Exam 4 Flashcards
timing of behavior/physiology tightly correlated w/
environment (i.e. day and night)
optimal times for behavior vary, but there’s a general pattern
behaviors cycle in oscillatory pattern
how to demonstrate circadian rhythm experimentally
- Put animal in constant environment (temp, noise, light)
- example: constant darkness (DD)
- Measure activity (wheel running)
-
Results: every day, activity starts a little sooner (cycles, but slightly less than 24 hours)
- Activity is circadian (periodic) but not exactly 24 hours
circadian pattern of humans in constant darkness
circadium rhythm slightly longer than 24 hours (but there’s genetic variation)
those w/ most genetic variation likely to have sleep disorders
circadian rhythm in constant light
rhythm of more than 24 hours (unlike less than 24 hours in constant darkness)
causes more health problems than in dark since nocturnal
properties of circadian rhythms
- Found in all organisms
- Period in constant conditions is close to, but not exactly 24 hours
- Can depend on light dark cycle and other cues (zeitgebers)
- Many single cells can display circadian rhythm; multiple cells release hormones to synchronize
nocturnal, diurnal, and crepuscular animals: definition + example
nocturnal: sleep during day (mice)
diurnal: sleep at night (humans)
crepuscular: sleep middle of day and middle of night, active dawn and dusk (fruit flies)
who discovered molecular mechanisms controlling circadian rhythms
Hall, Rosbash, Young
Konopka
identified gene that when mutated, change cycle duration
called period (per mutants)
Hall and Rosbash
discovered the function of the per gene
Young
determined function of a second clock gene, called timeless (tim)
found that per and tim proteins bind to each other
observations about per protein made by Rosbash
- per mRNA levels vary in cyclic fashion
- mRNA cycling is circadian (cycle b/w high mRNA and protein levels)
- per protein levels are also cyclic, peak level reached several hours after mRNA peak
initial model proposed by Rosbash about per protein function (5 steps)
- Transcribe per mRNA
- make per protein in cytosol
- import per protein into nucleus
- Per protein inhibits its own promoter (mRNA and protein levels fall)
- Inhibition is relieved, begin making per mRNA again
what is the actual mechanism w/ per and tim proteins that regulate timing of cycle
proteins clock (dCLK) and cycle (CYC) are required to activate transcription of per and tim
per and tim proteins dimerize, are phosphorylated, then enter the nucleus
phosphorylated dimer inhibits transcription of per and tim mRNA
how is timing of clock mechanism regulated
via phosphorylation of per and tim proteins by Doubletime (per) and crytochrome genes (tim)
Doubletime gene
produces kinase that regulates per protein in cytosol
degrades per: extends cycle duration by preventing dimer formation
Cryptochrome (Cry) gene
produces kinases that regulate tim proteins in both cytosol and nucleus
degrading tim in cytosol: extends cycle duration by preventing dimer formation
degrading tim in nucleus: destroys dimer, removing inhibition, next cycle starts
how does light entrain the clock
in flies (not mammals): cryptochrome is light sensing protein
light activated cryptochrome promotes rapid degradation of tim protein in cytosol (can’t form dimer, cycle stops)
phase shifting from travel
normally: light during day breaks down cytosolic tim, extends cycle duration
travel: light during night breaks down nuclear tim, advancing onset of next cycle (shortens cycle duration, phase advance)
how is mammalian clock different from flies
there are proteins homologous in structure/function to dCLK, CYC, and PER
PER forms dimer with CRY, not TIM (timeless lost)
cryptochrome (CRY) not light sensing
light input in mammals is different from flies how
CRY isn’t light sensitive
light input based on retinal projections to the SCN (suprachiasmatic nucleus)
evidence for role of SCN to generate circadian rhythms
- isolated SCN cells are sufficient to generate circadian rhythms
- electrical synapses b/w SCN neurons synchronizes entire nucleus
- intact SCN is necessary for whole animal rhythms
- hamsters w/o SCN have no circadian rhythm
- when transplant SCN back in, circadian rhythm comes back
melatonin and the SCN
melatonin (hormone) gives feedback to SCN
melatonin can phase shift SCN clock depending on when it’s present
circadian control in mammals by SCN: what are 4 things SCN can affect
autonomic innervation
body temperature
glucocorticoids
feeding
drosophila and melatonin
drosophila also use hormones to synchronize brain to body, but NOT same hormones as mammals
sleep definition
reversible quiescence
increased arousal threshold (need more intense sensation)
homeostatic regulation
related to circadian clock
sleep and circadian rhythm experiment w/ fruit flies
if sleep deprive fruit flies, they rest a lot more
example of homeostatic regulation
neuron firing during stages of sleep
during slow wave sleep: action potentials (in cortical or thalamic neurons) occur in bursts; high amp, low frequency
during REM sleep: APs and EEG look like wake; no communication b/w thalamus and cortex
experiment with rats sleep deprivation
experimental rats: turntable moves whenever rat shows EEG signs of beginning of sleep
control rats: can sleep, less sleep deprived
all experimental rats died after a month
sleep deprivation and cognitive tasks
performance worse on selective attention and arithmetic tasks after 1 night of sleep deprivation
activation is less
synaptic homeostasis hypothesis (SHY)
- when awake, more synapses are potentiated than depressed
- during sleep, synapses downscaled to lessen metabolic burden
- problem: some pathways show net LTP during sleep
glymphatic system: how does it work?
