Introduction to the Nervous System
A brief introduction to understanding the nervous system with remarks re
the autonomic nervous system.
The nervous system works like this: There are many kinds of receptors in the
body, each sensitive to a specific stimulus: heat, cold, pressure, light of the
visible spectrum, blood pressure, CO2 tension (amount of CO2 dissolved) in the
blood, pH, osmolarity, etc. Every one of these receptors is innervated by an
afferent neuron at a neuroreceptor contact-place called a neuroreceptor synapse.
When stimulated the receptor generates an excitatory state that is then
transmitted at the synapse to the process of its innervating afferent neuron.
The afferent neuron conveys the excitatory state to the central nervous system
(CNS); that is, to either the brain or the spinal cord. Over any particular period of
time, the excitatory state generated by the multitude of active receptors of the
body results in a varying pattern of excitation arriving at the central nervous
system. Within the central nervous system, the different paths of excitation are
conveyed by interneuronal synapses to interneurons* that, different from the
afferent neurons, may be excitatory or inhibitory to succeeding neurons.**
Afferent neurons are, with few exceptions, virtually all excitatory to the neurons
with which they synapse in the CNS. But the neurons with which they synapse in
the CNS may be excitatory or inhibitory to succeeding neurons. In that way, the
excitation that is conveyed by afferent neurons is significantly altered within the
CNS. Within the CNS, the contacts of afferent neurons at interneuronal synapses
give rise to multiple pathways (“circuits” if you will), and the neurons of those
pathways within the CNS may be excitatory; or they may be inhibitory. This
arrangement results in a modulation of the input to the CNS and a varying
pattern of efferent neuronal discharge. Efferent neurons are neurons that convey
the excitatory state from the brain and spinal cord to the muscles and glands that
carry out the actions of the body.
*The term interneuron is given various meanings in the literature. It is used
here to signify a neuron that lies entirely within the central nervous system.
**A single exception to the contact of afferent neurons with interneurons is the
direct contact of an afferent with an efferent neuron in the case of the stretch reflex. The
“patellar reflex” is an example. This is not exclusive in that the afferent neuron’s axon will
also give off branches that synapse with interneurons in addition to its synapse with one
or more efferent neurons.
The peripheral contact of the efferent neuron pathway is at a neuroeffector
synapse; that is, at the place of contact of the axon of an efferent neuron with an
effector of the body. The term “effector” refers to the organs that ultimately carry
out the actions of the body: muscle and gland. Whereas there are many different
kinds of receptors, each sensitive to a particular kind of stimulus, there are only
two kinds of effectors, muscle and gland. There are, however, three kinds of
muscle: skeletal (striated), cardiac, and smooth.
The fundamental concept of the nervous system, like all fundamental concepts,
is remarkable in its simplicity: A pattern of receptor stimulation is conveyed to the
brain and spinal cord by afferent neurons. Within the brain and spinal cord the
excitatory state is modulated resulting in a particular pattern of efferent discharge
to the body effectors, muscle and gland. The animal responds to its environment
by the contraction of muscle and the secretion of its glands.
interneuronal synapse
interneuronal synapse
efferent neurons
afferent neurons
interneuronal synapse
Diagram showing the basic pattern of the nervous system.
Yellow = excitatory interneuron. Green = inhibitory
All of us recognize other states of being that we refer to as “activities”; an
example is the state of consciousness in which we are aware of some of the
stimuli that excite our receptors. (We are not aware of all of the stimuli that excite
our receptors. For example, we are not aware of the O2 tension of our blood; but
receptors convey this information to the brain 24 hours a day.) We recognize that
we can think; we recognize that there can be a state of dreaming, that there are
mechanisms of attention in which awareness of certain stimuli is accentuated
and others suppressed. All of these so-called activities are different from the
contraction of muscle and the secretion of gland. Activities such as
consciousness, thinking, dreaming, etc. take place within the central nervous
system itself. These kinds of activities achieve a visible result only by the
contraction of muscle and the secretion of gland. With the contraction of muscle,
you get a change in the form of the muscle and, unless the contraction is
isometric (contraction without a change in length of the muscle), some kind of
movement of the structures to which the muscle attaches. With the secretion of
gland, you achieve the formation of a secretion.
The law of the nervous system is this: Input equals output. And a corollary
is this: Varying inputs yield varying outputs. We want to appreciate that the
output, the contraction of muscle and the secretion of glands, at any particular
time is due not only to afferent input that is then arriving at the CNS but also to
input that may have arrived minutes and possibly many minutes ago. The time
difference is strictly a function of the time that the “circuitry” spends within the
CNS before ultimately leading to efferent neurons that convey the excitatory state
to effectors.
