3 ways molecules can enter cells
1. diffuse through phospholipid bilayer (simple or facilitated)
2. active transport
3. vesicle transport (e.g. bacteria)
factors influencing the rate/direction of flux
1. size of molecule
2. temperature
3. concentration gradient in/out of cell
4. membrane permeability
5. membrane surface area
Mechanism of Na+/K+ ATPase
1. Binding Na+
2. Phosphorylation
3. Conformational change and release of Na+
4. K+ binding -> dephosphorylation
5. Releases K+ and returns the pump
2 domains that all ion channels have
1. P loop (provides specificity)
2. Transmembrane domain
how ion channels have specificity
1. charge exclusion
2. size exclusion - determined by the strength of interactions between ion and surrounding oxygens vs. ion's propensity to dissociate back into water
3. binding sites
conserved sequence for potassium channels
-TVGYG-
SCID
-severe combined immunodeficiency disease
-caused by mutations in Orai1 (channel protein) and STIM1 (pathway protein)
What makes one cell different from another?
differential gene expression
3 functions of membranes
1. selective barrier to passage of molecules
2. detecting chemical signals from other cells
3. anchor cells to adjacent cells and extracellular matrix
rate of diffusion of molecules across membranes vs in water
1,000 to a million times slower
What is the major factor limiting diffusion across a membrane?
hydrophobic interior of lipid bilayer
What molecules can diffuse through lipid bilayer?
1. Water
2. Lipids
3. Steroids
osmotic strength
[concentration] x # ions
familial hemiplegic migraine (FHM)
-throbbing pain with nausea, vomiting, aura (visual disturbance) and light/sound sensitivity
-caused by loss-of-function mutations in Na+, K+ ATPase
Luigi Galvani
Electrical stimulation of the nerve in a frog's leg produced contraction of the leg muscle
oscilloscope
-used to measure potential across membrane
-insert electrode in cell, grounded outside
How does membrane potential vary amongst cell types?
proliferating (and cancer cells) have potentials closer to zero compared to non-proliferating cells (e.g. neurons, skeletal muscle)
K+, Na+, and protein concentration in/out cell
Intracellular: high K+, low Na+, high protein
Extracellular: low K+, high Na+, low protein
Ouabain (toxin from Acokanthera)
-shuts down Na/K ATPase
-membrane potential doesn't change instantaneously, takes time for ions to diffuse in/out of cell
What controls and maintains the resting membrane potential?
-Na+ and K+ electrochemical gradients along the membrane
-kept constant by Na+/K+ ATPase and K+ leak channels
ion hydration
In water, interactions between water molecules and ions form, making a very stable configuration. Because of Na+ small size, it has even stronger hydration. It takes energy to "break" the hydration; oxygens on amino acid residues in channels provide the e
K+ channel
-excludes Na+ (mostly) by spacing size of binding sites (more oxygens stabilize larger K+ ion)
-potential binding sites for 4 K+ ions, but repulsion between ions makes it favorable to have only 2 K+ ions
What besides electrochemical gradient drives ion permeation?
Repulsion between ions in channel
Nernst potential
equilibrium potential of one ion; the potential where electrical and chemical gradient is equal and there is no net ion flow
Goldman-Hodgkin Katz potential
membrane potential (Vm) of the cell established by relative contributions of several ions
types of stimuli that open/close ion channels
Ligand gating, voltage gating, phosphorylation gating, stretch or pressure gating
Why is membrane potential not constant?
1. ionic composition in/out of cell
2. permeability of channels changes
voltage-gated ion channels
-have S4 resting voltage sensor oriented in a V shape that blocks ion flow
-during depolarization, S4 domain rises to open channel
-charges go to S4-S5 linker to cause conformational change
batrachotoxin
a toxin, secreted by poison arrow frogs, that selectively interferes with Na+ channels causing depolarization
cell-attached patch recording
-can record a single or several channels
-glass micropipette surrounds channel
voltage-clamp recording
-apply voltage, measure current to see response
-shows channels have different rates of response
How to tell what ion is involved in a response?
Ion substitution (e.g. K+/Ca2+ replaced leads to normal physiological solution, Na+ replaced leads to no response: toxin opens sodium channel)
neuron doctrine
nervous system is an interconnected network of cells
afferent neuron
sends info from PNS (sensory receptor) to CNS
efferent neuron
sends info from CNS to PNS (muscle, gland, neuron)
steps of neuronal signaling
1. stimulate cell
2. generate electrical signal
3. signal travels through the cell
4. signal passed to next cell
Is response or perception faster?
response is faster
graded potentials
1. can be hyperpolarizing or depolarizing
2. magnitude depends on strength of stimulus
3. distance from site of stimulus affects magnitude
action potential membrane depolarization
-ligand gated channels and mechanically gated channels often serve as initial stimulus for AP
-voltage gated channels allow APs to occur by allowing rapid depolarization and repolarization
states of Na+ channels
closed<=>open->inactivated->closed; open and inactivate rapidly
states of K+ channels
closed<=>open, open and close slowly
steps of action potential
1. steady resting potential due to K+ leak channels, P(K) > P (Na)
2. depolarizing stimulus brings to threshold
3. voltage-gated Na+ channels open causing rapid depolarization
4. inactivation of Na+ channels and delayed opening of K+ channels halts depola
Do Na+/K+ concentrations change significantly during an action potential?
