The nervous system is divided into the central nervous system (brain and spinal cord) and peripheral nervous system (sensory and motor nerves). Sensory receptors detect stimuli and transmit signals as action potentials along nerves. Receptors are classified based on location, stimulus detected, and physiological function. Mechanoreceptors detect mechanical stimuli while thermoreceptors detect temperature. Pain is transmitted by specialized pain receptors and is perceived as fast, localized pain or slow, diffuse pain depending on the location and type of stimulus. The brain modulates the emotional perception of pain signals from receptors.

1. Nervous system
by Dr. Dina Merzeban

2. Nervous system
•The nervous system is divided into central and peripheral parts
• The central nervous system contains the brain and the spinal cord.
•The peripheral nervous system contains
sensory nerves and motor nerves.
•The motor nerves are divided into autonomic and somatic peripheral nerves.
•The autonomic (involuntary) nervous system: Contains the sympathetic and
parasympathetic nervous systems.

4. SENSORY NERVOUS SYSTEM
Sensory receptors
• Definition: they are modified nerve endings of afferent fibers which
receive and convert different stimuli into action potentials (nerve
impulses).
• Functions:
 Detectors: detect changes in the surrounding environment.
 Transducers: transform any form of energy in the stimulus into
action potentials.

5. Classification of sensory receptors
(A) According to site of receptor: special sense –
cutaneous - visceral- deep
(B) Site of stimulus: teleceptors - Extroceptors-
Introceptors - proprioceptors.
(C) Physiological classification.

6. (C) Physiological classification
1. Mechano- receptors
2. Thermo- receptors
3. Chemo- receptors
4. Pain
5. Electromagnetic receptors

7. [1] Mechanoreceptors
•Detect mechanical deformity in the receptors as:
– touch,
– pressure,
– sound (cochlear receptors),
– acceleration (vestibular receptors) &
– blood pressure (baroreceptors).
•They are found in skin, mucous membrane, muscle,
tendons, joints, blood vessels, lungs and inner ear.

8. (C) Physiological classification
[2] Thermoreceptors:
In hypothalamus for body temperature regulations
In skin, mucous membrane: detect cold & warm energy .
[3] Chemoreceptors: Taste-smell - osmoreceptors-
glucoreceptors - for CO2 &O2 &H+
[4] Pain receptors

9. Properties of receptors
•Specificity
•Excitability
•Adaptation

10. Properties of receptors
• 1. Specificity
– Each receptor is most sensitive to one type of stimulus and
sensation E.g. light.
– Other forms of energy can stimulate the receptor, but it needs
stronger stimulus.

11. Properties of receptors
2. Excitability: (Receptor potential)
•It is a state of partial depolarization in the receptor
membrane which occurs when the receptor is stimulated.
•Receptor potential spreads passively.
•If depolarization reaches the firing level → it leads to action
potential in adjacent nerve fiber.

12. Ionic basis of receptor potential
•The energy of the stimulus causes non specific opening of
Na+ channels → Na+ entry leading to partial depolarization.
•The number of opened channels is directly proportional to
intensity of stimulus.

14. Resting

15. mechanical
Stimulus
Physical stimulus causing mechanical deformation on the capsule

16. Physical Stimulus
Mechanical deformation is transmitted to the inside
Opens up mechanosensitive Na+ channel
Causes depolarisation and thus receptor potential

17. Physical Stimulus
local current
Current flow through a local circuit

18. Properties of receptors
(3) Adaptation of receptors:
•Definition: Adaptation is decline in the receptor potential
and frequency of impulses in spite of constant maintained
application of the stimulus.

19. Adaptation of receptors
(a) Slowly (non) adapting receptors:
e.g. Pain receptors, muscle spindle & alveolar stretch receptors.
Their function is : to keep the brain continuously alert to
state of body & to protect the body from dangerous
changes in internal environment.
(B) Moderately adapting receptors: As temperature, smell & taste
(C) Rapidly adapting receptors: They adapt rapidly to continuously applied
stimuli e.g. touch receptors.

