Neuroscience Chapter 2: Physical and Electrical Properties of Cells in the Nervous System

Axoplasmic transport convergence

(1) Multiple inputs from a variety of different cells terminating on a single neuron. (2) Movement that directs the eyes toward the midline.

Afferent neuron

(1) Neuron that brings information into the CNS. (2) Neuron that transmits information toward a structure.

Axon

(1) Process that extends from the cell body of a neuron and conducts signals away from the cell body. (2) Distal process of a primary afferent neuron, which conducts signals toward the cell body.

Efferent neuron

1) Neuron that relays commands from the CNS to the smooth and skeletal muscles and glands of the body. (2) Neuron that transmits information away from a structure.

17. List two ways in which glial cells differ from nerve cells.

1Glial cells have no dendrites or axons, and glial cells cannot conduct an electrical potential.

6. If a membrane cha nnel opens when it is bound by a neu-rotransmitter, what type of membrane channel is it?

A membrane channel that opens when it is bound by a neurotransmitter is a ligand-gated channel.

Astrocytes

A. Cells in the central nervous system (CNS) with nutritive and clean-up functions

Modality-gated

A. Mechanical force, temperature change, or chemical stimulus

Retrograde transport: A. Recycles substances from the axon back to the soma. B. Moves neurotransmitters from the dendrites to the cell body. C. Moves substances from the soma toward the axon terminal. D. Moves neurotransmitters across the synaptic cleft. E. Moves information from astrocyte to astrocyte.

ANS: A Rationale: Axoplasmic transport occurs in two directions: anterograde and retrograde. Anterograde transport moves neurotransmitters and other substances from the soma down the axon toward the presynaptic terminal. Retrograde transport moves substances from the synapse back to the soma.

Depolarization occurs when: A. The membrane potential becomes less negative than the resting membrane potential. B. The membrane potential becomes more negative than the resting membrane potential. C. Cl- influx hyperpolarizes the membrane. D. The presynaptic terminal of a neuron is inhibited by another neuron. E. All membrane channels are closed, preventing the influx of Na+.

ANS: A Rationale: Sudden changes in membrane potential result from the flow of electrically charged ions through gated channels spanning the cell membrane. The membrane is depolarized when the potential becomes less negative than the resting potential.

20. Which one of the following is a feature of the nodes of Ranvier? A. Are distributed approximately every 1 to 2 millimeters (mm) along the membrane of the cell axon. B. Contain a high density of modality-gated K+ channels for rapid depolarization of the membrane. C. Contain a high density of voltage-gated Na+ channels for rapid repolarization of the membrane. D. Have low membrane capacitance, preventing the accumulation of electrical charge. E. Are heavily myelinated, which allows for rapid diffusion of an electrical potential.

ANS: A Rationale: The nodes are specialized for active propagation of an action potential by allowing ion flow across the membrane. Nodes of Ranvier are distributed every 1 to 2 mm along the axon and contain high densities of voltage-gated Na+ and K+ channels. An action potential spreads rapidly along a myelinated region and then slows when crossing the high-capacitance, unmyelinated region of the nodes of Ranvier.

27. Which one of the following is an example of divergence? A. Contraction of the hip flexor muscles when a painful stimulus is applied to the toe B. Integration of taste and smell information in the temporal lobe C. Multiple different cells synapsing with a single neuron in the spinal cord D. All of the above

ANS: A Rationale: Convergence is the process during which multiple inputs from a variety of neurons are integrated into a single neuron, whereas divergence is the process during which a single neuron communicates with multiple other neurons. The integration of multiple sensory inputs by the cerebral cortex is an example of convergence; conversely, an example of divergence is the stimulation of a single sensory neuron evoking a withdrawal reflex in a large group of muscle cells.

19. Propagation of an action potential along an axon is dependent on a(n): A. Complete myelination of the axon by glial cells B. Anterograde diffusion of the electric potential with active generation of new potentials C. Rapid repolarization associated with passive diffusion of Cl- D. Retrograde diffusion of the electrical potential E. Na+/K+ pump moving sufficient quantities of Na+ into of the cell and K+ out of the cell

ANS: B Rationale: An action potential is a brief, large depolarization in electrical potential that is repeatedly regenerated along the length of an axon. Regeneration allows an action potential to actively spread long distances, transmitting information down the axon to the presynaptic chemical release sites of the presynaptic terminal.

