CHAPTER 15
AUTONOMIC NERVOUS SYSTEM
This
is the part that regulates the activities of smooth muscle, cardiac muscle
and glands. It depends on sensory input from visceral organs and blood vessels,
which travels to the CNS, is integrated and results in an autonomic motor nerve
impulse. It operates without conscious control and is regulated by the hypothalamus
and medulla.
TABLE 15.1 P. COMPARISON OF AUTONOMIC AND SOMATIC DIVISIONS
INPUT into
the ANS comes from autonomic sensory neurons, most of which are associated with
interoceptors and is below conscious awareness.
OUTPUT from
the ANS consists of motor impulses that either excite or inhibit smooth
muscle, cardiac muscle, or glands. We have little or no voluntary
control over these responses.
There
are 2 major divisions of autonomic nervous system output:
1. Sympathetic
2. Parasympathetic
Most
organs receive impulses from both divisions--dual innervation. In general, impulses
from one division cause excitation and impulses from the other cause
inhibition. Which division does what depends on the organ. The hypothalamus
regulates the ANS and determines which division will dominate at any given
time.
Autonomic
motor pathways always consist of 2 motor neurons plus a ganglion.
1. Preganglionic neurons--cell body
is located in the CNS (brain or spinal cord). It sends its myelinated axon
(also called the preganglionic fiber) out of the CNS as
2. Postganglionic neuron--lies
entirely outside the CNS. Its cell body is in an autonomic ganglion and its
unmyelinated axon (postganglionic fiber) travels to a visceral effector.
COMPARISON
OF SYMPATHETIC/PARASYMPATHETIC
|
|
SYMPATHETIC |
PARASYMPATHETIC |
|
PREGANGLIONIC NEURON CELL BODIES |
CNS—LATERAL GRAY HORNS OF THORACIC OR UPPER
LUMBAR AREA OF SPINAL CORD (THORACOLUMBAR DIVISION) |
CNS—NUCLEUS OF A CRANIAL NERVE OR SACRAL AREA OF SPINAL CORD (CRANIOSACRAL DIVISION) |
|
LENGTH OF PREGANGLIONIC AXONS |
SHORT |
LONG |
|
PREGANGLIONIC AXONS LEAVE THE CNS AS A PART
OF |
ANTERIOR ROOT OF A SPINAL NERVE. AS THE
SPINAL NERVE BRANCHES, THESE AXONS
ENTER THE RAMI COMMUNICANTES. |
CRANIAL NERVE OR ANTERIOR ROOT OF A SPINAL
NERVE |
|
GANGLION |
SYMPATHETIC TRUNK GANGLION OR PREVERTEBRAL GANGLION |
TERMINAL GANGLION |
|
WITHIN THE GANGLION, THE PREGANGLIONIC AXON
SYNAPSES WITH: |
A LARGE NUMBER OF POSTGANGLIONIC NEURONS |
A SMALL NUMBER OF POSTGANGLIONIC NEURONS |
|
POSTGANGLIONIC NEURON CELL BODIES |
IN GANGLION |
IN GANGLION |
|
LENGTH OF POSTGANGLIONIC AXONS |
LONG |
SHORT |
|
EFFECTS |
WIDESPREAD |
NARROW |
|
GENERAL DESCRIPTION OF EFFECTS |
ENCOURAGES PROCESSES THAT INVOLVE USE OF
ENERGY—"FIGHT –OR-FLIGHT”
RESPONSE |
PROCESSES THAT RESTORE & CONSERVE
ENERGY---"HOUSE-KEEPING" |
|
MOST BODY STRUCTURES RECEIVE DUAL INNERVATION,
BUT A FEW ARE INNERVATED BY ONE DIVISION
ONLY |
SWEAT GLANDS, ARRECTOR PILI MUSCLES, FAT
CELLS, KIDNEYS, BLOOD VESSELS |
LACRIMAL (TEAR) GLANDS |
MORE
ABOUT GANGLIA
|
|
SYMPATHETIC TRUNK GANGLIA |
PREVERTEBRAL GANGLIA |
TERMINAL GANGLIA |
|
ALTERNATE NAMES FOR THIS TYPE |
VERTEBRAL CHAIN PARAVERTEBRAL |
COLLATERAL |
INTRAMURAL |
|
DIVISION |
SYMPATHETIC |
SYMPATHETIC |
PARASYMPATHETIC |
|
LOCATION |
VERY NEAR VERTEBRAL COLUMN ON EACH SIDE |
ANTERIOR TO VERTEBRAL COLUMN NEAR A LARGE
ABDOMINAL ARTERY |
VERY NEAR OR IN THE WALL OF THE VISCEERAL
ORGAN |
|
INNERVATES |
MOSTLY ABOVE THE DIAPHRAGM |
MOSTLY BELOW THE DIAPHRAGM |
