The Hindbrain and Midbrain

Objectives

When you have completed this section, you should be able to

• list the components of the hindbrain and midbrain and their functions;

• discuss the role of the cerebellum in movement and equilibrium;

• define the term brainstem and describe its anatomical relationship to the cerebellum and forebrain; and

• describe the location and functions of the reticular formation.

Our study of the brain is organized around the five secondary vesicles of the embryonic brain and their mature derivatives. We proceed in a caudal to rostral direction, beginning with the hindbrain and its relatively simple functions and progressing to the forebrain, the seat of such complex functions as thought, memory, and emotion.

The Medulla Oblongata

As noted earlier, the embryonic hindbrain differentiates into two subdivisions, the myelencephalon and meten-cephalon (see fig. 14.4). The myelencephalon develops into just one structure, the medulla oblongata (meh-DULL-uh OB-long-GAH-ta). The medulla is about 3 cm long and superficially looks like an extension of the spinal cord, but slightly wider. Significant differences are seen, however, on closer inspection of its gross and microscopic anatomy.

The anterior surface bears a pair of clublike ridges, the pyramids. Resembling two side-by-side baseball bats, the pyramids are wider at the rostral end, taper caudally, and are separated by an anterior median fissure continuous with that of the spinal cord (fig. 14.8). The pyramids

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Diencephalon

Thalamus Infundibulum

Cerebral peduncle Mammillary body

Pons

Medulla oblongata

Pyramid

Anterior median fissure Pyramidal decussation

Spinal cord (a)

Diencephalon

Thalamus Infundibulum

Cerebral peduncle Mammillary body

Pons

Medulla Oblongata Pons Mesencephalon

Diencephalon

Thalamus Pineal gland

Midbrain

Superior colliculus Inferior colliculus

Hindbrain

Fourth ventricle

Medulla oblongata ptic tract

Cranial nerves

Optic nerve (II) Oculomotor nerve (III) Trochlear nerve (IV) Trigeminal nerve (V) Abducens nerve (VI) Facial nerve (VII) Vestibulocochlear nerve (VIII) Glossopharyngeal nerve (IX) Vagus nerve (X) Accessory nerve (XI) Hypoglossal nerve (XII)

Spinal nerves

Colliculus Superior

Diencephalon

Thalamus Pineal gland

Midbrain

Superior colliculus Inferior colliculus

Hindbrain

Fourth ventricle

Medulla oblongata

Pineal Gland Midbrain

Figure 14.8 The Brainstem. (a) Ventral aspect; (b) right dorsolateral aspect. Some authorities do not include the diencephalon in the brainstem.

Lateral geniculate body Optic tract

Medial geniculate body Cerebral peduncle Pons

Superior cerebellar peduncle

Middle cerebellar peduncle

Inferior cerebellar peduncle

Olive

Cuneate fasciculus

Gracile fasciculus Spinal cord

Figure 14.8 The Brainstem. (a) Ventral aspect; (b) right dorsolateral aspect. Some authorities do not include the diencephalon in the brainstem.

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526 Part Three Integration and Control contain corticospinal tracts of nerve fibers that carry motor signals from the cerebrum to the spinal cord, ultimately to stimulate the skeletal muscles. Most of these motor nerve fibers decussate at a visible point near the caudal end of the pyramids. As a result, each side of the brain controls muscles on the contralateral side of the body. (This pertains only to muscles below the head.)

Lateral to each pyramid is an elevated area called the olive. It contains a wavy layer of gray matter, the inferior olivary nucleus, which is a center that receives information from many levels of the brain and spinal cord and relays it mainly to the cerebellum. Dorsally, the medulla exhibits ridges called the gracile fasciculus and cuneate fasciculus. These are continuations of the spinal cord tracts of the same names that carry sensory signals to the brain.

In addition to ascending and descending nerve tracts, the medulla contains sensory nuclei that receive input from the taste buds, pharynx, and viscera of the thoracic and abdominal cavities, and motor nuclei that control several primitive visceral and somatic functions. Some motor nuclei of the medulla discussed later in the book are:

• the cardiac center, which adjusts the rate and force of the heartbeat;

• the vasomotor center, which adjusts blood vessel diameter to regulate blood pressure and reroute blood from one part of the body to another; and

• two respiratory centers, which control the rate and depth of breathing.

Other nuclei of the medulla are concerned with speech, coughing, sneezing, salivation, swallowing, gagging, vomiting, gastrointestinal secretion, sweating, and movements of the tongue and head. Many of the medulla's sensory and motor functions are mediated through the last four cranial nerves, which begin or end here: cranial nerves IX (glossopharyngeal), X (vagus), XI (accessory), and XII (hypoglossal).