- materials brought to brains via arteries
- nutrients flow out
- waste goes into vein from interstitial fluid (extraceullar fluid)
- glymphatic system function varies if sleep or awake
glymphatic system: proposed role in sleep
awake: reduced interstitial space, restricted CSF flow, metabolites accumulate
asleep: increased interstitial space, better CSF flow, get rid of waste
what triggers sleep
interaction b/w thalamus and cortex
narcoleptic dogs: mutated gene
orexin (peptide, aka hypocretin)
essential for normal wakefulness
5 parts of somatosensory system
touch
temperature
pain
itch
propioception
sensation: 3 overall steps
- sensory fiber activation
- processing in spinal cord and brain
- perception of pain, touch, etc.
where are the bodies of sensory neurons located
dorsal root ganglion (DRG, sensation of body): located in spinal cord
trigeminal ganglion (TG, sensation of face): located in brainstem
sensory neurons’ two axons
each has unique termination pattern of peripheral axon in skin/organs, but also projects its central axon to specific laminae of spinal cord (DRG) or brainstem (TG)
4 main types of somatosensory fibers and where in spinal cord they terminate
Aα: muscle spindle; terminate in central/ventral spinal cord
Aβ: hair follicle, merkel cell; terminate in lamina IIv-V
Aδ: free nerve ending, D-hair follicle; terminate in lamina I,II,III, and V
C: free nerve ending; terminate in lamina I, II
which receptors sense touch
mechanoreceptors
mechanoreceptors function optimally w/:
light contact
two types of skin receptors based on speed of adaptation
slow adapting (SA) receptors: receptors that detect constant stimulus (e.g. pressure, skin stretch)
rapidly adapting (RA): detect only short pulses (e.g. initial contact, vibration)
4 types of mechanoreceptors in glabrous skin (no hairs)
merkel cell-neurite complex
meissner corpuscles
ruffini endings
pacinian corpuscles
merkel cell neurite complex
basal layer of epidermis, assoiate w/ nerve terminals branching from a single Aβ fibers
fine tactile discrimination, texture perception
meissner corpuscles
vibration, handgrip
ruffini endings
skin stretch
pacinian corpuscles
skin motion, skin slipping
touch circuit in central nervous system: how is touch info transmitted to brain for both glabrous and hair skin mechanoreceptors
- Post synaptic dorsal column (PSDC) neurons in dorsal spinal cords get info from glabrous and hairy skin LTMR
- Project to dorsal column nuclei (DCN), which synapse on to ventral posterior nuclear (VPN) complex of the thalamus
- Thalamus to somatosensory cortex
touch circuit in central nervous system: circuit exclusive to hairy skin
- spinocervical tract (SCT) neurons get info exclusively from hair skin
- synapse onto lateral cervical nucleus (LCN)
- LCN neurons snapse onto ventral posterior nucleus (VPN) complex of thalamus
mechanosensitive neurons in DRG
most DRG neurons are mechanosensitive:
some quickly desensitize
some are slow, have sustained current
piezo 2
peripheral mechanotransduction channel: mediates rapid adapting current
only piezo 2 is expressed in DRG
piezo 2 is expressed in
merkel cells, which are slow adapting (also DRG)
piezo 2 KO (merkel cell KO): loss of slow-adaptive firing
piezo 2 KO: main deficit
loss of touch sensation (major mechanoreceptor in DRG and merkel cells)
loss of mechanical pain
Piezo 1 KO: main deficits
broadly expressed in internal organs
vascular development deficits, sickle cell disease
thermoreceptors are:
free nerve endings
2 classes of thermoreceptors
cold fibers: respond to a decrease in temp
heat fibers: respond to an increase in temp
each fiber has a preferred temperature
what channels detect temperature change
thermal TRP channels
different TRP channels detect different temperatures and chemicals
TPRA1
NOT a cold sensor in vivo
cold sensing remains intact in KO mice
TRPV1
hot channel
activated by capsaicin (gives spicy hot sensation)
capsaicin and heat both trigger Ca influx
TRPV1 KO mice
impaired pain sensation (latency for withdrawing from heat pain)
worse heat detection, but can still detect some heat
what are the hot channels
TRPV1, TRPM3, TRPA1
TKO (triple knockout): complete loss of withdrawal response from heat
cool channel
TRPM8
KO mice can’t detect cold (mouse has no preference for cold vs hot chamber)
GluK2
