Transcript Peripheral Nervous System

Peripheral Nervous System
Chapters 14
Anatomy 32
The peripheral nervous system and its subdivisions
The PNS includes all nervous structures found outside the CNS. It requires
Three components: sensory receptors (pick up stimuli and send signals to
CNS), motor endings (axons synapse with effectors like muscle and glands),
and nerves and ganglia (neurons and cell bodies outside of CNS). Note that
almost all nerves are mixed, meaning theycarry both sensory and motor
neurons. Receptors can be classified by location, stimulus detected, or
structure
I. Peripheral sensory receptors- Receptors are either dendritic ends of
sensory neurons that monitor general sensory information over wide areas,
Receptors may also associate with cells that transfer the signal to a sensory
neuron (ex. merkel cells) and monitor specialized senses by existing in
localized regions.
A. Classification by location of sensory receptors1. Receptors that receive stimulus from the environment are sensitive
to pressure, pain, smell, sight, and hearing are called exteroceptors.
2. Receptors that are sensitive to the body’s internal changes, either
visceral or chemical changes, taste, and temperature are called
interoceptors.
3. Receptors found within skeletal muscles, tendons, joints, and
ligaments monitor the amount of stretching and send input on
movement. They are called proprioceptors.
B. Classification by stimulus detected1. Mechanoreceptors are stimulated by mechanical forces such as
touch, pressure, ect.
2. Thermoreceptors are stimulated by temperature changes
3. Chemoreceptors respond to chemicals changes in blood, taste, or
smell.
4. Photoreceptors respond to light such as those found in the eye.
C. Classification by structure- the specialized sense organs have receptors
with special structures for detecting certain stimuli, the ones listed below are
general sensory receptors found throughout the body that detect
mechanical forces.
1. Free dendritic endings- naked dendritic endings are in every tissue and
most often in the integument. They sense pain and temperature changes
and some sense light touch. Itch receptors (newly discovered) are also free
dendritic ends but are very thin in diameter.
2. Encapsulated dendritic endings- these are nerve endings wrapped in
connective tissue and serve as mechanoreceptors.
Meissner’s corpuscles- detect light touch in the skin
Krause’s End bulbs- detect fine touch in mucus membranes
Pacinian Corpuscles- found in deep connective tissue (hypodermis)
respond to vibration and briefly respond to deep pressure.
Ruffini’s Corpuscles- found in dermis respond to continuous pressure
Proprioceptors- found in muscle tissue, tendons, and ligaments, they
monitor stretch in locomotory organs
II. Peripheral motor endings- Axons that synapse with either muscle
or glands to activate them.
A. Innervation of skeletal muscles- each muscle fiber is
associated with a neuromuscular junction (synapse between a
motor axon and muscular tissue). A motor neuron branches in order
to innervate individual skeletal muscle fibers and is called a motor
unit. At the synapse the axon releases synaptic vesicles filled with a
neurotransmitter called acetylcholine which depolarizes the
sarcolemma. The basal lamina of the sarcolemma grooves releases
acetylcholinesterase, an enzyme that immediately breaks down
acetylcholine to prevent further stimulation of muscle twitch. The
motor unit signal causes an overall contraction of the muscle. The
finer the motor movement the more motor neurons and each
innervates less muscular fibers.
B. Innervation of visceral muscle and glands- the visceral motor
axon forms a row of axon knobs (varicosities) and have a larger
distance in the synaptic cleft. The response is slower because of
the time it takes for the neurotransmitter to diffuse. The heart is also
innervated by these motor neurons, but there are no varicosities.
III. Cranial Nerves- These nerves serve the head and neck; they originate in
the brain and most of them exit the skull through cranial foramina not
vertebral foramina. They are numbered I – XII rostrally to caudally.
• A. Olfactory nerve I- sensory nerve for smell, it runs
below the frontal lobe, purely sensory, cerebrum
– B. Optic nerve II- a brain tract exiting through the optic chiasma,
it sends signals of images to the brain, purely sensory, cerebrum
C. Oculomotor nerve III- caudal to optic chiasma it innervates internal
eye muscles to move the eye (superior, inferior, lateral, medial rectus)
and eyelids. It adjust the pupil and lens. Motor nerve, visceral motor,
and proprioceptive, midbrain
D. Trochlear nerves IV- (pulley) it inervates the superior oblique
muscles of the eye. Motor nerve, midbrain
E. Trigeminal nerves V – (three fold)- it has three branches that carry
sensory information from the face (superficial and internal) and motor
information for chewing muscles. Mixed, pons
F. Abducens nerves VI- innervates a muscle that abducts the eye (
lateral rectus), motor nerve, pons
G. Facial nerves VII- innervates muscles of facial expression, activates
facial glands, conveys sensory from taste buds. Mixed and visceral
motor, pons
H. Vestibulocochlear nerves VIII- sensory nerve for hearing and
equilibrium, purely sensor, medulla oblongata
I. Glossopharyngeal nerves IX. innervates the tongue and pharynx,
controls a muscle used for swallowing, activates salivary gland,
conducts taste, and other facial sensory. Mixed and visceral motor,
medulla oblongata
J. Vagus Nerves X- (wanders) controls muscles of swallowing extends
beyond the face and neck into the thorax and abdomen to innervate
internal organs for motor and sensory impulses, some sensory near ear
area. Mixed and visceral motor, medulla oblongata.