Which one of the following neuron structures is specialized for receiving synaptic input from other neurons? A. Cell body B. Dendrite C. Axon D. Axon hillock E. Presynaptic terminal

ANS: B Rationale: Dendrites, with branchlike extensions that serve as the main input sites for the cell and project from the soma, are specialized to receive information from other cells.

Afferent neurons convey information: A. Between interneurons B. From the CNS to skeletal muscles C. From peripheral receptors to the CNS D. Between the soma and presynaptic terminal E. From the CNS to smooth muscles

ANS: C Rationale: Afferent neurons carry sensory information from the outer body toward the CNS. Efferent neurons relay commands from the CNS to smooth and striated muscles and glands. Interneurons, the largest class of neurons, act throughout the nervous system, processing information locally or conveying information across short distances.

25. Which one of the following is not one of the primary components of a neuron? A. Axon B. Soma C. Postsynaptic membrane D. Dendrite

ANS: C Rationale: The primary components of a neuron consist of dendrites, which transmit information toward the cell body; the soma or cell body, which synthesize neurotransmitters; axons, which transmit information away from the cell body to a target cell; and presynaptic terminals, which release neurotransmitters into the synaptic cleft.

15. The resting membrane potential is: A. The same as the membrane equilibrium potential B. The voltage difference across a neuron's cell membrane, maintained by an unequal distribution of one specific ion C. Maintained by active transport of sodium ions (Na+) and potassium ions (K+) and passive diffusion of Na+, K+, and chloride ion (Cl-) through leak channels in the cell membrane D. Typically measured at +70 millivolts (mV) because the intracellular environment is more positively charged than the extracellular environment E. Created by a more negative charge inside the membrane than outside because Na+ is continuously moved inside the cell membrane by an active transport pump

ANS: C Rationale: The resting membrane potential is maintained via passive diffusion of ions across the cell membrane and via active transport of Na+ and K+ by Na+/K+ pumps.

26. The strength of local electrical potentials is modulated and integrated via: A. Spatial summation, the combined effect of potentials generated in other parts of the neuron B. Temporal summation, the combined effect of small potential changes occurring over several milliseconds C. Both A and B D. None of the above. A local potential is an all-or-none phenomenon.

ANS: C Rationale: The strength of local potentials is increased, and the strength of multiple potentials is integrated via spatial summation and temporal summation. Spatial summation refers to the process of summing potentials generated in different parts of the neuron, whereas temporal summation refers to the adding together of small potentials that occur in a period of several milliseconds. If the summation of these local potentials reaches a specific threshold level, an all-or-none action potential is generated.

Which of the following is the structural part of a neuron that releases a neurotransmitter? A. Dendrite B. Axon hillock C. Soma D. Presynaptic terminal E. Postsynaptic terminal

ANS: D Rationale: Axons end in presynaptic terminals, or fingerlike projections, which are the transmitting elements of the neuron. Neurons transmit information about their activity via the release of chemicals called neurotransmitters from the presynaptic terminal into the synaptic cleft.

22. Peripheral demyelination: A. Typically affects small diameter axons before large diameter axons. B. Is a characteristic feature of multiple sclerosis. C. Affects the structure of oligodendrocytes. D. Typically affects the Schwann cells of large, well-myelinated axons. E. Typically affects the axon at the ventral root of the spinal cord.

ANS: D Rationale: Peripheral neuropathies often involve destruction of the myelin surrounding the largest, most myelinated sensory and motor fibers, resulting in disrupted proprioception (awareness of limb position) and weakness. Guillain-Barré syndrome, metabolic abnormalities, viruses, trauma, and toxic chemicals can cause peripheral demyelination.

28. Glial cells contribute which one of the following? A. Communication between neurons and blood vessels B. Neural cell death C. Action potential propagation D. Both A and B E. All of the above

ANS: D Rationale: Glial cells, known as astrocytes, participate in cellular signaling with other astrocytes, neurons, and cells such as vascular smooth muscle. Glial cells also play an important role in phagocytosis and CNS development. Oligodendrocytes and Schwann cells aid in the propagation of action potentials generated by the neuron. Macroglial cells clean the extracellular environment; however, hyperactivity of these and other glial cells may result in neurologic damage.