Na+ concentration goes from 15mM to 15.01 mM, no
Nav1.7
large protein that is a voltage-gated Na+ channel; 4 subunits that have 6 transmembrane domains each
Mutations of Nav1.7
gain of function mutations lower threshold and cause APs to fire easier; can lead to [pain in hands and feet] primary erythermalgia (PE), [pain in face] paroxysmal extreme pain disorder (PEPD), or [no pain] channelopathy-associated pain disorder (CIP)
How does the nervous system encode stimulus strength?
strong stimuli cause more action potentials
absolute refractory period
The minimum length of time after an action potential during which another action potential cannot begin; due to Na+ channels
relative refractory period
the period of time following an action potential, when it is possible, but difficult, for the neuron to fire a second action potential, due to the fact that the membrane is hyperpolarized; due to K+ channels
What is the effect of a supra-threshold stimulus?
the membrane is maintained above threshold and AP firing depends only on Na+ channels
Node of Ranvier
site of AP
myelin
wraps neuron, preventing ion leakage; greater degree of myelination leads to greater nerve conduction velocity (e.g. pain receptors not myelinated=slow)
Why do action potentials only travel in one direction?
Na+ channel inactivation
Multiple Sclerosis (MS)
-destruction of the myelin sheath on neurons in the CNS
-T cells can pass blood-brain barrier causing autoimmune response that destroys glial cells
-leads to decrease in conduction due to leakage of ions
How to diagnose MS?
1. Blood test - rule out diseases with similar symptoms
2. Lumbar puncture
3. MRI - can reveal lesion areas of MS on brain and spinal cord
4. Evoked potential test - electrical signals produced by NS in response to stimuli are recorded
How does the mouse model of MS work?
1. immunize mouse with MBP-specific peptide
2. T cell mutates and causes characteristic neuroinflammation
anatomy of chemical synapse
mechanism of neurotransmitter release
1. action potential reaches terminal
2. voltage-gated Ca2+ channels open
3. Ca2+ enters terminal
4. neurotransmitter is released and diffuses into cleft
5. neurotransmitter binds to postsynaptic receptors
6. neurotransmitter removed from cleft
inhibitory synapse
causes hyperpolarization
excitatory synapse
causes depolarization
EPSP
excitatory post synaptic potential - depolarization (towards an AP)
IPSP
inhibitory postsynaptic potential - hyperpolarization (further from AP)
temporal summation
when one presynaptic neuron releases neurotransmitter many times over a period of time
spatial summation
when multiple presynaptic neurons together release neurotransmitter
How do we know that neurotransmitter release is mediated by vesicle fusion?
1. post-synaptic cell responds in a quantal fashion: weak presynaptic signals cause tiny, quantal depolarizations corresponding to a # of vesicles
2. vesicles can be seen at the synapse with electron microscopy
What is the evidence that calcium regulates vesicle release?
1. AP evokes an increase in calcium levels at active zone between presynaptic and postsynaptic
2. Ca2+ is required for vesicle fusion (no fusion when Ca2+ removed)
3. Caged Ca2+ is sufficient to trigger vesicle fusion
Describe the caged Ca2+ experiment
Ca2+ was put in a molecular "cage." The cage would open and release Ca2+ when exposed to light (photocleavage). Vesicle fusion occurred in the light but not in the dark.
SNARE hypothesis
intertwining of v-SNARES (on vesicle) and t-SNARES (on presynaptic membrane) allows fusion of the vesicle to allow for neurotransmitter release
Which SNARE proteins are involved in fusion?
synaptobrevin (vesicle), Munc18, syntaxin (membrane)
Which protein is involved in Ca2+ triggering?
synaptotagmin - entering Ca2+ binds it and it catalyzes membrane fusion
What happens if Ca2+ cannot enter axon terminal?
No Ca2+ influx through channels -> no vesicle fusion -> no signal across synapse
What are the 2 kinds of glutamate receptors?
1. Metabotropic
2. Ionotropic (AMPA and NMDA)
What is the difference between metabotropic and ionotropic receptors?
