Sensory Systems Lecture Notes PDF

Reminders Stage 2 quiz at end of class Midterm viewing after reading week Last Class: Sensory Receptors Involves four steps Cerebral 1. Stimulation of the sensory receptor...

Reminders Stage 2 quiz at end of class Midterm viewing after reading week Last Class: Sensory Receptors Involves four steps Cerebral 1. Stimulation of the sensory receptor cortex ◦ Receptive field, location Interneuron Thalamus 2. Transduction of the stimulus Interneuron 4 Integration of ◦ Receptor (Graded) potentials Stimulus Axon of sensory input in sensory 1 Stimulation Sensory neuron the CNS 3. Generation of action potentials of sensory receptor receptor ◦ Coding for intensity, duration ◦ adaptation Graded Action 4. Integration of sensory input 2 potential 3potentials Transdu Generation ◦ Modality ction of of action the potentials stimulus Last Class: Sensations Tactile sensations Thermal sensations Why? ◦ hot and cold receptors Pain sensations ◦ Nociceptors Section 1: https://forms.gle/bNpGNSpjpxVQ1UaK7 Section 2: https://forms.gle/YN4WW3q3mLeBfGkZ9 Review Questions Visual sensory receptors (photoreceptors) are examples of: ◦ Slow adapting receptors ◦ Fast adapting receptors ◦ High-intensity receptors ◦ Somatic receptors Overlapping receptive fields: ◦ Increase stimulus location acuity ◦ Are not possible ◦ Are created by one sensory receptor ◦ Apply only to touch receptors Pain Sensations: Referred pain Referred pain: Pain of a visceral organ that appears to be of somatic origin. ◦ E.g., left arm pain during a heart attack ◦ Occurs because both somatic sensory and visceral sensory neurons often converge on second-order neurons of the same ascending pathway to the brain  Referred Pain Examples i.e. Areas where visceral sensory neurons converge with somatic sensory neurons Proprioceptive Sensations Provide information about muscle and joint position Muscle spindles, tendon Sensory axons organs, joint kinesthetic receptors Sensory axon Tendon Muscle spindle sensory endings Muscle spindle capsule Sensory nerve endings Tendon organ capsule Muscle Spindle Detects: ◦ Static muscle length Activated when ◦ Velocity of shortening/lengthening muscle stretches Muscle Spindle: The Stretch Reflex Spindle firing causes ◦ Facilitation or a stretch reflex contraction in homonymous muscle (muscle that was stretched) ◦ Reciprocal inhibition in antagonist. Demo: ◦ Knee stretch reflex Far more complex than this ◦ Plays a role in almost every movement Tendon Organ and Reflex Tendon Organs: Free sensory endings intertwined within tendon Force detector (tension in tendon) Primary “reflex” action is to: ◦ Inhibit homonymous muscle and synergists Opposite of spindle ◦ Override muscle spindle facilitation ◦ Facilitate antagonist muscle and its synergists Prevents injury! E.g. drop from low box versus drop from high box! Joint Kinesthetic Receptors ◦ Detect joint movement and pressure ◦ Inhibit muscles when excessive strain Somatic Sensory Pathways Most pathways decussate (cross over to other side of body) ◦ Connect to primary somatosensory cortex  Primary somatosensory cortex Remember: Somatosensory Cortex Somatotopic organization of different regions of the body Shoulder Leg ◦ Adaptable Foot Toes ◦ Areas do overlap Genitals Project to somatosensory association area (subregions) ◦ e.g. recognize objects Summary: Somatic Sensations 1. Tactile sensations 2. Thermal sensations 3. Pain sensations 4. Proprioceptive sensations Reflexes Labelled lines to somatosensory cortex Checkpoint How does a tactile stimulation of a receptor change membrane potential? What causes referred pain? How does the activation of muscle spindles differ from tendon organs? Special Senses Smell Taste Vision Hearing Equilibrium Olfactory System Olfactory epithelium Receptors for olfaction (sense of smell) located in 5cm2 patch called olfactory epithelium ◦ Upper nasal cavity ◦ Olfactory glands produce mucus to dissolve odorants Parts of olfactory (I) nerve Olfactory gland Odorant binds to olfactory receptors linked to a G-protein Olfactory receptor