Hindbrain

The rhombencephalon, or hindbrain, is composed of two regions: the metencephalon and the myelencephalon. Each of these regions will be discussed separately.

Metencephalon

The metencephalon is composed of the pons and the cerebellum. The pons can be seen as a rounded bulge on the underside of the brain, between the midbrain and the medulla oblongata (fig. 8.19). Surface fibers in the pons connect to the cerebellum, and deeper fibers are part of motor and sensory tracts that pass from the medulla oblongata, through the pons, and on to the midbrain. Within the pons are several nuclei associated with specific cranial nerves—the trigeminal (V), abducens (VI), facial (VII), and vestibulocochlear (VIII). Other nuclei of the pons cooperate with nuclei in the medulla oblongata to regulate breathing. The two respiratory control centers in the pons are known as the apneustic and the pneumotaxic centers.

The cerebellum, containing over a hundred billion neurons, is the second largest structure of the brain. Like the cerebrum, it contains outer gray and inner white matter. Fibers from the cerebellum pass through the red nucleus to the thalamus, and then to the motor areas of the cerebral cortex. Other fiber tracts connect the cerebellum with the pons, medulla oblongata, and spinal cord. The cerebellum receives input from proprioceptors (joint, tendon, and muscle receptors) and, working together with the basal nuclei and motor areas of the cerebral cortex, participates in the coordination of movement.

The cerebellum is needed for motor learning and for coordinating the movement of different joints during a movement. It is also required for the proper timing and force required for limb movements. The cerebellum, for example, is needed in order to touch your nose with your finger, bring a fork of food to your mouth, or find keys by touch in your pocket or purse.

The Central Nervous System

Caudate -

nucleus

Putamen

Caudate nucleus

(tail) Corpus

Ventral , tegmental area

Prefrontal cortex

Putamen

Caudate nucleus

(tail) Corpus

Ventral , tegmental area

Caudate -

nucleus

Prefrontal cortex

Corpus Striatum And Substantia Nigra
Pons

Corpus striatum

Mesolimbic dopamine system

Nigrostriatal dopamine system

■ Figure 8.18 Dopaminergic pathways in the brain. Axons that use dopamine as a neurotransmitter (that are dopaminergic) leave the substantia nigra of the midbrain and synapse in the corpus striatum. This is the nigrostriatal system, used for motor control. Dopaminergic axons from the midbrain to the nucleus accumbens and prefrontal cortex constitute the mesolimbic system, which functions in emotional reward.

Table 8.3

Synaptic Effects of Some Abused Drugs

Drug

Action

How Synaptic Transmission Is Affected

Opiates

Stimulates opioid receptors

Exogenous opioids bind to and stimulate the G-protein-coupled receptors for the endogenous opioids.

Cocaine

Inhibits transporter needed for reuptake of dopamine (and serotonin and norepinephrine) into presynaptic axon terminals

Receptors for monoamines are stimulated indirectly because more neurotransmitters remain in the synaptic cleft.

Amphetamines

Stimulates the release of dopamine from dopaminergic neurons

Receptors for dopamine are stimulated indirectly because more dopamine is released into the synaptic cleft.

Ethanol (alcohol)

Facilitates GABA receptor function (promoting inhibition) and inhibits NMDA glutamate receptor function (decreasing excitation)

Receptors for GABA and the NMDA receptors for glutamate are ligand-gated channels, opened directly by binding to these neurotransmitters.

Nicotine

Stimulates nicotinic acetylcholine receptors

Nicotinic ACh receptors are ligand-gated channels, opened directly by binding to ACh or nicotine.

Source: Reprinted by permission from Nature Reviews Neuroscience: Vol. 2, No. 2, p. 120 (2001). Copyright 2001 Macmillan Magazines Ltd.

Source: Reprinted by permission from Nature Reviews Neuroscience: Vol. 2, No. 2, p. 120 (2001). Copyright 2001 Macmillan Magazines Ltd.

Brainstem Hindbrain Midbrain

Midbrain

Brain stem i— Pneumotaxic area respiratory — centers

Medulla oblongata-

Midbrain

Brain stem i— Pneumotaxic area respiratory — centers

Apneustic area L Rhythmicity area -

Reticular formation -

Medulla oblongata-

■ Figure 8.19 Respiratory control centers in the brain stem. These are nuclei within the pons and medulla oblongata that control the motor nerves required for breathing. The location of the reticular formation is also shown.

Damage to the cerebellum produces ataxia—lack of coordination resulting from errors in the speed, force, and direction of movement. The movements and speech of people afflicted with ataxia may resemble those of someone who is intoxicated. This condition is also characterized by intention tremor, which differs from the resting tremor of Parkinson's disease in that it occurs only when intentional movements are made. People with cerebellar damage may reach for an object and miss it by placing their hand too far to the left or right; then, they will attempt to compensate by moving their hand in the opposite direction. This back-and-forth movement can result in oscillations of the limb.

Myelencephalon

The myelencephalon is composed of only one structure, the medulla oblongata, often simply called the medulla. About 3 cm (1 in.) long, the medulla is continuous with the pons superiorly and the spinal cord inferiorly. All of the descending and ascending fiber tracts that provide communication between the spinal cord and the brain must pass through the medulla. Many of these fiber tracts cross to the contralateral side in elevated triangular structures in the medulla called the pyramids. Thus, the left side of the brain receives sensory information from the right side of the body and vice versa. Similarly, because of the decus-sation of fibers, the right side of the brain controls motor activity in the left side of the body and vice versa.

Many important nuclei are contained within the medulla. Several nuclei are involved in motor control, giving rise to axons within cranial nerves VIII, IX, X, XI, and XII. The vagus nuclei (there is one on each lateral side of the medulla), for example, give rise to the highly important vagus (X) nerves. Other nuclei relay sensory information to the thalamus and then to the cerebral cortex.

The medulla contains groupings of neurons required for the regulation of breathing and of cardiovascular responses; hence, they are known as the vital centers. The vasomotor center controls the autonomic innervation of blood vessels; the cardiac control center, closely associated with the vasomotor center, regulates the autonomic nerve control of the heart; and the respiratory center of the medulla acts together with centers in the pons to control breathing.

Reticular Formation

The reticular formation (fig. 8.19) is a complex network of nuclei and nerve fibers within the medulla, pons, midbrain, thalamus, and hypothalamus that functions as the reticular activating system, or RAS. Because of its many interconnections, the RAS is activated in a nonspecific fashion by any modality of sensory information. Nerve fibers from the RAS, in turn, project diffusely to the cerebral cortex; this results in nonspecific arousal of the cerebral cortex to incoming sensory information.

The RAS, through its nonspecific arousal of the cortex, helps to maintain a state of alert consciousness. Not surprisingly, there is evidence that general anesthetics may produce unconsciousness by depressing the RAS. Similarly, the ability to fall asleep may be due to the action of specific neurotransmitters that inhibit activity of the RAS.

Test Yourself Before You Continue

1. List the structures of the midbrain and describe their functions.

2. Describe the functions of the medulla oblongata and pons.

3. Locate the reticular formation in the brain. What is the primary function of the reticular activating system and how is this function accomplished?

The Central Nervous System

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Responses

  • Naomi
    Is ventral tegmental area part of hindbrain?
    5 years ago
  • Delores Begley
    What is corpus striatum?
    5 years ago
  • Castore
    What channels the ras from the medulla to the midbrain?
    5 years ago
  • PORRO
    What is the midbrain composed of?
    5 years ago
  • Taneli
    What neurotransmitter is in the corpus striatum?
    4 years ago

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