Unit 2 Flashcards
What are the ways to see the structure of the brain?
- MRI
-CAT/CT
What does MRI stand for?
magnetic resonance imaging
What are the advantages to an MRI?
- no x-rays or radioactive material
- provides detailed view of the brain in different dimensions
- shows the structure of the brain
What are the disadvantages to the MRI?
-expensive
-gross anatomy only
What does a CAT scan stand for?
- computerized axial tompography
What are the ways to measure the functional brain?
- PET Scan
-FMRI
What does PET stand for?
- positional emission tomography
What are the advantages to an FMRI?
- non-invasive
-detects changes in blood flow
(brain consumes the most O2 and glucose) - provides both anatomical and functional view of the brain
What are the disadvantages to the FMRI?
-blood flow is an indirect measure of neural activity
-large scale only
-heavy reliance on algorithms to standardize and produce data
What is a conectome?
a comprehensive map of neural connections
What are the levels of analysis for the connectome?
-microscale
-mesoscale
-macroscale
What does a microscale show?
- neurons immediate vicinity
What does the mesoscale show?
single neuron projections and circuit
What does the macroscale show?
connections between 1 brain region to another brain region
What scale is the serial electron microscopy on?
- microscale- shows neurons immediate vicinity
What is serial electron microscopy?
- only technique that can probe at the nanometer scale to elucidate a synaptic cleft
- done by sectioning tissue into thin slices and image
- aligns consecutive images to create 3-D volumes of individuals
- used to be done manually
What scale is trans-synaptic tracing on?
- mesoscale
What is trans-synaptic tracing?
- a strategy to determine local and long-distance connections between individual neurons
What are the two types of trans-synaptic tracing?
- anterograde
-retrograde
What is anterograde trans-synaptic tracing?
- starter cell to post synaptic neurons
What is retrograde trans-synaptic tracing?
- starter cell to pre-synaptic neurons
What is rabies viral trans-synaptic tracing?
- adapted from natural phenomenon
- retrograde tracer
- adapted to survive and spread in CNS
- transmission restricted to connected neurons (circuit info)
What scale is clarity on?
macroscale
What is clarity?
-allows visualization of deep structures without sectioning the brain
- Light-absorbing lipids are replaced with H20 soluble gel that turns the brain transparent
- neurons are labeled with flourescent molecules
(employs transgenic mice)
What’s the problem with connectome imaging?
all these techniques involve dead brains
What is the human connectome project?
- construct a map of the structural and functional; neural connections across individuals in vivo
- can see where genes are
- can help map how everything connects
- mapping is 1 cubic cm at a time
What does ipsilateral mean?
same side
What does contralateral mean?
different sides
How is the peripheral nervous system divided?
autonomic and somatic
What is the autonomic peripheral nervous system?
- controls self-regulated action of internal organs and glands
What is the somatic peripheral nervous system?
controls voluntary movements of skeletal muscles
What makes up the nervous system?
-meninges
-brainstem
-cerebral cortex
-ventricular system
-corpus callosum
-cerebellum
- spinal cord
What makes up the meninges?
-dura mater
-subdural space
-arachnoid membrane
-subarachnoid space
- pia matter
What is dura matter?
-thick tough inflexible covering
What is the subdural space?
-doesn’t exist under normal conditions but present in pressure disorders
What is the arachnoid membrane?
- resembles a spider web, holds layers close together
What is the subarachnoid space?
filled with CSF close to blood vessles
What is pia matter?
-adheres to surface of the brain
What is the ventricular system?
-contains CSF
- moves ions and nutrients
- no nerves or ganglion
What is in the cerebral cortex?
-sulcus
- fissure
-gyrus
What is the sulcus and an example of one?
-grooves
-increases area
-central sulcus
-valleys
What is a fissure and an example of one?
- a really deep sulcus
-increases area
-lateral fissure
-valleys
What is a gyrus and an example of one?
-ridges
-hills
-increases area
-precentral and postcentral gyrus
What are the types of cerebral cortex?
- molecular layer (surface)
-external granular layer
-external pyramidal layer
-internal granular layer
-internal pyramidal layer
-multiform layer
-all different sizes and densities depending on which part you’re looking at
What are Brodmann’s areas?
-distinguished 50 zones of cerebral cortex based on differences in cell
What do broadmann’s areas depends on?
-cytoarchitecture which depends on the type, size, density, and lamination (layer size) of the cell
- done using nissal stain
What is the nucleus?
- cluster of neuronal cell bodies with roughly similar functions and connections
What is the cortex?
- sheet-like arrangement (layers) of neuronal cell bodies
What is a tract?
- bundle of axons with same origin and destination
What is a commissure?
- a collection of axons that cross the midline
-largest is corpus callosum
What is the corpus callosum?
- beneath cortex
-connects left and right cerebral hemispheres - largest white matter structure in brain
- has colossal commissure
What is the colossal commissure?
- collection of axons crossing the midline
What is the cerebellum?
-part of CNS
- bilaterally symmetric (symmetrical off midline)
-coordinates precision, accuracy, timing of movements
What is the brainstem?
