16. - DISORDERS OF THE SENSE ORGANS

16. - DISORDERS OF THE SENSE ORGANS

 

A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic sensation (touch), taste and olfaction (smell). In short, senses are transducers from the physical world to the realm of the mind where we interpret the information, creating our perception of the world around us.

The receptive field is the specific part of the world to which a receptor organ and receptor cells respond. For instance, the part of the world an eye can see, is its receptive field; the light that each rod or cone can see, is its receptive field. Receptive fields have been identified for the visual system, auditory system and somatosensory system, so far.

The Human sensory system consists of the following sub-systems:

• Visual system consists of the photoreceptor cells, optic nerve, and V1.
• Auditory system
• Somatosensory system consists of the receptors, transmitters (pathways) leading to S1, and S1 that experiences the sensations labelled as touch or pressure, temperature (warm or cold), pain (including itch and tickle), and the sensations of muscle movement and joint position including posture, movement, and facial expression (collectively also called proprioception).
• Gustatory system
• Olfactory system

Human sensory receptors are:

• Chemosensor
• Mechanoreceptor
• Nociceptor
• Photoreceptor
• Thermoreceptor

 

DISEASES:

Amblyopia, also known as lazy eye, is a disorder of the visual system that is characterized by a vision deficiency in an eye that is otherwise physically normal, or out of proportion to associated structural abnormalities of the eye. It has been estimated to affect 1–5% of the population.

Amblyopia means that visual stimulation either fails to transmit or is poorly transmitted through the optic nerve to the brain for a continuous period of time. It can also occur when the brain "turns off" the visual processing of one eye, to prevent double-vision, for example in strabismus (crossed-eyes). It often occurs during early childhood, resulting in poor or blurry vision. Amblyopia normally affects only one eye in most patients. However, it is possible, though rare, to be amblyopic in both eyes, if both fail to receive clear visual images. Detecting the condition in early childhood increases the chance of successful treatment, especially if detected before the age of five. The earlier it is detected, and the underlying cause corrected with spectacles and/or surgery, the more successful the treatment in equalizing vision between the two eyes.

Anacusis is a state of complete deafness

There are two types of anacusis:

• Unilateral anacusis meaning only one ear is completely deaf.
• Bilateral anacusis meaning there is complete deafness in both ears.

Astigmatism is an optical defect in which vision is blurred due to the inability of the  optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. The two types of astigmatism are regular and irregular. Irregular astigmatism is often caused by a corneal scar or scattering in the crystalline lens, and cannot be corrected by standard spectacle lenses, but can be corrected by contact lenses. Regular astigmatism arising from either the cornea or crystalline lens can be corrected by a toric lens. A toric surface resembles a section of the surface of an American football or a doughnut where there are two regular radii, one smaller than the other one. This optical shape gives rise to regular astigmatism in the eye.

 

 The refractive error of the astigmatic eye stems from a difference in degree of curvature refraction of the two different meridians (i.e., the eye has different focal  p oints in different planes.) For example, the image may be clearly focused on the retina in the horizontal (sagittal) plane, but not in the vertical (tangential) plane. Astigmatism causes difficulties in seeing fine detail, and in some cases vertical lines (e.g., walls) may appear to the patient to be tilted. The astigmatic optics of the human eye can often be corrected by spectacles, hard contact lenses or contact lenses that have a compensating optic, cylindrical lens (i.e. a lens that has different radii of curvature in different planes), or refractive surgery.

 

 Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under normal lighting conditions. Color  blindness affects many people in a population.^[1] "Color blind" is a term of art; there is no actual blindness but there is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. Color blindness is usually a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome (in females, a good gene on only one of the two X chromosomes is enough to yield the needed photopigments).

 Symptoms like those of color blindness can also be produced by physical or chemical damage to the eye, the optic nerve, or parts of the brain. These impairments are not true color blindness, as defined. For example, people with achromatopsia are unable to see colors, but they are not strictly color blind; they suffer from a completely different disorder.

 The English chemist John Dalton published the first scientific paper on this subject in 1798, "Extraordinary facts relating to the vision of colours", after the realization of his own color blindness. Because of Dalton's work, the general condition has been called daltonism, although in English this term is now used more narrowly for deuteranopia alone.

 Color blindness is usually classed as a mild disability, but there are situations where it can give an advantage. Some studies conclude that color blind people are better at penetrating certain color camouflages. Such findings may give an evolutionary reason for the high prevalence of red–green color blindness.

 

AUDITORY SYSTEMS

In the human body, the system responsible for taking in sound from the environment is known as the "auditory system." The human auditory system is made up of the group of structures used in the process of hearing sound. This includes structures all the way from the outer ear to the brain's auditory cortex. Some of the ear's structures are also involved in the vestibular system, which helps the body maintain balance. The auditory system can experience problems and breakdowns at any level, but many options are available to individuals with these auditory problems.

 

Hearing Loss

Hearing loss is a reduction in the ear's ability to transmit or process sound. Problems in any part of the auditory system can lead to hearing loss. Hearing loss can range from mild to profound, and also includes deafness. Deafness is the absence of functional hearing. People who are deaf may be able to detect some sounds, but they are unlikely to be able to use sound alone as a component of the learning system. Also, they may not have the means for understanding and identifying auditory input, especially speech.

Many diseases and conditions can cause or contribute to hearing loss, including:

• Aging
• Certain drugs
• Congenital diseases
• Infection
• Injury
• Noise exposure.

These conditions damage different components of the auditory system. They can cause varying degrees of hearing loss that can be temporary or permanent, sudden or progressive.

Solutions for Hard of Hearing Individuals

Even mild hearing loss can have a significant impact on a person's life. However, individuals who are deaf or hard of hearing have many options for hearing and communication. Individuals with hearing loss may choose to use hearing aids or other amplification devices. Other individuals choose to have a cochlear implant, which helps the hearing system transmit sound signals to the brain. People with profound hearing loss may choose to communicate manually (with a system such as American Sign Language), or to use speech and speechreading training to communicate orally.