Review of Key Concepts

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Overview of the Brain (p. 516)

1. The adult brain weighs 1,450 to 1,600 g. It is divided into the cerebrum, cerebellum, and brainstem.

2. The cerebrum and cerebellum exhibit folds called gyri separated by grooves called sulci. The groove between the cerebral hemispheres is the longitudinal fissure.

3. The cerebrum and cerebellum have gray matter in their surface cortex and deeper nuclei, and white matter deep to the cortex.

4. Embryonic development of the brain progresses through neural plate and neural tube stages in the first 4 weeks. The anterior neural tube then begins to bulge and differentiate into forebrain, midbrain, and hindbrain. By the fifth week, the forebrain and hindbrain show further subdivision into two secondary vesicles each.

Meninges, Ventricles, Cerebrospinal Fluid, and Blood Supply (p. 519)

1. Like the spinal cord, the brain is surrounded by a dura mater, arachnoid mater, and pia mater. The dura mater is divided into two layers, periosteal and meningeal, which in some places are separated by a blood-filled dural sinus. In some places, a subdural space also separates the dura from the arachnoid.

2. The brain has four internal, interconnected cavities: two lateral ventricles in the cerebral hemispheres, a third ventricle between the hemispheres, and a fourth ventricle between the pons and cerebellum.

3. The ventricles and canals of the CNS are lined with ependymal cells, and each ventricle contains a choroid plexus of blood capillaries.

4. These spaces are filled with cerebrospinal fluid (CSF), which is produced by the ependyma and choroid plexuses and in the subarachnoid space around the brain. The CSF of the ventricles flows from the lateral to the third and then fourth ventricle, out through foramina in the fourth, into the subarachnoid space around the brain and spinal cord, and finally returns to the blood by way of arachnoid villi.

5. CSF provides buoyancy, physical protection, and chemical stability for the CNS.

6. The brain has a high demand for glucose and oxygen and thus receives a copious blood supply.

7. The blood-brain barrier and blood-CSF barrier tightly regulate what substances can escape the blood and reach the nervous tissue.

The Hindbrain and Midbrain (p. 524)

1. The medulla oblongata is the most caudal part of the brain, just inside the foramen magnum. It conducts signals up and down the brainstem and between the brainstem and cerebellum, and contains nuclei involved in vasomotion, respiration, coughing, sneezing, salivation, swallowing, gagging, vomiting, gastrointestinal secretion, sweating, and muscles of tongue and head movement. Cranial nerves IX through XII arise from the medulla.

2. The pons is immediately rostral to the medulla. It conducts signals up and down the brainstem and between the brainstem and cerebellum, and contains nuclei involved in sleep, hearing, equilibrium, taste, eye movements, facial expression and sensation, respiration, swallowing, bladder control, and posture. Cranial nerve V arises from the pons, and nerves VI through VIII arise between the pons and medulla.

3. The cerebellum is the largest part of the hindbrain. It is composed of two hemispheres joined by a vermis, and has three pairs of cerebellar peduncles that attach it to the medulla, pons, and midbrain and carry signals between the brainstem and cerebellum.

4. Histologically, the cerebellum exhibits a fernlike pattern of white matter called the arbor vitae, deep nuclei of gray matter embedded in the white matter, and unusually large neurons called Purkinje cells.

5. The cerebellum is concerned with motor coordination and judging the passage of time, and plays less-understood roles in awareness, judgment, memory, and emotion.

6. The midbrain is rostral to the pons. It conducts signals up and down the brainstem and between the brainstem and cerebellum, and contains nuclei involved in motor control, pain, visual attention, and auditory reflexes. It gives rise to cranial nerves III and IV.

7. The reticular formation is an elongated cluster of nuclei extending throughout the brainstem, including some of the nuclei already mentioned. It is involved in the control of skeletal muscles, the visual gaze, breathing, swallowing, cardiac and vasomotor control, pain, sleep, consciousness, and sensory awareness.

The Forebrain (p. 529)

1. The forebrain consists of the diencephalon and cerebrum.

2. The diencephalon is composed of the thalamus, hypothalamus, and epithalamus.

3. The thalamus receives sensory input from the brainstem and first two cranial nerves, integrates sensory data, and relays sensory information to appropriate areas of the cerebrum. It is also involved in emotion, memory, arousal, and eye movements.

4. The hypothalamus is inferior to the thalamus and forms the walls and floor of the third ventricle. It is a major homeostatic control center. It synthesizes some pituitary hormones and controls the timing of pituitary secretion, and it has nuclei concerned with heart rate, blood pressure, gastrointestinal secretion and motility, pupillary diameter,

Saladin: Anatomy & I 14. The Brain and Cranial I Text I © The McGraw-Hill

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

Chapter 14 The Brain and Cranial Nerves 559

thermoregulation, hunger and thirst, sleep and circadian rhythms, memory, and emotion.