-part of the cns
-controls fundamentals of life
-contains midbrain, pons, medulla
What is the midbrain?
- involved in vision, hearing, eye movement, body movement
What is the pons?
- involved in motor control and sensory analysis
What is the medulla?
-maintains vital functions like heart rate and heart rate)
What are the parts of the spinal cord?
- cervical
-thoracic - lumbar
-sacral
(can turkeys leave scars)
What is a simple circuit?
- signal from the cerebrum initiates this
-underlies knee-jerk reaction
-organized by dorsal and ventral horns
What are dorsal horns in charge of?
sensory circuits
What are ventral horns a part of?
- motor circuits
What are sensory/afferent signals?
- neurons send signals from teh senses, skin, muscles, internal organs to CNS
What are motor/ efferent signals?
neurons transmit commands from the CNS to muscles, glands, organs
How is the PNS organized?
- autonomic and somatic
What is the autonomic PNS?
- controls self-regulated action of internal organs and glands
What is the somatic PNS?
- controls voluntary movements of skeletal muscle
What are the parts of the PNS?
- nerves and ganglion
What are nerves?
- bundles of axons in the PNS
What are cranial nerves?
-part of PNS
-nerves go to both sides
- damage can effect one side and not the other
What are ganglions?
cluster of neuronal cell bodies in PNS
What is plasticity?
-adaptability of an organism to change in its environment/ ability to alter the neural connections of the brain as a result of experience in the process of learning
When was the term plasticity used?
- first used in the biological sense by R owen in 1850s
- “Animals in a state of nature is required to show their degree of plasticity
-wasn’t used to reference to the nervous system until the early 1950s
What is neural plasticity?
the ability of the NS to change in structure and or function
What is synaptic plasticity?
- neural plasticity that occurs at the synapse
What are specific examples of synaptic plasticity?
- synaptic depression at the neuromuscular junction
- presynaptic facilitation in aplysia gill withdrawal reflex
- long term potentiation (LTP) in the hippocampus
Describe changes at synapses?
-long or short time
-can be presynaptic or postsynaptic
- structural (spines) or electrical (stronger synapse)
What does synaptic plasticity allow for?
- changes in behavior, not just at the synaptic level, but at the behavioral level too
-experience changes in brain function - can enhance memories and change NT receptors
How does feedback at synapses work?
- through NT autoreceptors located on the presynaptic terminal
- autoreceptors are not always on same cell, just same type of cell
-respond to NT released by presynaptic terminal - located on presynaptic membrane
What kind of synapse does synaptic feedback happen on?
- typically ligand-gated metabotropic receptors
- ligands can be ionotropic or metabotropic
What kind of feedback happens at synapses?
- negative feedback to decrease NT released by the presynaptic terminal
- positive feedback but that’s rare
What are examples of NT feedback at synapses?
- dopamine
-serotonin
-norepinephrine (noradrenaline)
-glutamate neurons
How does noradrenaline synaptic feedback work?
- presynaptic neuron releases NT (noradrenaline) into the synaptic cleft
- transmitter acts on receptors of the postsynaptic neuron, but also on autoreceptors typically inhibits further release of NT
- Activation of these autoreceptors typically inhibits further release of NT
How does synaptic plasticity occur through NT receptors on glial cells?
-glial cells are uniquely placed to mediate synaptic plasticity
How does synaptic plasticity occur through retrograde signals like endocannabinoids?
-originates from the postsynaptic terminal and goes back
- several types of messengers
-presynaptic neuron is usually GABAergic or glutamatergic
-may include anandamide and 2AG
- usually act on CB1 receptor to reduce (turn down) the opening of presynaptic v-gated Ca2+ channels
- mechanism for synaptic plasticity is NT receptors on glial cells
What type of receptors are endocannabinoids?
metabotropic
Are endocannabinoids autoreceptotrs?
no, this is a signal released by the post-synaptic neuron
Describe synaptic depression at neuromuscular junction?
- some patterns of stimulation of presynaptic motor neuron result in a decrease in the size of EPSPs recorded in the muscle fiber (details vary by species and muscle)
How does synaptic depression at the Neuromuscular junction occur?
- a pattern of NT release detected by Mach receptors in Schwann cells leading to feedback into the terminal to cause less NT release for Each AP
Why is a reduction in synaptic activity important?
- a reduction in synaptic activity is important to minimize synaptic fatigue and rundown of NT
What can synaptic depression at the neuromuscular junction be blocked by?
- preventing Ca increases within the perisynaptic glia
- treating with Adenosine A1 receptor antagonist
- treating with mACH antagonist
- NOT by treating with nACHR antagonist
Where does the glial withdrawal reflex occur?
- in aplysia
What is the glial withdrawal reflex?
- learns to keep gills closed in the presence of stimulus
- learned response
-more time creates a stronger response - underlined by a simple monosynaptic circuit
- results in a stronger reaction by the muscles but the actual change is occurring at the sensory to the MN synapse, not the neuromuscular junction
What is a neural circuit?
- functional entity of interconnected neurons to accomplish some sort of task
What does the glial withdrawal reflex depend on?