The innervation of skeletal muscle by efferent neurons is different from that of
cardiac muscle, smooth muscle, and gland. A single efferent neuron extends
between the CNS and skeletal muscle cells. The axon of that neuron will branch
at its distal end to supply a few skeletal muscle fibers (ocular muscles) or a
moderate number (digital extensor and flexor muscles) or a number as great as
several hundred skeletal muscle fibers (biceps femoris, semitendinosus, gluteus
medius, etc.). But, always, a single efferent neuron extends between the CNS
and any skeletal muscle fiber. The innervation of cardiac muscle, smooth muscle
and gland is quite different. In the case of these effectors, designated autonomic
effectors, a chain of two efferent neurons extends from the central nervous
system to the autonomic effector.* The first efferent neuron has its cell body in
the brain or spinal cord; its axon leaves the CNS in the root of a cranial nerve or
the ventral root of a spinal nerve. It will synapse on the dendrites and/or cell body
of a second neuron that is outside the CNS. The second efferent neuron of the
two-neuron chain has its cell body and dendrites in a peripheral autonomic
ganglion. It is the axon of the second neuron that extends to cardiac muscle
cells, smooth muscle cells, or the gland cells that it innervates.
*Note: There are exceptions to this arrangement. The one is an exception with
respect to certain local circuitry that is present in the gastrointestinal tract and possibly
other visceral systems. This local circuitry takes place without connection in the CNS. It
is not exclusive; but works with the efferent autonomic innervation that proceeds from
the CNS to these same viscera. The local circuitry probably (key word) functions in the
coordination of activities such as peristalsis. The two-neuron efferent innervation from
the CNS is probably functional in overall regulation of such local activity. A second
exception is the neuroeffector synapse of some first efferent neurons on the medullary
cells of the adrenal gland. These medullary cells are in fact modified second efferent
neurons that secrete the hormone norepinephrine or epinephrine.
Functionally, there is this large and important difference between autonomic
efferent innervation and the innervation of striated muscle: The innervation of
striated muscle is essential for its function. Loss of innervation means immediate
loss of function and rapid and significant atrophy (first apparent in about five
days) of affected myofibers. The innervation of smooth muscle, heart muscle,
and gland is not essential for their function. Smooth and cardiac muscle will
continue to contract and the heart will continue to beat and can sustain life;
glands will continue to secrete. But regulation is lost. Autonomic innervation is
chiefly regulatory.
The autonomic nervous system is the name given to the system of the twoneuron efferent chain that extends from the CNS to innervate cardiac muscle,
smooth muscle and gland. The pathways within the CNS that convey impulses
from afferent neurons to the first neurons of the two-neuron chain are not a part
of the autonomic nervous system. These pathways within the CNS are
sometimes described as “autonomic pathways”; they should not be confused with
the system of two-neuron efferent innervation of autonomic effectors that is
designated the autonomic nervous system.
autonomic efferent
neuron 1 (red)
efferent neurons (pink)
innervating striated myofibers
autonomic efferent
neurons 2 (in an
autonomic ganglion)
The autonomic nervous system gets its name from the fact that the structures
that it innervates, heart, stomach and intestines, blood vessels, etc., were
thought to act independently of the will; that is involuntarily or autonomously. We
understand now that, with the exception of the local circuitry of the
gastrointestinal tract and perhaps other visceral systems, the nerve impulses
carried by the autonomic nervous system to effector organs have their ultimate
origin in the excitatory state conveyed to the CNS by afferent neurons from
receptors of the body.
Our objectives: This is what we want you to know and understand from this
1. CNS = central nervous system = brain and spinal cord;
2. Receptor organs are sensitive to a particular stimulus:
rod and cone cells of the retina are sensitive to electromagnetic radiation
of the visible spectrum;
hair cells of the internal ear are sensitive to bending of stereocilia;
tactile receptors: sensitive to deformation, bending of hairs, etc.;
pain receptors are sensitive to the release of substances originating from
injured cells;
temperature receptors are sensitive to the infrared electromagnetic
O2 and CO2 receptors: sensitive to oxygen tension and carbon dioxide
tension in the blood;
pH receptors are sensitive to hydrogen ion concentration;
stretch receptors are sensitive to stretch, etc., etc.
There are many different kinds of receptors, each kind sensitive to a
particular stimulus…
3. A single sensory neuron (afferent neuron) passes from a
neuroreceptor synapse to synapse with a neuron (interneuron**) within
the CNS; sensory neurons are excitatory (stimulate one or more
interneurons within the CNS);
**The term “interneuron” is variously used in the literature; in this writing,
it signifies a neuron that is entirely within the CNS.
4. Within the CNS, interneurons synapse with one another in a variable
circuitry. An interneuron will be excitatory (stimulate) or inhibitory
(inhibit stimulation) to all of the neurons with which it synapses.
Ultimately, the interneuronal circuitry ends on neurons (efferent
neurons) whose axons pass from the CNS;
5. Efferent neurons that pass to striated muscle form an excitatory
neuroeffector synapse with a variable number of muscle fibers at their
motor end-plate;
6. Autonomic efferent neurons pass to autonomic ganglia. The ganglion
cells’ axons (postganglionic fibers) end at neuroeffector synapses with
gland, smooth muscle, and heart muscle cells.
7. The fundamental path is: receptor > sensory (afferent) neuron) >
interneuronal circuitry within the CNS > efferent neurons that pass from
the CNS to striated muscle; or, following a peripheral synapse, the
postganglionic fiber passes to gland, smooth muscle, and heart
8. Many receptors. Only four effectors.
The law of the nervous system: Input equals
output. Receptors initiate the input … Muscle and
gland bring about the output…