VISCERA SERVED BY CRANIAL NERVES OR LOCATED WITHIN
THE ABDOMINOPELVIC CAVITY |
|
IMPULSES REACH THE GANGLION |
DIRECTLY FROM RAMI COMMUNICANTES (BRANCHES
OF SPINAL NERVES) |
AFTER FORMING NERVES KNOWN AS SPLANCHNIC
NERVES |
BY WAY OF CRANIAL NERVES OR SPINAL NERVES |
NEUROTRANSMITTERS
Autonomic
motor neurons release either acetylcholine or norepinephrine as their
neurotransmitter. Based on the neurotransmitter they release, ANS neurons are:
1.
Cholinergic neurons--release acetylcholine and include:
a. ALL preganglionic ANS neurons--both
sympathetic & parasympathetic
b. ALL parasympathetic postganglionic
neurons
c. A few sympathetic postganglionic neurons
(to sweat glands and a few blood vessels in skeletal muscle)
The
acetylcholine released is excitatory at some synapses and inhibitory at others.
Effects tend to be brief.
2.
Adrenergic neurons--release norepinephrine (noradrenalin) or epinephrine
(adrenalin). Includes most sympathetic postganglionic neurons. Effect is also
excitatory at some synapses and inhibitory at others. Effects are
longer-lasting and more widespread for 3 reasons:
a. More divergence of sympathetic fibers
b. Norepinephrine is broken down slowly
c. Epinephrine and norepinephrine are also
secreted into the blood by the adrenal glands and intensify the effects of
these fibers
The
parasympathetic division encourages activities that conserve or restore energy
during rest. One of its major concerns is digestion. In addition, it causes
secretion of tears and urination, and slows the heart rate.
The
sympathetic division prepares the body for emergencies. It encourages the use
of energy. During normal quiet function the sympathetic activity is
minimal--just enough so that body processes requiring energy can proceed. In time
of stress, fear, anger, exercise or physical danger, the sympathetic output
greatly increases and a series of events called the fight-or-flight response
occurs:
1. Pupils dilate
2. Increase in heart rate and force
3. Blood vessels to skin and viscera
contract
4. Blood vessels of skeletal muscle and
cardiac muscle dilate
5. Rapid deep breathing and dilation of air
passages
6. Blood sugar rises
7. Adrenal glands secrete epinephrine and
norepinephrine
8. Digestive tract secretion and motility
decrease
TABLE 15.4 P. 538 - 539
Visceral
autonomic reflexes adjust the activity of visceral effectors (cardiac muscle,
smooth muscle, glands). These reflexes play a key role in homeostasis by
regulating such functions as heart action, blood pressure, respiration,
digestion, etc.
A
visceral autonomic reflex arc consists of:
1. Receptor
2. Sensory neuron
3. Interneuron
4. Autonomic motor neurons (2)
a. Preganglionic--CNS to ganglion
b. Postganglionic--ganglion to effector
5. Visceral effector
Visceral
sensations often do not reach the cerebral cortex, so we are not aware of many
of the activities of the ANS. The hypothalamus is the major control and integration
center of the ANS. Other integrating centers include centers such as:
Cardiovascular center
Respiratory center
Vasomotor center Medulla
Swallowing center
Vomiting center
The
hypothalamus is the major control and integration center. It sends output to
the autonomic centers in the medulla and spinal cord. Input to the hypothalamus
regarding emotions, visceral functions, smell, taste, changes in temperature,
and blood concentration results in various adjustments of both divisions of the
ANS.
