Control and coordination

📚The biology

Two coordination systems

The endocrine system consists of glands that secrete hormones directly into the blood, which carries them everywhere; only cells with a complementary receptor respond. ADH, glucagon and insulin (Topic 14) are the syllabus’s examples.

• Nervous — electrical impulses along neurones; very fast; travels to a specific target; effect is short-lived; effectors are muscles or glands.
• Endocrine — chemical messengers in the blood; slower; potentially widespread; effect is longer-lasting; effectors are target cells with receptors.

Neurones

A sensory neurone carries impulses from a receptor to the central nervous system; its cell body lies to one side of the axon, on a branch. A motor neurone carries impulses from the CNS to an effector; its cell body is at one end with many dendrites. Intermediate neurones connect the two. Many axons are wrapped in a myelin sheath of Schwann cells, with gaps called nodes of Ranvier.

Receptor cells detect a stimulus and convert its energy into an electrical response — they are transducers. In a chemoreceptor of a taste bud: a chemical binds to a receptor protein in the membrane, which opens channels so that sodium ions enter; the cell depolarises, producing a receptor potential; if this reaches threshold, calcium channels open, calcium enters, and neurotransmitter is released onto the sensory neurone, which then generates an action potential.

Resting potential

The inside of the axon is about −70 mV relative to the outside. It is maintained by:

• Sodium–potassium pumps, which use ATP to move 3 Na⁺ out for every 2 K⁺ in, so more positive charge leaves than enters;
• the membrane being much more permeable to K⁺ than to Na⁺, so potassium leaks back out down its gradient, taking positive charge with it;
• negatively charged proteins inside the axon that cannot leave.

Action potential

1. Depolarisation. A stimulus opens some voltage-gated sodium channels. Na⁺ enters, making the inside less negative. If the potential reaches the threshold of about −55 mV, many more sodium channels open — a positive feedback — and the potential rises rapidly to about +40 mV.
2. Repolarisation. Sodium channels close; voltage-gated potassium channels open, so K⁺ leaves and the inside becomes negative again.
3. Hyperpolarisation. Potassium channels are slow to close, so a little too much K⁺ leaves and the potential briefly falls below −70 mV.
4. Recovery. The sodium–potassium pump restores the original ion distribution and the resting potential.

Action potentials are all-or-nothing: below threshold nothing happens; at or above it, the size of the action potential is always the same. A stronger stimulus is therefore coded not by a bigger impulse but by a higher frequency of impulses, and by more neurones firing.

The refractory period

Immediately after an action potential the sodium channels are inactivated and cannot reopen, so no further action potential can be generated for a short time. Two consequences:

• it sets an upper limit on impulse frequency, and so on the maximum stimulus intensity that can be coded;
• it ensures impulses travel in one direction only, because the region just behind cannot be re-excited, and it keeps them discrete rather than merging.

Saltatory conduction

In a myelinated axon the myelin sheath is an electrical insulator, so ions cannot cross the membrane except at the nodes of Ranvier. The local currents therefore flow from node to node and the action potential jumps between them — saltatory conduction. Since the membrane is depolarised only at the nodes rather than along its whole length, transmission is much faster (and uses less ATP, since fewer ions must be pumped back).

The cholinergic synapse

1. An action potential arrives at the presynaptic knob.
2. Voltage-gated calcium channels open and Ca²⁺ enters.
3. Calcium causes vesicles of acetylcholine to fuse with the presynaptic membrane and release the transmitter into the synaptic cleft by exocytosis.
4. Acetylcholine diffuses across and binds to receptors on the postsynaptic membrane.
5. Sodium channels open, Na⁺ enters, and if threshold is reached a new action potential is generated.
6. Acetylcholinesterase in the cleft hydrolyses acetylcholine, and the products are reabsorbed and resynthesised using ATP. Without this the postsynaptic neurone would fire continuously.

Synapses ensure one-way transmission — vesicles are only on the presynaptic side and receptors only on the postsynaptic side.

Muscle: structure

A muscle fibre contains many myofibrils, divided along their length into sarcomeres between Z discs. Each sarcomere contains thin actin filaments anchored to the Z discs and thick myosin filaments in the middle.

• I band — actin only, appears light.
• A band — the full length of the myosin, including where actin overlaps it; appears dark.
• H zone — the middle of the A band, myosin only, no overlap.

The fibre membrane (sarcolemma) folds inwards as T-tubules that carry the action potential deep into the fibre, and the sarcoplasmic reticulum that surrounds each myofibril stores calcium ions.

Muscle: the sliding filament model

1. An action potential arrives at the neuromuscular junction, acetylcholine is released, and an action potential spreads over the sarcolemma and down the T-tubules.
2. The sarcoplasmic reticulum releases Ca²⁺ into the sarcoplasm.
3. Ca²⁺ binds to troponin, changing its shape so that it moves tropomyosin away from the myosin-binding sites on the actin.
4. Myosin heads bind to actin, forming cross-bridges.
5. The heads tilt (the power stroke), pulling the actin past the myosin towards the centre of the sarcomere; ADP and phosphate are released.
6. ATP binds to the myosin head, causing it to detach from actin.
7. ATP is hydrolysed, and the energy released returns the head to its upright position, ready to bind further along. The cycle repeats while calcium is present.
8. When stimulation stops, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum, tropomyosin covers the binding sites again, and the muscle relaxes.

ATP is needed twice over: for detachment of the head, and for repumping calcium. This is why rigor mortis occurs — no ATP means the heads cannot detach.

Control and coordination in plants

The Venus fly trap. Sensory hairs on the lobes of the modified leaves are bent by an insect. Bending generates a receptor potential; if a hair is touched twice, or two hairs are touched, within a short interval, an action potential is triggered and spreads across the lobes. This causes a rapid loss of turgor in cells on the inner surface and a gain on the outer, together with acid-induced wall loosening and rapid cell expansion, so the lobes snap shut. Requiring two stimuli prevents the trap closing on a raindrop.

Auxin. Auxin promotes elongation growth by stimulating proton pumping into the cell wall. The resulting low pH activates enzymes that loosen the bonds between cellulose microfibrils, so the wall becomes more plastic and the cell expands as water enters by osmosis.

Gibberellin in barley. On germination, the embryo produces gibberellin, which diffuses to the aleurone layer and switches on genes for amylase. Amylase is secreted into the endosperm and hydrolyses the stored starch to maltose, which is absorbed by the embryo and used for respiration and growth. The gene control mechanism — gibberellin causes the breakdown of DELLA protein repressors, releasing transcription factors — belongs to Topic 16.