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1. Central Nervous System
General Design of the Nervous System
The CNS contains more than 100 billion neurons.
For different types of neurons, there may be only a few hundred
or as many as 200,000 such synaptic connections from input
fibers.
Conversely, the output signal travels by way of a single axon
leaving the neuron.
Then, this axon has many separate branches to other parts of
the nervous system or peripheral body.
A special feature of most synapses is that the signal normally
passes only in the forward direction (from the axon of a preceding
neuron to dendrites on cell membranes of subsequent neurons).
This forces the signal to travel in required directions for
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performing specific nervous functions.

2. Divisions of the Nervous System
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3. The Central nervous system
• Consists of:
Brain.
Spinal cord.
The CNS:
 Receives input from sensory neurons.
 Directs activity of motor neurons.
 Association neurons (interneurons) maintain
homeostasis in the internal environment
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4. The Spinal Cord
• The spinal cord has two main functions:
1. Common passageway for ascending and descending tracts.
Neurons in the white matter of the spinal cord transmit
sensory signals from peripheral regions to the brain and
motor signals from the brain to peripheral regions.
2. Center for reflexes. Neurons in the gray matter of the spinal
cord integrate incoming sensory information and respond
with motor impulses that control muscles (skeletal, smooth,
or cardiac) or glands.
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5. The Spinal Cord cont’d….
• The SC is an extension of
the brain stem that begins
at the foramen magnum
and continues down
through the vertebral canal
to the L1.
• It is held in position at its
inferior end by the filum
terminale, an extension of
the pia mater that attaches
to the coccyx.
• Along its length, the SC is
held within the vertebral
canal by denticulate
ligaments, lateral
extensions of the pia mater
that attach to the dural
sheath.
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6. The Spinal Cord cont’d….
• Cross section of the SC
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7. Spinal Cord Tracts
• The spinal cord white matter contains
 Ascending and
 Descending tracts
 Ascending tracts
 emerge from the first order (1°) neuron located in the dorsal
root ganglion (DRG).
 transmit sensory information from the sensory receptors to
higher levels of the CNS.
 These ascending tracts are:
gracile and cuneate fasciculi occupying the dorsal column, and
sometimes are named the dorsal funiculus.
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8. Spinal Cord Tracts cont’d….
 These fibers carry information related to
 Tactile
 Two point discrimination of simultaneously applied pressure
 Vibration sense
 Position sense
 Movement sense
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9. Spinal Cord Tracts cont’d….
• In the ventral column (funiculus) there are four prominent tracts:
1) the paleospinothalamic tract (or anterior spinothalamic tract)
carry
 pain, temperature, touch to the brain stem nuclei and to the
diencephalon
2) the spinoolivary tract carries information from Golgi tendon
organs to the cerebellum
3) the spinoreticular tract –carries information to the RF
4) the spino-tectal tract. Intersegmental nerve fibers travelling for
several segments and are located as a thin layer around the
gray matter is known as fasciculus proprius, spinospinal or
archispinothalamic tract.
 carries pain information to the brain stem and
diencephalon.
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10. Spinal Cord Tracts cont’d….
• In the lateral column (funiculus), the neospinothalamic
tract (or lateral spinothalamic tract) is located more
anteriorly and laterally, and carries pain, temperature
and crude touch information from somatic and visceral
structures.
• Nearby laterally, the dorsal and ventral spinocerebellar
tracts carry unconscious proprioception information
from muscles and joints of the lower extremities to the
cerebellum.
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11. Spinal Cord Tracts cont’d….
• Descending tracts. The descending tracts originate
from different cortical areas and from brain stem nuclei.
• The descending pathway carry information associated with
maintenance of motor activities such as posture, balance,
muscle tone, and visceral and somatic reflex activity.
• These include the lateral corticospinal tract and the rubrospinal
tracts located in the lateral column (funiculus).
• These tracts carry information associated with voluntary
movement.
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12. Sensory Part of the Nervous System—
Sensory Receptors
 Most activities of the nervous system are initiated by
sensory experience exciting sensory receptors.
 This sensory experience can either cause
immediate reaction from the brain or the SC
 memory of the experience can be stored in the brain for
minutes, weeks, or years and determine bodily reactions
at some future date.
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13. Sensory Part cont’d…..
• As shown fig. (next slide) the somatic portion of the sensory system
transmits
sensory information from the receptors of the entire body surface
and from some deep structures.
This information enters the CNS through peripheral nerves and is
conducted immediately to multiple sensory areas in
the spinal cord at all levels
the reticular substance of the medulla, pons, and
mesencephalon of the brain
 the cerebellum
the thalamus
 areas of the cerebral cortex.
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14. Fig Somatosensory Axis of the Nervous System
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15. Motor Part of the NS -Effectors
Eventual role of the NS is to control the various bodily activities.
This is achieved by controlling
 contraction of appropriate skeletal muscles throughout the body
 contraction of smooth muscle in the internal organs
 secretion of active chemical substances by both exocrine and
endocrine glands in many parts of the body.
 Figure on the slide 17 shows the “skeletal” motor nerve axis of
NS controlling collectively called motor functions of the NS, the
muscles and glands are called effectors.
 Operating parallel to this axis is another system, called the
autonomic nervous system, for controlling smooth muscles,
glands, and other internal organs.
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16. Motor Part of the NS cont’d…..
 Note in Figure that the skeletal muscles can be
controlled by the :
 spinal cord
 Reticular substance of the medulla, pons, and mesencephalon
 basal ganglia
 Cerebellum
 motor cortex
 Each of these areas plays its own specific role
 the lower regions are concerned primarily with automatic,
instantaneous muscle responses to sensory stimuli
 the higher regions with deliberate complex muscle movements
controlled by the thought processes of the brain.
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17. Fig Skeletal Motor Nerve Axis of the Nervous System
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18. Major Levels of CNS Function
 The human nervous system has inherited special functional
capabilities from each stage of human evolutionary
development.
 There are three major levels of the CNS having specific
functional characteristics:
 the spinal cord level
 the lower brain or subcortical level
the higher brain or cortical level.
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19. Major Levels of CNS Function Cont’d….
Spinal Cord Level
We often think of the spinal cord as being only a
conduit for signals from the periphery of the body to
the brain, or in the opposite direction from the brain
back to the body. This is far from the truth.
• Neuronal circuits in the cord can cause:
walking movements
reflexes that withdraw portions of the body
reflexes that stiffen the legs to support the body against
gravity
 reflexes that control local blood vessels, gastrointestinal
movements, including defecation reflex or urinary
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excretion.

