Nervous System
Part III: PNS
I. The peripheral nervous system
A. The peripheral nervous system is comprised of:
1. sense receptors-sense organs, referred to as biological
transducers--a device that transfers energy, usually converting
it into a different form when it does so.
a. senses are; hearing, smell, sight, taste, balance,
touch (a compound sense-pain, heat, cold and
pressure) and senses dealing with joint position,
muscle tension, blood pressure, blood pH, blood
oxygen content, blood osmotic pressure,
cerebrospinal-fluid pH, and others.
b. senses are divided based on where they receive their
stimuli.
1) exteroceptors-sense the external environment and
changes in it.
2) interoceptors-unconscious senses, maintain the internal
environment by sensing the changes in the internal body.
2. sensory nerves: the nerves that link
receptors with the CNS. and
3. motor nerves: the nerves that
link the CNS with the effectors.
sense organ
effector
B. There are several divisions within the peripheral nervous
system (PNS).
1. the somatic system-designed to keep the body in balance
with the external environment, provides communication
between various parts of the body and somatic effectors
(skeletal muscle cells), and
2. the autonomic system-designed to maintain the internal
environment, conducts impulses to visceral effectors
(autonomic effectors-smooth muscle, cardiac muscle, and
glandular tissue.
C. somatic nervous system:
1. contains:
a. receptors that react to changes in the external environment.
b. the sensory neurons that keep the CNS informed of those
changes, and
c. the motor neurons that adjust the positions of the skeletal
muscles in order to maintain the body's integrity and wellbeing.
2. sensory neurons and motor
neurons- cranial and spinal nervesthese nerves carry the sensory and
motor neurons of the somatic system,
as well as neurons of the autonomic
system.
Cranial nerves
Spinal nerves
a. cranial nerves
1) 12 pairs of cranial nerves-10 pairs come from the
brainstem, 1 pair comes from the cerebrum and 1 pair comes
from the thalamus.
2) Function:
a) transmit information to the brain from the special sensory
receptors regarding the sense of smell, sight, hearing, and
taste, and from general sensory receptors, especially in the
head region,
b) also bring orders from the CNS to the voluntary muscles
that control movements of the eyes, face, mouth, tongue,
pharynx, and larynx,
c) and provide communication between the CNS and some of
the internal organs.
3) designated by roman numerals and by name. names
suggest their distribution or function, numbers indicate the
order in which they emerge from the front to back.
a) I Olfactory-sensory; smell
b) II Optic-sensory; vision
c) III Oculomotor-motor; eye control
d) IV Trochlear-motor; movement of eye
e) V Trigeminal-mixed; sensations of head and face, chewing
f) VI Abducent-motor; eye movement
g) VII Facial-mixed; taste; facial expression, saliva secretion
h) VIII Vestibulocochlear (Acoustic)-sensory; hearing and
equilibrium
i) IX Glossopharyngeal-mixed; taste, swallowing; saliva
secretion, blood pressure and respiration control
j) X Vagus-mixed; sensations and movement of organs
supplied
k) XI Spinal accessory-motor movements of shoulder and
head
l) XII Hypoglossal-motor; tongue movements
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b. spinal nerves
1) 31 pairs of spinal nerves emerge from the spinal
cord and exit out of the spinal cavity horizontally
through the intervertebral foramina of their respective
vertebrae. Each spinal nerve innervates a specific
region of the body known as a nerve dermatome.
2) Even though each segment of the body surface is
innervated by a particular spinal nerve, there is sufficient
overlap so that should a nerve be injured, the segment will
still be innervated by nerves that serve segments on either
side of it.
3) identified by the general region of the vertebral column
from which they originate,
a) 8 pairs of cervical spinal nerves (C1-C8).
b) 12 pairs of thoracic sn (T1-T12).
c) 5 pairs of lumbar sn (L1-L5).
d) 5 pairs of sacral sn (S1-S5).
e) 1 pair of coccygeal sn (Co1).
Nerve Dermatomes
Cervical nerves C1– C8
Thoracic nerves T1-T12
Lumbar nerves L1-L5
Sacral nerves S1-S5
4) attaches indirectly by means of 2 short roots, anterior
(ventral) and posterior (dorsal) roots.
a) dorsal root:
1. consists of sensory
(afferent) fibers.
2. there is a swelling,
dorsal-root ganglion
containing the cell
bodies of the sensory
neurons.
Dorsal root
Dorsal-root ganglion
Cell bodies (sensory neurons)
spinal nerve
b) ventral root:
1. consists of motor
(efferent) fibers.
2. cell bodies are found in
the spinal cord, there are no
ganglion here.
