BIOL 0800 Reading- Chapter 6 Flashcards
Why are neurons said to serve as integrators?
Because their output reflects the balance of inputs they receive from up to hundreds of thousand of other neurons
What are processes, referring to neurons?
Long extensions, which connect neurons to each other and perform the neuron’s input and output functions
What are dendrites and dendritic spines?
Highly branched outgrowths that receive most of the inputs from other neurons; and knoblike outgrowths that increase the surface area of dendrites even more, and often contain ribosomes
What is contained in dendritic spines that generally isn’t contained in dendrites? Why?
Ribosomes that allow dendritic spines to remodel their shape in responses to variation in synaptic activity
What is the axon?
Also called a nerve fiber; long process that extends from the cell body and carries output to its target cells
What is the initial segment or axon hillock?
The region of the axon that arises from the cell body; the “trigger zone” where electrical signals are usually generated
What are collaterals?
Branches of the axons that increase the cell’s sphere of influence
What is the axon terminal?
The end of each axon branch that is responsible for releasing neurotransmitters from the axon
What are varicosities?
A series of bulging areas along the axon that can also release chemical messengers from the neuron
What is myelin?
The layers of highly modified plasma membrane wrapped around the axon by a nearby supporting cell called an oligodendrocyte; shields the axon from excess electrical signal and prevents loss of the axon’s own electrical signal
What is an oligodendrocyte?
In the brain and spinal cord, supporting cell that branches to for myelin on axons; can support up to 40 different axons
What is a Schwann cell?
In the PNS, cells that form individual myelin sheaths surrounding small segments of the axons at regular intervals; don’t cover the nodes of Ranvier
What are the nodes of Ranvier?
The spaces between adjacent sections of myelin where the axon’s plasma membrane is exposed to extracellular fluid
What is the purpose of the myelin sheath?
To speed up conduction of the electrical signals along the axon and to conserve energy
What is axonal transport?
The movement of various organelles and other materials as far as a meter between the cell body and the axon terminals; depends on scaffolding of microtubule rails running along the axon and motor proteins called kinesins and dyneins
What are kinesins and dyneins?
Motor proteins that facilitate axonal transport along the microtubule “rails”
What is the difference in kinesin and dynein movement?
Kinesins usually undergo anterograde movement (from cell body to axon terminals) and dyneins usually undergo retrograde movement (from axon terminals to cell body)
What is anterograde movement with kinesins useful for?
Moving nutrient molecules, enzymes, mitochondria, neurotransmitter-filled vesicles, growth factors, and other organelles
What is retrograde movement with dyneins useful for?
Carrying recycles membrane vesicles, growth factors, and other chemical signals that can affect the neuron’s morphology, biochemistry, and connectivity; also for harmful agents can invade the central nervous system
What are afferent neurons?
Convey info from the tissues and organs of the body toward the central nervous system
What are efferent neurons?
Convey info away from the CNS to effector cells like muscle, gland, or other nerve cells
What are interneurons?
Connect neurons within CNS
What are sensory receptors?
At the peripheral ends of afferent neurons, to respond to various physical or chemical changes in their environment by generating electrical signals in the neuron
How do sensory receptors work?
Generating electrical signals in the neurons; propagate electrical signals from their receptors into the brain or spinal cord
Why are afferent neurons unusual?
Only have a single process, usually considered an axon; after the cell body, the axon divides into the peripheral (begins where dendritic branches converge from receptor endings) and central (enters the CNS to form junctions with other neurons) processes
What parts of the afferent neurons are in the CNS?
Only a little bit of the central process; NOT the cell body or rest of the axon
What parts of the efferent neurons are in the CNS?
The cell bodies and dendrites, but usually not the axons
What is the different between nerve and nerve fiber?
Nerve fiber is generally referring to a single axon, but nerve is a bundle of axons (fiber) bound together by connective tissue
What parts of interneurons are in the CNS?
All of it! They’re 99% of all neurons
What is an example of a process that requires no interneurons?
Knee-tap reflex: the afferent neuron interacts directly with efferent neurons
What are the two characteristics of afferent neurons?
Transmit info into CNS from receptors’ peripheral endings; cell body and long peripheral process of the axon in the PNS, but short bit of central process of the axon in the CNS
What are the two characteristics of efferent neurons?