Metabotropic receptors are indirectly linked to ion channels (e.g. G-protein coupled), while ionotropic receptors are channels themselves
mechanism of glutamate binding
1. Glutamate binds to AMPA and NMDA channels
2. Na+ enters through AMPA, causing depolarization
3. Depolarization ejects Mg2+ from the loose binding site on NMDA
4. Ca2+ enters through NMDA channel
5. Ca2+ activates second messenger pathways
Why does depolarization from AMPA R cause ejection of the Mg2+ ion in NMDA R?
accumulation of positive charge (from Na+ entering cell) causes repulsion of Mg2+ ion
What is special about dendritic spines?
they have many AMPA/NMDA receptors
What is the difference between EPSP contributions from AMPA and NMDA?
AMPA = fast and sharp
NMDA = slow and sustained
What does the hippocampus do?
consolidates short term memory to long term memory
long term potentiation
After repeated stimulation, the same stimulus leads to increased neurotransmitter release (and bigger EPSP in postsynaptic cell).
mechanism of synaptic strengthening
1. Activation of NMDA R triggers Ca2+ influx
2. Ca2+ binds calmodulin
3. Calmodulin activates CamKinase
4. CamKinase promotes phosphorylation of AMPA R and addition of more AMPA R in the membrane, leading to bigger EPSP
5. Ca2+ also activates NO synthase,
retrograde amnesia
an inability to retrieve information from one's past
anterograde amnesia
an inability to form new memories
Henry Molaison
had his hippocampus removed to control epilepsy, gave him severe anterograde amnesia
Hebb's Rule
neurons that fire together wire together
What happens to dendritic spines due to LTP?
The spine heads grow and gain more AMPA receptors, and more vesicles containing neurotransmitters are released for the same stimulus.
CT scan
computed tomography scan - a noninvasive diagnostic imaging that uses X-rays and tech to produce images of the body
What are the layers of the head (out to in)?
1. Scalp
2. Skull
3. Dura mater
4. Venous blood
5. Arachnoid mater
6. Subarachnoid space of brain
7. Pia mater
8. Brain
What is the function of the choroid plexus?
Maintains ionic balance needed for CSF production
subdural hemmorhage
Blood leaks into dural area, shows up opaque in a CT scan
subarachnoid hemmorhage
Neurons are exposed to blood in place of CSF. Blood is toxic to neurons so it kills the neurons.
positron emission tomography
Inject a positron emitting tracer that is active: fluor-deoxy-glucose. Monitor levels of FDG in terms of glucose uptake as a measure of metabolic activity.
What are symptoms of Parkinson's disease?
1) Resting tremor (shaking)
2) Slowed movement
3) Rigidity of movements of the face
4) Shuffling gait
5) Forward tilt of trunk
What is the function of Broca's area?
generating words
What is the function of Wernicke's area?
speech comprehension
fMRI
Measures haemodynamic response to neural activity: BOLD (blood oxygen level dependent) contrast. Utilizes different magnetic properties of iron when O2 is bonded and not bonded.
What causes Parkinson's disease?
degeneration of dopaminergic neurons in substantia nigra
What do colors in PET represent?
red=high, blue=low
What is the fovea's role in vision?
It is the part of the retina required for high acuity vision because of the high concentration of cones. It is depleted of other cell types so there is minimal light scattering. It also has high spatial resolution.
What is the blind spot?
Optic disc is where all nerves in eye converge to form optic nerve, creating an area where there are no photoreceptors for vision.
What does the retina create?
It makes a point-to-point map of the visual field, which is actually inverted.
What is convergence in vision?
Receiving input from multiple cells which is passed down through other types of cells.
What cell involved in vision is the first to fire an action potential?
retinal ganglion cells
Ganglion vs Nucleus
Collection of neuronal cell bodies in periphery and CNS, respectively.
visual pathway
retinal ganglion cells -> optic nerve -> optic chiasm -> optic tracts -> lateral geniculate nucleus (LGN) of thalamus -> primary visual cortex (V1)
What has to happen for something to be "seen"?
action potentials in neurons of the primary visual cortex have to fire
What is special about the outer segment of photoreceptors?
They have a special membrane that allows for enhanced protein expression.
What kind of light protein do rods have?
Rhodopsin, which is blue/green sensitive
What kinds of light protein do cones have?
S opsin - for blue light
M opsin - for green light
L opsin - for red light
What molecule absorbs light and how?
11-cis retinal, which converts to all-trans retinal when it absorbs light. There is also a conformation change in the transducin peptide.
Mechanism of phototransduction
light hits rhodopsin (visual pigment), which activates transducin (G-protein), which activates cGMP phosphodiesterase, which degrades cGMP, resulting in the closing of Na+, Ca2+ cGMP-gated channels
What does guanylyl cyclase do?
converts GTP to cGMP
How do we know channels are closed in the light?
