Nerve Impulses, Synapses and Muscle

The resting potential and the action potential

A resting axon has a potential difference of about $-70$ mV across its membrane, the inside being negative. This is maintained by the sodium–potassium pump, which moves three sodium ions out for every two potassium ions in, and by the fact that the membrane is far more permeable to potassium than to sodium, so potassium leaks out. When a stimulus depolarises the membrane past the threshold of about $-55$ mV, voltage-gated sodium channels open and sodium floods in, reversing the potential to about $+40$ mV. These channels then close and voltage-gated potassium channels open, so potassium leaves and the potential falls again; the overshoot below the resting value is hyperpolarisation. During the refractory period no new impulse can start, which makes the impulse travel in one direction only and sets an upper limit on frequency.

Speed, synapses and the all-or-nothing law

An impulse is faster in an axon of larger diameter and much faster in a myelinated axon, where depolarisation jumps between the nodes of Ranvier — saltatory conduction. The all-or-nothing law means a stimulus either reaches threshold and produces a full-sized impulse, or it does not; a stronger stimulus is signalled by a higher frequency of impulses, not a bigger one. At a synapse, depolarisation opens calcium channels, calcium ions enter, vesicles fuse with the presynaptic membrane and release acetylcholine, which diffuses across and binds to receptors on the postsynaptic membrane, opening sodium channels. Transmission is one way because only the presynaptic neurone has vesicles and only the postsynaptic membrane has receptors.

Muscle and plant responses

A muscle fibre is made of myofibrils divided into sarcomeres between $Z$ discs. In the sliding filament model, calcium ions released from the sarcoplasmic reticulum bind to troponin, moving tropomyosin off the binding sites on actin. Myosin heads attach, tilt to pull the actin past, then detach when ATP binds. The I band and H zone shorten but the A band does not, because the filaments themselves do not change length. In plants, abscisic acid released in water stress causes guard cells to lose solutes, so water leaves them by osmosis, they become flaccid and the stoma closes.