调节与协调

The body has two coordination systems. The endocrine system 内分泌系统 sends chemical hormones 激素 (such as ADH, glucagon and insulin) in the blood. The nervous system 神经系统 sends fast electrical signals along nerve cells.

A neurone 神经元 is a nerve cell. There are three kinds:

• a sensory neurone 感觉神经元 carries impulses from a receptor towards the brain or spinal cord.
• a motor neurone 运动神经元 carries impulses out to an effector, such as a muscle 肌肉.
• intermediate neurones 中间神经元 connect sensory neurones to motor neurones.

A neurone has a long fibre (the axon) along which the impulse travels.

This is what real neurones look like in a stained slice of brain tissue:

A sensory receptor 感受器 cell detects a stimulus 刺激 and starts an impulse in a sensory neurone. For example, a chemoreceptor 化学感受器 cell in a taste bud 味蕾 detects chemicals in food, and this triggers an action potential 动作电位 in the sensory neurone.

The impulse is a change in the voltage across the neurone's membrane.

• resting potential 静息电位 — when no impulse passes, the inside is negative compared to the outside. This is maintained by a pump that moves sodium ions out and potassium ions in, and by the membrane being more permeable to potassium.
• action potential — a stimulus makes sodium channels open, so sodium ions rush in and the inside briefly becomes positive (depolarisation). Then the sodium channels close and potassium channels open, potassium ions leave, and the membrane potential 膜电位 returns to negative (repolarisation). The potassium channels are slow to close, so for a moment the inside becomes even more negative than at rest: this is hyperpolarisation 超极化.
• refractory period 不应期 — just after an action potential, the sodium channels cannot open again for a short time while the membrane returns to its resting potential. So each impulse is separate (discrete), travels in one direction only, and there is a limit on how often impulses can be sent (their frequency).

Faster impulses: myelin

Some neurones are wrapped in a fatty myelin sheath 髓鞘. The impulse cannot cross the sheath, so it jumps from one gap to the next. This jumping, called saltatory conduction 跳跃式传导, makes the impulse travel much faster.

Common mistakes.

• Saying the sodium–potassium pump causes depolarisation. The pump keeps up the resting potential; depolarisation is caused by sodium ions diffusing in through voltage-gated channels 电压门控通道.
• Saying the ions are "pumped" during an action potential. Na$^+$ and K$^+$ move by facilitated diffusion 易化扩散 through channels, down their concentration gradients.
• Saying a stronger stimulus gives a bigger action potential. Every action potential has the same size (all-or-nothing); a stronger stimulus gives more frequent impulses.
• Saying the impulse "jumps along the myelin". It jumps between nodes of Ranvier, where there is no myelin and ions can cross the membrane.

A synapse 突触 is a tiny gap between two neurones. A cholinergic synapse 胆碱能突触 passes the signal like this (the numbers match the figure):

1. an action potential arrives at the end of the first (presynaptic) neurone.
2. the depolarisation opens voltage-gated calcium ion 钙离子 channels, and calcium ions diffuse in.
3. this makes vesicles move to the membrane and fuse with it, releasing the neurotransmitter 神经递质 acetylcholine 乙酰胆碱 (ACh) into the gap by exocytosis 胞吐作用.
4. ACh diffuses across the gap and binds to receptors on the next (postsynaptic) neurone.
5. the receptors open sodium ion channels. Sodium ions diffuse in and depolarise the membrane; if the threshold is reached, a new action potential starts.
6. the enzyme acetylcholinesterase 乙酰胆碱酯酶 breaks ACh down, so it stops stimulating the receptors. The choline is taken back into the first neurone and used to make more ACh.

Because only the presynaptic side has vesicles and only the postsynaptic side has receptors, a synapse passes impulses in one direction only.

A neuromuscular junction 神经肌肉接头 is like a synapse, but between a motor neurone and a muscle.

Striated muscle 横纹肌 is made of long fibres. Each fibre is divided into repeating units called sarcomeres 肌节. Inside are two kinds of filament 肌丝: thick ones (myosin) and thin ones (actin). The T-tubule T小管 system carries the impulse deep into the fibre, and the sarcoplasmic reticulum 肌质网 stores and releases calcium ions.

The sliding filament model 肌丝滑动模型 explains contraction:

1. an impulse causes the sarcoplasmic reticulum to release calcium ions.
2. the calcium ions bind to troponin 肌钙蛋白, which makes tropomyosin 原肌球蛋白 move and uncover the binding sites on the actin.
3. the myosin heads attach to the actin and pull it inwards, using energy from ATP.
4. the thin filaments slide over the thick ones, so each sarcomere gets shorter and the muscle contracts.

The Venus fly trap

The Venus fly trap 捕蝇草 has tiny hairs on its trap-like leaves. When an insect touches the hairs, a fast electrical signal spreads, the cells quickly lose water and change shape, and the trap snaps shut to catch the insect.

Auxin and growth

Auxin 生长素 is a plant hormone that makes cells grow longer (elongation 伸长 growth). It does this by making the cell pump protons 质子 (hydrogen ions) into the cell wall 细胞壁. This acidifies 酸化 the wall, which loosens it, so the cell can stretch as water enters.

Gibberellin and germination

Gibberellin 赤霉素 controls the germination 萌发 of barley 大麦 seeds. It switches on the genes that make amylase, which then breaks down stored starch into sugars to feed the growing seedling.

Worked example. Explain why a nerve impulse crosses a synapse in one direction only, and why myelination speeds it up along an axon. At a synapse the vesicles of neurotransmitter are found only in the presynaptic neurone, and the receptors only on the postsynaptic membrane - so transmitter can only ever cross one way, which makes the synapse a one-way valve. Along a myelinated axon the myelin sheath insulates the membrane, so ions can only cross at the nodes of Ranvier; the impulse therefore jumps from node to node (saltatory conduction) instead of depolarising every part of the membrane in turn, which is far faster and uses less ATP. Locate the structure responsible for each effect: vesicles and receptors give the direction, insulation and nodes give the speed.

• Explain the resting potential (Na+/K+ pump; more negative inside) then the action potential (depolarisation → repolarisation) as all-or-nothing.
• Saltatory conduction (myelin, nodes of Ranvier) speeds the impulse; the refractory period makes it one-way and discrete.
• Synapse order: Ca2+ in → vesicles fuse → neurotransmitter → receptors → new impulse.
• Muscle contraction: the sliding-filament model (actin, myosin, ATP, Ca2+).