20. Major Levels of CNS Function Cont’d….
Lower Brain or Subcortical Level
 Many, if not most, of what we call subconscious
activities of the body are controlled in the lower areas
of the brain:
 medulla
 pons
mesencephalon
Hypothalamus
thalamus
cerebellum
Basal ganglia
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21. Major Levels of CNS Function Cont’d…
For instance, subconscious control of arterial
pressure and respiration is achieved mainly in
the medulla and pons
 Control of equilibrium is a combined
function of the portions of the cerebellum
and the reticular substance of the medulla,
pons, mesencephalon.
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22. Major Levels of CNS Function Cont’d…
• Higher Brain or Cortical Level
• After the preceding account of the many NS functions
that occur at the cord and lower brain levels, one may
ask, what is left for the cerebral cortex to do?
The answer to this is complex, but it begins with
the fact that the cerebral cortex is an extremely large
memory storehouse and responsible for many other
intellectual functions.
The cortex never functions alone but always in
association with lower centers of the NS.
Without the cerebral cortex, the functions of the
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lower brain centers are often imprecise.

23. Higher Brain or Cortical Level cont’d..
The vast storehouse of cortical information usually converts these
functions to determinative and precise operations.
The cerebral cortex is essential for most of our thought processes,
but it cannot function by itself.
It is the lower brain centers, not the cortex, that initiate wakefulness
in the cerebral cortex, thus opening its bank of memories to the
thinking machinery of the brain.
But it is the cortex that opens a world of stored information for use
by the mind.
Thus, each portion of the NS performs specific functions.
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24. Central Nervous System Synapses
Obviously information is transmitted in the NS mainly in the form
of nerve action potentials
In addition, each impulse may be:
blocked in its transmission from one neuron to the next,
changed from a single impulse into repetitive impulses,
integrated with impulses from other neurons to cause
highly intricate patterns of impulses in successive
neurons.
All these functions can be classified as synaptic functions of
neurons.
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25. CNS Synapses Cont’d…
Types of Synapses
There are two major types of synapses:
 the chemical synapse and
 the electrical synapse.
Almost all the synapses used for signal transmission in the CNS
of the human being are chemical synapses.
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26. CNS Synapses cont’d……
More than 40 important transmitter substances have
been discovered thus far.
Some of the best known are:
 Acetylcholine
 Norepinephrine
 Epinephrine
 Histamine
 Gamma aminobutyric acid (GABA)
 Glycine
 Serotonin, and
 Glutamate.
Electrical synapses, in contrast, are characterized by direct open
fluid channels that conduct electricity from one cell to the next.
This conduction is via the gap junctions that allow free movement
of ions from the interior of one cell to the interior of the next.
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27. Sensory Receptors
The are five basic types of sensory receptors:
 Mechanoreceptors-which detect mechanical compression or
stretching of the receptor or of tissues adjacent to the receptor
 Thermoreceptors, which detect changes in temperature, some
receptors detecting cold and others warmth
 Nociceptors (pain receptors), which detect damage occurring
in the tissues
 Electromagnetic receptors, which detect light on the retina of
the eye
 Chemoreceptors, which detect taste in the mouth, smell in the
nose, oxygen level in the arterial blood, osmolality of the body
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fluids, carbon dioxide concentration etc.

28. Sensory Receptors cont’d…
Differential Sensitivity of Receptors
 The first question that must be answered is, how do two types
of sensory receptors detect different types of sensory stimuli?
 The answer is, by “differential sensitivities.” That is, each type
of receptor is highly sensitive to one type of stimulus
