* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Laboratory Exercise 12: Sensory Physiology
Axon guidance wikipedia , lookup
Synaptogenesis wikipedia , lookup
Aging brain wikipedia , lookup
Neurotransmitter wikipedia , lookup
Embodied cognitive science wikipedia , lookup
Perception of infrasound wikipedia , lookup
NMDA receptor wikipedia , lookup
Time perception wikipedia , lookup
Proprioception wikipedia , lookup
Neuromuscular junction wikipedia , lookup
Sensory substitution wikipedia , lookup
End-plate potential wikipedia , lookup
Microneurography wikipedia , lookup
Feature detection (nervous system) wikipedia , lookup
Evoked potential wikipedia , lookup
Psychophysics wikipedia , lookup
Endocannabinoid system wikipedia , lookup
Signal transduction wikipedia , lookup
Clinical neurochemistry wikipedia , lookup
Molecular neuroscience wikipedia , lookup
Laboratory Exercise 12: Sensory Physiology An organism's survival depends on its continual adjustment to the environment. Changes in the environment are called stimuli. Receptors (sense organs) are nerve cells specialized for the detection of stimuli. The receptors convert stimulus energy to electrical energy of the nerve impulse to enable communication within the nervous system. This is known as transduction. Stimulus energies are of various types, such as mechanical (touch), thermal (heat, cold), chemical, sound, light are transduced into the electrically energy of the nerve impulse. The receptors transduce the stimuli to electrical energy of the nerve impulse (NI). The sensory messages (NI) via the sensory neurons are conveyed to the CNS. The CNS interprets and integrates the messages and sends out motor messages (NI) via motor neurons to the muscles and glands so the organism gives a meaningful, coordinated response to the stimulus. Receptors located on or near the body surface receive stimuli from the outside of body are exteroceptors. Receptors that receive stimuli from within the body are interoceptors (proprioceptors). They are stimulated by pressure (stretch), position and movements. Proprioceptors provide information about body position and movements. These receptors are located in muscles, tendons and joints. A. Skin (Cutaneous) Sensations Skin receptors are distributed in a pinpoint (punctiform) fashion. Touch receptors are more numerous than temperature receptors. The cold receptors are more numerous than hot receptors for a given area of skin. B. Distribution of Skin Receptors Receptors are not uniformly distributed on the body surface. Some areas of skin have a higher density of receptors than others and are more sensitive to a particular stimulus. For example, the fingers have a relatively high density of touch receptors and have great sensitivity, compared to the back. The back is not as sensitive, because of a relative low number of touch receptors per unit area of skin. C. Adaptation of Receptors When a receptor is excited a stimulus causes some change of its structure. If the stimulus remains constant over a period of time, the degree of change decreases and the number of nerve impulses (action potentials) fired by the receptor diminishes. This is known as adaptation of the sense organ that results in a decrease in the intensity of the sensation to the point where the stimulus is no longer felt at all. For example, touch receptors are excited when a mechanical (pressure) stimulus causes a deformation (change in shape) of the nerve cells in the receptor. If the stimulus is of sufficient strength a sensation will be felt. If the stimulus is of low strength and is kept constant, the receptor returns to its original shape and the sensation disappears. D. Taste Receptors Taste buds are specialized for transducing chemical stimuli (energy) into electrical energy of nerve impulses. Taste buds are found on the tongue. Taste is impossible on a dry tongue, the chemicals must be dissolved. Taste buds for each taste are not located to a specific area of the tongue as once thought. 1 2 E. Hearing Anatomy External Ear Sound waves are directed through the external auditory meatus of the outer ear to the middle ear. Middle Ear The sound waves vibrate the tympanic membrane or eardrum. This in turn vibrates the three small middle ear bones. Malleus (hammer) - attached to the tympanic membrane. Incus (anvil) - middle bone. Stapes (stirrup) - the inner bone up against the oval window. The middle ear is connected to the nose and the nasopharynx by the Eustachian (auditory) tube. The Eustachian tube prevents abrupt changes in air pressure from rupturing the eardrum as it equalizes air pressure on both sides of the tympanic membrane. Inner Ear Equilibrium The oval window separates the middle ear from the inner ear. From the vestibule there are three semi-circular