Computing with Neurons Flashcards
What did Golgi believe about the granules stained by the stain he invented?
That each individual granule fused together with one another to for a massive web (reticulum).
He concluded that the nervous system is NOT composed of discrete cells (he was wrong).
Who challenged Golgi’s view?
Santiago Ramon y Cajal
What conclusion did Cajal reach that opposed Golgi’s view of the nervous system?
That axons and dendrites are separated by a gap. This meant that each granule with its process is a DISTINCT CELL (neuron).
summary:
1. Neurons are functionally polarized
2. neurons are distinct cells
Who coined the word “synapse?”
Sherrington
Sherrington found that the reflex pathways were _______
Unidirectional
What is the name of the major branches given off the axon?
axon collateral
What does a voltmeter measure?
potential difference between two points
Near the end of axon collaterals, typical axons branch repeatedly to form a set of _____________
terminal arborizations
how can action potentials be recorded?
- intracellular electrodes: sharp tip of electrode is inserted into cell’s interior.
- Extracellular electrodes: close to the neuron, but don’t penetrate cell membrane.
How many Na+ ions and K+ ions does ATPase pump transport?
3 Na+ ions out of cell
2 K+ ions into cell
Sodium and calcium ions are more concentrated _____ neurons
Sodium and calcium ions are more concentrated OUTSIDE OF neurons
Define “electrolyte”
an aqueous solution of a material that ionizes (soluble acid, base, or salt)
Define “electrostatic pressure”
the attractive forces between atomic particles of opposite charge or repulsive forces between atomic particles of the same charge
The concentration gradient for K+ tends to push it ____ the neuron and the electrochemical potential gradient across the cell membrane tends to move the potassium ions _____ the neuron.
The concentration gradient for K+ tends to push it OUT OF the neuron and the electrochemical potential gradient across the cell membrane tends to move the potassium ions BACK INTO the neuron.
Define the equilibrium potential for potassium
The membrane potential at which the concentration gradient for K+ and the electrochemical potential gradient across the cell membrane (2 opposing forces) are balanced.
What is the major cause of neuronal resting potential?
The push and pull of potassium ions across the neuronal membrane
The voltage clamp technique
Developed by Hodgkin and Huxley.
- one set of electrodes records the potential across the neuronal membrane.
- second set of electrodes is used to inject current into the neuron.
- any time the first electrode detects a change in membrane potential, the second set of electrodes injects current into the cell to compensate for the change, holding the voltage across the membrane steady.
- by keeping track of how much current is being injected into the cell, the experimenter can figure out how much current is being carried across the membrane by ions.
Hodgkin and Huxley experiment with changes in sodium and potassium permeability during action potentials.
- Hodgkin and Huxley depolarized a voltage-clamped squid axon to 0mV.
- They then measured current flow across the axonal membrane both in SEA WATER and SOLUTION LACKING SODIUM IONS.
- Lack of Na+ ions eliminated Na+ influx and thus showed only the current due to K+.
- By subtracting the curve generated from the current due to K+ (Na+-free solution) from the curve of total current (in sea water), they INFERRED THE CURRENT THAT IS NORMALLY CARRIED OUT BY SODIUM.
When do voltage-dependent Na+ channels open?
When the depolarization reaches threshold value
The neuronal membrane at rest is relatively impermeable to ___, but some ___ flows out.
The neuronal membrane at rest is relatively impermeable to Na+, but some K+ flows out.
What is saltatory conduction?
conduction of APs by myelinated axons. The AP appears to jump from one node of Ranvier to the next.
During a significant portion of an action potential, the axon’s membrane is permeable to both Na+ and K+. Why is this overlap metabolically inefficient?
How have mammals reduced this inefficiency?
During this overlapping period, sodium ions are entering while potassium ions are leaving the cell simultaneously, causing the two currents to cancel each other out. It takes energy to set up concentration gradient, therefore this is a waste of energy.
Mammals have solved this problem by inactivating the sodium channels more quickly, before the potassium channels open.
If a neuron has one AP initiate at the axon hillock and another at a distal dendrite simultaneously, what would happen when the resulting AP waves “collide?”
Both APs would disappear after a collision because the axon segments behind them are in refractory state. They cannot support AP propagation.
What determines the rate at which APs are propagated down an axon?
Strength of stimulus. Higher strength = more APs
What is the neurotransmitter for most excitatory neurons in the CNS?
L-glutamate (ionic form of glutamic acid)
What happens when an AP wave reaches an axon terminal (in chemical synapses)
AP wave causes voltage-gated calcium channels to open. Ca2+ ions travel INTO the terminal along their concentration gradient. The rise in [Ca2+] causes the synaptic vesicles to fuse with the cell membrane and release (through exocytosis) their neurotransmitter content into the synaptic cleft.
EPSP’s resulting from synapse activation close to the cell body are much _____ than EPSP’s due to input from dendrites.
EPSP’s resulting from synapse activation close to the cell body are much LARGER than EPSP’s due to input from dendrites.
What occurs when two distal inputs are activated simultaneously?
the two EPSP’s tend to sum when they reach the cell body, producing a SINGLE LARGE EPSP that can trigger an AP.
This is known as SPATIAL SUMMATION.
What occurs when several subthreshold EPSPs occur in rapid succession?
They can summate and reach threshold (TEMPORAL SUMMATION)
Integrate-and-fire model
neurons sum their inputs and then “fire” action potentials only if this sum exceeds a threshold.
What happens to neurotransmitters after they bind to postsynatic receptors?
- they float back into the synaptic cleft where they are free to bind to other receptors.
- In order to avoid neuronal hyperactivity, 2 strategies are employed:
1. some neurons have enzymes in the synaptic cleft that break down and inactivate neurotransmitters.
2. Other neurons contain specialized proteins that actively transport neurotransmitters from the synaptic cleft back into the presynaptic terminal.
Some neurons combine INACTIVATION and REUPTAKE.
Inhibitory synapses are often located close to the _________ of the postsynaptic cell, whereas excitatory synapses are mainly found in the _________.
Inhibitory synapses are often located close to the AXON HILLOCK of the postsynaptic cell, whereas excitatory synapses are mainly found in the DENDRITES.
Activation: influx of ____
Inhibition: influx of ____
efflux of ___
Activation: influx of Na+
Inhibition: influx of Cl-
efflux of K+
