Ch 2- Physical and Electrical Properties of Cells in Nervous System Flashcards
What are neurons (nerve cells)?
Specialized cells that conduct messages in the form of electrical impulses throughout the body
What does the Cell Body (soma) do?
Synthesizes proteins used as neurotransmitters
What are the types of neuron processes?
Multiple dendrites
a single axon with axon-terminal branches (Telodendria)
What structures are usually found within neurons?
- Cell body
- Neuron process
- Nucleus
- Cytoplasm
- Mitochondria
- Nissl bodies
- Smooth ER
- Cytoskeleton
What are nissl bodies?
Clusters of RER - Synthesize proteins
What structures are found within the cytoskeleton?
Microtubules (Neurotubules): Help to transport substances between the cell body and axon terminals
Microfilaments
Neurofibrils: bundles of intermediate filaments (neurofilaments) that provide structural support
Describe neuron processes
Arm-like processes that extend from the cell body
What are the dendrites of a neuron?
The main receptive regions
Usually have branches called dendritic spines
Convey incoming messages toward the cell body
What is found on the plasma membranes of dendrites?
Contain numerous receptor sites for binding neurotransmitters from other cells
What is a neuronal axon?
A long, thin process
What is an axon hillock of a neuron?
A cone-shaped region where the axon joins the cell body
What is the Initial Segment of a neuron?
The part of the axon that joins the axon hillock
Has lots of voltage gated channels
What is the axoplasm of a neuron?
The cytoplasm of the axon
Contains various organelles
What is the axolemma of a neuron?
The plasma membrane of the axon
What are the branches along the length of an axon called?
axon collaterals
How do the axon and its axon collaterals end?
by dividing into many fine processes called terminal branches (telodendria)
What are axon terminals?
Aka synaptic terminals or synaptic knobs
Knob-like distal ends of the terminal branches
What is a neuronal synapse?
A specialized site where a neuron communicates with another neuron or effector cell
What 2 cells are involved in a neuronal synapse?
Presynaptic Cell: the neuron conducting the electrical signal toward the synapse
Postsynaptic Cell: the cell which receives the signal
What is a Neuromuscular Junction?
A synapse between a neuron and a muscle cell
What is a Neuroglandular Junction?
A synapse between a neuron and a gland
What is a neuronal synaptic cleft (gap)?
A narrow space
Separates the Presynaptic Cell and Postsynaptic Cell
Filled with interstitial fluid
What is Axoplasmic Transport?
The transport of materials between the cell body and axon terminals
In which direction along neurotubules within the axon does axoplasmic transport occur?
In both directions
Anterograde: Movement of materials from the cell body to the synaptic knob
Retrograde: Movement of materials from the synaptic knob to the cell body
What powers axoplasmic transport?
Powered by mitochondria, kinesin, and dynein
How are neurons parts organized?
The organization of the parts of a neuron varies with the type of neuron
What are 3 common types of neurons based on structure?
Bipolar Neurons
Pseudounipolar Neurons (Unipolar Neurons)
Multipolar Neurons
How many processes do bipolar neurons have?
2 primary processes that extend from the cell body
- A Dendritic Process
- An Axon
The cell body is between the 2 processes
Where are bipolar neurons found?
in special sensory organs (sight, smell, hearing)
How many processes do Pseudounipolar Neurons (Unipolar Neurons) have?
One process extends from the cell body and divides T-like into 2 axons
The 2 axons form 1 continuous process
No true dendrites
Where are Pseudounipolar Neurons (Unipolar Neurons) found?
Found in sensory neurons of PNS
How many processes do Multipolar Neurons have?
Multiple processes extend from the cell body
2 or more dendrites and 1 axon
Where are Multipolar Neurons found?
All motor neurons that control skeletal muscles are Multipolar Neurons
What are the most common type of neurons in the CNS?
Multipolar Neurons
Where are Purkinje Cells (Neurons) found?
In the cerebellum, Multipolar Neurons are called Purkinje Cells
What do the apical dendrites of Purkinje Cells form?
a large fan of finely branched processes
that enables them to receive input from a large number of neurons
What are the 3 Functional Classification of Neurons?
Sensory (Afferent) Neurons
Motor (Efferent) Neurons
Interneurons
Describe the function of sensory neurons?
