The next several podcasts will deal with nervous tissue and give you a brief
overview of the nervous system. The learning objectives would like you to be
able to recognize neurons in the central nervous system and in the peripheral nervous system.
I would like you to be able to recognize the major components of the central
nervous systems that would be different regions in the brain and spinal cord of
course we want you to be able to do that on histologic preparations. We'd
like you to be able to recognize component parts of the peripheral
nervous system so for example ganglia and peripheral nerves, would like you to
be able to compare and contrast white matter and gray matter as we talked
about these in the central nervous system and would hope that you begin to
integrate some neuro histology with things you've learned in anatomy so try
to integrate histology with anatomy and physiology. If we talk about the nervous
system in relation to tissues and organs, we can consider the central nervous
system as separate from the peripheral nervous system. The central nervous
system of course consists of the brain and the spinal cord and now here's an
unfamiliar term probably, we talk about nuclei and here when we use the term
nuclei we're talking about collections of nerve cell bodies, so if we say a
nucleus in the brain we're talking about a collection of nerve cell bodies in the
brain. We also talked about tracts or collections of axons and we'll talk
about gray matter, that region in the central nervous system where the cell
bodies of neurons are located, as opposed to white matter, that region of the
central nervous system that primarily just contains the axons of neurons, and
of course there are other cell types in the white and gray matter and we'll talk
about that as we go along in these discussions and now the peripheral
nervous system contains ganglia and ganglia are essentially collections of
nerve cell bodies and then nerves themselves and nerves in the
peripheral nervous system refer to tracts of axons or collections of axons.
So you remember for example in gross anatomy you may have talked about
cranial nerves or spinal nerves. We're not gonna ask you to be able to identify
histologically a cranial nerve or a spinal nerve, we would hope that you'd
recognize peripheral nerves though in histological sections and we'll review a
little bit later some of the nerve endings that we'd like you to be able
to recognize as well. Now just as a brief overview, we can make the general
statement that peripheral nerves conduct impulses from efferent or motor nerves
and they conduct impulses to afferent or sensory nerves of the central nervous
system and I think it's not a surprise to you that interactions between sensory
nerves so called afferent nerves which receives stimuli, the central nervous
system neurons that interpret those stimuli and motor or efferent nerves
that initiate responses create what we call neural pathways or reflex arcs and
in humans most sensory neurons that are out in the periphery of the body
don't pass directly to the brain, instead they communicate with motor neurons in
the spinal cord and you'll appreciate this more as you go through
neurophysiology and anatomy next year. We can talk about the somatic nervous
system and the autonomic nervous system, the somatic nervous system would be
neurons in the central nervous system that project to say the skin or to sense
organs or to primarily skeletal muscles and the somatic nervous system controls
the conscious voluntary functions of our skeletal muscles. The somatic nervous
system also controls involuntary movements of significant reflex arcs
like the knee jerk reflex or moving your hand away from a hot flame, those would
all be controlled by the somatic nervous system, on the other hand the autonomic
nervous system controls efferent involuntary functions like
heartbeat, breathing, digestion, maybe secretion of glands but also would
control afferent sensations from the viscera, so for example if you were
sensing pain from some of the visceral organs. Now as we said the somatic
nervous system that neuron cell bodies are usually within the central nervous
system, in the autonomic nervous system there are ganglia that are outside the
central nervous system. We can talk about subdivisions of the autonomic nervous
system, we're not gonna make a big deal about them in this course because you
will study them to some extent in Physiology into a much greater extent
next year in neurobiology but we talk about the subdivisions of the autonomic
nervous system as the sympathetic subdivision, the parasympathetic, and the
enteric subdivisions of the autonomic nervous system. As you probably have some
knowledge, the sympathetic nervous system is involved in the fight-or-flight
reflex, the parasympathetic division of the autonomic nervous system controls
and maintains normal bodily function so this actually regulates homeostasis
maybe like to control resources, the enteric component of the autonomic
nervous system of those components that are actually within the alimentary canal
and those nerves can actually function independently of the central nervous
system although as you will remember from gross anatomy, there are inputs
obviously from the central nervous system into the enteric nerves of the
alimentary canal. Note in this figure, the somatic motor neurons have cell bodies
