1. Lectures 10-11: Sensory Systems I-IIILectures 10-11: Sensory Systems I-III
Reading: chapter 6, except section on special senses
2. 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
3. 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
4. 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 Cortex
Primary
Cerebral Cortex
Secondary
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.
5. Concepts in Receptor Physiology
Receptor potential
Modality specificity
Receptive field
Maps
Lateral inhibition
Acuity
6. receptor potential - change in membrane potential at the site of
transduction
(example – photoreceptor) (example – pacinian corpuscl)
7. modality specificity - category of stimuli to which a receptor is
sensitive
(example – photoreceptor)
(example – pacinian corpuscl)
8. 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)
9. map: organized array of surface of receptor cells on the sensory surface
somatotopic map
tonotopic map
retinotopic or visuotopic map
10. lateral inhibition - inhibition of adjacent neurons in a map
- facilitates localization of stimuli
11. 2 point discrimination threshold
simultaneous presentation of stimuli
minimum audible angle
sequential presentation
1st 2nd
MAA
acuity
15. Optics of the Eye
- the lens inverts and focuses the visual stimulus onto the
surface of the retina
16. 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
17. - 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
19. Phototransduction
cell membrane of outer segment
disc membrane
rhodopsin
PDE
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)
20. Phototransduction
cell membrane of outer segment
disc membrane
rhodopsin
PDE
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)
21. 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.
25. Primary visual cortex, V1, area17: binocularity, orientation selectivity
Dorsal stream: where pathway e.g. area MT involved in motion detection
Ventral stream: what pathway e.g. area IT involved in face recognition
31. 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
32. Auditory System
Just as the auditory nerve branches to innervate
several targets in the cochlear nuclei, the neurons
in these nuclei give rise to several different
pathways.
Beyond the cochlear nucleus, there is a high degree
of 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
33. Somatosensory Receptors
Specific receptors for different modalities/sensations
- touch (mechanoreceptors)
- pain (nociceptors)
- temperature (thermoreceptors)
- proprioception (mechanoreceptors)
The Somatosensory System