cells ◦ Signal transduction leads to Olfactory depolarization (similar to other sensory epithelium receptors) Odorant molecule Olfactory System Olfactory cortex Action potentials carry information along the olfactory tract to the olfactory cortex The threshold for detecting odors is low Olfactory ◦ Only a few molecules need to be present to be tract perceived as odor ◦ E.g. Small amount of methyl mercaptan added to natural gas to provide an odor, and prevent lethal leaks from going unnoticed Olfactory tract Olfactory system rapidly adapts ◦ 50% reduction in first second ◦ Slowly thereafter Part of olfactory (I) nerve Olfactory epithelium Olfactory receptor cell Gustatory System Taste bud 5 primary tastes ◦ Salty – Na+ ◦ Sour – H+ ◦ Sweet – sugars Taste pore ◦ Bitter – variety (including poisons) Microvilli ◦ Umami – amino acids Gustatory receptor cells Gustatory ◦ Responds to only 1 type of tastant receptor cell ◦ Each taste bud contains all types of gustatory receptors ◦ Synapse with taste neurons First-order taste neuron (c) Components of a taste bud The Visual System Retina Functional components of the eye: Iris – gives you eye colour. Controls the diameter of pupil by constricting or dilating Iris Lens changes shape to focus the light Fovea on the retina light Lens Retina – converts light into action potentials Optic (II) nerve Lens focuses light The lens forms images of objects on the retina ◦ Flipped upside down and side to side Lens focuses light from the same source to one area on the retina ◦ Otherwise object would be blurry ◦ Accommodation: adjusting focus point (e) Example of an image focused on the retina Organization of the Retina Photoreceptors – include rods and cones. Detect light and convert to Rod receptor potentials Cone Signals passed to bipolar cells, then ganglion cells (whose axons give rise to optic nerve) Biopolar cell Ganglion cell Optic (II) nerve axons Path of Direction of light visual Action potentials through processing propagate along retina optic (II) nerve axons toward optic disc Photoreceptors OUTER SEGMENT OUTER SEGMENT Two are required for normal vision ◦ Rods ◦ Cones Both are sensitive to light INNER SEGMENT INNER SEGMENT ◦ Rods = highly sensitive, “night vision” ◦ Black and white Nucleus ◦ Cones = low sensitivity to light, “day vision” ◦ colours SYNAPTIC TERMINAL ◦ Do rods contribute to “day vision”? Synaptic vesicles ROD CONE LIGHT DIRECTION Types of Cones and Colour Blindness 3 cone types: ◦ Blue cones ◦ green cones ◦ red cones Colour Blindness: Inherited inability to distinguish between certain colours Red-green colour blindness ◦ Lack of either red cones or green cones ◦ Test → Distribution of Rods and Visual axis Cones on the Retina 80˚ 60˚ 60˚ Fovea – centre of retina 40˚ 40˚ ◦ Contains only cones 20˚ 0˚ 20˚ Blind spot ◦ Sharpest vision (Optic disc) Fovea Cones Rod Reasons for highest acuity (i.e., sharpest Cone vision) ◦ High photoreceptor density ◦ Bipolar and ganglion cells do not cover fovea Photoreceptor density (number/mm2) ◦ One to one ratio of synapses between retinal Blind spot cells and bipolar and ganglion cells Rods Light Ganglion cell Cones Fovea Bipolar cell Fovea Cone Distance across retina (degrees) NOT FOVEA FOVEA What seeing with only Rods and only cones might look like Rods Only Cones Only What it’s like combined  Left visual Right field visual field The Visual Pathway Monocular Monocular portion potion Binocular visual Extends from photoreceptors to the visual areas field of the brain Both eyes receive input from the left and right visual fields Optic (II) nerves Optic tract Primary visual cortex of occipital lobe Visual System Summary Light enters the eye through the pupil ◦ Iris adjusts the amount of light The lens focusses light on the retina The retina contains photoreceptors ◦ Rods and cones ◦ Fovea contains highest density of cones Checkpoint What is an example of a substance that gives rise to the umami taste? Sour taste? Why is vision sharpest in the fovea? How does the eye focus light from different distances? What is binoccular vision?