in the central nervous system and the axons travel directly to skeletal muscle
where they innervate the skeletal muscle, on the other hand note in the autonomic
nervous system, there are ganglia outside the central
nervous system. Here would be a what we would call a presynaptic neuron cell
body in the central nervous system but it comes out into the peripheral nervous
system, synapses with a postsynaptic neuron
and that then innovates an effector and in this case we're defining effector as
the target for the efferent neurons. We can talk about these effectors in
various locations in the body, the other point I want you to take away from here
besides the fact that they are a ganglion that are outside of the central
nervous system where where these autonomic nerves meet and synapse, but
there's at least a two neuron chain which is going to convey impulses to the
visceral effectors, so there's a neuron cell body that originates in the central
nervous system, the so called presynaptic neuron, which is going to synapse in a
ganglion outside of the central nervous system to the so called postsynaptic
neuron and that will then innervate the target organ. We're not gonna go over
these details in our course here, you've covered some of this in your anatomy
course, you'll cover it in much more detail next year in the neurobiology
course but this slide basically shows the divisions between the sympathetic
autonomic nervous system and the parasympathetic division of the
autonomic nervous system, just some basic things you recall the parasympathetic
divisions are basically from the cranial nerves and the sacral nerves, the
sympathetic divisions are basically from the cervical thoracic and lumbar regions
and as I said I'm not going to spend any time in our course here going over these,
this is just for your information to help you integrate and put things
together from what you've already talked about a little bit in anatomy and what
you will talk about in much greater detail later in your training. I think as
we appreciate though the autonomic nervous system does regulate the
function of organs by specific effectors so for example an effector could be
smooth muscle. Smooth muscle can modify the diameter or shape of the tubular
organs or hollow viscera, smooth muscle might under the influence of the
autonomic nervous system alter the diameter of blood vessels, might alter
the behavior of cells in the gut, the gallbladder, or the
urinary bladder. Of course the Cardiac conducting cells or Purkinje fibers,
their inherent depolarizations regulate the rate of cardiac muscle contraction.
If you think of glandular epithelium, the synthesis even composition and release
of some of the glandular secretions are under the control of the autonomic
nervous system and I don't think it's a surprise to you that neurons in the
brain and in other parts of the body can behave as secretory cells and so you're
gonna talk about in physiology and will allude to it later
in this course that some tissues have a neuroendocrine capability or a neuro
endocrine function and we'll leave that to discuss later in the course when we
talk about some of the endocrine organs. If we look at the development of the
nervous system, most of the nervous system develops from neuroectoderm, the
neural crest does play a role in the development of sensory and autonomic
ganglia, clearly Schwann cells and satellite cells which are supporting
cells of nerves developed from neural crest and then regions like the
adrenal medulla also originate from the neural crest and that might help you to
understand why we talk about neuro endocrine functions both of for example
cells and the adrenal medulla which we'll get to much later in the course,
but also the fact that some neurons can behave like endocrine cells. Composition
of nerve tissue, two major cell types are the neurons and the supporting cells. The
neurons or nerve cells are actually the functional cell of nerve tissue. You can
think of them almost as like the parenchymal cells, there are essentially
two major classes of neurons. We can talk about them as principal neurons
so-called projection or Golgi type one neurons, and interneurons or local
circuit or Golgi type two neurons, and neurons function to receive stimuli, to
conduct impulses and they form synapse from one neuron to another neuron or
from a neuron on to an effector cell so for example a neuron onto a say a
skeletal muscle cell, and then we talk about the supporting cells,
neuroglial cells or simply glial cells for short, these are the supporting cells
and in a loose way you might think of these as the stromal cells in nervous
tissue. These supporting cells or glial cells provide physical support for
neurons and protection for neurons, they can insulate the neurons as you'll come
to appreciate membrane depolarization is very important in the conduction of
nerve impulses so insulation is important. The supporting cells play a
role in repair after neural injury, you may remember that neurons by in large
there are some exceptions but neurons by in large don't divide but need some
glial cells maybe neural stem cells. Some of the glial cells regulate the internal
milieu or fluid environment of the central nervous system. Some of them play a role
in clearing neurotransmitters from synaptic clefts and some of them are
important in metabolic exchange between the vascular system and neurons.