23. Guillain-Barré syndrome: A. Involves demyelination of peripheral axons. B. Results from an autoimmune attack on Schwann cells. C. May affect cranial nerves controlling the muscles involved in swallowing, breathing, and facial expression. D. Both A and B E. A, B, and C

ANS: E Rationale: Both Guillain-Barré syndrome (acute idiopathic polyneuritis) and multiple sclerosis are autoimmune disorders that cause demyelination. Guillain-Barré syndrome is a peripheral neuropathy that affects sensory and motor function and, in severe cases, peripheral autonomic function. Lower cranial nerves may also be affected, resulting in facial weakness and difficulty swallowing and breathing.

24. Multiple sclerosis: A. Results from an autoimmune attack on oligodendrocytes. B. Involves demyelination of axons in the CNS. C. Has signs and symptoms associated with both motor and sensory impairment. D. Both A and B E. A, B, and C

ANS: E Rationale: Both Guillain-Barré syndrome (acute idiopathic polyneuritis) and multiple sclerosis are autoimmune disorders that cause demyelination. In multiple sclerosis, demyelination in the CNS produces plaque in the white matter. Because multiple sclerosis attacks the CNS, a greater variety of symptoms occur, including weakness, lack of coordination, visual problems, impaired sensation, slurred speech, memory problems, and abnormal emotional affect.

Local potentials: A. Are either receptor or synaptic potentials. B. Spread passively only a short distance along the cell membrane. C. Result from stimulation of sensory receptors or from the binding of a neurotransmitter with chemical receptor sites on a postsynaptic membrane. D. Both A and B E. A, B, and C

ANS: E Rationale: Local potentials are categorized as either receptor potentials or synaptic potentials, depending on whether they are generated at a peripheral receptor of a sensory neuron or at a postsynaptic membrane. These local potentials can only spread passively and therefore are confined to a small area of the membrane.

21. Demyelination of an axon: A. Results in decreased membrane resistance, allowing a leakage of electrical current. B. Results in slowed propagation of action potentials. C. May prevent propagation of action potentials. D. Both A and B E. A, B, and C

ANS: E Rationale: Myelination increases the speed of action potential propagation and the distance a current can passively spread. Thicker myelin leads to faster conduction and improved action potential propagation. Demyelination allows leakage of electrical current across the membrane, decreasing the amplitude and velocity of the signal as the action potential travels down the axon. Similarly, when a hose has a leaky wall, the flow diminishes as the distance from the faucet increases.

18. Which of the following change the electrical potential across the cell membrane? A. Activation and opening of ligand-gated K+ channels B. Activation and opening of modality-gated Na+ channels C. Activation and opening of voltage-gated Cl- channels D. Leak channels, which allow continuous diffusion of small ions E. All of the above

ANS: E Rationale: Neurons function by undergoing rapid changes in electrical potential across the cell membrane. An electrical potential across a membrane exists when the distribution of ions creates a difference in electrical charge on each side of the cell membrane. Four types of membrane channels allow ions to flow across the membrane: leak channels, modality-gated channels, ligand-gated channels, and voltage-gated channels.

Pseudounipolar cells: A. Have two dendrites. B. Have two somas. C. Are not neurons. D. Are glial cells. E. Have two axon extensions.

ANS: E Rationale: Pseudounipolar cells, a subclass of bipolar cells, appear to have a single projection from the cell body that divides into two axonal roots. Pseudounipolar cells have two axons and no true dendrites.

Guillain-Barré syndrome

Acute, autoimmune peripheral polyneuropathy characterized by progressive paralysis, burning and tingling sensations, and pain.

18. What are the four function s of astrocytes in the mature nervous system?

Astrocytes increase or decrease signaling between neurons, clean up ions and transmitters in the extracellular space and synapses, supply nutrition, and protect the central nervous system by serving as part of the blood-brain barrier.

