Structural Organization of the Brain

The brain is composed of an enormous number of association neurons and accompanying neuroglia, arranged in regions and subdivisions.These neurons receive sensory information, direct the activity of motor neurons, and perform such higher brain functions as learning and memory.

The central nervous system (CNS), consisting of the brain and spinal cord (fig. 8.1), receives input from sensory neurons and directs the activity of motor neurons that innervate muscles and glands. The association neurons within the brain and spinal cord are in a position, as their name implies, to asso-

Chapter Eight ciate appropriate motor responses with sensory stimuli, and thus to maintain homeostasis in the internal environment and the continued existence of the organism in a changing external environment. Further, the central nervous systems of all vertebrates (and most invertebrates) are capable of at least rudimentary forms of learning and memory. This capability—most highly developed in the human brain—permits behavior to be modified by experience and is thus of obvious benefit to survival. Perceptions, learning, memory, emotions, and perhaps even the self-awareness that forms the basis of consciousness, are creations of the brain. Whimsical though it seems, the study of brain physiology is the process of the brain studying itself.

The study of the structure and function of the central nervous system requires a knowledge of its basic "plan," which is established during the course of embryonic development. The early embryo contains an embryonic tissue layer known as ectoderm on its surface; this will eventually form the epidermis of the skin, among other structures. As development progresses, a groove appears in this ectoderm along the dorsal midline of the embryo's body. This groove deepens, and by the twentieth day after conception, has fused to form a neural tube. The part of the ectoderm where the fusion occurs becomes a separate structure called the neural crest, which is located between the neural tube and the surface ectoderm (fig. 8.2). Eventually, the neural tube will become the central nervous system, and the neural crest will become the ganglia of the peripheral nervous system, among other structures.

By the middle of the fourth week after conception, three distinct swellings are evident on the anterior end of the neural tube, which is going to form the brain: the forebrain (prosencephalon),

Gyrus

Gyrus

Human Spinal Cord Structure

■ Figure 8.1 The CNS consists of the brain and the spinal cord. Both of these structures are covered with meninges and bathed in cerebrospinal fluid.

The Central Nervous System midbrain (mesencephalon), and hindbrain (rhombencephalon). During the fifth week, these areas become modified to form five regions. The forebrain divides into the telen-cephalon and diencephalon; the mesencephalon remains unchanged; and the hindbrain divides into the metencephalon and myelencephalon (fig. 8.3). These regions subsequently become greatly modified, but the terms described here are still used to indicate general regions of the adult brain.

The basic structural plan of the CNS can now be understood. The telencephalon (refer to fig. 8.3) grows disproportion ately in humans, forming the two enormous hemispheres of the cerebrum that cover the diencephalon, the midbrain, and a portion of the hindbrain. Also, notice that the CNS begins as a hollow tube, and indeed remains hollow as the brain regions are formed. The cavities of the brain are known as ventricles and become filled with cerebrospinal fluid (CSF). The cavity of the spinal cord is called the central canal, and is also filled with CSF (fig. 8.4).

The CNS is composed of gray and white matter, as described in chapter 7. The gray matter, consisting of neuron cell bodies and dendrites, is found in the cortex (surface layer) of the

Dorsal View Days Embryo
Figure 8.2 Embryonic development of the CNS. This dorsal view of a 22-day-old embryo shows transverse sections at three levels of the developing central nervous system.

Three primary vesicles

Wall Cavity

Five secondary vesicles

Adult derivatives of

Prosencephalon

(forebrain)

Mesencephalon

(midbrain) Rhombencephalon

(hindbrain)

Prosencephalon

(forebrain)

Mesencephalon

(midbrain) Rhombencephalon

Telencephalon

Diencephalon

- Mesencephalon

(hindbrain)

■ Metencephalon

• Myelencephalon

Five secondary vesicles

Telencephalon

Diencephalon

- Mesencephalon

■ Metencephalon

• Myelencephalon

Forebrain Midbrain Hindbrain Cephalon

Spinal cord

Walls

Cerebral hemisphere

Thalamus Hypothalamus

Midbrain Pons

Cerebellum

Medulla oblongata

Cavities

Lateral ventricles

Third ventricle

Aqueduct

Upper-portion

Lower portion.

of fourth ventricle

Spinal cord

■ Figure 8.3 The developmental sequence of the brain. (a) During the fourth week, three principal regions of the brain are formed. (b) During the fifth week, a five-regioned brain develops and specific structures begin to form.

Interventricular foramen

Mesencephalic aqueduct

Fourth ventricle

Mesencephalic aqueduct

Fourth ventricle

Interventricular Foramen

Lateral ventricle

Third ventricle

To central canal of spinal cord

Lateral ventricle

Third ventricle

To central canal of spinal cord

Lateral I ventricle

Lateral I ventricle

Central Canal Spinal Cord

Mesencephalic aqueduct

To central canal of spinal cord

Interventricular foramen

Third ventricle

Interventricular foramen

Third ventricle

Mesencephalic aqueduct

To central canal of spinal cord

■ Figure 8.4 The ventricles of the brain. (a) An anterior view and (b) a lateral vie brain and deeper within the brain in aggregations known as nuclei. White matter consists of axon tracts (the myelin sheaths produce the white color) that underlie the cortex and surround the nuclei. The adult brain contains an estimated 100 billion (1011) neurons, weighs approximately 1.5 kg (3 to 3.5 lb), and receives about 20% of the total blood flow to the body per minute. This high rate of blood flow is a consequence of the high metabolic requirements of the brain; it is not, as Aristotle believed, because the brain's function is to cool the blood. (This fanciful notion—completely incorrect—is a striking example of prescientific thought, having no basis in experimental evidence.)

Test Yourself Before You Continue

1. Identify the three brain regions formed by the middle of the fourth week of gestation and the five brain regions formed during the fifth week.

2. Describe the embryonic origin of the brain ventricles. Where are they located and what do they contain?

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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Responses

  • fulgenzia rizzo
    How the structures of the nervous system form in the early week after fertilization?
    8 years ago
  • Jens
    What do the three primary (embryonic) brain vesicles look like?
    7 years ago
  • Kathrin Jager
    What is the function of the interventricular foramen of the brain?
    7 years ago
  • Marc
    Which is an incorrect association of brain region and ventricle?
    6 years ago
  • abaalom mustafa
    What are the adult derivatives of each of the five vesicles?
    2 years ago
  • doda marino
    Is the mesecephalon an incorrect association of brain region and ventricle?
    6 months ago
  • niina
    How is brain organized?
    12 days ago

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