Chapter 7 - Human Physiology
Chapter 7 - Human Physiology BIOL 2213
Popular in Human Physiology
Popular in Biology
This 8 page Class Notes was uploaded by Celine Notetaker on Sunday February 28, 2016. The Class Notes belongs to BIOL 2213 at University of Arkansas taught by Dr. Hill in Fall 2014. Since its upload, it has received 45 views. For similar materials see Human Physiology in Biology at University of Arkansas.
Reviews for Chapter 7 - Human Physiology
Report this Material
What is Karma?
Karma is the currency of StudySoup.
Date Created: 02/28/16
Chapter 7 – Sensory Physiology Five Classes of Sensory Receptors – There are 5 classes of sensory receptors that detect different stimuli. One important thing to note is that the term receptor has two distinct meanings. One refers to sensory receptors and the other refers to proteins on plasma membranes or inside cells. So we have sensory receptors and protein receptors. A stimulus is the energy or chemical that impinges upon and activates a sensory receptor. An adequate stimulus is the type of stimulus to which a particular sensory receptor responds in normal functioning. A sensory transduction is when the stimulus is transferred into an electrical response. Furthermore, any given sensory receptor gives rise to only one sensation. The following lists the 5 classes of sensory receptors: 1. Mechanoreceptors – respond to mechanical stimuli such as pressure and stretch, and are responsible for touch, blood pressure, and muscle tension. 2. Thermoreceptors – detect sensations of cold or warmth 3. Photoreceptors – respond to particular ranges of light wavelengths 4. Chemoreceptors – respond to the binding of particular chemicals to protein receptors on the sensory receptor membrane 5. Nociceptors – specialized neuronal endings that respond to a number of different stimuli, such as heat or tissue damage Receptor Potential – In sensory receptors, the transduction process begins with the opening of ion channels either directly or through a secondmessenger system. These channels are either located on the distal end of the axon or on a special receptor cell, that transfers neurotransmitters to the dendrite of another afferent neuron. Receptor potential is the change in graded potential caused by ion channels allowing an influx across the sensory receptor membrane. So, to reiterate, ion channels are the main source of creating receptor potential (graded potential) on the receptor membrane to eventually an action potential if the graded potential is strong enough to reach the axon’s voltagegated Na channels. Also, the more intense the graded potential, the more frequently the action potential fires, up to its limit due to the absolute refractory period. Adaptation is a decrease in sensory receptor sensitivity, which results in a decrease in action potential frequency. Primary Sensory Coding – Coding is the conversion of stimulus energy into a signal that conveys the relevant sensory information to the central nervous system. Important characteristics of a stimulus are as follows: type of energy, intensity, and location. 1. Stimulus Type – This is called modality. The type of sensory receptor a stimulus activates plays the primary role in coding the stimulus modality. A given sensory receptor is particularly sensitive to only one modality. Also, the receptive fields for different modalities overlap, so that a single stimulus, such as an ice cube on the skin, can give rise to sensations of pressure and temperature at the same time. 2. Stimulus Intensity – As the strength of a local stimulus increases, receptors on adjacent branches of the same afferent neuron are activated, resulting in a summation of their local currents. Stronger stimuli also tend to affect a larger area and activate similar receptors on the endings of other afferent neurons. 3. Stimulus Location – This refers to where the stimulus is being applied. Stimulus location is coded by the site of sensory receptor, which sends an action potential along specific anatomical pathways, called labeled lines, to a specific region of the CNS associated with that particular modality and location. Sensory Information Pathway – A sensory pathway is also known as an afferent sensory pathway. These chains of afferent neurons travel in parallel bundles to the CNS. In the CNS, they synapse with ascending pathways, which project up to the brain. There are 2 types of ascending pathways: 1. Specific Ascending Pathway – the ascending pathways in the spinal cord and brain that carry information about only single types of stimuli (different types of receptors). They pass through the brainstem and thalamus and then go to specific sensory areas of the cerebral cortex (except olfactory pathways). These specific sensory areas are called primary cortical areas. a. Somatosensory Cortex – a strip of cortex that lies in the parietal lobe which gets information from somatic receptors from the outer parts of the body including skin, skeletal muscle, tendons, and joints. b. Visual Cortex – located in the occipital lobe which gets information from the eyes. c. Auditory Cortex – located in the temporal lobe which gets information from the ears. d. Olfactory Cortex – located on the undersurface of the frontal and temporal lobes which gets information from the olfactory projections. 