- a neuromodulator
- depends on input from sensation modulatory interneuron L29 which forms a synapse onto the terminal of the sensory neuron
- results in a stronger reaction by the muscles but the actual change is occurring at the sensory to the MN synapse, not the neuromuscular junction
What is presynaptic facilitation?
- cellular/ synaptic mechanism that underlies sensitization of the gill withdrawal reflex (behavioral change
What does presynaptic facilitation with the gill withdrawal reflex cause?
- causes a decrease in K+ conductance in the sensory neuron terminal (fewer K channels open) it then stimulates synaptic growth
- longer refractory period
- less K+ efflux
- slower falling fase, less/ no undershoot, broader/ taller AP
- more depolarization of the terminal membrane
- could even build another synapse
What is the hippocampus necessary for?
- learning, particularly spatial and short-term memory
What is LTP?
- lasting potentiation of the EPSP
How can LTP be induced?
-strong depolarization of input one
- exact parameters vary depending on brain region
- there is input specificity
-NMDA can be essential
What inputs are unique to the hippocampus?
- CA3 inputs to CA1 neurons
What does a strong depolarization for LTP require?
- Postsynaptic CA1 neuron at the same time as pre-synaptic activity
- synapses not active, when the postsynaptic neuron is strongly depolarized, will not experience LTP (example of input specificity) (hebbian plasticity)
How do NMDA receptors be responsible for LTP
- ligand-gated by glutamate and voltage-gated y depolarization
- only allow CA2+ in when it is bound by glutamate, the membrane is strongly depolarized, and glycine is present
what are the additional mechanisms underlying LTP?
- increased effectiveness of AMPA receptors
- increased number of AMPA receptors in the postsynaptic membrane
- in some cases an increase in NT release and an increase in synaptic number
- must be communication between postsynaptic terminal to presynaptic terminal
How does retrograde signaling induce plasticity via endocannabinoids?
-retrograde messenger is endocannabinoids
-cannabinoid receptors are metabotropic
- presynaptic and postsynaptic neurons are usually gababergic or glutamatergic
- can vary by sex
- usually act on CB1 receptor to reduce the opening of presynaptic v-gated Ca channels
Are endocannabinoids an example of autoreceptors?
no, this is a signal released by the postsynaptic neuron
What form of receptors are cannabinoids?
metabotropic
How do endocannabinoids vary by sex?
- AEA is only in female hippocampus
What are examples of endocannabinoids?
- anandamide, AEA, 2-AG
How long can LTP last?
- over a year
- very long time
How can LTP be mediated by structural changes?
- more spines can grow
- sex and cocaine affect nucleus accumbens core medium spine density
- spine density is sexually dimorphic (females have enhanced response if they try but are less likely to try)
What is the effect of cocaine on spine density?
- increased density in all genders
What are the methods of synaptic plasticity?
- LTP and depression
- morphological changes in dendrite. axon structure, including spines
- genetic (changes in gene expression)
- molecular/ neurochemical (chemical phosphorylation)
- synaptic connections (new synapses, get rid of synapses, synaptic rearrangement)
- intrinsic properties (changes in excitability)
- neurogensis and neural recruitment
- structural plasticity
What is plasticity history?
- adult neural plasticity wasn’t widely accepted until 1970s
-1793: malacrane est. training a dog increased amt of cerebellar folding
-1890: william james proposed brain and its function aren’t fixed - 1894: ramon y cajal said adult brain is fixed but mental exercise leads to more dendrites
Why did it take so long for neural plasticity to be accepted?
- neuron doctrine isnt formed until 1890s
- synapse wasnt coined until 1897 (can’t name what you don’t know)
- 19th centruy evidence showed brain had less individual variation in size than other organs and is less effected by body weight changes
- thought adult brian was anatomially fixed bc they couldn’t see microscopic vidence
What was the plasticity paradigm shitf?
-early 1960s altman and DAS reported neurogenesis in rat hippocampus (canaries too later)
- LTP observed by Lomo in 1966
- 1963: Hubel and wisel discover ocular deprivation changes number of cortical neurons responding to light
-1961: rosenzweign said total ACHE activity higher in trained rats given harder problems and alll groups easier than rats given no training
What is structural plasticity?
- brain’s ability to change its neuronal connections an anatomy
- usually on microlevel (spines, dendrite shape, synapse, sometimes new neurons)
How do some insects have macrochanges like metamorphisis?
- incorporates new neurons into CNS at highly divergent rates depending on species, brain region and developmental stage
- often studies effect of various internal or external stimuli on brains anatomical recognition
What is the cortex?
- outer layered layer of the brain
What does the nissil stain do?
- shows cell bodies and ribosomes of neurons
What does the golgi stain do?
shows dendrites and axons of a random subset of neurons
What is neurogenesis?
- process by which new neurons are formed in the brain via neural stem cells
- crucial when an embryo is developing, but continues in certain brain regions throughout life
How does neurogenesis happen in adults?
- proliferation: new cells generated from stem cells
- survival of a portion of these new cells and their migration toward target areas
- terminal differentiaion into a neuronal or glial phenotype
- recruitment into the existing neuronal circuit
- forms new synapses
What is neural recriuitment?
