Human Sensory Organs and Their Functions
The Human Skin and Sensory Receptors
The skin consists of two layers: the epidermis (the superficial layer) and the dermis (the deep layer). The epidermis is an epithelial tissue made of several layers. Its cells are dead and impregnated with substances like keratin; these cells continually divide to renew those that are lost. In this layer, melanocytes are also found, which are cells containing a pigment called melanin that gives the skin its color.
Touch Receptors
Free nerve endings are located throughout the skin:
- Meissner's corpuscles: Sensitive to light pressure and contact.
- Pacinian corpuscles: Sensitive to heavy pressure and vibration.
- Ruffini endings: Sensitive to touch, pressure, and heat sensations.
- Krause corpuscles: Sensitive to a decrease in temperature (cold).
- Free nerve endings: Sensitive to heat sensations.
The Sense of Smell
The nasal cavity is lined by a mucosal epithelium called the pituitary membrane:
- Red pituitary: Located in the lower half of the nose, it contains blood capillaries to heat the breathed air.
- Yellow pituitary: Located in the upper part, it contains olfactory receptors formed by ciliated cells responsible for receiving and transporting stimuli.
The Sense of Taste
Taste receptors are found in the taste buds of the tongue, as well as on the palate, pharynx, and larynx. These receptors are sensitive to substances dissolved in saliva. There are four primary types of receptors: salty, bitter, acid, and sweet.
The Ear: Hearing and Balance
Hearing
Sound waves are picked up by the pinna and directed toward the ear canal, causing the eardrum (tympanum) to vibrate. This vibration is extended through the ossicles and transmitted to the cochlea (snail), which is filled with liquids. Here, mechanoreceptors sensitive to vibration convert stimuli into nerve impulses sent to the brain via the auditory nerve.
Balance
Moving the head moves the endolymph that fills the semicircular canals. This movement is detected by receptors on their walls, turning it into nerve impulses that send information about these movements to the brain.
The Sense of Sight
Accessory Organs
These include the eyebrows, eyelashes, eyelids, and lacrimal glands.
The Eyeball
- Conjunctiva: A transparent epithelium that covers the sclera and cornea.
- Sclera: The white coat that protects and maintains the eye's shape. At the front, it forms the cornea, which is oval-shaped.
- Choroid: An opaque, dark layer inside the eye. At the front, it forms the iris, a circular muscle beneath the cornea with the pupil in the center.
- Retina: Contains the visual receptors. The internal layer includes cones (sensitive to colors) and rods (perceive light).
The Lens and Eye Fluids
The lens is located behind the iris. The eye contains two main fluids:
- Aqueous humor: A liquid located between the iris and the lens.
- Vitreous humor: A gelatinous and transparent substance.
Operation of the Eye
Light enters through the pupil, and the lens focuses images that form on the retina. Retinal receptors catch the reflected light from objects and produce nerve impulses, which are carried by the optic nerve to the brain.
Common Eye Diseases
- Myopia: Difficulty seeing distant objects. It is caused by an excessive length of the eyeball, which makes objects focus in front of the retina.
- Hypermetropia: Good vision from afar, but difficulty focusing on close objects because they are focused behind the retina.