Co-ordination and Response

You have previously learned that one of the 7 characteristics of living organism is irritability or sensitivity. And this is the ability to detect a change in the outer environment and respond to it. A change in the environment is also called a stimulus (plural stimuli). Actions taken by the body in order to co-operate with a stimuli are called responses. The body detects a stimulus by parts in the body called receptors and is able to respond to it through other parts called effectors. Two organ systems are continuously working to detect and respond to stimuli, these organ system are called the nervous system and the endocrine system.

The Nervous System:

The nervous system is a system of organs working together to detect and respond to stimuli. The nervous system is made up of two systems, the Central Nervous System (C.N.S) and the Peripheral Nervous System (P.N.S) the peripheral nervous system connects the central nervous system to the other parts of the body.

Central Nervous System (CNS):

The central nervous system is made up of the brain and the spinal cord. The spinal cord is basically a big bundle of nerve cells running through a tunnel inside the backbone which protects it while the brain is protected by the skull. The central nervous system is what gives out orders to other parts of the body to perform certain jobs.

The Peripheral Nervous System PNS:

The peripheral nervous system is the other part of the nervous system. The main job of the PNS is to detect stimuli and send impulses to the CNS according to the stimuli. The PNS is made of receptors and nerves that carry the impulses.

Receptor cells are ones whose function is to detect something about its environment. There are many receptors in the body that are able to detect many changes like temperature, touch, light, sound and chemicals. There are some organs in the body that are there to detect just one stimulus, like the eye for example. These are called sensory organs and they can be defined as a group of receptor cells responding to specific stimuli.

Effectors are the opposite of receptors. Receptors are two detect the stimuli while effectors are two respond to it. Effectors are usually muscles and glands.

Neurons (Nerve Cells):

Neurones are one of the most important structures of the nervous systems. Neurones act as a wire that transmits electrical impulses all over the body. Like a cable that consists of many wires, a bundle of neurones is called a nerve. There are 3 types of neurones, each type is to transmit electrical impulses from a specific place to another.

Motor Neurone: This is a neurone that transmits electrical impulses from the Central nervous system to the effectors.

This neurone is made up of three segments; the cell body which is the start of the motor neurone and is in the CNS, axon which stretches out from the cell body all the way to end of the neuron, and the motor plate which is the end of the neurone and is in the effector muscle.
Neurones have features that are common between most animal cells like a nucleus, cymiddlelasm and cell surface membrane, but they also have some exclusive features like the axon. The axon is an extended cymiddlelasm thread along which electrical impulses travel. Some motor neurones have axons of length 1 metre. Axons are coated by a layer of myelin called myelin sheath, this is an electrically insulating layer which is essential for the proper functioning of the nervous system.

Another exclusive feature of neurones is dendrites, these are several short threads of cymiddlelasm coming out of the cell body. Their function is to pick up electrical impulses from other cells.

The last exclusive feature of motor neurones only is motor end plate. This is just the end of the axon which is in the muscle. It passes the electrical impulses from the neurone to the muscle fibres.

Sensory Neurones: like other neurones, sensory neurones carry electrical impulses from one place to another. But sensory neurones carry electrical impulses in the direction different to that of motor neurones, from the receptors to the CNS.

The sensory neurone’s shape is unique. This is because it is made of a cell body, with two arms extending out of it. The first arm is the axon which’s other end is in the CNS. The second arm is dendrite which’s other end is in the receptor. The dendrite is similar in structure to the axon except that it joins the receptor with the cell body. The electrical impulses of the sensory neurone flow from the receptor, through the dendrite to the cell body, then from the cell body to the CNS through the axon.

Relay Neurone: Relay neurones are located in the CNS. Their job is to pass electrical impulses from the sensory neurone onto the motor neurone, so it acts like a diversion.

Where neurones meet, they are not actually touching each other. Instead there is a gap called synapse or junction box. When the electrical impulses reach the end of a neurone, the neurone secretes a chemical transmitter which passes by diffusion to the other neurone causing the impulses to be carried from the first neurone to the second.

Reflex Arc (Nervous System in Action):

If your finger touches a hot surface, receptor cells in the skin of your finger detect a stimulus, which is a sudden rise in the temperature. The receptor uses the energy of the stimulus to generate electrical impulses. These impulses are then carried by the axons of the dendrites of the sensory neurone through cell body to axon and from the axon to the CNS. At the CNS the electrical impulses travel through the synapse to the relay neurone, which passes it onto the motor neurone. The nerve impulses are transmitted through the axon of the motor neurone to the targeted muscle which contracts when electrical impulses reach it, resulting in your finger being pulled away from the hot surface. This pathway is called the reflex arc and happens in about a fraction of a second.

