Lecture 12: Neural Tissue 2 Flashcards
Ependymal Cells
Produce and circulate,
Line ventricles and central canal,
Produce cerebrospinal fluid,
CNS
Astrocytes
Maintain the blood-brain barrier,
Provide structural framework for the neurons of the CNS,
CNS
Astrocytes Act As _______ For Substances Entering The CSF/Interstitial Fluid
Gatekeepers
How Do Astrocytes Act As Gatekeepers?
Each ‘foot’ wraps around a capillary in CNS and act as gatekeepers of CNS bloodstream
Ventricle
Fluid filled
Oligodendrocytes
Extensions wrap around neuron axons, usually more than one,
CNS
Microglia
“Garbage trucks”
Engulf (phagocytize) cellular debris, waste products and infective organisms,
CNS
Satellite Cells
Similar to astrocytes,
Surround neuron cell bodies in ganglia,
Regulate the environments around neurons,
PNS
Schwanns Cells
Similar to oligodendrocytes,
Myelinate sections of a single axon or
Surround sections of many unmyelinated axons,
PNS
Temporary Crushing Injury To An Axon Results In _______
An unexcitable axon for days to weeks
Sustained Crushing Or Severing Of An Axon Results In _____
Death of an axon distal to the injury
Wallerian Degeneration
Axon distal to injury site degenerates and macrophages clean up debris,
Schwann cells help form a path for new axon growth,
Axon may or may not grow back
Resting Potential For A Neuron
-70mV,
Net negative charge inside the cell
Sodium Goes
In
Potassium Goes
Out
Passive Channels
Protein channels that are always open,
‘Leaky’,
Na+ and K+
Gated Channels
Active channels,
Chemically-gated,
Voltage-gated,
Mechanically-gated
Chemically-Gated Channels
Open or close when they bind specific chemicals (dendrites and cell body),
Have chemical receptors,
Gates opened by neurotransmitters
Voltage-Gated Channels
Open or close in response to changes in transmembrane potential (axon),
Change in membrane voltage,
Activation gate opens at -60mV,
Inactivation closes at 30mV
Mechanically-Gated Channels
Open or close in response to physical distortion,
Touch receiving neurons,
Pressure opens channel, allowing A.P. and/or depolarization
Graded Potentials
Local change in transmembrane potential,
Produced from any stimulus that opens gated ion channels,
May or may not result in action potential,
Causes depolarization or hyperpolarization
Hyperpolarize
Open more K+ channels to make more - because it is +,
Shifting negative
Depolarize
Open Na+ channels,
Shifting positive
Action Potentials
Propagated changes in transmembrane potential that spread across an excitable membrane
Once Threshold Is Reached At ______, An Action Potential Occurs
Axon hillock
Steps Of Action Potential (5)
- Depolarization to threshold
- Opening of activation gates on voltage-gated sodium channels
- Na+ inactivation gates close and K+ channels open at +30mV
- K+ channels start closing at -70mV
- Hyperpolarization to -90mV and then return to resting potential
How Does Returning To Resting Potential Happen?
It occurs through the action of passive channels and the Na+/K+ exchange pump (Na+/K+ ATPase pump)
How Many Action Potentials Can A Neuron Undergo Per Second?
1000
Refractory Period
Period of time during an action potential, when another action potential cannot or is unlikely to occur
Absolute Refractory Period
Voltage-gated sodium channels are either open or are inactivated,
No action potential possible
Relative Refractory Period
Sodium channels are in normal resting state, but potassium channels are open or membrane is hyperpolarized,
Action potential is possible but requires a larger stimulus because it is harder to reach threshold
Propagation Of Action Potentials
Movement of an action potential along the length of an axon
Continuous Propagation
Occurs in unmyelinated axons,
Action potential spreads from ‘segment’ to ‘segment’ down length of axon,
Saltatory Propagation
Occurs in myelinated axons,
Depolarization of action potential occurs only at nodes
Propagation Speed Is Determined By…
Myelination status and the diameter of an axon
Larger Axon Diameter =
Much faster propagation speed,
Less resistance for sodium in axoplasm
Type A Fibers
Largest diameter, myelinated,
Carry sensory information about position, balance, light touch, pressure, and motor information to skeletal muscles,
Carries most important info because it is fastest
Typer B Fibers
Smaller diameter, myelinated,
Type C Fibers
Smallest diameter, unmyelinated
Type B & C Fiber Functions
Carry sensory information about general touch, temperature and motor information to smooth and cardiac muscle and glads
