pediagenosis: Sensory
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Showing posts with label Sensory. Show all posts
Showing posts with label Sensory. Show all posts

Thursday, April 1, 2021

Special Senses: Vision

Special Senses: Vision

Special Senses: Vision
Vision in humans involves the detection of a very narrow band of light ranging from about 400 to 750 nm in wavelength. The shortest wave-lengths are perceived as blue and the longest as red. The eye contains photoreceptors that detect light, but, before the light hits the receptors responsible for this detection, it has to be focused onto the retina (200 μm thick) by the cornea and the lens (Fig. 57a).

Special Senses: Vision


The photoreceptors can be divided into two distinct types called rods and cones. Rods respond to dim light and cones respond in brighter conditions and can distinguish red, green or blue light. The rods and cones are found in the deepest part of the retina, and light has to travel through a number of cellular layers to reach these photoreceptors. Each photoreceptor contains molecules of the visual pigments (rods: rhodopsin; cones: erythrolabe (red), chlorolabe (green) and cyanolabe (blue)); these absorb light and trigger receptor potentials which, unlike other receptor systems, lead to a hyperpolarization of the cell and not depolarization.
Special Senses: Hearing And Balance

Special Senses: Hearing And Balance

Special Senses: Hearing And Balance
Hearing
The young healthy human can detect sound wave frequencies of between 40 Hz and 20 kHz, but the upper frequency limit declines with age. When sound waves reach the ear, they pass down the external auditory meatus (the external ear) to the tympanic membrane that vibrates at a frequency and strength determined by the magnitude and pitch of the sound. The vibration of the membrane causes three ear ossicles (malleus, incus and stapes) in the middle ear (an air-filled cavity) to move, which, in turn, displaces fluid within the cochlea (the inner ear) as the foot of the stapes moves the oval window at the base of the cochlea. This mechanical link prevents the incoming sound energy from being reflected back, and the ossicles improve the efficiency with which the sound energy is transferred from the air to the fluid. Small muscles are attached to the ossicles and contract reflexly in response to loud sounds, thereby dampening the vibration and attenuating the transmission of the sound (Fig. 58a).


Special Senses: Hearing And Balance

The inner ear includes the cochlea and also the vestibular organs responsible for balance (see later). The receptors involved in both hearing and balance are specialized mechanoreceptors called hair cells. Projecting from the apical surface of the hair cell is a bundle of over 100 small hair-like structures called stereocilia and a larger stere- ocilium called the kinocilium. Deflection of the stereocilia towards the kinocilium leads to a potential change in the cell (depolarization), the release of a transmitter substance from the base of the hair cell, and activation of the nerve fibres that convey impulses to the higher centres of the brain.
Sensory Receptors

Sensory Receptors

Sensory Receptors
The sensory receptor is a specialized cell. In mammals, receptors fall into five groups: mechanoreceptors, thermoreceptors, nociceptors, chemoreceptors (Chapter 56) and photoreceptors (Chapter 57). There is further specialization within these groups. Each receptor responds to one stimulus type; this property is called the specificity of the receptor. The stimulus that is effective in eliciting a response is called the adequate stimulus.
Transduction processes. Some receptors consist of a nerve fibre alone (e.g. free nerve endings), others consist of a specialized accessory structure (e.g. olfactory receptors, Pacinian corpuscles), and others are more complex and consist of a specialized receptor cell which synapses with a neurone, in other words a secondary sensory cell (e.g. gustatory receptors and Merkel’s discs).
Sensory Receptors

Mechanoreceptors. These are found all over the body. Those in the skin have three main qualities: pressure, touch and vibration (or acceleration) (Fig. 55a,b). When the responses to constant stimuli are studied in the various receptors, the receptors can be divided into three types on the basis of their adaptive properties: slowly adapting receptors that continue to fire action potentials even when the pressure is maintained for a long period (e.g. Ruffini’s endings, tactile discs, Merkel’s discs); moderately rapidly adapting receptors that fire for about 50–500 ms after the onset of the stimulus, even when the pressure is maintained (e.g. hair follicle receptors, Meissner’s corpuscles); and very rapidly adapting receptors that fire only one or two impulses (e.g. Pacinian corpuscles) (Fig. 55b). These three types of receptor are examples of receptors in the skin that detect intensity, velocity and vibration (or acceleration), respectively.
Special Senses: Taste and Smell

Special Senses: Taste and Smell

Special Senses: Taste And Smell
The so-called special senses comprise the sensations of taste, smell, vision, hearing and balance. The receptors involved in taste and smell are chemoreceptors, those in vision are photoreceptors, and those in hearing and balance are mechanoreceptors.

