Chapter 12 Flashcards

Question

Neuroglia CNS • Microglia

Answer 1

•function as phagocytes •remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nerve tissue

Answer 2

• cuboidal or columnar cells arranged in a single layer that possess microvilli or cilia •Line the ventricles of the brain and the central canal of the spinal cord •Monitor and assist in the circulation of cerebrospinal fluid

Answer 3

•Encircle PNS axons and form the myelin sheath around axons •Each Schwann cell myelinates only a single axon •1 Schwann cell can also enclose as many as 20 or more UNmyelinated axons

Answer 4

•Flat cells that surround the cell bodies of the neurons of PNS ganglia •Provide structural support and regulate the exchange of materials between the neuronal cell bodies and interstitial fluids

Answer 5

•multilayered lipid and protein covering produced by Schwann cells (PNS) an oligodendrocytes (CNS) that surrounds the axons of most neurons •Axons without this sheath are called unmyelinated

Answer 6

•Schwann cells begin to form myelin sheaths around axons during fetal development: •Each Schwann cell wraps about 1 mm of a single axon’s length by spiraling many times around the axon. •Eventually, multiple layers of glial plasma membrane surround the axon, with the Schwann cell’s cytoplasm and nucleus forming the outermost layer. • The inner portion, consisting of up to 100 layers of Schwann cell membrane, is the myelin sheath. •The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath, is the neurolemma which is found only around axons in the PNS. •When an axon is injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon. •Gaps in the myelin sheath, called nodes of Ranvier, appear at intervals along the axon. Each Schwann cell wraps one axon segment between two nodes

Answer 7

- oligodendrocyte myelinates parts of several axons: •Each oligodendrocyte puts forth about 15 broad, flat processes that spiral around CNS axons, forming a myelin sheath. •A neurolemma is not present, because the oligodendrocyte cell body and nucleus do not envelop the axon. •Nodes of Ranvier are present, but they are fewer in number. •Axons in the CNS display little regrowth after injury. •This is thought to be due, in part, to the absence of a neurolemma, and in part to an inhibitory influence exerted by the oligodendrocytes on axon regrowth.

Answer 8

•allow communication over short and long distances whereas -Production of an AP or a GP depends upon the existence of a resting membrane potential and the existence of certain ion channels

Answer 9

•allow communication over short distances only -Production of an AP or a GP depends upon the existence of a resting membrane potential and the existence of certain ion channels

Answer 10

• an electrical potential difference (voltage) across the membrane. In excitable cells this voltage is called resting membrane potential

Answer 11

• gated channels that randomly open and close

Answer 12

• gated channels that open in response to binding of ligand (chemical) stimulus

Answer 13

• gated channels that open in response to mechanical stimulus (such as touch, pressure, vibration, or tissue stretching)

Answer 14

• gated channels that open in response to voltage stimulus (change in membrane potential)

Answer 15

•a small buildup of negative ions in the cytosol along the inside of the membrane, and an equal buildup of positive ions in the extracellular fluid (ECF) along the outside surface of the membrane •Such a separation of positive and negative electrical charges is a form of potential energy, which is measured in volts or millivolts •The buildup of charge occurs only very close to the membrane. The cytosol or extracellular fluid elsewhere in the cell contains equal numbers of positive and negative charges and is electrically neutral.

Answer 16

• In neurons, the resting membrane potential ranges from −40 to −90 mV. A typical value is −70 mV. •A cell that exhibits a membrane potential is said to be polarized. Measured by- •The tip of a recording microelectrode is inserted inside the cell, and a reference electrode (device that conducts electrical charges) is placed outside the cell in the extracellular fluid. •The recording microelectrode and the reference electrode are connected to an instrument known as a voltmeter, which detects the electrical difference (voltage) across the plasma membrane.

Answer 17

• across the plasma membrane and the selective permeability of the neuron’s membrane to Na+ and K+ - non-conducting neuron is positive outside and negative inside

Answer 18

• Most anions inside the cell cannot leave freely (unlike K+) because they are attached to non diffusible molecules such as ATP and large proteins - non-conducting neuron is positive outside and negative inside

Answer 19

• Na+/K+ ATPase expels 3 Na+ for every 2 K+ imported. Since this pump removes more positive charges from the cell than it can bring in the pump is electrogenic-contributes to the negative resting membrane potential - non-conducting neuron is positive outside and negative inside

Answer 20

• A graded potential is a small deviation from the resting membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative)

Answer 21

• When the response makes the membrane more polarized (inside more negative)

Answer 22

• When the response makes the membrane less polarized (inside less negative)

