General Bio 1: Chapter 43- Neurons, Glia, and Nervous Systems

What are the two types of cells neurons have?

Neurons and glia 

Neurons

Nerve cells

What do neurons do?

Generate and conduct electric signals

What are the two kinds of glia?

Macroglia and microglia 

Macroglia

Modulate neuron activity and provide support

Microglia

Small phagocytic cells; major immune defense mechanism in the nervous system

Central Nervous System

Brain and spinal cord

What are the brain and spinal cord sites of?

Information processing and storage 

Peripheral nervous system (PNS)

Provides communication between the central nervous division and all of the rest of the body

Where do neurons and macroglia originate from?

Neural stem cells in the neural tube in early embryo

What happens when neural stem cells divide?

One daughter cells remains a stem cell and the other becomes a neuroblast or glliablast  

What is in the neuron structure

Cell body, dendrites, axon, axon terminals

Cell body

Contains nucleus and organelles

Dendrites

Bring information to the cell body

Axon 

Carries information away from the cell body

Axon terminals

At the tip of the axon

What does the number of dendrites reflect?

The number of sources of information coming to the neuron

How do neurons process and communicate information?

Through changes in electric potential across membranes

Action potentials (APs)

Large, sudden, transient, rapidly reversed large changes in membrane potential 

What do small changes in membrane electrical potential generate?

Action potentials

Synapse

Forms when axon terminals come extremely close to the membrane of a target cell

What happens when action potential reaches axon terminals?

Synapse transfers the information from presynaptic cell to postsynaptic cell

What do electrical synapses allow for?

Action potentials to pass directly between two neurons 

What does the AP cause?

Terminal to release neurotransmitter chemicals

What do neurotransmitter chemicals do?

Diffuse to receptors on postsynaptic cell

What happens when the neurotransmitter binds to postsynaptic cell?

Neurotransmitter may be excited or inhibited

How do neurons integrate information?

By summing excitatory and inhibitory inputs 

What are the glia in brain and spinal cord called?

Oligodendrocytes

What do oligodendrocytes do?

Wrap around neuron axons which form concentric layers of insulating cell membrane

Schwann Cells

Glia that wrap the axons of other nerves

Myelin 

Covers axons 

What do some diseases do?

Affect myelin and impair conduction of action potentials

Multiple Sclerosis

Autoimmune disease where antibodies to proteins in myelin in the brain and spinal cord are produced

What do aquaporins in astrocytes do?

Take up fluid and distribute it to interstial spaces in the brain

Where does the intersial fluid leave through?

Perivascular spaces of the veins, taking metabolic waste products with it

How do astrocytes control communication between pre and postsynaptic cells?

They take up neurotransmitter from synapse 

What do astrocytes store?

Glycogen that can supply neurons with fuel

What do astrocytes release?

Neurotransmitters that can alter the activities of neurons

What do astrocytes aid in?

Repair and regeneration of neurons

Tripartite synapse 

A synapse includes pre and postsynaptic neurons as well as connections from astrocytes 

What do Sodium-Potassium pumps in all animal cells create?

Gradients of Na+ and K+ across cell membrane

Why is the inside of the cell usually negative relative to the outside in all animal cells?

Because “leak channels” allow some ions (K+) to diffuse out

Membrane potential

Electrical charge difference across a cell membrane

What causes a membrane potential?

Balance between the tendency of K+ ions to diffuse down their concentration gradient and the electrical potential that holds them back 

Resting potential

The steady state membrane potential of a neuron

Voltage

Electric potential difference- force that causes charged particles to move between two points

What are the major ions in neurons?

Sodium (Na+), potassium (K+), Calcium (Ca 2+), Chloride (Cl-)

Resting potential of an axon

-60 to -70 millivolts mV

How is electric current carried in solutions and across all membranes?

By ions

What allows ions to move quickly across the membrane?

Stimulus that changes the permeability of the membrane

Action potential 

A sudden, rapid, reversal in voltage across a portion of the ell membrane 

What happens to the ions during an action potential

They flow into the cell, making the inside more positive than outside = depolarization

What are ion transporters and ion channels responsible for?