Schwann cells

B. Cells in the peripheral nervous system that produce myelin

Leak

B. No stimulus required

21. Compare and contrast Guillain-Barré syndrome and mul-tiple sclerosis.

Both Guillain-Barré syndrome (acute idiopathic polyneuritis) and multiple sclerosis are autoimmune disorders that cause demyelination. Guillain-Barré syndrome is a peripheral neuropathy that affects sensory and motor function and, in severe cases, peripheral autonomic function. Most people recover completely from a single bout of Guillain-Barré syndrome. In multiple sclerosis, demyelination in the central nervous system produces plaques in the white matter. Because multiple sclerosis attacks the central nervous system, a greater variety of symptoms occur, including weakness, lack of coordination, visual problems, impaired sensation, slurred speech, memory problems, and abnormal emotional affect. Multiple sclerosis is progressive. Its course is characterized by exacerbations and remissions.

Divergence

Branching of a single neuronal axon to synapse with a multitude of neurons.

Neurons

C. Cells that receive and transmit information via propagation of an electrical signal

Voltage-gated

C. Change in electrical potential across the cell membrane

Case 1 I.D., a 19-year-old man, suffered severe flu symptoms, requiring him to stay home from work for 2 days. Four days after his return to work, I.D. noted tingling and numbness in his fingers. By the end of the day, he noticed his hand movements were clumsy. The following day, I.D. returned to work. Midday he was unable to stand and could not use his hands. At the hospital, he experienced respiratory weakness and was placed on a ventila-tor. He had nearly total paralysis of voluntary muscles, including facial and swallowing muscles. He was unable to close his eyes and required tube feeding. Nerve conduction studies for both motor and sensory pathways were conducted. (To test peripheral sensory nerve pathways, an electrical stimulus is given to the skin at a distal point over a nerve and is recorded with surface electrodes at a more proximal point over the same nerve. The time required to transmit the signal between the two points indicates the conduction velocity. Peripheral motor conduction studies are similar, except that the electrical stimulus is given proximally over the nerve and is recorded from the skin over an associated muscle.) For I.D., studies indicated that peripheral sensory and motor conduction times were significantly prolonged bilaterally. I.D. had suffered peripheral nerve demyelination, presumably due to an autoimmune response to viral infection. With loss of myelin, nerve conduction was severely impaired. I.D. had sensory loss and muscular weakness that significantly impaired his ability to move. Occupational therapy trained I.D. to use sip-and-puff switches to control an electric wheelchair, prescribed durable medical equipment, and instructed caregivers on skin care and positioning in the wheelchair and bed and, later, on activities of daily living training. Physical therapy included postural drainage positions for lung hygiene, increasing tolerance to the upright position, range-of-motion exercises, breathing exercises, low-load low-repetition gradual strengthening exercises, and, later, functional mobility training. Questions 1. The disease was confirmed to involve the peripheral nervous system. Did the loss of myelin involve oligoden-drocytes or Schwann cells? 2. How does loss of myelin along peripheral sensory fibers affect the propagation of action potentials in the affected axons? 3. Would loss of myelin in sensory neuron fibers impair the generation of local receptor potentials or the propaga-tion of action potentials?

CASE 1 1. 1. Loss of myelin in the peripheral nervous system involves destruction of Schwann cells. 1. 2. Loss of myelin lowers membrane resistance and allows leakage of ions across the membrane. Loss of myelin decreases the speed or blocks the propagation of action potentials. 1. 3. The propagation of action potentials is impaired because of leakage of ions across the membrane. Receptor potentials are not impaired, because nerve endings of the sensory neurons are not damaged.

Case 2 J.R. is a 27-year-old woman with MS who was admitted to the hospital twice in the past year with complaints of bilateral lower extremity weakness and blurred vision. Upon examination, she exhibited about 30% of normal muscle strength in the left lower extremity and about 50% of normal strength in the right lower extremity. She exhibited mild left foot drop during the swing phase of gait and slight knee hyperextension during the stance phase. At the hospital, visual evoked potentials were evaluated to assess nerve conduction velocity along the visual tracts. Evoked potentials are extracted from an electroencephalogram (EEG) recorded during repetitive presentation of a flash of light. The time from the stimulus to the appearance of the potential on the EEG indicates the central conduction time. For J.R., decreased visual sensory conduction times were determined. J.R. was referred to physical therapy for strengthening exercises and gait training with an ankle-foot orthosis. The physician's orders specified low-repetition exercises and avoidance of physical overexertion. Questions 1. Delayed conduction times for the evoked potentials suggest a problem with sensory conduction within the central nervous system. What nervous system abnormality can explain delayed sensory nerve conduction times? 2. What mechanism related to generation of the action potential may be directly impaired by increases in body temperature associated with overexertion?

