Section 4: Neuro 1A + 1B + 2

Neurons

• specialized for communication and singaling

• receive, process, and transmit information

• generate and transmit electrical signals (nerve impulses

• communicate with other neurons, muscles, or glands

Glia (neruglia)

• support and protect neurons

• provide structural and metabolic support

• maintain chemical environment around neurons

• assist and optimize neuronal signaling and conduction

Dendrites

branching extensions of soma convey information towards cell body

Soma

the cell body; produces neurotransmitters

In Nuerons; coded information is passed from one neuron to the next by ________

synaptic transmission

Trigger Zone

on a neuron; where action potentials are generated

Axon

on a neuron; conducts electrical signals to next neuron or muscle

Oligodendrocytes

a type of glial cell in the CNS; form myelin around axons → insulation and faster conduction

Astrocytes

a type of glial cell in the CNS; structural support; regulate chemical environment around cell

Microglia

a type of glial cell in the CNS; immune defence; remove debris and damaged cells by phagocytosis

Schwann Cells

a type of glial cell in the PNS; form myelin around peripheral axons

Satellite cells

a type of glial cell in the PNS; surround and support neuron cell bodies; help regulate their local environment

Ions

atoms or molecules with an electrical charge:

• cations (+)

• anions (-)

Valence

electrical charge carried by an ion

Electrical Current

movement of electrical charge. In biological systems: movement of ions

Electrical potential energy

enemy available to drive movement of electrical charges

Similar to Electrical potential energy, electrical gradients can drive __________

ion movement

Voltage (V)

difference in electrical potential between two locations, provides driving force for movement of chafed particles

Voltage across the cell membrane

• cells maintain unequal distribution of ions across membrane, creates an electrical potential difference between inside and outside the cell

• inside the cell is usually negative relative to outside

Membrane potential (Vm)

• electrical potential difference across cell membrane

• Vm = V(inside) - V(outside)

Resting membrane potiential

• membrane potential when cell is at rest

• stable, typically -60 to -90 mV

Origin of the Resting Membrane Potential

1. Unequal distribution of ions (high K inside, high Na outside)

2. Selectively permeable membrane (membrane is more permeable to K than Na, leaving outside more negative)

Na/K pump

• uses ATP to transport 3 Na out and 2 K in

• maintains Na and K concentration gradients and preserves the negative interior (more positives charges leave than eneters)

Why is K more permeable than Na

more open K+xE channels open the Na+ or Cl-

Excitable Cells

can change their membrane potential and can generate propagate electrical impulses (neurons)

Chemically (ligand)-gated ion channel

open/close in response to chemical signal binding to channel

Mechanically gated ion channel

open/close in response to physical force

Voltage Gated Ion channel

open/close in response to changes in membrane potential

Deploarization

• membrane potential becomes less negative

• absolute value of Vm decreases

  • EX: Na enters the cell

Repolarization

• membrane potential returns toward its resting value

  • EX: K leaves the cell

Hyperpolarizaiton

• membrane potential becomes more negative than resting value

• absolute value of Vm increases

  • EX: K leaves the cell or Cl- enters the cell

Nernst Equation

• calculates equilibrium potential (E_ion) for a single ion

• At constant T; E = [58/z] log₁₀(C_out/C_in)

  • Z = valence

  • C = concentration

relative contribution of each ion depends on:

1. concentration gradient

2. relative permeability

Goldman-Hodgkin-Katz (GHK) Equaiton

• calculates the membrane potential when several ions are present

• E_m=(58) log₁₀ ([P_{a out}/ P_{a in}] + [ P_{b out/} P _{b in] …)}

• P = permeability to []

Graded Potentials

small localized changes in membrane potential that result from the opening of ion channels. The magnitude of depends on the stimulus (stronger=larger change) with variable polarization

Graded Potential Pathway

initiated in dendrites/soma and only travel a short distance with some of the current ‘leaking out’ and the graded potential dies. The signal can travel in both directions

Action Potential

can travel long distances, it is initiated at hillock (trigger zone) and is mediated by voltage-gated channels:

• if depolarization reaches the threshold:

  • voltage-gated channels open and let Na+ in and membrane is further polarized

All Or None Response

if the threshold is reached, action potential. If no threshold is reached, no action potential