Afferent neurons of the PNS
Deliver impulses from sensory receptors to the CNS
Where are the cell bodies of sensory neurons located?
in peripheral sensory ganglia
What are the 2 types of sensory neurons?
Somatic Sensory Neurons: monitor the external environment- Body position and movement
Visceral Sensory Neurons: monitor internal conditions- status of organs
Describe the function of motor neurons?
Efferent neurons of the PNS
Deliver impulses from CNS to peripheral effectors
What do Somatic Motor Neurons innervate and where are their cell bodies?
Innervate: Skeletal Muscles
Cell Bodies: Are located in the CNS
What do Visceral Motor Neurons innervate?
Smooth Muscle
Cardiac Muscle
Glands
Adipose Tissue
What kind of system is the Visceral Motor system?
A 2-neuron system:
- Preganglionic Neuron: Cell body located in the CNS
- Postganglionic Neuron: Cell body is located in peripheral autonomic ganglia
What is the location of interneurons (Internuncial Neurons)?
Located between neurons
Example: can process incoming sensory information from sensory neurons and stimulate a motor response by activating appropriate motor neurons
Only found in the CNS
Are interneurons sensory or motor?
neithersensoryor motor
What are the most common neuron in the body?
Interneurons make up over 99% of the neurons in the body
What is convergence in term of interactions between neurons?
Several neurons synapse on a single postsynaptic neuron
What does diverging mean in terms of interactions between neurons?
One neuron spreads stimulation to many neurons or neuronal pools in the CNS
What are glial cells?
Cells of the nervous system
Do glial cells generate or propagate action potentials?
No
How much of the volume of the CNS do glial cells make up?
Make up about half of the volume of the CNS
What is the function of glial cells?
Support neuronal survival and activities
What are the 4 types of Neuroglia in CNS?
Ependymal
Astrocytes
Oligodendrocytes
Microglia
What are the 2 types pf Neuroglia in PNS?
Satellite Cells
Schwann Cells
What are ependymal cells?
Cilia (or microvilli) lined cells that form an epithelial membrane (ependyma)
What do ependymal cells line?
Central canal of the spinal cord
Ventricles of the brain
What is the function of ependymal cells?
Produce, monitor and circulate CSF
Do ependymal cells have a basement membrane?
No basement membrane
Basal ends of ependymal cells are elongated and extend into underlying gray mater
What are Astrocytes?
The most abundant glial cells
Contain a numerous radiating processes
What are the functions of Astrocytes?
- Maintaining the Blood-Brain-Barrier: radiating processes with expanded perivascular feet wrap around capillary endothelial cells; Secrete chemicals that control the permeability of the capillary endothelial cells
- Provide a structural framework for CNS neurons
- Repair damaged neural tissue
- Guide neuron development in the embryonic brain
- Help adjust composition of interstitial fluid in the brain
What are Micoglia?
Immune system cells act as phagocytes
Macrophages of the CNS
When do Microglia exist?
if the Resting state as Ramified Microglia
if the Active state as Reactive Microglia
What are ramified microglia?
aka “resting microglia”
Monitor CNS environment: explore CNS tissue via fine processes that continuously extend and retract
If they detect damage, disease or infection, they are activated and become Reactive Microgli
What activate Reactive Microglia?
Injury
Infection
Disease (e.g., MS, Parkinson Disease, Alzheimer Disease)
What are the functions of Reactive Microglia?
Engulf and destroy
- Pathogens
- Foreign material
- Necrotic tissue
- Cellular debris and waste products
Signal other immune cells for assistance
What are oligodendrocytes?
Cells with processes that extend out and coil around axons of CNS
The processes produce and maintain myelin sheaths around segments of multiple axons of CNS
How does CSF form in ependymal cells?
forms as filtrate containing glucose, oxygen, vitamins, and ions (Na+, Cl-, Mg2+, etc)
What is the function of satellite cells?
Surround neuron cell bodies in ganglia of PNS
Regulate the exchange of materials between the neuronal cell bodies and interstitial fluid
What do schwann cells do?
Encircle a segment of an axon
Can then either
- Coil around the axon forming myelin sheath around a segment of a single axon (myelinated internode); Several Schwann Cells are needed to enclose an axon along its entire length
- Not coil around the axon
What are myelin sheaths composed of?
lipid and protein
What are Myelinated Internodes?
