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CRANIAL NERVES & PNS
CRANIAL NERVES
1. Twelve pairs of cranial nerves originate from the
nose, eyes, inner ear, brain stem, and spinal
cord.
2. They are named primarily based on their
distribution and are numbered I – XII in order of
attachment to the brain. Table 14.4 summarizes
the types, locations, functions, and disorders of
the cranial nerves.
Terminal§
0
Olfactory
1
Optic
2
Oculomotor
Trochlear
Trigeminal
Abducens
3
4
5
6
Brain
Region
Near the
olfactory
Cerebral
Cortex
Limbic
System
Midbrain
Pons
"
"
Facial
7
"
Auditory
8
Medulla
Glossopharyngeal
9
"
Vagus
10
"
Spinal Accessory 11
Hypoglossal
12
"
"
Cranial Nerve CN#
Major Functions
Reception of pheromone for sex
Smell
Vision
Eyelid & eyeball movement; pupil dilation
Control downward & lateral eye movement
Chewing; sensation of face & mouth
Control lateral eye movement
Control most facial expressions; secretion of tears & saliva;
taste; ear sensation
Hearing; balance
Taste; swallowing; sensation from tongue, tonsil, pharynx,
carotid blood pressure
Sensory, motor and autonomic functions of viscera - glands,
digestion,
heart rate, breathing rate, aortic blood pressure
Controls muscles used in head movement
Controls tongue movements
DEVELOPMENT OF THE
NERVOUS SYSTEM
1. The development of the nervous system begins
with a thickening of a region of the ectoderm
called the neural plate.
2. During embryological development, primary
brain vesicles form from the neural tube and
serve as forerunners of various parts of the
brain.
3. The telencephalon forms the cerebrum, the
diencephalons develops into the midbrain, the
metencephalon develops into the pons and
cerebellum, and the myelencephalon froms the
medulla.
AGING AND THE NERVOUS
SYSTEM
1. The brain grows rapidly during the first
few years of life.
2. Age-related effects involve loss of brain
mass and decreased capacity for sending
nerve impulses.
AUTONOMIC NERVOUS
SYSTEMS
OBJECTIVES
• Compare the structural and functional
differences between the somatic and
autonomic parts of the nervous system.
• Describe preganglionic and postganglionic
neurons of the autonomic nervous system.
• Compare the anatomical components of
the sympathetic and parasympathetic
divisions of the autonomic nervous
system.
OBJECTIVES
• Describe the neurotransmitters and receptors
involved in autonomic responses.
• Describe the major responses of the body to
stimulation by the sympathetic and
parasympathetic divisions of the ANS.
• Describe the components of an autonomic
reflex.
• Explain the relationship of the hypothalamus to
the ANS.
 It is easy to forget that much of the human nervous
system is concerned with routine, involuntary jobs, such
as homeostasis, digestion, posture, breathing, etc. This
is the job of the autonomic nervous system, and its
motor functions are split into two divisions, with
anatomically distinct neurones. Most body organs are
innervated by two separate sets of motor neurones; one
from the sympathetic system and one from the
parasympathetic system. These neurones have opposite
(or antagonistic) effects. In general the sympathetic
system stimulates the “fight or flight” responses to
threatening situations, while the parasympathetic system
relaxes the body. The details are listed in this table:
Organ
Eye
Tear glands
Salivary glands
Lungs
Heart
Gut
Liver
Bladder
Sympathetic System
Dilates pupil
No effect
Inhibits saliva production
Parasympathetic
System
Constricts pupil
Stimulates tear secretion
Stimulates saliva
Dilates bronchi
production
Speeds up heart rate
Constricts bronchi
Inhibits peristalsis
Slows down heart rate
Stimulates peristalsis
Stimulates glucose
production
Inhibits urination
Stimulates bile production
Stimulates urination
Autonomic Nervous System
• One division of the autonomic nervous system, called
the sympathetic nervous system, dominates in times of
stress. It controls the "fight or flight" reaction, increasing
blood pressure, heart rate, breathing rate, and blood flow
to the muscles. Another division, called the
parasympathetic nervous system, has the opposite
effect. It conserves energy by slowing the heartbeat and
breathing rate, and by promoting digestion and
elimination (of waste). Most glands, smooth muscles,
and cardiac muscles constantly get inputs from both the
sympathetic and parasympathetic systems. The CNS
controls the activity by varying the ratio of the signals.
Depending on which motor neurons are selected by the
CNS, the net effect of the arriving signals will either
stimulate or inhibit the organ.
• Motor fibers that govern involuntary responses,
do not lead directly to the organs they innervate.