 Thus, the rods and cones of the eyes are highly responsive to
light but are almost completely non-responsive to other stimuli
 The osmoreceptors of the supraoptic nuclei in the
hypothalamus detect minute changes in the osmolality of the
body fluids but have never been known to respond to sound.
 Pain receptors in the skin are almost never stimulated by
usual touch or pressure stimuli but do become highly active
the moment tactile stimuli become severe enough to damage
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the tissues.

29. Sensory Receptors cont’d…
Modality of Sensation— The “Labelled Line”
Principle
Each of the principal types of sensation that we can experience—
pain, touch, sight, sound, and so forth—is called a modality of
sensation.
Yet despite the fact that we experience these different modalities
of sensation, nerve fibers transmit only impulses.
Therefore, how is it that different nerve fibers transmit different
modalities of sensation?
The answer is that each nerve tract terminates at a specific point
in the CNS, and the type of sensation felt when a nerve fiber is
stimulated is determined by the point in the NS to which the
fiber leads.
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30. Sensory Receptors cont’d…
For instance, if a pain fiber is stimulated, the person perceives
pain regardless of what type of stimulus excites the fiber.
The stimulus can be electricity, overheating of the fiber, crushing
of the fiber, or stimulation of the pain nerve ending by damage
to the tissue cells. In all these instances, the person perceives
pain.
Likewise, if a touch fiber is stimulated by electrical excitation of a
touch receptor or in any other way, the person perceives touch
because touch fibers lead to specific touch areas in the brain.
Similarly, fibers from the retina of the eye terminate in the vision
areas of the brain, fibers from the ear terminate in the auditory
areas of the brain, and temperature fibers terminate in the
temperature areas.
• The fact that every sensory information is conducted to
definitive areas in the brain is called The “Labelled Line”
Principle
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31. CLASSIFICATION OF SOMATIC SENSES
The somatic senses can be classified into three
physiologic types:
 the mechanoreceptive somatic senses, which include both
tactile and position sensations that are stimulated by
mechanical displacement of some tissue of the body
 the thermoreceptive senses, which detect heat and cold
 the pain sense (nociceptive), which is activated by any factor
that damages the tissues.
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32. Other Classifications of Somatic Sensations
 Somatic sensations are also often grouped
together in other classes, as follows.
Exteroreceptive sensations are those from the surface of
the body.
Proprioceptive sensations are those having to do with the
physical state of the body, including
position sensations
tendon and muscle sensations
pressure sensations from the bottom of the feet
the sensation of equilibrium (which is often
considered a “special” sensation rather than a
somatic sensation).
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33. Other Classifications cont’d…
Visceral sensations are those from the viscera of
the body; in using this term, one usually refers
specifically to sensations from the internal organs.
Deep sensations are those that come from deep
tissues, such as from fasciae, muscles, and bone.
These include mainly
o deep pressure,
o pain
o vibration
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34. Tactile Receptors.
There are at least six entirely different types of tactile receptors,
but many more similar to these also exist.
1. Free nerve endings, found everywhere in the skin and in many
other tissues, can detect:
 touch and
 pressure
2. Touch receptor called Meissner’s corpuscles, elongated
encapsulated nerve endings of a large (type Aß) myelinated
sensory nerve fiber
 They are rapidly adapting receptors
 sensitive to movement of objects over the surface of the skin
as well as to low frequency vibration.
3. Merkel’s discs, responsible for giving steady-state signals that
allow one to determine continuous touch of objects against the
skin.
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 These receptors are slowly adapting type

