Lecture 3 & 4 - Special Senses (HLTH1030)

Special senses I HLTH1030 – Anatomy & Physiology of Body Systems Associate Professor Ryan Anderton School of Health Sciences | University of Notre Dame Australia  Importance of sensory systems Exploring our senses is a HUGE concept The ability to sense changes...

Special senses I HLTH1030 – Anatomy & Physiology of Body Systems Associate Professor Ryan Anderton School of Health Sciences | University of Notre Dame Australia  Importance of sensory systems Exploring our senses is a HUGE concept The ability to sense changes in the environment and in the body allows survival How we interpret sensation determines how – We develop (physically, cognitively) – Our memories are formed – We interact socially – We become unique  The brain, behaviour and senses Early descriptions of behaviour were dependent on stimulus- response modelling Along with the the sensation of pain and touch, special sense constantly inform us of our environment As we will see, how senses are processed depend on the Visual input is acting as the special sense, the integration process response is driven through a complex process  The influence of sense - Simple organisms Simple organisms such as Hydra respond to light, pH and nutrient presence They have basic nerve connections and primitive histology However, Hydra defy standard SR modeling Hydra show endogenous activity when kept in a homogenous environment Bossert et al., 2012, Int. J. Dev. Biol  General Principles in higher animals Specialized receptor cells are distributed throughout eukaryotic organisms Purpose – to convert physical or chemical signal into electrical signal Electrical signals travel via peripheral nervous system to central nervous system (CNS) Afferent signals processed by CNS Efferent signals from CNS elicit appropriate response STIMULUS RECEPTOR SENSORY NEURON INTEGRATION  Cell body Dendrites (biosynthetic center (receptive regions) and receptive region) Neurons (Nerve Cells) Characteristics: – Long-lived ( 100 years or more) – Amitotic—with few exceptions – High metabolic rate—depends on Nucleolus continuous supply of oxygen and glucose – Plasma membrane functions in: Axon Electrical signaling (impulse generating Cell-to-cell interactions during and conducting region) development Nucleus Impulse direction Node of Ranvier Nissl bodies Axon Axon hillock Schwann cell terminals Neurilemma(one inter- (secretory node) Terminal region) branches Types of Sensory Receptors: Mechanoreceptors Stretching of cell membrane opens ion channels Examples: - pressure, touch, vibration - cell stretch (osmoreceptors) - balance (equilibrium) - sound - muscle length and tension - joint position and movement Principles of Animal Physiology, Moyes & Schulte, 2006, 1 st Edition, Benjamin Cummings, Fig 7.1, p 250  Example of Touch in humans Detect discriminative touch and body position Sensory nerve endings in the skin etc. relay to sensory neurons Reach DRG and fibres run only in dorsal column (posterior funiculi) Sensory decussation in medulla, relayed to VPL nucleus of thalamus prior to input to somatosensory cortex  Types of Sensory Receptors: Chemoreceptors Chemicals bind to specific receptors on cell membrane Examples: - CO2 - pH - various organic/inorganic molecules Principles of Animal Physiology, Moyes & Schulte, 2006, 1 st Edition, Benjamin Cummings, Fig 7.1, p 250  Thermoreceptors Free nerve endings, mainly in skin, lining of oral cavity and on tongue Have both warm and cold receptors/sensory neurons Allows ions to enter cell, eliciting generator potential TRPV1 receptor responds to heat, TRPM8 responds to cooling and menthol Most snakes have highly sensitive thermoreceptors In rattlesnakes, thermoreceptors are bunched in small pits  Nociceptors on Body Surface Free nerve ending chemoreceptors that respond to tissue-damaging stimuli In the brain the signals are perceived as pain Respond to variety of stimuli – Intense mechanical stimuli – Intense heat and cold – Chemicals released from damaged tissue (eg, chemical mediators of inflammation) Neuroscience, Purves et al., 3rd Edition, 2004, Sinauer Associates, Fig 9.6 p 220 Photoreceptors Respond to light Initiate chain of reactions Causes breakdown of 2nd messenger molecules and closure of ion channels O’Connor et al., 2010, Proceedings of Royal Society of Biol  Neuronal communication after detection How do sensory receptors communicate with sensory nerve? The transmission of a nerve impulse across a synapse is by means of a chemical called a neurotransmitter The minute gap between the two neurons = synaptic cleft.  