_____ has a larger response than a graded potential

action potential

Action potential moves _____

unidirectionally

What is responsible for the rising of action potential

the influx of sodium ions through voltage-gated sodium channels → fast

What is responsible for the falling of action potential

the efflux of potassium ions through voltage-gated potassium response → slow

Depolarization Phase

• rising

• Na+ influx through voltage-gated sodium channels

• Na+ channels open quickly

• inside of membrane becomes less negative

Repolarization Phase

• falling

• K+ efflux through voltage-gated potassium channels that open slowly

Absolute Refractory Period

• occurs when Na+ channels are open or innactivated

• no new action potential can be initiated during this time

• the duration of action potential is always the same

Conductivity Velocity

the speed at which AP travels down the length of an axon, relies on:

• axon diameter

• myelinization

• temperature

If the larger the diameter of axon, why aren’t all axons big?

• very costly (large volumes of cytoplasm, membranes, organelles, etc)

• take up a lot of space

Propagation of Action Potential in Unmyelinated Axon

has continuous propagation → repeatedly regenerating itself at every single patch of the membrane

Propagation of Action Potential in Mylenated Axon

• uses Saltatory Conduction → impulse jumps from one node of ranvier to another

• depolarization spreads faster

Frequency Coding

the amplitude and duration of action potential is constant and the strength of stimulus is coded into the frequency

Maximum frequency is limited by __________

absolute refractory period

Action potential frequency increases with _______ stimuli

longer

Electrical Synapse

• 2 neurons directly connected by gap junctions where ions flow from one neuron to another

• very fast transmission with rapid synchronization

  • depolarization of one neuron caused an immediate depolarization in the next

Electrical Synapse is found where?

Where speed or synchronization is critical like the brain, spinal cord, or retina

Chemical synapses have _____ communication between the cytoplasm of two cells

no direct

Neurotransmitters

are exocytosed in synaptic gapfrom presynaptic cell and bind to R on postsynaptic cell

Steps of Synapse expelling Neurotransmitter

• initiated by AP at presynaptic terminal

• Ca channels open → rise in Ca within terminal button

• causes the fusion of vesicles containing NT with plasma membrane

• NT expelled into synaptic cleft

Fast (ionotropic) synapses

receptors are trans membrane ion channels and the binding of NT directly opens or closes

Slow (metabotrophic) synapses

receptors are G-protein-coupled and activate intracellular second messenger cascades → indirectly modulate ion channel

Synaptic delay

0.5 - 1 ms

Steps of Neurotransmitter removal from synaptic cleft

• Re uptake → actively transported back into the presynaptic neuron

• Enzymatic Degradation → broken down be specific enzymes

• Diffusion → NT diffuses away from synaptic cleft and taken up by surrounding cells

Factors influencing signal strength

• amount of NT released (more vesicles → larger response)

• Density and sensitivity of postsynaptic receptors

Post Synaptic Potential

graded, temporary, and small in amplitude. but can be excitatory or inhibitory

EPSP (Excitatory post-synaptic potential)

small depolarization, brings postsynaptic cell closer to threshold for the generation of AP → positive charge enters cell

IPSP (inhibitory post-synaptic potential

small hyperpolarization that brings postsynaptic cell away from threshold → negative charge in or positive out

Charge in Membrane Polarization Equals

the sum of all PSPs (EPSP + IPSP) in post-synaptic neuron

Temporal Summation

• one synapse activated many times

• addition of PSP’s over time

Spatial Summation

several synaptic contacts are activated simultaneously

Neuronal/Synaptic Integration

neuron adds all IPSPs and EPSP to determine if an AP is generated

Neuromuscular Junction Signaling vai Acetylcholine (ACh)

• Ach binds to cholinergic R (chemical gated) and the Na and K channels open

• depolarization = EPP = end plate potential

• local current between depolarized end plate and adjacent area

• muscle AP initiated and propagated via Na voltage-gated channels

• contraction of muscle

• ACh destroyed by Ach-esterase

Tetrodotoxin (TTX)

a neurotoxin that blocks voltage-gated sodium channels

Botulinum Toxin

a neurotoxin that prevents the release of acetylcholine

Curare

a neuromuscular blocker that binds to ACh on skeletal muscle

Fluoxetine (Prozac)

an antidepressant that inhibits the reabsorption of serotonin