2. Nonspecific Ascending Pathway – activated by sensory units of several different types and therefore signals general information. They just indicated that something is happening. Cortical Association – Cortical association areas are brain areas that lie outside the primary cortical areas but are adjacent to them. Unlike primary cortical areas, cortical association areas are not considered part of the sensory pathways, but are important in the perception of senses. After information is sent to the primary cortical areas, it is then relayed to the cortical association areas for further processing. Some of the neurons in the in the cortical association areas can receive and integrate input from more than one type of sensory stimuli (multiple primary cortical areas get processed by a single cortical association area). An example is vision and neck position causing an awareness of head position. Therefore, cortical association areas are involved in complicated perception. Factors that affect perception include: 1. Receptor adaptation 2. Emotions, experience and personality 3. Not all stimuli give rise to conscious sensation a. Blood pressure in arteries 4. Lack of receptors for certain stimuli a. Radio waves 5. Damaged neural pathways 6. Drugs Short Summary of Neural Happenings: 1. 1+ type of receptors can exist in a sensation area (eyes, skin, ears, etc.) 2. The action potentials from the sensation area travel to their respective primary cortical area via ascending pathways 3. 1+ primary cortical areas can relay information to 1+ cortical association areas a. Ability of cortical association areas gives rise to various perceptions Somatic Sensation – This describes sensations from the skin, muscles, bones, tendons, and joints and is initiated by a variety of sensory receptors (mechanoreceptors, chemoreceptors, and thermoreceptors all give rise to the sensation of ‘touch”). Touch and pressure are associated with mechanoreceptors. Some of these mechanoreceptors adapt quickly, so that they only fire when the stimulus is changing. Fastadapting mechanoreceptors give rise to touch, movement, and vibration. On the other hand, some mechanoreceptors adapt slowly which give rise to pressure. Spatial discrimination happens according to the size of the receptor field. For example, some receptors have small, well defined receptor fields, like in the fingertips. 4 receptors are prevalent in somatosensory systems: 1. Meissner’s Corpuscle – fastadapting mechanoreceptor (touch and pressure) 2. Merkle’s Corpuscle – slowadapting mechanoreceptor (touch and pressure) 3. Pacinian Corpuscle – fastadapting mechanoreceptor (vibration and deep pressure) 4. Ruffini Corpuscle – slowadapting mechanoreceptor (skin stretch) Neural Pathways of Somatosensory Systems – Peripheral afferent nerve fibers from somatic receptors synapse on neurons that form specific ascending pathways projecting into the somatosensory cortex via the brainstem and thalamus. All peripheral afferent nerve fibers either synapse on the anterolateral pathway (ascending pathway) in the gray matter in the spinal cord, or enter the dorsal column pathway (ascending pathway) in the white matter where the fibers do not synapse until they reach the brainstem. Once inside the somatosensory cortex, the endings of axons from specific somatic pathways are grouped according to peripheral location. Visible Light – Light is anything in the visible spectrum. The wavelengths capable of stimulating receptors in the eye range from about 400 to 750 nm. Different wavelengths of light within this band are perceived as color. Anatomy of the Eye: Photoreceptors – First of all, accommodation refers to the adjustments made for distance due to changes in lens shape. The retina is an extension of the central nervous system that contains photoreceptors. The two types of photoreceptors are called cones and rods. 1. Rods – respond to very low levels of illumination 2. Cones – respond only to high levels of illumination Light passes all the way through all cell layers into the back of the retina, where the rods and cones are located. Directly behind the retina is a layer called the pigment epithelium. This layer absorbs light so that there is no reflection and scattering back on the photoreceptors, which would cause an image to blur. Photoreceptors contain molecules called photopigments, which absorb light. Rods contain Rhodopsin. All photopigments contain membrane proteins called opsins, which surround and bind to a chromophore (retinal) molecule. Chromophores are light sensitive molecules. The opsin in each photoreceptor is different and binds to chromophores in different ways, causing each photopigment to absorb light at a specific energy. Photoreceptors ← Photopigments (opsins) ← Chromophore (retinal) PhotoTransduction – Each photoreceptor contains over a billion photopigments (opsins). Photoreceptors act in reverse to many other sensory receptors in that its resting state is depolarized. Photoreceptors hyperpolarize in response to an adequate stimulus. The following outlines the steps of phototransduction: 1. Guanylyl cyclase (an enzyme) converts GTP into a high concentration of intracellular cGMP. 2. cGMP (a ligand) maintains the ligandgated ion cation channels open, so there is a + 2+ persistent influx of Na and Ca . a. Therefore, in the dark, cGMP concentrations are high and the photoreceptor is depolarized. Also, there are a lot of intracellular cations in the dark. 3. Light shines on a chromophore (retinal) molecule, changing its shape. 4. The change in chromophore shape alters the shape of the opsin. 5. Opsin interacts with transducin, a Gprotein receptor. 6. Transducin activates the enzyme, cGMPphosphodiesterase, which degrades cGMP. 7. Cation channels close. 8. The membrane potential hyperpolarizes to produce the sensation of light. Photoreceptor Adaption – There are 2 types of adaptation: 1. Dark Adaptation – Vision is only supplied by the rods in a dark room. In a lighted area, rhodopsin is completely activated. Restoration to resting state (of the chromophore) takes several minutes. Once at its resting state, retinal becomes light sensitive again. 