The sensations of taste and smell are two modalities of sense that are very closely related. What the layperson calls ‘taste’ is really a combination of taste and smell, and probably a number of other modalities. Taken together, a better term would be flavour. The modalities of flavour are taste (gustation), smell (olfaction), touch (texture), temperature (thermoreception) and common chemical sense (chemoreception).

Special Senses: Taste And Smell


Gustation
Taste buds (the gustatory end organs) (Fig. 56a) are found in the tongue, soft palate, pharynx, larynx and epiglottis, and are unevenly distributed around these regions. Those in the tongue are associated with three of the four types of papillae (fungiform, foliate and circumvallate) (Fig. 56b). Those associated with the other oral tissues are found on the smooth epithelial surfaces. The different papillae occupy specific areas of the tongue. Their associated taste buds are innervated by either the glossopharyngeal (IX) nerve (posterior one- third of the tongue) or the chorda tympani branch of the facial (VII) nerve (anterior two-thirds of the tongue). In humans, the number of taste buds varies considerably: on average in the range 2000–5000, but can be as low as 500 or as high as 20 000 (Fig. 56c). Each taste bud is made up of 50–150 neuroepithelial cells arranged in a compact, pear-shaped structure (intragemmal cells). There is general agreement that there are four types of intragemmal cells: basal, type I (dark cells), intermediate and type II (light cells). Each taste bud comprises a dynamic system in which there is a rapid turnover of cells within each bud. The lifespan of an individual receptor cell is about 10 days. There is a small opening in the surface, the taste pore, where the cells have access to the gustatory stimuli (Fig. 56a).
Introduction To Sensory Systems

Introduction To Sensory Systems

Introduction To Sensory Systems
Sensation and perception
The brain obtains its information about the external and internal environment and about the body’s relation to the external environment by sensory experience emanating from sensory receptors (sense organs). There are a number of common steps in sensory reception: (i) a physical stimulus (i.e. touch, pressure, heat, cold, light, etc.); (ii) a transduction process (i.e. the translation of the stimulus into a code of action potentials); and (iii) a response (i.e. taking a mental note or triggering a motor reaction).

Monday, September 21, 2020

EYE ANATOMY

EYE ANATOMY

EYE ANATOMY

The structure of the human eye is so complex that it’s hard to believe that it’s not the product of intelligent design. But by looking at the eyes of other animals, scientists have shown that it evolved very gradually from a simple light-dark sensor over the course of around 100 million years.

Saturday, September 15, 2018

Cochlear Implant

Cochlear Implant


Cochlear Implant
Nucleus 6, Cochlear A cochlear implant has four main components. A microphone, worn near the ear, detects audio and transmits a signal to a sound processor. The processor then arranges the signal and sends it to a built-in transmitter. The transmitter passes the signal to an implanted receiver/stimulator, which transforms it into electrical stimuli for the electrodes. Finally these signals are relayed to the auditory nerve.

Thursday, September 13, 2018

Retinal Implant

Retinal Implant


Retinal Implant
Argus II, Second Sight A camera mounted on a pair of glasses captures real-time images and transmits them wirelessly to an implant on the retina. The implant contains 60 electrodes and, depending on the image, will generate different patterns of electrical signals, which are then sent to the remaining healthy retinal cells. These cells are activated by the signals, and carry the visual information to the brain for processing.

Thursday, September 6, 2018

Sensory Systems An Overview

Sensory Systems An Overview

Sensory Systems: An Overview
A sensory system is one in which information is conveyed to the spinal cord and brain from peripheral sensory receptors, which in themselves are either specialized neurones or nerve endings.

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