35. Tactile Receptors cont’d….
4. Hair end-organ,
slight movement of any hair on the body stimulates a
nerve fiber entwining its base.
Like Meissner’s corpuscles, detects mainly
movement of objects on the surface of the body
initial contact with the body.
5. Ruffini’s end-organs, which are
 Multi-branched
 Encapsulated endings
 Located in the deeper layers of the skin and in deeper internal tissues
 In joint capsules and help to signal the degree of joint rotation.
 Adapt very slowly
 Detect heavy and prolonged touch and pressure signals.
6. Pacinian corpuscles
 Adapt in a few hundredths of a second.
 Particularly important for detecting tissue vibration or other rapid
changes in the mechanical state of the tissues.
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36. Pain
Pain Is a Protective Mechanism. Pain occurs




whenever any tissues are being damaged, and it causes the individual
to react to remove the pain stimulus.
Even such simple activities as sitting for a long time on the ischia can
cause tissue destruction because of lack of BF to the skin where it is
compressed by the wt of the body.
When the skin becomes painful as a result of the ischemia, the person
normally shifts wt subconsciously.
But a person who has lost the pain sense, as after spinal cord injury,
fails to feel the pain and, therefore, fails to shift.
This soon results in total breakdown and desquamation of the skin
(pressure sore) at the areas of pressure.
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37. Types of Pain
Pain has been classified into two
 Fast Pain
 Slow Pain
 Fast pain is felt within about 0.1 second after a pain stimulus is
applied
 Slow pain begins only after 1 second or more and then increases
slowly over many seconds and sometimes even minutes.
 Fast pain is also described by many alternative names, such as sharp
pain, pricking pain, acute pain, and electric pain.
 Slow pain also goes by many names, such as slow burning pain,
aching pain, throbbing pain, nauseous pain, and chronic pain.
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38. Pain Receptors and Their Stimulation
 The pain receptors in the body tissues are all free nerve endings.
 They are widespread in:
 the superficial layers of the skin
 certain internal tissues
 the periosteum, the arterial walls




the joint surfaces
the falx and tentorium in the cranial vault
most other deep tissues are only sparsely supplied with nerve endings
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No nociceptors in the brain.