Areas of the axon wrapped in myelin that:
- Insulate axons
- Increase speed of nerve impulse conduction
What are Nodes of Ranvier?
The gaps between adjacent myelinated internodes
Present on neurons with:
- Myelinating Schwann Cells
- Oligodendrocytes
What is Neurilemma?
The outer nucleated cytoplasmic layer of the Schwann Cell
Lies outside of the Myelin Sheath
Encloses the myelin sheath
Helps in the regeneration of an injured axon
What is include in Neurilemma?
A thin cell membrane forming the outer layer
Cytoplasm
Nucleus of the Schwann Cell
Where are Neurilemma found?
Only present around axons of the PNS
Oligodendrocytes dont have a neurilemma b/c their cell body and nucleus dont envelop the axon so, they generally dont regenerate after an injury
What are Nonmyelinating Schwann Cells?
Schwann Cells that surround axons of neurons in the PNS but coiling process doesnt occur
Neurons associated are said to be unmyelinated
Support and physically separate unmyelinated axons
Do Nonmyelinating Oligodendrocytes exist?
yes, but Few in number and Function is unknown
What are the steps in schwann cell myelination?
- schwann cell starts to wrap around a portion of an axon
- schwann cell cytoplasm and plasma membrane begin to form consecutive layers around axon
- the overlapping inner layers of the schwann cell plasma membrane form the myelin sheath
- eventually, schwann cell cytplasm and nucleus are pushed to periphery of the cell as the myelin sheath is formed
What are the steps in unmyelinated schwann cell axon formation?
- schwann cell starts to envelop multiple axons
2. the unmyelinated axona are enveloped by the schwann cell, but there are no myelin sheath wraps around each axon
What are he functions of Transmembrane Channels?
Serve as openings through the cell membrane
Control the movement of ions across the membrane
What are the 2 Main Types of Membrane Channels?
Leak Channels (passive)
Gated Channels (active)
Describe Leak (passive) membrane channels
Always open
Allow for diffusion of a small number of ions through the cell membrane
The plasma membrane has many more K+ leak channels than Na+ leak channels
Help maintain a stable resting membrane potential
How are K+ leak channels in comparison to Na+ leak channels/
K+ are “leakier” than Na+ leak channels
So, its easier for K+ to leak out than for Na+ to enter
Describe Gated (active) membrane channels?
Open in response to a specific stimulus
Close when the stimulus is removed
What are the 3 gated channels?
Modality (Mechanically)-Gated Ion Channels
Ligand (Chemically)-Gated Ion Channels
Voltage-Gated Ion Channels
Describe Modality (Mechanically)-Gated Ion Channels
Are specific to sensory neurons
Open in response to
- Mechanical Forces
- Stretch
- Touch
- Pressure
- Temperature Changes
The force distorts the channel, causing the gate to open
Describe Ligand (Chemically)-Gated Ion Channels
A ligand is a chemical substance that binds to specific receptor
Open in response to the binding of a ligand (e.g., chemical)-examples:
- Neurotransmitters
- Hormones
- Specific Ions
Are most abundant on dendrites and cell body of a neuron
Describe Voltage-Gated Channels
Open and close in response to changes in membrane potential (voltage)
Participate in generation and conduction of APs
Located in Axolemma of axons and Plasma
membranes of muscle fibers
When open, what do membrane channels allow flow of?
Allows flow of ions between the extracellular and intracellular environments
This results in a change in membrane potential
How is gradient defined?
The degree of inclination
How do ions, molecules and particles move in a gradient?
Tend to move down a gradient (from high concentration to low) until the concentration gradient is removed - dynamic equilibrium (uniform concentration)
The steeper the gradient, the faster the rate of movement
What are 3 examples of gradients?
Concentration Gradients
Electrical Gradients
Pressure Gradients
How do molecules move in the state of dynamic equilibrium?
Molecules will still move around (and cross a membrane) but there will be no net movement of the number of molecules from one area to another
What is a concentration gradient?
A state in which a substance is found indifferent concentrations eitherover a region of space or on opposite sides of a membrane
A difference in ions, molecules, or particles
between 2 regions
How do substances move in a concentration gradient?
substance will move from an area of high concentration to an area of low concentration (down its concentration gradient)
What is an electrical gradient?