Ventral root
Motor cell bodies
spinal nerve
Spinal Nerve Roots
Dorsal root
Dorsal-root ganglion
Cell bodies (sensory neurons)
Ventral root
Motor cell bodies
spinal nerve
5) each spinal nerve exits from the vertebral column through
an intervertebral foramen and is named for the vertebra
immediately below that foramen.
a) dorsal and ventral roots unite to form a spinal nerve in the
intervertebral foramen,
Dorsal root
Dorsal root ganglion
Ventral root
Spinal nerve
b) each spinal nerve divides into rami (branches).
1. dorsal ramus- fibers that innervate the muscles and skin of
the dorsal portion of the body in that area.
2. ventral ramus- consists of fibers that innervate the ventral
portion of the body in the area as well as the limbs.
a. form plexuses (networks) with one another, each
plexuses consists of interconnections of several
ventral rami, these then give rise to peripheral nerves.
3.Meningeal branch- divides into 2 communicant rami that
form part of the sympathetic chain ganglion.
a. white rami communicants- white because they
are myelinated
b. gray rami communicants- gray because the fibers
are not myelinated.
6) all spinal nerves are mixed or both sensory and motor.
Sensory Neurons
Motor Neurons
Sense organs
Dorsal root
Dorsal root ganglion
Ventral root
Spinal nerve
Sympathetic chain
ganglion
effectors
Dorsal ramus
Ventral ramus
White ramus
Gray ramus
Dorsal ramus
Ventral ramus
White ramus
Gray ramus
sympathetic
ganglion
Gray ramus
White ramus
Ventral ramus
Dorsal ramus
body wall,
muscle
layers
D. Autonomic system:
1. maintains a steady state within the internal environment.
works automatically and without voluntary input.
2. effectors are smooth and cardiac muscle and glands.
3. the autonomic system has a series of 2 efferent neurons
between the spinal cord and the effector. (the somatic system
has a single efferent neuron)
a. preganglionic neurons--conduct impulses before they reach
a ganglion, the first to emerge from the spinal cord.
b. postganglionic neurons--conduct impulses after they reach
a ganglion, the second in the series.
4. The autonmonic system is subdivided into two categories.
a. sympathetic system-is associated with mobilizing energy
during stress situations.
1) this system operates continually, but it effects are most
dramatic during emergency situations.
2) Sympathetic neurons:
a) the preganglionic neurons emerge from the spinal cord
through the motor roots of the thoracic and 2 lumbar spinal
nerves. (thoracolumbar)
1. they pass through the white rami communicants to the
paravertebral sympathetic ganglion chain. (series of ganglia
located along the length of the vertebral column) most end
within the ganglia and synapse with the postganglionic
neurons. they innervate blood vessels and organs in the head,
neck, and thoracic region.
paravertebral
sympathetic ganglion
chain
3) some sympathetic ganglia are located a short distance from
the sympathetic chain ganglia and are called collateral
ganglia.
a) these ganglia have their origins in the lower 6 thoracic and
first 2 lumbar levels of the spinal cord.
b) neurons from the collateral ganglia innervate smooth
muscles and glands of the abdominal and pelvic viscera and
their blood vessels.(includes organs of the digestive, urinary,
and reproductive systems)
4) sympathetic effects tend to be quite widespread rather than
precise, involving many organs and not just one.
5) generally, sympathetic neurons release norepinephrine
(exception--neurons that innervate sweat glands generally
release acetylcholine)
6) the sympathetic nervous system helps the body adjust to
the continuous stresses of life.
sympathetic system
thoracolumbar
preganglionic neuron
postganglionic neuron
spinal nerves
sympathetic chain ganglia
collateral ganglion
b. parasympathetic system-usually acts to conserve and
restore energy.
1) neurons emerge from the brain as part of the III,
VII, IX, & X cranial nerves and from the sacral region
of the spinal cord as part of the 2, 3, & 4 sacral spinal
nerves. (craniosacral outflow)
2) preganglionic fibers synapse with postganglionic neurons
in terminal ganglia located near or within the visceral
structures that they innervate.
3) those from the cranial region innervate the eye,
structures of the head, and thoracic and abdominal
viscera.
4) the sacral nerves innervate the lower portion of the large
intestine, urinary system and reproductive system, including
the erectile tissues.
5) parasympathetic nerves do not innervate the blood vessels
or sweat glands.
6) parasympathetic fibers release acetylcholine as their
transmitter substance, known as a cholinergic system
7) the effects of the parasympathetic system are brief, but
more precise than the sympathetic.
8) this system is most active when you are emotionally calm
and physically at rest.
parasympathetic system
craniosacral
terminal ganglion
preganglionic neuron
postganglionic neuron