Chapter Nervous System. Chapter Musculoskeletal System. Chapter Endocrine System. Chapter Circulatory and Pulmonary Systems. Chapter Osmoregulation and Excretion. Chapter Immune System. Chapter Reproduction and Development. Chapter Behavior. Chapter Ecosystems. Chapter Population and Community Ecology.
Chapter Biodiversity and Conservation. Chapter Speciation and Diversity. Chapter Natural Selection. Chapter Population Genetics. Chapter Evolutionary History. Chapter Plant Structure, Growth, and Nutrition. Chapter Plant Reproduction. Chapter Plant Responses to the Environment. Full Table of Contents. This is a sample clip. Sign in or start your free trial. JoVE Core Biology. Next Video. Embed Share. Cells of the lens lose their organelles and are therefore transparent.
The lens becomes progressively opaque as newer cells replace older ones, which accumulate in the lens. This is called a cataract. If the lens loses its elasticity due to age and cannot assume a spherical shape, it leads to loss of near vision, and this is called presbyopia. If images of far objects focus at a point in front of the retina, the eye is nearsighted or myopic and far vision is poor. If images of near objects focus at a point behind the retina, the eye is farsighted or hyperopic and near vision is poor.
If the lens or cornea is not smooth, it is called astigmatism. The lens separates an anterior chamber filled with aqueous humor and a posterior chamber filled with vitreous humor. If aqueous humor is formed faster than it is removed, it results in increased pressure within the eye. This can cause irreversible blindness with the death of optic nerves and it is called glaucoma. The pigmented, opaque iris that has a central hole, the pupil, controls the amount of light entering the eye.
The iris has smooth muscles, innervated by autonomic nerves. Stimulation of the sympathetic nerves dilates the pupil to let in more light when light is poor and stimulation of the parasympathetic nerves constricts the pupil to allow in less light when light is bright.
Rods — sensitive and responding to low light and cones — less sensitive and responding to bright light. There are three kinds of cones containing red-, green-, or blue-sensitive pigment.
Photoreceptors contain photopigments, which absorb light. There are 4 photopigments, rhodopsin in the rods, and one in each of the 3 cone types.
Each photopigment contains an integral membrane protein, opsin, which binds a light-sensitive chromatophore molecule. The chromatophore — retinal a derivative of vitamin A is the same in all the 4 photopigments. The opsin is different in each type of photopigment, absorbing light at different wavelengths of the spectrum. The light activates retinal, causing it to change shape and triggering a hyperpolarization in the bipolar cells, which synapse with the photoreceptor cells.
After its activation, retinal changes back to its resting shape by light-independent mechanisms and the photoreceptor cell is depolarized. Photoreceptor cells synapse with neurons called bipolar cells which, in turn, synapse with ganglion cells that produce the first action potentials in the chain. Axons from ganglion cells form the optic nerve, which crosses over to the opposite side of the optic chiasm.
Receptor cells of the organ of Corti, the hair cells, are mechanoreceptors that have hairlike stereocilia. The vibration of the basilar membrane, with which the hair cells are attached, stimulates the hair cells and the pressure waves are transformed into receptor potentials.
The entire audible range extends from 20 to 20, Hz. Human ears can tolerate sounds not exceeding 85 decibels. 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. Receptive fields have been identified for the visual system, auditory system and somatosensory system, so far.
Reference Terms. Related Stories. The autonomic nervous system comes in two opposing parts, sympathetic and parasympathetic.
The sympathetic division stimulates bodily processes in response to information about the body and the external environment received by the autonomic nervous system, whereas the parasympathetic division has an antagonistic effect by inhibiting bodily functions. Principally, the sensory nervous system with its different sensory systems is part of the peripheral nervous system or, better, it starts in the periphery and ends in the central nervous system.
As a whole, the sensory nervous system detects and encodes stimuli and then sends signals from receptors, that is, sense organs or simple sensory nerve endings, to the central nervous system, that is, it transduces environmental signals into electrical signals that are propagated along nerve fibers. In contrast, the motor systems respond to information provided by the sensory systems to generate movements and other forms of behavior.
The main function of the sensory nervous system is to inform the central nervous system about stimuli impinging on us from the outside or within us. By doing so, it informs us about any changes in the internal and external environment. The central nervous system integrates the sensory information and communicates the information to target organs in our body.
Therefore, a given sensory system comprises receptor cells in sense organs, neurons that project from sense organs to the brain, and specific brain areas that process the afferent information coming from the periphery. For each of the five classic senses vision, touch, hearing, smell, and taste , a corresponding cortical area exists in the brain [ 5 ] referred to as sensory cortex, namely visual cortex, somatosensory cortex, auditory cortex, olfactory cortex, and gustatory cortex.
Our brain also houses a vestibular cortex to process information from the vestibular organs, the utricle and saccule with the maculae, and the semicircular ducts with the crista ampullaris. In addition to the sensory cortices, the brain or, more specifically, the cerebral cortex is involved in the control of voluntary movement, for example, in the frontal lobe [ 6 ].
Parts of the brain are responsible for encoding sensory information and controlling motor behavior. These are the primary sensory and motor cortices, and they constitute only about one-fifth of the cerebral cortex [ 2 ]. Not all brain areas can be assigned easily to either sensory or motor functions.
These areas are involved in processing complex stimuli, forming relations between objects and planning adaptive responses including memory formation. The functions are referred to as cognition and are carried out in the association cortices in the parietal, temporal, and frontal lobes such as the prefrontal cortex, posterior parietal cortex, and inferotemporal cortex [ 2 ]. As pointed out so poignantly by Barth et al. Sensing and sensory systems are a characteristic property of living animals and have evolved over millions of years by selective pressures to develop many sense organs for specific tasks with magnificent precision [ 1 ].
As a result, animals use a stunning diversity of sensory systems to extract information from their environment [ 8 ] and have many sensory abilities not know to humans such as ultraviolet, infrared, ultrasound, electromagnetic reception, and skeletal strain detection [ 7 ]. On the one hand, the differences between the sensory systems in terms of complexity are obvious. On the other hand, despite all the differences, there are commonalities that have been discovered in sensory systems and the brains [ 1 ].
As indicated by these authors, while some animals such as insects and mollusks may vastly differ from humans, they share a surprising number of basic properties of living organisms.
The similarities extend to brain functions such as learning and memory and advanced cognitive abilities which traditionally have been associated with primates rather than snails, bees, or birds.
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