figure you see a cross section of the spinal cord similar to the view
you learned earlier when studying the spinal cord for the first time
(see Spinal Cord).
The difference between this drawing and the first one is that this one
includes SANS ganglia and visceral sensory input. If you have not
already done so, you should review several other pages before you worry
about this one. These include 1) Overview,
and 2) SANS. The drawing above can be
broken into two areas of physiology: INPUT via afferent pathways, and
OUTPUT via efferent pathways.
and Unconscious Sensations via Neurons F and G:
you have learned about sensory input from skin, muscle and joint
sensors that send action potentials toward the CNS via spinal nerves
with a final entry into the spinal cord via the dorsal root.
sensory neurons have their cell bodies located in the dorsal root
ganglion. These unusual neurons are called pseudounipolar
neurons. In the drawing above, neuron F is this type.
as you probably have experienced, your inner organs carry pain
sensors. Your inner organs are referred to as your
viscera. Visceral sensors therefore exist to tell your brain
about what is going on deep inside you.
Just like more peripheral sensors such as those in your skin and
joints, visceral sensors exist that are perceived consciously. Have
you ever felt gastrointestinal pain? Other examples are menstrual
cramps that occur because of uterine smooth muscle contractions.
more intense painful feedback comes from uterine muscle during the
In addition, there are sensors that provide the brain with feedback
about homeostatically controlled variables such as blood pressure,
oxygen content, hydrogen ion concentration, etc. These sensors
may be found in and around your visceral organs. The feedback from
these sensors is not consciously perceived, but is certainly perceived
by the brain as its control centers try to maintain normal levels of
blood pressure, etc. (Blood pressure regulation sensory
information can be found at my webpage: Baroreceptor
Reflex.) Both types of sensors send their action
potentials back to the CNS either via the spinal cord or via cranial
nerves that attach directly to the brain. In the diagram
you can see a visceral sensory afferent axon (from the stomach symbol)
enter the SANS ganglion,
pass to the spinal nerve, and then enter the spinal cord via the dorsal
root. This type of neuron is labelled G.
As you saw previously,
ascending tracts in the white matter of the spinal cord carry axons up
to the brain so that sensory input to the CNS is perceived either
consciously or unconsciously. In the drawing above, neuron G
crosses over to the opposite side of the spinal cord and ascends to the
brain in an ascending tract in the white matter.
you know from the overview,
efferent pathways carry commands from the CNS to effectors such as
muscle and glands. The two efferent systems used to control
effectors are 1) the voluntary Somatic Nervous System (SomNS) with
connections to the
skeletal muscles and 2) the involuntary Autonomic Nervous System (ANS)
with connections to the
smooth and cardiac muscles and endocrine and exocrine glands.
Autonomic Nervous System has two sub-systems called the Sympathetic
(SANS) and Parasympathetic Nervous Systems (PANS).
figure above shows how the SANS
nerves exit the spinal cord, but does not show the PANS nerves.
On another website, you can see the many different nerves associated
with the SANS and PANS respectively. To compare the
of the SANS
vs PANS stimulation on effector organs, please read: EFFECTS.
with the SomNS, SANS axons (e.g., neuron C) leave the CNS via
the ventral (anterior) roots of the spinal nerves. Note that the
SANS exits the spinal cord in the thoracic,
and first two sections of the lumbar regions (see SANS). It is therefore also referred to
as the thoracolumbar nervous system.
usual SANS output from the brain to the effectors is carried by FOUR
specific example can be seen at Baroreceptor
beginning of the action potential
journey begins with a neuron which lives in the hypothalamus of the brain. Its axon
the medulla (brain stem).
second neuron then descends down through
tracts of the spinal cord's white matter (e.g. neuron I) and enters the
lateral horn of the grey matter near the level of a particular spinal
the grey matter in the lateral horn, it synapses with the third
neuron, the preganglionic neuron cell body (e.g. neuron C).
The preganglionic neuron send its axon to exit the spinal cord via
the ventral root.
axons travel a short distance
and then take a branch from the spinal nerve to a sympathetic ganglion
where they synapse with the postganglionic neuron's cell body (e.g. neurons E and / or D).
In some cases neural commands begin in the medulla, so the path
involves only three neurons.
drawing of the 31 spinal
To learn more
of the spine: SPINE.
learn more about
division of the Autonomic Nervous System: SANS.
To learn more
division of the Autonomic Nervous System: PANS.
To compare the
of the SANS
vs PANS stimulation on effector organs: EFFECTS.