Download Sensory Physiology: A Comprehensive Overview and more Lecture notes Human Physiology in PDF only on Docsity!
Sensory physiology o Sensory input Vital to the integrity of personality and intellectual function o Sense organs Initiate somatic and visceral reflexes that are indispensable to homeostasis and to our survival o Sensory receptor A structure specialized to detect a stimulus Some are bare sensory nerve endings Others are true sense organs Nervous tissue surrounded by other tissues that enhance response to a certain type of stimulus An example would be the eye o Inside eye has photoreceptors – which detect the light o Also has extraocular muscles Help us turn the eyes so you can see better, so muscles here are accessory structures that help in vision Receptors act as transducers Generate graded potentials called receptor potentials in response to a stimulus Receptors are either specialized nerve endings of afferent neurons or separate cells that signal the afferent neuron Sensory cell, that is not sensory neuron, is very capable of detecting stimulus and will release NT to bind with neuron to produce action potential o Examples-Rods and cones of the eye that are capable of detecting light The cells in the eye that can form action potentials are ganglion cells and they not sensitive to light Just respond to NT Simple receptors Neurons with free nerve endings Complex neural receptors Have nerve endings enclosed in connective tissue capsules Special sense receptors Cells that release NT onto sensory neuron, causing an action potential
Muscle spindles o Always inform NS about how stretched (length) the muscles are Tendon organs (detect muscle tension) o Also known as Golgi tendon organs o Present between muscles and tendons o Always give information about how much the muscle has contracted Sensory Receptor classification o Based on Modality – Nature of stimulus Thermoreceptors Varying degrees of heat Photoreceptors Photons of light Nociceptors Pain Chemoreceptors Oxygen, pH, various organic molecules like glucose Mechanoreceptors Pressure (baroreceptors), cell stretch (osmoreceptors), vibration, acceleration, and sound o Based on Origin of stimuli Exteroreceptors Detect external stimuli Skin, eyes Interoreceptors Detect internal stimuli Baroreceptors (blood pressure) Proprioceptors Sense body position and movements Also internal Always telling body how tensed the muscles are, how much they are stretched, etc o Based on Distribution General senses Also known as somesthetic or somatic Widely distributed Special senses Limited to head Vision, hearing, equilibrium, taste, and smell Sensory definitions o Transduction The conversion of one form of energy to another Fundamental purpose of any sensory receptor Conversion of stimulus energy (light, heat, touch, sound, etc.) into nerve signals o Receptor potential
Sensory transduction o Stimulus energy converted into information to be processed by CNS o Ion channels or second messengers initiate change in membrane potential o Adequate stimulus Preferred form of stimulus for a receptor For eyes the preferred form of stimulus is light For ears the preferred form of stimulus is vibrations from sound waves o Threshold Minimum stimulus needed to trigger an action potential o Receptor potential Change in sensory receptor membrane potential, a graded potential in a receptor cell Somatosensory projection pathways o From receptor to a specific destination in the brain, most somesthetic signals travel by way of three neurons o First-order neuron Afferent neuron From body Enter the dorsal horn of spinal cord via spinal nerves From head Enter pons and medulla via cranial nerve Touch, pressure, and proprioception on large, fast, myelinated axons Heat and cold on small, unmyelinated, slow fibers o Second-order neuron ( they are interneurons) Decussation to opposite side in spinal cord, medulla or pons End in thalamus Except for proprioception which ends in cerebellum o Third-order neuron (interneurons) Thalamus to primary somesthetic cortex of cerebrum Receptive field o Is the area or region which is under a sensory neuron’s jurisdiction of sensing o Receptive field of secondary neuron is the combined areas of all of the primary neurons’ receptive fields Primary neurons converge onto secondary