协调与反应

Your body must coordinate 协调 all its parts and react to changes. The nervous system 神经系统 does this using fast electrical impulses 电脉冲 that travel along neurones 神经元 (nerve cells). It has two parts:

• the central nervous system 中枢神经系统 (CNS) — the brain 大脑 and the spinal cord 脊髓.
• the peripheral nervous system 周围神经系统 (PNS) — all the nerves outside the brain and spinal cord.

The nervous system coordinates and regulates 调节 the body's functions.

There are three kinds of neurone:

• sensory neurones 感觉神经元 — carry impulses from receptors to the CNS.
• relay neurones 中间神经元 — pass impulses on inside the CNS.
• motor neurones 运动神经元 — carry impulses from the CNS to effectors.

A reflex 反射 action is a fast, automatic response 反应 to a stimulus 刺激 (a change). It follows a fixed path, the reflex arc 反射弧:

receptor 感受器 → sensory neurone → relay neurone → motor neurone → effector 效应器 (a muscle 肌肉 or gland 腺体)

A synapse 突触 is a junction (a tiny gap) between two neurones. At the end of one neurone are vesicles 囊泡 holding neurotransmitter 神经递质 molecules. Across the synaptic gap 突触间隙, the next neurone has receptor proteins 受体蛋白.

Events at a synapse:

1. an impulse arrives and makes the vesicles release neurotransmitter into the synaptic gap.
2. the neurotransmitter molecules diffuse 扩散 across the gap.
3. they bind to the receptor proteins on the next neurone.
4. this starts a new impulse in the next neurone.

Synapses make impulses travel in one direction only.

Sense organs 感觉器官 are groups of receptor cells that respond to one kind of stimulus — light, sound, touch, temperature or chemicals. The eye responds to light.

Parts of the eye

The pupil reflex

In bright light the pupil gets smaller, to protect the retina. In dim light it gets wider, to let more light in. (Supplement) This is done by two sets of antagonistic 拮抗 muscles in the iris: the circular muscles 环肌 and the radial muscles 辐射肌. In bright light the circular muscles contract and the pupil narrows; in dim light the radial muscles contract and the pupil widens.

Accommodation (Supplement)

Accommodation 视觉调节 is changing the shape of the lens to focus on near or far objects, using the ciliary muscles 睫状肌 and the suspensory ligaments 悬韧带:

• near object: the ciliary muscles contract, the ligaments slacken, and the lens becomes fatter — more refraction.
• far object: the ciliary muscles relax, the ligaments pull tight, and the lens becomes thinner — less refraction.

Rods and cones (Supplement)

The retina has two kinds of light receptor:

• rods 视杆细胞 — very sensitive, good for vision 视觉 in dim light (night vision); they do not detect colour.
• cones 视锥细胞 — three kinds, each absorbing a different colour, giving colour vision; they work best in bright light.

Cones are packed most densely at the fovea 中央凹, the part of the retina that gives the sharpest image.

A hormone 激素 is a chemical, made by a gland and carried in the blood, that changes the activity of one or more target organs 靶器官. Hormones are made by endocrine glands 内分泌腺, which release them straight into the blood.

Adrenaline is released in 'fight or flight' situations, such as fear or danger. It increases the breathing rate, the heart rate 心率, the blood glucose concentration 浓度 and the pupil diameter, getting the body ready to act.

Homeostasis 稳态 is keeping a constant internal environment 环境 inside the body — for example a steady temperature and a steady blood glucose level.

(Supplement) Homeostasis works by negative feedback 负反馈: when a value moves away from its set point 设定点, the body acts to bring it back. The change itself triggers the correction.

Controlling blood glucose

The liver 肝脏 and the pancreas keep blood glucose steady:

• insulin lowers blood glucose after a meal — the liver stores the glucose as glycogen 糖原.
• (Supplement) glucagon raises blood glucose between meals — the liver releases glucose.

In Type 1 diabetes 糖尿病 the pancreas cannot make insulin, so blood glucose rises too high. It is treated with insulin injections and a carefully planned diet.

Worked example. Blood glucose rises sharply after a meal, then falls back to normal over the next two hours. Name the organs and the hormone involved, and say what they do. The pancreas detects the rise and releases insulin. Insulin makes the liver take glucose out of the blood and store it as glycogen, so the level falls back to normal. Between meals the opposite loop runs: the pancreas releases glucagon and the liver releases glucose again. Keep the two words apart - insulin is the hormone, glycogen is the store. Swapping them is the classic error.

Controlling body temperature (Supplement)

The brain detects the body temperature 温度 and keeps it steady. The skin helps:

When you are too hot: you sweat, and the arterioles 小动脉 widen (vasodilation 血管舒张) so more blood flows to the skin capillaries 毛细血管 and loses heat.

When you are too cold: you shiver 颤抖 (the muscles make heat), and the arterioles narrow (vasoconstriction 血管收缩) so less blood reaches the surface, keeping heat in.

A tropism 向性 is a plant's growth response to a direction:

• gravitropism 向地性 — growth in response to gravity 重力 (roots grow down, shoots grow up).
• phototropism 向光性 — growth in response to the direction of light (shoots grow towards the light).

(Supplement) These responses are controlled by a chemical called auxin 生长素:

• auxin is made in the shoot 嫩枝 tip.
• it diffuses down through the plant.
• light or gravity makes the auxin spread unequally to one side.
• in shoots, auxin promotes growth: cells elongate 伸长 (grow longer). The side with more auxin grows faster, so the shoot bends.

Common mistakes.

• Mixing up glucagon (a hormone) and glycogen (the store of glucose in the liver). Check the spelling: the two words differ by one letter.
• Saying insulin turns glucose into glycogen. Insulin is a message carried in the blood; it makes the liver cells change glucose into glycogen.
• (Supplement) Saying an electrical impulse jumps across a synapse. The impulse makes neurotransmitter molecules diffuse across the gap; they bind to receptors and start a new impulse.
• (Supplement) Saying blood vessels move nearer to the skin when you are hot. The vessels do not move: the arterioles widen, so more blood flows through the capillaries near the surface.
• (Supplement) Saying a shoot bends towards light because the lit side grows faster. Auxin collects on the shaded side, so the shaded side grows faster and the shoot bends towards the light.

• Nervous system: CNS (brain + spinal cord) and PNS (the nerves). Impulses travel along neurones.
• Learn the reflex arc in order: receptor → sensory → relay → motor → effector.
• At a synapse, neurotransmitter diffuses across the gap — this makes impulses go one way only.
• Eye: cornea and lens refract light; the lens changes shape (accommodation); rods for dim light, cones for colour.
• Hormones are slower but longer-lasting than nerves. Insulin lowers blood glucose; glucagon raises it.
• Too hot → sweating and vasodilation; too cold → shivering and vasoconstriction.
• Auxin from the shoot tip controls phototropism and gravitropism by making cells elongate.