Irritability
we found that irritability is one of the seven characteristics of living things. It
means that living organisms can respond to changes in their internal
environment and the world around them. These responses usually increase
their chances of survival. Animals and plants react to changes in the
environment, not only drastic climate changes, but also simple everyday
changes. For example, a snake looking for food will move toward the scent of
a rat, and the shoots of a seedling will grow towards light.
Stimulus
A stimulus is a change in the environment that an organism reacts or
responds to. It could be light, temperature, a texture, a chemical in the air or
moisture, a response is the change in the organism brought about by the
stimulus.
Response of green plants
In chapter 17, we saw that seedlings respond to unilateral (one-directional)
stimuli of light and gravity. The roots grew in the direction of gravity and the
shoots grew towards light. Plants need water and minerals from the soil, so
the roots must grow down into the soil to reach them. Green plants, including
seedlings, also need light for photosynthesis. It is therefore important for the
survival of green plants to grow towards light (figure 18.3).A plant in a room
will grow towards the window where there is sunlight. A seed may be taken
into a cave by a bird or bat. It may germinate and then the seedling will grow
towards light and out of the cave’s entrance or any other opening.
Otherwise, the plant will die for lack of food in the darkness.Invertebrates,
like millipedes, earthworms and woodlice, need certain conditions to survive.
They respond to variations in light intensity, temperature and moisture. The
investigation illustrated in figure 18.4 shows that these invertebrates
respond by moving towards a cooler temperature, moist soil and away from
bright light. These responses ensure that they do not dehydrate and are
hidden from predators, that is, the responses help to ensure their survival.
The sense organs of humans
Humans have five senses: hearing, sight, smell, taste and touch. In humans,
the main sense organs are the eyes, ears, nose, tongue and skin. A group of
sense cells and other tissues form a sense organ.
• Eye – At the back of the eye is the retina which is a layer of sensory cells
thatrespond to light. Impulses are sent from these cells to the brain by the
optic nerve so that changes in shape, colour, brightness and distance are
detected.
• Ears – Sensitive hairs in the inner ear respond to vibrations in the air
(sound waves). Impulses are sent from these hairs to the brain by the
auditory nerve so that changes in the quality, tone, pitch and loudness are
detected.
• Nose – As air flows into the nose during breathing, chemical molecules in it
touch sensitive hairs. These send messages to the brain so that changes in
scent are detected.
• Tongue – Groups of receptor cells, called taste buds, respond to chemicals
in the food (figure 18.5). Different parts of the tongue are sensitive to
different flavours like salt, sweet, bitter and sour. These send messages to
the brain so that changes in flavour of the food are detected.
• Skin – This is the largest organ of the body. Nerves ending as sensory cells
are scattered throughout the skin. These are sensitive to pain, touch, change
in temperature, light pressure and heavy pressure. They send impulses to
the brain so that it can detect what has been touched.
The nervous system
The nervous system is made up of neurones or nerve cells. Neurones
transmit electrical impulses to and from the brain. The nervous system is
made up of:
• the central nervous system (CNS) which consists of the brain and spinal
cord;
• the peripheral nervous system (PNS) which consists of all the nerves
outside the central nervous system (figure 18.6).
The peripheral nervous system forms a vast communication network linking
the reception of the stimuli to a response. Receptors receive stimuli from the
environment and responses are brought about by effectors.Sensory neurones
conduct impulses from receptors to the central nervous system. Motor
neurones conduct impulses from the central nervous system to the effectors.
Intermediate or relay neurones link sensory and motor neurones. They are
found in the central nervous system .
A typical pathway, from the stimulus touching the receptor to the effector
bringing about a response. The numbers in the following paragraphs refer to
figure 18.9.
1.)The stimulus is, say, a hot object touching a pain receptor in the skin of
the hand.
2.) A signal travels along the sensory neurone to the central nervous system
(CNS).
3.) In the CNS, a relay neurone carries the signal through the brain.
4.) The relay neurone passes the signal to the motor neurone.
5.) The signal travels along the motor neurone to the effector (biceps
muscle) which responds (contracts).
6 .)The hand is moved away from the hot object.
The nervous system is adapted to carry messages quickly between specific
locations in the body, so that quick responses can be made. Sometimes the
effector may be a gland. Endocrine glands are found throughout the body
and they regulate a wide range of activities, including heart rate, metabolism
and reproduction. Together, the nervous system and endocrine system co-
ordinate all of the body’s activities.
The synapse
Signals travel along neurones as electrical impulses, which are very fast.
However, there are millions of neurones in your body, and where the ends of
two neurones meet there is a small gap called a synapse (Figure 18.10).
Electrical impulses cannot cross these gaps, so they are converted to a
chemical signal in order to cross the synapse. As they reach the other
neurone, they are converted back into electrical impulses so that they can
continue quickly on their way.
Reflex actions
A reflex action is a rapid and automatic response to a stimulus. It does not
require conscious control (you do not think about doing it). Examples of
reflex actions are the knee jerk, sneezing, the pupil reflex and blinking. The
pathway between the receptor and effector is called the reflex arc. There are
two kinds of reflex:
• spinal reflexes are nerve impulses that pass through the spinal cord and do
not go to the brain (e.g. the knee jerk response, figures 18.12 and 18.13);
• cranial reflexes are reflexes in the head region (e.g. blinking and the
response of the pupil in the eye to light.
The brain
The brain is the most important part of the nervous system. It enables
humans
to ‘think’ or ‘reason’, a skill which is supposedly lacking in most animals.
The brain has grey matter on the outside and white matter on the inside. It
is surrounded by tough membranes, called meninges, and cerebrospinal fluid
which cushion it from knocks. It is also surrounded by the bones of the skull.
Your brain is very well protected (figure 18.14). Humans can perform
complex
mental and physical activities co-ordinated by different areas of the brain
(figure 18.15). They receive stimuli from the environment and the brain
brings
about the appropriate response.
Autonomic nervous system
The autonomic nervous system is the n ame given to all the nerves which
automatically control the normal functioning of internal organs like the
heartwithout conscious control. For example, your heart keeps beating,
peristalsis occurs, breathing occurs, pupils dilate and blood vessels constrict,
without you having to think about any of these responses – they occur even
when you sleep.The internal environment of the body must be kept stable
(chapter 16). Homeostasis, the maintenance of a constant internal
environment, depends on the autonomic nervous system. All cells, and
therefore tissues and organs, function effi ciently in certain conditions of
temperature, pH and water. Any change in these conditions must be
remedied: for example, if there is a lack of water, cells become dehydrated,
so the body responds to increase the amount of water available. Animals and
plants respond to internal changes in ways that lead to stabilising the
internal environment.
