Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 4Excitable Tissue: Nerve 91

actually depolarize neurons. BDNF-deficient mice lose
peripheral sensory neurons and have severe degenerative
changes in their vestibular ganglia and blunted long-term
potentiation.

OTHER FACTORS AFFECTING

NEURONAL GROWTH

The regulation of neuronal growth is a complex process.
Schwann cells and astrocytes produce ciliary neurotrophic
factor (CNTF). This factor promotes the survival of damaged
and embryonic spinal cord neurons and may prove to be of
value in treating human diseases in which motor neurons de-
generate. Glial cell line-derived neurotrophic factor (GD-
NF) maintains midbrain dopaminergic neurons in vitro.
However, GDNF knockouts have dopaminergic neurons that
appear normal, but they have no kidneys and fail to develop an
enteric nervous system. Another factor that enhances the
growth of neurons is leukemia inhibitory factor (LIF). In ad-
dition, neurons as well as other cells respond to insulinlike
growth factor I (IGF-I) and the various forms of transform-
ing growth factor (TGF), fibroblast growth factor (FGF),
and platelet-derived growth factor (PDGF).
Clinical Box 4–2 compares the ability to regenerate neurons
after central and peripheral nerve injury.

CHAPTER SUMMARY

■ There are two main types of microglia and macroglia. Microglia

are scavenger cells. Macroglia include oligodendrocytes,

Schwann cells, and astrocytes. The first two are involved in my-

elin formation; astrocytes produce substances that are tropic to

neurons, and they help maintain the appropriate concentration

of ions and neurotransmitters.

■ Neurons are composed of a cell body (soma) which is the meta-

bolic center of the neuron, dendrites that extend outward from

the cell body and arborize extensively, and a long fibrous axon

that originates from a somewhat thickened area of the cell body,

the axon hillock.

■ The axons of many neurons acquire a sheath of myelin, a pro-

tein–lipid complex that is wrapped around the axon. Myelin is

an effective insulator, and depolarization in myelinated axons

jumps from one node of Ranvier to the next, with the current

sink at the active node serving to electrotonically depolarize to

the firing level the node ahead of the action potential.

■ Orthograde transport occurs along microtubules that run the

length of the axon and requires molecular motors, dynein, and

kinesin.

■ Two types of physicochemical disturbances occur in neurons:

local, nonpropagated potentials (synaptic, generator, or electro-

tonic potentials) and propagated potentials (action potentials).

■ In response to a depolarizing stimulus, voltage-gated Na+ chan-

nels become active, and when the threshold potential is reached,

an action potential results. The membrane potential moves to-

ward the equilibrium potential for Na+. The Na+ channels rap-

idly enter a closed state (inactivated state) before returning to

the resting state. The direction of the electrical gradient for Na+

is reversed during the overshoot because the membrane poten-

tial is reversed, and this limits Na+ influx. Voltage-gated K+

channels open and the net movement of positive charge out of

the cell helps complete the process of repolarization. The slow

return of the K+ channels to the closed state explains after-

hyperpolarization, followed by a return to the resting mem-

brane potential.

■ Nerve fibers are divided into different categories based on ax-

onal diameter, conduction velocity, and function.

■ Neurotrophins are produced by astrocytes and transported by

retrograde transport to the neuronal cell body, where they foster

the production of proteins associated with neuronal develop-

ment, growth, and survival.

MULTIPLE-CHOICE QUESTIONS

For all questions, select the single best answer unless otherwise directed.

1. The distance from between one stimulating electrode to record-
   ing electrode is 4.5 cm. When the axon is stimulated, the latent
   period is 1.5 ms. What is the conduction velocity of the axon?
   A) 15 m/s
   B) 30 m/s
   C) 40 m/s
   D) 67.5 m/s
   E) This cannot be determined from the information given.


2. Which of the following has the slowest conduction velocity?
   A) Aα fibers
   B) Aβ fibers
   C) Aγ fibers
   D) B fibers
   E) C fibers


3. A man falls into a deep sleep with one arm under his head. This
   arm is paralyzed when he awakens, but it tingles, and pain sensa-
   tion in it is still intact. The reason for the loss of motor function
   without loss of pain sensation is that in the nerves to his arm,
   A) A fibers are more susceptible to hypoxia than B fibers.
   B) A fibers are more sensitive to pressure than C fibers.
   C) C fibers are more sensitive to pressure than A fibers.
   D) motor nerves are more affected by sleep than sensory nerves.
   E) sensory nerves are nearer the bone than motor nerves and
   hence are less affected by pressure.


4. Which part of a neuron has the highest concentration of Na+
   channels per square millimeter of cell membrane?
   A) dendrites
   B) cell body near dendrites
   C) initial segment
   D) axonal membrane under myelin
   E) none of the above


5. Which of the following statements about nerve growth factor is
   not true?
   A) It is made up of three polypeptide subunits.
   B) It facilitates the process of apoptosis.
   C) It is necessary for the growth and development of the sym-
   pathetic nervous system.
   D) It is picked up by nerves from the organs they innervate.
   E) It is present in the brain.