oxytocin strengthens the contractions, the contractions stretch the cervix more, and the loop ends with the birth. Blood clotting is the other example the course names: the first clotting factors activate more, and the cascade ends when the clot is formed.

The swap: positive is not "good" and negative is not "bad". The words say only whether the response opposes the change (negative) or reinforces it (positive). Almost every homeostatic loop is negative because a variable that is to be held steady needs a response that pushes back. Positive loops exist for the few processes that need to be driven to completion.

The nervous system in one screen

The central nervous system is the brain and spinal cord. The peripheral nervous system is everything else: sensory neurons carrying information in, motor neurons carrying instructions out. The motor side is split into the somatic system (skeletal muscle, voluntary) and the autonomic system (organs and glands, involuntary), and the autonomic into the sympathetic branch (the body's alarm setting) and the parasympathetic (rest and digestion).

A neuron has dendrites (receive signals), a cell body, an axon (carries the signal away), often a myelin sheath around the axon with gaps between the segments, and axon terminals that pass the signal on at synapses.

The simplest loop is the reflex arc: receptor → sensory neuron → interneuron in the spinal cord → motor neuron → effector. The knee-jerk is the shortest version, so short that it skips the interneuron: the sensory neuron connects straight to the motor neuron in the spinal cord. A tap on the tendon below the kneecap stretches the thigh muscle, the sensory neuron fires, and the muscle contracts before the brain has heard about it. That is a homeostatic loop at its fastest, and the signal running along each of those neurons is the same one: the action potential.

The resting membrane potential: about −70 mV, and the sodium potassium pump that keeps it

A resting neuron's inside is negative relative to its outside, by about 70 millivolts: the resting membrane potential is about −70 mV. (A potential difference in millivolts is the same quantity as the volts across a battery in electric fields; the scale is just smaller.) Three things set it.

First, potassium leaks out. K⁺ is concentrated inside the cell and leak channels let it drift out down its gradient, carrying positive charge away. Second, large negatively charged proteins inside cannot leave. Third, the sodium potassium pump keeps the gradients in place: for each ATP it spends, it moves 3 Na⁺ out of the cell and 2 K⁺ in, against their gradients, so sodium stays high outside and potassium high inside. Say what the pump is for now, before the trace: it maintains the gradients the action potential will spend. It is not the thing that makes the voltage fall during the signal.

The action potential traced: one message, five steps, the millivolts

An action potential is all-or-none: below threshold nothing happens, and at threshold the full event runs the same way every time. The axes of the graph are membrane voltage in mV against time in ms.

1. Depolarisation to threshold. A stimulus (a signal at a synapse, or the action potential arriving from the stretch of axon behind) lets some positive charge in and the voltage rises from about −70 mV toward the threshold, about −55 mV. A stimulus that does not reach threshold fades and nothing fires.
2. The rising phase. At threshold, voltage-gated Na⁺ channels open. Sodium is high outside and the inside is negative, so Na⁺ rushes in and the inside goes positive, to a peak a few tens of millivolts above zero; textbooks print about +30 to +40 mV. This is depolarisation.
3. The falling phase. At the peak the Na⁺ channels inactivate and voltage-gated K⁺ channels open. Potassium is high inside and the inside is now positive, so K⁺ rushes out, carrying positive charge away, and the voltage falls back toward −70 mV. This is repolarisation, and it is potassium leaving through channels.
4. The undershoot. The K⁺ channels close slowly, so a little too much potassium leaves and the voltage dips below the resting value for a moment. This is hyperpolarisation, and the stretch of membrane is in its refractory period: its Na⁺ channels are inactivated and it cannot fire again yet. That is why the signal travels one way, forward along the axon into membrane that has not fired.
5. Back to rest. The K⁺ channels close, the leak channels bring the voltage back to about −70 mV, and the sodium potassium pump, running all the time, keeps the gradients topped up for the next signal.

The signal moves along the axon because each depolarised patch brings the next patch to threshold. With a myelin sheath the action potential jumps from one gap to the next (saltatory conduction), which is much faster than depolarising every patch in turn.

At the end of the axon, a chemical synapse in four lines: the action potential opens Ca²⁺ channels in the terminal; Ca²⁺ in makes vesicles release neurotransmitter into the gap; the neurotransmitter binds receptors on the next cell and changes its voltage; and the neurotransmitter is removed, by an enzyme or by reuptake, so the signal is brief.

The two things students swap

Which ion does what. Na⁺ moving in makes the rise; K⁺ moving out makes the fall. Both are positive ions, so the direction is everything: sodium in, up; potassium out, down. A mnemonic helps only if it carries the direction with the ion.

The pump and the fall. Repolarisation is potassium leaving through voltage-gated channels. The pump does not repolarise the membrane; it maintains the gradients afterwards and all the time, 3 Na⁺ out and 2 K⁺ in per ATP. "The sodium-potassium pump repolarises the neuron" is the sentence a marker circles.

A third, from above: positive and negative feedback as good and bad.

Check: the graph cold, then one sibling

With the page closed, draw the action potential with the axes labelled (mV against ms), mark the resting value, the threshold, the peak and the undershoot, and label which channel opens at each turn and which ion moves which way. Write the five-part temperature loop in five lines. Write one positive-feedback example and what ends it. Then open the page and mark it; Karpicke and Blunt (2011) found that drawing and recalling from memory produced more learning than re-studying the figure, and how to study biology has the redraw-from-memory routine for a whole unit.

Then a sibling, cold: trace the five parts of the loop for a fall in core temperature on a winter walk, and name the ion and its direction at the rise and at the fall of the action potential.

Answer. Stimulus: core temperature falls below about 37 °C. Receptor: thermoreceptors in the skin and hypothalamus. Control centre: the hypothalamus. Effectors: skeletal muscles shiver and skin arterioles constrict (and you put on a coat). Response: heat is made and kept, temperature rises toward the set point, the stimulus is removed. Rise: Na⁺ in. Fall: K⁺ out. If you wrote "the pump" at the fall, that is tonight's card.

Where this sits in the course

These are ideas 22 and 23 of the 27 in the SBI4U course map: Homeostasis and negative feedback, and The nervous system and the action potential, the opening of strand E. Before them idea 21, Patterns of inheritance, closes strand D. After them come idea 24, The endocrine system and hormonal control, which runs the same loops with hormones over minutes instead of milliseconds, and idea 25, The kidney, osmoregulation and excretion. And at the molecular scale, the first strand's feedback inhibition, where a pathway's product slows its first enzyme, is this page's loop on one enzyme.

Homeostasis and the action potential on one screen

• Five parts: stimulus, receptor, control centre, effector, response.
• Negative feedback opposes the change; positive feedback reinforces it. Neither means good or bad.
• The voltages: rest about −70 mV; threshold about −55 mV; a positive peak (about +30 to +40 mV in the textbooks); a brief undershoot below rest.
• Sodium in, up; potassium out, down.
• The pump's job: 3 Na⁺ out, 2 K⁺ in, per ATP; it maintains the gradients and does not repolarise.
• The two swaps: which ion makes the fall; the pump against the K⁺ channels.