In
time of great emotional stress, the cerebral cortex can take control over
autonomic functions, probably working through the limbic system.
Anxiety--increased rate & force of
heartbeat, increased blood pressure
Bad news, unbearable sight--cortex may cause
widespread vasodilation--resulting drop in BP may cause fainting.
While
ANS functions are considered involuntary, it appears that some people are able
to learn to take control of certain areas on a voluntary basis.
Yoga
Biofeedback
CHAPTER 16 SENSORY, MOTOR & INTEGRATIVE SYSTEMS
Homeostasis
requires a continuous input of sensory information, some at the conscious level
and some below the conscious level.
Sensation--conscious
or subconscious awareness of external or internal stimuli.
Sensations
and the reactions they bring about may:
1.
Reach no higher than the spinal cord and result in spinal reflexes (no brain
activity)
2.
Reach the lower brain stem and result in subconscious motor reactions (changes
in heart rate, breathing rate, etc.)
3.
Reach the thalamus and result in crude identification of type and crude
localization
4.
Reach the cerebral cortex and result in precise identification of type and
precise localization
Perception--conscious
awareness and interpretation of sensations, based on past experience and stored
information. Not all sensory input results in perception.
Modality--type
of sensation, such as pressure, touch, pain, temperature change, etc. Each type
of sensory neuron carries information for only one modality. Sensory modalities
include:
1. General senses
a. Somatic senses
1) Tactile (touch, pressure,
vibration)
2) Thermal
3) Pain
4) Proprioception
b. Visceral senses—conditions in internal
organs
2. Special senses
4
events are required for a sensation to occur:
1.
Stimulation of sensory receptor
2.
Transduction--stimulus converted to a graded potential
3.
Impulse generation & conduction—if the graded potential reaches threshold strength,
a nerve impulse results. This impulse travels to the CNS.
4.
Integration--CNS translates the impulse into a sensation. You actually see,
feel, hear, etc. in the brain (cerebral cortex)
Projection--the
cortex interprets the sensation as coming from the body part where sensory
receptors were stimulated and projects it back to that area, so it seems that
we feel pain in the injured part, hear in the ear, etc.
Sensory
receptors respond strongly to one kind of sensation (their special one) and weakly
or not at all to others--selectivity. A very intense stimulus of the
"wrong" type may activate some.
Receptors
may be:
1. Free nerve endings—bare dendrites
Pain, temperature, tickle, itch, some
touch
2. Encapsulated nerve endings—dendrites
enclosed in a CT capsule
Some touch receptors
3. Separate cells that synapse with
first-order sensory neurons—these are associated with special senses
Simple
receptors are associated with the general senses. They may be free nerve endings,
the simplest type, or may have some distinctive structures. General senses:
Touch Temperature
Pressure Pain
Vibration Proprioception
Complex
receptors are associated with the special senses. They are located only in
specific locations (eye, ear, etc.) and have distinctive structures.
Smell Equilibrium
Taste Hearing
Vision
The
electrical response of a sensory receptor to a stimulus is either a receptor potential
or a generator potential.
Receptor potentials---do not
trigger action potentials---vision, hearing, equilibrium, taste. Receptor
potentials cause the release of neurotransmitter that may trigger an impulse in
associated sensory neurons, but the cell that receives the stimulus does not
itself generate an impulse.
Generator potentials---these do
trigger action potentials (nerve impulses)---all other receptors respond this
way. If the stimulus reaches threshold, a nerve impulse is generated.
Types of receptors classified by location:
1. Exteroceptors--at or near body
surface
2. Interoceptors (visceroceptors)—deeper--
in blood vessels, viscera, muscles and the nervous system. Give information
about internal conditions, which may not reach the conscious level.
3. Proprioceptors--in muscles, tendons,
joints, the internal ear. They give information about body position and
movement.
