Biomedical Engineering Reference
In-Depth Information
Clusters of nerve cells located in the CNS are called nuclei, and clusters of nerve cells in
the PNS are called ganglion. On the other hand, nucleons located in the PNS have been
designated as nerves, while those in the CNS are called tracts.
The nervous system has also been divided into the somatic and autonomic nervous
systems. Each of these systems consists of components from both the central and peripheral
nervous systems. For example, the somatic peripheral nervous system consists of the sen-
sory neurons, which convey information from receptors for pain, temperature, and mechan-
ical stimuli in the skin, muscles, and joints to the central nervous system, and the motor
neurons, which return impulses from the central nervous system to these same areas of
the body.
The autonomic nervous system is concerned with the internal meter of the body, includ-
ing involuntary regulation of smooth muscle, cardiac muscle, and glands and is further
divided into the sympathetic and parasympathetic divisions. The sympathetic division
causes blood vessels in the viscera and skin to constrict, vessels in the skeletal muscles to
dilate, and the heart rate to increase, whereas the parasympathetic division has the opposite
effect on the vessels in the viscera and skin, provides no innervation to the skeletal muscles,
and causes the heart rate to decrease. Thus, the sympathetic division prepares the body for
“fight or flight,” while the parasympathetic division returns the body to normal operating
conditions.
Brain function is dependent on neuronal circuits. Neurons interconnect in several differ-
ent types of circuits. In a divergent circuit, each branch in the axon of the presynaptic
neuron connects with the dendrite of a different postsynaptic neuron. In a convergent
circuit, axons from several presynaptic neurons meet at the dendrite(s) of a single postsyn-
aptic neuron. In a simple feedback circuit, the axon of a neuron connects with the dendrite
of an interneuron that connects back with the dendrites of the first neuron. A two-neuron
circuit is one in which a sensory neuron synapses directly with a motor neuron, whereas
a three-neuron circuit consists of a sensory neuron, an interneuron in the spinal cord, and a
motor neuron. Both of these circuits can be found in reflex arcs (Figure 3.29). The reflex arc
is a special type of neural circuit that begins with a sensory neuron at a receptor (e.g., a pain
receptor in the fingertip) and ends with a motor neuron at an effector (e.g., a skeletal muscle).
FIGURE 3.29
This reflex arc begins with a sensory neuron in the finger that senses pain when the fingertip is
pricked by the pin. An action potential travels from the sensory neuron to an interneuron and then to a motor neu-
ron that synapses with muscle fibers in the finger. The muscle fibers respond to the stimulus by contracting and
removing the fingertip from the pin.
