21 - Peripheral Nervous System
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Transcript of 21 - Peripheral Nervous System
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Lecture 21 Peripheral Nervous SystemKardong Chapter 16, Hildebrand Chapter17
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Basic Layout of the Nervous SystemThe nervous system stimulates the action of muscles in light of information
from sense organs.
sense organs central nervous
system
afferent or
sensory neurons
muscles
efferent or motorneurons
sensory and motor neurons = peripheral nervous system
association
neurons
Nerve cells (neurons) are the basic units of the nervous
system. Bundles of neurons are called nerves in the
peripheral nervous system or tracts in the central nervous
system.
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Cell Body or perikaryonwhere the nucleus resides.
Axonthe cellular extension
that transmits information.
Axon cylinder = axon plus
Schwann cells which wrap theaxon in myelin.
Dendrites - the connection to
adjacent nerves, muscles, or
sense organs.
Ganglion - a group of cellbodies outside the CNS.
The morphology of a motor neuron
KK 16.2, H&G (Fig. 17.2)
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Other Types of Neurons
While motor neurons are typicallymonopolar(cell body at one end), sensory
neurons are typicallybipolar(cell body in
the middle of the axon).
To complete a circuit between sense organ
and muscle, motor neurons and sensoryneurons are linked in the CNS by association
neurons, which are typically multipolar.
KK 16.3, H&G 17.2
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Synapses and Neurotransmitters
Dendrites of neurons do not
touch; transmission is across a
synapse via diffusion of a
neurotransmitter produced by
presynaptic vesicles.
At least 3 neurons are
involved in going from
sensory information
(stimulus) to muscleresponse. Such a circuit from
sensor to effector is called a
reflect arc.
KK 16.5, H&G 17.1, 17.8
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Sensory and motor peripheral nerves
Sensory nerves enter the CNS via
dorsal roots. Their cell bodies are indorsal root ganglia outside the CNS.
Motor nerves leave the CNS via
ventral roots. In spinal nerves, these
roots come together just outside the
cord and separate again into dorsaland ventral rami.
KK 16.7, H&G 17.8
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Embryological OriginsCNSThe brain and spinal
cord are derived from the
neural tube (neurectoderm).
PNSMotor nerves have
their cell bodies in the CNS,
and their axons grow out of
the CNS to targeted musclesand organs.
The dorsal root ganglia
containing the cell bodies of
sensory nerves are derived
from the neural crests.Bipolar axons grow towards
the CNS and towards their
target sense organs.
KK 16.8, H&G 17.7, 17.8
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Relationship of peripheral nerves to
embryonic mesoderm
The dorsal ramus serves
the epaxial musclesand
dermis, whereas the ventral
ramus serves the hypaxial,appendicular, and visceral
muscles.
KK 16.9
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1) Central Nervous System: brain and spinal cord
2) Peripheral Nervous System: spinal and cranial nerves
A) Somatic Nerves- sensory - information from the integument, skeletal muscles
- motor - information to integument and muscles
B) Visceral Nerves- sensory - information from receptors in the viscera
- motor - information to visceral muscles (gut, heart)
Divisions of the Nervous System
Visceral motor nerves comprise autonomic system. They are of two sets that actin opposition to each other. One type comprises the sympathetic nervous system
and the other the parasympathetic nervous system.
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Again :
Sensory neurons travel via
dorsal root into the spinalcord. Their cell bodies are in
the dorsal root ganglia.
Motor neurons travel via
ventral root out of the spinal
cord. Their cell bodies are inthe grey matter of the cord.
Somatic neurons (sensory
or motor) use somatic or
dorsal ramus.
Visceral neurons use
visceral or ventral ramus.
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A complication:
The previous diagram
illustrates the pattern for
spinal nerves in amniotes,but in lampreys only
somatic motor fibres use
the ventral root.
Non-amniotes may havevisceral motor neurons using
either root, as below left.
Knowing this helps us
understand the cranial
nerves (which are not so
neatly arranged as spinal
nerves!)
Fig. 17-12
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Cranial Nerves - peripheral nerves emerging from the brain
The cranial nerves are numbered with Roman numerals I through up
to XII, and some recognize an anterior nerve 0 (terminalis).
Special Sensory Nerves. Three cranial nerves are associated with the
special sense organs. They are not serially homologous with the rest
of the peripheral nervous system. These are the olfactory nerve (I),
the optic nerve (II) and the auditory (statoacoustic) nerve (VIII).
Dorsal Root Cranial Nerves. These cranial nerves are sensory and
visceral motor in function, and serially homologous with dorsal roots
of spinal nerves.
Ventral Root Cranial Nerves. These are somatic motor in function,
and serially homologous with the ventral roots of spinal nerves.
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Ventral view of shark brain showing the cranial nerves.
Dorsal Root (sensory or
visceral motor)
Special
Visceral Root (somatic motor)
KK 16.14, H&G 17.14
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Cranial nerves help us
understand the
segmentation of the
vertebrate head, which has
been highly modifed and no
longer appears segmented.
The diagram illustrates the
hypothesized ancestralsituation of the dorsal roots,
and the dorsal root cranial
nerves of a fish. The
vertebrate head has at least
7 segments (see Table 16.2in Kardong) but if the upper
diagram is correct it would
be 8 or more.
KK 16.16, H&G 10.7
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