Unit 2 Week 2 Flashcards
How is the somatic sensory system unique from other sensory systems?
Receptors: broadly distributed
-responds to many kinds of stimuli, rather than one (mechanical, thermal, and chemical)
-different types of sensory neurons encode diverse somatosensory stimuli
What does the somatic sensory system provide?
-enables the body to feel pressure and sense pain and temperature
-touch, pain, itch, and thermosenstation
-proprioception (sense position/movement of body parts)
-interoception (sense of internal organ function)
Describe the touch sensation
-starts at the skin (largest sensory organ)
-touch stimuli: pressure on the skin
-use mechanoreceptors to detect touch (convert mechanical force to neural signals)
What somatic sensory receptor is found within the epidermis?
Merkel’s disks
What somatic sensory receptors are found within the dermis?
Pacinian corpuscles, Ruffini’s endings, Meissner’s corpuscles
What is a receptive field?
the region of a sensory surface (retinal, skin), when stimulated changes the membrane potential of a neuron
How do the receptive fields of different mechanoreceptors compare?
they are different sizes
Discuss each mechanoreceptor type including their receptive field size and adaptation
Meissner’s: small, rapid
Pacinian: large, rapid
Merkel’s: small, slow
Ruffini’s: large, slow
What does it mean if a mechanoreceptor has a rapid adaptation to a stimulus?
transient response mostly at the beginning and the end of the stimulus
What does it mean if a mechanoreceptor has a slow adaptation to a stimulus?
more sustained response during the stimulus
What is responsible for the response profile of Pacinian corpuscle?
special ending, respond differently when the corpuscle is removed
Describe Mechanosenssitive ion channels
-mechanoreceptors express different mechanosensitive channels to detect touch
-mechanosensitive ion channels convert mechanical force into receptor potential of mechanorceptors
-specific types of channels in most somatic sensory receptors are still unidentified
What are the different ways that mechanosensitive ion channels can be opened?
-stretching of lipid membrane
-force on extracellular structures
-force on cell’s cytoskeleton
What are Piezo1 and Piezo 2?
Mechanosensitive ion channels
-non-selective cation channels
-important for touch sensation
What is Cre/LoxP?
Cre: a site-specific recombinase
LoxP: a short sequence from bacteriophage P1, which is recognized by Cre
How does the Cre/LoxP system work for gene knockout?
Cre will cut the DNA section between two LoxP sites in the same orientation
What is gene knockout?
a powerful genetic technique to understand gene function
What channel do Merkel cells require to transduce mechanical stimuli into electrical signals?
Piezo2
Describe primary afferent axons for somatic sensory system.
axons bringing information from the somatic sensory receptors to the spinal cord or brain stem
How many types of primary afferent axons for the somatic sensory system are there?
4, varying in properties like axons from skin, axons from muscle, diameter, speed, and sensory receptors
What is the trajectory of touch-sensitive AB axons in the spinal cord?
divisions of spinal gray matter: dorsal horn, intermediate zone, ventral horn
Describe the segmental organization of the spinal cord
spinal segments (30) - spinal nerves within four divisions of the spinal cord
Define dermatomes
the area of the skin innervated by the right and left dorsal roots of a single spinal segment
What is a sensory map?
one-to-on correspondence between spinal segments and dermatomes
What are the two different central touch pathways?
the dorsal column-medial lemniscal pathway and the trigeminal touch pathway (face and the top of the head)
Define somatotopy
the topographic organization of somatic sensory pathway in which neighboring receptors in the skin feed information to neighboring cells in a target brain structure
-a sensory map for the somatic sensory system, which helps determines the location of touch sensation
What is somatotopy- Homunculus?
a sensory map for touch sensation: the mapping of the body surface onto the primary somatosensory cortex
-more sensitive regions receive more CNS processing
What is Two-Point Discrimination used for? What is considered?
to measure spatial resolution of touch sensation
-receptive field density
-receptive field size
-computing power of the brain
-other spacial neural mechanisms
Describe what each of the primary afferent axons is used for.
