Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

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In common with all living organisms, especially other animals, Man exhibits irritability or sensitivity : we are aware of changes in our environment. These changes may act as stimuli, e.g. changes in light intensity, temperature, sound, pain. Our nervous system deals with this information and so may bring about a rapid response in the organism. A response is a change in an organism of part of it, produced by a stimulus. It is important to note that it is not actually powered by the stimulus, which merely acts as a trigger.
Stimulus Response
Hand feels extreme heat Remove it from source of heat
Bright light is shone on eye > pupil reduced (NOT close eyelid)
> moving object near eye
OR foreign object in eye
Eyelids blink
Sight of food > saliva production (NOT "eating")
> obstruction in throat
> getting hot
(due to exercise?)
> Exercise/fight/emotion Heart beats faster
Hear a sudden noise > turn to see, etc
> invited to eat food Eat food
> feel cold Put a sweater on

Responses to stimuli (involving the nervous system) can be of 2 types:-
a) Those over which you have no control, such as sneezing when an irritant enters your nose.
These "automatic" responses are called INVOLUNTARY RESPONSES. Such reflex actions may be unlearned (innate - instinctive) or learned (e.g. balance when riding a bicycle).
Give 2 more examples of such responses.
Stimulus Response
> >
> >

b) Those over which you have control and which require thought, such as speaking or running.
These responses are called VOLUNTARY RESPONSES (see previous work on the Brain and Voluntary and Reflex actions ).
Give 2 more examples of such responses.
Stimulus Response
> >
> >
However, the enormous number of responses, both voluntary and involuntary, cannot occur randomly, but must be co-ordinated to serve the body as a whole, and part of this co-ordination is brought about in part by the central nervous system.

Nervous impulses from sensory nerve cells (e.g. in skin , & other sensory organs) must travel in to the spinal cord and/or brain in order to be co-ordinated, so that impulses bringing about an appropriate response are sent out along fibres of motor nerve cells, which connect to muscles and glands.

These appropriate responses are of importance to the well being of the organism, e.g. they prevent harm; they prevent waste, but they must be carried out as quickly as possible.

A reflex arc

When a response is involuntary, it may be called a reflex action. One example is the knee-jerk reflex: the right leg is crossed over the left, and struck sharply just above or below the knee-cap, the lower leg jerks outward by reflex action.
Reflex responses are extremely fast because the impulses travel through a simple arrangement of nerve fibres called a reflex arc. This does not involve the brain. However, since we are usually aware that the response is occurring, nervous fibres must transmit the information up the spinal cord to the brain.

Use ALL of the following to label this diagram of a reflex arc: *stimulus; receptor; *effector;
motor-end plate (where motor nerves end inside muscles); pain receptor in skin; *ganglion (location of sensory nerve cell body); white matter; grey matter; muscle; sensory nerve fibre; motor nerve fibre, relay nerve fibre [*possibly not shown in the book].
Indicate, using arrows, the direction in which nervous impulses move.
reflex arc

How does the "white matter" differ from the "grey matter"?
(apart from colour) >white is (fatty sheaths of) nerve fibres
> grey is cell bodies
How does the arrangement of these parts differ in the the brain?
> grey matter is on outside of brain


Passage of nervous impulse along nerve fibres

Nervous impulses do not travel like an electric current passing along wires, and they do not travel along a direct, continuous pathway or circuit like the (terrestrial) telephone system (?).
Use the mouse to label the parts of the nerve cell indicated on the diagram below.

sensory neuron Nerve cell are also known as neurons or neurones. Most have processes - nerve fibres - called axons or dendrons. Usually there is a sheath of fatty material around the outside of each of these fibres. This acts as insulation, but also speeds up the impulses.

What sort of nerve cell is this?
> sensory nerve cell

The nervous impulse itself is a result of complicated electrochemical activity in the nerve fibre - a wave of change in membrane permeability causing movement of ions, and hence a change in electrical potential. This is why nervous activity can be monitored using (amplified) voltages (E.E.G, E.C.G.), and also why muscles contract in an uncontrollable way in electrocution!

What does EEG stand for? What does it measure?
> electroencephalogram >electrical activity in brain

What does ECG stand for? What does it measure?
>electrocardiogram > electrical activity in heart

Passage of nervous impulses from cell to cell

Interchange of information involves impulses crossing the small spaces between the nerve endings. These spaces are called synapses. The transfer of impulses across these synapses involves the release of a "transmitter" chemical from one nerve cell, which then quickly diffuses to the next nerve cell in the chain. There are several such neurotransmitter substances.

How many synapses are there in the reflex arc in the diagram on the previous page?
> 2

Action of drugs

Most drugs, both useful and harmful, may affect the body (especially brain), by altering the nerve cells' natural reaction to these chemicals, or by mimicking the body's normal compounds.
Some drugs slow down the passage of stimuli by affecting nerve cell membranes, and others act like neurotransmitters, perhaps passing stronger or longer-lasting impulses. As such they may alter the way sensory information is processed, or affect the thinking process.
Nerve gases, and insecticides which have been developed from them, affect the enzymes (cholinesterase enzymes) which recycle neurotransmitter substances after they have been released in he synapses.

Stimulants are drugs which speed up the action of the brain
e.g. caffeine, found in tea and coffee.
Sedatives are drugs which slow down the action of the brain
e.g. alcohol.

Why may it be difficult to sleep if you have been drinking coffee during the evening?
> caffeine stimulates brain
Even one alcoholic drink will have some effect on the brain.
Why do many people advocate the "Don't Drink and Drive" rule?
> alcohol affects judgement

Drug abuse

Many illegal chemicals have extreme effects on the function of the brain, e.g. LSD (lysergic acid diethylamide) causes hallucinations - objects around you may change colour, shape and size, or you may see and experience things that are not there at all. Such experiences may cause fear, depression, and mental disorders. Mostly, these substances alter one's perception of reality.

In addition, such chemicals confuse the mental faculties so that many drug users die in accidents, or overdose on the chemicals. Many drugs induce a feeling of dependency - addiction - and are linked with criminal activity. These are just some of the facts about drug abuse problems that people should think hard about in case they're considering abusing drugs.

Solvent abuse (sniffing glue, lighter fuel, etc.) - on which one can easily overdose - can also cause death by heart failure.

Other substances can incidentally affect the body's "thermostat" - located in the hypothalamus (refer to work on homeostasis and osmoregulation) and have caused death following disruption of the body's temperature control and regulation of the body's water content.

Even "legal" chemicals, such as alcohol (a "social" drug!) , have other adverse effects on the body.
List as many effects of alcohol on different parts of the body as you can.
> (brain) [reaction time increased] - pleasant relaxed feeling - dizziness slurred speech - double vision - becoming quarrelsome - unable to stand - unconscious - death
> brain shrinkage
stomach ulcers

>cancers - digestive system, liver damage leading to cirrhosis, heart disease

damage to unborn baby

It is important to be clear about the physiological effects of drugs but also also their potential consequences in personal and social contexts. Be prepared to say no, and not be ashamed of it.


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