A difference in electrical charges between 2 regions
The plasma membrane creates a difference in the distribution of positively charged ions (cations) and
negatively charged ions (anions) between the 2 sides of plasma membrane
Electrical Gradient example: if there is a higher concentration of Na+ in the ECF compared to the ICF where does Na+ move?
Na+ is favored to diffuse into a cell because of the concentration gradient (chemical gradient and electrical gradient)
This combination is referred to as the Electrochemical Gradient
Define Transmembrane Potential (Membrane Potential)
difference in electrical charge across plasma membrane due to differences in concentrations of ions inside and outside cell
What is Transmembrane Potential (Membrane Potential) important for?
Transmission of nerve signals
Muscle contraction
Glandular secretions
The function of various other activities of the body
What is Resting Membrane Potential?
The transmembrane potential in an undisturbed cell
There is no net flow of ions across the membrane
What are the charges of the inner surface and outer surface of the plasma membrane compared to each other?
Inner surface more negatively charged
Outer surface more positively charged
What is the charge range of the Resting Membrane Potential?
from -10 mV to -100 mV
Depending on cell type
What channels are open with Resting Membrane Potential?
Only leakage channels are open
All gated Na+ and K+ channels are closed
What is the average resting membrane potential?
varies by cell type, but averages -70 mV
How is Resting Membrane Potential maintained?
Unequal distribution of ions in the ECF and cytosol
Inability of most ions to leave the cell
Leak channels
Na+/K+ Pumps
What does Extracellular Fluid contain a high concentration of?
Na+ and Cl-
What does Cytosol contain a high concentration of?
K+ and negatively charged proteins
Are K+ leak channels or Na+ leak channels more abundant in the plasma membrane?
more K+ leak channels
What does a higher abundance of K+ leak channels as compared to Na+ leak channels lead to?
more K+ can diffuse down their concentration gradient and out of the cell than the amount of Na+ that can diffuse down their concentration gradient into the cell
Result in:
- Cytoplasmic surface of the plasma membrane is more negative
- ECF surface is more positive
What are most anions inside the cell attached to?
Non-diffusible molecules or Large proteins
most cannot leave
What is the function of Na+/K+ Pumps?
Help maintain Resting Membrane Potential by:
- Pumping out 3 Na+
- Pumping in 2 K+
Requires ATP
Define depolarization
Any shift from the Resting Membrane Potential toward a more positive potential
In what situation does depolarization occur?
Occurs when the resting membrane is exposed to a stimulus that opens the Chemical-Gated Na+ Channels
Na+ enters the cell and creates a voltage change
The positive charge of Na+ shifts the Transmembrane Potential toward 0 mV
What is the maximum change in Transmembrane Potential proportional to?
the size of the stimulus
What does a greater Transmembrane Potential stimulus lead to?
greater the number of chemical channels to open
more Na+ that enters the cell
greater the membrane area affected
greater the degree of depolarization
Define Hyperpolarization
A change in the Transmembrane Potential toward a more negative value than the Resting Membrane Potential (it “over shoots”)
Hyperpolarization example: If the RMP is -70 mV, how would hyperpolarization occur?
Hyperpolarization would occur if the membrane potential becomes more negative than its RMP, say to -80 mV
What can cause hyperpolarization?
Opening of voltage-gated K+ channels: Allows K+ to flow out of the cell
Opening of voltage-gated Cl- channels: Allows Cl- to flow into the cell
When Transmembrane Potential changes in the membrane potential what are the 2 types of signals can be produced?
Graded Potentials
Action Potentials
Describe graded potentials
Also called local potentials
Are a short-lived localized change in the resting membrane potential
How do graded potentials spread?
Can not spread far (1 – 2 mm) from the site of stimulation
These changes cause current flows that decrease in magnitude with distance
How does the magnitude of graded potentials vary?
Varies with strength of stimulus
The greater the stimulus, the greater voltage change and the farther the current will flow
What will any stimulus that opens a Chemical-Gated Ion Channel produce?
a graded potential
How are localized current (graded potentials) established?
A stimulus causes Chemically-Gated Ion Channels to open at site of stimulation
A small amount of a specific ion crosses the plasma membrane
A localized change in the resting membrane occurs
A localized current is established as the ions move along the cytoplasmic side of the plasma membrane
The flow of current dies out
Why does the flow of current die out in a localized current (graded potentials)?
