Ganong's Review of Medical Physiology, 23rd Edition

244
SECTION III
Central & Peripheral Neurophysiology

The conditions under which the human stimulation studies

were performed precluded stimulation of the banks of the

sulci and other inaccessible areas. Meticulous study has

shown that in monkeys there is a regular representation of the

body, with the axial musculature and the proximal portions of

the limbs represented along the anterior edge of the precentral

gyrus and the distal part of the limbs along the posterior edge.

The cells in the cortical motor areas are arranged in col-

umns. The ability to elicit discrete movements of a single

muscle by electrical stimulation of a column within M1 led to

the view that this area was responsible for control of individ-

ual muscles. More recent work has shown that neurons in sev-

eral cortical columns project to the same muscle; in fact, most

stimuli activate more than one muscle. Moreover, the cells in

each column receive fairly extensive sensory input from the

CLINICAL BOX 16–1

Lower versus Upper Motor Neuron Damage

Lower motor neurons

are those whose axons terminate on

skeletal muscles. Damage to these neurons is associated

with

flaccid paralysis, muscular atrophy, fasciculations

(visible muscle twitches that appear as flickers under the

skin),

hypotonia

(decreased muscle tone), and

hyporeflexia

or

areflexia.

An example of a disease that leads to lower

motor neuron damage is

amyotrophic lateral sclerosis

(ALS).

“Amyotrophic” means “no muscle nourishment” and

describes the atrophy that muscles undergo because of dis-

use. “Sclerosis” refers to the hardness felt when a pathologist

examines the spinal cord on autopsy; the hardness is due to

proliferation of astrocytes and scarring of the lateral columns

of the spinal cord. ALS is a selective, progressive degenera-

tion of

α

-motor neurons. This fatal disease is also known as

Lou Gehrig disease

because Gehrig, a famous American

baseball player, died of it. The worldwide annual incidence

of ALS has been estimated to be 0.5–3 cases per 100,000

people. Most cases are sporadic, but 5–10% of the cases are

familial. Forty percent of the familial cases have a mutation in

the gene for Cu/Zn superoxide dismutase

(SOD-1)

on chro-

mosome 21. SOD is a free radical scavenger that reduces oxi-

dative stress. A defective

SOD-1

gene permits free radicals to

accumulate and kill neurons. The disease has no racial, socio-

economic, or ethnic boundaries. The life expectancy of ALS

patients is usually 3–5 years after diagnosis. ALS is most com-

monly diagnosed in middle age and affects men more often

than women. The worldwide incidence of ALS is 2 per

100,000 of total population. The causes of ALS are unclear,

but possibilities include viruses, neurotoxins, heavy metals,

DNA defects (especially in familial ALS), immune system ab-

normalities, and enzyme abnormalities.

Upper motor neurons

typically refer to corticospinal

tract neurons that innervate spinal motor neurons, but they

can also include brain stem neurons that control spinal

motor neurons. Damage to these neurons initially causes

muscles to become weak and flaccid but eventually leads

to

spasticity, hypertonia

(increased resistance to passive

movement),

hyperactive stretch reflexes,

and abnormal

plantar extensor reflex

(Babinski sign).

The Babinski sign is

dorsiflexion of the great toe and fanning of the other toes

when the lateral aspect of the sole of the foot is scratched.

In adults, the normal response to this stimulation is plantar

flexion in all the toes. The Babinski sign is believed to be a

flexor withdrawal reflex that is normally held in check by

the lateral corticospinal system. It is of value in the localiza-

tion of disease processes, but its physiologic significance is

unknown. However, in infants whose corticospinal tracts

are not well developed, dorsiflexion of the great toe and

fanning of the other toes is the natural response to stimuli

applied to the sole of the foot.

FIGURE 16–3

A view of the human cerebral cortex, showing the

motor cortex (Brodmann’s area 4) and other areas concerned with control

of voluntary movement, along with the numbers assigned to the regions

by Brodmann.

(Reproduced with permission from Kandel ER, Schwartz JH, Jessell

TM [editors]:

Principles of Neural Science,

4th ed. McGraw-Hill, 2000.)

FIGURE 16–4

Hand area of motor cortex demonstrated by

functional magnetic resonance imaging (fMRI) in a 7-year-old

boy.

Changes in activity associated with squeezing a rubber ball with the

right hand are shown in white and with the left hand in black.

(Reproduced with permission from Waxman SG:

Neuroanatomy with Clinical

Correlations,

25th ed. McGraw-Hill, 2003.)

7

3,1,2

Posterior

parietal

cortex

Primary somatic

sensory cortex

Motor cortex

Supplementary

motor area

Premotor

cortex

Prefrontal

cortex

(^57)
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