Irritability, Sensitivity and Coordination

Irritability is one of the seven characteristics of living things. It means that living organism 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 towards 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. The response to stimuli is important for the survival of organisms.

Some examples of stimuli, the responses and the importance to the organism of responding in this way.

Stimulus Possible response Importance to organism of response

Chemical from an Moves towards organism Organism may be a potential mate or potential
organism prey

Moisture in soil Moves towards moist areas Prevent desiccation or dying, especially for
organisms without a waterproof outer covering

Light Move from light to darker areas Escape from predators since it is harder to be
seen in darker areas
 Cold temperatures Move away from cold Organism cannot survive in cold temperatures,
temperature body not adapted

(Desiccate: To remove the moisture from a thing that normally contains moisture, such as a plant; to dry out
completely; to preserve by drying. The process of desiccating a thing is called desiccation; an agent used to
bring about desiccation is called a desiccant or a desiccator.)

Unlike most humans, animals in the wild have to find food every day and maybe avoid being food for another
organism. They have to be very aware of stimuli coming from their environment and be able to make the
appropriate response. More often than not, these everyday changes in the environment are a matter of life or
death.

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 that respond 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. 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.
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.

Figure 18.8 is 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 (contract).
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.
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.
Some receptors, effectors and responses in humans.

Stimulus Receptor Effector Response

Object moving towards Retina of the eye receives Muscles of the neck Head turns away so the
the face and sends a message to object cannot hit the face
the brain

Very hot object which is Nerve endings in the skin Muscles of the arm Hand pulls away from the
about to be picked up sensitive to temperature hot object
send a message to the
brain

Chemicals from food Chemoreceptors in the Salivary gland Saliva secreted and body
reach the nose (smell) nose send a message to prepares to digest food
the brain

All activity involves the coordination of the brain, spinal cord, sensory and motor neurones. Stimuli are
constantly being received, sent to the brain where they are analysed and appropriate responses sent back.

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 (example
the knee jerk response
● Cranial reflexes are reflexes in the head region (example 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 had grey matter on the outside and white matter on the
outside. 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. Humans can perform
complex mental and physical activities co-ordinated by different areas of the brain. They receive stimuli from
the environment and the brain brings about appropriate response.
The spinal cord is also composed of grey and white matter, but here the white matter is on the outside and the
grey matter on the inside.

Autonomic nervous system

The autonomic nervous system is the name given to all the nerves which automatically control the normal
functioning of internal organs like the heart without 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. Homeostasis, the maintenance of a constant internal environment, depends on the autonomic nervous
system. All cells, and therefore tissues and organs, function efficiently 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 stabilizing the internal environment.

The endocrine system

In humans, the endocrine system consists of a number of glands called endocrine glands. The endocrine system
controls growth and development. A gland is a structure which secretes a specific chemical substance. In
humans, there are two types of gland: exocrine glands and endocrine glands.

Exocrine glands transport their secretions by ducts to other parts of the body. For example, salivary glands in
the mouth secrete saliva via ducts into the mouth; tear glands by the eye secrete fluid which passes through
ducts onto the eye’s surface. Endocrine glands secrete chemicals called hormones directly into the bloodstream.
These glands have a rich blood supply to collect the hormone and transport it to its target organ. The pancreas is
an organ of the digestive system. It contains two types of secretory cell. One type produces enzymes that make
up pancreatic juice which is secreted through a duct to the duodenum. The other type produces the hormone
insulin that diffuses into blood vessels which pass through the pancreas. The pancreas is therefore a structure
that is made up of both exocrine and endocrine glands.
Hormones help to control and coordinate many body activities, including growth and development. They are
produced by endocrine glands positioned in specific areas of the body. Hormones are produced in very small
amounts and travel through the body in the bloodstream to target organs. Hormones influence the activities of
these target organs.

The pituitary and the hypothalamus

Most, but not all, endocrine glands work under the influence of a single master gland – the pituitary, which is
situated beneath the brain. The hypothalamus is situated close to the pituitary. While the hypothalamus is not
an endocrine gland, it regulates the secretion of some of the pituitary gland hormones. If the pituitary is thought
of as the master gland of the endocrine system, then the hypothalamus can be thought of as the manager. The
hormones produced by the pituitary and their effects are shown below:

Hormone Functions

Pituitary growth hormone Stimulates growth of the entire body; too much causes
gigantism; too little causes dwarfism

Antidiuretic hormone (ADH) Stimulates the kidneys to reabsorb more water from
filtrate when the blood plasma becomes too
concentrated

Other hormones: eg. Follicle stimulating hormone Stimulate other glands such as the ovaries, thyroid,
(FSH), luteinising hormone (LH), thyroid and testes into activity
stimulating hormone (TSH)

Involuntary and Voluntary Actions

● Involuntary actions are automatic and do not involve thought.
● Voluntary actions we think about and decide what to do.

We can compare the actions in a table:

Involuntary Actions Voluntary Actions

Speed Very quick, as they are automatic May be slower, as we need to think about
them

Purpose To protect us from harm, especially To take account of experience and allow
physical danger us to think and develop

Control Spinal cord or hind part of the brain Cerebrum of the brain
 Outcome Same every time, as it is automatic Can vary with different people or
occasions

Examples Removing hand or foot from a hot or Any action based on thinking, memory or
sharp object. Blinking and pupil learning, example reading
reflex

Comparing the Nervous and Endocrine Systems

● The nervous system uses electrical impulses sent along nerves. Impulses are sent directly to an effector,
such as a muscle. The response is rapid and does not last long. The nervous system is important for
everyday interactions with the environment. It helps keep us safe and able to learn from our experiences.
● The endocrine system uses 30 chemical hormones, sent around in the blood. The hormones eventually
reach their target organs and have their effects. The response is usually slow and may include long
lasting changes, such as growth. The endocrine system helps to control many reactions in the body, to
keep the internal environment constant.

Quick Check Activity

1). Use the following terms to fill in the spaces to complete the sentences correctly: stimuli, brain, sound,
light, sensory, impulses:

____________________ cells in the sense organs respond to different _________________. They send nervous
__________________ along nerves to the spinal cord or __________________. Our eyes focus
_________________ waves; our ears respond to ___________________ waves.

2). Use the following terms to fill in the spaces to complete the sentences correctly: spinal cord, bone,
cerebrum, brain, involuntary, voluntary:

The central nervous system consists of the _______________ and ________________________, both protected
inside __________________. An automatic action is called _________________________, controlled by the
____________________ or the hind part of the _______________. Actions we think about are called
____________________, controlled by the fore _____________________.

3). Use the following terms to fill in the spaces to complete the sentences correctly: testes, pituitary,
thyroxine, adrenaline, growth, insulin:

The ______________________ is the master gland. It produces _________________ hormone and controls
several other organs, including the ovaries and __________________. The adrenal glands make
_______________. The thyroid makes ___________________ and the pancreas makes ___________________.