Sensory Organs and the Human Nervous System
Sensory Organs
Taste
Taste buds are located in the tongue papillae, and also on the palate, pharynx, and larynx. Gustatory sensory cells are sensitive to substances dissolved in saliva.
Ears: Hearing and Balance
The ears are the sensory organs of hearing and balance. Through them, we perceive sounds, and they report on the position of the head to maintain balance.
Structure:
- External ear: Formed by the pinna (the visible part of the ear), which is a skin fold supported by cartilage, and the ear canal, which is closed at its end by the eardrum membrane.
- Middle ear: A cavity located in the skull. Three small bones (ossicles) are found here: the malleus (hammer), incus (anvil), and stapes (stirrup). They are in contact with the eardrum on one end and the inner ear on the other. The middle ear communicates with the pharynx through the Eustachian tube, which allows air to enter and exit to equalize pressure on the eardrum.
- Inner ear: Formed by the bony labyrinth, which is filled with perilymph. The membranous labyrinth is inside the bony labyrinth and is filled with endolymph. Two zones are distinguished in the labyrinth: the semicircular canals (where the sense of balance resides) and the cochlea (where the sense of hearing resides).
Hearing
Sound waves are collected by the pinna, directed through the ear canal, and cause the eardrum to vibrate. This vibration is transmitted to fill the liquids of the cochlea, where mechanoreceptors, sensitive to vibration, convert the stimulus into nerve impulses that are sent to the brain via the auditory nerve.
Balance
When we move our head, the endolymph filling the semicircular canals also moves. This movement is captured by receptors at the base of the canals, which transform it into nerve impulses. These impulses send information about movement to the brain.
Eyes: Sight
The eyes are the sensory organs of sight.
Structure:
The eye is formed by a series of structures and accessory organs. The eyeball is housed in a cavity formed by the bones of the skull and face.
- Accessory Organs: Eyebrows, eyelids, lacrimal glands, and eyelashes.
- Eyeball: Formed by the iris (a circular muscle beneath the cornea), the pupil (an opening in the center of the iris), the conjunctiva (a transparent epithelium that covers the sclera and cornea), the lens (a transparent lens behind the iris), the retina (the layer where visual receptors are located), the choroid (a dark layer inside the eye), and the sclera (a hard, white layer that maintains the shape of the eye).
Operation
The retinal receptors pick up the light reflected by the objects we see and produce nerve impulses that are transported by the optic nerve to the brain.
There are two types of receptors in the retina:
- Rods: Perceive low-intensity light, but not colors.
- Cones: Are sensitive to colors, but not low light.
To see objects clearly:
- Accommodation: The lens adjusts its curvature to change the direction of light rays and allow the visualized objects to form correctly on the retina.
- Regulation of the quantity of light: The pupil closes or opens depending on the intensity of light.
Cerebral Cortex
Sensory areas: Receive impulses from the sensory organs through sensory nerves. There, the impulses are translated into sensations. The motor cortex areas are responsible for producing nervous stimuli that are conducted via motor neurons to effector organs, which are responsible for performing the response.
Voluntary Acts
Process: Receptors in the sensory areas of the cerebral cortex receive stimuli. Through nerve connections, the motor areas, located in other areas of the brain, receive information from the sensory areas, process it, and produce a response knowingly and voluntarily. This response is conducted through motor neurons to the corresponding effector organs.
Reflex Acts
Orders are elaborated by motor neurons in the spinal cord without the control of the cerebral cortex. They are conducted through a structure called a reflex arc, which involves the following elements: a receptor sensitive to stimulation and a sensory neuron.