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Introduction (page 395)

Bone and protective membranes called meninges surround the brain and spinal cord.

Meninges (page 396)

1. The meninges consist of a dura mater, arachnoid mater, and pia mater.

2. Cerebrospinal fluid occupies the space between the arachnoid and pia maters.

Ventricles and Cerebrospinal Fluid (page 397)

1. Ventricles are connected cavities within the cerebral hemispheres and brain stem.

2. Cerebrospinal fluid fills the ventricles.

3. Choroid plexuses in the walls of the ventricles secrete cerebrospinal fluid.

4. Ependymal cells of the choroid plexus regulate the composition of cerebrospinal fluid.

5. Cerebrospinal fluid circulates through the ventricles and is reabsorbed into the blood of the dural sinuses.

Spinal Cord (page 401)

The spinal cord is a nerve column that extends from the brain into the vertebral canal. It terminates at the level between the first and second lumbar vertebrae.

1. Structure of the spinal cord a. The spinal cord is composed of thirty-one segments, each of which gives rise to a pair of spinal nerves.

b. It is characterized by a cervical enlargement, a lumbar enlargement, and two deep longitudinal grooves that divide it into right and left halves.

c. White matter surrounds a central core of gray matter.

d. The white matter is composed of bundles of myelinated nerve fibers.

2. Functions of the spinal cord a. The spinal cord is the center for spinal reflexes.

(1) Reflexes are automatic, subconscious responses to changes.

(2) They help maintain homeostasis.

(3) The knee-jerk reflex employs only two neurons.

(4) Withdrawal reflexes are protective actions.

b. The cord provides a two-way communication system between the brain and structures outside the nervous system.

(1) Ascending tracts carry sensory impulses to the brain; descending tracts carry motor impulses to muscles and glands.

(2) Many of the fibers in the ascending and descending tracts cross over in the spinal cord or brain.

Brain (page 411)

The brain is the largest and most complex part of the nervous system. It contains nerve centers that are associated with sensations. The brain issues motor commands and carries on higher mental functions.

1. Brain development a. Brain structure reflects the way it forms.

b. The brain develops from a neural tube with three cavities—the forebrain, midbrain, and hindbrain.

c. The cavities persist as ventricles, and the walls give rise to structural and functional regions.

2. Structure of the cerebrum a. The cerebrum consists of two cerebral hemispheres connected by the corpus callosum.

b. Its surface is marked by ridges and grooves; sulci divide each hemisphere into lobes.

c. The cerebral cortex is a thin layer of gray matter near the surface.

d. White matter consists of myelinated nerve fibers that interconnect neurons within the nervous system and communicate with other body parts.

3. Functions of the cerebrum a. The cerebrum is concerned with higher brain functions, such as thought, reasoning, interpretation of sensory impulses, control of voluntary muscles, and memory storage.

b. The cerebral cortex has sensory, motor, and association areas.

c. The primary motor regions lie in the frontal lobes near the central sulcus. Other areas of the frontal lobes control special motor functions.

d. Areas that interpret sensory impulses from the skin are located in the parietal lobes near the central sulcus; other specialized sensory areas are in the temporal and occipital lobes.

e. Association areas analyze and interpret sensory impulses and provide memory, reasoning, verbalizing, judgment, and emotions.

f. One cerebral hemisphere usually dominates for certain intellectual functions.

g. Short-term memory is probably electrical. Long-term memory is thought to be encoded in patterns of synaptic connections.

4. Basal nuclei a. Basal nuclei are masses of gray matter located deep within the cerebral hemispheres.

b. They relay motor impulses originating in the cerebral cortex and aid in controlling motor activities.

5. Diencephalon a. The diencephalon contains the thalamus and hypothalamus.

b. The thalamus selects incoming sensory impulses and relays them to the cerebral cortex.

c. The hypothalamus is important in maintaining homeostasis.

d. The limbic system produces emotional feelings and modifies behavior.

6. Brain stem a. The brain stem extends from the base of the brain to the spinal cord.

b. The brain stem consists of the midbrain, pons, and medulla oblongata.

c. The midbrain contains reflex centers associated with eye and head movements.

d. The pons transmits impulses between the cerebrum and other parts of the nervous system and contains centers that help regulate rate and depth of breathing.

e. The medulla oblongata transmits all ascending and descending impulses and contains several vital and nonvital reflex centers.

f. The reticular formation filters incoming sensory impulses, arousing the cerebral cortex into wakefulness in response to meaningful impulses.

g. Normal sleep results from decreasing activity of the reticular formation, and paradoxical sleep occurs when activating impulses are received by some parts of the brain, but not by others.

7. Cerebellum a. The cerebellum consists of two hemispheres connected by the vermis.

b. A thin cortex of gray matter surrounds the white matter of the cerebellum.

c. The cerebellum functions primarily as a reflex center, coordinating skeletal muscle movements and maintaining equilibrium.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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