Biomedical Engineering Reference
In-Depth Information
average, 100 to 200 Angstroms (10
9
meters) wide. This is known as the
. Each
of these synaptic endings contains a large number of submicroscopic spherical structures
(
synaptic cleft
) that can be detected only under the electron microscope. These synaptic
vesicles, in turn, are essentially “chemical carriers” containing transmitter substance that
is released into the synaptic clefts on excitation. With this information in hand, let us
consider the sequence of events that enables one neuron to communicate with another.
When an individual neuron is excited, an electrical signal is transmitted along its axon
to many tiny branches, diverging fibers near its far end. These axonal terminals end or
synapse close to the “input terminals” (the dendrites and cell body) of a large number of
other neurons. When an electrical pulse arrives at the synapse, it triggers the release of a
tiny amount of transmitter substance. This chemical carrier floats across the synaptic cleft
between the axonal fiber and the cell body, thereby altering the status of the receiving
neuron. For example, the chemical emissions may urge the receiving neuron into a state
whereby this second cell is activated and conducts a similar electrical pulse to its axon. In
this way, the initial electrical signal may be propagated to a still more remote part of the
other hand. If the surfaces of muscle cells lie close enough to a number of such terminals
to receive a substantial supply of these chemical carriers, the muscle will experience a
resulting electrochemical reaction of its own that will cause it to contract and thereby per-
form some mechanical chore. In a similar manner, a gland can be stimulated to secrete
the chemical characteristic to its activity. Neurons with the ability to cause a muscular or
glandular reaction are known as effector neurons or motoneurons.
For most of us, the most pleasurable sensations come from our perception of the world
around us. This sense of awareness is made possible by still another group of specialized
neurons known as
synaptic vesicles
. Acting as input devices, these neurons accept and convert
various sensory information into appropriate electrical impulses that can then be properly
processed within the nervous system. These receptors measure such quantities as pressure,
warmth, cold, and displacement, as well as the presence of specific chemicals. Considering
that every minute of one's life the brain is virtually bombarded by such a voluminous
amount of incoming information, it is astounding that it can function at all.
So as we have seen, nerve cells are responsible for the following variety of essential
functions:
receptor cells
1.
Accepting and converting sensory information into a form that can be processed with the
nervous system by other neurons.
2.
Processing and analyzing this information so an “integrated portrait” of the incoming
data can be obtained.
3.
Translating the final outcome or “decision” of this analysis process into an appropriate
electrical or chemical form needed to stimulate glands or activate muscles.
The nervous system, which is responsible for the integration and control of all the body's
functions, has been divided by neuroscientists into the central nervous system (CNS) and
the peripheral nervous system (PNS) (Figure 3.27). The former consists of all nervous tissue
enclosed by bone (e.g., the brain and spinal cord), and the latter consists of all nervous
tissue not enclosed by bone, which enables the body to detect and respond to both internal
and external stimuli. The peripheral nervous system consists of the 12 pairs of cranial and
31 pairs of spinal nerves with afferent (sensory) and efferent (motor) neurons.
