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Chapter 8&9

Chapter 8 & 9

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Zachariah Campbell

Chapter 8: Organization of the Sensory System General Principles of Sensory System Function Sensory Receptors • Sensory receptors are specialized cells that transduce, or convert, sensory energy (for example, light photons) into neural activity. Receptors are Energy Filters • We refer to people who lack receptors for parts of the usual visual spectrum as being color deficient or color- blind. There are also differences in the visual receptors of individual people who see the usual range of color. Joris Winderickx and his colleagues report that about 60% of men have one form of the red receptor and 40% have another form. Many females may have both forms. Hence, different people may see different “ reds.” Receptors Transduce Energy • Each sensory system’s receptors are specialized to filter a different form of energy: • For vision, light energy is converted into chemical energy in the photoreceptors of the retina, and this chemical energy is in turn converted into action potentials. • In the auditory system, air- pressure waves are converted into a number of forms of mechanical energy, the last of which eventually activates the auditory receptors, which then produce action potentials. • In the somatosensory system, mechanical energy activates mechanoreceptors, cells that are sensitive, say, to touch or pain. Somatosensory receptors in turn generate action potentials. • For taste and olfaction, various chemical molecules carried by the air or contained in food fit themselves into receptors of various shapes to activate action potentials. • For pain sensation, tissue damage releases a chemical that acts like a neurotransmitter to activate pain fibers and thus produce action potentials. • The dendrite has Na channels that are “ stretch sensitive” and open in response to the stretching of the dendrite’s membrane. If the influx of sodium ions in the stretch- sensitive Na channels is sufficient to depolarize the dendrite to its threshold for an action potential, the voltage- sensitive K and Na channels will open, resulting in a nerve impulse heading to the brain. Receptive Fields Locate Sensory Events • Every receptor organ and cell has a receptive field, a specific part of the world to which it responds. • For each of the sensory systems, its receptors’ unique “ view” of the world is its receptive field. Receptive fields not only sample sensory information but also help locate sensory events in space. Because the receptive fields of adjacent sensory receptors may overlap, their relatively different responses to events help in localizing sensations. The spatial dimensions of sensory information produce cortical patterns and maps of the sensory world that form, for each of us, our sensory reality. Receptors Identity Change and Constancy • Sensory receptors differ in sensitivity. They may adapt rapidly or slowly to stimulation or react only to a specific type of energy. • Rapidly adapting receptors detect whether something is there. They are easy to activate but stop responding after a very short time. • If you push a little harder when you first touch your arm, you will feel the touch much longer because many of the body’s pressure- sensitive receptors are slowly adapting receptors that adapt more slowly to stimulation. In the visual system, the rapidly adapting rod- shaped receptors in the eye respond to visible light of any wavelength and have lower response thresholds than do the slowly adapting cone- shaped receptors, which are sensitive to color and position. Receptors Distinguish Self from Other • Receptors that respond to external stimuli are called exteroceptive; receptors that respond to our own activity are called interoceptive. For example, objects in the world that we see, that touch us, or that are touched by us and objects that we smell or taste act on exteroceptive receptors, and we know that they are produced by an external agent. • When we run, visual stimuli appear to stream by us, a stimulus configuration called optic flow. When we move past a sound source, we hear auditory flow, changes in the intensity of the sound that take place be-cause of our changing location. Receptor Density Determines Sensitivity • The ability to recognize the presence of two pencil points close together, a measure called two- point sensitivity or discrimination, is highest on the parts of the body having the most touch receptors. • In the fovea (a small area of the retina where color photoreceptors are concentrated), the receptors— all cone cells — are small and densely packed to make sensitive color discriminations in bright light. In the periphery of the retina, the rod cells that are the receptors for black– white vision are larger and more scattered, but their sensitivity to light (say, a lighted match at a distance of 2 miles on a dark night) is truly remarkable. Neural Relays Relays Determine the Hierarchy of Motor Responses • Some of the three to four relays in each sensory system are in the spinal cord, others are in the brainstem, and still others are in the neocortex. At each level, the relay allows a sensory system to produce relevant actions that define the hierarchy of our motor behaviour. • The pain