The ions entering the cell experience resistance to movement by contents of the cell
The ions leak out through leakage channels
Where can graded potentials be generated?
sensory neurons
motor neurons
interneurons
Describe graded potentials in sensory neurons
At peripheral sensory receptors, sensory receptors have Modality-Gated Ion Channels
When stimulated, these ion channels open allowing an influx of ions, generating a graded potential
Describe graded potentials at Motor Neurons and Interneurons
On postsynaptic membrane
graded potentials are generated in motor neurons and interneurons when they’re stimulated by input from other neurons
Presynaptic neuron releases neurotransmitter
Binding of neurotransmitter to receptor on postsynaptic membrane opens Ligand-Gated Ion Channels
This causes a change in the Resting Membrane Potential of postsynaptic cell
Action of neurotransmitter on membrane channel
determines whether synaptic potential will be depolarizing (excitatory) or hyperpolarizing (inhibitory)
What does a single nerve impulse produce?
a small local graded depolarization that cant induce an action potential
Define summation
The process of combining the electrical impulses
What does summation greatly increases the probability of?
that a postsynaptic membrane will reach threshold depolarization and result in an action potential
What are the 2 forms of summation?
Temporal Summation
Spatial Summation
What is temporal summation?
The combined effect of a series of small potential changes that occur within milliseconds of each other
Occurs when nerve impulses are received in rapid succession at a single synapse
Effects of the 2nd (excitatory) stimulus are added to those of the 1st (excitatory) stimulus
In temporal summation, what does the first impulse produce?
A small local graded potential
Before it dissipates, successive impulses trigger more small graded potentials
These impulses summate and produce a much greater depolarization of the postsynaptic membrane than would result from a single impulse
What happens in temporal summation if the postsynaptic membrane reaches threshold depolarization?
An action potential can occur
When does spatial summation occur?
When simultaneous stimuli at different locations have a cumulative effect on the transmembrane potential
Involves multiple synapses that are active simultaneously
What does the activity of one synapse produce in spatial summation?
a graded potential with localized effects
What happens if more than one synapse is active at the same time in spatial summation?
All will “pour“ Na+ across the postsynaptic membrane
What happens at each active synapse in spatial summation?
The Na+ that produce the EPSP spread out along the inner surface of the membrane and mingle with those entering at other synapses
The effects on the membrane are cumulative
What determines the degree of depolarization in spatial summation?
How many synapses are active at any moment
Their distance from the initial segment
When does an action potential occur in spatial summation?
When the transmembrane potential at the initial segment reaches threshold
What actions occur with no summation in temporal summation?
2 stimuli separated in time cause EPSPs that do not add together
What is spatial summation of EPSPs and IPSPs?
changes in membrane potential can cancel each other out
What are the steps in a summation action potential generation?
neurotransmitter release
receptor binding
ion channels open or close
conductance change causes current flow
postsynaptic potential changes
postsynaptic cells excite or inhibit
summation determines whether or not an AP occurs
What is an action potential?
A brief reversal of the membrane potential
Consists of propagated changes in the transmembrane potential that, once initiated, affect the entire excitable membrane
It is an electrical impulse that travels along the cell membrane and does not diminish as it moves away from its source
What must happen in terms of depolarization for an action potential to occur?
Depolarization has to be great enough to reach the membrane threshold causing the voltage-gated channels to open
What is threshold in terms of action potential?
The minimum voltage to stimulate an action potential
Varies, but average is about -55 mV in many neurons
What are the cells with excitable membranes that can generate action potentials?
Neurons
Muscle Cells
What is the principle way neurons communicate?
action potentials
What is the All-or-None principle?
A stimulus either triggers an action potential or does not produce one at all
You can not have a partial action potential
When the threshold is reached what occurs with the impulse?
The impulse generated will travel the entire length of the membrane
What kind of actions potentials are produced by strong stimuli as compared to minimally sufficient stimuli?
Strong stimuli produce action potentials of the same voltage and duration as are produced by minimally sufficient stimuli
What depolarization is sufficient to trigger and action potential?
15 mV depolarization (from -70 mV to -55 mV)
What is the trigger zone (Spike Initiation Zone)?
