The document discusses various types of sensations detected by the nervous system including pain, touch, proprioception, vibration, and thermal sensations. It describes: 1) The pathways for each sensation from receptors to the central nervous system, including the roles of first, second, and third order neurons. 2) Characteristics of different sensations such as poorly localized deep pain versus well-localized superficial pain. 3) Structures involved in each pathway such as dorsal root ganglia, dorsal columns, spinothalamic tracts, and various brain regions.

1. The Nervous System/Part III
Dr. Fawaz A. Mustafa
PhD in Medical Physiology and Pharmacology
Unlimited permission is granted free of charge to print or photocopy all pages of this
work for educational, not for profit use by university staff or students

2. Pain sensation
• The golden rule for any sensation is applying
stimulus which should fall on a receptor which
should be connected to a nerve fibre which
comes from nerve cell body, to be conducted
until the information reaches the final centre for
analysis.
• The pain is a sensation.

3. Pain sensation
• What the kinds of pain-producing
stimuli?
• They are:
a. Thermal stimuli:
b. Mechanical stimuli:
c. Chemical stimuli:

4. Pain sensation
• What is the histological structure of the
receptor?

5. Threshold and reaction of pain
• The term “threshold of pain” means the minimum
amounts of stimulus intensity that can stimulate
the nerve endings to produce pain sensation. This
is equal for both male and female and for all
people.
• The term “reaction of pain” differs from individual
to another. When applying a stimulus of the same
intensity on several persons, some will not care,
others cry, others become anxious or angry.
Persons with long nails, even weak stimulus
produce pain.

6. Pain sensation
• Therefore, whenever there is tissue damage,
two types of pain occur:
a. Fast, acute, sharp or quick pain at the
beginning. The fast pain awakes us to injury
(apprise body to the injury) by giving
collaterals to the reticular activating system.
Once this system is activated, the cerebral
cortex will awake us.
b. Chronic, prolong, continuous or aching pain,
later on. The slow chronic pain gives
collateral ends to the limbic system, which
makes us suffer the pain.

7. Pain sensation and the distribution of
pain receptors
a. The external surface of human bodies is well
protected by the dense distribution of pain
receptors because they alarm the body
whenever tissue damage is occurring.
b. The internal parts of the body (such as bone,
muscles, ligaments, joints and other internal
structures) also have pain receptors. They have
also pain receptors, but the distance between
one receptor and another is large.

8. Pain sensation and the distribution of
pain receptors
c. Internally, there are structures which have no
pain receptors. For example, liver cells contain
no receptors, but the surrounding capsule of the
liver contain pain receptors. So, any disease or
damage inside the liver cannot be felt, but that
concerning the capsule can be felt. Brain tissue
doesn’t contain any pain receptors. If there is a
cut part of brain, no pain felt. But the surrounding
blood vessels (around the brain) as well as the
maters (dura, arachnoid and pia maters) contain
pain receptors. Concerning lungs, the bronchi
and the blood vessels contain the pain receptors,
while alveoli do not.

9. Ischemic pain
• There is a special type of pain known as
“ischemic pain”. Ischemia means low blood
supply to a tissue.
• Chemicals are produced inside the tissue as a
result of low blood supply to the tissue. These
chemicals will stimulate he chemical receptors
for pain.

10. Ischemic pain
• In persons with angina pectoris, they feel pain
when they walk a lot, and the type of this pain is
ischemic.
• Ischemic pain is also felt in muscle cramp
• Cold could stimulate muscle contraction and if this
occurs for long duration, this prevents enough
blood supply and results in ischemic pain.

11. Pain pathway
• It consists of receptors on which stimulus is
applied, then receptor is connected to a nerve
fibre which belongs to a soma, and many
synapses occur till the pain sensation reaches
the final centre.
• To conduct a pain, there are two types of
conduction: fast by Ad fibres and slow by C
fibres.

12. Pain pathway
• If a stimulus for pain is applied on the receptors
in the skin of the thumb, this will travel through
the axon of median nerve and these axons
belong to neuron soma that are present in the
posterior (dorsal horn) of cervical segments of
the spinal cord, then these nerve fibres will send
collaterals. The part of the pathway from the
receptor to the dorsal (posterior) horn of the
spinal cord is the first order neuron for both
fast and slow pain.

13. Pain pathway
• The posterior horn of the spinal cord consists of
many layer (I, II, III, IV, V, VI) which contain
neurons that belong to the sensory system (in
the gray matter of spinal cord).
• The first order neuron ends (not present) in the
posterior horn, then it synapses with the second
order neuron whose axon passes to the
opposite side (from the right to the left or from
the left to the right), then it continues till it
reaches the thalamus.