Chemical Synapses Chemical synapses Axon terminal contains vesicles of chemical neurotransmitter Neuron secretes neurotransmitter that binds to specific receptors on cell membrane of second neuron Most common way in which neurons communicate Animal Physiology, Moyes & Schulte, 2nd Edition, 2008, Benjamin Cummings,  Signal Transmission at Chemical Synapse 1. AP’s arrive at axon terminal 2. Voltage-gated Ca2+ channels open 3. Ca2+ enters cell 4. Ca2+ signals to vesicles 5. Vesicles move to membrane 6. Vesicles dock and release neurotransmitter by exocytosis 7. Neurotransmitter diffuses across synaptic cleft and binds to specific receptors 8. Binding of neurotransmitter to receptor activates signal transduction pathway Animal Physiology, Moyes & Schulte, 2nd Edition, 2008, Benjamin Cummings  Chemical classification of neurotransmitters 1. Acetylcholine 2. Biogenic Amines – Dopamine, norepinephrine (NE), and epinephrine – Serotonin and histamine (5- Hydroxytryptamine or 5-HT) 3. Amino Acids – GABA, Glycine – Glutamate 4. Neuropeptides – Endorphins and Enkephalins – Somatostatin, gastrin, CCK, oxytocin, ADH 5. Purines 6. Gases and Lipids  Neurotransmitters Neurotransmitters can be either excitatory or inhibitory Excitatory NT Excitatory: – Initiate an action potential – E.g. acetylcholine, dopamine Inhibitory Inhibitory: NT – Inhibit an action potential – E.g. GABA, serotonin Glutamate is usually excitatory, however in phototransduction it is inhibitory  Sensory transduction Physical energy electrical energy Electrical energy produced by receptor = receptor potential Stimulus acts by affecting opening/closing of channels or affecting Ca2+ levels inside separate cell Principles of Animal Physiology, Moyes & Schulte, 2006, 1st Edition Separate cell causes Specialised nerve ending release of  Graded potentials and special senses A receptor potential is a graded response to a stimulus that may be DEPOLARIZING or HYPERPOLARIZING Receptor potentials have a threshold in stimulus amplitude that must be reached before a response is generated Graded means the amplitude of the receptor potential is proportional to the size of the stimulus The transduction process couples stimulus detection (i.e., activation of a receptor protein) to the opening or closing of ion channels  Stimulus Intensity: Generator potential Stimuli of increasing intensity activate, or recruit, greater number of receptors Stimulus Intensity: Receptor potential Human Physiology: An Integrated Approach, Silverthorn, 4 th Edition, 2009, Benjamin Cummings, Fig 8.13, p 263  When we get to the brain Principles of Human Physiology, Benjamin Cummings, Germann and Stanfield, 2nd International Edition © 2005, Fig 9.7, p 251 Any given neuron usually receives signals from many other neurons Special sense pathways can form complex neural pathways when they enter the brain When groups of neurons synapse there can always be integration  Lecture objectives Understand the importance of the sensory system in development, behaviour and daily activities List and describe the different sensory receptors Understand the role of graded potentials in sensory transduction Explain how sensory cells communicate, including the frequency and intensity  Special senses II HLTH1030 – Anatomy & Physiology of Body Systems Associate Professor Ryan Anderton School of Health Sciences | University of Notre Dame Australia  Animal responses to light The golden jellyfish follow the sunlight, as to follow the algae on which they feed The sensation of light therefore modulates the jellyfish’s behaviour  Patterns of Organization of Eyes Single cell Flatworms Higher Arthropods animals vertebrates Animal Physiology, Hill et al., 1st Edition, 2004,  Human Vision + eye evolution Our dominant sense – 70% of all sensory receptors in the body are in the eyes – Nearly ½ of the cerebral cortex is involved w/ processing visual information Darwin himself noted that “To suppose that the eye… could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree” However, statistical modelling shows the formation of our eye could occur in less than ½ a million years Recent studies have identified “eyeless genes” (Pax-6 and Aniridia) Structures of the eye Fibrous tunic – Outermost layer of the eye Vascular tunic – Choroid, ciliary body, iris – Pupil is the centre opening of the iris – Pupils are regulated by nervous imput Sensory/Neural tunic – Neural and pigmented layer  Outermost Layer Sclera - Tough connective tissue coat over majority of outside of eye - Makes up “white” of eye Cornea - Transparent