Computing with Neurons

Question 1

What did Golgi believe about the granules stained by the stain he invented?

Answer 1

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).

Question 2

Who challenged Golgi’s view?

Answer 2

Santiago Ramon y Cajal

Question 3

What conclusion did Cajal reach that opposed Golgi’s view of the nervous system?

Answer 3

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

Question 4

Who coined the word “synapse?”

Answer 4

Sherrington

Question 5

Sherrington found that the reflex pathways were _______

Answer 5

Unidirectional

Question 6

What is the name of the major branches given off the axon?

Answer 6

axon collateral

Question 7

What does a voltmeter measure?

Answer 7

potential difference between two points

Question 8

Near the end of axon collaterals, typical axons branch repeatedly to form a set of _____________

Answer 8

terminal arborizations

Question 9

how can action potentials be recorded?

Answer 9

• intracellular electrodes: sharp tip of electrode is inserted into cell’s interior.
• Extracellular electrodes: close to the neuron, but don’t penetrate cell membrane.

Question 10

How many Na+ ions and K+ ions does ATPase pump transport?

Answer 10

3 Na+ ions out of cell

2 K+ ions into cell

Question 11

Sodium and calcium ions are more concentrated _____ neurons

Answer 11

Sodium and calcium ions are more concentrated OUTSIDE OF neurons

Question 12

Define “electrolyte”

Answer 12

an aqueous solution of a material that ionizes (soluble acid, base, or salt)

Question 13

Define “electrostatic pressure”

Answer 13

the attractive forces between atomic particles of opposite charge or repulsive forces between atomic particles of the same charge

Question 14

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.

Answer 14

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.

Question 15

Define the equilibrium potential for potassium

Answer 15

The membrane potential at which the concentration gradient for K+ and the electrochemical potential gradient across the cell membrane (2 opposing forces) are balanced.

Question 16

What is the major cause of neuronal resting potential?

Answer 16

The push and pull of potassium ions across the neuronal membrane

Question 17

The voltage clamp technique

Answer 17

Developed by Hodgkin and Huxley.

1. one set of electrodes records the potential across the neuronal membrane.
2. second set of electrodes is used to inject current into the neuron.
3. 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.
4. 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.

Question 18

Hodgkin and Huxley experiment with changes in sodium and potassium permeability during action potentials.

Answer 18

• 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.

Question 19

When do voltage-dependent Na+ channels open?

Answer 19

When the depolarization reaches threshold value

Question 20

The neuronal membrane at rest is relatively impermeable to ___, but some ___ flows out.

Answer 20

The neuronal membrane at rest is relatively impermeable to Na+, but some K+ flows out.

Question 21

What is saltatory conduction?

Answer 21

conduction of APs by myelinated axons. The AP appears to jump from one node of Ranvier to the next.

Question 22

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?

Answer 22

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.

Question 23

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?”

Answer 23

Both APs would disappear after a collision because the axon segments behind them are in refractory state. They cannot support AP propagation.

Question 24

What determines the rate at which APs are propagated down an axon?

Answer 24

Strength of stimulus. Higher strength = more APs

Question 25

What is the neurotransmitter for most excitatory neurons in the CNS?

Answer 25

L-glutamate (ionic form of glutamic acid)

Question 26

What happens when an AP wave reaches an axon terminal (in chemical synapses)

Answer 26

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.

Question 27

EPSP’s resulting from synapse activation close to the cell body are much _____ than EPSP’s due to input from dendrites.

Answer 27

EPSP’s resulting from synapse activation close to the cell body are much LARGER than EPSP’s due to input from dendrites.

Question 28

What occurs when two distal inputs are activated simultaneously?

Answer 28

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.

Question 29

What occurs when several subthreshold EPSPs occur in rapid succession?

Answer 29

They can summate and reach threshold (TEMPORAL SUMMATION)

Question 30

Integrate-and-fire model

Answer 30

neurons sum their inputs and then “fire” action potentials only if this sum exceeds a threshold.

Question 31

What happens to neurotransmitters after they bind to postsynatic receptors?

Answer 31

• 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.

Question 32

Inhibitory synapses are often located close to the _________ of the postsynaptic cell, whereas excitatory synapses are mainly found in the _________.

Answer 32

Inhibitory synapses are often located close to the AXON HILLOCK of the postsynaptic cell, whereas excitatory synapses are mainly found in the DENDRITES.

Question 33

Activation: influx of ____
Inhibition: influx of ____
efflux of ___

Answer 33

Activation: influx of Na+
Inhibition: influx of Cl-
efflux of K+