- incorporation of new neurons into existing neural circuits and brain regions
- happens in fetal and infant brains
What are the rat experiments?
- altman and das in 1965
- thymidine H3 labeled granule cells in rat hippocampus
- used theymadine to show the amount of cell proliferation and neurogenesis
What is thymidine commonly used in?
- cell proliferation assays
- incorporatied into dividing cells proportionally to the amount of cell proliferation
Who completed the songbird experiments?
- fernado nottebohn
What are the songbird’s experiements?
-paper is a brain for all seasons
- seasonal changes in song stereotype in canaries
- og song and then song falls apart and then a new modified song is created
- the final conclusion is that the rate of neurogenesis seems to dramatically differ between environment, brain regions, species, human being substantially lower than some other species
What is the evidence of the songbird experiment?
- same technique as rats, but adults and longer time period after thymidine injection
- direct link to behavior
- clear evidence to hormone modulation
- clear and careful differentiation of neurons, glia, and ventricular cell
- electron microscopy and electrophysiology
How can adult brains undergo neurogenesis?
- stem cells means its possible to trigger neurogenesis and neural recruitment
What are the cab driver experiments?
- tested cab drivers since they have to navigate without maps
- showed an increase in spatial learning
- higher gray matter volume
- not enough proof because correlation doesn’t equal causation
What makes neurons more lilkely to survive?
- in the adult hippocampus they are more likely to survive if they are used
What are the misc. examples of neural plasticity?
- nursing rats have smaller somatostatin from ventral side
- neurons process smaller portions of skin meaning that precision and resolution are increased
- cats with cut whiskers adapt
What is hemifeild neglect:?
- unilateral attention deficit of right side of the brain
- contralateral damage
What kind of damage does hemifeild neglect create?
- contralateral
- damage to right side displays on left
What causes hemifield neglect?
- damage to righ parietal cortex
- after a stroke you can regain some function
What is the hemingeild italy experiement?
- patients were asked to imagine they were standing in different places and what they could see
- shows this is a problem with perception, not sensation
- since they were asked to recall it showed they had an internal field that couldn’t imagine the stuff on the left
What is sensation?
- processing of info through senses
What is perception?
- conscious experience of sensory info
What are the general principles of sensory systems?
- have specialized receptor cells
- has sensory transduction and sensory coding
- has receptive field and response properties
- The thalamus is a relay/ processing station
- thalamus is main part of every sense except olfaction
- primary sensory cortex
- has orderly representation (like brain maps)
- association cortex and higher-order processing
- has parallel processing
- descending regulation
-adaptation
How is the blind spot of the eye created?
– optic nerve is where axons and blood vessels go to and from the retina and the rest of the CNS creating it
What are photoreceptors?
- initial light receptors
Why is the retina a layered structure?
- light has to go thorugh several layers of cells before reaching photoreceptors
What is the adaptation to having photoreceptors in the back of the eye?
- very fragile so having them deeper protects them
- helps to stabilize them
- helps with faster transduction to optic nerve
what is the retina made of?
rods and cones
What are rods?
-structures with high sensitivity and low acuity
- processes black and white and dim light
-has rhodopsin
-usually used at night
What are cones?
- low sensitivity and high acuity
- bright, light, color vision
- 3 types of cones because of three types of opsin
What is the difference between normal vision and color blindness ?
- normal vision is trichromatic
-red/green colorblindness is dichromatic - red/green can have loss of red or green cones
What are the three types of opsin and their sensitivities?
- blue cone: peak sensitivity: 430
- green cone: peak sensitivity: 530
- red cone peak sensitivity:560
What are the parts of rods and cones?
- synaptic terminal that releases glutamate and no axon
- inner segments
- outer segment
What does the inner segments of rods and cones include?
- nucleus
- normal cell components
What does the outer segment of the rods and cones include?
-has photoreceptors specialized for phototransduction
- cone also has membranous disks containing photopigment
Where are rods and cones imbedded?
- pointy/comb end
What happens when photoreceptors see light
THEY HYPERPOLARIZE
What is the cycle of photoreceptors in the dark?
- cGMP keeps Na+ chanels open
- inward Na+ current in outer segment (dark current)
- RMP is -30 - -40mV
What is the cycle of photoreceptors in the light?
- cGMP levels decrease
- fewer Na+ channels open
- hyperpolarization
- graded decrease in NT release
What happens to photoreceptors in the light and the dark?
- no AP just graded potentials
- NT release is like any other neuron, depolarization= release
How does phototransduction happen in the dark?
-dark Na+ channels open mostly in outer segment is kept open by cGMP
- also K_ channels open mostly in the inner segment
- need more NaK+ pumps than typical to maintain ion gradients in comparison to the rest of the brain
How does phototransduction happen in the light?
- photon absorbed by photopigment (rhodopsin in rods, three different opsins)
- causes change in conformation
- activates an intracellular messenger called transducin (g-protein)
- transducin activates a phosphodiesterase (PDE)
- PDE hydrolyzes cGMP reducing its concentration
- decreased cGMP levels result in the closing of Na channels
- loss of Na current results in hyperpolarization (toward ek of -70)
- hyperpolarization leads to less Ca influx leading to less glutamate released
How is phototransduction turned off?