canals. Sensory receptors (hair cells) within the vestibule and the ampulla (the enlarged portion) of each semicircular canal, provide the sense of equilibrium or balance. Sensory neurons from these receptor cells form the vestibular branch of the vestibulocochlear nerve (VIII). Auditory The cochlea, shaped like a snail's shell, contains the sensory receptors for hearing. Sound vibrations cause movements of the hair cells. The hair cells set up nerve impulses in the cochlear branch of the VIII cranial nerve. The two branches of the VIII cranial nerve pass through the temporal bone. G. Vision Anatomy Retina - inner layer of the eyeball The receptors for sight, the rods and cones are located in the retina. Rods - allows for black-white vision and the ability to recognize shapes and movements. The rods are in the periphery of the retina. Cones - allows for color vision and for visual acuity. The cones are densely concentrated in the center of the retina, the fovea centralis. This region lacking in rods, makes the fovea the area of sharpest vision. Optic disk or blind spot - the area where the axons of nerves coming from the rods and cones leave the retina to form the optic nerve (II). The impulses are then conveyed to the brain. 3 Choroid - middle layer of the eyeball This is a thin, black membrane that absorbs light rays. The anterior part of the choroid is the ciliary body, consists of smooth muscle that changes the shape of the lens to adjust for near and far vision, and the iris. Sclera - outer layer of the eyeball This is a fibrous white layer (the white of the eye). It covers all but the anterior surface. This surface is the cornea the transparent area that allows light to enter. Aqueous humor - fills the anterior cavity with a watery fluid that bathes the eye. It provides for transport of substances and maintenance of pressure in the eye. Iris - accounts for eye color, contains circular smooth muscle fibers that control size of the pupil. The pupil is the hole in the center of the eye, the diameter of which controls the amount of light entering the eye. Lens - a clear, oval structure attached to the ciliary body by suspensory ligaments. For near vision - ciliary muscles contract, lessen tension on the suspensory ligaments lens thickens, becomes more convex. For far vision - ciliary muscles relax, increase tension on the suspensory ligaments lens thins, becomes flatter, less convex. The process of changing lens shape to compensate for near and far vision is accommodation. Vitreous humor - fills the posterior cavity with a jellylike substance. It maintains pressure and shape of the eyeball and keeps the retina in place. Physiology H. Visual Acuity To see an object, receptor cells in the retina must transduce light energy into electrical nerve impulses and convey these impulses to the visual center of the brain. Whether the image is clear or blurred depends on the eye's ability to focus light onto its photoreceptors. In normal vision, emmetropia, the refractive surfaces (cornea and lens), of the eye focus light directly onto the retina. In nearsightedness, myopia, the image falls short of the receptors. In farsightedness, hypermetropia or hyperopia. the image falls beyond the receptors. The Snellen test, tests for visual acuity. The Snellen chart uses different sized letters at a given distance (20 feet). Visual acuity is expressed as a ratio as the distance at which the individual sees letters clearly divided by the distance at which a normal eye reads the letters. Visual acuity = Distance individual sees letters Distance normal eye sees letters 4 Visual acuity of 20/100 indicates below normal vision - what the normal eye sees clearly at 100 feet, can be seen by the individual only clearly at 20 feet. A ratio of 20/10 indicates above normal acuity - what the normal eye sees at 10 feet, can be seen by the individual at 20 feet. I. Astigmatism Astigmatism - a refractive defect of the eye caused by an irregularly shaped cornea and/or lens. The light rays spread out over the retina instead of converging to a specific focal point. An astigmatic individual will see some radiating lines indistinctly, others appear sharply focused. J. Near Point Determination To function effectively, the lens has to make rapid adjustments to changes in viewing distance. Change in lens curvature keeps the image sharp on the retina. These shifts in lens contour are due to the elasticity of the lens. With age the near point of vision becomes greater due to loss of elasticity of the lens. The near point of vision is the distance an object is held from the eye at which the image begins to blur. With loss of elasticity the lens is less able to accommodate for near vision. This is known as presbyopia. K. Pupillary Reflexes The retinal sense cells initiate reflex responses to light intensity and viewing distance. Both reflexes adjust the amount of light that enter the eye to an optimal level. Light Reflex This reflex causes the smooth muscle of the iris to contract- Circular smooth muscle constricts and radial smooth muscle dilates the diameter of the pupil. Near Reflex This reflex keeps objects in sharp focus at different distances. If observe an object far away and then view another object up close, three things happen to get the near object into focus instanteously. 