pathway also has relays in the brainstem, especially in the midbrain periaqueductal gray matter (PAG) that surrounds the cerebral aqueduct. This region is responsible for a number of complex responses to pain stimuli, including behavioural activation and emotional responses. Pain re-lays in the neocortex not only localize pain in a part of the body, but also identify the kind of pain that is felt, the external cause of the pain, and potential remedies. • Recall that the superior colliculus is a major visual center of the brainstem and the inferior colliculus is a major auditory center. In animals without a neo-cortex, these brainstem regions are the main perceptual systems. For animals with visual and auditory areas in the neocortex, these subcortical regions still perform their original functions of: - Detecting stimuli and - Locating them in space Message modification Takes Place at Relays • The messages carried by sensory systems can be modified at relays. For example, descending impulses from the cortex can block or amplify pain signals at the level of the brainstem and at the level of the spinal cord. Many of us have had the experience when we are excited by an activity, as occurs when we are playing a sport, that we may not notice an injury only to find later that it is quite severe. This inhibition, or gating, of sensory information can be produced by descending signals from the cortex, through the periaqueductal gray matter, and on to lower sensory relays. • Descending messages from the brain gate the transmission of a pain stimulus from the spinal cord to the brain. Relays allow Sensory Interactions • Where relays take place in sensory pathways, systems can interact with one an-other. For example, we often rub the area around an injury to reduce the pain or shake a limb to reduce the sensation of pain after an injury. These actions increase the activity in fine touch and pressure pathways, and this activation can block the transmission of information in spinal- cord relays of the pain pathways. • A dramatic effect of sensory interaction is the visual modification of sound known as the McGurke effect. If a speech syllable such as “ ba” is played by a recorder to a listener who at the same time is observing someone whose lips are articulating the syllable “ da,” the listener hears not the actual sound ba, but the articulated sound da. The viewed lip movements modify the auditory perception of the listener. The potency of the McGurke effect highlights the fact that our perception of speech sounds is influenced by the facial gestures of a speaker. Central Organization of Sensory Systems • The code sent from sensory receptors through neural relays is interpreted and eventually translated into perception, memory, and action in the brain, especially in the neocortex. Sensory Information Is Coded • After it has been transduced, all sensory information from all sensory systems is encoded by action potentials that travel along peripheral- system nerves until they enter the brain or spinal cord and then on tracts within the central nervous system. Every bundle carries the same kind of signal. • The presence of a stimulus can be encoded by an increase or decrease in the discharge rate of a neuron, and the amount of increase or decrease can encode the stimulus intensity. • Some people hear in color or identify smells by how the smells sound to them. This mixing of the senses is called synesthesia. Anyone who has shivered when hearing certain notes of a piece of music or at the noise that chalk or fingernails can make on a blackboard has “ felt” sound. Early Sensory System Is Composed of Subsystems • The pathway from the eye to the suprachiasmatic nucleus (number 1) of the hypothalamus controls the daily rhythms of such behaviours as feeding and sleeping in response to light changes. The pathway to the pretectum (2) in the midbrain controls pupillary responses to light. The pathway to the pineal gland (3) controls long- term circadian rhythms. The pathway to the superior colliculus (4) in the midbrain controls head orientation to objects. The pathway to the accessory optic nucleus (5) moves the eyes to compensate for head movements. The pathway to the visual cortex (6) controls pattern perception, depth perception, color vision, and the tracking of moving objects. The pathway to the frontal cortex (7) controls voluntary eye movements. Sensory Systems Have Multiple Representations • Topographic organization is a neural– spatial representation of the body or areas of the sensory world perceived by a sensory organ. Vision Photoreceptors • Rays of light enter the eye through the cornea, which bends them slightly, and then through the lens, which bends them to a much greater degree so that the visual image is focused on the receptors at the back of the eye. The light then passes through the photoreceptors to the sclera, which reflects the light back into the photoreceptors. • The light’s having to pass through the layer of retinal cells (to be bounced back at them by the sclera) poses little obstacle to our visual acuity for two reasons. First, the cells are quite transparent and the photoreceptors are extremely sensitive; they can be excited by the absorption of a single photon. Second, many of the fibers forming the optic nerve bend away from the retina’s