Types
of receptors classified by type stimulus they respond to:
1. Mechanoreceptors--mechanical pressure or
stretching
Touch Hearing
Pressure Equilibrium
Vibration Blood pressure
Proprioception
2. Thermoreceptors--changes in temperature
3. Nociceptors--pain
4. Photoreceptors--light
5. Chemoreceptors--chemicals--taste, smell,
composition of body fluids such as blood
6. Osmoreceptors—osmotic pressure of body
fluids
Table 16.1
P. 550
Adaptation--change
in sensitivity to a long-lasting stimulus. Some receptors adapt quickly, some
not at all.
Rapidly adapting receptors---pressure,
touch, smell
Slowly adapting receptors---body position,
chemicals in blood
Pain receptors adapt most slowly or not at
all
SOMATIC SENSATIONS
Sensory
receptors are located on or near the surface of the body. They are more
numerous in areas where touch is especially important, such as fingertips,
tongue and lips. Other areas may have the same types of receptors, but in much
smaller numbers. If receptors are associated with skin, the sensation may be
called a cutaneous sensation.
1.
Tactile sensations
a. Touch
1) Crude touch—just know that
something has contacted the skin
2) Fine touch—very specific
information—exactly where, plus shape, size and texture of whatever
touched
a) Meissner’s corpuscles
(corpuscles of touch)—in dermal papillae of hairless skin—dendrites enclosed in
a CT capsule
b) Hair root plexuses—in hairy
skin—free nerve endings wrapped around hair follicles
c) Merkel’s discs (Type I
cutaneous mechanoreceptors)—saucer-shaped flattened free nerve endings—upper
dermis just below stratum basalis
d) Corpuscles of Ruffini (Type
II cutaneous mechanoreceptors)—deep in dermis—most sensitive to stretching
b. Pressure—sustained sensation over a
wider area than touch
1) Meissner’s corpuscles
2) Merkel’s discs
3) Lamallated (Pacinian
corpuscles)—dendrite enclosed in a large CT capsule
c. Vibration—rapidly repetitive signals
1) Meissner’s corpuscles—more sensitive to low
frequency vibrations
2) Pacinian corpuscles—more
sensitive to higher frequency vibrations
d. Itch—free nerve endings are stimulated
by chemicals
e. Tickle—free nerve endings and Pacinian
corpuscles are involved—why it works only when someone else does it is a
mystery
2.
Thermal sensations—receptors are thermoreceptors—free nerve endings
a. Cold receptors—in stratum basale
b. Warm receptors—in dermis
3. Pain--nociceptors are free nerve endings
found in almost every body tissue. They may respond to almost any type of
stimulus that is strong enough to cause tissue damage. Pain continues because
chemicals released by the damaged tissue continue to stimulate nociceptors.
Pain is classified as one of 2 types:
a. Fast (acute) pain---occurs rapidly and
is described as sharp, pricking pain. It comes only from the surface of the
body; it does not originate in deeper tissues. It travels on Type A fibers and
b. Slow (chronic) pain---begins more
slowly and increases in intensity. It is described as burning, aching,
throbbing. It occurs in both skin and deeper organs. It travels on Type C
fibers and a toothache is an example.
Another
way of classifying pain is:
a. Somatic pain---well localized sensations
1) Superficial---skin receptors
2) Deep---skeletal muscles, joints,
tendons and fascia
b. Visceral pain---from internal organs. It
is felt only when it involves large areas and often is poorly localized. It may
seem to be located in the skin over the organ or even in an area far away—if
this happens it is called referred pain.
Analgesia--relief
from pain
Aspirin, etc. block prostaglandins
(chemicals which stimulate nociceptors)
Novocaine and other local
anesthetics----block nerve impulse generation & conduction by blocking Na+
channels
Opiates--alter perception at synapses in
brain
Cordotomy--severing spinal cord tract
Rhizotomy--cutting spinal nerve sensory
roots
4.
Proprioception (kinesthetic sense—kinesthesia is the perception of body
movements)--awareness of activities of muscles, tendons, joints, balance and
equilibrium. Related to muscle tone, movement of body parts, body position.
