The interaction function enables people to perceive changes and stimuli that occur both inside their bodies and in their environment, to interpret or process them and generate coordinated responses in order to survive.

A stimulus is a physical or chemical change that takes place outside or inside the body and triggers a response in it.

Our bodies contain a series of specialist cells, called receptors, which perceive stimuli and send the information they detect to the coordination centres for processing.


Internal receptors are those that perceive changes inside the body, such as a rise in blood sugar levels.

External or sensory receptors are those that perceive stimuli in the external environment. Depending:

Mechanoreceptors, which perceive mechanical stimuli such as pressure, vibration or movement.

Thermoreceptors, which perceive changes in temperature.

  Chemoreceptors, which detect chemical substances in the air, water, etc.

Nociceptors, which perceive pain.

Photoreceptors, which are light-sensitive.

The nervous system is responsible for analysing the internal and external stimuli perceived by the receptors, generating orders and sending them to the effectors. It carries out these functions through nerve impulses.These cells are called neurons. The nervous system coordinates fast, short-lived responses, such as muscle contraction.

The endocrine system

The endocrine system is made up of a series of endocrine glands, ie. glands that release the substances they generate into the bloodstream. These substances are called hormones. Hormones coordinate the internal organs by triggering chemical reactions. This system coordinates responses that are slower but longer-lasting than those coordinated by the nervous system.


Effectors are the organs responsible for executing the body’s response. They include muscles, which respond by contracting, and endocrine glands, which respond by secreting substances.

Smell receptors or olfactory cells are chemoreceptors that are sensitive to volatile or gaseous substances in the air.

Our bodies contain a series of specialist cells, called receptors, which perceive stimuli and send the information.

How odours reach the brain

– The volatile or gaseous substances in the air enter the nasal passages.

• The olfactory cells, which are located in the upper lining of the nasal passages, called the olfactory mucosa, perceive the substances. This lining also contains glands that secrete mucus. We can only smell substances if they are dissolved in mucus and present in sufficient concentration.

• When the olfactory cells are stimulated, they send nerve impulses through the olfactory nerve to the brain, which interprets the information and identifies the smell


Thermoreceptors, which detect heat and cold; mechanoreceptors, which detect contact and pressure; and nociceptors, which perceive pain. Together, all these sensations constitute our sense of touch.

How tactile sensations reach the brain

Touch receptors are located in the skin and are not distributed in a uniform manner. This is why some parts of our bodies are more sensitive than others.

When stimulated, touch receptors send nerve impulses through different nerves to the brain, where they are interpreted and identified.


Taste receptors, known as gustatory cells, are chemoreceptors which are sensitive to the chemical substances of food dissolved in saliva.

How tastes reach the brain

Gustatory cells are located inside a series of bulb-shaped structures called taste buds.

Gustatory cells are stimulated by the chemical substances in food and send nerve impulses through the gustatory nerve to the brain, which interprets and identifies tastes.


The receptors in our ears are mechanoreceptors responsible for either hearing or balance. Hearing receptors are called auditory cells and are sensitive to vibrations in the air, balance receptors are called balance cells and are sensitive to movement.

How our ears work: hearing and balance

Hearing. Sound waves enter the outer ear and reach the eardrum, which begins to vibrate. This vibration is transmitted along the ossicles to the fluid inside the cochlea. The movement of the fluid stimulates the hearing receptor cells which send nerve impulses through the auditory or cochlear nerve to the brain, where they are transformed into sounds.

Balance. The sensory balance cells are located in the semicircular canals and the cavities found at their base. When we move, the fluid inside these structures also moves and stimulates the balance cells, which send nerve impulses through the vestibular nerve to the brain,which provides us with information about our body’s position.


Sight receptors are located in the eyes and are photoreceptors that are sensitive to variations in light intensity. They enable us to see.

How our eyes work: sight

Light reaches the cornea, which directs it towards the pupil. The pupil opens or closes depending on the intensity of the light at that particular moment. The light then travels through the lens, which focuses it onto the retina. In the retina, photoreceptor cells generate nerve impulses which they then send through the optic nerve to the brain, where they are converted into visual images.

Nervous coordination is the coordination carried out by the nervous system. This system is made up of nerve tissue whose cells, called neurons, are capable of transmitting information through nerve impulses. 

A nerve impulse is kind of like an electrical current, or an electrical signal, which is transmitted along the neuron membrane and then on to other neurons through synapses.

The neuron that transmits the message is called the transmitter neuron, and the neuron that receives the

message is called the receptor neuron.

In the majority of synapses, the transmitter and receptor neurons are not in physical contact with each other the message is transmitted from one to the other by a series of chemical substances called neurotransmitters, which are stored in the axon terminals

Nerve impulse transmission across a synapse

1 The nerve impulse which has been transmitted along the membrane of the transmitter neuron reaches the end of its axon.

2 At the end of the axon, the arrival of the nerve impulse causes neurotransmitters to be released into the synaptic left.

3 Neurotransmitters bind to the membrane of the receptor neuron and generate a new nerve impulse.

4 The new nerve impulse is transmitted along the membrane to the axon, where the whole process is repeated.


The nervous system receives information from the receptors, processes and coordinates it, and generates orders for the effectors.

The encephalon

The encephalon is protected by the skull or cranium and by three membranes, the meninges, between which a liquid called the cerebrospinal fluid circulates. 