14. Pain pathway
• This bundle called “tract”, and in this case it has
a special name which is “spinothalamic tract”.
• The thalamus is a region in the brain above the
hypothalamus, and it contains a group of nuclei,
some of which are sensory nuclei. The second
order neuron starts at the posterior horn of the
spinal cord, then passes to the medulla oblongata
and pons, then it ends in the nuclei of the
thalamus.

15. Pain pathway
• The third order neuron starts from the
thalamus and goes upward till it reaches the
central cortex (to the parietal lobe of the brain
in the postcentral gyrus). This is the final point
where the pathway ends.
• Since there is crossing of the fibres from one
side to the opposite side, therefore, the pain in
the right side goes to the left hemisphere of
brain.

16. Pain pathway
• Another example:
If the stimulus is applied to one toe instead of the
thumb, the receptor will conduct the action
potential to a nerve fibre which ends in a nerve cell
body in the posterior horn of one of the sacral
segments of the spinal cord. This is the first order
neuron.

17. Pain pathway
Then the second order neuron starts from the
posterior horn then passes to the lumbar, thoracic
and cervical segments, then reaches to the
medulla oblongata and pons to end in the
thalamus. A third order neuron will start to end in
the cerebral cortex.

18. Pain pathway
• Further example:
If the stimulus is applied to the jaw (a hit on the jaw), the
pathway will start, as usual, from the receptors which are
attached to the axons of the sensory part of the trigeminal
nerve, and these axons end in the neuron body which is
present in the sensory nuclei of the brainstem, and this is
the first order neuron. The second order neuron starts from
the cells present in the nuclei and the fibre cross to the
opposite side, then the second order neuron will reach the
thalamus and ends there. The tract here is called “the
nucleothalamic tract”. The third starts at the thalamus and
ends into postcentral gyrus in the parietal lobe (cerebral
cortex).

19. Deep or visceral pain
• It is the pain in structures and tissues inside the
thorax, abdomen, joints and bones.
• The distribution of the pain receptors is very
sparse, thus, if a stimulus is applied on an area
free of receptors, the result is no production of
pain.
• To produce a pain, a diffuse stimulus due to a
diffuse damage or abnormality is needed to
stimulate the scattered receptors.

20. Deep or visceral pain
• Difference between superficial and visceral pain
1. The most important difference is that highly
localised types of damage to the viscera
seldom cause severe pain.
2. In the superficial pain, there is a knowing the
location of the stimulus very well (good
localisation). In the visceral pain, we cannot
localise exactly where the pain is

21. Deep or visceral pain
3. Visceral pain is slow pain and almost all the time
4. The stimuli that produce the visceral pain differs from
those that produce superficial pain, (How?!)

22. Deep or visceral pain
• Visceral pain is not only poorly localised,
but it is also a referred type of pain.
• Referred pain is the pain in a place far away
from the place of the stimulus that produces this
pain. But why does this occur? And why does
refer to this place not to that?

23. Deep or visceral pain
Long story short, the characteristics of visceral pain are:
• Poorly localised
• Slow, chronic pain
• Referred pain
• It may be produced by stretching
• It is associated with autonomic function move than the
superficial pain does.

24. Analgesic system of the body
• There are some terms in nervous system such
as “algesia” which means “pain”, “analgesia”
which means loss in pain and “anesthesia”
which means suppression of all types of
sensations
• Analgesic system of the body is a system of the
body that is nowadays well known both
histologically and anatomically

25. Analgesic system of the body
• The analgesic system in the body involve many
things, such as:
1. Increasing the threshold for pain. When
somebody is fighting or he gets angry, he will
increase the threshold for pain. Since the
threshold is increased, the pain producing
stimuli of small intensity (that used to produce
pain) producing no more pain and no action
potential unless the stimulus intensity increases
more than the threshold for pain.

26. Analgesic system of the body
2. The other mechanism that can reduce painful
sensation is by suppression of transmission
of pain impulses. When a stimulus is applied
to the nerve endings, an action potential is
conducted by the first neuron to be transmitted
to the second. To suppress the pain, there is a
need to prevent the action potential from
reaching to the second order neuron, thus, pain
does not reach the final centre. But how can
this be done?

27. Analgesic system of the body
3. The third mechanism is by secretion of pain-
killing chemicals inside the body (e.g. morphine
and morphine-like drugs) which act to kill the
pain.

