Chapter 12 Review Questions

In the CNS, a neuron typically receives information from other neurons at its

dendrites

Regulation by the nervous system provides

swift, but brief, responses to stimuli

Depolarization of a neuron plasma membrane will shift the membrane potential toward

0 mV

In multiple sclerosis, there is intermittent and progressive damage to the myelin sheath of peripheral nerves. This results in poor motor control of the affected area. Why does destruction of the myelin sheath affect motor control?

Action potentials travel faster along fibers that are myelinated than fibers that are nonmyelinated. Destruction of the myelin sheath increases the time it takes for motor neurons to communicate with their effector muscles. This delay in response results in varying degrees of uncoordinated muscle activity. The situation is very similar to that of a newborn, who cannot control its arms and legs very well because the myelin sheaths are still being laid down. Since not all motor neurons to the same muscle may be demyelinated to the same degree, there would be some fibers that are slow to respond while others are responding normally, producing contractions that are erratic and poorly controlled.

What is the difference between anterograde flow and retrograde flow?

Anterograde flow is the movement of materials from the cell body to the synaptic terminals. Retrograde flow is the movement of materials toward the cell body.

Harry has a kidney condition that causes changes in his body's electrolyte levels. As a result, he is exhibiting tachycardia, an abnormally fast heart rate. Which ion is involved, and how does a change in its concentration cause Harry's symptoms?

Harry's kidney condition is causing the retention of potassium ions. As a result, the K+ concentration of the extracellular fluid is higher than normal. Under these conditions, less potassium diffuses from heart muscle cells than normal, resulting in a resting potential that is less negative(more positive). This change in resting potential moves the transmembrane potential closer to threshold, so it is easier to stimulate the muscle. The ease of stimulation accounts for the increased number of contractions evident in the rapid heart rate.

What is meant by saltatory propagation? How does it differ from continuous propagation?

In saltatory propagation, which occurs in myelinated axons, only the nodes along the axon can respond to a depolarizing stimulus. In continuous propagation, which occurs in unmyelinated axons, an action potential appears to move across the membrane surface in a series of tiny steps.

Which two types of neuroglia insulate neuron cell bodies and axons in the PNS from their surroundings?

Neuroglia in the PNS are 1) satellite cells and 2) Schwann cells

Why can't most neurons in the CNS be replaced when they are lost to injury or disease?

Neurons lack centrioles and therefore cannot divide and replace themselves

What is the difference between temporal summation and spatial summation?

Temporal summation is the addition of stimuli that arrive at a single synapse in rapid succession. Spatial summation occurs when simultaneous stimuli at multiple synapses have a cumulative effect on the transmembrane potential.

What factor determines the maximum frequency of action potentials that could be conducted by an axon?

The absolute refractory period limits the number of action potentials that can travel along an axon in a given unit of time. During the absolute refractory period, the membrane cannot conduct an action potential, so a new depolarization event cannot occur until after the absolute refractory period has passed. If the absolute refractory period for a particular axon is 0.001 sec, then the maximum frequency of action potentials conducted by this axon would be 1000/sec.

State the all-or-none principle of action potentials.

The all-or-none principle of action potentials states that any depolarization event sufficient to reach threshold will cause an action potential of the same strength, regardless of the amount of stimulation above threshold.

Describe the steps involved in the generation of an action potential.

The membrane depolarizes to threshold. Next, voltage-gated sodium channels are activated, and the membrane rapidly depolarizes. These sodium channels are then inactivated, and potassium channels are activated. Finally, normal permeability returns. The voltage-gated sodium channels become activated once the repolarization is complete; the voltage-gated potassium channels begin closing as the transmembrane potential reaches the normal resting potential.

Twenty neurons synapse with a single receptor neuron. Fifteen of the 20 neurons release neurotransmitters that produce EPSPs at the postsynaptic membrane, and the other five release neurotransmitters that produce IPSPs. Each time one of the neurons is stimulated, it releases enough neurotransmitter to produce a 2-mV change in potential at eh postsynaptic membrane. If the threshold of the postsynaptic neuron is 10 mV, how many of the excitatory neurons must be stimulated to produce an action potential in the receptor neuron if all five inhibitory neurons are stimulated?

To reach threshold, the postsynaptic membrane must receive enough neurotransmitter to produce an EPSP of +20 mV(1= 10 mV to reach threshold and 1=10 mV to cancel the IPSPs produced by the five inhibitory neurons). Each neuron releases enough neurotransmitter to produce a change of +2 mV, so at least 10 of the 15 excitatory neurons must be stimulated to produce this effect by spatial summation.

What are the structural and functional differences among type A, B, and C fibers?

Type A fibers are myelinated and carry action potentials very quickly (120 m/sec). Type B are also myelinated, but carry action potentials more slowly due to their smaller diameter. Type C fibers are extremely slow due to their small diameter and lack of myelination.

What is the functional difference among voltage-gated, chemically gated, and mechanically gated channels?

Voltage-gated channels open or close in response to changes in the transmembrane potential. Chemically gated channels open or close when they bind specific extracellular chemicals. Mechanically gated channels open or close in response to physical distortion of the membrane surface.

Receptors that bind acetylcholine at the postsynaptic membrane are

chemically gated channels

The neural cells responsible for the analysis of sensory inputs and coordination of motor outputs are

interneurons

If the resting membrane potential is -70 mV and the threshold is -55 mV, a membrane potential of -60 mV will

make it easier to produce an action potential

Phagocytic cells in neural tissue of the CNS are

oligodendrocytes

What three functional groups of neurons are found in the nervous system? What is the function of each type of neuron?

sensory neurons: transmit impulses from the PNS to the CNS motor neurons: transmit impulses from the CNS to peripheral effectors interneurons: analyze sensory inputs and coordinate motor outputs

What are the major components of (a) the central nervous system? (b) the peripheral nervous system?

the CNS: brain and spinal cord the PNS: all other nerve fibers, divided between the efferent division (which consists of the somatic nervous system and the autonomic nervous system) and the afferent division (which consists of receptors and sensory neurons)

Describe the events that occur during nerve impulse transmission at a typical cholinergic synapse.

the action potential arrives at the synaptic terminal, depolarizing it extracellular calcium enters the synaptic terminal, triggering the exocytosis of ACh ACh binds to the postsynaptic membrane and depolarizes the next neuron in the chain ACh is removed by AChE

What factor determines the direction that ions will move through an open membrane channel?

the electrochemical gradient