Nerve/Muscle Connections
Nerve/Muscle Connections
Nerves, known as sensory neurons, come from every part of the body to carry impulses to
the brain or spinal cord concerning the condition of every nook and cranny in the body,
including the muscles. In the same way, motor neurons send impulses to the muscles, often
through intermediate connections or "interneurons" in the spinal cord. These
messages cross the gap at a junction between nerves and muscles, setting off a chain of
events that ends in contraction. Within a second, millions of impulses reach the motor
neurons. Some of the impulses are sent from various parts of the brain and spinal cord;
some come from sense organs located in the joints, ligaments, and tendons; and some come
from the muscles themselves. The seeds of movement are sown by the brain, in its primary
cortex, an area of the brain's wrinkled surface which spans both cerebral hemispheres.
Another patch of cortex directly in front of the primary area also houses neurons which
are involved in movement. This area is thought to be important to speech and delicately
coordinated movements such as those performed by the hand. Electrical impulses from many
regions of the brain feed into the motor areas. The brain must collect and analyze all the
sensory messages it receives before it can direct a coordinated movement. This interplay
is continuous and elaborate - sight, sound, smell, pressure and pain are all important,
but so are messages bringing information about the angles and position of joints, the
length and tension of muscles, or even the speed of movements. At every point along the
descent from brain to muscle, impulses can influence interneurons to vary the precision of
muscular control. An average motor neuron may have as many as 15,000 connections each,
providing information from all over the body. Some parts, like the back, which have a
limited precision of motion, are only equipped with a few - perhaps 50,000. Hand muscles,
which perform very delicate and precise movements are driven by about 200,000 neurons. A
second major transmission network produces contractions of groups of muscles and is
responsible for larger muscular functions, such as running, walking or swimming. A
"muscle spindle" is a sensory end organ in a muscle that is sensitive to
stretching of the muscle.
Modificado em: 18-01-1999