The Pons and Cerebellum

The embryonic metencephalon develops into two structures, the pons and cerebellum.

The Pons

The pons17 appears as an anterior bulge in the brainstem rostral to the medulla (fig. 14.8). Its white matter includes tracts that conduct signals from the cerebrum down to the cerebellum and medulla, and tracts that carry sensory signals up to the thalamus. Cranial nerve V (trigeminal) arises from the pons, and cranial nerves VI (abducens), VII (facial), and VIII (vestibulocochlear) arise from the junc-

17 pons = bridge tion of the pons and medulla. The pons contains nuclei that relay signals from the cerebrum to the cerebellum, and nuclei concerned with sleep, hearing, equilibrium, taste, eye movements, facial expressions, facial sensation, respiration, swallowing, bladder control, and posture.

The Cerebellum

The cerebellum is the largest part of the hindbrain (fig. 14.9). It consists of right and left cerebellar hemispheres connected by a narrow bridgelike vermis.18 Three pairs of stalklike cerebellar peduncles19 (peh-DUN-culs) connect the cerebellum to the brainstem: the inferior peduncles to the medulla oblongata, the middle peduncles to the pons, and the superior peduncles to the midbrain. These are composed of the nerve fibers that carry all signals between the cerebellum and the rest of the brain. The cerebellum receives most of its input from the pons, via the middle peduncles, but the spinocerebellar tracts enter through the inferior peduncles. Motor output leaves the cerebellum by way of the superior peduncles.

Each hemisphere exhibits slender, parallel folds called folia20 (gyri) separated by shallow sulci. The cerebellum has a surface cortex of gray matter and a deeper layer of white matter. In a sagittal section, the white matter, called the arbor vitae,21 exhibits a branching, fernlike pattern. Each hemisphere has four pairs of deep nuclei, masses of gray matter embedded in the white matter. All input to the cerebellum goes to the cortex and all of its output comes from the deep nuclei.

The cerebellum contains about 100 billion neurons. The most distinctive of these are the Purkinje22 (pur-KIN-jee) cells—unusually large, globose neurons with a tremendous profusion of dendrites (see fig. 12.5, p. 448), arranged in a single row in the cortex. Their axons travel to the deep nuclei, where they synapse on output neurons that issue fibers to the brainstem.

The cerebellum is mostly concerned with muscular coordination and will be discussed in a later section on motor control. Evidence is also emerging that the cerebellum plays a role in judging the passage of time, in some other cognitive processes (awareness, judgment, and memory), and in emotion.

The Midbrain

The embryonic mesencephalon develops into just one mature brain structure, the midbrain—a short segment of the brainstem that connects the hindbrain and fore-

18verm = worm

20foli = leaf

2,"tree of life"

22Johannes E. von Purkinje (1787-1869), Bohemian anatomist

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Superior colliculus Inferior colliculus -

Cerebral-

aqueduct

White matter (arbor vitae)

Gray matter

Cerebellar hemisphere

Arbor Vitae Anatomy

Pineal gland Posterior commissure

-Mammillary body

-Midbrain

Anterior

Posterior

Oculomotor nerve Fourth ventricle Pons

Medulla oblongata

Anterior

Medulla oblongata

The Hindbrain Pons

Posterior

Anterior lobe

Vermis Posterior lobe

Folia

Figure 14.9 The Cerebellum. (a) Midsagittal section, showing relationship to the brainstem; (b) superior aspect.

Anterior lobe

Vermis Posterior lobe

Folia

Figure 14.9 The Cerebellum. (a) Midsagittal section, showing relationship to the brainstem; (b) superior aspect.

brain (see figs. 14.1d, 14.2b, and 14.8). It contains the cerebral aqueduct and gives rise to two cranial nerves that control eye movements: cranial nerve III (oculomotor) and IV (trochlear). Some major regions of the mid-brain are (fig. 14.10):

• The cerebral peduncles, which help to anchor the cerebrum to the brainstem. The corticospinal tracts pass through the peduncles on their way to the medulla.

• The tegmentum,23 the main mass of the midbrain, located between the cerebral peduncles and cerebral aqueduct. It contains the red nucleus, which has a pink color in life because of its high density of blood vessels. Fibers from the red nucleus form the rubrospinal tract in most mammals, but in humans its connections go mainly to and from the cerebellum, with which it collaborates in fine motor control.

• The substantia nigra24 (sub-STAN-she-uh NY-gruh), a dark gray to black nucleus pigmented with melanin, located between the peduncles and the tegmentum. This is a motor center that relays inhibitory signals to the thalamus and basal nuclei (both of which are discussed later). Degeneration of the neurons in the substantia nigra leads to the muscle tremors of Parkinson disease (see insight 12.4, p. 475).