K. Accessory nerves XI- accessory for the vagus nerve- it joins it, and
controls muscles that moves the head and some of the same as the
vagus. Motor and visceral motor, medulla oblongata.
L. Hypoglossal nerves XII- (below the tongue) runs below the tongue
and innervates the tongue muscles, motor nerves, medulla oblongata.
III. Spinal Nerves- There are 31 pairs of nerves exiting the spinal column: 8
cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. They innervate
the body in sections as seen on page 426. Each nerve has a dorsal
(sensory) and ventral root (motor) that attach to the spinal cord at the
rootlets. Each spinal nerve also has dorsal and ventral ramus that carries
motor and sensory nerves. The ventral ramus connects to rami
commicantes that connect to symphathetic chain ganglia. The dorsal rami
supplies the posterior parts of the body and ventral rami supplies the lateral
and anterior sides of the body.
•
A. Innervation of the back- the nerves follow a neat and simple
pattern.
B. Innervation of the anterior thoracic an abdominal wall- supply
intercostals muscles, skin or anterior and lateral thorax and
abdomen.
C. Introduction to nerve
plexuses- these are
networks of nerve
clusters formed by
ventral rami from
different spinal nerves.
Plexuses serve the limbs
and are designed to
prevent paralysis of a
limb muscle by the
distruction of just one
spinal nerve.
1. The cervical plexus
and innervation of the
neck- formed by C1-C4
nerves, most branches
are cutaneous nerves
and anterior neck
muscles and diaphragm.
2. The brachial plexus and
innervation of the upper
limb- formed by C5-C8
nerves, it supplies the upper
limbs. The plexus’ extremely
complex lies between the
cervical and axillary regions.
Roots run deep of the
sternocleidomastoid, they
unite to form trunks which
divide into anterior and
posterior divisions that break
into lateral, medial, and
posterior cords that divide
into the terminal branches
(around axilla) that innervate
the arm Pg 430. The nerves
of the arms are: axillary
nerve, musculocutaneous
nerve, median nerve, ulnar
nerve, and radial nerves.
3. The lumbar plexus and innervation of the lower limb- formed by L1-L4
nerves the main branches innervate the anterior thigh via the femoral nerve.
Medial thigh and adductor muscles are innervated by the obturator nerve.
4. The sacral
plexus and
innervation on
the lower limbformed by L4-S4
nerves, its many
branches
innervate the
buttock, lower
limb, pelvis, and
perineum. The
largest branch is
the sciatic nerve
that supplies
lateral and
posterior limb
regions. It
branches into the
tibial and fibial
nerve.
5. Innervation of joints of the body- As a
health professional you need to know the
nerves that innervate the joints. Use
Hilton’s Law: any nerve that innerates a
muscle producing movement at a joint also
innervates the joint itself and the skin over
it. Example: Knee joint is surrounded by
anterior and posterior thigh muscles that
are innervated by femoral, obturator, and
branches of the sciatic nerves.
6. Innervation
of the skin:
dermatomeseach spinal
nerves
innervates a
skin zone as
shown in page
436
Chapter 15: Introduction to the autonomic nervous
system
IV. nerves that innervate
smooth muscle, cardiac
muscle, and glands. The
stimulus they send is not
voluntarily controlled. It is
the visceral motor division
A. Comparison of autonomic and somatic motor systems- the motor unit of
autonomic system includes two motor neurons: preganglionic neuron
(myelinated) has an axon that synapses with the postganglionic neuron
(unmyelinated) in autonomic ganglion, then the axon of the postganglionic
neuron synapses with the viscera. A somatic motor unit contains one
myelinated axon per innervated muscle cell. **Note that autonomic ganglia
is motor not sensory.
• B. Divisions of the autonomic NS- both divisions have
motor units that cause opposite effects of the visceral
organs they innervate.
• Symphatheic division- mobilizes the body, responsible
for “fight or flight response”: increase heart rate,
breathing rate, higher blood pressure, dilate pupils,
vasoconstriction, etc. Nerves arise from thoracic and
lumbar regions. It releases norepinephrine
(noradrenaline). Innervates with greater branching.