– The brain is in turn divided into cerebral cortex, is rough and full of folds and furrows, known as circumvolutions. The brain converts information into awareness and houses the advanced functions,memory.

– The cerebellum controls balance and voluntary movements, such as walking, running or riding a bicycle.

– The brainstem links the encephalon to the spinal cord and controls diverse involuntary functions.

The spinal cord

The spinal cord is a long, thin bundle of nerves protected by the spine, which links the encephalon to the rest of the body. It serves as a conduit for nerve impulses from the receptors to the encephalon, and from the encephalon to the effectors. It also coordinates simple responses called reflex actions.


The PNS links the CNS to the body’s organs. It is formed by 31 pairs of nerves which stem from the spinal cord and 12 nerves which originate from the encephalon.

Nerves are divided into sensory nerves and motor nerves.

The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.

• The somatic PNS consists of the sensory nerves and the motornerves. It controls voluntary movements.

• The autonomic PNS consists of motor nerves which regulate the activity of the internal organs. It controls involuntary movements. Is divided into the sympathetic nervous system stimulates the body to react to stress, fear, etc. The parasympathetic nervous system causes the body to relax. 


The stimuli perceived by internal and external receptors reach central nervous system, where an appropriate order is generated in the environment.

Reflex actions

Reflex actions are fast, involuntary, automatic responses that are controlled by the spinal cord, and are triggered in emergency situations.

A series of nervous elements are involved in reflex actions. These are known as the reflex arc and are:

1. A receptor which perceives the stimulus; for example, a pin prick.

2 A sensory nerve which transmits the impulse to the spinal cord.

3 An association neuron or interneuron which generates an

immediate response order.

4 A motor nerve along which the order travels to the effector organ.

5 An effector organ which executes the response 


Voluntary actions are conscious, deliberate responses that are Coordinated by the encephalon.

1 A receptor which perceives the stimulus.

2 A sensory nerve which transmits the impulse to the spinal cord and from there to the encephalon.

3 The cerebral cortex, which turns the information into a conscious sensation and uses both this and stored prior experiences to generate a response order which it then sends through the spinal cord to a motor nerve.

4 A motor nerve along which the order travels to the effector organ. 

5 An effector organ  which contracts voluntarily. 


1 The hypothalamus regulates the pituitary gland The hypothalamus, which forms part of the nervous system, reacts to both external stimuli and variations in the concentration of hormones in the bloodstream. 

2 The pituitary gland regulates the other glands. When stimulated by the hypothalamus, the pituitary gland starts manufacturing hormones. The hormones of the pituitary gland enter the bloodstream and travel round the body. 

3 Hormones regulate the activity of the body’s organs. The hormones produced by glands are distributed through the bloodstream and regulate the activity of the body’s organs.

4 Hormones halt secretion in the pituitary gland. An increase in the level of hormones in the bloodstream sends a signal to the pituitary gland, telling it to stop manufacturing its hormones.


Vision-related diseasesBlindness is in one or both eyes. This can be caused by genetic problems, accidents or diseases such as diabetes, or due to the influence of diverse external agents.

. Conjunctivitis  is an inflammation of the conjunctiva. It is caused by both viral and bacterial infections.

. Cataracts occur when the lens of an eye becomes cloudy. 

Colour blindness is a congenital disease

• Refractive anomalies include short-sightedness, long-sightedness andastigmatism.

Hearing disorderHearing loss is a decrease or loss of hearing capacity. It can be a congenital condition or it can be caused by different factors.

Balance disorderVertigo is a sensation of movement or feeling of spinning around oneself.

Prevention •Diseases related to sight, it is important to see an ophthalmologist if you detect any eye abnormalities, vitamine A also help prevent eye illnesses.

•Diseases related to hearing, you should avoid noisy environments. Wear protective earmuffs.


Disorders of the nervous system.

Psychological disorders include stress and depression due to social or environmental factors.

Degenerative diseases include Alzheimer’s,in which patients lose their memory and their capacity to think and use language.Older people, no effective treatment

Brain or spinal trauma can cause irreversible damage to the brain or spinal cord. 

Addictions. The consumption of alcohol or other

drugs, such as cocaine, cannabis or club drugs may cause irreparable damage to the nervous system. You are dependent, difficult stop taking them.

Diseases of the endocrine system  Diabetes, which is caused by decreased secretion of insulin by the pancreas.

Hypothyroidism, thyroid gland producing less thyroxine than is needed by the body, reducing the metabolic activity of cells.

Hyperthyroidism, the thyroidgland producing too much thyroxine.Metabolic activity increases.

Prevention  is advisable to have a healthy diet, avoid alcohol and to exercise regularly.


Problems affecting the bones

Trauma:physical injures that broken bones.

Joint dislocations: occur when a bone is separated from its joint.

• Scoliosis is a deviation of the spine.

Osteoporosis is a disease of the bones that makes them weak and prone to fracture. 

Problems affecting the joints

Arthritis is an inflammation of the joints

Osteoarthritis is a degenerative joint disease that affects the mobility of the joints.

Gout is an inflammation that primarily affects the joints of the feet, ankles, knees, hands and elbows.

Problems affecting the soft tissues

Strains, which are injuries to muscles and tendons.

Sprains, which are injuries to the ligaments, mainly caused by twisting.

. Muscle contracture are the permanent and involuntary contraction of a muscle.