Control and coordination in mammals

Biology A2 · Control and coordination. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Mammals have two coordination systems. The nervous system sends fast electrical impulses along neurones to particular cells. The endocrine system sends hormones in the blood; it is slower, but the effects last longer and are more widespread.

An impulse is an action potential: a brief reversal of the potential across the neurone membrane, caused by ions moving through channels. A receptor cell starts it. In a taste bud, for example, sodium ions from salt diffuse into a chemoreceptor cell and depolarise it.

Impulses pass from one cell to the next at a synapse. At a neuromuscular junction they make striated muscle contract by the sliding filament mechanism.

Rules to remember

Resting potential (about −70 mV): the sodium–potassium pump uses ATP to move 3 Na⁺ out and 2 K⁺ in, and the membrane is more permeable to K⁺, which diffuses out
Action potential: voltage-gated Na⁺ channels open and Na⁺ diffuses in (depolarisation); then they close, voltage-gated K⁺ channels open and K⁺ diffuses out (repolarisation)
An action potential happens only if the threshold is reached, and it is always the same size; a stronger stimulus gives a higher frequency of impulses
Refractory period: no new action potential can start, so impulses stay separate, travel one way and have a maximum frequency (1 ÷ refractory period)
Myelin insulates the axon, so action potentials occur only at nodes of Ranvier and the impulse jumps from node to node (saltatory conduction)
Cholinergic synapse: impulse arrives → Ca²⁺ enters the presynaptic knob → vesicles release acetylcholine → it binds to receptors, Na⁺ channels open in the postsynaptic membrane → acetylcholinesterase breaks it down
Muscle: impulse passes down T-tubules → sarcoplasmic reticulum releases Ca²⁺ → Ca²⁺ binds to troponin → tropomyosin moves off the binding sites on actin → myosin heads attach and pull; ATP is needed for the heads to detach and reset

Common mistake

When a muscle contracts the filaments do not get shorter; they slide past each other. The A band stays the same length, while the I band and the H zone get shorter.

Worked example

In a muscle the thick filaments are 1.6 µm long. A sarcomere shortens from 2.5 µm to 2.1 µm. What is the total length of I band in one sarcomere before and after?

The A band is the length of the thick filaments, 1.6 µm, and it does not change.
I band = sarcomere length − A band. Before: 2.5 − 1.6 = 0.9 µm.
After: 2.1 − 1.6 = 0.5 µm.

Answer: The I band shortens from 0.9 µm to 0.5 µm; the A band stays at 1.6 µm.

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