Human Physiology, 14th edition (2016)

170 Chapter 7

capillaries is covered by the astrocyte end-feet. In addition, astro-

cytes have other extensions adjacent to the synapses between the

axon terminal of one neuron and the dendrite or cell body of another

neuron. The astrocytes are thus ideally situated to influence the

interactions between neurons and between neurons and the blood.

Here are some of the proposed functions of astrocytes:

1. Astrocytes take up K 1 from the extracellular fluid. Because

K^1 diffuses out of neurons during the production of nerve

impulses (described in section 7.2), this function may be impor-

tant in maintaining the proper ionic environment for neurons.

2. Astrocytes take up some neurotransmitters released

from the axon terminals of neurons. For example, the

neurotransmitter glutamate (the major excitatory neu-

rotransmitter of the cerebral cortex) is taken into astrocytes

and transformed into glutamine ( fig. 7.10 ). The glutamine

is then released back to the neurons, which can use it to

reform the neurotransmitter glutamate. The glutamine from

astrocytes can also be used by other neurons to produce

GABA, the major inhibitory neurotransmitter in the brain.

3. The astrocyte end-feet surrounding blood capillaries take

up glucose from the blood. The glucose is metabolized into

lactic acid, or lactate ( fig. 7.10 ). The lactate is then released

and used as an energy source by neurons, which metabolize

it aerobically into CO 2 and H 2 O for the production of ATP.

Thus, PET scans and MRI (chapter 8, section 8.2), which

visualize brain locations by their metabolic activities, are

based on the functions of astrocytes as well as neurons.

4. Astrocytes release lactate, which aids neuron function.

Neurons can take in both glucose and lactate, but active

neurons appear to rely on lactate to sustain a high rate of

aerobic cell respiration. Astrocytes, but not most neurons,

can store glycogen and use it to produce lactate, which is

released and transported into neurons. Evidence suggests

Neurotrophins

The 1986 Nobel Prize in Physiology or Medicine was awarded
for the discovery of nerve growth factor ( NGF ), a regulatory
molecule produced by neurons that promotes the survival and
growth of sympathetic and sensory neurons in the developing
fetal brain. NGF was the first of the neurotrophins to be dis-
covered. Other neurotrophins were later characterized, including
brain-derived neurotrophic factor ( BDNF ), neurotrophin 3, and
neurotrophin 4. A related molecule, glial-derived neurotrophic
factor ( GDNF ), released by astrocytes, has also been found.
Neurotrophins have important functions in the adult nervous
system. NGF is required for the maintenance of sympathetic
ganglia, and neurotrophins are required for mature sensory neu-
rons to regenerate after injury. Similarly, GDNF promotes the
survival of dopaminergic neurons (those that use dopamine as
a neurotransmitter) and spinal motor neurons. Neurotrophins
regulate the survival and differentiation of adult neural stem cells
(chapter 3, section 3.3; and chapter 8, section 8.1) in parts of
the brain involved in learning and memory. This may be related
to the observation that neurotrophin synthesis is stimulated by
neural activity and appears to play roles in activity-dependent
synaptic changes in learning and memory. These roles include
the growth of dendrites and axons, the formation of synapses,
and synaptic changes during learning.

Functions of Astrocytes

Astrocytes (from the Greek aster 5  star) are large stellate cells with
numerous cytoplasmic processes that radiate outward ( fig.  7.10 ).
They are the most abundant of the glial cells in the CNS, constitut-
ing up to 90% of the nervous tissue in some areas of the brain.
Astrocytes have processes that terminate in end-feet surround-
ing the capillaries of the CNS; indeed, the entire surface of these

Figure 7.10 Astrocytes have processes that end on capillaries and neurons. Astrocyte end-feet take up glucose from
blood capillaries and use this to help supply energy substrates for neurons. Astrocytes also take up the neurotransmitter glutamate
from synapses and convert it to glutamine (Gln), which is then recycled to the neurons.

Astrocyte

End-feet

Capillary

Gln

Glutamate

Lactate

Axon

Postsynaptic

cell

Glucose