Kainate (KA) glutamate receptor
cold channel
GluK2 KO almost completely loses ability to detect cold
acute vs chronic pain
acute (nocioceptive pain): good pain
chronic (pathological pain): abnormal changes to somatosensory system, bad pain
fast vs slow pain
fast pain: transmitted by myelinated A𝛿 fibers
slow pain: transmitted by unmyelinated C fibers
different methods of testing pain
mechanical pain: von frey assay
hot pain: hargreaves assay
cold pain: 0oC
sodium channel specifically expressed in nociceptor
Nav1.8+
4 types of nociceptors
mechanical
thermal
chemical
polymodel (combination of pain)
peripheral vs central terminals of sensory neurons
peripheral terminal: detects noxious stimuli (tranduction)
central terminal: transmits noxious info to brain (transmission)
labelled line hypothesis
specific DRG and spinal neurons process noxious information
different neurons transmit different sensations
simple labellined line hypothesis cannot explain:
noxious stimuli evoke pain in most cases, but not always
pain can be evoked by innocuous stimuli in chronic pain patients
gate control theory of mechanical pain
pain transmission neuron (T neuron) in spinal cord receives peripheral inputs from C/A𝛿 nociceptors to transmit acute pain
LTMRs activate T neurons, but inhibitory interneuron inhibits T, prevents touch from causing pain
under pathological conditions, inhibitor gate is gone, touch can trigger pain
gate control circuit: what properties of T neuron and inhibitory interneuron
Somatostatin (SOM) expressed in excitatory interneruons in dorsal spinal cord
Dynorphin (Dyn) expressed in inhibitor interneurons
mechanisms of chronic pain
peripheral sensitization: overactivate TRPV1, excitate Nav1.7 (spontaneous pain)
central sensitization (amplification): excitation, disinhibition
two forms of itch
chemical itch: activated by pruritogens, can be histamine dependent or independent, high threshold nociceptors (pruriceptor)
mechanical itch: activated by innocuous mechanical stimuli, histamine independent, low threshold mechanoreceptor
spinal circuits processing chemical itch vs chemical itch
chemical itch: there are parallel pathways transmitting chemical itch
3 subsets of itch neurons in the DRG
mechanical itch: pathway is distinct from chemical itch
population coding hypothesis: pain vs itch
- there are itch specific and pain specific circuits
- itch stimuli activate GRPR+ neurons in spinal cord
- itch sensing neurons can respond to painful stimuli
- itch neurons express TRPV1 like pain neurons
- pain suppresses itch
- activation of pain pathway can cause inhibition of itch path
reflex arc steps
- Activation of mechanical receptors
- Activation of sensory neurons in DRG
- Info processing in spinal cord
- Activation of motor neurons
- Response by effectors
hierarchical organization of movement control
motor cortex → brainstem nuclei → local circuit nuerons → motor neurons → skeletal muscles
spinal cord organization: which receptors go where
nocioceptors and mechanoreceptors project from dorsal root ganglion to specific laminae in dorsal horn (spinal cord)
proprioceptors project to ventrally located motor neurons in spinal cord, which connect back to muscle to drive movement
muscle pairs
extensor contraction: extends joint
flexor contraction: decreases joint angle
motor pools and motor units
each muscle fiber innervated by single motor neuron
single motor neuron innervates multple muscle fibers (motor unit)
motor pool: motor neurons that innervate same muscle
size principle of motor neurons
neurons w/ smaller motor units (small axon diameters and cell bodies) fire before neurons w/ large motor unit size
this difference used for fine motor control
motor columns
motor neurons are organized in motor columns in ventral spinal cord along rostral-caudal axis
medial: trunk muscles
lateral: limb muscles
rostral: forelimb
caudal: hindlimb
different spinal cord sections
cervical spinal cord: controls arms
lumbar spinal cord: controls legs
stretch in muscle fibers
Aα mechanoreceptors are activated by stretch of muscle spindles, info is sent to sensory neurons
stretch reflex
Proprioceptors detect stretch, trigger motor response to counteract stretch (negative feedback loop)
reciprocal inhibition
contraction of one muscle set is accompanied by relaxation of antagonistic muscle
