Human Physiology, 14th edition (2016)

228 Chapter 8

8.4 Midbrain and Hindbrain

The midbrain and hindbrain contain many relay centers

for sensory and motor pathways, and are particularly

important in the brain’s control of skeletal movements.

The medulla oblongata contains centers for the control of

breathing and cardiovascular function.

detect light passing through their skulls. In mammals, however,
the daily cycles of light and darkness influence the SCN by way
of tracts from the retina (the neural layer of the eyes) to the
hypothalamus (see chapter 11, fig. 11.33). These retinohypo-
thalamic tracts are activated not by the photoreceptors involved
in vision (the rods and cones), but rather by a population of
retinal ganglion cells that contain their own light-sensitive pig-
ment, melanopsin. These photosensitive ganglion cells in the
retina act, via the retinohypothalamic tracts, to entrain the cir-
cadian clocks of the SCN to daily cycles of light and darkness.
They are also responsibile for the pupillary reflex constriction
in response to light (chapter 10; see fig. 10.28).
Scientists have discovered circadian clock genes in neurons
of the SCN and other areas of the brain, as well as in the cells
of the heart, liver, kidneys, skeletal muscles, adipose tissue, and
other organs. The clock genes are transcribed into mRNA, which
are then translated into protein like other genes. However, there
appears to be complex networks of negative feedback loops that
suppress clock gene transcription after a delay, resulting in cir-
cadian oscillations of gene activity. Although the “peripheral
clocks” (the clocks outside of the SCN) have daily cycles of
activity, they would not be synchronized with other peripheral
clocks or with the environmental light/dark cycle without the
influence of the suprachiasmatic nuclei.
The SCN receive photic (light) information from the retino-
hypothalamic tracts and have neural outputs to other nuclei of the
hypothalamus, as well as to the thalamus, arcuate nucleus, amyg-
dala, and other brain regions. By means of these neural outputs,
the SCN influence circadian rhythms of body temperature, feed-
ing, locomotor activity (movements), the autonomic nervous sys-
tem, and the secretions of endocrine glands. Through autonomic
nerves, the SCN can regulate circadian rhythms in the liver and
other visceral organs. By indirectly influencing the secretions of
the anterior pituitary, the SCN entrains the adrenal glands to pro-
duce circadian rhythms in the secretion of cortisol.
The secretion of melatonin from the pineal gland is high-
est at night because of regulation by the SCN via sympathetic
nerves (see fig. 11.33). Melatonin is a major regulator of cir-
cadian rhythms, as discussed in chapter 11, section 11.6. For
example, the presence of melatonin receptors in the pancreatic
islets suggests that melatonin may influence the secretion of
insulin, which likewise follows a circadian rhythm. Also, mel-
atonin’s ability to promote relaxation of vascular smooth mus-
cles may contribute to the circadian rhythms of blood pressure.

| CHECKPOINT

7a. List the functions of the hypothalamus and indicate

the other brain regions that cooperate with the

hypothalamus in the performance of these functions.

7b. Explain the structural and functional relationships

between the hypothalamus and the pituitary gland.

LEARNING OUTCOMES

After studying this section, you should be able to:

8. Identify the structures and functions of the midbrain
   and hindbrain.


9. Describe the structure and function of the reticular
   activating system.

Midbrain

The mesencephalon, or midbrain, is located between the dien-

cephalon and the pons. The corpora quadrigemina are four

rounded elevations on the dorsal surface of the midbrain (see

fig.  8.19 ). The two upper mounds, the superior colliculi, are

involved in visual reflexes; the inferior colliculi, immediately

below, are relay centers for auditory information.

The midbrain also contains the cerebral peduncles, red

nucleus, substantia nigra, and other nuclei. The cerebral pedun-

cles are a pair of structures composed of ascending and descending

fiber tracts. The red nucleus, an area of gray matter deep in the

midbrain, maintains connections with the cerebrum and cerebel-

lum and is involved in motor coordination.

The midbrain has two systems of dopaminergic (dopamine-

releasing) neurons that project to other areas of the brain

(chapter 7, section 7.5). The nigrostriatal system projects from

the substantia nigra to the corpus striatum of the basal nuclei;

this system is required for motor coordination, and it is the

degeneration of these fibers that produces Parkinson’s disease.

Other dopaminergic neurons in the ventral tegmental area

(VTA) of the midbrain, adjacent to the substantia nigra, are

part of the mesolimbic system that projects dopaminergic input

to the limbic system of the forebrain ( fig. 8.21 ). This system is

involved in behavioral reward (reinforcing goal-directed behav-

ior), and has been implicated in drug addiction and psychiatric

disturbances.

The best-studied pathway of the brain’s reward system

is the dopaminergic neurons that project from the VTA to the

nucleus accumbens ( fig.  8.21 ). The nucleus accumbens is a

collection of neurons in each hemisphere that is located where

the head of the caudate nucleus meets the anterior putamen.

Though part of the ventral striatum, it can also be considered

part of the limbic system because of its function in emotional

reward. However, dopaminergic neurons from the VTA also

project to the prefrontal cortex, amygdala, hippocampus, and

other brain areas that have reciprocal neural interactions with

one another and participate in the reward system.