5. The epithalamus lies above the thalamus and includes the pineal gland (an endocrine gland) and habenula (a relay from limbic system to midbrain).

6. The cerebrum is the largest part of the brain. It is divided into two hemispheres, and each hemisphere into five lobes: frontal, parietal, occipital, and temporal lobes and the insula.

7. Nerve fibers of the cerebral white matter are bundled in tracts of three kinds: projection tracts that extend between higher and lower brain centers, commissural tracts that cross between the right and left cerebral hemispheres through the corpus callosum and the anterior and posterior commissures; and association tracts that connect different lobes and gyri within a single hemisphere.

8. The cerebral cortex is gray matter with two types of neurons: stellate cells and pyramidal cells. All output from the cortex travels by way of axons of the pyramidal cells. Most of the cortex is neocortex, in which there are six layers of nervous tissue. Evolutionarily older parts of the cerebrum have one- to five-layered paleocortex and archicortex.

9. The basal nuclei are masses of cerebral gray matter lateral to the thalamus, concerned with motor control. They include the caudate nucleus, putamen, and globus pallidus.

10. The limbic system is a loop of specialized cerebral cortex on the medial border of the temporal lobe. Some of its parts are the hippocampus, amygdala, fornix, and cingulate gyrus. It is important in smell, emotion, and memory.

Higher Brain Functions (p. 536)

1. The cerebral cortex generates brain waves that can be recorded as an electroencephalogram (EEG). Different types of brain waves (alpha, beta, theta, delta) predominate in various states of consciousness and certain brain disorders.

2. The cycle of sleep and waking is controlled by the suprachiasmatic nucleus of the hypothalamus and the reticular formation of the lower brainstem. Sleep progresses from stage 1 to stage 4 with characteristic changes in the EEG and other physiological values. Most dreaming occurs during a fifth type of sleep called rapid eye movement (REM) sleep.

3. Cognition (consciousness, thought, etc.) involves several association areas of the cerebral cortex, especially in the parietal, temporal, and frontal lobes.

4. The hippocampus of the limbic system processes information and organizes it into long-term memories (memory consolidation). These memories are then stored in other regions of the cerebral cortex, including the prefrontal cortex and the temporal lobe. The cerebellum is also involved in procedural memory (learning motor skills) and the amygdala in emotional memory.

5. The amygdala, hippocampus, and hypothalamus are important emotional centers of the brain, involved in such feelings as love, fear, anger, pleasure, and pain, and in learning to associate behaviors with reward and punishment.

6. Somesthetic sensation is controlled by the postcentral gyrus, where there is a point-for-point correspondence (somatotopy) with specific regions on the contralateral side of the body.

7. Special senses other than taste and equilibrium are controlled by other areas of primary sensory cortex: smell in the temporal and frontal lobes, vision in the occipital lobe, and hearing in the temporal lobe and insula. Taste signals go with somesthetic senses to the postcentral gyrus and equilibrium signals to the cerebellum.

8. The primary sensory areas are surrounded with sensory association areas that process sensory input, relate it to memory, and identify the stimuli.

9. Motor control resides in the motor association area and precentral gyrus of the frontal lobe. The precentral gyrus shows a somatotopic correspondence with muscles on the contralateral side of the body.

10. The basal nuclei and cerebellum play important roles in motor coordination and the conduct of learned motor skills.

11. Language is coordinated largely by Wernicke's and Broca's areas. Recognizing language and formulating what one will say or write occur in Wernicke's area; compiling the motor program of speech resides in Broca's area; and commands to the muscles of speech originate in the precentral gyrus.

12. The brain exhibits cerebral lateralization: Some functions are coordinated mainly by the left hemisphere and others by the right. The categorical hemisphere (in most people, the left) is responsible for verbal and mathematical skills and logical, linear thinking. The representational hemisphere (usually the right) is a seat of imagination, insight, spatial perception, musical skill, and other "holistic" functions.

The Cranial Nerves (p. 546)

1. Twelve pairs of cranial nerves arise from the floor of the brain, pass through foramina of the skull, and lead primarily to structures in the head and neck.

2. Cranial nerves (CN) I and II are purely sensory. All the rest are mixed, although the sensory components of some are only proprioceptive and aid in motor control, so they are often regarded as motor nerves (CN III, IV, VI, XI, and XII).

3. The functions and other characteristics of the cranial nerves are described in table 14.2.

Saladin: Anatomy & I 14. The Brain and Cranial I Text I © The McGraw-Hill

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

560 Part Three Integration and Control

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