Transmit info out of CNS to effector cells like muscle/gland/neurons; cell body, dendrites and part of axon in CNS< most of axon in the PNS
What are the four characteristics of interneurons?
Function as integrators and signal changers; integrate groups of afferent/efferent neurons into reflex circuits; lie entirely within CNS; more than 99% of all neurons
What is a synapse?
The anatomically specialized junction between two neurons where one alters the electrical and chemical activity of the other
What transmits the signal across synapses, usually?
Neurotransmitters
What are glial cells?
The other ~90% of the nervous system; surround the soma, axon, and dendrites of neurons and provide physical and metabolic support
What are the types of glial cells?
Oligodendrocytes (and Schwann cells), astrocytes, microglia, and ependymal
What is an astrocyte?
Glial cell: regulates the composition of the ECF in the CNS by removing potassium ions and neurotransmitters around synapses; stimulates formation of tight junctions between the cells that make up the walls of capillaries found in the CNS to form the blood-brain barrier (prevents toxins and other substances from other substances from entering the brain)
What is the blood-brain barrier?
Prevents toxins and other substances from entering the brain
How do astrocytes function in developing embryos?
Guide the neurons to their final destinations and stimulate neuronal growth by secreting growth factors
What are microglia?
Glial cells: specialized macrophage-like cells that perform immune functions in the CNS
What are ependymal cells?
Glial cells: line the fluid-filled cavities within the brain and spinal cord and regulate the production and flow of cerebrospinal fluid
How does development of the nervous system begin?
In embryo: division of stem cells develops into neurons or glia; differentiate and migrate and send out processes that will become axons/dendrites
What is the growth cone?
A specialized enlargement that forms the tip of each extending axon and is involved in finding the correct route and final target for the process
What are neurotrophic factors?
Growth factors for neural tissues that help guide the axon along the surfaces of other cells; along with cell adhesion molecules that reside on the surface of glia and embryonic neurons
What are two types of substances that help guide the developing axons along the surfaces of other cells, usually glial cells?
Cell adhesion molecules and soluble neurotrophic factors
What happens once the target of the advancing growth cone is reached?
Synapses form; BUT they’re active even before they get there
What strange phenomenon occurs with the majority of developing neurons?
Apoptosis! Probably to refine and fine-tune the connectivity in the nervous system
What factor affects the final function of neurons?
The neuron’s early activity before they’re done maturing
Axon regeneration depends on what?
If the damage occurs outside the CNS and doesn’t affect the neuron’s cell body
Does neuron function restoration occur quickly or slowly?
VERY slowly: 1 mm per day, approximately
What happens when electrical resistance is high?
Current flow will be low
What is Ohm’s Law?
I = V/R
What is the effect of high electrical resistance on current flow?
Lower flow; characteristic of insulators
What is the effect of low electrical resistance on current flow?
Higher flow; characteristic of conductors
What is water that contains dissolved ions, and what are lipids, in terms of conductivity? Why?
Water with ions is a conductor because ions can carry current; Lipids are insulators because they have few charged groups
What is the resting membrane potential?
The potential difference across the plasma membranes from in to out, negative
What is the convention when defining membrane potential?
The ECF is set at 0 mV, and the polarity of the membrane potential is in terms of the sign of the excess charge on the inside of the cell
How is the resting membrane potential defined?
From inside to outside
What can cause the resting membrane potential to change?
An electrical current that alters the potential
Why does the resting membrane potential exist?
Tiny excess of negative ions in the cell and an excess of positive ions outside; excess neg inside are electrically attracted to the excess pos outside, and the excess charges collect in a thin shell tight against the inner and outer plasma surfaces
For each of these three (Na+, K+, Cl-), determine whether they’re more concentrated in or out of typical neuron.
Out, In, Out
Why is Na+ concentrated outside of the cell, but K+ concentrated inside the cell?
Because of the sodium potassium pump that pumps Na+ out and K+ in
What two factors does the magnitude of the Respo depend on?
Difference in specific ion concentrations in the intra and extra cellular fluids, and difference in membrane permeabilities to the different ions, which reflect the number of open channels for the different ions in the plasma membrane
What is the equilibrium potential?
