Chapter 11 Anatomy and Physiology I

b

At which point of the illustrated action potential would voltage-gated Na+ channels be mostly open but voltage-gated K+ channels be mostly closed?

In the AP graph, an AP begins and ends at resting membrane potential. Depolarization to approximately +30 mV (inside positive) is caused by Na⁺ influx. Depolarization ends when Na⁺ channels inactivate. Repolarization and hyperpolarization are caused by K⁺ efflux. If threshold is reached, and AP is generated. If not, depolarization remains local. In nerve impulse propagation, each AP provides the depolarizing stimulus for triggering an AP in the next membrane patch. Regions that have just generated APs are refractory; for this reason, the nerve impulse propagates in one direction only. APs are independent of stimulus strength: Strong stimuli cause APs to be generated more frequently but not with greater amplitude.

Explain how action potentials are generated and propagated along neurons.

c

In which area of the neuron is an action potential initially generated?

Neuroglia (supporting cells) segregate and insulate neurons and assist neurons in various other ways. CNS neuroglia include astrocytes, microglial cells, ependymal cells, and oligodendrocytes. PNS neurolgia include Schwann cells and satellite cells.

List the types of neuroglia and cite their functions.

dendrite

What is the structure at A?

a

Where do most action potentials originate? a. Initial segment b. Cell body c. Nodes of Ranvier d. Axon terminal

negative greater

a positive ion is driven into the cell because the inside of the cell is _________ compared to the outside of the cell, and Na+ is driven into the cell because the concentration of Na+ is _____ outside the cell.

e

A molecule that carries information across a synaptic cleft is a a. synaptic cleft. b. sending neuron. c. synapse. d. receiving neuron. e. neurotransmitter.

Anatomically, neurons are classified according to the number of processes issuing from the cell body as multipolar, bipolar, or unipolar. Functionally, neurons are classified according to the direction of nerve impulse conduction. Sensory neurons conduct impulses toward the CNS, motor neurons conduct away from the CNS, and interneurons (association neurons) lie between sensory and motor neurons in the neural pathways.

Classify neurons by structure and by function.

Depolarization is a reduction in membrane potential (inside becomes less negative); hyperpolarization is an increase in membrane potential (inside becomes more negative). Graded potentials are small, brief, local changes in membrane potential that act as short-distance signals. The current produced dissipates with distance. An action potential (AP), or nerve impulse, is a large, but brief, depolarization signal (and polarity reversal) that underlies long-distance neural communications. It is an all-or-none phenomenon.

Compare and contrast graded potentials and action potentials.

During the absolute refractory period, a neuron cannot respond to another stimulus because it is already generating an AP. During the relative refractory period, the neuron's threshold is elevated because repolarization is ongoing.

Define absolute and relative refractory periods.

Neurons have a cell body and cytoplasmic processes called axons and dendrites. The cell body is the biosynthetic (and receptive) center of the neuron. Except for those found in ganglia, cell bodies are found in the CNS. Most neurons have many dendrites, receptive processes that conduct signals from other neurons toward the nerve cell body. With few exceptions, all neurons have one axon, which generates and conducts nerve impulses away from the nerve cell body. Axon terminals release neurotransmitters. Bidirectional transport along axons use ATP-dependent motor proteins "walking" along microtubule tracks. It moves vesicles, mitochondria, and cytosolic proteins toward the axon terminals and conducts substances destined for degradation back to the cell body.

Define neuron, describe its important structural components, and relate each to a functional role.

The major class of neurotransmitters based on chemical structure are acetylcholine, biogenic amines, amino acids, peptides, purines, dissolved gases, and lipids. Functionally, neurotransmitters are classified as (1) inhibitory or excitatory (or both) and (2) direct or indirect action. Direct-acting neurotransmitters bind to and open ion channels. Indirect-acting neurotransmitters act through second messengers. Neuromodulators also act indirectly presynaptically or postsynaptically to change synaptic strength. Neurotransmitter receptors are either channel-linked receptors that open ion channels, leading to fast changes in membrane potential, or G protein-linked receptors that oversee slow synaptic responses mediated by G proteins and intracellular second messengers. Second messengers most often activate kinases, which in turn act on ion channels or activate other proteins.

Define neurotransmitter and name several classes of neurotransmitters. Understand how they can be classified.

The measure of potential energy of separated electrical charges is called voltage (V) or potential. Current (I) is the flow of electrical charges from one point to another. Resistance (R) is hindrance to current flow. Ohm's law gives the relationship among these: I = V / R In the body, ions provide the electrical charges; cellular plasma membranes provide resistance to ion flow. The membranes contain leakage channels (nongated, always open) and gated channels. A resting neuron exhibits a resting membrane potential, which is -70 mV (inside negative). It is due both to differences in sodium and potassium ion concentrations inside and outside the cell and to differences in permeability of the membrane to the ions. The ionic concentration differences result from the operation of the sodium-potassium pump, which ejects 3 Na⁺ from the cell for each 2 K⁺ transported in.