Reflex Arc: RECEPTOR → Sensory Neurone → CNS → Motor Neurone → EFFECTOR


Voluntary and Involuntary Actions:

The reflex arc is a reflex action. Reflex means it is automatically done without your choice. This is because when the electrical impulses reach the relay neurone in the CNS from the receptors, some impulses are carried by other neurones to the brain, and some impulses are passes onto the motor neurone to the effector muscle and the response takes place. The electrical impulses going to your brain are much slower that the ones going to the effector muscle directly. This is why the reflex action takes place before you realise it, it is uncontrollable. Reflex actions are said to be involuntary actions. Involuntary actions start at the sense organ heading to the effector. They are extremely quick. Voluntary actions are the ones that you make the choice to do. Like picking up a bag from the floor for example. Your brain sends electrical impulses to the effector muscles ordering them to contract so you could pick the bag up. Voluntary actions are slower than involuntary actions and they start at the brain.


The Human Eye:

The human eye is a sensory organ. This means it is an organ of tissues working together to detect and respond to a specific stimulus, which is light.

Features of the Human Eye:

How We See:

When the light hits an object, it is reflected in all directions. When a light ray reflected from the object hits your eye you see that object. At the back of your eye, there is a spot on the retina called the fovea (blind spot). This spot is full of light sensitive cells. When the light ray falls on the fovea, the light sensitive cells generate electrical impulses that travel through the optic nerve to brain. When the electrical impulses reach the brain, the brain generates the image you see. This all happens in less than a fraction of a second.

But this is the general idea only. Light rays enter the eye from every direction. If they are not focused on the fovea, they will most probably not hit it and we won’t see. Here comes the role of the front part of the eye. When the light ray hits the eye at an angle, it first has to penetrate the cornea which refracts (bends) the light ray inwards. The cornea acts as a converging (convex) lens. Then the light penetrates the lens which refracts the ray a little more inwards focusing the light ray on the fovea. And thus the light ray is focused on the retina. When the ray hits the retina, the closer to the fovea the sharper the image is.


The angle at which the light ray hits the hits the eye depends on the distance of the object. Every light ray that hits the eye needs a certain amount of refraction in order to be directed to the fovea. This is why the lens has the ability to widen and narrow according to the distant of the object you’re looking at in order to make the light ray hit the retina at the right spot. This is called accommodation. Light rays refracted from close objects are diverging (spreading out), they need to be refracted inwards to be focused on the fovea. When you look at a close object, it takes some time till the vision becomes clear. This is because at first, the light ray is not correctly refracted, so it hits the retina away from the fovea. The electrical impulses are generated and sent to the brain which realises that the image is not clear. The brain then sends electrical impulses to the ciliary muscles making them contract. When the ciliary muscles contract the suspensory ligaments become loose, this makes the lens become thicker and rounder for more refraction of the light rays. Now the light rays are correctly refracted and hit the retina at the fovea, the image becomes clear.

For far visions it is the exact opposite. The rays reflected from far objects are almost parallel. Very little refraction should be done. The brain sends electrical impulses to ciliary muscles making them relax, the suspensory ligaments now tighten up and pull the lens which become narrow.



Ciliary muscles











Rods and Cones:

The retina is full of light sensitive cells called photoreceptors. There are two types photoreceptors, they are rods and cones. Rods and cones are specialised types of neurons. They look alike but they are a little different in function.

Rods are sensitive to dim light. At night or in dark places, most light detection electrical impulses transmission is done by rods. Vitamin A is essential for proper functioning of rods, if Vitamin A lacks it can lead to night blindness. Rods are spread all over the retina.

Cones are sensitive to bright and coloured light. All cones are packed in one area, the fovea.

The Pupil:

The pupil of the eye is the dark round area in the centre of it. It is surrounded by a coloured ring structure called the iris. The pupil and the together play a big role in protecting the eye from damage by limiting the amount of light entering the eye. If too much light fall on the retina, the rods and cones get damaged. The iris and pupil change their size to smiddle that happening. The iris contains two sets of muscles; Circular and Radial muscles. Circular muscles run around the iris and radial muscles run from the centre to the outside. When circular muscles contract they make the pupil smaller. When the radial muscles contract the stretch the pupil outwards making it wider.