1. Pupils decrease in diameter. 2. Convergence of the eyeballs. 3. Accommodation of the lens to the near object by increased refraction of light rays by the lens. L. Reaction Time to Auditory and Visual Stimuli The time required for response to a stimulus depends on: 1. sensitivity of the receptors; 2. nerve conduction velocity; 3. number of synapses in the path; 4. time it takes for the impulse to across a synapse (synaptic delay). 5 6 7 8 Sense Organs Receptor in General Receptors - Special nerve cells that respond to stimuli by initiating and transmitting action potentials or nerve impulses. A stimulus is a change in the environment. Those action potentials that reach a level of consciousness in the cerebral cortex are called sensations. Other action potentials that do not reach a level of consciousness go to spinal cord or sub-cortical areas of the brain. The different kinds of sensations - touch, temperature, sight, etc. are called modalities. Interpreting sensations from previous experience is perception. The sequence of events from stimulus reception to response are - detection of the stimulus by the receptor conduction of afferent impulses sensation and perception by the cerebral cortex conduction of efferent impulses to effectors response of the effectors. Only from the receptors can we perceive our environment, however, human receptors cannot detect all changes in the environment. Even if detect changes in the environment information may be lost or distorted before reaching the cerebrum. Interpretation of the sensation depends on information stored in the cerebral cortex. Properties of Receptors Receptors have properties in common. 1. Transduction - convert energies of the stimulus into electrical energy of the action potential or nerve impulse regardless of type of stimulus. Light receptors of eye transduce or changes light energy to electrical energy. Mechanical energy of pressure of the sound wave is transduced by the hearing receptors of the inner ear to electrical energy. The conversion of energy of the stimulus to electrical energy of the action potential is known as the law of specific energies. 2. Quality (modality or kind) of stimulus perceived is determined by: a. Type of receptor; and b. Brain destination of the action potential. If all receptors initiate action potentials, how does the nervous system recognizes different kinds of stimuli? Answer is that the action potentials from different receptors are relayed to specific areas of the sensory cortex that determines how the action potentials are interpreted. The kind of sensation depends on the part of the sensory cortex that receives the action potential not on the action potential itself. Thus, if hearing receptor's action potentials go to the visual cortex, you would see "sound" and if light receptor's action potentials go to the auditory cortex; you would hear "light". 3. Intensity of stimulus - Variations in the intensity of the stimulus is detected by: a. Number of receptors stimulated; and b. Frequency of the receptors firing off the action potentials. However, the stimulus must exceed a threshold or minimum strength - law of adequate stimulus. Increasing strength or frequency of the 9 stimulus increases frequency of receptors firing off action potentials and number of receptors activated. The sum of the action potentials reaching the sensory cortex is used to interpret the intensity of the stimulus. 4. Differential sensitivity of receptors Within normal range of strength of stimulus, each type of receptor has a low threshold for its response to its kind of stimulus. It has a high threshold and fails to respond to other kinds of stimuli. If a strong stimulus is applied can stimulate receptors that normally would not respond. For example, light receptors normally only respond to light, but if hit the eye this mechanical stimulus creates the sensation of light. 5. Adaptation Loss of receptor activity when a continual low intensity stimulus is applied which results in disappearance of sensation. Pressure and touch receptors adapt rapidly. This does not apply to pain receptors that are slow to adapt and is a protective device. The pain informs us of a dangerous situation and protects the body part from further injury. 10