Define resting membrane potential, and describe its electrochemical basis (what ions contribute, what channels contribute)

In nonlyelinated fibers, APs are produced in a wave all along the axon, that is, by continuous conduction. In myelinated fibers, APs are generated only at myelin sheath gaps and are propagated more rapidly by saltatory conduction.

Define salutatory conduction and contrast it to continuous conduction.

A synapse is a functional junction between neurons. The information-transmitting neuron is the presynaptic neuron; the information-receiving neuron is the postsynaptic neuron. Electrical synapses allow ions to flow directly from one neuron to another; the cells are electrically coupled. Chemical synapses are sites of neurotransmitter release and binding. When the impulse reaches the presynaptic axon terminals, voltage-gated Ca²⁺ channels open, and Ca²⁺ enters the cell and mediates neurotransmitter release. Neurotransmitters diffuse across the synaptic cleft and attach to postsynaptic membrane receptors, opening ion channels. After binding, the neurotransmitters are removed from the synapse by diffusion, enzymatic breakdown, or reuptake into the presynaptic terminal or astrocytes.

Define synapse. Distinguish between electrical and chemical synapses by structure and by the way they transmit information. Understand the steps of synaptic transmission of chemical synapses and how neurotransmitter effects are extinguished.

EPSPs and IPSPs summate temporally and spatially. The membrane of the axon hillock acts as a neuronal integrator. Synaptic potentiation, which enhances the postsynaptic neuron's response, is produced by intense repeated stimulation. Ionic calcium appears to mediate such effects, which may be the basis of learning. Presynaptic inhibition is mediated by axoaxonal synapses that reduce the amount of neurotransmitter released by the inhibited neurons.

Describe how synaptic events are integrated and modified.

-- A bundle of nerve fibers is called a tract in the CNS and a nerve in the PNS. -- A collection of cell bodies is called a nucleus in the CNS and a ganglion in the PNS.

Differentiate between (1) a nerve and a tract, and (2) a nucleus and a ganglion.

Binding of neurotransmitters at excitatory chemical synapses results in local graded potentials called EPSPs, caused by the opening of channels that allow simultaneous passage of Na⁺ and K⁺. Neurotransmitter binding at inhibitory chemical synapses results in hyperpolarizations called IPSPs, caused by the opening of K⁺ or Cl⁻ channels. IPSPs drive the membrane potential farther from threshold.

Distinguish between excitatory and inhibitory postsynaptic potentials.

a

During depolarization, which gradient(s) move(s) Na+ into the cell? a. both the electrical and chemical gradients b. only the chemical gradient c. only the electrical gradient d. Na+ does not move into the cell. Na+ moves out of the cell.

Large nerve fibers (axons) are myelinated. The myelin sheath is formed in the PNS by Schwann cells and in the CNS by oligodendrocytes. The myelin sheath gaps are also called nodes of Ranvier. Nonmyelinated fibers are surrounded by supporting cells, but the membrane-wrapping process does not occur.

Explain the importance of the myelin sheath and describe how it is formed in the central and peripheral nervous systems.

The major functional division of the PNS are the sensory (afferent) division, which conveys impulses to the CNS and motor (efferent) division, which conveys impulses from the CNS. The efferent division includes the somatic (voluntary) system, which serves skeletal muscles, and the autonomic (involuntary) system, which innervates smooth and cardiac muscle and glands.

Explain the structural and functional divisions of the nervous system.

eccrine organs

Hormones are released by __________. The nervous system does have some control over endocrine function, but the endocrine system is considered a separate signaling system.

a

If a signal from a sending neuron makes the receiving neuron more negative inside, a. the receiving neuron is less likely to generate an action potential. b. the receiving neuron is more likely to generate an action potential. c. the sending neuron becomes more positive inside. d. the sending neuron becomes more negative inside. e. the receiving neuron immediately generates an action potential

d

Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference? a. Positive membrane potential b. Threshold potential c. Action potential d. Resting membrane potential (RMP)

The nervous system bears a major responsibility for maintaining body homeostasis. It monitors, integrates, and responds to information in the environment. The nervous system is divided anatomically into the central nervous system (CNS) (brain and spinal cord) and the peripheral nervous system (PNS) (mainly cranial and spinal nerves).

List the basic functions of the nervous system.

b

Many neurons have many short, branching extensions called dendrites. What is the benefit of these structures for a neuron? a. There is a large surface area to send signals to other cells. b. The dendrites provide a large surface area for connections from other neurons. c. There is a large amount of space for myelin to form and make electrical conduction more efficient. d. There is a large area for production of chemicals used to signal other neurons.