Answer 23

electrical signals are graded means that they vary in (size), depending on the strength of the stimulus

Answer 24

• When a graded potential occurs in the dendrites or cell body of a neuron in response to a neurotransmitter

Answer 25

• The graded potentials that occur in sensory receptors

Answer 26

•The opening or closing of these ion channels alters the flow of specific ions across the membrane, producing a flow of current that is localized. •This mode of travel by which graded potentials die out as they spread along the membrane is known as decremental conduction. •Because they die out within a few millimeters of their point of origin, graded potentials are useful for short-distance communication only •Summation is the process by which graded potentials add together. If two depolarizing graded potentials summate, the net result is a larger depolarizing graded potential.

Answer 27

•impulse is a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state. •An action potential has two main phases: a depolarizing phase and a repolarizing phase.

Answer 28

• the negative membrane potential becomes less negative, reaches zero, and then becomes positive

Answer 29

• the membrane potential is restored to the resting state of −70 mV

Answer 30

• Following the repolarizing phase there may be an after-hyperpolarizing phase, during which the membrane potential temporarily becomes more negative than the resting level.

Answer 31

•Two types of voltage-gated channels open and then close during an action potential. •These channels are present mainly in the axon plasma membrane and axon terminals. •The first channels that open, the voltage-gated Na+ channels, allow Na+ to rush into the cell, which causes the depolarizing phase. •Then voltage-gated K+ channels open, allowing K+ to flow out, which produces the repolarizing phase. •The after-hyperpolarizing phase occurs when the voltage-gated K+ channels remain open after the repolarizing phase ends

Answer 32

•An action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed the threshold. •The generation of an action potential depends on whether a particular stimulus is able to bring the membrane potential to threshold. •An action potential will not occur in response to a subthreshold stimulus- a weak depolarization that cannot bring the membrane potential to threshold. •However, an action potential will occur in response to a threshold stimulus- a stimulus that is just strong enough to depolarize the membrane to threshold. •Several action potentials will form in response to a suprathreshold stimulus- a stimulus that is strong enough to depolarize the membrane above threshold. •Each of the action potentials caused by a suprathreshold stimulus has the same amplitude (size) as an action potential caused by a threshold stimulus but will have greater frequency

Answer 33

• The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus • During the absolute refractory period, even a very strong stimulus cannot initiate a second action potential.

Answer 34

• The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus - During the absolute refractory period, even a very strong stimulus cannot initiate a second action potential.

Answer 35

• period of time during which a second action potential can be initiated, but only by a larger-than-normal stimulus. •In contrast to action potentials, graded potentials do not exhibit a refractory period

Answer 36

• involves step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane. •In continuous conduction, ions flow through their voltage-gated channels in each adjacent segment of the membrane. •Note that the action potential propagates only a relatively short distance in a few milliseconds. •Continuous conduction occurs in unmyelinated axons and in muscle fibers

Answer 37

• (= leaping), the special mode of action potential propagation that occurs along myelinated axons, occur because of the uneven distribution of voltage-gated channels. •When an action potential propagates along a myelinated axon, an electric current (carried by ions) flows through the extracellular fluid surrounding the myelin sheath and through the cytosol from one node to the next. •The action potential at the first node generates ionic currents in the cytosol and extracellular fluid that depolarize the membrane to threshold, opening voltage-gated Na+ channels at the second node. •The resulting ionic flow through the opened channels constitutes an action potential at the second node. •Then, the action potential at the second node generates an ionic current that opens voltage-gated Na+ channels at the third node, and so on. •Each node repolarizes after it depolarizes

Answer 38

• (pre- = before) refers to a nerve cell that carries a nerve impulse toward a synapse. It is the cell that sends a signal

Answer 39

• cell that receives a signal • (post- = after) that carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse

Answer 40

Axon to dendrite

Answer 41

Axon to cell body

Answer 42

Axon to axon

Answer 43

•Gap junctions connect cells and allow the transfer of information to synchronize the activity of a group of cells •Each gap junction contains a hundred or so tubular connexons, which act like tunnels to connect the cytosol of the two cells directly. •As ions flow from one cell to the next through the connexons, the action potential spreads from cell to cell

Answer 44

• Because action potentials conduct directly through gap junctions, electrical synapses are faster than chemical synapses. -At an electrical synapse, the action potential passes directly from the presynaptic cell to the postsynaptic cell

Answer 45

• Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers. -A large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions

