Chapter 12 part 3 Flashcards
what are ganglia?
masses of cell bodies of neurons of the PNS
satellite cells could also be called…
amphicytes
Schwaan cells could also be called…
neurilemma cells
what is the function of satellite cells/amphicytes
they surround neuron cell bodies in the ganglia. they regulate the environment around the neurons, similar to astrocytes in the CNS
what is the function of Schwaan/neurilemma cells
form a sheath around peripheral axons
whenever a schwaan cell covers an axon, the outer surface of the schwaan cell is called the ____
neurilemma
most AXONS in the PNS are shielded from contact with the interstitial fluid by what?
Schwaan cells
what is the difference between the myelination capabilities of Schwaan cells and oligodendrocytes
Schwaan cells (PNS) can only myelinate 1 segment of a single axon
oligodendrocytes in the CNS can myelinate portions of several adjacent axons
explain what happens when a neuron is injured
in the cell body, Nissl bodies disperse and the nucleus moves away from its centralized location as the cell increases its rate of protein synthesis. if the neuron recovers its functional abilities, it will regain its normal appearance
the KEY to recovery of a neuron appears to be events where?
in the axon
if the pressure applied during a crushing injury reduces local blood flow, the axonal membrane will become unexcitable
if the pressure is alleviated after an hour or 2, the neuron will recover within a few weeks.
more severe or prolonged pressure produces effects similar to if an axon had been cut
in the PNS, ______ cells play a role in repairing damaged nerves
Schwaan cells
explain how Schwaan cells help to repair damaged nerves in the PNS and what this process is called
Wallerian degeneration
the axon distal to the site of injury degenerates, and macrophages migrate to the area to clean up the debris.
However, the Schwaan cells that were wrapping the axon do not degenerate. They proliferate and form a solid cellular cord that follows the path of the original axon. As the neuron recovers, its axon grows into the site of injury and the Schwaan cells wrap around the axon
if the axon grows along the appropriate cord of Schwaan cells, it may reestablish its normal synaptic contacts.
however, if it stops growing or wanders in a new direction, normal function will not return
the regenerating, growing axon is more likely to arrive at its appropriate destination if….
the cut edges of the original nerve bundle REMAIN IN CONTACT
WHY is regeneration more complicated in the CNS? give 3 reasons
-many more axons are likely to be involved
-astrocytes produce scar tissue that can prevent axon growth across the damaged area
-astrocytes release chemicals that prevent the growth of axons
what is the “resting potential”
the transmembrane potential of a resting cell
ALL NEURAL ACTIVITIES begin with what event?
a change in the resting potential of a neuron
what is the graded potential?
the effect of a stimulus decreases with distance from the stimulus
if the ______ is large enough, an action potential will be triggered in the membrane of the axon
graded potentia
what is an action potential?
an electrical impulse that is propagated (spread) along the surface of an axon and does NOT diminish as it moves away from its source
an action potential travels along 1 synapse or multiple?
one or more
explain what synaptic activity is
the presynaptic cell typically releases neurotransmitters. these chemicals bind to receptors on the postsynaptic plasma membrane, changing its permeability and producing graded potentials in the plasma membrane of the postsynaptic cell
the response of the postsynaptic cell ultimately depends on….
what the stimulated receptors do and what other stimuli are influencing the cell at the same time
this is called information processing
the extracellular fluid contains high concentrations of ___ and ___, whereas the cytosol contains high concentrations of _____ and ___
extracellular – sodium ions and chloride ions
intracellular – potassium ions and negatively charged proteins
can ions move freely across the lipid portions of the plasma membrane?
no – they need channels
at the resting potential, ions move through ______
leak channels – membrane channels that are always open
the sodium-potassium exchange pump is a ___ mechanism
active transport
the sodium potassium exchange pump brings what in and out?
2 k in
3 na out
is a cell more negative on the inside or outside
inside
explain the passive forces acting across the plasma membrane
chemical gradients and electrical gradients
how do…
potassium ions…..
sodium ions…..
move into/out of the cell based on CHEMICAL GRADIENT
potassium ion concentration tends to be high inside of the cell, so they tend to move out of the cell through open potassium channels
IN CONTRAST,
Na+ concentration tends to be high OUTSIDE of the cell, so a chemical gradient drives them INTO the cell
is the plasma membrane more permeable to potassium or sodium?
potassium
____ leave the cytoplasm more rapidly than ___ enter because the plasma membrane is more permeable to ____
POTASSIUM IONS leave the cytoplasm more rapidly than SODIUM IONS enter, because the plasma membrane is more permeable to POTASSIUM IONS
Whenever postitive or negative charges are held apart (as in the plasma membrane), a ___ arises
potential difference
the resting potential for a typical neuron is ___mV
-70mV
a movement of charges to eliminate potential differences is called…..
a current
the RESISTANCE of a membrane is a measure of….
how much the membrane restricts ion movement
if the resistance is very high, the current is…
very small
what is the electrochemical gradient?
the sum of the chemical and electrical forces acting on that ion across the plasma membrane
what is the PRIMARY FACTOR affecting the resting potential of most cells, including neurons?
the electrochemical gradients of sodium and potassium
the intracellular concentation of potassium ions is relatively ___
high
the CHEMICAL gradient for potassium ions tends to drive them….