neuron o Activity in a sensory unit (a sensory neuron and its receptive field is sensory unit) is altered by peripheral events and communicates information into the CNS o Some neurons have small receptive fields, and some have very large receptive fields o Overlapping stimulation between neighboring receptive fields provides general information about the location of a stimulus More peripheral terminals of B are stimulated compared to A and C so more action potentials from B are sent o Differential arrival time of action potential allows the brain to tell where the stimulus is coming from o Two-Point Discrimination Many primary sensory neurons converging onto a single secondary neuron creates a very large secondary receptive field Look at the image in the PowerPoint The two stimuli will be perceived as a single point because both stimuli fall within the same receptive field When fewer primary neurons converge, secondary receptive fields are much smaller The two stimuli activate separate pathways and hence are perceived as two distinct stimuli How different parts of the brain get information from different sensory organs o Eyes Optic nerve Sends information to the thalamus Thalamus sends it to visual cortex where actual processing happens
o Tongue Gustatory neurons Goes to thalamus and then goes to gustatory complex where actual processing happens o And so on for other sensory organs o All going to different parts of the cerebral cortex via the thalamus Thalamus acts as a relay station The only sense that does not go through the thalamus is smell (olfaction) Goes directly to olfactory complex Part of cerebral cortex responsible for processing smell is very close to the part of the cerebral cortex that is concerned with memory o The reason why when you smell something you get a flood of memory associated with that smell Properties/Attributes of a Stimulus to consider for decoding sensory information o Incoming information from all sensory receptors is coming to brain in the form of action potentials Brain will look for these 4 characteristics to decode what that information is o 1. Modality Indicated by the nature of the stimulus or activated receptor Activated sensory neurons If sensory information is delivered through optic nerve then the brain will equate that information to vision/light If sensory information is delivered through olfactory nerve then the brain will equate that information to smells Specific to receptor type Labeled line coding 1:1 association of a sensory receptor with a sensation When you rub your eye and you see bright spots because it stimulates the receptors mechanically due to rubbing and the CNS will assume this as information about light even though you stimulated the eyes mechanically by rubbing. o Location (areas of brain involved in processing of specific sensory information) Neural processing of sensory input occurs at specific locations in the brain, see image for various processing centers in the brain So, location in the brain where exactly that information (action potentials are delivered for processing is important in interpreting the incoming sensory information For example, during some brain surgeries patients complain of cold touch on hand, this happens due to accidental touching by a probe of a part of the brain responsible for sensing cold. Lateral line inhibition It helps in localizing the stimulus When a stimulus stimulates multiple receptive fields and the pathway closest to the stimuli inhibits the other pathways Enhances the perception of the stimulus and helps in localization of the stimulus Inhibitory pathways at secondary neuron Presynaptic inhibition at secondary neuron o Intensity Information about stimulus intensity is conveyed by increase in frequency of action potentials Higher frequency of actions potentials is generated during stronger stimulus and brings about more NT release The number of action potentials generated by a pressure-sensitive, sensory afferent neuron is directly proportional to stimulus intensity o Duration Longer stimulus result is generating action potential for longer duration. However, some neurons exhibit a property called adaptation. Adaptation If stimulus is there for long period of time, that will cause decrease or cessation of action potentials frequency.