In most neurons, where action potentials are generated at the junction of the axon hillock and initial segment as a result of a graded potential that occurred in the cell body or dendrites
What does the trigger zone have a high concentration of?
voltage-gated Na+ channels
What type of summation occurs with synaptic potentials initiated in the cell body and dendrites in the trigger zone?
spatially and temporally summated
What happens if the influx of Na+ reaches the Trigger Zone and is sufficient to depolarize the Trigger Zone to threshold?
The voltage-gated Na+ channels will open and an action potential will be propagated
What are the steps in an action potential at the trigger zone?
- a graded potential above threshold reaches the trigger zone
- voltage-gated Na+ channels open, and Na+ enters the axon
- positive charge flows into adjacent sections of the axon by local current flow
- local current flow from the active region causes new sections of the membrane to depolarize
- refractory period prevents backward conduction. loss of K+ from the cytoplasm repolarizes the membrane.
What are the 3 steps in generation of an action potential?
Depolarization
Repolarization
Hyperpolarization
What is involved in the depolarization step of an action potential?
A stimulus opens the chemical-gated Na+ channels
Opening of the voltage-gated Na+ channels occurs at the transmembrane potential known as threshold (–60 to –55 mV)
When the voltage-gated Na+ channels open plasma membrane becomes much more permeable to Na+
Due to their electrochemical gradient Na+ rushes in and rapid depolarization occurs
What happens with the transmembrane potential in depolarization?
The transmembrane potential changes from – 70 mV to a positive value
After depolarization what kind of ions are found on the cytoplasmic side of the plasma membrane like?
cytoplasmic side of the plasma membrane now contains more + ions than – ions
Do all depolarizations lead to action potentials?
No, the stimulus must be significant enough to cause the membrane potential to reach the threshold
The stimulus must cause a depolarization large enough to open voltage-gated Na+ channels to initiate an action potential
What happens as transmembrane potential approaches +30mV?
Repolarization begins
Voltaged-gated Na+ channels close and Voltaged-gated K+ channels open
K+ exit the cell and flows out, down its electrochemical gradient
Transmembrane potential shifts back towards its resting level
What is repolarization in an action potential?
The process of returning the transmembrane potential to normal resting levels
What occurs on the cytoplasmic side of the plasma membrane during repolarization?
Starts to move away from a positive value and towards its resting potential as a result of ion movement through the membrane channels
More positive ions are exiting the cell than entering
After repolarization how long do the voltage-gated Na+ channels remain inactivated?
Until the membrane has repolarized to near threshold levels
At that level they remain closed but capable of opening
After repolarization when do voltaged-gated K+ channels begin closing?
as the membrane reaches the normal resting potential (– 70 mV)
Until all of the K+ channels close K+ continue to exit the cell (until about – 90 mV) which causes a brief hyperpolarization
At what point does the repolarization phase end?`
when the membrane potential reaches its resting state (e.g., – 70 mV) but K+ will continue to exit until all K+ channels close
What is hyperpolarization of an action potential?
The phase during which the membrane potential temporarily becomes more negative than the resting potential
When are the K+ voltage channels finally closed?
at about – 90 mV
AP has ended
How is resting potential restored after repolarization?
K+ leaks into the cell to restore normal resting electrical potential
Na+/K+ pump restores normal resting ionic conditions
What is the absolute refractory period?
Period from opening (activation) of the voltage-gated Na+ channels until voltage-gated Na+ channels reset to original resting state
During this time, the plasma membrane cannot respond to further stimulation because the voltage-gated Na+ channels either are Already open or Inactivated
What is the relative refractory period?
Begins when voltage-gated Na+ channels have returned to resting state with some voltage-gated K+ channels still open and repolarizing occurring
Ends when the membrane has returned to resting potential
During this time, axon’s threshold for AP generation is substantially elevated and a stimulus that would normally have generated an AP is not sufficient
A very strong stimulus is needed to re-open the voltage-gated Na+ channels that have already returned to resting state and allow another AP to generate
What are the Sequence of Events that Transmit Sensory Information along an Axon?
- Deformation of a peripheral receptor
- Change in local membrane potential of the sensory ending
- Development of an action potential in the sensory axon
- Release of neurotransmitter from the sensory neuron presynaptic terminal
- Binding of neurotransmitter to the ligand-gated channel on the postsynaptic cell membrane
- Activation of postsynaptic membrane potential
What is the refractory period?