K+ Na+

More ______ moves out of the cell than _____ moves into the cell

c

On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you? a. The inside surface of the plasma membrane is much more positively charged than the outside surface. b. The outside surface of the plasma membrane is much more negatively charged than the inside surface. c. The inside surface of the plasma membrane is much more negatively charged than the outside surface. d. There is no electrical potential difference between the inside and the outside surfaces of the plasma membrane.

c

Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel? a. Voltage-gated channels b. Sodium-potassium ATPases c. Leak channels d. Ligand-gated channels

d

The Na+-K+ pump actively transports both sodium and potassium ions across the membrane to compensate for their constant leakage. In which direction is each ion pumped? a. Both Na+ and K+ are pumped out of the cell. b. Both Na+ and K+ are pumped into the cell. c. K+ is pumped out of the cell and Na+ is pumped into the cell. d. Na+ is pumped out of the cell and K+ is pumped into the cell.

dendrites soma

The ________ and ______ of the cell receive signals from other neurons.

c

The concentrations of which two ions are highest outside the cell. a. K+ and A- (negatively charged proteins) b. Na+ and A- (negatively charged proteins) c. Na+ and Cl- d. K+ and Cl-

K+ Na+

The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____

b

The plasma membrane is much more permeable to K+ than to Na+. Why? a. There are many more voltage-gated K+ channels than voltage-gated Na+ channels. b. There are many more K+ leak channels than Na+ leak channels in the plasma membrane. c. The Na+-K+ pumps transport more K+ into cells than Na+ out of cells. d. Ligand-gated cation channels favor a greater influx of Na+ than K+.

b

The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. Identify these two factors. a. The presence of concentration gradients and voltage-gated channels b. The presence of concentration gradients and leak channels c. The presence of a resting membrane potential and leak channels d. The presence of concentration gradients and Na+-K+ pumps

b

The small space between the sending neuron and the receiving neuron is the a. synaptic terminal. b. synaptic cleft. c. calcium channel. d. neurotransmitter. e. vesicle.

axon hillock axon

Voltage-gated Na+ and K+ channels allow for the triggering of an action potential at the ______ ________ and its propagation down the ______

d

What characterizes depolarization, the first phase of the action potential? a. The membrane potential changes to a much more negative value. b. The membrane potential reaches a threshold value and returns to the resting state. c. The membrane potential changes to a less negative (but not a positive) value. d. The membrane potential changes from a negative value to a positive value.

c

What characterizes repolarization, the second phase of the action potential? a. Once the membrane depolarizes to a threshold value of approximately -55 mV, it repolarizes to its resting value of -70 mV. b. As the membrane repolarizes to a negative value, it goes beyond the resting state to a value of -80 mV. c. Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV. d. Before the membrane has a chance to reach a positive voltage, it repolarizes to its negative resting value of approximately -70 mV.

d

What division of the nervous system is most specifically responsible for voluntary motor control? a. sympathetic nervous system b. parasympathetic nervous system c. central nervous system d. somatic nervous system

d

What event triggers the generation of an action potential? a. The membrane potential must return to its resting value of -70 mV from the hyperpolarized value of -80 mV. b. The membrane potential must hyperpolarize from the resting voltage of -70 mV to the more negative value of -80 mV. c. The membrane potential must depolarize from the resting voltage of -70 mV to its peak value of +30 mV. d. The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.

c

What is the first change to occur in response to a threshold stimulus? a. Voltage-gated Ca2+ channels change shape, and their activation gates open. b. Voltage-gated Na+ channels change shape, and their inactivation gates close. c. Voltage-gated Na+ channels change shape, and their activation gates open. d. Voltage-gated K+ channels change shape, and their activation gates open.

-70mV

What is the value for the resting membrane potential for most neurons?

a

What opens first in response to a threshold stimulus? a. Voltage-gated Na+ channels b. Ligand-gated cation channels c. Voltage-gated K+ channels d. Ligand-gated Cl- channels

a

What prevents the Na+ and K+ gradients from dissipating? a. Na+-K+ ATPase b. Na+ cotransporter c. Na+ and K+ leaks d. H+-K+ ATPase

b

When calcium ions enter the synaptic terminal, a. the inside of the receiving neuron becomes more positive. b. they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron. c. they cause an action potential in the sending neuron. d. the inside of the receiving neuron becomes more negative. e. neurotransmitter molecules are quickly removed from the synaptic cleft.

d

When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron, a. vesicles in the synaptic terminal fuse to the plasma membrane of the sending neuron. b. ion channels in the plasma membrane of the sending neuron open. c. the receiving neuron becomes more negative inside. d. ion channels in the plasma membrane of the receiving neuron open. e. the receiving neuron becomes more positive inside.

a and b

Which areas of this neuron would be classified as receptive regions?

a

Which neuron circuit pattern is involved in the control of rhythmic activities such as breathing? a. reverberating circuit b. parallel after-discharge circuit c. converging circuit d. diverging circuit

d

Which of the cell types shown is most associated with the production and flow of cerebrospinal fluid (CSF)?

b

Which of the following is NOT one of the basic functions of the nervous system? a. Integrate sensory input for decision making. b. Release hormones into the bloodstream to communicate with other cells in the body. c. Generate direct, electrical signals. d. Decode sensory information from the environment.

c and d

Which of the following membrane regions would have significant numbers of voltage-gated ion channels?

a

Which of the neuroglial cell types shown form myelin sheaths within the CNS?