Impulses travel to both the cerebral cortex and the cerebellum.
a. Muscle spindles--specialized skeletal
muscle fibers detect stretch
b. Tendon organs--detect stretch in tendons and
monitor force of contraction in muscles
c. Joint kinesthetic receptors--capsules of
synovial joints--respond to pressure, movement and strain on joint.
SUMMARY OF RECEPTORS TABLE 16.2
P. 555
SOMATIC SENSORY AND MOTOR
MAPS
Figure 16.6 P. 558
Sensory
input from specific areas always winds up in a very specific area of the
cerebral cortex. If the specific group of neurons that deals with sensory input
from a body area is lost, sensation in that area is also lost. Motor impulses
work the same way—a specific group of neurons is in charge of motor control of
a specific effector. If that group of neurons is unable to function, the
effector is also unable to function.
PHYSIOLOGY OF SENSORY
PATHWAYS
Somatic
sensory pathways involve 3 separate neurons:
1. First order neuron---receptor into spinal
cord or brain stem
a. If from face, mouth, teeth &
eyes—axon travels in cranial nerve
b. Everywhere else—axon travels in
spinal nerve
2. Second order neuron---from spinal cord or
brain stem to thalamus---axons of these cross to opposite side in the medulla
or spinal cord and continue to thalamus
3. Third order neuron---connects thalamus to
primary somatosensory area of cerebral cortex
All
sensory input travels in ascending pathways with specific kinds of input always
traveling in
1. Posterior column-medial lemniscus
pathways
Fine touch
Stereognosis
Proprioception
Weight discrimination
Vibration
a. First-order neurons extend from
sensory receptors in all parts of the body except the head to the spinal cord,
with their axons forming the posterior column tracts, and synapse with
second-order neurons in the medulla. Impulses from the head travel by way of
cranial nerves but behave the same way once they reach the medulla.
b. In the medulla, axons of
second-order neurons cross to the opposite side, enter the medial lemniscus,
and travel to the thalamus
c. In the thalamus, axons of
second-order neurons synapse with third-order neurons, which travel to the
proper specific area of the cerebral cortex
2. Anterolateral (spinothalamic) pathways
Pain
Temperature
Itch
Tickle
Some crude touch and pressure
a. First-order neurons connect
receptors to the spinal cord and synapse with second-order neurons located in
the spinal cord (posterior gray horn).
b. Second-order axons cross to the
opposite side in the spinal cord and ascend in the lateral spinothalamic or
anterior spinothalamic tracts to synapse with the third-order neuron in the
thalamus
c. Third-order neurons extend to the
proper area of the cerebral cortex
3. Anterior spinocerebellar and posterior spinocerebellar tracts carry impulses concerned with proprioception to the cerebellum.
SOMATIC
MOTOR PATHWAYS
All of these pathways go to skeletal
muscle. In general, these pathways consist of at least two motor neurons,
an upper motor neuron (UMN) and a lower motor neuron (LMN). In some cases there
is an additional neuron, an interneuron, between these two; in other cases
there is not. The UMN is always in the brain. The LMN is the one that connects
directly to the skeletal muscle. Some of these are in the spinal cord, others
are in the nuclei of origin of cranial nerves (for muscles in the head area).
There are two types of somatic motor pathways:
PYRAMIDAL (DIRECT) PATHWAYS
The
most direct motor pathways extend from the cortex to skeletal muscle. The
primary motor area (precentral gyrus) of the cerebral cortex is the major
control region for voluntary motor movements. Motor nerve impulses for
voluntary movements of skeletal muscle originate in the motor cortex and travel
by direct (pyramidal) pathways. Specific areas control specific muscles.
Upper motor neuron of cortex
Internal capsule of cerebrum
Medulla (pyramids)--cross to
opposite side
Interneuron
Lower
motor neuron in the Lower
motor neuron in the
nucleus
of a cranial nerve OR anterior gray horn of spinal cord
Skeletal
muscle of face/head
Other skeletal muscle
These
impulses direct precise, voluntary movements. They travel in 3 tracts down the
spinal cord:
1. Lateral corticospinal—axons of 90% of
upper motor neurons travel this pathway. They cross at the pyramids (medulla)
and go on to control muscles of the extremeties.