A-alpha fibers: proprioceptors of skeletal muscle
A-beta fibers: mechanoreceptors of skin
A-delta fibers: pain and temperature
C: temperature, pain, itch
What is pain?
feeling of sore, aching, throbbing sensations
What triggers pain?
stimuli that signal body tissue being damaged or have the potential of causing tissue damage
What are nociceptors?
pain receptor neurons
What are the types of nocicptors?
most are polymodal
-mechanical
-thermal
-chemical
How are nociceptors activated?
ion channels in nociceptors can be opened by
-strong mechanical stimulation, temperature extremes, oxygen deprivation, and chemicals
-substances released by damaged cells
proteases (bradykinin), ATP, K+ ion channels
histamine
What substance promotes swelling?
histamine
What are the primary afferent axons for pain?
First pain (fast) : A-delta fiber
Second pain (slow) - C fiber
What molecule is responsible for temperature sensation (peppers)?
capsaicin
What does Capsaicin activate?
TRPV1, responds to both hot peppers and hot temps
What are transient receptor potentials (TRP)?
-cation channels
-can be activated by various external stimuli e.g. temperature, chemicals, and light
-group 1 subfamily: TRPC, TRPV, TRPM, TRPN, and TRPA
-group 2 subfamily: TRPP and TRPML
What does TRPV1 detect?
hot and capsaicin sensory
What does TRPM8 detect?
cold and methanol sensor
What many distinct TRP channels are there in thermoreceptors?
6
What is the dorsal-column medial lemniscal pathway responsible for?
touch, vibration, two-point discrimination, proprioception
What is the spionthalmic pathway responsible for?
pain, temperature, some touch
Describe the major properties if light
-light is electromagnetic radiation
-gamma radiation and cool colors are high energy
-radio waves and hot colors are low energy
Describe the structure of the eye, specifically the retina.
-eye collects light, focuses on retina , and forms image
-fovea is the thinnest area in the retina
-retina is where we perceive light
Describe the laminar organization of the retina
-seemingly inside-out layers
-light passes through ganglion cells and bipolar cells before reaching photorecepters
What are the two types of photoreceptors in the retina?
-rods and cones
-converts light to neural signals
-different in morphology and function
Describe the morphology and function of rods
one types, more sensitive to light (for nighttime vision), NA
Describe the morphology and function of cones
three types, less sensitive to light (for daytime vision), color vision
There are regional differences in retinal structure and function. Describe the differences between the central retina and the peripheral retina
Central Retina (focea) :
-almost all cones
-an area of highest visual acuity
Peripheral Retina
-higher ration of rods to cones
-more sensitive to low light
What is the light sensor in rods?
Rhodopsin (GPCR)
What is retinal?
a vitamin A derivative that is light sensitive and serve as a co-factor for rhodopsin
How is GPCR activated?
light causes change in shape of retinal and this activates the rhodopsin (GPCR)
How does cGMP become hydrolyzed?
light activates phosphodiesterase, which hydrolyzes cGMP
Describe phototransduction in rods in the light vs the dark
Dark current: Na+ ions move into cells due to open cGMP-gated sodium channels. Cells are depolarized, and glutamate is released.
Light: depletes cGMP, closing sodium channels. Cells are hyperpolarized (turned OFF) and the glutamate release decreases
Describe the mechanism of visual transduction in rods
-light activates the rhodopsin (GPCR)
-transducin, the G-protein, is stimulated
-phosphodiesterase (PDE) is activated
-PDE reduces the cGMP level
-Na2+ channels close, and cells become hyperpolarized (turned OFF)
-the release of glutamate is decreased
Describe phototransduction in cones
-similar process to phototransduction in rods
-three different cones
blue (short wavelength)
green (medium wavelength)
red (long wavelength)
Describe how color vision works in cones (Young-Helmholtz trichromacy theory of color vision)
cones are sensitive to three different colors : green, blue, and red. When these colors are combined, eyes can tell a difference between millions of colors
Why can’t rods see color?
