Bio 462 Week 1

Dreams

What does not fit the input-output model for the brain?

Action

Brain is there to predict to lead to

Movement

Appearance of neurons coincided with appearance of

Nervous system

Builds a model of the world (internalizes the physical world)

Future

Nervous system builds a virtual model of the

Neuronal Ensembles

Groups of neurons get activated together to build internal symbolic structures

Thoughts

Symbols, representations, ideas, perceptions, memories, emotions, movement plans

exist internally without outside stimulus 

build an “internal brain state”

Hierarchy

The neuronal ensembles are organized in hierarchies (different levels)

Wiring

links different modules (specific diagrams that must be precise)

Precise connections

“labeled lines” design (neurons connect only with particular ones) ex: taste pathways

Distributed connectivity

Indiscriminate, many connections pruned and tweaked through learning

Ensembles

independent units that can construct more complex structures

Learning

shapes the distributed connectivity by pruning connections not needed

Some connections get stronger, some get weaker

The brain draws information from the outside world and stores it

a key component of the prediction-action model

Associative learning

two neurons responding to two separate stimuli

Two stimuli happening over and over at same time: connections strengthen,

neurons form a unit, stimuli get associated, new concept learned

Maps

The wiring diagrams are organized in maps which provide representations

ex: position of object in space correlates to specific position in retina

Control

Compares output with the prediction

if there is an error signal/wrong outcome then fine-tuning of output=feedback control

Optimization

Work must be performed effectively at minimal cost

ex) retina can react to a single photon

1/3 and 2/3

gene expressed uniquely in the nervous system; total genes expressed in the nervous system

Ion currents

Neurons send signals to each other via electricity provided by _____

Axon

Output of the signal, extends for long distance signaling

Dendrites

Receives the signal, axons usually connect to ____ of another neuron

Convergent connection

One neuron connects to another neuron

Divergent connection

One neuron connects to multiple neurons

Action potential

All or none. generated by neurons when they want to signal

results from integration of several inputs if the threshold of depolarization is reached

Its amplitude is constant

transient increased permeability to Na followed by slower and prolonged permeability to K⁺ to restore the resting potential

Synapses

Neurons connect to other neurons through

Chemical synapse

secretion of neurotransmitter between a pre-synaptic and a post-synaptic terminals

Post-synaptic cell

expresses the receptor of the neurotransmitter (chemical synapse)

Electrical synapse

direct link between neighboring cytoplasms

Local potential

slow and graded: their amplitude depends on the intensity of the input

ex: Synapses, sensory neurons (sensory receptors)

Ionotropic

-ion channels opening with ligand

Fast acting

Metabotropic

-coupled to second messengers

- Slow acting

Post-synaptic potentials

The movement of ions induces 

ESPS

excitatory post-synaptic potential

ISPS

inhibitory post-synaptic potential

Spines

The axons of excitatory terminals contact dendritic

helps signaling become more efficient 

Soma

Inhibitory neurons can contact ____, not spines

Central Nervous System

Brain and spinal cord

Cell bodies in brain and spinal cord

Peripheral Nervous System

Cell bodies outside of brin and spinal cord

Sensory systems

Receive and process information from environment (input)

Motor systems

Generate movement (control muscles) (output)

Associational systems

Complex brain functions

Astrocytes

Maintain a balanced environment around neurons, star shaped (“astra”)

Glial cells

Astrocytes, oligodendrocytes, and Schwann cells

Oligodendrocytes

CNS, form myelin around axons

can contact multiple axons at once

Schwann Cells

PNS, form myelin around axons

establish a 1:1 relationship with axons

Microglia 

“Immune” cells of the CNS, mediate inflammation, same origin as macrophages, participate in the pruning of synapses

Resting potential

All cells have unequal distribution of ions on both sides of the membrane: creating a

Electroneutral

Inside and outside of the cell

cytosol, extracellular fluid

-40 to -90 mV

Neurons’ resting membrane potential

Impermeable

Plasma membrane is ____ to ions

Electrodes

allow measurements of membrane potentials

There is always a reference __  (outside of the cell)

Electrical potential

amount of work necessary to move charged particles (reflects the difference of charge between two sides)

