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Lectures 10-11: Sensory Systems I-IIILectures 10-11: Sensory Systems I-III
Reading: chapter 6, except section on special senses
Conduction speed on myelinated axons ~ 120 m/s
Longest axon ~ meter long (big toe to spinal cord)
Total travel time for a nerve impulse from head to toe: 10 ms + 10 ms + 1-2 ms per synapse = ~ 25 ms
Brain
sensory inputs
motor outputs
Body
Sensory-motor transformation
Sensory Systems - 6 major sensory systems in the mammalian brain, each is organized according to a common anatomical plan.
- Visual System vision, sleep-waking cycle
- Auditory System hearing
- Somatosensory System touch, pain, temperature, proprioception
- Olfactory System* smell
- Gustatory System taste
- Vestibular System posture and balance
General Organization:
receptors - stimuli are transduced by receptors grouped together to form a sensory surface
transduction - the conversion of stimulus energy to a neuronal signal
receptor - a cell whose axon or dendrite is capable of transduction in a particular sensory modality
Receptor Relay Nuclei Thalamus Cerebral CortexPrimary
Cerebral CortexSecondary
relay nuclei - Groups of neurons located in the central nervous system that process signals from receptor neurons and transmit signals to the thalamus.
thalamus - Groups of neurons, organized into nuclei within the thalamus, that process signals from relay nuclei and transmit signals to the cerebral cortex.
primary cerebral cortex - Anatomically defined areas of the cerebral cortex that process signals from the thalamus and transmit signals to secondary cerebral cortex.
secondary cerebral cortex - Anatomically defined areas of the cerebral cortex that process signals from primary sensory cortex and transmit signals to association cortex, motor cortex and subcortical structures.
Concepts in Receptor Physiology
Receptor potential
Modality specificity
Receptive field
Maps
Lateral inhibition
Acuity
receptor potential - change in membrane potential at the site of transduction
(example – photoreceptor) (example – pacinian corpuscl)
modality specificity - category of stimuli to which a receptor is sensitive
(example – photoreceptor)
(example – pacinian corpuscl)
receptive field - location on the sensory surface within which a stimulus (of the appropriate modality)
can influence the activity of a sensory neuron
(example – auditory)
(example – somatosensory)
map: organized array of surface of receptor cells on the sensory surface
somatotopic map
tonotopic map
retinotopic or visuotopic map
lateral inhibition - inhibition of adjacent neurons in a map - facilitates localization of stimuli
2 point discrimination threshold
simultaneous presentation of stimuli
minimum audible angle
sequential presentation
1st 2nd
MAA
acuity
Visual Receptors :
Photoreceptors (rods and cones) located in the retina
The Visual System
Structure of the Eye
- Cornea- Pupil- Lens
- Aqueous humor- Vitreous humor
- Retina- Choroid- Sclera
- Optic disc- Optic nerve
Surface of the Retina
= Optic disc = Location of Fovea
Optics of the Eye
- the lens inverts and focuses the visual stimulus onto the surface of the retina
Anatomy of the Retina
- light passes through the neural circuitry of the retina before making contact with the photoreceptors in the back of the eye
- Cell Types: photoreceptors, bipolar cells, ganglion cells, horizontal cells, amacrine cells
- Two classes of photoreceptors - rods and cones (serve scotopic and photopic vision)
- 3 regions of the photoreceptor - outer segment, inner segment, synaptic terminal
- Outer segment contains discs that house photopigment molecules (rhodopsin or coneopsin)
- Outer segments are embedded in the pigment epithelium to reduce the absorption of scattered light
3 Classes of Cones: Red, Green, and Blue
Phototransductioncell membrane of outer segment
disc membrane
rhodopsinPDE
Na+
cGMP
In Dark - high levels of intracellular cGMP keep the cGMP-gated Na+ channel opens- cell depolarized- cell constituitively releases neurotransmitter
transducin
(Adapted from Figure 6-25)
Phototransductioncell membrane of outer segment
disc membrane
rhodopsinPDE
In Light: 1) light stimulation of rhodopsin leads to activation of a G protein, transducin
Na+
light
transducin
2) activated G protein activates cGMP phosphodiesterase
3) PDE hydrolyzes cGMP, reducing its concentration
4) cGMP-gated Na+ channels close, the cell hyperpolarizes, and the cell stops releasing neurotransmitter
cGMP GMP
(Adapted from Figure 6-25)
Summary - Photoreceptors
- Structure of the eye.
- Anatomy of the retina (rods, cones, bipolar, horizontal, amacrine, ganglion).
- Anatomy of photoreceptors (outer segment, inner segment, synaptic terminals).
- Phototransduction cascade: light absorption leads to hyperpolarization and a decrease in the
amount of neurotransmitter release.
Visual System
Visual System – (cont.)
Visual System – (cont.)
Primary visual cortex, V1, area17: binocularity, orientation selectivityDorsal stream: where pathway e.g. area MT involved in motion detectionVentral stream: what pathway e.g. area IT involved in face recognition
Auditory Receptors: Hair Cells located in Cochlea
The Auditory System
Structure of the Ear
- External ear: pinna, external auditory meatus, tympanic membrane- Middle ear: tympanic membrane, ossicles, oval window- Inner ear: oval window, cochlea, vestibular apparatus, round window
Cochlea – Structure and Sound Transduction
Cochlea – Structure and Sound Transduction
Cochlea – Tonotopic Organization
Sound Waves
Vibration of Tympanic Membrane
Vibration of Oval Window
Vibration of Middle Ear Bones
Fluid Movement within the Cochlea
Vibration of Basilar Membrane
Bending of Hair Cells
Action Potentials Generated in Auditory Nerve
Graded Receptor Potential
Propagation to Auditory Cortex
Vibration of Round Window
Dissipation of Energy (no sound perception)
Summary – sound transduction
Auditory SystemJust as the auditory nerve branches to innervateseveral targets in the cochlear nuclei, the neuronsin these nuclei give rise to several different pathways.
Beyond the cochlear nucleus, there is a high degreeof bilateral connectivity.
The best-understood function mediated by the auditory brainstem nuclei is sound localization.
Humans use two binaural cues to localize the horizontal position of sound sources, depending on the frequencies in the stimulus.- interaural timing differences (if < 3 kHz)- interaural intensity differences (if > 3 kHz)
This figure is not in any version of the textbook
Somatosensory Receptors
Specific receptors for different modalities/sensations
- touch (mechanoreceptors)
- pain (nociceptors)
- temperature (thermoreceptors)
- proprioception (mechanoreceptors)
The Somatosensory System
Pacinian corpuscle
Somatosensory System – (cont.)
Spinal Cord
Relay Nuclei(dorsal columns)
Thalamus
Dorsal Root Ganglion
1o Somatosensory Cortex
Skin
Somatosensory System
Sensory HomunculusSomatosensory System – (cont.)
Summary – Sensory Systems, Organization and Receptor Physiology
6 Major Sensory Systems - visual, auditory, somatosensory, olfactory, gustatory, vestibular.
General Organization - receptor, relay nuclei, thalamus, primary cerebral cortex, secondary cerebral cortex.
Receptor Physiology – signal flow, receptor potential, modality specificity, receptive field, maps, lateral inhibition.
Visual System
Somatosensory System
Auditory System
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