THE NERVOUS SYSTEM II
I. Myelination and action potential conduction
A. Axons are myelinated by the activities of oligodendrocytes in the central nervous
system and Schwann cells in the peripheral nervous system.
B. Perhaps the most important reason for this is that myelination allows for higher
velocities of nervous impulse or action potential conduction.
C. Action potentials are generated by ion permiability changes in the axon membrane.
Once an action potential is initiated, it is self-propagating. In nonmyelinated axons, the
permiability changes must be induced along the entire length of the axon. The action
potential can only be conducted as fast as the permiability changes can occur.
D. Because myelinated axons are well insulated by their myelin coverings, the action
potential can skip the myelinated lengths of the axon and it is only necessary for the
permiability changes to occur at the nodes of Ranvier (between myelinated portions of
the axon). So, myelination shortens the length of axon membrane in which permiability
changes must be induced and thereby speeds up transmission of the nervous
impulse.
E. The fewer the nodes of Ranvier for a given length of axon, the faster the
transmission.
II. Involuntary component of the nervous system
A. The voluntary component of the nervous system is important in effecting the
movement of skeletal muscle and sensing changes in our external environment.
B. There is also an involuntary component to the nervous system called the
autonomic nervous system. The neurons of this part of the central and peripheral
nervous systems are important in,
1. modulation of smooth muscle contraction - particularly the digestive
and reproductive tracts.
2. secretion of some glands
3. modulation of cardiac rhythm
C. Thus, the autonomic component of the nervous system relieves us of the necessity
of tying up large parts of our voluntary circuitry simply to maintain vital body functions.
If conscious thought were necessary every time our heart beat and every time the
muscles of our digestive tract contracted, we would not have much time to think about
anything else.
II. Involuntary component of the nervous system
D. Autonomic nervous system is divided into two parts
1. Sympathetic - neurotransmitter is
norepinephrine (noradrenaline) for
postganglionic axons, has adrenergic
synapses - stimulates (enhances) activity.
Preganglionic axon synapses use acetyl
choline.
http://faculty.washington.edu/chudler/auto.html
II. Involuntary component of the nervous system
2. parasympathetic - neurotransmitter is
acetylcholine, has cholinergic synapses generally slows activity
http://faculty.washington.edu/chudler/auto.html
E. These two components of autonomic nervous system are generally antagonistic, so in a
given organ that they innervate, one will stimulate and the other will inhibit the activities of the
organ [like an accelerator (sympathetic) and a brake (parasympathetic)] . There are
exceptions where both stimulate such as in some salivary glands.
III. Ganglion structure
A. Ganglion (pl. - ganglia) - an aggregation of
neurons and associated glial cells outside the central
nervous system.
http://millette.med.sc.edu/Lab%209&10/F5%20image%20page.htm
III. Ganglion structure
B. Types of ganglia,
medulla
oblongata
1. Dorsal root (spinal) ganglia and cranial
ganglia (craniospinal ganglia) - neurons
are purely sensory, considered part of
voluntary nervous system. Connective
tissue capsule surrounds ganglion.
a. Cranial ganglia - nerves running through them
may have both motor and sensory components sensory components (axons/dendrites) from
neurons in ganglia, motor components (axons)
from neurons in brain.
spinal
ganglia
dorsal rootlets of
spinal nerves
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab9/Examples/exsomarc.htm
III. Ganglion structure
1. Dorsal root (spinal) ganglia and
cranial ganglia (craniospinal ganglia)
a. Neurons mainly cortical, small (15-30 um)
and large (120 um) neurons. Ganglion medula
is mainly axonal processes and a few glial and
neuronal cells.
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Nervous/Nervous.htm#labganglia
1. Dorsal root ganglia and cranial ganglia
b. If properly stained, neurons of these ganglia
usually show fine Nissl bodies and may have
lipofuscin deposits.
c. Neuron perikarya are surrounded by glial
cells called satellite cells. Form a sort of
capsule around the perikaryon
d. Fibroblasts are present between the satellite
cell surrounded perikarya.
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Nervous/
Nervous.htm#labganglia
1. Dorsal root ganglia and cranial ganglia
e. Neurons of dorsal root and cranial ganglia are
generally pseudounipolar. Sensory stimulation of
dendritic end causes nerve impulse that bypasses the
neuron perikaryon.
f. An exception are neurons of the vestibular ganglion
of the inner ear (associated with cranial nerve #8)
which are bipolar
http://www.csus.edu/org/nrg/carter/NeurosylActive/histology/neuron/bipolar.htm
III. Ganglion structure
B. Different types of ganglia,
2. Autonomic ganglia - associated with nerves of the autonomic nervous system.
Both sensory and motor cell bodies. Connective tissue capsule may surround
autonomic ganglia.
a. intramural ganglia
• generally considered part of parasympathetic
system
(recent work suggests both parasym-pathetic and
sympathetic neurons)
• small (just a few neurons)
• located within viscera particularly in the walls of
digestive tract
• no connective tissue capsule surrounding intramural
ganglia
• A lot fewer satellite cells than in dorsal root and
cranial ganglia
III. Ganglion structure
B. Different types of ganglia,
2. Autonomic ganglia
b. Other autonomic ganglia
• may have connective tissue capsule
• neuron perikarya not just cortical
• both sympathetic and parasympathetic neurons
• mainly multipolar neurons with fine Nissl bodies
• innervation to both smooth muscle and secretory
components of mucosa.
