Info

eptember 13, 1848, was a momentous day for Phineas

SGage, a young man who worked in Vermont evening out terrain for railroad tracks. To blast away rock, he would drill a hole, fill it with gunpowder, cover that with sand, insert a fuse, and then press down with an iron rod called a tamping iron. The explosion would go down into the rock.

But on that fateful September day, Gage began pounding on the tamping iron before his coworker had put down the sand. The gunpowder exploded outward, slamming the inch-thick, 40-inch-long iron rod straight through Gage's skull. It pierced his brain like an arrow propelled through a soft melon, shooting out the other side of his head. Remarkably, Gage stood up just a few moments later, fully conscious and apparently unharmed by the hole just blasted through his head.

As it turned out, Gage was harmed in the freak accident, but in ways so subtle that they were not at first evident. His friends reported that "Gage was no longer Gage." Although retaining his intellect and abilities to move, speak, learn, and remember, Gage's personality dramatically changed. Once a trusted, honest, and dedicated worker, the 25-year-old became irresponsible, shirking work, cursing, and pursuing what his doctor termed "animal propensities."

Researchers as long ago as 1868 hypothesized that the tamping iron had ripped out a part of Gage's brain controlling personality. In 1994, computer analysis more precisely pinpointed the damage to the famous brain of Phineas Gage, which, along with the tamping iron, wound up in a museum at Harvard University. Researchers reconstructed the trajectory of the tamping iron, localizing two small areas in the front of the brain that control rational decision making and processing of emotion.

More than a hundred years after Gage's accident, in 1975, 21-year-old Karen Ann Quinlan drank an alcoholic beverage after taking a prescription sedative, and her heart and lungs stopped functioning. When she was found, Quinlan had no pulse, was not breathing, had dilated pupils, and was unresponsive. Cardiopulmonary resuscitation restored her pulse, but once at the hospital she was placed on a ventilator. Within twelve hours, some functions returned—her pupils constricted, she moved, gagged, grimaced, and even opened her eyes. Within a few months, she could even breathe unaided for short periods.

Because Quinlan's responses were random and not purposeful, and she was apparently unaware of herself and her environment, she was said to be in a persistent vegetative state. Her basic life functions were intact, but she had to be fed and given water intravenously. Fourteen months after Quinlan took the fateful pills and alcohol, her parents made a request that was to launch the right-to-die movement. They asked that Quinlan be taken off of life support. Doctors removed Quinlan's ventilator, and she lived for nine more years in a nursing home before dying of infection. She never regained awareness.

Throughout Quinlan's and her family's ordeal, researchers tried to fathom what had happened to her. A CAT scan performed five years

Rod Through Head
A rod that blasted through the head of a young railway worker has taught us much about the biology of personality.

after the accident showed atrophy in two major brain regions, the cerebrum and the cerebellum. But when researchers analyzed Karen Ann Quinlan's brain in 1993, they were surprised. The most severely damaged part of her brain was the thalamus, an area thought to function merely as a relay station to higher brain structures. Quinlan's tragic case revealed that the thalamus is also important in processing thoughts, in providing the awareness and responsiveness that makes a person a conscious being.

The cases of Phineas Gage and Karen Ann Quinlan dramatically illustrate the function of the human brain by revealing what can happen when it is damaged. Nearly every aspect of our existence depends upon the brain and other parts of the nervous system, from thinking and feeling; to sensing, perceiving, and responding to the environment; to carrying out vital functions such as breathing and heartbeat. This chapter describes how the billions of neurons and glia comprising the nervous system interact to enable us to survive and to enjoy the world around us.

The central nervous systems (CNS) consists of the brain and the spinal cord. The brain is the largest and most complex part of the nervous system. It includes the cerebrum, the diencephalon, the brain stem, and the cerebellum, which will be described in detail in the sec tion titled "Brain." The brain includes about one hundred billion (1011) multipolar neurons and countless branches of the axons by which these neurons communicated with each other and with neurons elsewhere in the nervous system.

The brain stem connects the brain and spinal cord and allows two-way communication between them. The spinal cord in turn provides two-way communication between the central nervous system (CNS) and the peripheral nervous system (PNS).

Bones, membranes, and fluid surround the organs of the central nervous system. More specifically, the brain lies within the cranial cavity of the skull, whereas the spinal cord occupies the vertebral canal within the vertebral column. Beneath these bony coverings, membranes called meninges, located between the bone and the soft tissues of the nervous system, protect the brain and spinal cord (fig. 11.1a).

meninges

The meninges (sing., meninx) have three layers—dura mater, arachnoid mater, and pia mater (fig. 11.1b). The dura mater is the outermost layer. It is primarily composed of tough, white, dense connective tissue and contains many blood vessels and nerves. It attaches to the inside of the cranial cavity and forms the internal periosteum of the surrounding skull bones (see reference plate 53).

In some regions, the dura mater extends inward between lobes of the brain and forms supportive and protective partitions (table 11.1). In other areas, the dura mater splits into two layers, forming channels called dural sinuses, shown in figure 11.1.b Venous blood flows through these channels as it returns from the brain to vessels leading to the heart.

The dura mater continues into the vertebral canal as a strong, tubular sheath that surrounds the spinal cord. It is

Partition

Location

Falx cerebelli

Separates the right and left cerebellar hemispheres

Falx cerebri

Extends downward into the longitudinal fissure, and separates the right and left cerebral hemispheres (fig. 11.1b)

Tentorium cerebelli

Separates the occipital lobes of the cerebrum from the cerebellum (fig. 11.1a)

attached to the cord at regular intervals by a band of pia mater (denticulate ligaments) that extends the length of the spinal cord on either side. The dural sheath terminates as a blind sac at the level of the second sacral vertebra, below the end of the spinal cord. The sheath around the spinal cord is not attached directly to the vertebrae but is separated by an epidural space, which lies between the dural sheath and the bony walls (fig. 11.2). This space contains blood vessels, loose connective tissue, and adipose tissue that provide a protective pad around the spinal cord.

A blow to the head may rupture some blood vessels associated with the brain, and the escaping blood may collect in the space beneath the dura mater. This condition, called subdural hematoma, can increase pressure between the rigid bones of the skull and the soft tissues of the brain. Unless the accumulating blood is promptly evacuated, compression of the brain may lead to functional losses or even death.

Scalp

Cranium

Cerebrum

Tentorium cerebelli

Cerebellum

Vertebra Spinal cord Meninges

Cerebellum

Vertebra Spinal cord Meninges

Connective Tissue Surrounding Skull

Skin

Subcutaneous tissue

Bone of skull

Dural sinus (superior sagittal sinus)

Arachnoid granulation

Dura mater

Arachnoid mater

Pia mater _

Subarachnoid space Falx cerebri Gray matter White matter

Figure 11.1

(a) Membranes called meninges enclose the brain and spinal cord. (b) The meninges include three layers: dura mater, arachnoid mater, and pia mater.

Skin

Subcutaneous tissue

Bone of skull

Dural sinus (superior sagittal sinus)

Arachnoid granulation

Dura mater

Arachnoid mater

Pia mater _

Subarachnoid space Falx cerebri Gray matter White matter

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.

Get My Free Ebook


Post a comment