- when light goes away
1. transducin inactivates itself and also gets inactivated by arrestin
2. Calcium based mechanisms are in place to re-open the Na channels even if light elvels don’t change - part of this is light adaptation
-returns Vm to -35mv or so ( in part Ca through Na channels inhibits cGMP synthesis enxyme guantyl cyclase
What are the types of neurons in the retina?
- photoreceptors
-interneurons
-horizontal
-bipolar
-amacrine cells - retinal ganglion cells
What is the basic neuronal circuit?
- photoreceptors to bipolar cells to retinal ganglion cells to the thalamus via the optic nerve
What is the visual field of one eye?
- each visual point corresponds to a point of the retina
-each photoreceptor in the retina has a receptive field - both eyes see almost all of the visual field not split into two halves
- visual space maps onto the retina in an orderly manner
What is the receptive field?
- a portion of visual field within which a stimulus causes a change in activity
What is the fixation point?
- where we are looking which corresponds to the fovea
-where we direct light when we are focusing
What is the fovea?
- related to the fixation point
-layers are pushed to one side to allow a more direct path for light to get to the photoreceptors (less scattering of light
-strong direct line for information
-minimal light scattering
What is the photorecepter RGC ratio in the fovea?
1:1
Are there rods in the fovea?
no
Are there blood vessels in the fovea?
no
Do bipolar cells have a receptive field?
yes
Does the fovea have a receptive field?
yes
Do bipolar cells have an AP?
no
What is the direct pathway?
- info carried directly from photoreceptors to RGCs
- not necessarily 1:1
What is the indirect pathway?
-receive info indirectly from photoreceptors vis horizontal cells
What do the indirect and direct pathways have in common?
- usually have opposite effects on the bipolar cells
- create a center-surround field organization for bipolar cells
What are the two types of bipolar cells?
- on center and off center
- creats on center and off center RGCs
What is the direct on center pathway cell cycle?
- light onto the center of the receptive field hyperpolarizes the photoreceptors
- photoreceptors release less glutamate (NT)
- less glutamate binds to mgluR6 then mgluR6 deactivation gate opens and then CGMP-gated Na+ channels which result in depolarization
- glu is inhibitory
How is the action of the NT determined?
- by what receptors are present on the postysynaptic terminal
What is the indirect pathway of an on center cell?
- light onto the surround of the receptive field hyperpolarizes the photo pathway
- photoreceptors release less glutamate (NT)
- horizontal cell is then hyperpolarized
- horizontal cell forms inhibitory gaba synapse on the center photoreceptor this hyperpolarizing the horizontal cell causing less gaba release at inhibitory synapse, depolarizing center photoreceptor
- the center photoreceptor then releases more glutamate
6.since the synapse onto the bipolar cell is inhibitory, this hyperpolarizes the bipolar cell
-horizontal- gaba
Where do bipolar cells transmit info to?
RGCs
What would the strongest stimulus on a RCG be?
- a tiny dot of light or donut shape, but the visual field doesn’t work like this
- actual strongest stimuli are those that have edges, contrast between a light area and a dark area
- an edge in the right place is the best stimulus
Do RCGS have center surround receptive cells?
yes
How do bipolar cells create a a response?
- bipolar cells form excitatory glutamatergic synaps on RCG, then the RGC express AMPA, NMDA, kainate receptors
-RCGS make AP to get a signal to thalamus
What are the types of RGCs?
-m-type
-p-type
Describe p-type RCGs
-smaller cells
-smaller receptive field
-slower AP conduction
-slower adapting
-sensitive to light wavelentghs (cones have to go through light wavelengths)
- discerns stimulus forms and fine details - high resolution visual
-cells are sustained
Describe M-type RCGS?
- stimulus movement
-low resolution vision - big
- info from rods
- causes a transient response
Can M-type and P-type cells generate AP?
no
Where do RCG axons project to through the optic nerve?
- Lateral geniculate nucleus of thalmus (primary visual pathway
- midbrain (superior colliculus)
- hypothalamus (circadian clock)
How do RCG axons go to the LGN, midbrain, and hypothalamus?
- the optic nerve
What does the lateral geniculate nucleus allow us to do?
percieve information
What does the midbrain do?
maps that link points in space
What part of the brain is sensitive to blue light waves?
the hypothalamus which controls circadian rhythm
What is the primary visual pathway?
photoreceptors send dignals to bipolars which go to RGCs which then goes to the GLN in the thalamus, the primary visual cortex, the secondary visual cortex and then other associaiton cortexes
Where is the thalamus located?
part of the diancephalon region
Where do retinal inputs go?
the lateral geniculate nucleus of the thalamus
Does information from the parts ot the RGCs mix?
no
(P and M are kept seperate)
What type of cells are LGNs?
mononucleic
How is info from the LGN organized, segregated, and processed?
parallel
Wat is the primary visual pathway made of?
