Sense Organs: Smell, Touch, and Taste in Biology

SENSE ORGANS

Sensory receptors (receptors that perceive stimuli from the external environment) can be grouped together and form the sense organs.

SMELL

Smell receptors or olfactory cells are chemoreceptors that are sensitive to volatile or gaseous substances in the air. Odours are sensations that are created in three stages:

• The volatile or gaseous substances in the air enter the nasal passages.
• The olfactory cells, located in the upper lining of the nasal passages, called the olfactory mucosa, perceive the substances. This lining also contains glands which 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.

SMELL Olfactory Nerve

Close-up view of the olfactory mucosa Olfactory nerve Neuron Skull bone Skull bone Olfactory mucosa Nasal passages Olfactory receptor cells Mucus Volatile substances

TOUCH

There are various kinds of touch receptors:

• thermoreceptors (detect heat and cold)
• mechanoreceptors (contact and pressure)
• nociceptors (perceive pain)

All these sensations constitute the sense of touch. Touch receptors are located in the skin but they are not distributed in a uniform manner. Some parts are more sensitive than others. When stimulated, touch receptors send nerve impulses through different nerves to the brain, where they are interpreted and identified. Mechanoreceptor for contact Thermoreceptor for cold Thermoreceptor for heat Nerve endings that perceive pain Mechanoreceptor for pressure

TASTE

Taste receptors, known as gustatory cells, are chemoreceptors which are sensitive to the chemical substances of food dissolved in saliva. Gustatory cells are located inside a series of bulb-shaped structures called taste buds. Taste buds are located on the palate, pharynx and, mainly, the tonque, which contains a series of small bumps called papillae. Gustatory cells have sensory cilia that are stimulated by the chemical substances in food. They send nerve impulses through the gustatory nerve to the brain, which interprets and identifies tastes. Papillae Close-up view of a papilla Gustatory cells Tongue Taste bud

HEARING

The receptors in our ears are mechanoreceptors responsible for either hearing or balance. Compressão Rarefação VARIAÇÕES DE PRESSÃO VARIAÇÃO DE PRESSÃO A -Crista da onda Vale da onda REPRESENTAÇÃO DA ONDA SONORA Hearing receptors, called auditory cells, and are sensitive to vibrations in the air. Balance receptors are called balance cells and are sensitive to movement.

HEARING Human Ear Divisions

The human ear is divided into: the outer ear, the middle ear and the inner ear.

• The outer ear is formed by the visible ear or auricle and the ear canal
• The middle ear is made up of the eardrum and a chain of tiny bones called the ossicles (malleus, incus and stapes), which link the eardrum to the inner ear.
• The inner ear is formed by the cochlea, which contains the hearing receptor cells, and the semicircular canals which house the balance cells.

All these structures are filled with fluid.

HEARING Ear Anatomy

Semicircular canals Incus Ossicles Vestibular nerve Malleus Stapes Auditory nerve Cochlea Eardrum Hearing receptor cells Inner ear Middle ear Outer ear Auricle Ear canal

HEARING Sound Wave Transmission

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

Sight receptors are located in the eyes and are photoreceptors that are sensitive to variations in light intensity. They enable us to see. Sclera Choroid Retina Iris Conjunctiva Optic nerve Lens Cornea Vitreous humour Pupil

SIGHT Human Eye Structure

The human eye has two parts: the eyeball and the auxiliary structures. . The eyeball is a slightly flattened sphere filled with transparent fluids (the aqueous and the vitreous humours). It has three layers:

• The sclera or outer layer (white). Its front part is transparent and is called the cornea. The cornea has a thin protective membrane called the conjunctiva.
• The choroid or intermediate layer (darker, with many blood vessels). Its front part contains the iris, which has an orifice in it, known as the pupil. The pupil opens or closes depending on the amount of light reaching the eye. Behind the iris is a lens that changes shape to focus the image received onto the retina.
• The retina is the internal layer. It contains two types of photoreceptors, rods and cones.

. The auxiliary structures are the eyebrows, eyelids, eye muscles and lacrimal glands. They protect the eyeball and enable it to move.

SIGHT How Our Eyes Work

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 send through the optic nerve to the brain, where they are converted into visual images.

• Retina Nervio Óptico Left Visual Field Right Visual Field Left Eye Right Eye Optic Nerve Optic Chiasm Lateral Geniculate Nucleus Brain Primary Visual Cortex