OUTSIDE of the cell
HOWEVER, the electrical gradient opposes this movement because K+ is attracted to the negatively charged proteins inside of the cell
the ____ gradient for K+ is strong enough to overcome the ____ gradient
the CHEMICAL gradient is strong enough to overcome the ELECTRICAL GRADIENT.
BUT the electrical gradient weakens the force driving K+ out of the cell
the transmembrane potential at which there is no net movement of a particular ion across the plasma membrane is called the….
EQUILIBRIUM POTENTIAL
For potassium ions, the equilibrium occurs at a potential of about….
-90mV
there is a strong CHEMICAL GRADIENT driving Na+ ______ the cell
in
there is an ELECTRICAL GRADIENT driving Na+ __ the cell
into
the equilibrium for Na+ is approximately…
+66mV
an electrochemical gradient is a form of ___ energy
potential
if a plasma membrane did not exist, would there be any electrochemical gradients?
no
any stimulus that increases the permeability of the plasma membrane to sodium or potassium ions does what?
produces sudden and dramatic ion movement – like a dam being opened
a stimulus that opens sodium ion channels will do what?
trigger a rush of Na+ ions into the cell (both chemical and electrical gradients drive it into the cell)
does the nature of the stimulus that increases the permeability of the plasma membrane to a certain ion determine the amount of ion movement?
no.
if a stimulus opens the door, the electrochemical gradient does the rest
at a resting potential of -70mV, is the net potassium electrical gradient pushing potassium ions into or out of the cell?
out
does the transmembrane potential always remain the same?
NO – it is dynamic. changes in response to temporary changes in membrane permeability
active channels could also be called ______
when do they open/close
gated channels. they open/close in response to specific stimuli
each gated (active) channel can be in 1 of 3 states:
- closed but capable of opening
- open (activated)
- closed and INCAPABLE of opening (inactivated)
what are the 3 classes of gated channels
- chemically gated channels
- voltage-gated channels
- mechanically gated channels
chemically gated channels are most abundant where?
on the dendrites and cell body of a neuron, where most synaptic communication occurs
voltage gated channels open/close in response to….
changes in the transmembrane potential
voltage gated channels exist in areas of….
excitable membrane (a membrane capable of generating and conducting an action potential)
give some examples of excitable membranes (where voltage-gated channels are found)
axons of unipolar and multipolar neurons
sarcolemma of skeletal muscle fibers and cardiac muscle
what are the most important voltage gated channels
sodium channels
potassium channels
calcium channels
at a normal resting potential of -70mV, will the voltage gated sodium channel be opened or closed?
closed
at a resting potential of -60mV, will the voltage gated sodium channel be opened or closed?
opened — sodium enters the cell
at a resting potential of +30mV, what will the voltage gated sodium channel do?
it will be inactivated
mechanically gated channels open or close in response to….
physical distortion of the plasma membrane
chemically gated sodium channels are widespread on the surfaces of a ____, but voltage gated sodium channels are most abundant ____
chemically gated – widespread on the surface of a neuron
voltage-gated – abundant on the axon, its branches, and synaptic terminals
mechanically gated channels are typically only located on…..
the dendrites of sensory neurons
any stimulus that opens a gated channel produces a ___ potential
graded (local) – cannot spread far from the site of stimulation
any shift from the resting potential toward a more positive potential is called….
depolarization
when sodium ions enter the cell, the transmembrane potential shifts towards ___ and this is called ____
0mV
depolarizaiton
explain what a local current is
the spread of sodium ions INSIDE the plasma membrane produces a LOCAL CURRENT that depolarizes adjacent portions of the plasma membrane
the more open channels, the more sodium ions enter the cell, the greater the area of the membrane affected, and _____
GREATER THE DEGREE OF DEPOLARIZAITON
The process of restoring the normal resting membrane potential after depolarization is called….
repolarization
explain what happens when a gated potassium channel opens
opposite effect of opening a sodium channel.
potassium goes rushing out of the cell, making the interior of the cell LOSE POSITIVE IONS
this is called hyperpolarization (ex: -70mV to -80mV)