most inputs to motor neurons are mediated by
spinal interneurons
interneurons and rabies virus
transsynaptic retrograde labeling:
inject into motor neuron, crosses synapse, and labels premotor interneurons
does NOT label proprioceptor projection to spinal cord (bc retrograde only)
excitatory premotor interneurons + example
amplify signaling, excitatory input
example: flexor withdrawal reflex (withdraw limb from aversive stimuli)
inhibitory premotor interneurons examples
stretch reflex: reciprocal inhibition
crossed extensor reflex: activation of extensor muscles and inhibiton of flexor muscles on opposite side of the body
central pattern generators (CPG)
circuit that is capable of producing rhythmic output w/o sensory feedback
cat experiment about central pattern generators
- disconnect cortex/thalamus from brainstem/spinal cord
- cat can’t control its motion
- electrically stimulate brainstem motor center to initiate movement
- result: recordings show similar walking pattern (rhythmic/coordinated muscle contraction in absence of sensory feedbacjk)
how do central pattern generators work
flexor and extensor motor neurons are excited by excitatory premotor neurons
excitatory premotor neurons inhibit each other and the other motor neuron
right left alteration in central pattern generators
excitatory premotor neurons are required for left-right alternation (inhibits the opposite side)
MdV neurons
MdV premotor neurons are presynaptic to only specific subset of forelimb motor neurons
receive inputs from multiple brain regions (motor cortex, superior colliculus, cerebellum)
important for reaching and grasping (skilled motor learning)
what does ablating purkinje cells in cerebellum do
disorganized walking
organization of the cerebellar circuit
mossy fibers → granule cells and climbing fibers → purkinje cells (output of cerebellum)
basket cells and stellate cells are inhibitory interneurons
motor functions of cerebellum
skilled motor learning
forward modeling: combines sensory and motor info to predict where an object will be in the future
nonmotor functions of cerebellum
cerebellum sends projections to the frontal lobe and influences cognition, emotion, motivation, judgement
damage impairs language perception, cognition
basal ganglia
projects to area involved in motor control ,cognition, judgement
initiate and maintain activity in the cortex
organization of the basal ganglia
striatum receives input from cortex and thalamus
sends inputs to dopaminergic neurons in SNc and VTA (send modulatory output back to striatum)
also output to superior colliculus and brainstem
basal ganglia: two major GABAergic outputs
Direct (D1+): excitatory, enhances movement
Indirect (D2+): inhibitory, suppresses movement
motor cortex is where?
premotor regions include?
M1 is in frontal lobe
premotor regions include premotor cortex, supplementary motor area, supplementary eye field, presupplementary motor area
upper motor neurons of M1 project to:
lower motor neurons via corticospinal tracts
also connect to interneurons of spinal cord to influence reflexes and CPGs
M1 seems to use ____ to encode direction of movement
population coding (combination of neurons signal direction of movement)
movement coding in M1: firing rate of neuron determines:
direction of movement
what is neurodegeneration
progressive loss of structure or function of neurons, including death of neurons
physical changes of alzheimer’s disease
brain shrinks (nerve cell death and tissue loss)
plaques (clumps of beta amyloid protein)
tangles (twisted strands of another protein
treatments of alzheimer’s
early stages: acetylcholinesterase inhibitors
severe stages: NMDA receptor antagonist
amyloid plaques made up of
beta amyloid protein
what are neurofibrillary tangles made fo
microtubule associated tau protein (taupathy)
amyloid hypothesis
amyloid beta protein disrupts communication b/w cells and activate immune cells, which trigger inflmamation
small soluble aggregates of it are more toxic than large accumulations
production of amyloid beta
Aβ is part of transmembrane protein called amyloid precursor protein (APP), cleaved by α-secretase or β-secretase to form APP-α or APP-β
γ-secretase then produces Aβ (Aβ42 most likely to form aggregates)
what evidence implicates Aβ
mice w/ mutation in 3 genes for Aβ production develop amyloid plaques