-rgc axons
What RGC axons are part of the primary visual pathway?
- eye
-optic nerve
-optic chiasm - stalk of pituitary gland
-optic tract
-brain stem
What does the optic nerve do?
connects eye to optic nerve
What does the optic chiasm do?
connects to optic tract
Can Lateral RCGS cross the midline of the visual hemifields?
-no
- sends info to ipsilateral thalmus
-stays on the same side
What are the visual hemifields?
-right and left vision fields with fixation point in the middle
Do medial RCGs cross midline?
- sends info to contralateral thalamus
- criss crosses
What part of the visual field is only seen by one eye?
- only the peripheral edges, most is binocular
Where does info from the right visual field go?
left cortex
Where does info from the left visual field go?
the right cortex
What broadmans area is the primary visual cortex?
17
Where do outputs from the primary visual cortex go?
- mapped back to LGN
- if you modify the map you modify perception
-signals will go up and down the map for more processing
How many layers does the primary visual cortex have and describe the layers?
- 6
- neocortex
- layers are striated
- right eye input and left eye inputs are seperate in layer four because they are in the LGN
What is the primary visual cortex?
- part of the primary visual pathway
How are the layers of the primary visual cortex monocular?
one eye at a time in mapped in layers but binocular overall
What are ocular dominance columns?
- across layers, cells within a column respond more strongly to 1 eye or the other
What is orientation selectivity?
- many neurons in primary visual cortex show a preference for bars of light of a particular orientation
- response properties tend to get more complex as we progress from neurons in periphery to neurons in primary then the secondary association cortex
What is the dorsal processing streams?
- into parietal association cortex
- critical for processing object potion
-portions arranged in direction of motion columns
How is the dorsal ventral processing stream organized?
- direction of motion columns
What is the dorsal processing stream critical for?
object motion
What is the ventral parallel processing stream?
- into temporal association cortex
- few direction ion selective cells
What is the ventral parallel processing critical for?
-object recognition including shape, color, and force perception
What do the cells in Layer 4 of the primary visual cortex like?
- bars of light of a particular orientation
What is the oval of excitation with bordering regions of inhibition in the primary visual cortex?
- kind of like on-off RCGS
- much of this info comes from M-type RCGs
- on a graph it peaks is what it prefers
-has to do with orientation selectivity
What is direction selectivity?
the response properties of some cortical neurons include a preference for movement in a particular direction (magnocellular stream)
- specific for one column of light moving in one direction
How is directon sensitivity formed?
direction cells with adjacent fields synapse on a single simple cortical cell
How is the somatosensory system divided?
- 2 subsystems distinct in function and anatomy also different sensory receptor cells and pathways to cerebral cortex
- mechanical and tactile
What is the mechanical stimuli of the somatosensory system?
tough presssure vibration and joint position
What is the tactile stimuli of the somatosensory system?
- light, touch, vibration, and joint position
What is proprioception?
- position of body in space
What is the somatosensory system made of?
- specialized cells (somatosensory receptor neurons)
- not proteins
What do somatosensory receptor neurons do?
convert stimuli to the language of the nervous system
What is sensory transduction?
-transduces mechanical signals to electrical signals
What does sensory transduction depend on?
- stimulus, quantity/ size, strength, frequency of AP
(more Ap= stronger signal)
-stimulus location (sensory receptor and cortex location) - stimulus quality and native type (sensory receptor and CNS neuron properties)
What are the three functional categories of somatosensation?
- mechanoreceptors
- nocireceptors
-thermoreceptors
What are mechanoreceptors?
-touch, proprioceptive and tactile, a wide variety of specialized strucutres
What are proprioception organs
- muscle spindle, golgi tendon orgon, joint receptors
How are the tactile mechanoreceptors different?
- each tactile mechanoreceptor has a different response property and receptive field on the skin
- every sensory neuron has different response properties
- most sensory neurons have a receptive field
-type of stimulus that excites them - axon diameter
- threshold
- adaptation
What are response properties?
describes the stimulus to which a sensory neuron responds (what, where, etc)
What is the somatosensory receptive field?
- tactile somatosensory the receptive field is the area on the skin within which the appropriate stimulus will cause a depolarization of the neuron
Why do somatosensory organs vary in their receptive fields?
- if they didn’t we wouldn’t be able to distinguish a touch on the face from a touch on the hand
What are nocireceptors?
- pain, all free nerve endings
What are thermoreceptors?
- heat/cold and all free nerve endings
What is phasic adaptation?
fast adaptation
What is tonic adaptation?
slow adaptation
Where does adaptation of somatosensory neurons occur?
-pacinion corpsucles
What are pacinion corpsucles?
- specialized structure around nerve endings that are responsible for adaptations because it contains somatosensory ion channels that detect membane stretching
What are the three types of ion channels that happen iwth somatosensory adaptations?
- some are sensitive to lipid stretching
- some open when a cytoskeletal protein is moved
-some open when a extracellular protein is moved
What does a high threshold mean for somatosensory receptors?
high= less sensitive
What does a low threshold mean for somatosensory receptors?
- more sensitive
What does axon diameter do to somatosensory cells?
