Neurophysiology Flashcards
What is a cell membrane composed of?
A phospholipid bilayer
What can cross a cell membrane?
Lipid-soluble molecules and gases diffuse through, water-soluble molecules pass with help
What does permeability of the membrane depend on?
Molecular size, lipid solubility, and charge
What is simple diffusion?
The movement of a substance down its concentration gradient using passive transport (no ATP)
What impacts the speed of diffusion?
The size of the concentration gradient. The larger the gradient, the faster the diffusion
What is facilitated diffusion?
Movement of molecules
down the concentration gradient across the membrane with the help of a carrier protein. Passive transport
What is saturated diffusion?
When the concentration of molecules exceeds the number of transport molecules
What transport types are passive? (No ATP)
Simple diffusion and facilitated diffusion
What is primary active transport?
The movement of molecules against the concentration gradient using ATP
What is secondary active transport?
Movement of molecules up its concentration gradient powered by the transport of another molecule down its gradient
How does active transport work?
Causes a conformational change in the membrane protein
What are pore loops?
Portions of a membrane protein that dangle inside the channel to create a selectivity filter based on size and charge
What is a ligand-gated channel?
The binding of a receptor with its ligand triggers a membrane event.
What is a voltage-gated channel?
Channels that open in response to changes in membrane polarity
What is the voltage sensing mechanism in voltage-gated channels?
The 4th transmembrane domain of the protein, S4 segment
How does the S4 segment function?
S4 segments are positively charged and stick out to the sides like wings. During resting potential the wings are held downwards, closing the channel.
What is endocytosis
inward pinching of the membrane to create a vesicle to capture proteins from the outside
What is exocytosis 1?
Kiss and run. Secretory vesicles dock and fuse at fusion pores and connect and disconnect several times before contents are emptied
What is exocytosis 2?
Complete fusion of the vesicle with the membrane, releasing the vesicle contents at once.
2 conditions for MPs?
1) create a concentration gradient with ATPases
2) Semi-permeable membrane to allow one ion species to move across more than another
Na+ / K+ pump
ATPase enzyme that moves 3 Na+ out and 2 K+ in for each ATP used
What contributes to resting membrane potential?
K+ has eq potential of -90mV and Na+ and Cl- diffusion make it -70mN instead
What is the concentration of ions inside and outside of the membrane?
Inside has more anions (proteins) and K+ and outside has more Cl- and Na+
How does depolarization occur?
Once membrane is depolarized to -55mV, S4 segments move up and Na+ gates open
What is an action potential?
A change in membrane potential that moves from -70mV to +30mV in cells with Na+ channels
Subthreshold stimuli
Below 15mV and open some Na+ channels but not enough to start action potentials
Suprathreshold stimuli
Produce more than enough to change to cause an action potential but don’t impact magnitude
Frequency coding
Information pertaining to stimulus intensity coded by changes in AP frequency
Refractory period
Period after an AP where some or all Na+ channels are inactivated
Relative refractory period
Some of the Na+ channels are reconfigured and can produce a smaller AP
What happens if more K+ is added outside the cell?
The K+ concentration gradient will be destroyed and the cell will stay depolarized
After-hyperpolarization
Allow Na+ channels to reconfigure fast enough to generate another AP. Both K+ leak channels and voltage-gated K+ channels are open to hyperpolarize the cell
Impulse conduction
When a patch of excitable membrane generates an AP, producing a depolarizing current for adjacent membrane.
How is lambda (length constant) improved?
1) increasing the diameter causes less resistance, less voltage lost
2) increasing membrane resistance causes less current to be leaked out
What is the Node of Ranvier?