The period from the time an action potential begins until the normal resting potential has stabilized
The membrane will not respond normally to additional depolarizing stimuli
What are the 2 parts of the refractory period?
Absolute Refractory Period
Relative Refractory Period
What does the Absolute Refractory Period prevent?
Prevents the neuron from generating a second action potential
Ensures that each action potential is a separate “all or none” event and one-way transmission
What is the speed of nerve impulse transmission is affected by?
Myelin
Diameter of Axon
-The larger the diameter, the lower the resistance and the faster the propagation speed (b/c cytosol offers less resistance than the plasma membrane)
What are the 2 ways an AP travel along an axon?
Continuous Propagation (unmyelinated axons)
Saltatory Propagation (myelinated axons)
What is Continuous Propagation (unmyeliated axons)?
The AP moves along the axon membrane in segments, starts at initial segment of axon
The initial segment has an abundance of voltage-gated Na+ channels
The local current spreads in all directions
Why cant an axon cell body respond to an AP in continuous propogation?
Because it lacks voltage-gated Na+ channels
In continuous propagation what happens to the previous segment as the next segment depolarizes?
an AP is generated and the previous segment enters Refractory period
so, APs only move in one direction (forward)
In continuous propagation, how does an AP move?
across the surface of the membrane in a series of tiny steps
What is Saltatory Propagation (myelinated axons)?
Myelin increases the resistance to the flow of ions across the membrane
Voltage-gated Na+ channels are concentrated at the nodes so only the nodes respond to depolarization
Ions can only cross the plasma membrane at the Nodes of Ranvier
The action potential “jumps” from node to node
Are nerve impulses carried along an axon faster in continuous propagation or saltatory propagation?
Saltatory Propagation
What are the three steps of Saltatory Propagation?
- local potentials: local potentials usually generated in the neuron’s dendrites accumulate and reach the trigger zone of the axon
- action potentials: trigger zone depolarizes to threshold and generates an action (AP)
- action potential propagation: the action potential is propagated down the axon to the axon terminals
What is Neuroinflammation?
The response of the CNS to Infection, Disease, and Injury
What is the Neuroinflammation response mediated by?
Reactive Microglia
Astrocytes
When are reactive microglia beneficial?
Engulf and destroy: Pathogens, Foreign material, Necrotic tissue, Cellular debris and waste products
Signal other immune cells for assistance
Produce neurotrophic factors (proteins) that support axonal regeneration and remyelination
Mobilize astrocytes to: Reseal the blood-brain barrier and Provide nutritional support
What kind of harmful effects can Neuroinflammation cause?
Death of neurons and oligodendrocytes
Inhibit neural regeneration
In what conditions can a correlation between abnormal glial activity and neural damage be seen?
Stroke
Alzheimer’s Disease
Parkinson’s Disease
MS
What kind of harmful correlation can reactive glial cells cause?
correlation between abnormal glial activity and neural damage
When Microglia and Astrocytes become excessively activated this can cause them to what?
Lose their physiologic buffering function
Release harmful substances such as: Inflammatory Cytokines and Proteinases
Contribute to neuronal damage
What is the reason for the dual role of a delicate balance between the protective roles and destructive roles of microglia?
not fully known
For what function is myelin a critical component?
conduction of information in the nervous system
What happens to an AP as it travels along an axon from a myelinated region to an area where myelin has been damaged?
Resistance to the electrical signal increases as Na+ leaks out
Propagation of the electrical current slows and eventually may stop before it reaches the next site of conduction
Cell implantation advances have been made to enhance neuronal regeneration where/when?
In demyelinating disease
and
Following nerve trauma
In animals, what does Schwann cell implants result in?
significant regeneration of axons across a spinal cord transection
What is the significance of regeneration of axons across a spinal cord transection in animals?
It’s often associated with improved motor function
It has great potential as a medical intervention for individuals with spinal cord injury
Following trauma to the spinal cord what is the migration path of Schwann cells and why?
From periphery into spinal cord injury site, where they participate in endogenous repair processes
B/c of Schwann cell’s intrinsic characteristics including the ability to secrete a variety of neurotrophic factors that help in the repair process
What are the capabilities of Transplanted Schwann Cells?
