Sensitivity: responding to stimuli

All living organisms must be able to detect changes in their environment and respond appropriately. Changes in the environment are called stimuli(singular: stimulus). A stimulus may be in either the external environment (outside the organism) or the internal environment (inside the organism). Sensitivity,the ability to respond appropriately to stimuli, is one of the characteristic features of life. Each organism has its own specific type of sensitivity that improves its chances of survival. A single-celled amoeba, for example, can move away from a harmful stimulus such as very bright light, and move towards a favourable stimulus such as food molecules, but it can only distinguish between a limited number of different stimuli.

In an amoeba, the detection of the stimulus and the response to the stimulus must both take place in a single cell. However, in large multicellular animals such as mammals, stimuli are detected in sense organs,and organs that respond are called effectors. The sense organs and effectors may be in quite different parts of the body. In addition, responses usually involve the coordinated actions of many different parts of the body. To achieve this coordination, one part of the body must be able to pass information to another part. In mammals, there are two major systems that convey information: the nervous system and the endocrine (hormonal) system.

The nervous system

Nervous systems range from the simple nerve nets of jellyfish and sea anemones, which have no brain and relatively few interconnections, to the nervous system of humans, with brains of staggering complexity. The human brain contains many millions of cells, each of which may communicate with thousands of other nerve cells. Their interconnections form circuits which enable us to control our muscles, think, remember, and even study our own brains.

All the various animal nervous systems are fast-acting communication systems containing nerve cells, neurones,which convey information in the form of nerve impulses (electrochemical changes). Neurones take various forms but each has a cell body,containing a nucleus, and nerve fibres,long extensions that transmit nerve impulses rapidly from one part of the body to another. Fibres carrying impulses away from the cell body are called axons;those carrying impulses towards the cell body are called dendrons.Apart from the main nerve fibre, there may be small dendrons (dendrites)extending from the cell body.

In mammals, sensory neuronescarry messages from peripheral sense organs to a central nervous system(CNS) consisting of the brain and spinal cord. The CNS acts as an integration centre and processes information from many sources. Motor neuronesconvey instructions from the CNS to effector organs (mainly muscles and glands).

A mammalian motor neurone can convey information rapidly over considerable distances; for example, a single nerve impulse may be transmitted from the spinal cord to the feet in a few milliseconds. These fast-conducting neurones are enclosed along most of their length by a thick insulating material called the myelin sheath. The myelin sheath is produced by special supporting cells called Schwann cells. The sheath is essentially a series of cell membranes, each produced by a Schwann cell and wrapped many times around the axon. Gaps between the membranes of each Schwann cell, called the nodesof Ranvier,are the key to the fast transmission of nerve impulses.

Fast transmission enables mammals to respond almost instantaneously to stimuli. Nerve impulses can be directed along the nerve fibres to specific points in the body so that responses can be very localised.

The endocrine system

Typically, the nervous system is adapted to convey messages rapidly between specific locations so that quick responses can be made. In contrast, the endocrine systemis adapted to carry information from one source to many destinations to bring about long-lasting responses.

The endocrine system consists of a number of glands that secrete hormones(organic chemicals, usually proteins or steroids). The glands of the endocrine system are called endocrine glandsor ductless glandsbecause they secrete their hormones directly into the bloodstream. Once inside a blood vessel, a hormone is carried in the bloodstream so that it can reach almost any cell in the body. However, each hormone has its own target cellson which it acts. Therefore, although all the hormones are transported together in the bloodstream, each has its own specific effect on the body. In some cases, a target cell has specific receptor molecules on its cell surface membrane which bind the hormone molecule. Once bound onto the membrane, the hormone brings about its response.

Endocrine glands occur at strategic points around the body. Their hormones regulate a wide range of activities, including blood glucose concentration, gastric secretion, heart rate, metabolism, growth rate, reproduction, and water balance.

■ Glossary of essential terms for you to know

N	English term	Russian equivalent
1.	stimulus	зд. раздражитель
2.	single-celled	одноклеточный
3.	multicellular	многоклеточный
4.	effector	нервное окончание; эффектор
5.	mammals	млекопитающие
6.	favourable	благоприятный
7.	sense organs	органы чувств
8.	relatively	относительно; довольно
9.	spinal cord	спинной мозг
10.	characteristic features	отличительные признаки; характерные черты
11.	to transmit nerve impulses	передавать нервные импульсы
12.	the myelin sheath	миелиновая оболочка
13.	ductless glands	железы внутренней секреции
14.	bloodstream	кровоток
15.	blood vessel	кровеносный сосуд
16.	heart rate	пульс (частота сердцебиений)
17.	to respond to stimuli	реагировать на раздражители
18.	cranial nerve	черепно-мозговой нерв
19.	to process information	обрабатывать информацию
20.	to enclose	окружать; окаймлять
21.	insulating material	изолирующий материал
22.	gastric secretion	желудочная секреция

■ Your Essential Assignments

I. Quick check

1. What is an axon?

2. In what form is information conveyed in:

a) the nervous system

b) the endocrine system?

II. Using monolingual English dictionary write down what the words below mean:

The nodes of Ranvier, proteins, muscle, reproduction.

III. Match the words with their definitions:

Words	Definitions
1. neurones	a) the ability to respond appropriately to stimuli
2. stimuli	b) fibres carrying impulses away from the cell body
3. axons	c) nerve cells which convey information in the form of nerve impulses
4. schwann cells	d) fibres carrying impulses towards the cell body
5. dendrons	f) a thing that produces a reaction in living things
6. sensitivity	g) special supporting cells which produce the myelin sheath