In the previous video we talked about all the ways in which the nervous system is physically
structured into central and peripheral, sensory and motor, somatic and autonomic, and sympathetic
and parasympathetic. We also got into the basic functionalities of the different parts
and divisions, like what parts of the body the somatic goes out to vs. the autonomic.
Here we’re going to dwell a bit more on how the nerve cells or neurons in all these
parts work together to make a system that gives us such a vast range of abilities—spectacularly
complex yet performing in a relatively (though not entirely) orderly fashion that ultimately
allows an animal to do the work that needs to be done in getting its genes into the next
generation of animals. Yes, it does boil down to natural selection shaping the evolution
of the nervous system of animals, of which ours—the human nervous system—is the most
complex and most sophisticated of the animal world.
The basic idea is incredibly simple. Sensory devices like photoreceptors in your eyes,
touch receptors in your skin, or chemical receptors on your tongue or in your olfactory
bulb send nerve impulses via nerve cells to the central nervous system where the sensory
peripheral nerve cells connect with other nerve cells within the central nervous system
that connect with other nerve cells that connect with others in a way that processes and maybe
stores the information. Meanwhile your motor nervous system is activated by motor centers
in the brain and spinal cord and its neurons go out and tell the motor units to do whatever
it is that they’ll do.
There are some places where the wiring between sensory and motor is very short. The classic
example is a spinal reflex in which a sensory impulse is received by a single spinal neuron
that then just reaches over to the motor neurons that can then cause a muscular contraction.
Here is your hand that touches a hot fajita plate—the one the waitress warns you not
to touch, but you forgot. The sensory neuron from your hand travels the length of your
arm and half your torso width to your spinal cord, where it triggers an interneuron—that’s
the term used for any neuron with both its receipt and delivery of nerve impulses entirely
within the central nervous system. The interneuron sends its signal only a short distance within
the spinal cord to the motor neurons that send a signal down to the muscles of your
arm causing it to pull back quickly. With this kind of simple wiring, the response can
be very fast. In contrast, any kind of more sophisticated response than pulling your hand
away from a hot surface will likely require a complex pattern of nerve impulses getting
sent up to the brain, and it will probably be a different part of the brain that sends
impulses down the spine to your somatic motor peripheral nerves.
At the center of all of this action is the nerve cell, and it should be no surprise to
you that we’ll be spending the next couple of lessons learning about how the neuron does
its job of receiving nerve impulses from other cells and sending nerve impulses down its
axon to other cells.
