neurons and action potentials

ventricles

four spaces filled by CSF, interconnected

• constant flow of CSF, providing cushioning/buoyancy

• flow of nutrients to and out of brain

brain-blood barrier

semi-permeable separation between blood and brain

• pathogen protection in sensitive brain tissue

  • endothelial cells, astrocytes, pericytes

  • passes: water, gas, hydrophobic, glucose, amino acids

  • blocks: large, hydroPHILIC, bacteria

• challenge for drug delivery

human vs rodent brain

similarities

• genes, neurons, proteins, structures, connectivity patterns

differences

• size and neuron no.

• brain structural proportions, small organizational diffs

• smooth rodent brain cortex

glia cells (50% of cells)

do not fire AP, support

3 types

1. astrocytes: touch neurons/BV cells; anchoring, balance chem. concentration outside neurons, injury repair

2. microglia: immune defense by destroying foreign

   1. oligodendrocytes: wrap neurons with myelin to increase electrical signaling

neurons

Ramon y Cajal’s Neuron Doctrine that says the brain is made up of small, discrete intersecting bodies

• soma (cell body), dendrites, axon terminals, terminal branches

parts of the neuron

dendrites: branching projections that collect info

soma: contains the nucleus and integrates info

axon: conducts neural signal across a distance (long distances such as SC to toe)

synapse stages

presynaptic: sending side

postsynaptic: receiver

voltage

difference in electrical potential between 2 points

• voltage at resting: -70 mV

  • higher negative charge inside the cell

Na/K pump

high K+ and low Na+ inside b/c NA/K pump ejects 3 Na+ and pulls 2 K+, using 1 ATP

• overall negative charge inside the neuron

inactivation of neuron voltage

equal concentrations inside and out —> death, no electrical activity

• Ouabin: african arrow poison that stops Na/K pump

action potential

spikes to +30 mV at peak AP

action potential graph

time (ms) vs voltage (mV)

depolarization

membrane potential becomes less negative

hyperpolarization

membrane potential becomes more negative

all or nothing principle

an action potential is initiated after threshold, -55mV, is met

• Na+/K+ pump brings back to baseline during weak stimulations

  • strong stimulation (depolarization) at +30mV

steps in action potental

1. depolarization: above -55mV —> opens voltage dependent Na+ channel and Na+ rushes in due to low concentration in the cell

2. at peak voltage of 30mV, Na+ channels close

   1. stops depolarization (transitioning to repolarization) and channels cannot open for 1 ms

3. high voltage opens voltage dependent K+ channels

   1. repolarization and K+ move out

4. Hyperpolarization closes K+ channels, resulting in afterpolarization

5. Inactive Na+ channels cause a refractory period where no new action potentials occur and Na+/K+ pump continuously brings back to baseline

afterpolarization

being less than -70mV of baseline following an action potential

tetrodotoxin (TTX)

caused by puffer fish in eyes

inactivates Na+ channels by blocking them

• contribute to paralysis

lidocaine/novocaine

occuring in pain-sensitive neurons where Na+ channels do not cause signaling

beginning of an action potential, location

begins at the axon hillock, traveling along the axon to axon terminals

axon hillock

rich in Na+ channels, causing volatile movement

• depolarizing formed by Na+ influx flows down the neuron, depolarizing the next section

  • Na+ channels close prior, then travel, preventing A.P. from moving backward

saltatory conduction

use of myelin sheaths (fatty substance) surrounding the axon

• in CNS: oligodendrocytes

• in PNS: schwann cells

• speeds up AP from 2 m/s to 120 m/s

multiple schlerosis

breakdown of myelin sheath

slows down neural communication, impairing vision, muscle weakness, movement/coordination issues, and cognitive impairments

neurotransmission steps

general: higher Ca2+ extracellularly

1. Action potential opens up Ca2+ channels and they rush in the axon terminal

2. vesicles get fused to the extracellular membrane and open up at the synaptic cleft

   1. neurotransmitters bind to the receptors on dendrite of postsynaptic neuron

what determines neurotransmission

types of neurotransmitters, type of receptors (ionotropic vs metabotropic), types of ion channels opening