Chapter 11 Nervous System and Nervous Tissue

Where in the neuron is an action potential initially generated?

axon hillock

Binding of the neurotransmitter to its receptor causes the membrane to __________.

chemically gated; postsynaptic

A molecule that carries information across a synaptic cleft is a ____________.

neurotransmitter

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

K+, Na+

What prevents the Na+ and K+ gradients from dissipating?

Na+-K+ ATPase

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.

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)

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.

During depolarization, which gradient(s) move(s) Na+ into the cell?

both the electrical and chemical gradients

Which component has a role in the postsynaptic cell during synaptic activity?

chemically gated channels

Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.

chemically gated; postsynaptic

Which neurotransmitter(s) is/are the body's natural pain killer?

endorphins

Hyperpolarization results from

slow closing of voltage-gated K+ channels

The small space between the sending neuron and the receiving neuron is the _______.

synaptic cleft

What component of the reflex arc determines the response to a stimulus?

integration center

Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?

Leak 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

Unmyelinated fibers conduct impulses faster than myelinated fibers. True or False

False; myelinated fibers conduct impulses faster than unmyelinated fibers.

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 best characterizes depolarization?

small consecutive steps of Na+ penetration into the axon along its length

Which statement best describes exocytosis?

Membrane organelles fuse with the membrane and release contents out of the cell.

Neurons are also called nerve cells. True or False

True

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

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.

What conditions will increase the diffusion of molecules, such as neurotransmitters?

An increase in the amount of neurotransmitter exocytized by the presynaptic axon.

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.

What type of channel on the postsynaptic membrane binds neurotransmitter?

a chemically gated channel

A postsynaptic cell can be a neuron, a muscle cell, or a secretory cell. What is an example of a presynaptic cell?

a neuron

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

Which of the following is the conducting region of the neuron?

axon

Which part of the neuron is responsible for generating a nerve impulse?

axon

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 neuroglial cell types shown control the flow of cerebrospinal fluid within the CNS?

ependymal cells

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 is NOT a functional classification of neurons?

multipolar

Which best represents synaptic transmission?

presynaptic axon to synapse to dendrite or postsynaptic cell body

In addition to diffusion, what are two other mechanisms that terminate neurotransmitter activity?

reuptake and degradation

An action potential releases neurotransmitter from a neuron by opening which of the following channels?

voltage-gated Ca2+ channels

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

-70 mV

3 nervous system functions

1. Sensory input 2. Integration 3. Motor output

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.

Which of the following is NOT true of association neurons?

Most association neurons are confined within the peripheral nervous system (PNS).

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

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

Oligodendrocytes

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

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 is the first change to occur in response to a threshold stimulus?

Voltage-gated Na+ channels change shape, and their activation gates open.

Which membrane potential occurs because of the influx of Na+ through chemically gated channels in the receptive region of a neuron?

excitatory postsynaptic potential

Neurotransmitter is released from presynaptic neurons through what mechanism?

exocytosis

Where do most action potentials originate?

initial segment

In a synapse, neurotransmitters are stored in vesicles located in the __________.

presynaptic neuron

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.

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

The concentrations of which two ions are highest outside the cell?

Na+ and Cl-

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 characterizes depolarization, the first phase of the action potential?

The membrane potential changes from a negative value to a positive value.

When calcium ions enter the synaptic terminal,

They cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron.

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.

What is the magnitude (amplitude) of an action potential?

100 mV

What is the role of calcium in synaptic activity?

Calcium influx into the synaptic terminal causes vesicle fusion.

Which neuroglia are the most abundant and versatile of the glial cells?

astrocyte

Which of the neuroglial cell types is the most abundant in the CNS?

astrocyte

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.

Which of the following types of neurons carry impulses away from the central nervous system (CNS)?

motor

Which of the following are bundles of neurofilaments that are important in maintaining the shape and integrity of neurons?

neurofibrils

Which neuron circuit pattern is involved in the control of rhythmic activities such as breathing?

reverberating circuit

Which of the neuroglial cell types shown are found in the PNS?

satellite cell

Which criterion is used to functionally classify neurons?

the direction in which the nerve impulse travels relative to the central nervous system

The repolarization phase of an action potential results from

the opening of voltage-gated K+ channels

Which of the following allows us to consciously control our skeletal muscles?

the somatic nervous system

The depolarization phase of an action potential results from the opening of which channels?

voltage gated Na+ channels

What opens first in response to a threshold stimulus?

voltage gated Na+ channels

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