o Extraocular muscles (6) Muscles outside the eye o 6 skeletal muscles for each eye Help in the movement of the eyeball o Movement is important to turn eye and fix gaze on object to bring it into focus We want the light rays entering eye to fall on fovea o Continuously scan the image don’t just look at one part constantly o Saccades Quick, jerky movements of the eyeball Purpose is to cause light rays to fall on fovea and also prevents overstimulation of rods and cones for long time Don’t want fatigue or adaptation of sensory receptors o Movements of the eyes are tightly regulated by skeletal muscles whose neural controls are influenced by head position and operated in ways that assure convergent image formation o Lacrimal glands are on upper lateral sides of eyes Secrete tears o Tears are very important to keep eye from getting dry o Tears are antibiotic Lysozyme Hydrolytic enzyme that can break down walls of bacteria Peptidoglycans of bacterial walls are digested by lysozyme IgA (immunoglobulin A) Secretory immunoglobulin o Usually present in secretions such as sweat, tears, milk, etc Also contain electrolytes o Excess of tears comes out nasolacrimal duct o Tarsal glands or Meibomian glands in the eyelashes Purpose is to produce oil to prevent evaporation of the tears Internal anatomy of the eye
o Eye is ball filled with fluid o Three layers Sclera o Outermost, tough, made up of connective tissue o Offers structural support o White of the eye o Becomes transparent and forms cornea in the front Cornea When light rays move from one medium to another the waves bend o Light refraction Cornea focuses light onto fovea centralis o Area of acute vision Most of the refraction (2/3 of refraction) is done by cornea o The remaining 1/3 of refraction is done by lens The refraction in cornea is fixed (or constant) o While Lens can change its shape and, thus refract the light and accommodate for different distances (offers flexibility) Choroid layer o Highly vascularized o Perforation in middle is called pupil o Forms ciliary body and iris as it comes to frontside of eye o Pigmented with melanin Color of the iris varies depending on the amount of melanin pigment we have Purpose of pigmented middle layer is to absorb stray light rays to prevent reflection and re-reflection so no blurred images formed while vision processing o Iris is made up of circularly arranged smooth muscles If they constrict less light enters the eye Iris constrictors Parasympathetic NS Also has radiating smooth muscles When they constrict they will dilate the eye (let more light in) o Iris dilators Sympathetic NS (fight or flight need more light to get more information about the surroundings) o Suspensory ligaments are connective tissue cords Inelastic Suspends lens When ciliary muscles contract they will loosen the suspensory ligaments The lens will become rounder o Lens is bioconvex structure o Can be very round or very thin disc like based on how much pull is exerted on it by suspensory ligaments o Should be very round when looking at close object Ciliary muscles contracted o Presbyopia Lens loses its flexibility so cannot focus properly in older individuals Why older people have to hold stuff farther away The contraction state of the ciliary muscles determines the amount of tension that the zonular fibers exert on the lens Zonular fibers = suspensory ligaments Contracted = lower tension and more rounded lens Relaxed = higher tension and more flattened lens Retina
Jelly-like substance Mostly transparent Sometimes can see floaters o Aqueous humor Quite watery compared to vitreous humor Fills both posterior and anterior chambers o Posterior chamber is from iris to lens o Anterior chamber is from iris to cornea Produced from ciliary body Excess fluid is constantly drained through Canal of Schlemm Glaucoma o If aqueous fluid is not drained due to blockage of canal, the intraoccular pressure will increase and that can damage vision o Leading causing of blindness in developed world
Summary of the eye Layer of the eye Description Sclera White part of the eye Outer covering of the eye Thick in relation to the other layers Made up of connective tissue Functions to protect the inner structures of the eye Choroid Pigmented layer that helps to absorb stray light Middle layer Retina Inner layer that contains the photoreceptor cells - rods, cones Fovea/Fovea centralis - area with the highest concentration of cones; directly behind the pupil Structure of the eye Description Cornea Extension of the sclera At front of eye Clear, allows light to pass through Convex shape that helps the focusing of light Anterior chamber Right behind cornea Filled with aqueous humor (fluid) Iris Made up of pigmented smooth muscle Colored part of your eye Controls size of pupil Regulates the amount of light that enters the eye by controlling the size of the pupil Pupil Opening in the middle of the iris Posterior chamber Space between the iris and the lens Filled with aqueous humor (fluid) Lens Help focus light on the retina Accommodation - allows us to focus light Can become rounder or thinner Ciliary muscle and suspensory ligaments Suspensory ligaments connect the lens to the ciliary muscle; control thickness and thinness Vitreous chamber Large chamber behind the lens Filled with vitreous humor - jelly-like fluid Holds the retina in place Accommodation o The process by which the eye adjusts the shape of the lens to keep objects in focus o Emmetropia Default state of eyes while you are looking at object which is about 20 feet far Eye is relaxed When looking at far objects Light rays reflected by the object is nearly parallel from distant objects so don’t have to work hard
