Bio 14 Week 2 Nervous System Pt. 2

Neurons and Glia

Two Broad Categories of Nerve Cells

Neurons

electrically excitable nerve cells that can receive and transmit signals

Glia

nerve cells that provide mechanical and metabolic support to neurons

Astrocytes/ependymal cells/oligodendrocytes/microglia

CNS neuroglial cell types in vertebrates

satellite cells/schwann cells

PNS neuroglial cell types in vertebrates

Astrocytes

CNS glia that are

• involved in formation of the blood brain barrier

• deliver nutrients

  • maintain extracellular K+ homeostasis

Ependymal cells

CNS glia that line the spinal cord and brain ventricles; produce cerebrospinal fluid (CSF).

oligodendrocytes

CNS glia that forms myelin in the CNS

microglia

CNS glia that are the immune cells of the brain; responsible for inflammatory responses and removing debris and pathogens.

satellite cells

PNS glia that surrounds neuron cell bodies in the ganglia of the peripheral nervous system (PNS).

Schwann cells

PNS glia that forms myelin in the peripheral nervous system (PNS) and assist with neuron regeneration after injury.

Dendrites

• Often more than one per cell.

• Have a tapering diameter, getting smaller the further they are from the cell body.

• Receive information on the cell body and on the ends of small "spines."

• Do not have a myelin sheath.

Axon Initial Segment (AIS)

• Located near the cell body.

• Expresses a specialized combination of proteins and ion channels.

• Segregates the axon from the dendrite and soma.

• Typically shapes and generates information before it is conducted down the axon.

Axons

• Usually only one per cell.

• Have a constant diameter, unlike dendrites.

• Sometimes have a myelin sheath.

• Conduct/send information.

Axon Terminal

The site of neurotransmitter release to generate signals in other cells, such as muscle cells.

K+

Ions with greater concentration inside the cell

Na+, Cl-, Ca2+

Ions with greater concentration outside the cell

negative (-70 mV)

the cell interior is _____ relative to the outside of the cell

Graded potentials, action potentials

General types of membrane potential changes in neurons

Graded potentials

membrane potential changes in neurons that are due to something external: sensory stimulus or neurotransmitters

action potentials

membrane potential changes in neurons that are intrinisic: generated by the cell

passive/dendrites/cell body

Graded potentials are ______ responses that occur primarily in the _______ and _______ of neurons. The size of the change depends on the strength of the stimulus.

self-propagating

Generally, graded potentials are not _________. They are confined to the area where the stimulus was applied, and the size of the change in potential depends on the strength of the stimulus. A stronger stimulus will create a larger graded potential.

Na+ , K+ , Ca2+

Action potentials are generated by activity of voltage-gated ion channels selective for ______

Action Potential Threshold

The membrane potential at which an action potential will occur. In neurons, this is often around -50 mV.

Threshold Depolarization

The unstable membrane potential where Na⁺ ions flowing into the cell exceed the flow of K⁺ ions out of the cell, triggering the action potential. It is referred to as a "point of no return," initiating the feedforward cycle of the action potential.

Feedforward Cycle

The action potential process that is initiated once the threshold is reached, leading to an explosive depolarization followed by repolarization of the cell membrane. This cycle is self-propagating and continues down the axon without further external stimuli.

Na⁺ Channels

• Open Briefly (~1 millisecond): The ______ open in response to membrane depolarization, allowing Na⁺ to flow into the cell.

• Inactivate Rapidly: After opening, the _______ close quickly even if depolarization is still happening.

• Closed Until Resting Potential: The _______ remain closed until the resting membrane potential is restored.

K⁺ Channels

• Open in Response to Depolarization: These channels open slower than Na⁺ channels after depolarization.

• K⁺ Flow Out of the Cell: K⁺ moves out of the cell through the open channels, helping to repolarize the membrane.

• Close When Membrane is Repolarized: The _______ close when the membrane potential returns to its resting values.

Hyperpolarization

The slower opening of K⁺ channels is indeed a key factor in __________

Depolarizing phase

• Increasing number of Na+ channels opening

• (K+ channels are also stimulated by the depolarization, but are slower to open)

Repolarizing phase

• Increasing numbers of Na+ channels inactivating

• More K+ channels are now open than Na+ channels are open

passive spread/action potential

The _______ of membrane potential changes extends only short distances. So the membrane potential change would diminish rather quickly over a relatively short distance if the _______ was not generated

integral membrane

Voltage-gated channels are _______ ______ proteins

4 homologous domains/6 membrane spanning regions

Voltage-gated channels usually have ________ with _________ in each domain

symmetry

Voltage-gated channels typically have 4-fold ____

Na+ channels/Ca2+ channels

Types of voltage-gated channels that have 4 very highly homologous domains in 1 channel protein

K+ channels

Types of voltage-gated channels made up of 4 distinct, yet homologous, subunits

Pore loops (ion selectivity)

These ________ determine which ions the channel is selective for. For example, the Na+ channel has a specific _______ that allows only Na+ to pass through while excluding other ions.

pore loop

each domain of a voltage-gated ion channel typically has its own ________

S4 transmembrane region

each domain in a voltage-gated ion channel contains an _______ for voltage sensing/activation

third/positively/hydrophilic

Every ______ amino acid in the S4 region is a _______ charged amino acid (arginine or lysine) → S4 region is _______

outward helical twist of S4

For S4, membrane depolarization causes the _____

channel opening (more + charges in cell)

the outward helical twist of S4 leads to _____

Inactivation Loop (Na⁺ Channel)

• Closes the channel by "plugging" the pore of the channel once it has been activated (opened).

• It closes very quickly even if the membrane remains depolarized to ensure that the ion channel is only open for a short period

Ball and Chain Mechanism (K⁺ Channel)

• inactivation mechanism where a segment of amino acids moves into the pore and blocks it after the channel opens

axon diameter/myelination

The conduction velocity (speed) of action potentials vary as a function of _____ and _____

decreased internal resistance

Axons with larger diameters have higher conduction velocities due to __________ to flow of charge

Myelin

a lipid-rich layer coating the axons of some neurons

Vertebrates

Myelin is only found in _______

Myelin formation

• Myelin is formed by glial cells that wrap themselves around the axon multiple time

• As each additional layer is formed, the cytoplasm continues to be squeezed out and the glial membranes get more closely packed together

ONE

Individual Schwann cells form myelin sheath around ___ axon

30%

In about ___ of the peripheral axons, Schwann cells help form a myelin sheath around a single axon

loose wraps/multiple axons

Majority of Schwann cells form _______ around _______ THIS IS NOT MYELIN!

individual/multiple

In the CNS, ______ oligodendrocytes form myelin sheaths around ______ axons

Capacitance

the ability to store charge

decreases/capacitance

Myelin _____ the _______ of the axon membrane

separation of charges/attraction of ions

Greater ________ myelin, decreased _______ across membrane and decreased ability to store charge

few/myelin sheath

there are very ___ Na and K channels under the ______

saltatory conduction

process in myelinated axons where action potentials jump from one Node of Ranvier to the next along the axon, instead of traveling the entire length of the axon

Node of Ranvier

periodic gaps in the myelin sheath that insulates nerve axons

energy/signal transmission

Saltatory conduction allows the action potential to move much faster by "jumping" between nodes. This reduces the _____ required and speeds up ______.