rods only have a single opsin- rhodopsin
What is scotopic condition?
e.g. nighttime lighting
rods contribute to vision ( no color information)
What is photopic condition?
e.g. daytime lighting
cones contribute to vision
What is mesopic conditon?
e.g. indoor lighting
both rods and cones contribute to visions
What is meant by duplex retinal?
two complementary systems in the eye (rods + cones)
What is the advantage of duplex retinal?
our vision can operate in a wide range of light intensities
Describe the central visual pathway
- photoreceptors in eye
- other retinal neurons
- LGN
- visual cortex
Which axons project into the forebrain?
ganglion cell axons
What are the five types of cells within the retina?
vertical pathway
-ganglion cells
-bipolar cells
-photoreceptor cells
indirect pathway
-amacrine cells
-horizontal cells
Which retinal cells fire action potentials?
ganglion cells
What generates AP of ganglion?
neural computation with the retinal generate AP of ganglion cells to relay visual information to the brain
What is the direct pathway of information flow in the retina?
vertical pathway
1. photoreceptors
(excitatory or inhibitory)
2. bipolar cells
(excitatory)
3. retinal ganglion cells
What is the indirect pathway of the flow of information in the retina?
modulate retinal processing in the direct pathway by lateral connections
amacrine cells and horizontal cells
What are Amacrine cells responsible for?
receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells
What are horizontal cells responsible for?
receive input from photoreceptors and provide inhibitory feedback signals to other photoreceptors and bipolar cells
What is the receptive field?
an area of the retina where light changes neurons firing rate
both direct and indirect pathways count
Describe the two types of center/surround receptive fields
ON-Center/ OFF-Surround = A neuron’s activity is highest when light is on in the center and off in the surround
OFF-Center/ON-Surround = A neuron’s activity is lowest when light is on in the center and off in the surround
What does it mean to be “ON”?
Depolarized - turned on
What does it mean to be “OFF”?
Hyperpolarized - turned off
Where are Center-Surround receptive fields found?
observed in both bipolar cells and ganglion cells
Explain the different center-surround receptive fields for bipolar cells and RGCs
For ON-center or OFF-center: the nature of the synapse between photoreceptor and bipolar cell
For antagonistic center-sound receptive fieldsL: lateral inhibition via horizontal cells
What determines whether a receptor field of a bipolar call is ON-center or OFF-center?
the synapse between photoreceptors and bipolar cells
How do photoreceptors respond to light?
always off (-)
What is the cone circuit?
surround photoreceptors (PRs) are not directly connected to bipolar cells
What types of synapse do Surround PRs have on horizontal cells?
excitatory (+)
What types of synapse do Horizontal cells have on central PRs?
inhibitory (-)
What is lateral inhibition?
a general mechanism to sharpen the differences between neighboring neurons
lateral inhibition via horizontal cells enhances the differences between light-stimulated photoreceptors and the surrounding unstimulated photoreceptors
Describe the circuit mechanism for center-surround receptive fields
The synapse type between center photoreceptors and bipolar cells determines Center ON vs Center OFF
-excitatory synapse: Center OFF
-inhibitory synapse: Center ON
Lateral inhibition via horizontal cell yields opposite response at receptive field Center vs. Surround
Bipolar cells and RGCs has the same type of center-surround receptive fields
Describe ganglion receptive fields
-responses to light-dark edge crossing an OFF-center ganglion cell receptive field
Why have center surrounded fields?
such receptive fields leads to a neural response that emphasizes the contrast at the light-dark edges
getting most information from edges where intensities change and mat serve the following purposes:
-contrast enhancement
-edge detection
-motion detection
Describe parallel processing in the visual system
simultaneous input from two eyes
-input from eyes compared in cortex
-determines depth and distance of object
information about light dark: ON-center and OFF-center ganglion cells
information about color vision
Where does the dorsal root axon cross over in the dorsal column-medical lemniscal pathway?
medulla
Where does the dorsal root axons cross over in the spinothalamic pathway
in the spinal cord