- changes conduction velocity, some types of info reach CNS quicker and then more later
What is the size of a primary sensory receptor axon?
varies in size and in myelination
What type of azons to pain and temperature have?
- slower but there are two types of pain receptors because we have different types of pain
What kind of axon whould a chronic, lasting, throbbing pain have?
-unmyelinated because it lasts longer
What kind of axon would a sharp pain have?
myelinated
What is two point discrimination?
- ability to discern that two nearby objects touching the skin are truly two distinct points, not just one
- describes a particular aspect of perception of touch, not general sensitivity to touch, acuity in terms of the location of touch
- ACUITY NOT SENSITIVITY
How is two point discrimination measured?
- minimal interstimulus distance required to perceive 2 simultaneous stimuli as distinct
What is the 2 point discrimination distance for fingertips?
2mm apart
What is the 2 point discrimination distance for the forehead?
17mm apart
What is the 2 point discrimination distance for the back?
40 mm apart
What is the 2 point discrimination distance for the calf?
45 mm apart
What factors affect 2 point descrimination?
receptor density
receptive field size (plasticity because more receptors can be added or subtracted)
What is one segment of the spinal cord segment of the somatosensory pathway made of?
bipolar neurons
What are the names for the peripheral somatosensory neurons?
- primary
-first order - sensory
-afferent
Are cell bodies ganglion or dorsal?
- ganglion
- the peripheral branch of the axon is a spinal nerve and the central branch of the axon is a dorsal root
How are spinal cord segments defined?
doorsal and ventral roots
What is the order of the spinal cord segments?
-cervical
-thoracic
-lumbar
-sacral
How many cervical spine segments are there?
8
How many thoracic spine segments are there?
12
How many lumbar spine segments are there?
5
How many sacral spine segments are there?
5
What are dermatomes?
-territory innervated by a given spinal nerve
-reveals the segmental organization of our bodies
What is the anterior head portion of our body mediated by?
- cranial nerves which extend from brainstem, not spinal cord
Is the dorsal column-medial lemnisical pathway contralateral or ipsilateral?
ipsilateral
What is the dorsal column-medial lemnisical pathway?
-a mechanosensory pathway
-primary sensory afferents involved in mechanosynchronization make both local synapses and rostral projections
Where do local synapses involved in spinal reflex go?
- local synapses involved in spinal reflexes and rostral projections run in the ipsilateral dorsal column of the spinal cord and medulla
Where do excitatory synapss on 2nd order turn contralateral?
- medulla
-this crossover is different then pain and temp
Where do pain and temperature cross over signals?
- crosses over in the spinal cord and ignores medulla
Describe first order afferent signals?
- cell bodies are in dorsal root ganglion
- project to ipsilateral second degree afferent
Describe 2nd order afferent signals?
- cell bodies are in the spinal cord
-project to contralateral thalamus
Why do we care about where crossing over occurs?
- because of spinal cord damage
What is the difference between the mechanosensory and pain and temp pathways?
-its a matter of where the secondary sensory neuron has its cell body
How are the mechanosensory and pain/temp pathways the same?
info in both pathways end up in contralateral to thalamus and then cerebral cortex
Where is the cell body in the mechanosensory pathway?
medulla
Where is the cell body for pain and temp pathway?
spinal cord
where is the thalamus?
part of diancephalon
Where does sensory info go?
-almost all sensory info passes through the thalamus and continues to the cerebral cortex in the thalamocortical projections
Where do somatosensory secondary afferents synapse?
- 2 regions of the thalamus
-ventral posterior nucleus and intralaminar nuclei
Where is the primary somatosensory cortex?
- in the postcentral gyrus
-part of the cerebral cortex that first receives sensory info
-each sensory modality goes to a different region of the cerebral cortex?
Does the primary somatosensory cortex have broadman’s areas?
yes
What is a somatotopic map?
-simmilar to orderly representation of the visual field and retina in the primary visual cortex
somatosensory version of a cortical map
Is the somatosensory map ipsilateral or contralateral of the body?
contralateral
What is the somatotopic map of the visual cortex?
-retinotopic map
What are cortical maps?
- representational of the body, where the senses are perceived in the cerebral cortex
-orderly (fingers are all near each other
How is the somatosensory cortex organized?
-columnar
- like the visual cortex
- neurons within a column process info from the same region of the body
Where do thalamocortic projections synapse onto neurons?
- layer 4 of the primary sensory cortex (like the primary visual cortex)
Does the somatosensory map only apply to humans?
no
How does phantom limb syndrome work?
-there is a perception of pain even though the limb is gone
-doesn’t matter how this perception arises
- perception may get mixed so tough on the face could be perceived on the phantom limbs because of the somatotopic map
Does phantom limb syndrome happen because of sensation or perception?
-perception causes pain
- sensation is gone when limb is lost
What can people with phantom limb syndrome feel?
- proprioception, nocireception, touch
How can you treat phantom limb syndrome?
- mirrorbox treatment
How many somatosensory maps are there?
many maps for many different sensory cortices
How are response properties of S1 neurons different than sensory receptors?