Expressed as voltage

Greater

A ____ difference of potential will move more charges (more current will flow)

Equilibrium potential

Two compartments separated by impermeable membrane

Only K⁺ allowed to go through

Same concentrations on both sides = same amounts of ions = no difference of potential

Outside

Which has a higher concentration? outside or inside

Down

K⁺ ions diffuse ____ their gradient of concentration

Negative

Inside compartment: loses ions = becomes more

Positive

Outside compartment: gains ions = become more

Electric field

Newly established difference of potential creates an _____

Diffusion force

will push the K⁺ ions from inside to outside

Electric force

will push K⁺ ions from outside to inside

net flow of ions null + equilibrium potential

When electric force = diffusion force 

Nernst equation

predicts the equilibrium potential for a given ion across a membrane

Giant squid nerve cells

Insert electrodes into ____

large axon conducts electrical signal fast, allowing rapid excitation and contraction of muscles to escape predators more easily

Current

What is the direction of cation movement called?

Inward

Positive current

depolarization 

Outward

Negative current (pos charges leaving the cell)

hyperpolarization (inside is more negative)

ionic concentrations, membrane potential

Small changes in ______ can cause large changes in

Uneven

____ charges align close to the membrane

Capacitance

The membrane “stores”charges

Active transporters

move ions against their concentration gradient

happens when Outside and inside concentrations of ions are different

Lipidic membranes

selectively permeable: ions need to pass through specific channels to diffuse

Equilibrium

 balance between chemical and electrical forces

  Ion gradient can determine the electrical potential

  OR

  Electrical potential can determine the ion fluxes

Goldman equation

takes into account the relative permeabilities to ions

-58 mV

Membrane permeable to K+ only

+58 mV

Membrane permeable to Na+ only

Ion transporters

Maintain different concentration of ions on both sides of the membrane

Rest

Permeability to K+ greater

More K inside than out = negative membrane potential

Depolarization

Na flows in  (permeability increases)

 positive membrane potential and generation of action potential

Increases resting potential

Increasing the concentration of K⁺ outside of the neurons causes

Smaller amplitude

Decreasing the external concentration of Na⁺ induces an action potential with a

True

The resting potential is not affected by the external Na⁺ concentration

Opening of ion-gated channels that will let in Na+ or K+. Ion permeability is sensitive to the membrane potential which needs to be above a certain threshold to generate an action potential.

What causes these changes in membrane permeabilities ?

Voltage Clamp

to study ion permeability at any level of membrane potential

enables control of the membrane voltage of the cell

Amount of injected current = direct measurement of the sum of all ionic currents across the membrane.

Examines how the membrane potential affects ion flows (currents) across the membrane.

No current flows except for brief ones

What happens after hyperpolarization?

Inward current

Positive charges from outside to inside

voltage changes

What does a Capacitive current compensate for?

Time-dependent

For axons, __________ stimulus will generate an additional capacitive current.

rapid inward current then slow delayed outward current

What happens at depolarization?

voltage-dependent

the membrane permeability of the axons is

Early vs late current

two types of current

Early current: has a U shape, increases with increased membrane potential but starts to decrease with membrane potentials above 0 mv

Late current: increases with increasing membrane potentials

Early current

reverses from inward to outward at around +55 mV = equilibrium potential of Na⁺

^{Extracellular concentration of Na is high=early current is inward}

^{extracellular concentration of Na is low= early current is outward (Na leaves the cell)}

Late current

Efflux/outward current of K+ affects

External Na+ does not affect this

Early influx of Na+ and delayed efflux of K+ when increasing the membrane potential

Current is affected by

Tetrodotoxin (Na) tetraethylammonium (K)

Toxins that block either Na+ or K+ currents

Membrane Conductance

reciprocal of resistance, related to permeability

Na⁺: rapid increase, rapid decrease (activation then inactivation after K+ is activated)

K⁺: slower delayed increase

Both conductances are activated time-dependently

How do the conductances of Na+ and K+ change over time?

Small

Second action potentials are ___ due to 4.5 ms interval between two stimuli

Refractory period

axon is not as excitable

Na+ conductance increasing/depolarization creating a feedback loop

Voltage increase leads to