39. Stimuli Excite Pain Receptors

o
o
o
o
o
o
Three Types of Stimuli Excite Pain Receptors
Mechanical
Thermal
Chemical.
Fast pain is elicited by the mechanical and thermal types of stimuli
Slow pain can be elicited by all three types.
Some of the chemicals that excite pain receptors include:
bradykinin
serotonin
histamine
potassium ions
Acids
Acetylcholine
proteolytic enzymes
 prostaglandins
 substance P
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40. Dual Pathways for Transmission of Pain Signals
into the CNS
• Even though all pain receptors are free nerve endings, these
endings use two separate pathways for transmitting pain
signals into the CNS.
 a fast-sharp pain pathway
 a slow-chronic pain pathway.
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41. Pain pathway……
The fast sharp pain signals are elicited by
mechanical or
thermal pain stimuli
transmitted to the spinal cord by small type Aσ fibers at
velocities between 6 and 30 m/sec.
The slow-chronic type of pain is elicited mostly by
persisting mechanical or thermal stimuli
is transmitted to the spinal cord by type C fibers at
velocities between 0.5 and 2 m/sec.
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42. Pain pathway……
Because of the double system of pain innervation, a sudden
painful stimulus often gives a “double” pain sensation:
The sharp pain apprises the person rapidly of a damaging
influence and, plays an important role in making the person react
immediately to remove himself or herself from the stimulus.
The slow pain tends to become greater over time.
This sensation eventually produces the intolerable suffering of
long continued pain and makes the person keep trying to relieve
the cause of the pain.
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43. Pain pathway……
Dual Pain Pathways in the Cord and Brain Stem takes two
pathways:
 the neospinothalamic tract
 the paleospinothalamic tract.
Neospinothalamic tract conducts fast pain and terminate mainly in
lamina I (lamina marginalis) of the dorsal horns and there excite
second-order neurons of the neospinothalamic tract.
These give rise to long fibers that cross immediately to the
opposite side of the cord through the anterior commissure and
then turn upward, passing to the brain in the anterolateral
columns.
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44. Pain pathway……
Capability of the NS to Localize Fast Pain in the Body.
The fast-sharp type of pain can be localized much more exactly in
the different parts of the body than can slow chronic pain.
However, when only pain receptors are stimulated, without the
simultaneous stimulation of tactile receptors, even fast pain may
be poorly localized, often only within 10 cm or so of the stimulated
area.
Yet when tactile receptor that excite the dorsal column–medial
lemniscal system are simultaneously stimulated, the localization
can be nearly exact.
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45. Pain pathway……
Probable NT of the Type Aσ Fast Pain Fibers.
It is believed that glutamate is a neurotransmitter
substance secreted in the spinal cord at the type
Aσ pain nerve fiber endings.
This is one of the most widely used excitatory
transmitters in the CNS, usually having a duration
of action lasting for only a few milliseconds.
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46. Pain pathway……
Pathway for Transmitting Slow-Chronic Pain.
The paleospinothalamic pathway transmits pain mainly from the
peripheral slow-chronic type C pain fibers, although it does
transmit some signals from type Aσ fibers as well.
In this pathway, the peripheral fibers terminate in the spinal cord
almost entirely in laminae II and III of the dorsal horns, which
together are called the substantia Gelatinosa (the location of
the 1st synapse in pain pathway).
Most of the signals then pass thru one or more additional short
fiber neurons within the dorsal horns themselves before
entering mainly lamina V.
Here the last neurons in the series give rise to long axons that
mostly join the fibers from the fast pain pathway, passing first
thru the anterior commissure to the opposite side of the cord,
then upward to the brain in the anterolateral pathway.
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47. Pain pathway……
Neurotransmitter of Type C Nerve Endings- Substance P
Research experiments suggest that type C pain fiber terminals
entering the SC secrete both glutamate transmitter and
substance P transmitter.
The glutamate transmitter acts instantaneously and lasts for only
a few milliseconds.
Substance P is released much more slowly, building up in
concentration over a period of seconds or even minutes.
It has been suggested that the ‘’double” pain sensation one feels
after a pinprick is due to the glutamate transmitter giving a
faster pain sensation and the substance P transmitter giving a
more lagging sensation.
It seems clear that glutamate is the neurotransmitter mostly
transmitting fast pain into the CNS
Substance P is concerned with slow-chronic pain.
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48. Pain Suppression (Analgesia System) in the CNS
The degree to which
a person
reacts to pain varies
tremendously.
This results partly
from a
capability of the
brain itself to
suppress input of
pain signals
to the nervous
system by
activating a pain
control
system, called an
analgesia system.
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49. Pain Suppression….
The analgesia system consists of three major components:
1. The periaqueductal gray and periventricular areas of the
mesencephalon send signals to
2. The raphe magnus nucleus, a thin midline nucleus located in the
lower pons and upper medulla, and the nucleus reticularis
paragigantocellularis, located laterally in the medulla.
From these nuclei, second-order signals are transmitted down the
dorsolateral columns in the spinal cord to
3. a pain inhibitory complex located in the dorsal horns of the SC
At this point, the analgesia signals can block the pain before it is
relayed to the brain.
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50. Pain Suppression….
It has been observed that electrical stimulation either in the
periaqueductal gray
area or in the raphe magnus nucleus can suppress
many strong pain signals entering by way of the dorsal
spinal roots.
Stimulation of areas at still higher levels of the brain that
excite the periaqueductal gray area can also suppress
pain.
Some of these areas are:
1. the periventricular nuclei in the hypothalamus, lying adjacent
to the third ventricle
2. to a lesser extent, the medial forebrain bundle
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51. Gate Control Theory Of Pain
• The gate control theory of pain argues that the
sensory messages travel through the body’s
pain highway i.e. from the stimulated nerves to
the spinal cord.
• The messages undergo reprocessing here and
then get transferred to thalamus, the brain’s
depot for tactile information.
• The laminae II & III (substantia gelatinosa), the
pain sensation is somewhat blocked so that the
amount of pain a person feels is lessened.
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52. 52