down syndrome people w/ 3 fcopies of chromosome carrying APP gene develop amyloid plaques
mutations in APP gene causes
familial Alzheimer’s disease (FAD)
mutation in _____ and ____ increase Aβ production
App gene and presenilin (PS1 and PS2)
presenilins (PS) and catalytic activity
catalytic components of γ-secretase complexes
complexes contain PEN2 and APH1 also
aspartyl residues in transmembrane domains 6 and 7 required for catalytic activity
toxic Aβ effects
impaired synaptic plasticity
neuron death
LTD (learning/memory affected)
spine loss
3 phases of AD
- First phase (preclinical AD): Aβ accumulates w/o symptoms
- Second phase (mild cognitive impairment MCI): taupathy and neurodegeneration, predementia
- Third phase (AD): neurodegeneration eliminates neurons irreversibly, serious dementia
mouse models to study pathogenesis of AD:
App knock in and APP overexpressing mice
exhibit extensive Aβ pathology w/o taupathy and neurodegeneration
(mutation of tau protein are not cause of AD)
ApoE hypothesis
ε4 allele of ApoE is major risk factor for AD
parkinsons disease is the most common
neurodegenerative movement disorder
second most common progressive neurodegenerative disorder (after alzheimers disorder)
two symptoms of PD
bradykinesia (slow movement)
tremors
direct pathway
D1+ GABAergic striatum neurons project to GPi and SNr in basal ganglia
promote movement
indirect pathway
D2+ GABAergic striatum neurons project GPe
inhibits movement
SNc and VTA
SNc: movement
VTA: motivation, reward prediction
cellular mechanism of PD
loss of SNc dopaminergic neurons
D1+ neuron activation reduces (hyperactivation of GPi and SNr inhibitory projection neurons)
protein problem w/ PD
misfolded α-synuclein
regulates DA storage (reduced number of vesicles available for storage, DA in cytoplasm increase, oxidative stress)
what does α-synuclein do normally
lots at presynaptic terminal, participates in vesicle recycling
degraded by the UPS and by lysosomes
interacts strongly w/ membranes (plasma, mitochondrial)
PINK1 and parkin
normally, PINK1 degraded in mitochondria
mitochondrial damage: PINK1 and Parkin accumulate in outer membrane of mitochondria; PINK1 causes ubiquitination of Parkin, causing mitophagy
oxidized and aggregated α-synuclein inhibits mitophagy, inducing cell apoptosis (bad)
PINK1 KO and parkin
PINK1 KO: no mitophagy (Parkin not ubiquitinated, enlarged mitochondria)
PINK1 KO + overexpressed Parkin: mitophagy, like WT
PD treatment
L-dopa (tyrosine is precursor)
deep brain stimulation
cell replacement therapy (induced pluripotent stemp cell)
electron microscopy
uses electrons to image
high resolution images
can image very small tings (1-20nm)
brainbow
ratio of RGB expression allows for unique cell type identification (express RGB construct in tandem to get different colors)
can make each cell type a different color
relies upon Cre-LoxP system
default color is red
Rett syndrome is what kind of genetic disorder
X-linked disorder
patients usually are girls because males tend to die early since they only have one copy on the X chromosome
Rett syndrome caused by mutations in what gene
Mecp2 gene, which encodes methyl-CpG-binding protein 2
located on X chromosome
involved in chromatin remodeling and transcriptional regulation
Mecp2 domains
methyl-CpG-binding domain (MBD)
transcriptional repression domain (TRD)
what Mecp2 do
recruits transcriptional corepressor complex containing Sin3A and histone deacetylase (HDAC) to methylated CpG islands
results in target gene transcription inhibition
what else Mecp2 do
is also able to activate gene transcription by recruiting CREB and other transcriptional factors to non-CG methylated DNA regions
how does Mecp2 cause neurological deficits: summary
MeCP2 binds to methylated DNA and regulates gene expression
can cause splicing, missense, nonsense, deletion, insertion (any of these can cause loss of function)
Mecp2 KO mice
mimic symptoms of Rett:
- slow development of brain
- mobility problems, breathing problems
- rescue: overexpressing MeCP2 in the KO mice allows them to have normal brain weight
MeCP2 conditional KO mice
KO gene in adults
- Gad67-Cre: targets GABAergic neurons
- Cross w/ Mecp2f/f
- KO Mecp2 only in GABAergic neurons
- overgrooming, skin lesions
- Mecp2 regulates GABA synthesis