The distribution of charges across the membrane that determine membrane potential

What does the sodium potassium pump do?

Moves Na+ to outside and K+ to the inside, establishes concentration gradients

What do ion channels in the membrane allow for?

Ions to pass through

What does net movement of ions depend on?

Concentration gradient and voltage difference

Electrochemical gradient

Two motive forces- Concentration gradient and voltage difference that ion net movement depends on

Potassium equilibrium potential (Ek)  

Membrane potential at which net movement of K+ stops 

What influences resting potential?

K+ leaky channels and movements of Na+ and Cl-

Patch clamping

Allows ion channels and their properties to be studied

What happens during patch clamping?

Electrode/pipette places against membrane, slight suction removes a patch of membrane with ion channels intact

Ion movement and channel opening and closing recorded as electric currents

How are ion movement and channel opening and closing recorded as in patch clamping?

Electric currents 

What are the types of gated ion channels?

Voltage gated, Chemically gated, Mechanically gated

Voltage gated channels

Respond to change in voltage across membrane

Chemically gated channels

Depend on specific molecules that bind or alter the channel protein

Mechanically gated channels 

Respond to force applied to membrane 

What alters membrane potential?

Opening and closing of gated channels

What happens when Na+ channels open suddenly?

Na+ diffuses in and inside of cell becomes less negative

Depolarized

Inside of neuron becomes less negative (more positive) compared to resting condition

Hyperpolarized

Membrane potential becomes even more negative

What happens when gated K+ channels open?

K+ efflux increases over normal leak rate = membrane becomes more negative

Graded membrane potentials

Changes from resting potential proportional to stimulus magnitude

What generates action potentials?

Action of voltage-gated Na+ and K+ channels

What happens to gates at resting potential?

Channels are closed 

What causes channels to open at resting potential?

Slight depolarization

Where are voltage-gated ion channels concentrated in?

Hillock

When is a threshold reached?

When membrane is depolarized about 5 to 10 mV above resting potential

When can voltage-gated Na+ channels not open?

During refractory period

Activation gate

Closed at rest but opens quickly at threshold

Inactivation gate

Open at rest and closes at threshold; reopens 1-2 milliseconds later than activation gate closes

How do voltage gated K+ channels contribute to refractory period?

Remaining open 

What happens with the opening of voltage gated K+ channels?

Efflux of K+ ions makes membrane potential less negative than resting potential

After-hyperpolarization/Undershoot

Dip in membrane potential after action potential

Why is it important that an action potential can travel over long distances with no loss of signal?

It is an all or none event to voltage gated Na+ channels that ensures maximum action potential

Action potential in myelinated vs nonmyelinated 

Travels faster in myelinated 

Nodes of Ranvier

Regularly spaced gaps in myelin along an axon

Self regenerating

Action potential spreads to adjacent membrane regions

Why can’t action potential reverse direction?

Because of refractory period of channels behind it

Where are action potentials generated at?

The nodes and positive current flows down the inside of the axon

What happens when positive current reaches next node?

Membrane is depolarized and another axon is generated

Saltory conduction

Propagation formed when action potentials appear to jump from node to node

How do neurons communicate with other neurons or target cells?

At synapses

Electrical synapse 

Action potential spreads directly to postsynaptic cell 

Chemical synapse

Neurotransmitters from a presynaptic cell that induce changes in membrane potential of postsynaptic cell

What are the most common synapse in vertebrates?

Chemical synapses

Neuromuscular junctions

Chemical synapses between motor neurons and skeletal muscle cells

What is the neurotransmitter of neuromuscular junctions?

Acetylcholine (ACh)

Where does ACh diffuse?

Across synaptic cleft to motor end plate on muscle cell

How do vesicles release ACh into synaptic cleft?

Exocytosis

When does an action potential cause the release of ACh?

When voltage gated Ca 2+ channels open and Ca 2+ enters axon terminal