53. Pain neurtrasmitter substances
 Several transmitter substances are involved in the analgesia
system; especially involved are enkephalin and serotonin.
Many nerve fibers derived from the periventricular nuclei
and from the periaqueductal gray area secrete enkephalin at
their endings.
The endings of many fibers in the raphe magnus nucleus
release enkephalin when stimulated.
Fibers originating in this area send signals to the dorsal horns of
the spinal cord to secrete serotonin at their endings. serotonin →
local cord neurons to secrete enkephalin.
The enkephalin is believed to cause both presynaptic and
postsynaptic inhibition of incoming type C and type Aσ pain fibers
where they synapse in the dorsal horns.
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54. Pain NT Substances
Brain’s Opiate System
 Endorphins and
 enkephalins are involved
About a dozen of opiate-like substances have now
been found at different points of the nervous system; all
are breakdown products of three large protein
molecules:
proopiomelanocortin,
Proenkephalin
prodynorphin.
Among the more important of these opiate-like
substances are
ß-endorphin,
met-enkephalin,
leuenkephalin,
dynorphin.
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55. Trigeminal Neuralgia
 Trigeminal neuralgia causes facial pain.
 Develops in mid to late life.
 Characterized by feeling like bursts of sharp, stabbing, electricshocks.
 Can last from a few seconds to a few minutes
 Interferes with common daily activities such as eating and sleep
.
 Leads to irritability, severe anticipatory anxiety and depression,
and life-threatening malnutrition.
 Often called "tic douloureux" because of a characteristic muscle
spasm that accompanies the pain.
 In almost all cases (97%), pain will be restricted to one side of
the face.
55

56. What Is Tabes Dorsalis?
• Tabes dorsalis is a slow degeneration of the covering of nerve
cells and nerve fibers that carry sensory information to the
brain. The degenerating nerves are in the dorsal column of the
spinal cord (the portion closest to the back of the body) and
carry information that help maintain a person's sense of
position. Tabes dorsalis results when a syphilis infection goes
untreated.
• Treatment
The treatment of choice for tabes dorsalis is antibiotics. The drug
that is normally recommended is penicillin, given intravenously.
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57. Motor Functions of the Spinal Cord -the Cord
Reflexes
 Sensory info is integrated at all levels of the NS → appropriate
motor responses that begin in the SC with relatively simple
muscle reflexes → the brain stem with more complicated
responses, and finally extend to the cerebrum, where the most
complicated muscle skills are controlled.
 Without the special neuronal circuits of the cord, even the most
complex motor control systems in the brain could not cause any
purposeful muscle movement.
 To give an example, there is no neuronal circuit anywhere in
the brain that causes the specific to-and-fro movement of the
legs that is required in walking.
 Instead, the circuits for these movements are in the cord, and
the brain simply sends command signals to the spinal cord to
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set into motion the walking process.

