L11: Nervous System II

What is resting membrane potential?

The voltage difference across plasma membrane of resting cell

Which side of the cell is negative at rest?

The outside

Why is the inside of the cell more negative?

• there is more K+ inside, and more Na+ on the outside

• negatively charged molecules can’t readily cross the membrane

What are the steps of an action potential?

1. Depolarization

2. Repolarization

3. Hyperpolarization

What happens in depolarization?

• if trigger reaches threshold, then voltage-gated Na+ channels open

• Na+ influx —> action potential begins

What happens in repolarization?

• Na+ channels are inactive, K+ channels open

• K+ leaves the cell, membrane polarizes

What happens in hyperpolarization?

K+ channels close slowly

How does nerve signal conduction begin?

Dendrites receive synaptic input

What does synaptic input create?

Graded potential

What is graded potential?

A small temporary change in a neuron’s electrical charge

What do graded potentials spread to?

The trigger zone (axon hillock + first portion on axon)

What happens when an axon reaches the threshold?

There is an action potential

Can dendrites receive multiple synaptic inputs for local potentials?

Yes

What do neurotransmitters bind to on local potentials?

Postsynaptic membrane

What is the difference between a local potential and an action potential?

Local potentials are short distance and based on graded changes

Which potentials are ligand-gated?

Local potentials

Do all ligand-gated ion channels need to open for a local potential?

No

What can strengthen a synapse?

Repeated or coordinated synaptic activity

How does repeated activity increase synaptic strength?

Increases intracellular signaling, which increases the number of AMPA receptors

When does dendritic spine enlarge?

With a stronger synaptic activity

What structural and/or functional changes can learning produce (3)?

1. Formation of new dendritic spines

2. Elimination of synaptic connections— weak or inactive ones

3. Changes in existing synaptic strength

How does glutamate activate AMPA receptors?

Glutamate binds to AMPA receptors, AMPA channels open, Na+ enters postsynaptic membrane causing depolarization

How do NDMA receptors become activated?

Glutamate binds to the NMDA receptors, but blocked by Mg2+ at resting potential

Strong activity causes Mg2+ block to be removed so Ca2+ can enter

How does calcium trigger synaptic strengthening?

Activates intercellular signaling that promotes:

• insertion/activation of AMPA receptors

• changes in dendritic spine structure

What do AMPA and NMDA receptors do?

Mediate fast communication between cells

What determines if a neurotransmitter is excitatory or inhibitory?

The receptor and ion channels it activates

What causes EPSPs (excitatory postsynaptic potentials)?

Neurotransmitters bind to receptors that open channels allowing positive ions to flow into the postsynaptic neuron (causes depolarization)

What causes IPSPs (inhibitory postsynaptic potentials)?

Neurotransmitters bind to receptors that open channels allowing negative ions to enter/positive ions to exit (causes hyperpolarization)

What does the trigger zone contain?

High density of voltage-gated Na+ channels

What is spatial summation?

Inputs from multiple synapses at approx. same time

What is temporal summation?

Repeated input from the same synapse in rapid succession?

What does a neuron fire according to the summation of?

Net sum of excitatory and inhibitory inputs

What is myelin?

A lipid-rich wrapping around the plasma membrane of glial cells

What are the functions of myelin?

Insulates axon, reduces current leaking, increases conduction speed

What does each Schwann cell form around?

One myelin segment around one axon

What does each oligodendrocyte cell form around?

Myelin around segments of multiple axons

What is an internode?

Myelin-covered segment between two nodes

What kind of conduction is a myelinated axon?

Saltatory conduction

What kind of conduction is a nonmyelinated axon?

Continuous conduction

Where are Na+ channels concentrated at in a myelinated axon?

Nodes of Ranvier

Where are Na+ channels concentrated at in a nonmyelinated axon?

Along the axon

Where are action potentials regenerated at on a myelinated axon?

Successive nodes

Where are action potentials regenerated at on a nonmyelinated axon?

Successive regions of axon

Why does action potential signal usually move forward?

The membrane behind the action potential is in refractory period

What is the resting membrane potential?

-70mV

Which potential has summation?

Local potentials

What is the main function of local potentials?

Integrate incoming signals (to decide if an action potential gets fired)

What is the main function of action potentials?

Trasmit signals over long distance