2. Light Adaptation – Rods are overwhelmingly activated as the retinal receives a lot of light energy at once. Eventually, rhodopsin is inactivated and the rods become unresponsive so that only less sensitive cones are operating. Neural Pathways of Vision – Light signals are converted to action potentials through the interaction of photoreceptors with bipolar cells and ganglion cells (clusters of neurons in the CNS). Photoreceptors and bipolar cells undergo graded potentials in response to the hyperpolarization of rods and cones. Photoreceptors interact with bipolar and ganglion cells in 2 ways, called the ONpathway and the OFFpathway. In both pathways, photoreceptors are depolarized in the absence of light, causing the neurotransmitter glutamate to be released onto bipolar cells. In the presence of light, photoreceptors are hyperpolarized and glutamate is not released. From here, the pathways differ: 1. ONpathway – Light hyperpolarizes photoreceptors, causing glutamate release to stop. In these bipolar cells, glutamate receptors are inhibitory, so the absence of glutamate depolarizes the bipolar cell. This causes an increase in the frequency of action potentials. 2. OFFpathway – Light hyperpolarizes photoreceptors, causing glutamate release to stop. In these bipolar cells, glutamate receptors are excitatory, so the absence of glutamate hyperpolarizes the bipolar cell, causing a decrease in the frequency of action potentials. The axons of the ganglia form the output from the retina, which is called the optic nerve (cranial nerve II). The two optic nerves meet at the base of the brain to form the optic chiasm, where the fibers cross and travel within the optic tracts to the opposite side of the brain into the visual cortex. Color Vision – This begins with the activation of photopigments in cone photoreceptor cells. There are 3 types of cones: red, green, and blue. With any wavelength of incoming light, the different cones respond with different graded potentials. Anatomy of the Ear: Sound Transmission – Sound waves enter the external auditory canal. Air molecules push against the tympanic membrane, causing it to vibrate at the same frequency as the sound wave. The middle ear cavity is exposed to atmospheric pressure via the Eustachian tube which is connected to the pharynx through slits. When the slits are closed, and the pressure changes, the pressure stretches the tympanic membrane and causes pain. 1. Sound waves vibrate the tympanic membrane. 2. Vibrations enter the middle ear. 3. The malleus, incus, and stapes amplify the sound pressure through the oval window and into the cochlea. The total force applied to the tympanic membrane and oval window are the same, but the force per unit area in the oval window is much higher. 4. The cochlear duct is a membranous tube within cochlea. The cochlear duct is filled with fluid called endolymph, which is similar in electrolyte composition to intracellular fluid. Outside the cochlear duct is filled with perilymph. The scala vestibuli is above the cochlear duct and begins at the oval window. The scala tympani are below the cochlear duct and begin at the round window. Anyways, waves of pressure are created in the scala vestibuli. This pressure is transmitted into the cochlear duct and dissipated by the round window at the scala tympani. 5. The side of the cochlear duct nearest the scala tympani is the basilar membrane, which sits the organ of Corti that contains the ear’s sensory receptor cells. The sound waves from step 4 cause the basilar membrane to vibrate. Hair Cells of the Organ of Corti – Receptor cells of the organ of Corti are called hair cells. Hair cells are mechanoreceptors. There are two types of hair cells, inner and outer hair cells. The tectorial membrane overlies the organ of Corti, and embeds the projections of the outer hair cells. When there is pressure in the cochlea, the basilar membrane is displaced, and mechanically gated ion channels open in the hair cells. This causes depolarization, creating influx of Ca from voltagegated calcium channels, triggering release of the neurotransmitter, glutamate. This causes generation of an action potential in the neurons, whose axons form the cochlear branch of the Vestibulocochlear nerve (cranial nerve VIII). Finally, low frequency sounds distort the helioctrema region while high frequency sounds distort the region closest to the middle ear. Cochlear fibers synapse in the brainstem. The interneurons send information through the thalamus and into the auditory complex in the temporal lobe. Vestibular System – The vestibular system consists of the vestibular apparatus, a series of membranous tubes that connect with the cochlear duct. The vestibular apparatus consists of 3 semicircular canals and two saclike swellings called the utricle and saccule. 1. Semicircular Canals – The 3 semicircular canals are oriented in the 3 main 3D planes so rotation can be detected. Receptor cells are located in 3 gelatinous masses (1 for each semicircular canal) called the cupula, which is encapsulated by a bulge called the ampulla. During head movement, the ampulla is pushed against the stationary endolymph fluid, causing the bending of hair cells, which alters the release of neurotransmitters. 2. The Utricle and Saccule – The utricle and saccule provide information about linear movement due to changes in head movement relative to the forces of gravity. The hair cells of the utricle and saccule are covered by a gelatinous substance in which tiny stones called otoliths are embedded. The otoliths (calcium carbonate crystals) make the gelatinous fluid heavier than the endolymph, so the gel will move according to the forces of gravity and will pull against the hair cells so the mechanoreceptors are stimulated. a. Utricle – hair cells point straight up (respond when you tip your tip) b. Saccule – hair cells are at right angles (respond when you go from lying to standing)