58. Motor Functions of the Spinal Cord cont’d…
However, the role of the brain in the control of movement can not
be undermined.
The brain gives directions that control the sequential cord
activities:
to promote turning movements when they are required
to lean the body forward during acceleration
to change the movements from walking to jumping
to monitor continuously and control equilibrium.
All this is done through “analytical” and “command” signals
generated in the brain.
But it also requires the many neuronal circuits of the spinal cord
that are the objects of the commands.
These circuits provide all but a small fraction of the direct control
of the muscles.
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59. Organization of the Spinal Cord for Motor fxns
The cord gray matter
is the integrative
area
for the cord reflexes.
Figure shows the
Typical organization
of the cord gray
matter in a single
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60. Organization of the SC for Motor fxns…
Anterior Motor Neurons. Located in each segment of the
anterior horns of the cord gray matter are several thousand
neurons that are 50 to 100% larger than most of the others and
are called anterior motor neurons.
They give rise to the nerve fibers that leave the cord by way of the
anterior roots and directly innervate the skeletal muscle fibers.
The neurons are of two types
alpha motor neurons and
gamma motor neurons.
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61. Organization of the SC for Motor fxns…
 Interneurons. Interneurons are present in all areas of the cord
gray matter in the
 Dorsal horns
 Anterior horns
 Intermediate areas between them
 Are small and highly excitable, often exhibiting spontaneous
activity and capable of firing as rapidly as 1500/s
 Have many interconnections with one another, and many of
them also synapse directly with the anterior motor neurons
 Interneurons and anterior motor neurons are responsible for
most of the integrative functions of the SC
61

62. Organization of the SC for Motor fxns…
Alpha Motor Neurons- give rise to large type A alpha (Aα) motor
nerve fibers, averaging 14 micrometers in diameter and branch
several times after they enter the Muscle and are collectively
called the motor unit.
The alpha motor neurons innervate extrafusal muscle fibers
Gamma Motor Neurons- The gamma motor neurons are located
in the spinal cord anterior horns, transmit impulses through
much smaller type A gamma (Aγ) motor nerve fibers, which go
to small, special skeletal muscle fibers called intrafusal fibers
See next fig.
62

63. Organization of the SC for Motor fxns…
Fig
63

64. Organization of the SC for Motor fxns…
Renshaw Cell Inhibitory System.
Renshaw cells are located in the anterior horns of the spinal cord,
in close association with the motor neurons.
These cells transmit inhibitory signals to the surrounding motor
neurons, which is known as lateral inhibition.
It was found out that the Renshaw cells contain glycin and GABA
receptors.
64

65. Organization of the SC for Motor fxns…
Collateral inhibition effect is important for the
following major reason:
The motor system uses this lateral inhibition to focus, or
sharpen, its signals in the same way that the sensory
system uses the same principle—that is, to allow
unabated transmission of the primary signal in the
desired direction while suppressing the tendency for
signals to spread laterally.
65

66. Muscle Sensory Receptors
Proper control of muscle function requires not only excitation of
the muscle anterior motor neurons but also continuous
feedback of sensory information from each muscle to the SC,
indicating the functional status of each muscle at each instant.
To provide this information, the muscles and their tendons are
supplied abundantly with two special types of sensory receptors:
 Muscle spindles found in the belly of the muscle and send
information to the NS about muscle length or rate of change of
length, and
 Golgi tendon organs which are located in the muscle tendons
and transmit information about tendon tension or rate of change
of tension.
66

67. Muscle Sensory Receptors…......
 The signals from these two receptors are either entirely or
almost entirely for the purpose of intrinsic muscle control.
Operate almost completely at a subconscious level.
 Transmit tremendous amounts of information to:
the spinal cord
the cerebellum
even to the cerebral cortex, helping each of these
portions of the NS function to control muscle contraction.
67

68. Receptor Function of the Muscle Spindle Structure
and Motor Innervation of the Muscle Spindle.
Skeletal muscle contains two types of muscle fibers:
Small intrafusal skeletal muscle fibers
Large extrafusal skeletal muscle fibers.
 The area midway between two ends of intrafusal fibers has few
or no actin and myosin filaments.
 Hence does not contract
 Instead, it functions as a sensory receptor, as described later.
 These portions are excited by small gamma motor nerve fibers
also called gamma efferent fibers
 The large alpha efferent fibers (type A alpha nerve fibers)
innervate the extrafusal skeletal muscle.
68

69. Receptor Function of the Muscle…..
Sensory Innervation of the Muscle Spindle.
The receptor portion of the muscle spindle is its central portion.
They are stimulated by stretching of this mid portion of the spindle
and are excited in two ways:
1. Lengthening the whole muscle stretches excites the mid
portion of the spindle
2. Even if the length of the entire muscle does not change,
contraction of the end portions of the spindle’s intrafusal fibers
stretches the mid portion of the spindle and therefore excites
the receptor.
Two types of sensory endings are found in this central receptor
area of the muscle spindle.
 the primary ending and
 the secondary ending.
Primary Ending. transmits sensory signals to the spinal cord
Secondary Ending. Innervates receptor region on one or both
69
sides of the primary ending.

