生活百态：为什么转圈时会头晕

 What makes people dizzy when they spin?

Rolling down a hill or spinning around in endless circles causes dizziness or vertigo¹ because hair-like sensory² nerve cells in our ears send wrong messages to our brain.

The motions of our bodies are detected by the vestibular system found in the upper part of our inner ear. The vestibular system senses whether we are standing³ up or lying down.

When we spin in circles or get up too fast from the couch, we feel dizzy and lightheaded due to the vestibular system's nerve reactions to these unusual motions.

Here's how our vestibular system reacts to a slight change in direction when we bend our heads: The vestibular system senses motion through three semicircular canals that are at right angles to one another. Not only do these canals have hair-like sensory nerve cells, but they also contain a fluid called endolymph.

Following the principle of inertia⁴, endolymph resists changes in motion. As a result of this resistance, the endolymph lags behind and stimulates⁵ hair cells to send nerve signals to the brain. Our brain interprets the nerve messages and knows which direction the head moved.

However, when we spin, our brain receives mixed messages. The endolymph slowly begins to move in the same direction we are spinning. As time goes by, the endolymph catches up to the rate of speed we are spinning and no longer stimulates the hair-like nerve cells. This causes our brain to quickly adapt to the nerve signals. However, when we stop spinning, the endolymph continues to move and stimulates hair cells in the opposite direction.

These hair cells send wrong messages to the brain making it think that the head is still spinning although it actually has stopped. As a result of this inaccurate⁶ signal, we experience dizziness. Eventually, the endolymph stops moving and no more signals are sent to the brain. The brain interprets that the spinning motion has ceased and the dizziness disappears.

note:

vertigo: 晕头转向

sensory nerve: 感觉神经

vestibular system: 前庭系统

lightheaded: 头昏眼花的

fluid: 液体

endolymph: 内淋巴

inertia: 惯性

1 vertigo   

n.眩晕

参考例句：

• He had a dreadful attack of vertigo.他忽然头晕得厉害。
• If you have vertigo it seems as if the whole room is spinning round you.如果你头晕，就会觉得整个房间都旋转起来

2 sensory   

adj.知觉的，感觉的，知觉器官的

参考例句：

• Human powers of sensory discrimination are limited.人类感官分辨能力有限。
• The sensory system may undergo long-term adaptation in alien environments.感觉系统对陌生的环境可能经过长时期才能适应。

3 standing   

n.持续，地位；adj.永久的，不动的，直立的，不流动的

参考例句：

• After the earthquake only a few houses were left standing.地震过后只有几幢房屋还立着。
• They're standing out against any change in the law.他们坚决反对对法律做任何修改。

4 inertia   

adj.惰性，惯性，懒惰，迟钝

参考例句：

• We had a feeling of inertia in the afternoon.下午我们感觉很懒。
• Inertia carried the plane onto the ground.飞机靠惯性着陆。

5 stimulates   

v.刺激( stimulate的第三人称单数 )；激励；使兴奋；起兴奋作用，起刺激作用，起促进作用

参考例句：

• Exercise stimulates the body. 运动促进身体健康。 来自《简明英汉词典》
• Alcohol stimulates the action of the heart. 酒刺激心脏的活动。 来自《现代英汉综合大词典》

6 inaccurate   

adj.错误的，不正确的，不准确的

参考例句：

• The book is both inaccurate and exaggerated.这本书不但不准确，而且夸大其词。
• She never knows the right time because her watch is inaccurate.她从来不知道准确的时间因为她的表不准。