CASE 2 1. 1. Destruction of oligodendrocytes causes demyelination of axons within the central nervous system. 1. 2. Increases in body temperature may alter the activity of voltage-gated Na+ channels, preventing the generation of an action potential.

Soma

Cell body; metabolic center of a cell.

Temporal summation

Cumulative effect of a series of either receptor potentials or synaptic potentials occurring within milliseconds of each other.

Spatial summation

Cumulative effect of receptor or synaptic potentials occurring simultaneously in different regions of the neuron.

Oligodendrocytes

D. Cells in the CNS that produce myelin

Ligand-gated

D. Neurotransmitter binding to the surface of a channel receptor on a postsynaptic membrane

1. Do dendritic projections function as input units or output units for a neuron?

Dendritic projections are input units for the neuron.

5. Define the terms depolarization and hyperpolarization with respect to resting membrane potential.

Depolarization occurs when the membrane potential becomes less negative with respect to the resting membrane potential. Hyperpolarization occurs when the membrane potential becomes more negative with respect to the resting membrane potential.

Resting membrane potential

Difference in electrical potential across the cell membrane of a neuron when the neuron is neither receiving nor transmitting information (i.e., the electrical state of a neuron's cell membrane when the cell is at rest [neither electrically excited nor inhibited]).

Multiple sclerosis

Disease characterized by random, multifocal demyelination limited to the CNS; signs and symptoms include numbness, paresthesia, Lhermitte's sign, asymmetric weakness, and/or ataxia.

Microglia

E. Cells that clean the neural environment and contribute to the destruction of injured or aging axons

Equilibrium potential

Electrical membrane potential where any diffusible ion is electrically and chemically distributed equally on the two sides of the membrane.

Hyperpolarization

Electrical state of a neuron's cell membrane when the membrane potential becomes more negative than its resting potential.

Neuron

Electrically excitable nerve cells of the nervous system.

Presynaptic terminal

End projection of an axon specialized for releasing neurotransmitters into the synaptic cleft.

9. Why is h yperpolarization of a neuronal membrane consid-ered inhibitory?

Hyperpolarization of a neuronal membrane is considered inhibitory, because hyperpolarization makes the neuron less likely to generate an electrical signal.

Nodes of Ranvier

Interruptions in the myelin sheath that leave small patches of axon unmyelinated; unmyelinated patches contain a high density of voltage-gated Na+ channels that contribute to the generation of action potentials.

Action potential

Large changes in the electrical potential of a neuron's cell membrane, resulting in the rapid spread of an electrical signal along the cell membrane.

Macroglia

Large support cells of the nervous system, including oligodendrocytes, Schwann cells, and astrocytes.

13. Do large-diameter or small-diameter axons promote faster conduction velocity of an action potential?

Large-diameter axons promote faster conduction velocity of an action potential.

Modulation

Long-lasting changes in the electrical potential of a neuron's cell membrane that alters the flow of ions across the cell membrane.

Schwann cells

Macroglia that form myelin sheaths enveloping only a single neuron's axon or partially surrounding several axons; found in the peripheral nervous system.

Oligodendrocytes

Macroglia that form myelin sheaths, enveloping several axons from several neurons; found in the CNS.

Astrocytes

Macroglia that play a critical role in nutritive and cleanup functions in the CNS.

Voltage-gated channels

Membrane ion channels that open in response to changes in electrical potential across a neuron's cell membrane.

Modality-gated channels

Membrane ion channels, specific to sensory neurons that open in response to mechanical forces (e.g., stretch, touch, pressure) or thermal or chemical changes.

Anterograde transport

Movement of proteins and neurotransmitters from the soma to the axon.

