Essentials of Anatomy and Physiology

Yet another type of glial cell is the astrocyte(liter-
ally, “star cell”). In the embryo, these cells provide a
framework for the migrating neurons that will form
the brain. Thereafter, the extensions of astrocytes are
wrapped around brain capillaries and contribute to the
blood–brain barrier, which prevents potentially
harmful waste products in the blood from diffusing
out into brain tissue. These waste products are normal
in the blood and tissue fluid, but brain tissue is much

more sensitive to even low levels of them than are

other tissues such as muscle tissue or connective tis-

sue. The capillaries of the brain also contribute to this

barrier, because they are less permeable than are other

capillaries. A disadvantage of the blood–brain barrier

is that some useful medications cannot cross it, and

the antibodies produced by lymphocytes cross only

with difficulty. This becomes an important considera-

tion when treating brain infections or other diseases

The Nervous System 167

Afferent (sensory) neuron

Axon terminal

Axon

Nucleus

Cell body

Functional dendrite

Myelin sheath

Receptors

Dendrites

Nucleus

Axon terminal

Efferent (motor) neuron

Cell body

Axon

Schwann cell nucleus

Myelin sheath

Node of

Ranvier

A B

Schwann cell

Axon

Neurolemma

Layers of myelin sheath

C

Figure 8–1. Neuron struc-
ture. (A) A typical sensory neu-
ron. (B) A typical motor neuron.
The arrows indicate the direc-
tion of impulse transmission.
(C) Details of the myelin sheath
and neurolemma formed by
Schwann cells.
QUESTION:The axon terminal
of the motor neuron would be
found at what kinds of effec-
tors?