The membrane potential at which the two fluxes (ions down their concentration gradient, and ions back against their concentration gradient due to electrical gradient) become equal in magnitude but opposite in direction
What is characteristic of the equilibrium potential for different ion species?
Can be different in magnitude and direction among ion species, depending on the concentration gradients between intracellular and extracellular compartments for each ion
What is the Nernst equation?
Describes the equilibrium potential for any ion species: aka the electrical potential necessary to balance a given ionic concentration gradient across a membrane so that net flux of that ion is 0; Eion = (61/Z)log(Cout/Cin) where Z is the valence of the ion and C out and C in are the extra/intracellular concentrations of the ion
For a given concentration gradient, how does membrane permeability to an ion species affect that ion species’ contribution to the membrane potential?
The greater the membrane permeability to an ion species, the greater the contribution that ion species will make to the membrane potential
What is the Goldman Hodgkin Kats equation?
V= 61log{ ([PsubKKout] + [PsubNaNaout] + [PsubClClin])/([PsubKKin] + [PsubNaNain] + [PsubClClout])}
What do the Ps stand for in the GHK equation?
Permeabilities; essentially creates a weighted average
What is largely the reason for the resting membrane potential across the plasma membrane?
The movement of K+ out of the cell down its concentration gradient through open K+ “leak” channels, so that the inside of the cell becomes negative with respect to the outside; BUT the resting membrane potential isn’t equal to the K+ concentration, because a small number of Na+ channels are open in resting state
What isn’t the Respo equal to the K+ equilibrium potential if the K+ is the primary reason for the negative charge inside the cell?
Because there are some Na+ channels open in resting state, so some sodium ions continually move into the cell, canceling the effect of an equivalent number of potassium ions simultaneously moving out
Why doesn’t the concentration of intracellular sodium and potassium ions change over time if the ions are constantly moving to create the resting potential?
Because the sodium potassium pump maintain the concentrations at stable levels
How does the NaK pump work?
Moves 3 Na+ out, and 2 K+ in: makes the inside of the cell more negative than it would be from ion diffusion alone
What is an electrogenic pump?
A pump that moves net charge across the membrane and contributes directly to the membrane potential
How does the NaK pump indirectly contribute to the membrane potential, since its electrogenic contribution is small?
It creates the concentration gradients down which the ions diffuse to produce most of the charge separation that makes up the potential
How is Cl ion concentration affected by the difference in Na and K concentration?
The membrane has Cl channels, but not pumps. So, Cl concentrations shift until the equilibrium potential is equal to the resting membrane potential: moves it out of the cell
What are the two ways neurons process and transmit information?
Graded potential and action potentials
What are graded potentials import for, vs action potentials?
Graded for signaling over short distances, but action for long-distance signals of neuronal and muscle membranes
What is depolarization?
When the membrane potential becomes less negative, closer to zero
What is overshoot?
A reversal of the membrane potential polarity, that is when the inside of a cell becomes positive relative to the outside
What is repolarization?
When the membrane potential that has been depolarized returns toward the resting value
What is hyperpolarization?
When the membrane potential gets more negative than at resting level
What are graded potentials?
Changes in membrane potential that are confined to a relatively small region of the plasma membrane; usually produced when some specific change in the environment acts on a specialized region o the membrane
True of False, graded potentials are always depolarizing and aren’t related to the magnitude of the initiating event.
FALSE! They can be depolarizing OR hyperpolarizing, and are related to the magnitude of the initiating event
What does it mean that local current is decremental, and why?
That the flow of change decreases as the distance from the site of origin of the graded potential increases, because the plasma membranes are leaky to ions so the currents die out
What is summation?
When additional stimuli occur before the graded potential has died away and they’re added to the depolarization from the first stimulus; particularly important for sensation
What are action potentials?
Large alternations in the membrane potential; generally very rapid and may repeat at frequencies of several hundred per second; used often in neurons and muscle cells, as well as of some endocrine, immune, and reproductive cells; are also all-or-none, in that superthreshold stimuli elicit the same effect as threshold stimuli
What are excitable membranes?