Unmyelinated portions of the axon that have Na+ channels to generate action potentials
Schwann cell function
Wrap around a single portion of one axon, squeezing out cytoplasm
Oligodendrocyte function
Wrap around a whole bunch of axons individually
Saltatory conduction
APs on one node can create a depolarizing current strong enough to last 5-10 nodes
Remark bundle
Insulation in unmyelinated axons caused by Schwann cells and oligodendrocytes engulfing 5-30 axons without winding
Electrical synapses
Electrical signals are transmitted from one cell to the next and don’t involve the release of neurotransmitters
Chemical Synapse
Transmitter is released into the extracellular space between adjacent cells
Vesicle release
Triggered by Ca++ ions that are released into the bouton in response to depolarizing AP currents
Ionotropic effects
Ligand binds to an ion channel to directly produce a post-synaptic potential
Ionotropic receptor ligands
Acetylcholine, glutamate, GABA (IPSPs) and glycine *can also act on metabotropic
Metabotropic effects
Binding of the ligand to post-synaptic receptor to activate a G-protein coupled enzyme to activate a 2nd messenger
Enzyme facilitation in 2nd messangers
cAMP, cGMP or InP3 is activates to activate phosphokinases to phosphorylate membrane proteins like Ca++ channels
Beta-adrenoreceptor
Metabolic receptor for noradrenalin which activates adenylyl cyclase to increase cAMP production
Post-synaptic potentials
Generated in inexcitable membranes such as dendrites and cell bodies to produce graded potentials
PSP Summations
Many PSPs are needed to depolarize the trigger zone to -55mV and summation allows PSPs to be added
Spatial Summation
Larger number of EPSPs occurring together in dendrites that overlap to be added on
Temporal summation
Long EPSPs that allow successive inputs on any given synapse to generate other EPSPs to add onto pre-existing EPSPs
Ihibitory IPSPs
Located between the EPSP generation site and trigger zone to stop depolarizing EPSP currents
How do IPSPs work?
Involve the opening of Cl- channels to hyperpolarize the cell, clamping the MP
Spike trains
EPSPs at the trigger zone generate an AP (spike). Caused by a long-lasting stimulus at the post-synaptic neuron
Receptor potential
Change in MP due to the signal from an exterior sensory cue which causes depolarization
How do receptor proteins function?
Function similar to PSPs and follow pathways similar to ionotropic and metabotropic
Olfactory Receptor
A metabotropic receptor that produces currents similar to EPSPs
Amplification
Occurs in metabotropic mechanisms and can make cells sensitive
Sensory cell signal transmission
The branch point between different axons usually has the trigger zone and is depolarized by summation
Sensory cell vesicles
Alone, the depolarizing current can cause an influx of Ca++, allowing vesicles to be released
Slow adaptation
Slow decay of receptor potential for the duration of the stimulus. Magnitude based
Rapid adaptation
When stimulus is constant, the receptor potential is zero and can elicit a response when stimulus is removed. Velocity of stimulus based.
Habituation
Repeated identical stimuli in close succession elicits a weaker response each time
Coding of stimulus intensity (graded potentials)
For graded potentials, greater stimulus intensity leads to greater receptor depolarization, releasing more transmitter
Increasing stimulus intensity effect
Higher threshold sensory neurons are recruited
What happens to APs when a stimulus is intense?
The frequency of APs increases
Labeled line modality
Requires different receptors for qualities such as colours, texture, etc
Population code
Specific stimuli are coded by the ratio of activity across a population of receptors. Ex, a stimulus activates one receptor stronger than others
Receptive field
A spatial area each sensory neuron responds to. The more neurons, the higher the sensitivity
Blood brain barrier function
Regulates the extracellular fluid in the neuronal environment
Areas lacking the BBB
Hypothalamus and pituitary glans are connected to the bloodstream to secrete hormones. Circumventricular organs sense chemicals
Brain encasings
Skull, meninges, reticular formation
Meninges
1) Dura mater: sac containing the brain and spinal cord next to skull
2) arachnoid membrane
3) pia mater: lies on brain
Reticular formation
Loose nerve cells that connect the brain to the spinal cord
Subarachnoid space
Between the aracnoid space and pia matter. Filled with CSF to cushion the brain and also has blood vessels leading to the brain tissue
BBB structure
Endothelial lining in blood vessels are tightly bound by gap junctions in the BBB but throughout the body have gaps
Ventricles
Cavities in the brain filled with CSF
CSF pathway
Lateral ventricle empties into the 3rd ventricle which connects to the 4th ventricle by the Aqueduct of Sylvius. 4th ventrical drains into the central canal into the spinal cord
CSF drainage
Central canal > subarachnoid space > large venous sinus (midline) > venous system or arachnoid villi in dura matter
Choroid plexus
Produces most CSF and is a network of capillaries
CSF composition
Same osmolarity as blood but reduced K+, Ca++, Mg++
How much CSF is there and how often is it replaced?
140ml volume (25ml in ventricles, 115ml in subarachnoid, 75ml in spinal cord) and is replaced 3x a day
Astrocytes
Provide a bridge between neurons and blood vessels, wrap capillaries, produce lactate, remove neurotransmitters
Local blood flow
Regulated by astrocytes to obtain more nutrients or less
Prostaglandin (PGE2)
Triggered by glutamate synapses to release Ca++ in astrocytes to cause vasodilation
