1. The blood-brain barrier is a barrier between the bloodstream and brain tissue formed by tight junctions between the capillary endothelial cells, the basement membrane of the endothelium, and to some extent the perivascular feet of the astrocytes. It prevents many molecules in the blood, including many drugs, from getting into the interstitial spaces of the brain. Thus, it presents a difficulty for drug treatment of brain disorders.

2. A lesion in the cerebellum would cause voluntary muscle contractions to be uncoordinated, whereas a lesion in the basal nuclei would cause tremors or other unwanted (involuntary) contractions.

3. One would tend to turn the head quickly to the right in response to the sound, the headlights, or both. The visual reflex is mediated by the superior colliculi, whereas the auditory reflex is mediated by both the superior and inferior colliculi. (The corpora quadrigemina, a collective term for the superior and inferior colliculi, would also be an appropriate answer for the auditory reflex.)

4. The loss of a cerebral hemisphere is survivable, but would seriously compromise a variety of sensory, motor, and cognitive functions. For example, it would cause unilateral paralysis, blindness, and loss of somatic sensation, and could cause a loss of speech or language comprehension. The hypothalamus, however, controls more fundamental functions indispensible to life, such as water balance, hunger, thermoregulation, regulation of the heart rate, and pituitary hormone secretion. Destruction of a relatively small amount of hypothalamic tissue is fatal.

5. (a) Partial to complete paralysis of the voluntary muscles of the right side of the body. (b) Paralysis and loss of sensation on both sides of the body from the neck down. (c) Paralysis and sensory loss in the pelvic region and lower extremities.

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