I. Gross Organization of the Nervous System
The nervous system is the organ system responsible for producing, controlling and guiding our
acts, thoughts and responses to the world around us. During embryological development it is
derived from cells similar to those which go onto form our skin, but those precursor cells to the
nervous system become incredibly specialized and diverse, working together more complexly
and intricately than any other organ system in the body. The basic plan arises from the form of a
simple hollow tube that expands and differentiates itself into something that looks and functions
very different from the tube it once was. We will examine its major divisions (the central, CNS,
and peripheral, PNS, nervous systems) and its subdivisions, both in architecture and in function.
The most "available" division of the nervous system to the early anatomists for examination is
peripheral nervous system
(PNS). It consists of the nerves which directly connect to the skin,
muscles, blood vessels and organs of the body. As a general simplification, if nerve tissue not
encased in bone (skull, spinal column), it is part of the PNS.
somatic nervous system
at one time was called the
voluntary nervous system
accurately describes the role and distribution of its connections. The somatic nervous system
innervates the muscles, and connective tissues attached to the skeleton and our skin. It is
responsible for our voluntary movements and the physical sensations (heat, cold, pressure,
vibration, pain) we experience. The individual nerves are typically made up of
nerve fibers. Such a nerve can be roughly thought of as a bidirectional
cable that has wires, some of which send impulses out to the body from the nervous system and
some of which carry impulses from the body to the nervous system.
a. Afferent Nerves
To describe a nerve fiberas afferent means simply that the direction of the impulses it transmits
the nervous system from the body's muscles and skin. Therefore, afferent nerves
information towards the nervous system.
b. Efferent Nerves
An efferent nerve fiber sends impulses
from the nervous system in the direction of the
body's muscles. The efferent fibers generate movements of the skeleton and hence
nerve fibers since their activation causes the locomotion of our limbs, torso and facial
autonomic nervous system
used to be called the
involuntary nervous system
autonomic system is responsible for sensory and motor functions outside of our voluntary
control, such as internal organs and glands, smooth muscles in our gastrointestinal tract and
blood vessels and the smooth muscles attached to our skin. Its subdivisions, the sympathetic and
parasympathetic, have opposite and complementary actions on our bodies' organs and tissues.
Autonomic nervous system activity is constantly balancing between its two components. Neither