• Parasymphathetic division- relaxes or unwinds the body,
is active when body is at rest, it conserves body energy,
directs digestion and waste elimination, opposite effects
of fight or flight response. Nerves arise from the brain
and sacral regions. It releases acetylcholine
(cholinergic). Innervates with less branching.
•
V. The parasympathetic division- (all summarized in figure 15.5,
15.6, and table 15.2
•
A. Cranial outflow- these are ANS nerves belonging to the
parasympatehtic division (relaxing) that innervate the organs in the
head, neck, thorax, and abdomen.
1. Oculomotor outflow—smooth muscles in the eye (pupil /lens
adjustment)
•
•
2. Facial nerve outflow-stimulate secretion of glands of the
head (tears, mucus, saliva)
•
3. Glossopharyngeal outflow- stimulates secretion of a large
salivary=
parotid gland
– 4. Vagus nerve outflow- innervate visceral organs of the thorax
(heart and lungs) and most of the abdomen (liver, gallbladder,
pancreas, stomach, small intestine, and stops half way through
large intestine) through the autonomic nerve plexuses.
•
•
B. Sacral outflow- This section runs from s2-s4 and innervates the
abdominal and pelvic organs the vagus nerve outflow did not
innervate, including large intestine, rectum, bladder, and reproductive
organs.
• VI. The
sympathetic
division- This
division innervates
more organs and is
more complex than
the
parasympathetic.
• A. basic
organization- The
sympathetic
system innervates
the integument: its
glands and the
arrector pili in
addition to internal
organs and blood
vessels. There is
also more glanglia
due to the
preganglionic and
postganglionic cell
bodies.
•
1. Sympathetic trunk gangliaThere are 22-24 sympathetic
trunk ganglia that run along the
sides of the vertebral column
(next to the centrum) in a
vertical direction from neck to
pelvis. The ganglia connect to
long synaptic chains called
sympathetic trunks so the
overall appearance a
“beaded”chain. The sides of the
ganglia (contains cell bodies)
connect so spinal nerves
(running horizontally) by gray
and white rami communicantes.
Thus the axon of the
postganglionic neuron joins the
spinal nerves.
•
2. Prevertebral ganglia- Some
ganglia does not lie on the sides
of the vertebral column but
anterior to it along a large
abdominal artery (abdominal
aorta). They are not paired and
occur only in the abdomen and
pelvis.
• B. Symphathetic
pathways- Typically
the preganglionic
neuron sends the
axon from the spinal
cord out through the
ventral root to spinal
nerve to the white
ramus
communicantes into
the sympathetic trunk
where it synapses
with the
postganglionic
neuron that sends its
axon through the
gray communicantes
out through the spinal
nerves and finally to
the organ it
innervates.
• C. Role of the
adrenal medulla
in the
sympathetic
division- A gland
that sits on the
superior aspect of
the kidney. It
releases
norepinephrine
(noradrenaline)
and epinephrine
(adrenaline) into
the blood stream
to excite the body
in time of “fight,
flight, or fright”
response.
•
•
VII. Central control of the autonomic nervous system- The CNS
controls the activity of the ANS
A. Brain stem and spinal cord- medulla oblongata regulates heart rate,
blood pressure, and digestive activity. The midbrain controls the
sympathetic response to fear. The spinal cord control visceral reflexes
(defication and urination reflex can be voluntarily controlled).
•
B. Hypothalamus and amydala- The hypothalamus is the main integration
center of ANS and the amygdala coordinates sympathetic learned fear
responses
•
C. Cerebral cortex- Voluntary control of ANS may happen via meditation
when deep thoughts of relaxation activate the limbic system which in turns
activates the hypothalamus to activate the parasympathetic NS. Thoughts
of frightful experiences may stimulate the amygdala to activate the
hypothalamus that activates the sympathetic NS.
VIII. Visceral sensory neurons- Receptors in the viscera are free
dendritic ends that send afferent signals caused by stretching, temperature
and chemical changes, and irritation. Integration translates these signals
into hunger, fullness, pain, or nausea. Visceral sensation may be hard to
localize. Sometime pain is referred pain, a problem with an organ like the
heart may send pain down the arm (not an area where the heart is located.)
IX. Visceral reflexes- These are also called peripheral reflexes because
they may not involve the CNS a visceral sensory branches may synapse
with postganglionic motor neurons in sympathetic ganglia. This makes the
system independent of CNS control.
•
•
• A map of
referred pain:
these are
skin or body
regions that
present pain
when there is
visceral pain.
The organ
and site of
referred pain
are
innervated by
the same
nerve.