• The central (periaqueductal) gray matter, a large arrowhead-shaped region of gray matter surrounding the cerebral aqueduct. It is involved with the

24 substantia = substance + nigra = black

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Physiology: The Unity of Nerves Companies, 2003 Form and Function, Third Edition

528 Part Three Integration and Control reticulospinal tracts in controlling our awareness of pain (see chapter 16).

• The tectum,25 a rooflike region dorsal to the aqueduct. It consists of four nuclei, the corpora quadrigemina,26 which bulge from the midbrain roof. The two superior nuclei, called the superior colliculi27 (col-LIC-you-lye), function in visual attention, visually tracking moving objects, and such reflexes as turning the eyes and head in response to a visual stimulus, for example to look at something that you catch sight of in your peripheral vision. The two inferior colliculi receive afferent signals from the inner ear and relay them to other parts of the brain, especially the thalamus. Among other functions, they mediate the reflexive turning of the head in response to a sound.

• The medial lemniscus, a continuation of the gracile and cuneate tracts of the spinal cord and brainstem.

The reticular formation, discussed next, is a prominent feature of the midbrain but is not limited to this region.

25tectum = roof, cover

26corpora = bodies + quadrigemina = quadruplets

_Think About It_

Why are the inferior colliculi shown in figure 14.9 but not in figure 14.10? How are these two figures related?

The Reticular Formation

Running vertically through the core of the midbrain, pons, and medulla is a loosely organized core of gray matter called the reticular formation, composed of more than 100 small nuclei, including several of those already discussed, mingled with bundles of nerve fibers (fig. 14.11). The functions of the reticular formation include the following:

• Somatic motor control. Some motor neurons of the cerebral cortex send their axons to reticular formation nuclei, which then give rise to the reticulospinal tracts of the spinal cord. These tracts modulate (adjust) muscle contraction to maintain tone, balance, and posture. The reticular formation also relays signals from the eyes and ears to the cerebellum so the cerebellum can integrate visual, auditory, and vestibular (balance and motion) stimuli into its role in motor coordination. Other reticular formation motor nuclei include gaze centers, which

Reticular Formation Cerebellum

Superior colliculus

Cerebral aqueduct

Medial geniculate nucleus

Reticular formation Central gray matter Oculomotor nucleus Medial lemniscus Red nucleus Substantia nigra

Oculomotor nerve (III)

Cerebral peduncle

Anterior

Cerebral peduncle

Superior colliculus

Cerebral aqueduct

Medial geniculate nucleus

Reticular formation Central gray matter Oculomotor nucleus Medial lemniscus Red nucleus Substantia nigra

Oculomotor nerve (III)

Anterior t Figure 14.10 Cross Section of the Midbrain.

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Arbor Vitae Lateral View Brain
Figure 14.11 The Reticular Formation. The formation consists of over 100 nuclei scattered through the brainstem region indicated in red. Arrows represent the breadth of its projections to and from the cerebral cortex and other CNS regions.

enable the eyes to track and fixate on objects, and central pattern generators—neuronal pools that produce rhythmic signals to the muscles of breathing and swallowing.

• Cardiovascular control. The reticular formation includes the cardiac center and vasomotor center of the medulla oblongata.

• Pain modulation. The reticular formation is the origin of the descending analgesic pathways mentioned in the earlier description of the reticulospinal tracts.

• Sleep and consciousness. The reticular formation has projections to the cerebral cortex and thalamus that allow it some control over what sensory signals reach the cerebrum and come to our conscious attention. It plays a central role in states of consciousness such as alertness and sleep. Injury to the reticular formation can result in irreversible coma. General anesthetics work by blocking signal transmission through the reticular formation.

The reticular formation also is involved in habitua-tion—a process in which the brain learns to ignore repetitive, inconsequential stimuli while remaining sensitive to others. In a noisy city, for example, a person can sleep through traffic sounds but wake promptly to the sound of an alarm clock or a crying baby. Reticular formation nuclei that modulate activity of the cerebral cortex are called the reticular activating system or extrathalamic cortical modulatory system.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

8. Name the visceral functions controlled by nuclei of the medulla.

9. Describe the general functions of the cerebellum.

10. What are some functions of the midbrain nuclei?

11. Describe the reticular formation and list several of its functions.

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Responses

  • oona
    What runs vertically through the core of the midbrain, pons, and medulla?
    7 years ago
  • Bisirat
    Is the thalamus in the midbrain or the hindbrain?
    5 years ago
  • sinit
    Which hindbrain structure has a black with with pink in the middle?
    4 years ago

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