structure in front of eye - Allows light to enter The corneal injury is Principles of Human Physiology, Germann and Stanfield visualised as a white patch on the cornea Middle layer Choroid - vascular, pigmented layer under sclera - stops reflection of light that reaches back of eye Lens - focuses light on the retina Ciliary body - contains ciliary muscles, which attach to lens by zonular fibres - change shape of lens to focus light Iris - regulates amount of light entering eye by adjusting diameter of pupil  Inner Layer: Retina Neural tissue that detects light 2 types of photoreceptors - rods and cones Fovea - where light from center of visual field strikes retina - area of greatest visual acuity Optic disk (papilla) - where optic nerve and blood vessels supplying eye pass through retina - no photoreceptor cells - also called “blind spot”  The retina Neural layer contains: – Photoreceptors that transduce light energy – Bipolar and ganglion cells – Amacrine and horizontal cells Ganglion cell axons: – Run along the inner surface of the retina – Leave the eye as the optic nerve The optic disc: – Is the site where the optic nerve leaves the eye  Rods and Cones Rods – Respond to dim light – Are used for peripheral vision – Black and white vision Cones – Respond to bright light – Have high-acuity color vision Kawamura, 2012, Cell membrane and transport – Are found in the macula lutea – Are concentrated in the fovea centralis  Internal Chambers of Eye Lens and ciliary body separate eye into 2 chambers Anterior (front) chamber contains clear, watery fluid (aqueous humour) - Supplies nutrients to cornea and lens Posterior (rear) chamber contains firm, jelly-like material (vitreous humour) - Maintains spherical structure of eye  Refraction of Light Waves by Eye Both cornea and lens have convex surfaces, causing light waves entering eye to converge onto retina A given point in visual field comes to focus on a single point on retina Passage of light waves through convex lens causes retinal image to be inverted and reversed  Mammals and birds focus (accommodate) their eyes by changing shape of lens Life: The Science of Biology, Purves et al., 6th Edition  Regulation of Light Entering Eye Inner circular and outer radial layer of iris smooth muscle controls pupil size, determining how much light enters eye In bright light, inner circular muscle contracts, causing pupillary constriction,  light entering eye In low light, outer radial muscle contracts, causing pupillary dilation,  light entering eye  Phototransduction Light-sensitive pigment (rhodopsin) located in “disks” in outer segment Rhodopsin comprises protein called opsin On exposure to light, retinal dissociates from opsin, initiating sequence of reactions that activates phosphodiesterase Phosphodiesterase degrades levels of cGMP inside cells In the dark In light Principles of Human Physiology, Stanfield,  How vision occurs Transmitters released from rod and cone cells communicate to bipolar cells in retina Varying degrees of convergence occur – determines visual acuity Ganglion cell axons make up the optic nerve Two optic nerves combine and partially decussate at the optic chiasm, just in front of brainstem Travel in optic tracts (“pathway”) to terminate in nucleus of thalamus called lateral geniculate body From here, projections are sent to many areas such as the visual cortex or suprachiasmatic nucleus  Suprachiasmatic Nucleus Part of the hypothalamic pathway Degeneration studies show direct input from the retina Thought to be part of circadian clock (jet lag) What roles does melatonin play? Circadian rhythms Correlation with Cancer Linked to seasonal stress Immune system Pineal SEX! SCN Light  Visual fields Lateral retina sees the medial visual field Medial retina sees the lateral visual field Medial optic nerve crosses over at the optic chiasm, lateral continues on Left visual cortex codes for ride visual field Right visual cortex codes for left visual field Lesions of the visual pathway The visual pathway was elucidated in animals Depending on where the lesion is, the visual field is affected differently Bedny, 2011, PNAS  Lecture objectives Ability to describe the cellular structure of the retina Ability to describe the processing of information from visual fields to the brain Understanding of the function of different receptors for light (photoreceptors) in the retina Understanding of the changes which occur to allow vision in dim versus bright light Ability to describe common problems in refraction of light within the eye and how these can be corrected