Instead, they make their trips in two stages. The
first set of fibers leads from the CNS to ganglia
(which are collections of nerve cell bodies) that
lie outside the CNS (the preganglionic fibers). At
the ganglia the fibers form synaptic junctions
with the dendrites of as many as twenty different
cell bodies. The axons of these cell bodies form
a second set of fibers, the postganglionic fibers.
It is these postganglionic fibers that lead to the
organs.
• The chief ganglia involved in the autonomic nervous
system form two lines running down either side of the
spinal column. They are outside the bony vertebrae.
These two lines of ganglia outside the column resemble
a pair of long beaded cords. At the lower end, the two
cords join and finish in a single central stretch. These
lines of ganglia are sometimes called the sympathetic
trunks (used by the sympathetic nervous system). Not all
ganglia are located in the sympathetic trunks. Some are
not; and it is possible for a preganglionic fiber to go right
through, making no synaptic junction there at all, joining
instead with ganglia located in front of the vertebrae. For
the parasympathetic nervous system, some of the
ganglia separating the preganglionic fibers from the
postganglionic fibers are actually located within the
organ the nerve is servicing. In that case, the
preganglionic fiber runs almost the full length of the total
track, whereas the postganglionic fiber is at most just a
few millimeters long.
• The splanchnic nerves, which originate
from some of the thoracic nerves, have
their preganglionic fibers ending in a mass
of ganglia lying just behind the stomach. It
represents the largest mass of nerve cells
that is not within the CNS and is
sometimes called the "abdominal brain". It
is a vital spot to be protected during
boxing.
COMPARISON OF SOMATIC AND
AUTONOMIC NERVOUS SYSTEMS
• The somatic nervous system operates under conscious control; the
ANS usually operates without conscious control.
• Sensory input for the somatic nervous system is mainly from the
special senses and somatic senses; sensory input for the ANS is
from interoceptors, in addition to special senses and somatic
senses.
• The axons of somatic motor neurons extend from the CNS and
synapse directly with an effector. Autonomic motor pathways consist
of two motor neurons in series. The axon of the first motor neuron
extends from the CNS and synapses in a ganglion with the second
motor neuron; the second neuron synapses with an effector.
• The output (motor) portion of the ANS has two divisions;
sympathetic and parasympathetic. Most body organs receive dual
innervation; usually one ANS division causes excitation and the
other causes inhibition.
• Somatic nervous system effectors are skeletal muscles; ANS
effectors include cardiac muscle, smooth muscle, and glands.
ANATOMY OF AUTONOMIC
MOTOR PATHWAYS
• Preganglionic neurons are myelinated; postganglionic neurons are
unmyelinated.
• The cell bodies of sympathetic preganglionic neurons are in the
lateral gray horns of the 12 thoracic and the first two or three lumbar
segments of the spinal cord; the cell bodies of parasympathetic
preganglionic neurons are in four cranial nerve nuclei (III, VII, IX,
and X) in the brain stem and lateral gray horns of the second
through fourth sacral segments of the spinal cord.
• Autonomic ganglia are classified as sympathetic trunk ganglia (on
both sides of vertebral column), prevertebral ganglia (near or inside
visceral effectors).
• Sympathetic preganglionic neurons synapse with postganglionic
neurons in ganglia of the sympathetic trunk or in prevertebral
ganglia; parasympathetic preganglionic neurons synapse with
postganglionic neurons in terminal ganglia.
ANS NEUROTRANSMITTERS
AND RECEPTORS
•
•
•
•
•
•
Cholinergic neurons release acetylcholine, which binds to nicotinic or
muscarinic cholinergic receptors.
In the ANS, the cholinergic neurons include all sympathetic and
parasympathetic preganglionic neurons, all parasympathetic postganglionic
neurons, and sympathetic postganglionic neurons that innervate most sweat
glands.
In the ANS, adrenergic neurons release norepinephrine. Both epinephrine
and norepinephrine bind to alpha and beta adrenergic receptors.
Most sympathetic postganglionic neurons are adrenergic.
Table 15.2 summarizes the types of cholinergic and adrenergic receptors.
An agonist is a substance that binds to and activates a receptor, mimicking
the effect of a natural neurotransmitter or hormone. An antagonist is a
substance that binds to and blocks a receptor, thereby preventing a natural
neurotransmitter or hormone from exerting its effect.
PHYSIOLOGICAL EFFECTS OF
THE ANS
• The sympathetic division favors body functions
that can support vigorous physical activity and
rapid production of ATP (fight-or-flight
response); the parasympathetic division
regulates activities that conserve and restore
body energy.
• The effects of sympathetic stimulation are longer
lasting and more widespread than the effects of
parasympathetic stimulation.
• Table 15.3 compares structural and functional
features of the sympathetic and parasympathetic
divisions.