The nervous system

Neurons are among the most excitable cells in the body. As a group, they respond to a wide range of electrical, chemical, thermal, or mechanical stimuli, transmitting messages to one another, to muscles, and to endocrine organs (hormone-secreting glands). Together, all the neurons and their supporting cells (glial cells) compose the central nervous system.

Neurons do not exist in sponges (phylum Porifera) or more primitive organisms. The first neurons appear among the coelenterates (phylum Cnidaria), which include jellyfish, hydra, and anemones. Of all living organisms, coelenterates have the simplest nervous arrangement, with only two types of nerve cells: receptor-conductor cells (those that respond to the stimuli and pass it on) and effector cells (those that contract when the stimulus reaches them). They have none of the alternative types that allow for the increased flexibility of response typical of higher organisms.

As the neurons in a nervous system increase in number, so does the complexity of behavioral responses an animal can have. Since one neuron communicates with nearby neurons, which in turn communicate with other neurons, the total number of possible neuron connections increases exponentially as the total number of neurons increases.

A roundworm (phylum Nematoda) is an organism that moves very little. It has only about 160 neurons. The leech (phylum Annelida), slightly more mobile, has about 13,000 nerve cells. An octopus (phylum Cephalopoda), which has considerable control over its movements and behavior, has over 1 billion neurons. And humans (phylum Chordata) have more than 10 billion neurons.

Changes in the physical or chemical environment (i.e., due to motion, sound, light, heat, or chemicals) can be converted into nervous impulses. The environmental change is known as the stimulus, and the neuronal response is the neural impulse. When part of the nervous system receives a neural impulse, it may respond by sending another impulse to the appropriate effectors. Many effectors are muscles, which respond by contracting. However, there are many other types of effectors, such as photoreceptor cells or glandular cells. In addition, the nervous impulse may reach another neuron, which triggers it to the next neuron, and so on, although such an impulse may eventually dissipate to the point that it can longer elicit a response from an effector.

II. POST-READING TASKS.

1. Give the definition to the concept “neural impulse” and explain its function.

2. Sum up what you have learned about neurons and prepare a report on the topic: “The neuron is an essential constituent of the central nervous system in organisms of different types”.

UNIT 6

PRE-READING AND READING TASKS.

Study the meaning and pronunciation of these words:

anchor (v)	['xNkq]	закреплять, фиксировать
marrow	['mxrqV]	костный мозг
muscularity	["mAskjV'lxrItI]	мускулатура
musculature	["mAskjV'lxrItI]	мускулатура
pelvis	['pelvIs]	таз, тазовые кости
periosteum	["perI'PstIqm]	надкостница
spine	[spaIn]	позвоночник,позвоночный столб
rib	[rIb]	ребро
female	['fi:meIl]	женский
fragile	['frxdZaIl]	хрупкий, ломкий
hollow	['hPlqV]	полый, пустой
male	[meIl]	мужской
muscular	['mAskjVlq]	мышечный
spongy	['spAndZI]	пористый, губчатый
underneath	["Andq'ni:T]	ниже

Try to guess the meaning of the following words and phrases:

to fit one’s function	to be responsible for smth
blood vessels	to range from … to…
compact bone	gluteus maximus

Read and translate the text.

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