In bright light, too much light starts entering the eye, which is dangerous for the rods and cones, which detect the high light intensity. The rods and cones start a reflex arc by sending electrical impulses to the brain via sensory neurone. The brain responds by sending electrical impulses to the muscles of the iris via motor neurone. These impulses make the circular muscles contract and the radial muscles relax limiting the amount of light entering the eye, thus protecting the rods and cones from damage.

If you walk into a dark room, the rods and cones sense the little amount of light. They start another reflex arc and send electrical impulses to the brain which responds by sending electrical impulses the muscles of the iris. The radial muscles contract and the circular muscles relax widening the pupil to let more light in.

Antagonistic Muscles:

You have just learned that in order for the pupil to get narrower or wider, two muscles work simultaneously, when one contracts the other relaxes. Pairs of muscles like that are called antagonistic muscles.

The most known antagonistic muscle pair is the biceps and triceps of the arm. The bi and the tri for short, they are what causes the movement of the arm. They work simultaneously to bend or straighten the arm. The biceps is located in front of the humerus bone of the upper arm. The biceps is joined to the radius bone of the lower arm and the triceps is joined to the ulna bone of the lower arm. Muscles are attached to bones by strong fibres called tendons.

When you want to bend your arm the brain send two electrical impulses, one to the bi making it contract and one to the tri telling it to relax. When the bi contracts, it becomes shorter pulling the bones to which it is attached close and bending the arm. This causes the fibres of the tri to stretch while they are relaxed.

To straighten your arm, the brain send electrical impulses to both muscles making the bi relax in order to leave the muscle it is attached to free. The tri contracts and becomes shorter pulling the muscle it is attached to into place and straightening the arm.

The biceps can be called a flexor because it flexes (bends) the arm. The triceps can be called an extensor because it extends (straightens) the arm.


A drug is a chemical substance that modifies and affects chemical reactions of the body when taken in. Many drugs are useful to us like antibiotics, painkillers and caffeine.

Some drugs however are abused by users to feel relaxed, or reach euphoria. Euphoria is a state of mind at which the abuser feels extremely happy and relaxed. These drugs include alcohol and heroin.


Alcohol is a depressant drug. This means that it reduces the activity of the brain and slows down the nervous system and reflex actions. Alcohol can be extremely dangerous when the user is in a situation in which they need fast reflex actions.  Alcohol is addictive. The more you drink it the more you need it. The user may reach a point where they cannot do without alcohol, this is when they become alcoholics. Alcohol is broken down into fats by the liver. If the abuser drinks too much alcohol, the cells of the kidney may die shortening their life.


Heroine is a narcotic drug. This means that it relieves pain and induces sleep. Heroine is extracted from a plant called opium poppy. Most heroine abusers become addicts. For the addicts heroine become the number one priority in their lives. They would do anything to get the drug even become criminals and possess a threat to their society. If not rehabilitated, a heroine abuser will end up homeless or dead. Some heroine users inject the drug in their veins by an unsterilized, shared needle, this increases the risk of getting AID/HIV.

The Endocrine System:

You have previously learned that messages are delivered around body as electrical impulses by the nervous system. Another way messages are transported around the body is by chemicals called hormones secreted by the endocrine system.

Hormones are chemical substances produced by a gland, carried by the blood, which alters the activity of one or more specific target organs and is then destroyed by the liver.

Hormones are produced in organs called endocrine glands which make up the endocrine system. The following diagram shows the glands of a human body.

Glands are organs made of secretory cells which’s function is to produce hormones and secret them into the bloodstream. Glands have a dense network of blood capillaries in them to secret the hormones in. hormones are carried around the plasma like all other content of the blood but certain organs are able to use them, these are target organs.