2. Anterior corticospinal--do not cross at
medulla but may cross in spinal cord—coordinate movements of neck and trunk
muscles--10% of upper motor neurons.
3. Corticobulbar--terminate in nuclei of 9
pairs of cranial nerves (all EXCEPT I,II,VIII) and control movements of face,
head, and neck. Some of these cross and some do not.
EXTRAPYRAMIDAL (INDIRECT)
PATHWAYS
All
other descending somatic motor tracts. These do not cross at the medulla:
Rubrospinal
Tectospinal
Vestibulospinal
Medial reticulospinal
Lateral reticulospinal
Nerve
impulses that travel these pathways follow complicated circuits that involve
not only the motor cortex but also the basal ganglia, thalamus, cerebellum,
reticular formation, and various nuclei in the brain.
BASAL GANGLIA
Remember, these are nuclei in
the cerebrum. Although they have no direct connection to skeletal muscle, they
enter into its control by influencing the upper motor neurons. The basal
ganglia seem to regulate the initiation and termination of movements and
suppress unwanted movements. Damage to the basal ganglia leads to uncontrolled
movements and an undesirable increase in muscle tone. Parkinson disease and
CEREBELLUM
The
cerebellum is a center for posture and equilibrium, but is also involved in
learning and performing rapid, skilled movements such as swimming, speaking,
typing, etc. It receives two kinds of input:
1. What muscles SHOULD be doing (from
cerebrum)
2. What muscles ARE doing (from PNS)
If
there is a difference between these, the cerebellum sends signals to the
cerebrum, which will then adjust the situation.
Damage
to the cerebellum causes ataxia, which includes:
Jerky, uncoordinated movements
Slurred speech
Tremors
Loss of the awareness of where body parts
are (proprioception)
INTEGRATIVE FUNCTIONS OF
THE CEREBRUM
1. Wakefulness
and sleep---humans sleep and wake in a fairly constant 24 hour cycle called
circadian rhythm. This is regulated by an area of the hypothalamus called the
suprachiasmatic nucleus. Neurons show a higher level of activity when we are
awake. When neurons become fatigued, we feel a desire to sleep, so that neurons
may become less active and recover.
One
portion of the reticular formation, the RAS, causes an increase in cortical activity.
This is the part that keeps us awake and must become less active so that we can
sleep. Arousal from sleep seems to be brought about by stimulation of the
RAS--this is what the alarm clock gets going.
Stimulus (alarm clock or other noise, light, pain)
RAS
Cortex
Arousal
Apparently,
without activity of the RAS we would tend to drop off to sleep very easily. The
RAS is what maintains consciousness. Damage to the RAS can lead to coma. Some
factor seems to reduce the activity of the RAS when it is time to sleep.
2
types of sleep:
Non-rapid eye-movement (NREM) sleep--slow
wave sleep--in 4 different stages
Rapid eye-movement (REM) sleep--dreams occur
In
normal sleep the two types alternate with a REM period about every 90 minutes.
Both types of sleep appear to be needed. Apparently increases and decreases in
the amount of serotonin and norepinephrine may regulate this cycle.
2.
Learning and memory
Learning--ability to acquire new knowledge
or skill through instruction or experience
Memory--ability to retain that knowledge.
For an experience to become part of memory it must produce persistent changes
in the brain. Plasticity—ability of the brain to change. Parts of the brain
involved:
Association cortex of all 4 lobes of the
cerebrum
Parts of the limbic system—especially the
hippocampus and the amygdaloid nucleus
Diencephalon
Short-term
memory—lasts seconds to hours—looking up
Long-term
memory—days to years. Information is stored and can be retrieved. About 1% of
our incoming information goes into long-term memory. Long-term memory involves anatomical and
biochemical changes at synapses. This may include:
Enlargement of existing synaptic end bulbs
Decrease in the rate of removal of a
neurotransmitter