Chapter 11 Nervous System and Nervous Tissue

Levels of organization in the nervous system
· Central nervous system (CNS)
· Peripheral nervous system (PNS)
o Sensory (afferent) division
o Motor (efferent) division
1. Somatic nervous system
2. Autonomic nervous system (ANS)
– Sympathetic division
– Parasympathetic division
Which of the neuroglial cell types is the most abundant in the CNS?
astrocyte
Where in the neuron is an action potential initially generated?
axon hillock
The depolarization phase of an action potential results from the opening of which channels?
voltage gated Na+ channels
The repolarization phase of an action potential results from
the opening of voltage-gated K+ channels
Hyperpolarization results from
slow closing of voltage-gated K+ channels
What is the magnitude (amplitude) of an action potential?
100 mV
The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.
K+, Na+
During depolarization, which gradient(s) move(s) Na+ into the cell?
both the electrical and chemical gradients
What is the value for the resting membrane potential for most neurons?
-70 mV
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?
Na+ is pumped out of the cell and K+ is pumped into the cell
The concentrations of which two ions are highest outside the cell?
Na+ and Cl-
Local anesthetics block voltage-gated Na+ channels, but they do not block mechanically gated ion channels. Sensory receptors for touch (and pressure) respond to physical deformation of the receptors, resulting in the opening of specific mechanically gated ion channels. Why does injection of a local anesthetic into a finger still cause a loss of the sensation of touch from the finger?
Touch stimulation of this sensory receptor will open the mechanically gated ion channels, but action potentials are still not initiated because propagation of an action potential requires the opening of voltage-gated Na+ channels.
Steps in transferring across chemical synapses
1. An action potential arrives at the synaptic terminal.
2. Calcium channels open, and calcium ions enter the synaptic terminal.
3. Vesicles containing neurotransmitter fuse with the plasma membrane of the sending neuron.
4. Neurotransmitter molecules diffuse across the synaptic cleft.
5. The neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron causing ion channels there to open.
The small space between the sending neuron and the receiving neuron is the _______.
synaptic cleft
A molecule that carries information across a synaptic cleft is a ____________.
neurotransmitter
When calcium ions enter the synaptic terminal,
They cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron.
When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron,
ion channels in the plasma membrane of the receiving neuron open.
If a signal from a sending neuron makes the receiving neuron more negative inside,
the receiving neuron is less likely to generate an action potential.
Which membrane potential occurs because of the influx of Na+ through chemically gated channels in the receptive region of a neuron?
excitatory postsynaptic potential
Which neurotransmitter(s) is/are the body’s natural pain killer?
endorphins
Which neuron circuit pattern is involved in the control of rhythmic activities such as breathing?
reverberating circuit
What component of the reflex arc determines the response to a stimulus?
integration center
Which of the following allows us to consciously control our skeletal muscles?
the somatic nervous system
3 nervous system functions
1. Sensory input
2. Integration
3. Motor output
Which of the neuroglial cell types shown control the flow of cerebrospinal fluid within the CNS?
ependymal cells
Which of the neuroglial cell types shown form myelin sheaths within the CNS?
Oligodendrocytes
Which of the neuroglial cell types shown are found in the PNS?
satellite cell
Which of the following types of glial cells monitors the health of neurons, and can transform into a special type of macrophage to protect endangered neurons?
microglia
Which of the following peripheral nervous system (PNS) neuroglia form the myelin sheaths around larger nerve fibers in the PNS?
Schwann cells
Which of the following are bundles of neurofilaments that are important in maintaining the shape and integrity of neurons?
neurofibrils
Which of the following is true of axons?
A neuron can have only one axon, but the axon may have occasional branches along its length.
Which of the following is the conducting region of the neuron?
axon
Which criterion is used to functionally classify neurons?
the direction in which the nerve impulse travels relative to the central nervous system
Which of the following is NOT a functional classification of neurons?
multipolar
Which of the following is NOT true of association neurons?
Most association neurons are confined within the peripheral nervous system (PNS).
Neurons are also called nerve cells. True or False
True
Unmyelinated fibers conduct impulses faster than myelinated fibers. True or False
False; myelinated fibers conduct impulses faster than unmyelinated fibers.
Which neuroglia are the most abundant and versatile of the glial cells?
astrocyte
Which part of the neuron is responsible for generating a nerve impulse?
axon
Which of the following types of neurons carry impulses away from the central nervous system (CNS)?
motor
Where do most action potentials originate?
initial segment
What opens first in response to a threshold stimulus?
voltage gated Na+ channels
What characterizes depolarization, the first phase of the action potential?
The membrane potential changes from a negative value to a positive value.
What characterizes repolarization, the second phase of the action potential?
Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV.
What event triggers the generation of an action potential?
The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.
What is the first change to occur in response to a threshold stimulus?
Voltage-gated Na+ channels change shape, and their activation gates open.
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?
Resting membrane potential (RMP)
Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?
Leak channels
On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you?
The inside surface of the plasma membrane is much more negatively charged than the outside surface.
The plasma membrane is much more permeable to K+ than to Na+. Why?
There are many more K+ leak channels than Na+ leak channels in the plasma membrane.
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.
The presence of concentration gradients and leak channels
What prevents the Na+ and K+ gradients from dissipating?
Na+-K+ ATPase
Arrange these parts in order, from left to right, of a successful direct depolarization path within one neuron.