Answer 46

•One-way transfer of information from a presynaptic neuron to a postsynaptic neuron •Plasma membranes of synapses are close but do not touch, separated by a synaptic cleft •The presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter). The postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential). •The time required for these processes at a chemical synapse, a synaptic delay of about 0.5 msec, is the reason that chemical synapses relay signals more slowly than electrical synapses

Answer 47

1. A nerve impulse arrives at a synaptic end bulb (or at a varicosity) of a presynaptic axon. 2. The depolarizing phase of the nerve impulse opens voltage-gated Ca2+ channels on the synaptic end bulbs. Because calcium ions are more concentrated in the extracellular fluid, Ca2+ flows inward through the opened channels. 3. An increase in the concentration of Ca2+ inside the presynaptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles. As vesicle membranes merge with the plasma membrane, neurotransmitter molecules within the vesicles are released into the synaptic cleft. 4. The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron’s plasma membrane. 5. Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particular ions to flow across the membrane. 6. As ions flow through the opened channels, the voltage across the membrane changes. This change in membrane voltage is a postsynaptic potential. •Depending on which ions the channels admit, the postsynaptic potential may be a depolarization (excitation) or a hyperpolarization (inhibition). 7. When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron.

Answer 48

•A depolarizing postsynaptic potential •Opening of Na+ channels

Answer 49

•A hyperpolarizing postsynaptic potential •Opening of Cl- or K+ channels

Answer 50

receptor that contains a neurotransmitter binding site and an ion channel

Answer 51

receptor that contains a neurotransmitter binding site but No ion channel

Answer 52

1. Diffusion 2. Enzymatic degradation 3. Uptake into cells

Answer 53

• If several presynaptic end bulbs release their neurotransmitter at about the same time, the combined effect may generate a nerve impulse

Answer 54

•postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time. • results from the buildup of neurotransmitter released simultaneously by several presynaptic end bulbs

Answer 55

•postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times. •results from buildup of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession

Answer 56

• If the total excitatory effects are greater than the total inhibitory effects but less than the threshold level of stimulation, the result is an EPSP that does not reach threshold. •Following an EPSP, subsequent stimuli can more easily generate a nerve impulse through summation because the neuron is partially depolarized

Answer 57

• If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, one or more nerve impulses (action potentials) will be triggered. •Impulses continue to be generated as long as the EPSP is at or above the threshold level

Answer 58

• If the total inhibitory effects are greater than the excitatory effects, the membrane hyperpolarizes (IPSP). •The result is inhibition of the postsynaptic neuron and an inability to generate a nerve impulse

Answer 59

• amino acid that acts as the main inhibitory neurotransmitter in the central nervous system • reduces neuronal excitability by blocking nerve transmission • binds to post-synaptic GABA receptors which hyperpolarizes the cell and prevents an action potential from being transmitted

Answer 60

• the nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit •This arrangement amplifies the signal

Answer 61

•the postsynaptic neuron receives nerve impulses from several different sources.

Answer 62

•the incoming impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on. •Branches from later neurons synapse with earlier ones. •This arrangement sends impulses back through the circuit again and again. •The output signal may last from a few seconds to many hours, depending on the number of synapses and the arrangement of neurons in the circuit

Answer 63

•a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell. •A differing number of synapses between the first and last neurons imposes varying synaptic delays, so that the last neuron exhibits multiple EPSPs or IPSPs. •If the input is excitatory, the postsynaptic neuron then can send out a stream of impulses in quick succession. •may be involved in precise activities such as mathematical calculations

Answer 64

•In the CNS, there is little or no repair due to: •Inhibitory influences from neuroglia, particularly oligodendrocytes •Absence of growth-stimulating cues that were present during fetal development •Rapid formation of scar tissue

Answer 65

•repair is possible if the cell body is intact, Schwann cells are functional, and scar tissue formation does not occur too rapidly •Steps involved in the repair process are: Chromatolysis •About 24 to 48 hours after injury the Nissl bodies break up into fine granular masses. Wallerian degeneration •By the third to fifth day, the part of the axon distal to the damaged region becomes slightly swollen and then breaks up into fragments; the myelin sheath also deteriorates but the neurolemma remains. Degeneration of the distal portion of the axon and myelin sheath -Macrophages phagocytize the debris. Synthesis of RNA and protein accelerates, which favors rebuilding or regeneration of the axon. Formation of a regeneration tube •The Schwann cells on either side of the injured site multiply by mitosis, grow toward each other, and may form this •The tube guides growth of a new axon from the proximal area across the injured area into the distal area previously occupied by the original axon. -New axons cannot grow if the gap at the site of injury is too large or if the gap becomes filled with collagen fibers.