Hormones, the Senses, and the Structure of the Eye and Skin

Hormones: Specific Chemical Messengers

A hormone is a chemical that acts on target cells, specifically those that have receptors for it. Hormones are very efficient: a minimal amount is enough to fulfill its function. Its production is controlled, and both excess and deficit of a hormone can cause disease.

Sensory Organs and Their Characteristics

• Sight: the eye, which detects light.
• Hearing: the ear, which detects vibrations.
• Taste: the tongue, which detects dissolved chemical substances.
• Smell: the nasal fossae, which detect gaseous chemical substances.
• Touch: the skin, which detects pressure.

The Skin

The skin has two main layers: the superficial epidermis and the deep dermis.

The Epidermis

The epidermis is an epithelial tissue made up of layers of cells. The most superficial layers are made of dead cells impregnated with keratin, a substance that makes the skin impermeable. The deepest layer contains dividing cells that continually produce new cells, which move toward the surface. Among them are melanocytes, cells that produce melanin, the pigment that gives skin its color.

The Dermis

The dermis contains sweat glands, which produce sweat to cool the skin, and sebaceous glands, which produce fat that lubricates and waterproofs the skin and hair. It also has nerve endings that detect touch.

The Sense of Smell

The olfactory receptors are located in the upper part of the nasal cavity, in the olfactory mucosa. This mucosa contains olfactory cells with cilia, which are responsible for receiving and transmitting smell stimuli. The lower part of the nasal cavity, the respiratory mucosa, is lined with abundant blood capillaries that warm the air we breathe.

The Ear

Outer Ear

The outer ear is made up of the auricle, a cartilage covered by skin, and the external auditory canal, which ends at the tympanic membrane.

Middle Ear

The middle ear is an air-filled cavity containing three ossicles (the hammer, anvil and stirrup). One end is in contact with the tympanic membrane and the other with the inner ear. It communicates with the pharynx through the Eustachian tube, which allows air to enter and leave, equalizing pressure on both sides of the eardrum.

Inner Ear

The inner ear is formed by a series of channels called the bony labyrinth. Inside it is a fluid called perilymph, and within the membranous labyrinth is the endolymph. The labyrinth has two distinct zones: the semicircular canals, responsible for the sense of balance, and the spiral cochlea, responsible for hearing.

How the Ear Works

• Hearing: sound waves travel through the ear canal and make the tympanic membrane vibrate. These vibrations pass through the ossicles to the cochlea, where they are transformed into nerve impulses.
• Balance: when we move, the endolymph moves within the semicircular canals, stimulating receptors that transform this movement into nerve impulses.

The Eye and Vision Disorders

The eye is the organ where light is detected. Several vision anomalies can occur:

• Myopia: the eyeball is too long, so images focus in front of the retina. Nearby objects are seen clearly, while distant objects look blurred.
• Hyperopia: the eyeball is too short, so images focus behind the retina. Distant objects are seen clearly thanks to accommodation, but nearby objects appear blurred, especially in people who cannot fully focus the lens.
• Astigmatism: the cornea has irregular curvature, so light does not focus properly on the retina. This causes blurred and distorted vision.