o Myopia Nearsightedness o Can see near objects clearly Eyeball is too long/big Light rays are focused in front of retina o Need to defract the light rays Corrected by concave lens
Source of this picture-https://www.asiapacificeyecentre.com.sg/hyperopia/ Photoreceptors o Rods and cones o Color-blindness Results from a defect in one or more of the three types of cones o Rods are responsible for night vision and cones are responsible for day vision and color vision o Structure of photoreceptors Outer segment o Consists of stacks of membrane which has visual pigments o Visual pigments are collectively called photopsins In cones – the pigments are specifically called iodopsins Three different types of iodopsins are present in three types of cones o there are three types of cones S cones Short wavelength cones Detect Blue light M cones Medium wavelength Green L cones Long wavelength Red Three primary colors – red, blue, green o Red has longest wavelength, green has medium, blue has shortest Each cone is very sensitive to a specific wavelength But if you send a wavelength that it isn’t sensitive to, it will still stimulate just to a lesser degree Differential stimulation Helps us to analyze various shades of colors due to differential stimulation of these 3 types of cones In rods – visual pigment is called rhodopsin o Visual pigment has two parts 1. Protein part called Opsin 2. Retinal – this is derived from Vit A Vitamin A is also referred to as retinol It exist in two states, cis-retinal and all trans-retinal Cis form is bent form When retinal becomes straight in response to light it is called trans-retinal Aka bleached form Activated Inner segment o Location of major organelles and metabolic operations such as photopigment synthesis and ATP production Synaptic terminal o Synapses with bipolar cells Phototransduction o The principles of phototransduction is same for both rods and cones o There are two pathways, 1. Light-on pathway and 2. Light-off pathway o In Light-on pathways action potentials are generated in ganglion cells when the photoreceptors are stimulated by light o In light-off pathways action potentials are not generated in ganglion cells when the photoreceptors are stimulated by light
Bipolar cells For some bipolar cells glutamate is inhibitory for others it is excitatory Glutamate inhibits bipolar cell, causing it to be hyperpolarized; bipolar cell therefore does not release NT No glutamate to inhibit bipolar cell, so bipolar cells are depolarized, and they release NT Glutamate excites bipolar cell, causing it to depolarize and release NT No glutamate so bipolar cell is in hyperpolarized state (inhibits); do not release NT Ganglion cells Only cells that generate action potentials Does not create action potential Creates action potential Creates action potential Does not create action potential Phototransduction in Rods o When there is no light = rhodopsin is in cis-retinal form. Rhodopsin contain an opsin part and a retinal part Opsin and retinal fit well together cGMP is a second messenger whose levels are high during darkness Causes the opening of sodium channels When sodium enters the membrane will be depolarized (-40 mV) Dark current o Sodium entering the cell during darkness Calcium channels open in response to depolarization and that causes exocytosis of NT Tonic release of NT onto bipolar cell o When light falls, light bleaches rhodopsin Converted from cis form to trans-retinal form Can no longer fit with opsin so they dissociates Opsin is bleached and activates transducing, a membrane protein When transducin is activated it will activate phosphodiesterase (enzyme) This enzyme neutralizes cGMP Thus lower levels of cGMP results in closing of sodium channels o Potassium continues to leave causing hyperpolarization Hyperpolarization causes less or no exocytosis of NT After some time the pigment is converted back into active form Come back and reassociate with opsin
Processing visual information o Visual fields If you close one eye, what can you see with one eye is its visual field When open both the eyes there is a region where the visual fields of the two eyes overlap This overlapping is important because it helps us to have binocular vision and depth perception o Binocular vision Because of visual field overlap