-response properties of S1 neurons tend to be more complex
- ex. some prefer movement while some prefer movement in a particular direction
- more cells in S1
What does s1 stand for?
primary somatosensory
What does s2 stand for?
secondary somatosensory
Where are S1 signals sent and why?
-s1 signals are projected to s2 signals
- this continues the processing of sensory info including the association of it with other sensory modalities and motor and emotional processes
Are there more ascending or descending projections in the somatosensory system?
- descending
What are descending projections?
-projections sent down from the brain to the motr output
How are descending projections sent?
-sent using projections from the cortex to the thalamus, brainstem and spinal cord
What is an example of a descending projection?
pain
How is pain a descending projection?
- endogenous pain system involves neurons in the PAG which express opioid at every level
-also manufactures endogenous opioids
What does PAG stand for?
periaqueductal gray
What does opioids being expressed at every level of pain projection do?
- affects tolerance, addiction and resistance
What are endogenous opioids?
-neuropeptides
- endorphins
- intrinsic pain management
What are tastants?
- chemicals in food that are detected by taste buds which consist of special sensory cells
What are odorants?
-airborne odor molecules
- nose version of tastants
How do odorants work?
- stimulate receptor proteins found on the cilia at the tips of sensory signals
How do we taste the flavor of food?
-taste and smell messages converge
What are the types of taste receptors on the tongue?
- papillae
- taste buds
-taste receptor cells - basal cells and gustatory afferent axons
What are papillae and what do they contain?
-papillae are the little taste-sensitive bumps on the tongue
- contain between 1-100s of taste buds
What do taste buds contain?
- between 50-150 taste receptor cells
- basal cells
-gustatory and afferent axons
Is the tongue map real?
no. taste perception is intermixed with sensations coming from all regions of the tongue
Why do people think the tongue map is real?
different parts of the tongue are more sensitive to certain flavors
Why are different parts of the tongue more sensitive to flavors than other flavors?
- different tastants will trigger depolarization and sometimes an AP in the taste cell
- the particular transudtion method will vary in each taste cell and that’s what gives them more or less chemical specificity
- most taste buds are sensitive to one basic taste
When are you able to perceive a taste?
when the taste receptor cells reach threshold of an AP
How many basic tastes are there?
-number is under debate
- we know there is sweet, sour, umami (protein), and maybe CO2
What does synsepalum dulcificum?
- causes sour foods to taste sweet by having miraculin (a glycoprotein) that binds and activates sweet taste preceptors at a low pH
Where is the gustatory nucleus?
in the medulla
How do gustatory afferents make it to the gustatory nucleus?
travel through at least three of the cranial nerves
IS the central taste pathway contralateral or ipsilateral?
primary ipsilateral unlike vision
Are taste info and tongue somatosensation the same thing?
no. two different processes
How is the central taste pathway organized?
-we don’t know yet
What is ageusis?
-loss of taste preception via a lesion of cortex or thalamus
What is anosmia?
loss of olfaction
What is broca’s aphasia?
- impaired language production not due to impaired motor control
What is wernickes aphasia?
impaired language comprehension despite normal auditory function
What is asterognosia?
- inability to recognize objects by feeling despite a normal senese of touch
-can be limited ot the contralateral hand?
What is hemifeild neglect?
-can’t see left half of visual field even though there is no visual damage
What is prospagnosia?
-inability to recognize faces (faceblindness)
What type of sensation is olfaction?
-chemical sensation
How does olfaction occur?
- odorants in mucus bind directly to (or can be shuttled via odorant binding proteins) to one of many receptor molecules located in the membranes of cilia
- this then activates odorant-specific g-protein triggering an intracellular signaling pathway that causes membrane depolarization
What is population coding?
- different patterns of receptor cells respond to each odorant, the overall pattern of many cells provides specificity for each particular cell
- similar to taste
What does orderly representation in the olfactory bulb come from?
glomeruli
What are glomeruli?
- spherical structures inside of which around 25,000 primary olfactory axons converge
How are glomeruli organized?
- by odorant being etected
- cortex organization is less clear
What is the central olfactory pathways?
-usually thalamus to orbitofrontal neocortex
-unusual: direct projection to pyriform cortex (temporal lobe, in parahippocampal cortex) which plays a role in odor identification
- amygdala for social functions
entohinal cortex for memory
What is the sensory receptor for autidory and vestibular systems?
hair cells
Where are auditory system receptors?
- in specialized organs that contain fluid like the chchlea
How do auditory system receptors send signals?
- movement causes waves in that fluid which causes movement of stereocilia on hair cells
- movement in one direction causes depolarization and movement in the other direction causes hyperpolarization
-K+ flows in which is why there is an unusually high amount of surrounding fluid
What is the primary difference for auditory systems vs the other systems?
-auditory has more processing before cortex
- multiple regions process
- heavy descending projections
How is the auditory info organized?
- into tonotopic map
How is context conveyed to the auditory system?
neuromodulators
What is the association cortex?
- most of cerebral cortex
- expanded the most during the evolution of primates
- has the highest levels of processing, so the most complex disorders