70. Receptor Function of the Muscle…..
Division of the Intrafusal Fibers
There are also two types of muscle spindle intrafusal fibers:
1. Nuclear bag muscle fibers (one to three in each spindle), in
which several muscle fiber nuclei are congregated in expanded
“bags” in the central portion of the receptor area,
2. Nuclear chain fibers (three to nine), which are about half as
large in diameter and half as long as the nuclear bag fibers and
have nuclei aligned in a chain throughout the receptor area
(See figure on the next slide)
The primary sensory nerve ending is excited by both the nuclear
bag intrafusal fibers and the nuclear chain fibers.
Conversely, the secondary ending is usually excited only by
nuclear chain fibers.
70

71. 71

72. Response of Both the 1º and the 2º Endings to the
Length of the Receptor
Static Response Vs Dynamic Response
When the receptor portion of the muscle spindle is stretched
slowly, the No of impulses transmitted from both the 1º and the 2º
endings ↑almost directly in proportion to the degree of stretching
and the endings continue to transmit these impulses for several
minutes.
This effect is called the static response of the spindle receptor,
meaning simply that both the 1º and 2º endings continue to
transmit their signals for so long.
When the length of the spindle receptor ↑ suddenly, the 1º ending
(not the 2º ending) is stimulated, this excess stimulus of the 1º
72
ending is called the dynamic response

73. Muscle Stretch Reflex
• The simplest
manifestation of
muscle spindle
function is the
muscle stretch reflex
(monosynaptic)
• Whenever a muscle
is stretched
suddenly, excitation
of the spindles
causes reflex
contraction of the
large skeletal muscle
fibers of the
stretched muscle
and also of closely
allied synergistic
muscles.
73

74. Neuronal Circuitry of the Stretch Reflex.
 Figure on the previous slide demonstrates the basic
circuit of the muscle spindle stretch reflex.
Figure shows a type Ia proprioceptor nerve fiber
originating in a muscle spindle
and entering a dorsal root of the spinal cord.
A branch of this fiber then goes directly to the AH of the
cord gray matter and synapses with these neurons
AH neurons send motor nerve fibers back to the same
muscle from which the muscle spindle fiber originated.
Thus, this is a monosynaptic pathway that allows a
reflex signal to return with the shortest possible time
delay back to the muscle after excitation of the
74
spindle.

75. Flexor Reflex (Withdrawal Reflex)
A sensory stimulus from a limb
→cause the flexor muscles of
the limb to contract
→withdrawing the limb from
the stimulating object.
This is called the flexor reflex.
Most powerfully by stimulation
of pain endings, such as by
a pinprick, heat, or a wound,
for which reason it is also
called a nociceptive reflex,
or simply a pain reflex.
Fig
75

76. • Neuronal Mechanism of the Flexor Reflex.
Figure on slide 75 shows the neuronal pathways for the flexor
reflex.
A painful stimulus is applied to the hand → excitation of the flexor
muscles of the upper arm → withdrawing the hand from the
painful stimulus.
The pathways for eliciting the flexor reflex pass to the spinal cord
interneuron pool of neurons then to the motor neurons.
Most of the signals of the reflex traverse many more neurons and
involve the following basic types of circuits:
 Diverging circuits to spread the reflex to the necessary
muscles for withdrawal
 Circuits to inhibit the antagonist muscles, called reciprocal
inhibition circuits
 Circuits to cause after discharge lasting many fractions of a
76
second after the stimulus is over

77. Knee Jerk Reflex
• Clinically, a method used to determine the sensitivity
of the stretch reflexes
The knee jerk can be elicited by simply striking the
patellar tendon with a reflex hammer; this stretches
the quadriceps muscle and excites a dynamic stretch
reflex that causes the lower leg to “jerk” forward.
A sudden stretch of muscle spindles is all that is
required to elicit a dynamic stretch reflex.
The muscle jerks are used by neurologists to assess
the degree of facilitation of spinal cord centers.
77

78. 78