central part, or fovea, so as not to interfere with the passage of light through the retina. • The retina contains two types of photoreceptive cells that transduce light energy into action potentials. Rods, which are sensitive to dim light, are used mainly for night vision. Cones are better able to transduce bright light and are used for daytime vision. Three types of cones, each type maximally responsive to a different set of wavelengths— either red or blue or yellow— mediate color vision. • Rods and cones differ in their distribution across the retina: cones are packed together densely in the foveal region, whereas rods are absent from the fovea entirely and more sparsely distributed in the rest of the retina. Thus, to see in bright light, acuity is best when looking directly at things and, to see in dim light, acuity is best when looking slightly away. • The photoreceptive cells synapse with a simple type of neuron called a bipolar cell and induce graded potentials in such cells. Bipolar cells, in turn, induce action potentials in ganglion cells. Retinal ganglion cells send axons into the brain proper (remember that the retina is a part of the brain). Visual Pathways • Just before entering the brain, the two optic nerves (one from each eye) meet and form the optic chiasm. • Optic Chiasm: Point at which the optic nerve from one eye partly crosses to join the other, forming a junction at the base of the brain. • So the right half of each eye’s visual field is represented in the left hemisphere of the brain, and the left half of each eye’s visual field is represented in the right hemi-sphere of the brain. • Having entered the brain proper, the optic tract, still consisting of the axons of retinal ganglion cells, diverges to form two main pathways to the visual cortex. Both pathways relay through the thalamus. The larger projection synapses in the lateral geniculate nucleus (LGN) of the thalamus, on neurons that then project to the primary visual cortex, V1. • The LGN has six well- defined layers: layers 2, 3, and 5 receive fibers from the ipsilateral eye, and layers 1, 4, and 6 receive fibers from the contralateral eye. The topography of the visual field is reproduced in the LGN: the central parts of each layer represent the central visual field, and the peripheral parts represent the peripheral visual field. • The LGN cells project mainly to layer IV of the primary visual cortex. This layer is very large in primates and has the appearance of a stripe across the visual cortex; hence the name striate (striped) cortex. • The major visual pathway from the retina to the LGN to the striate cortex is the geniculostriate pathway, bridging the thalamus (geniculate) and the striate cortex. • The geniculostriate pathway takes part in pattern recognition and conscious visual functions. The second main visual pathway takes part in detecting and orienting to visual stimulation. This tectopulvinar pathway relays from the eye to the superior colliculus in the midbrain tectum and reaches the visual areas in the temporal and parietal lobes through relays in the lateral posterior-pulvinar complex of the thalamus. Hearing • Hearing is the ability to construct perceptual representations from pressure waves in the air. Hearing abilities include sound localization— identifying the source of pressure waves— and echo localization— detecting pressure waves reflected from objects— as well as the ability to detect the complexity of pressure waves, through which we hear speech and music. Auditory Receptors • Sounds are changes in air- pressure waves. The frequency, amplitude, and complexity of these changes determine what we hear. We hear the frequency, or speed, of pressure changes as changes in pitch; we hear the amplitude, or intensity, of pressure changes as loudness; and we hear the complexity of pressure changes as timbre, the perceived uniqueness of a sound. • The human ear has three major anatomical divisions: outer ear, middle ear, and inner ear. The outer ear consists of the pinna and the external ear canal. The pinna catches waves of air pressure and directs them into the external ear canal, which amplifies them somewhat and directs them to the eardrum. The middle ear consists of the eardrum and, connected to it, the hammer, anvil, and stirrup, a series of three little bones (the ossicles). The ossicles in turn connect to the oval window of the inner ear. • In short, pressure waves in the air are amplified and transformed a number of times in the ear: by deflection in the pinna, by oscillation as they travel through the external ear canal, and by the movement of the bones of the middle ear. • In the inner ear is the cochlea, which contains auditory sensory receptors called hair cells. The cochlea is rolled up into the shape of a snail. It is filled with fluid, and floating in the middle of this fluid is the basilar membrane. The hair cells are embedded in a part of the basilar membrane called the organ of Corti. • When the oval window vibrates, a second membrane within the cochlea, the round window, bulges, sending waves through the cochlear fluid that cause the basilar membrane to bend and thus to stimulate the hair cells to produce action potentials. The larger the air- pressure changes, the more the basilar membran
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