Chapter4

　Human brain performs one of the major functions: It must be informed about what is happening both in the external environment and within the body, by means of sensory stimuli. Such information is received by the sensory system. The simplest sensory receptors are specialized peripheral endings of afferent neurons. Sensory organs include receptors and some specialized structures housing the sensitive receptors essential for special perception. These receptors have differential sensitivities to various stimuli, respond to particular stimuli, called the adequate stimuli, and translate the energy forms of the stimuli into bioelectrical signals. There are discrete pathways from the receptors to the CNS so that information about the type and location of the stimuli can be deciphered by the CNS, even though all the information arrives in the form of action potentials.
　All sensory receptors have one feature in common, the sensory stimulus produces a graded membrane potential called a receptor potential. The strength and rate of change of the stimulus are reflected in the magnitude of the receptor potential, which in turn determines the frequency of action potentials generated in the afferent neuron. A special characteristic of almost all sensory receptors is that they adapt either partially or completely to their stimuli when a continuous sensory stimulus is applied. The receptors respond at a very high impulse rate at first, and then at a progressively lower rate until finally many of them no longer respond at all.
　The eye is designed to focus the visual image on the retina with minimal optical distortion. Light is focused by the cornea and the lens, and then traverses the vitreous humour that fills the eye cavity before reaching photoreceptors in the retina, which are called the rods and cones. Rods and cones are activated when the photopigments they contain differentially absorb various wavelengths of light. Light absorption causes a biochemical change in the photopigment that is converted into a change in the release of transmitter from the photoreceptors and ultimately the rate of action potential propagation by the retinal ganglion cells which provide axons for the visual pathway leaving the retina .The cones are responsible for color vision and display high acuity but can be used only for day vision because of their low sensitivity to light. Different ratios of stimulation of three cone types by varying wavelengths of light lead to colour vision. The rods provide only indistinct vision in shades of grey, but because they are very sensitive to light, they can be used for night vision. When the rods and cones are excited, signals are transmitted through successive neurons in the retina itself and finally into the optic nerve fibres and cerebral cortex.
　The ear performs two unrelated functions: hearing, which involves the external ear, middle ear, and cochlea of the inner ear; and sense of balance, which involves the vestibular apparatus of the inner ear. The ear receptors are located in the cochlea and vestibular apparatus, which are mechanoreceptors and are called hair cells. Hearing depends on the ear's ability to convert airborne sound waves into mechanical deformations of receptive hair cells, thereby initiating neural signals that are transmitted to the auditory cortex in the brain for sound perception. High frequency resonance of the basilar membrane occurs near the base, where the sound waves enter the cochlea through the oval window; and low frequency resonance occurs near the apex mainly because of difference in stiffness of the fibers but also because of increased "loading" of the basilar membrane with extra amounts of fluid that must vibrate with the membrane at the apex.
　The vestibular apparatus in the inner ear consists of the semicircular canals for detecting rotational acceleration or deceleration in any direction, and the utricle and saccule for detecting the orientation of the head with respect to gravity. Neural signals are generated in response to mechanical deformation of hair cells caused by specific movement of fluid and related structures within these sense organs. This information is important for the body to obtain the sense of equilibrium and maintain posture.
　Taste and smell are chemical senses. Taste is mainly a function of the taste buds in the mouth. For practical analysis of taste the receptor capabilities have been collected into four general categories called the primary sensations of taste. They are sour, salty, sweet, and bitter. Olfactory receptors are located in the mucosa in the upper part of the nasal cavity. About seven different primary classes of olfactory stimulants preferentially excite separate olfactory cells. These classes of olfactory stimulants are characterized as camphoraceous, musky, floral, pepperminty, ethereal, pungent, putrid. Both sensory pathways include two routes: one to the limbic system for emotional and behavioral processing and one through the thalamus to the cortex for conscious perception and fine discrimination.
　Cutaneous sensations include touch-pressure sense, temperature sense and pain. Touch is mediated by mechanoreceptors in the skin. At least six entirely different types of tactile receptors are known: free nerve endings, Meissner's corpuscle, Merkel's disk, hair follicle receptors, Ruffini's corpuscles and Pacinian corpuscle. The two-point touch threshold, which is the minimum distance that can be distinguished between two points of touch, is a measure of the distance between receptive fields. The two-point touch threshold varies for different body regions. The mechanoreceptors are silent in the absence of tactile stimuli. They are excited to cause receptor potentials by mechanical deformation of them, which stretch the membrane and open sodium ion channels, which in turn will lead to action potentials when the receptor potential rises above threshold. The more and the faster the receptor potential rises above the threshold level for action potential, the greater their frequency becomes.
　Cold receptors and warmth receptors fire action potentials continuously at 2~5 Hz when the skin temperature is set at its normal value of 34℃. Cold receptors respond to steady-state temperatures of 5~40℃ and fire at highest rates at a skin temperature of 25℃.Warmth receptors are tonically firing at steady temperatures of 29~45℃ and most active at 45℃. There are far more cold receptors than warmth receptors, in fact, 10 times as many in most parts of the skin.
　Pain is mediated by nociceptors, which respond selectively to stimuli that can damage tissue. These classes of nociceptors can be distinguished on the basis of the type of stimulus: mechanical nociceptors, thermal nociceptors and polymodal nociceptors. All pain receptors are free nerve endings which use neural pathway for transmitting pain signals into the central nervous system.