Retrograde transport

Movement of some substances from the axon back to the soma for recycling.

3. What is the specialized function of multipolar cells?

Multipolar cells are specialized to receive and accommodate huge amounts of synaptic input to their many dendrites.

16. Are networks composed of interneuronal convergence and divergence found throughout the central nervous system or only in the spinal cord?

Networks composed of interneuronal convergence and divergence are found throughout the central nervous system.

Pseudounipolar neuron

Neuron with two axonal roots. The two roots appear to leave the cell body as a single process, so the neuron is called pseudounipolar.

Ligand-gated channels

Neuronal membrane ion channels that open in response to the binding of a chemical neurotransmitter.

Multipolar cells

Neurons having multiple dendrites arising from many regions of the cell body and possessing a single axon.

Interneurons

Neurons that either process information locally or convey information short distances from one site in the nervous system to another.

19. T o what critical function do both oligodendrocytes and Schwann cells contribute in the nervous system?

Oligodendrocytes and Schwann cells form the myelin sheath around axons to promote the propagation of an action potential.

20. What are the differences between o ligodendrocytes and Schwann cells?

Oligodendrocytes are located in the central nervous system, and their processes completely wrap several axons from different neurons. Schwann cells partially or completely wrap a single axon in the peripheral nervous system. In addition to providing insulation, Schwann cells clean up the cellular environment and provide structural support to neurons.

10. Perip heral receptors have what types of ion channels?

Peripheral receptors have modality-gated channels.

Dendrites

Process that extends from the cell body of a neuron; conducts information toward the cell body.

Saltatory conduction

Rapid propagation of an action potential by jumping from one node of Ranvier to the next along a myelinated axon.

Myelin

Sheath of proteins and fats formed by oligodendrocytes and Schwann cells to envelop the axons of nerve cells; provides physical support and insulation for conduction of electrical signals by neurons.

Local potential

Small changes in the electrical potential of a neuron's cell membrane that is graded in both amplitude and duration.

Microglia

Small support cells of the nervous system.

4. What are the thr ee major ions that contribute to the elec-trical potential of a cell membrane in its resting state?

Sodium (Na+), potassium (K+), and chloride (Cl) contribute to the resting potential of the cell membrane.

Glia

Support cells of the nervous system, including oligodendrocytes, Schwann cells, astrocytes, and microglia.

11. List two types of local potential summation that can result in depolarization of a membrane to the threshold level.

Temporal summation and spatial summation of local potentials can bring the membrane to the threshold level.

12. The generation of an action potential requires the influx of what ion? Is the influx mediated by a voltage-gated channel?

The generation of an action potential requires the influx of Na+. This influx is mediated by a voltage-gated channel.

14. What are the unique features of the nodes of Ranvier that promote generation of an action potential?

The nodes of Ranvier have a high density of voltage-gated Na+ channels, which promote the generation of an action potential.

8. How is the resting membrane potential maintained?

The resting membrane potential is maintained by the unequal distribution of specific ions and charged molecules across the membrane. The unequal distribution is produced by the presence of negatively charged molecules inside the neuron that are too large to diffuse through membrane channels, by passive diffusion of ions through membrane channels, and by the Na+-K+ pump.

2. Name one example of a pseudounipolar cell. Why is it called pseudounipolar ?

The sensory neurons that convey information from the body to the spinal cord are pseudounipolar cells. They appear to have one process; however, one axon connects the periphery to the cell body, and a second axon connects the cell body to the spinal cord.

15. Names of tracts in the central nervous system identify the origin and termination of the tract. Where does the spino-thalamic tract originate? Where does this tract terminate?

The spinothalamic tract originates in the spinal cord and terminates in the thalamus.

22. How could naturally occurring stem cells in the brain assist recovery after brain injury?

The stem cells in the brain could be activated to produce new neurons that could migrate into the damaged area and begin to make new synapses.

7. What does the term graded mean with respect to the gen-eration of local receptor and synaptic potentials?

The term graded means that both the amplitude and the duration of the electrical potential can vary depending on the stimulus, and that it is not an all-or-none event

Refractory period

Time period during an action potential when no stimulus, no matter how strong, will elicit another action potential.