Membranes that are capable of producing action potentials
What is the mechanism that the nervous system uses to communicate over long distances?
Propagation of action potentials
How are ligand-gated, mechanically-gated, and voltage-gated channels involved in propagation of action potentials?
Ligand and mechanical are usually the initial stimulus for an action potential, but voltage-gated channels give a membrane the ability to undergo action potentials
What kind of channel enables a membrane to propagate action potentials?
Voltage-gated
How do voltage-gated Na+ and K+ channels operate for action potential propagation?
Depolarization of the membrane causes Na+ channels to open, and then are inactivated by the opening of K+ channels; when the membrane repolarizes to negative voltages, both channels return to the closed state
What are the two key differences that allow Na+ and K+ channels to play different roles in the production of action potentials?
1) Na+ channels are faster to respond to changes in voltage; they open before K+ channels do; 2) Na+ channels have an inactivation gate that limits the flux of sodium ions by blocking a channel shortly after depolarization opens it and allows the channel to return to the closed state after repolarization
What are the steps that occur in an action potential?
1) resting membrane potential is close to K+ equilibrium potential because there are more open K+ than Na+ channels; 2) depolarizing stimulus occurs; 3) depolarization opens some voltage-gated Na+ channels and Na+ enters to add to local depolarization; 4) when the threshold potential is reached, depolarization becomes positive feedback: Na+ causes depolarization, which opens more Na+ channels to cause more depolarization; 5) when membrane potential approached peak value, Na+ permeability declines when inactivation gates step in to block Na+; sluggish K+ gates have started to open up; 6) once the K+ gates are opened, K+ rushes in and repolarizes the membrane and allows the Na+ channels to close again; 7) slowly closing K+ channels allow a little too much positive charge back in, resulting in afterhyperpolarization
How are anesthetics related to action potentials?
Prevent action potentials from occurring because they block voltage-gated Na+ channels so that they can’t open in response to depolarization
What is the absolute refractory period?
Occurs during the period when the voltage-gated Na+ channels are either already open or have proceeded to the inactivated state; when the second stimulus can’t produce a second action potential
What is the relative refractory period?
The interval during which a second action potential can e produced but only if the stimulus strength is considerably greater than usual, after the absolute refractory period
Why is the only direction that propagation can occur away from the region of membrane that has recently been active?
Because a membrane area that has just undergone an action potential is refractory and can’t immediately undergo another
What happens when membrane after action potentials aren’t refractory?
Then the action potential can be conducted both ways, like when action potentials in skeletal muscle cells are initiated in the middle and propagate towards both ends
What affects the velocity with which an action potential propagates?
Fiber diameters and whether the fiber is myelinated or note (larger fiber diameter, faster propagation)
Why do larger fibers propagate action potentials faster?
Because they offer less resistance to local current, more ions flow in a given time, bringing adjacent regions of the membrane to threshold faster
Where do action potentials occur?
Only at the nodes of Ranvier, because there are fewer voltage-gated Na+ channels in the myelinated regions of axons
What is saltatory conduction?
The way action potentials appear to jump from one node to another as they propagate across a myelinated fiber
Which is faster, saltatory conduction or propagation in nonmyelinated fibers of the same axon diameter?
In saltatory conduction, because less charge leaks out through the myelin-covered sections of the membrane and more charge arrives at the node adjavent to the active node, to generate the action potential there sooner than if the myelin wasn’t present
How is initial depolarization achieved in afferent neurons?
By graded potential: receptor potential, generated in the sensory receptors at the peripheral ends of the neurons
What is receptor potential?
The graded potential that starts in the sensory receptors at the peripheral ends of the neurons for start action potentials from afferent neurons
What is synaptic potential?
The graded potential generated by synaptic input to the neuron that starts action potentials from neurons other than afferent neurons
What s pacemaker potential?
The spontaneous change in neuron’s membrane potential that starts action potentials from neurons other than afferent neurons
Why is there no stable, resting membrane potential in cells that can trigger action potentials with pacemaker potentials? What kinds of cells are they?
Cells like smooth muscle and cardiac muscle cells; because of the continuous change in membrane permeability
Where are pacemaker potentials usually seen?
In rhythmic activity, like breathing, heartbeat, and peristalsis