Hormone produced

Function of hormone

Adrenal gland


Prepares the body for activities that need energy and quick reflex actions



Makes liver reduce blood glucose level


Makes liver increase blood glucose level



Produces male secondary sexual characteristics



Produces female secondary sexual characteristics


Helps control menstrual cycle and maintain pregnancy



When you get a fright you feel some changes in your body like a sudden increase in heart beat rate, blood flowing quickly in veins and your breath becomes deeper and faster. This is because the fright you got caused the brain to send electrical impulses to the adrenal glands making them secrete adrenaline hormone in your bloodstream. Adrenaline is a hormone that is secreted from the adrenal glands to prepare the body for situations that need lots of energy and fast reflex action, like fights or running away for example. Adrenaline’s main objective is to increase your metabolic rate so that you have enough energy for fighting or running away etc. This is why adrenaline is called the three Fs hormone (Fight, fright, flight). One of adrenaline’s target organs is the heart. When adrenaline reaches the heart it causes the cardiac muscle to contract and relax much rapider so that oxygen and glucose reach the muscles of the body faster. Adrenaline also makes the liver convert glycogen into glucose and secret it in the blood to be used in respiration. When adrenaline reaches the diaphragm and the intercostals muscles of the ribs, they make it contract and relax faster too to increase rate of breathing. These changes cause an increase in the respiration rate so that lots of energy is being released. Generally, adrenaline is secreted when you are nervous or anxious.

Use of Hormones in Food Industries:

Technologies and science have advanced enough that we can now gut much more money out of farming and animal keeping. Hormones are now being used in farms to increase milk yields in cows and growth rate in cattle and fish.

In farms, the cows are being injected with a hormone called Bovine Somatotropin or BST. BST is a hormone that is naturally produced in cows. The function of BST is to produce milk. Injecting cows with extra BST will boost milk production and bring in more money for the farmers. Some people however are against the use of BST and claim it is safer for both the cows and the consumer to keep it natural and keep more cows if we want an increased milk yield.

Growth hormones are also being mixed with the food fed to cattle to increase their growth rate and make them grow larger. But again many people are against this and prefer buying meat and fish that were naturally grown.


Comparing Nervous and Endocrine Systems:

Nervous System

Endocrine System

Information sent in form of electrical impulses

Information sent in form of chemical hormones

Information travel neurones

Information travel in bloodstream

Information travels extremely rapidly

Information travels relatively slow

Information is headed to one target (effector)

Information may be used by several targeted organs

Electrical signals have an effect that ends quickly

Hormones have a longer lasting effect


Coordinates and Responses in Plants:

Plants cannot move themselves to areas of preferable conditions. This is why plants have the ability to detect a stimulus and respond to it by growing or bending in its direction or away from it. These responses are called tropisms. For example a plant tends to grow its stem in the direction of sunlight for more photosynthesis, this is a tropism. There are two types of tropism, these are phototropism and geotropism.

A tropism can be either positive or negative. If a tropism is in the direction of the stimulus, it is positive. If the tropism is away from the stimulus it is negative.

For example, a plant’s shoot tends to grow in the direction of sunlight, this is positive phototropism. But the plant’s root grows in the opposite direction deeply into the soil, this is negative phototropism. However, positive phototropism can also be described as negative geotropism because it involves the plant growing in the direction opposite to gravity. And negative photo tropism can be described as positive geotropism because it involves the plant growing towards gravity.


Tropisms are controlled by a chemical called Auxin. Auxin is a plant hormone. It is produced by cells at the tip of roots and shoots of plants. At the tip of a shoot, there is an area in which cells are being produced by dividing so that the shoot grows. Old cells do not divide, but they grow longer instead. The growth of these cells longer is controlled by auxins. Auxins is what makes the plant grows this is why a plant doesn’t grow if you cut it’s tip off.

Auxins’ Role in Phototropism:

If the sun shines on the right side of a plant’s shoot, auxins will accumulate on the dark opposite left side. Auxins accumulating there makes the cells on the left side grow much faster than the cells on the right side. When the left side of the shoot starts growing faster than the right side, the shoot will start to bend to the right side towards sunlight. This is phototropism.

Auxins’ Role in Geotropism:

Auxins tend to settle at the bottom end of the root. However, this does not make the sells of the tip of the root grow longer. Instead, auxins prevent the cells at the bottom tip of the root from growing, making the cells at the middle of the root grow faster. When the cells of the middle of the root grow faster, they push the root deeper into the soil and the root gets longer. The root grows in the direction of the gravitational pull. This is geotropism.

Roots show positive geotropism and negative phototropism because they grow towards gravity and away from sunlight at the same time. Shoots show positive phototropism and negative geotropism because they grow towards the sunlight and away from gravity at the same time.

Advantages of Positive Phototropism:

Advantages of Positive Geotropism:

Selective Weed Killers:

Auxins can be used to kill weeds that grow over grass or cereal crops. If weed grows on crops, auxins are sprayed everywhere. Weeds absorb auxins faster than crops or grass. Auxins accumulate in the weeds making them grow very rapidly. Fast growth of weed kills it leaving the crops or grass alive. Auxins are used ass selective weed killers.