Human Sensory System: Stimuli, Receptors, and Common Disorders
The Human Sensory System
A stimulus is a change in our environment or within us that we perceive and which provokes a response from our body. Coordination involves electrochemical processes and nerve impulses, producing high-speed electrochemical responses. Endocrine coordination involves glands discharging substances (hormones) into the blood when receiving nerve impulses or chemical information from the internal environment. Responses executed by muscles and glands are coordinated by the endocrine system, which are slow but very durable.
Receptors: Internal and External
Receptors capture internal changes in the internal environment, such as a rise in CO2 or damage to an organ. External receptors detect stimuli from the external environment, such as sound or light waves, and chemical substances. Some receptors, such as those for touch or temperature, are scattered throughout the skin, while others are grouped in sensory organs.
Sensory Organs
Sensory organs include those for touch, smell, taste, hearing, balance, and vision.
The Skin and Touch
The skin contains a large number of different receptors, each sensitive to a particular stimulus. Together, they give us the sense of touch.
The Sense of Smell
The sense of smell, though not the most developed, allows us to distinguish approximately 10,000 different smells. The sequence of processes is as follows:
- Volatile molecules enter the nostrils and dissolve in the mucus.
- Olfactory cells detect these molecules via chemoreceptors.
- The olfactory cells produce a nerve impulse transmitted to olfactory bulb neurons.
The Tongue and Taste
When we bring something into our mouth, we experience taste sensations.
Balance
Balance allows us to perceive turns, acceleration, and the position of our body. This is facilitated by the saccule, utricle, and three semicircular canals oriented in three planes of space.
Audition (Hearing)
Audition allows us to listen and pick up vibrations transmitted through the air (sound waves). These are collected by the ear canal and vibrate the eardrum. This membrane vibration is transmitted to the ossicles, which multiply the force of motion and move a small membrane of the cochlea in the inner ear.
Vision: The Eye and Light
The eye receives light and focuses it onto the retina. The eye works like a camera, with two lenses. The cornea directs light toward the pupil, which is located in the middle of the iris, a colored disk with muscles that open and close the pupil as a function of light intensity.
The Retina
The retina transforms light stimuli into nerve impulses. In the retina, thousands of photoreceptor cells called rods and cones are excited to receive the focused light.
Common Sensory Disorders
Hearing Loss
Hearing loss is the diminution or loss of hearing. It may be congenital or caused by ear infections.
Vertigo
Vertigo is a sensation of dizziness, often accompanied by nausea, due to infections or accidents.
Eye Injuries
Eye injuries are damage to the eye due to infections or accidents.
Cataracts
Cataracts are an opaque layer that develops on the lens and prevents the passage of light.
Refractive Anomalies
Common refractive anomalies include myopia and hyperopia.
Myopia (Nearsightedness)
With myopia, objects situated far from the eye look blurry due to an anomalous elongation of the eyeball, causing images to focus in front of the retina.
Hyperopia (Farsightedness)
With hyperopia, people see blurry objects located near the eye due to an anomalous shortening of the eyeball, which causes the image to focus behind the retina.
Risk Factors and Prevention
- Staying in very noisy environments.
- Prolonged exposure to intense light.
- Excessive use of screens.
- Insufficient light for reading.
- A diet deficient in vitamin A.
Other Receptors
Thermoreceptors capture differences in temperature between the external environment and the body itself. Nociceptors cause us the sensation of pain. Mechanoreceptors detect pressure changes on the skin.