28. Touch sensation
• The receptors for touch sensation are
histologically modified structures which are
connected to a nerve fibre.
• When a stimulus is applied to the receptor, there
is a kind of depolarisation or release of chemicals
that produce action potential in the nerve fibre
which belongs to a nerve cell body.
• These receptors differ in structure, some called
“Pacinian corpuscle”, others “Ruffini’s end-organ”
or “Merckel’s disc” or “Meissner’s corpuscle”

29. Touch sensation
• There are two types of touch sensation:
a. Fine touch
b. Crude touch

30. Touch sensation
• The pathway for fine touch starts by applying
stimulus to the receptor, this initiates an impulse
in the attached nerve fibre which does not go to
the dorsal horn but to the white matter of the
spinal cord and then it ascends upward without
crossing to the opposite side till it reaches the
medulla oblongata.
• The nerve fibre from the receptor to the medulla
oblongata is the first order neuron and is called
“dorsal column”.

31. Touch sensation
• In the medulla oblongata, a second order
neuron starts from the two nuclei: nucleus
gracillis and nucleus cuneatus which
contain neuron bodies whose axons will
form the nerve fibres of the second order
neuron that crosses to the opposite side.
• Crossing is in the medulla oblongata not in
the spinal cord. Then it ascends upward till
reaching the thalamus.

32. Touch sensation
• The second order neuron starts from the
nucleus gracillis and nucleus cutanus and ends
in the thalamus.
• The second order neuron is called by a special
name “medial leminscal system”.
• A third order neuron starts from the thalamus
to reach the post central gyrux.
• The most important type of touch is the fine
touch. Since, it is important, its receptors must
be connected to a large-diameter, myelinated
nerve fibre (usually type Aβ).

33. Touch sensation
• The crud touch is not that important as the fine.
Its receptors are connected to a nerve fibre of
small diameter (myelinated or non-myelinated)
the ends of dorsal horn of the spinal cord. This is
the first order neuron.
• The second order neuron stars at the posterior
(dorsal) horn and crosses to the opposite side
and ascends as anterolateral spinothalamic
tract, till it reaches the thalamus.
• The third order neuron arises from the
thalamus and go to the postcentral gyrux.

34. Proprioception sensation
• Proprioception or position sense is the sense by
which we can know where is our body in the
surrounding and where is each part of our body
relatively to the other.
• What is the type of stimuli of this sensation? And
what’s the type of receptors?

35. Proprioception sensation
• This sensation is very important, the first
order neuron’s nerve fibre must be of large
diameter and myelinated type, so that
information carried by which goes quickly
to the brain. The first order neuron passes
through the dorsal column till it reaches
the nucleus gracillis and cuneate nucleus
in the medulla oblongata.

36. Proprioception sensation
• Then a second order neuron from the medulla
oblongata and ends at the thalamus. The third
order neuron starts from the thalamus and ends in
the postcentral gyrux which tries to build a body
image from the information (coming from
mechanical receptors in joints, muscle spindles
and touch receptors) by combining and integrating
this information.
• This body image tells the cerebral cortex whether
the body is stabilized or not. If it is not, some of
information goes to the cerebellum for balance.

37. Proprioception sensation
• This sensation has two pathways:
i. To cerebral cortex: to build a body image
ii. To cerebellum: to build the balance of body
(when it is not in balance) by increasing the
contraction of some muscle around the joints
for example.

38. Vibratory sensation
• It is a kind of repetitive mechanical stimuli of
certain frequency. The pathway is similar to that
of fine touch.
• Guyton and hall 13th Ed.: page 615

39. Thermal sensation
• The human being can perceive different gradations
of cold and heat, from freezing cold to cold to
cool to indifferent to warm to hot to burning hot.
• Thermal gradations are discriminated by at least
three types of sensory receptors: cold receptors,
warmth receptors, and pain receptors..

40. Thermal sensation
• Thermal stimuli stimulate these receptors which
are connected to nerve fibres (which are either
of large diameter and non-myelinated, or small
diameter and myelinated), which belong to soma
near the spinal cord. This is the first order
neuron which ends in the posterior horn of the
spinal cord.

41. Thermal sensation
• The second order neuron starts at the posterior
horn and crosses to the opposite side and goes
through the anterolateral spinothalamic tract till it
reaches to the thalamus, where a third order
neuron starts and ends in the post central gyrux.
• This pathway is long, because thermal sensation
is not that important. The receptors of thermal
sensation are of the adaptive type.

42. Thermal sensation
• The thermal sensation usually occurs between
10 and 40 degree centigrade.
• The cold receptor are activated from 10 to 30
degree centigrade, while the warmth receptors
are activated from 20 to 40 degree centigrade

43. Thermal sensation
• Below and above this range, there will be no
adaptation, and below 10 degree centigrade and
above 40 degree centigrade, there is a feeling of
discomfort.
• At zero degree centigrade, the feeling is not
thermal sensation, but it is pain sensation because
at zero degree centigrade, tissue damage occurs,
and in this case the action potential goes in the
pain pathway whose receptors are nonadapting.
• The same thing (Pain, tissue damage) is happened
at 45 degree centigrade and above.