• perikarya with incomplete layer of satellite cells
• nuclei of neurons offset to one side of perikaryon
Autonomic ganglion in the wall of the ureter.
http://millette.med.sc.edu/Lab%209&10/F7%20image%20page.htm
http://www.med.mun.ca/anatomyts/digest/gut97a.htm
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Muscle/Muscle.htm#LABSMOOTH
Invertebrate ganglia
CNS consists of ganglia with characteristic
structural organization
Neurons typically unipolar.
All neuron perikarya in cortex, just under
connective tissue sheath
Medulla of ganglion, the neuropil, consists of
closely packed dendrites/axons and synapses
Nerve tracts extend from ganglia to peripheral
regions of body. Both motor and sensory function.
Ganglia exhibit highly specific organization, often
possible to identify the same specific neuron in
different individuals of same species (identifiable
neurons)
http://slugsite.us/bow/nudiwk76.html
IV. CNS, Gray Matter, White Matter
Review+
A. White matter - cortically located (except in cerebrum and
cerebellum). Composed of
1. myelinated and unmyelinated axons,
2. oligodendrocytes,
3. astrocytes, mainly fibrous
4. microglial cells
5. White color due to presence of large
amounts of myelin.
6. no neuron perikarya!
cortical - peripheral
medullary - middle
Review+
B. Gray matter - usually medullary (except cerebrum and cerebellum). Composed
of
1. neuron perikarya,
2. dendrites, some lightly myelinated
cortical - peripheral
medullary - middle
3. mostly unmyelinated axons
4. some myelinated axons,
5. astrocytes, mainly protoplasmic
6. oligodendrocytes,
7. microglial cells.
C. Consider spinal cord structure versus cerebellum structure.
1. Spinal cord
a. cortical white matter
cortical - peripheral
medullary - middle
b. medullary gray matter in a butterfly shape
Posterior median sulcus
c. central canal lined with ependymal cells
d. gray matter divided into
* ventral (anterior) horns ventral roots, motor
function
* dorsal (posterior) horns dorsal roots, sensory
function
Anterior median fissure
http://www.lab.anhb.uwa.edu.au/mb140/
IV. CNS, Gray Matter, White Matter
2. Cerebellum
a. composed of many folds called
folia (sometimes gyri).
b. these folds have cortical gray matter,
medullary white matter
c. Cerebellum - the cortical gray matter is divided into 3 regions.
* Most external - molecular layer with few perikarya and many unmyelinated nerve fibers.
* Middle - Purkinje neuron/cell layer - single layer of neurons with perikarya lying
between molecular layer and granular layer. Highly branched dendritic tree that extends
into molecular layer. Axon extends into granular layer. Dendritic branching in one plane
(fan-shaped).
* Inner - granular layer - Smallest neurons in human body (5 um diameter). Neurons are
multipolar with 3 - 6 dendrites and one axon.
Cerebellum - silver nitrate
Cerebellum - hematoxylin
http://www.histol.chuvashia.com/atlas-en/nerv-04-en.htm
IV. CNS, Gray Matter, White Matter
3. Cerebrum also has cortical gray matter and medullary white matter.
a. General gross structure similar to that of
cerebellum (folds called gyri), but neurons are
different and the gray matter only consists of one
layer.
b. Perikarya of neurons in cerebrum are mostly
pyramidal, stellate (multipolar), or spindle shaped
(bipolar).
http://www.kumc.edu/instruction/medicine/anatomy/histoweb/nervous/ne
rve11.htm
http://www.histology.wisc.edu/histo/uw/histo.htm
V. Sensory structures
Somatic receptors - receptors associated with the skin, i.e. involved in reception of
cutaneous senses.
Visceral receptors - receptors associated with the internal tissues and organs, i.e. the
viscera.
A. There are 5 principle types of receptors.
1. nociceptors (somatic and visceral receptors) that are sensitive to pain, pressure,
vibration and temperature.
2. proprioceptors that are sensitive to ones position in space. Inner ear,
Pacinian corpuscles, and receptors of muscular info (muscle spindles).
3. chemoreceptors - taste and smell
4. audioreceptors - hearing
5. photoreceptors - sight.
B. Your slide set in lab only has an example of Pacinian corpuscles (proprioceptor). We
will only discuss Pacinian corpuscles, the end=bulb of Krause (nociceptor), and
Meissner's corpuscles (nociceptor) here.
V. Sensory structures
1. TOUCH - Meissner's corpuscles
a. the most numerous sensory structure in
your body.
b. Common in hairless skin such as tips
of fingers, palms, soles, nipples, and
lips.
c. Elongated structures consisting of
fibroblasts and/or modified Schwann cells.
that form a multilayered stack.
d. Naked (unmyelinated) dendrite/axon enters one
end and zigzags up through stack. This structure
is encapsulated in connective tissue and is usually
located in a dermal papilla just below the
epidermis. The naked dendritic endings are what
perceive touch.
http://www.udel.edu/Biology/Wags/histopage/colorpage/cin/cinmcs.gif
Meissner's corpuscles
http://www.udel.edu/Biology/Wags/histopage/colorpage/cin/cintpmc.GIF
V. Sensory structures
Meissner's corpuscle
V. Sensory structures
2. PRESSURE - Pacinian corpuscles
a. Central unmyelinated nerve ending
(dendrite/axon) surrounded by 30 or more
concentric layers of connective tissue composed
of epithelioid fibroblasts.
b. Found in deep layer of dermis, in loose
connective tissue, in mesenteries, and in external
genitalia of both males and females.
c. Function in propriocetion, i.e. sense/signal
mechanical deformations of limbs and other body
parts.
d. Respond to vibrations and pressure
http://www.udel.edu/Biology/Wags/histopage/colorpage/cne/cnepc.GIF
V. Sensory structures
http://www.udel.edu/Biology/Wags/histopage/colo
rpage/cne/cnepc.GIF