axon, axon hillock, cell body, dendrite, presynaptic terminal

dendrite, cell body, axon hillock, axon, presynaptic terminal
Which of the following best characterizes depolarization?
small consecutive steps of Na+ penetration into the axon along its length
When an action potential arrives at the end of the axon terminal, a series of events take place that result in the release of neurotransmitter from the presynaptic axon. Select the answer that correctly describes the primary stimulus for vesicles to move towards the cell membrane and eventually release their contents.
voltage-gated membrane channels open, and Ca+2 enters the cytoplasm, increasing intracellular calcium
Which statement best describes exocytosis?
Membrane organelles fuse with the membrane and release contents out of the cell.
What conditions will increase the diffusion of molecules, such as neurotransmitters?
An increase in the amount of neurotransmitter exocytized by the presynaptic axon.
If the membrane of a postsynaptic dendrite is setting up a graded potential, what must have happened after neurotransmitter was released by the presynaptic terminal?
The neurotransmitter bound at postsynaptic receptors to open postsynaptic ion channels
Sequence at the chemical synapse
1. Action potential sweeps down presynaptic axon.
2. Calcium channels open in axon terminal.
3. Synaptic vesicles fuse and exocytize neurotransmitter.
4. Diffusino of neurotransmitter into extracellular fluid separating two neuron’s membranes.
5. Graded potential at postsynaptic membrane
Which best represents synaptic transmission?
presynaptic axon to synapse to dendrite or postsynaptic cell body
Predict the possible effect of a drug that totally blocks the neurotransmitter receptor on the postsynaptic membrane.

For example, curare is a neurotoxin used by several South American cultures. The primary effect of curare is that acetylcholine, a major
neuromuscular neurotransmitter, cannot bind at its receptor because curare is blocking it. Predict the possible effects of curare on the
postsynaptic membrane and muscle.

Local graded potentials and action potential transmission is blocked and there is no
response by the postsynaptic cell, the muscle.
A postsynaptic cell can be a neuron, a muscle cell, or a secretory cell. What is an example of a presynaptic cell?
a neuron
Which component has a role in the postsynaptic cell during synaptic activity?
chemically gated channels
What is the role of calcium in synaptic activity?
Calcium influx into the synaptic terminal causes vesicle fusion.
What is the role of neurotransmitter at a chemical synapse?
Neurotransmitter binds to receptors on the postsynaptic cell membrane and allows ions to diffuse across the membrane.
Neurotransmitter is released from presynaptic neurons through what mechanism?
exocytosis
What type of channel on the postsynaptic membrane binds neurotransmitter?
a chemically gated channel
In addition to diffusion, what are two other mechanisms that terminate neurotransmitter activity?
reuptake and degradation
Events that occur during synaptic activity are listed here, but they are arranged in an incorrect order. Choose the correct order of these events below.

(a) Voltage-gated calcium channels open
(b) Neurotransmitter binds to receptors
(c) Action potential arrives at axon terminal (d) Neurotransmitter is removed from the
synaptic cleft
(e) Neurotransmitter released into synaptic cleft
(f) Graded potential generated in postsynaptic cell

(c) Action potential arrives at axon terminal
(a) Voltage-gated calcium channels open
(e) Neurotransmitter released into synaptic cleft
(b) Neurotransmitter binds to receptors
(f) Graded potential generated in
postsynaptic cell
(d) Neurotransmitter is removed from the synaptic cleft
In a synapse, neurotransmitters are stored in vesicles located in the __________.
presynaptic neuron
An action potential releases neurotransmitter from a neuron by opening which of the following channels?
voltage-gated Ca2+ channels
Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.
chemically gated; postsynaptic
Binding of the neurotransmitter to its receptor causes the membrane to __________.
chemically gated; postsynaptic
The mechanism by which the neurotransmitter is returned to a presynaptic neuron’s axon terminal is specific for each neurotransmitter. Which of the following neurotransmitters is broken down by an enzyme before being returned?
acetylcholine