Current in photoreceptors is decreased by light, measured through voltage-clamps
How does hyperpolarization in photoreceptors lead to depolarization of bipolar cells?
In the dark, glutamate is released from photoreceptors, which binds to metabotropic glutamate receptors in the bipolar cell. This activates phosphodiesterase so cGMP is cleaved and Na+ channels are closed in the bipolar cell. In the light, glutamate is no
How does neuron diameter effect conduction speed?
larger diameter = faster
primary erythermalgia
caused by increased Nav1.7 function, pain in feet and hands
Paroxysmal extreme pain disorder (PEPD)
caused by increased Nav1.7 function, ocular, mandibular, and rectal pain
Channelopathy-associated pain disorder
absent Nav1.7 function, experience no pain
About how long is the synaptic cleft?
~50nm
Why do axons have a lot of mitochondria?
Signal transduction requires a lot of ATP.
What is learning and what is its cellular basis?
Memory is the change in behavior after acquiring new knowledge. It is stored as the weight of synaptic connections based on experience
What is memory and what is its cellular basis?
the process by which knowledge is stored and retrieved. It is stored as the strength of synaptic circuits
The Lost Mariner
About a man from Pennsylvania who used to be in the military. He was old but thought he was young. He could remember his past clearly, but he used present tense to describe his military days. He was intelligent but struggled with tasks that took longer as
Why do sodium channels have 2 gates?
The ball and chain mechanism determines the length of time between consecutive action potentials, which encodes for stimulus strength.
How does loss of dopamine in the basal ganglia lead to the phenotypes we observe in Parkinson's patients?
The basal ganglia is involved in motor control, so when dopaminergic neurons in the basal ganglia die, control over movement is lost. Leads to shuffling gait, trembling, and rigidity.
How to measure receptive fields in the retina?
1. fix subject's gaze on some fixation point on a screen
2. record from cell in the pathway
3. shin spots of light in specific locations of screen
4. note where in space a light elicits a response
photoreceptor cell receptive fields
small and non-overlapping
retinal ganglion cell receptive fields
larger and overlapping
What happens when it is dark in the center of the receptive field?
1. cone depolarizes
2. glutamate released
3. a) on-center bipolar cell hyperpolarized
3. b) off-center bipolar cell depolarized (AMPA active)
What happens when there is light in the center of the receptive field?
1. cone hypolarized
2. less glutamate released
3. a) on-center bipolar cell depolarized
4. b) off-center bipolar cell hyperpolarized (AMPA inactive)
horizontal cells
Retinal neurons that mediate lateral interactions between photoreceptor terminals and the dendrites of bipolar cells.
retinitis pigmentosa
a progressive degeneration of the retina that affects night and peripheral vision
channelrhodopsin
A protein that, in response to light of the proper wavelength, opens a channel to admit sodium ions, which results in excitation of neurons.
neural pathway of somatosensory system
somatosensory receptors -> anterolateral column of spinal cord -(passes through medulla)-> thalamus > somatosensory cortex (in parietal lobe)
*3 total neurons
*there is an afferent pain fiber through dorsal root ganglion
A? fibers
-myelinated
-large diameter
-proprioception, light touch
-no thermal threshold
A? fibers
-lightly myelinated
-medium diameter
-nociception (mechanical)
C fibers
-unmyelinated
-small diameter
-innocuous temperature, itch
-nociception (mechanical, thermal, chemical)
- ~43 C temperature threshold
neurogenic inflammation
inflammatory peptides bind and cause Na+/Ca2+ influx, leading to action potentials
innocuous stimuli
nonpainful stimuli
noxious stimuli
painful stimuli, has S-shaped curve for pain
pain allodynia
pain to a normally innocuous stimulus
hyperalgesia
increased sensitivity to pain
voltage and current graphs for heat-evoked depolarization
voltage is normal upward "hill" shape
current is inward (a downward peak) and is faster
TRPV1
Na+/Ca2+ ion channel that opens from capsaicin or heat >43 C
heat-sensitive neuron mutations
TRPV1 and/or Nav1.7 KO's do not release NT
How can capsaicin be used to treat some conditions?
can kill sensory neurons to stop pain
TRPM8
cold sensor, menthol receptor
volt meter
a device used to measure voltage in the cell (like a thermometer)
voltage clamp
maintains a specific voltage (like a thermostat) so current/ion flow can be measured
List all the cells in the visual pathway and their locations.
1. photoreceptor - deep in retina
2. bipolar cell - medium deep in retina
3. retinal ganglion cell - least deep in retina, axon in optic nerve, and optic tract
4. interneuron - LGN (lateral geniculate nucleus) to visual cortex