Myelinating Regenerating Axons
Remyelinating Demyelinated Axons
What does the transplantation of Schwann Cells result in?
significant regeneration of axons across a spinal cord
What do axons remyelinated by transplanted Schwann cells exhibit?
restoration of conduction through the lesion with reestablishment of normal conduction velocity
How do demyelinating diseases affect conduction?
Reduce or block conduction when current leaks out of previously insulated regions between the nodes
What is Guillain-Barré Syndrome?
A life-threatening group of autoimmune disorders that attack the PNS?
What are the subtypes of Guillain-Barré Syndrome?
Acute Inflammatory Demyelinating Polyneuropathy
Acute Axonal Neuropathy
What is the onset and cause of Guillain-Barré Syndrome?
Onset: rapid and often preceded by a Viral Infection or Bacterial Infection
Cause: Unknown
What is the suspected pathogenesis of Guillain-Barré Syndrome?
Depends on subtype
Autoantibodies may attack the: Myelin causing demyelination of the axon OR
Axon causing axonal degeneration
What are the motor clinical manifestations of Guillain-Barré Syndrome?
Rapidly progressive ascending muscle weakness of the limb
Starts in the legs
Spreads upward to the arms and face
Can lead to flaccid paralysis
What are the sensory clinical manifestations of Guillain-Barré Syndrome?
Paresthesia
Numbness
Neuropathic Pain
What are the ANS clinical manifestations of Guillain-Barré Syndrome?
Hypotension
Arrhythmias
Abnormalities in sweating
Why is Guillain-Barré Syndrome usually a medical emergency?
Because it may involve respiratory muscles in which rapid respiratory failure can occur requiring use of a ventilator
What population does Guillain-Barré Syndrome affect?
anyone at any age, male/female
When are peak symptoms seen in Guillain-Barré Syndrome?
Often within 10 - 14 days
What methods are used to diagnose Guillain-Barré Syndrome?
Examination
Signs/Symptoms
Loss of Deep Tendon Reflexes
Nerve Conduction Studies, EMG
Analysis of CSF: High levels of protein
What is plasmapheresis?
A process that filters blood, separating plasma from the blood cells
Blood cells are returned to the person
Plasma, which contains the antibodies is removed
Plasma is replaced with: Donated plasma or A plasma substitute
What treatment methods are used for Guillain-Barré Syndrome?
High-dose Immunoglobulin Therapy
Plasmapheresis
Do people with Guillain-Barré Syndrome ever recover?
80% - 90% of persons achieve a full and spontaneous recovery within 6 - 12 months
Explain the physiological issue that occurs with Guillain-Barré Syndrome?
Autoantibodies bind to and attack myelin
Macrophages subsequently invade myelin
Autoantibodies bind to the axolemma at the Nodes of Ranvier and damage the axon
What is OT for Guillain-Barré Syndrome? directed at?
Activities of daily living, including self-care
Stretching and range of motion exercises
Return of functional mobility
If voluntary movement is present in a person with Guillain-Barré Syndrome why should exercise be gentle?
To avoid causing damage in partially denervated muscles
Because exercise of partially denervated muscles can interfere with axonal regrowth
What is the most common demyelinating disorder?
Multiple Sclerosis
What is Multiple Sclerosis?
A chronic inflammatory autoimmune demyelinating disease of the CNS
Characterized by: Inflammation and Selective destruction of CNS myelin
How does Multiple Sclerosis affect the PNS?
it doesnt
Multiple sclerosis is the leading cause of what in early adulthood?
neurologic disability
What is the age of onset for Multiple Sclerosis?
20 – 40 years of age
Does MS affect men or women more?
Women affected twice as frequently as men
What is the cause of MS?
Unknown but possibly related to:
- Genetic Mutation: A group of genes that affect many immune functions
- Environmental Factors like Exposure to the Epstein-Barr Virus, Smoking and Low levels of Vitamin D
How can low levels of vitamin D cause MS?
Vitamin D activates receptors on immune regulatory cells
Once activated, these cells decrease immune activity, potentially decreasing the likelihood of developing an autoimmune disease like MS
When is MS more likely to occur?
If there is a family history of MS: 15% of individuals with MS have a first-degree relative with MS
In individuals who live in a region where MS is more common - regions farther away from the equator