20. Properties of receptors
(3) Adaptation of receptors:
• Mechanism of adaptation:
Remodeling (readjustment) of the receptor structure and
inactivation of Na+ channels.

21. Physical Stimulus
local current

22. Pain Sensation
•Pain is unpleasant sensation. It occurs when there is
tissue damage.
•Pain receptors : Free nerve endings of three types:
A. Mechanical pain receptors: mechanical injurious stimuli.
B. Thermal pain receptors : cold pain- warm pain .
C. Chemical pain receptors: stimulated by chemical
injurious stimuli e.g. Hcl in case of peptic ulcer.

23. Distribution of pain receptors
Widely
distributed
Sup. Layers of skin
Pleura
Periosteum
periosteum
Arterial walls
Joint surface
duraof cranial cavity
Less
distributed
Deep tissues
& Viscera
Absent
Liver
Parenchyma
Lung alveoli
Brain tissue

24. Pain Sensation
•Adaptation: slowly or non adaptive receptors.
Types of pain:
1- According to quality of pain:
Fast (acute) & slow (chronic).
2- According to site of origin of pain:
Coetaneous, Deep, Visceral.

25. Fast pricking pain (Immediate,
acute)
Slow burning pain (chronic,
aching, throbbing)
1- Onset Felt within 0.1 second. After one second.
2- Duration Short duration. Long duration.
3- Characters Well localized. Poorly localized.
4- Receptors. Mechanical & thermal. Elicited by all types of receptor.
5- Fibers Aδ fibers. C fibers.
6- Chemical
transmitter.
Glutamate. Substance P.
7- Pathway Neospinothalamic tract (of lateral
spinothalamic)
Paleospinothalamic (of lateral
spinothalamic).
8- The end of
fibers
Ends in sensory cortex Ends in reticular formation, then to
non specific thalamic nuclei then to
whole cortex .
9- Deep tissue occur usually in skin & may in
pleura & peritoneum
Occur in skin & deep tissue.
10- Blocked By hypoxia & pressure. By Local anesthesia (Cocaine).

26. Clssification of pain according to the
site of its origin

27. (A) Cutaneous pain
• Pain produced by stimulation of pain receptors in the
skin.
• It occurs in 2 phases of fast pricking
followed by slow burning
• It is accurately localized due to: high number of pain
receptors in skin & the fast pain reaches the sensory cortex.

28. (B) Deep pain
•Characters: Diffuse, dull aching and depressor effects.
• •Produced from muscle, tendons, ligaments, joints &
periosteum.
• •Causes: inflammation, ischemia or muscle spasm.
• •Ischemic pain: it is type of deep pain caused by
decreased blood supply to a tissue by thrombosis,
inflammation of vessel or its narrowing. Examples: 1-
Angina pectoris: ischemia of cardiac muscle. 2-
lntermittent claudication: ischemia of skeletal muscle.

29. (C) Visceral pain:
•It is pain from different viscera of the abdomen & chest.
• •Sharp cut in the viscera does not cause pain
• •Pain from peritoneum, pleura or pericardium.

30. (C) Visceral pain:
Characters of visceral pain:
1- Dull aching or rhythmic cramps (colic)
2- Diffuse & poorly localized
3- Depressor (parasympathetic) autonomic changes: decrease
heart
rate, ABP , nausea & vomiting.
4- Rigidity of the overlying muscles
5- Referred to the surface area i.e. referred pain.

31. (C) Visceral pain:
Causes of visceral pain:
1- Ischemia: accumulation of acidic metabolites and proteoyltic
enzymes.
2- lnflammation of peritoneal covering of viscera.
3- Irritation: chemical irritation by Hcl in peptic ulcer.
4- Over-distension of a hollow viscous (e.g. urinary bladder)
5- Spasm of a hollow viscous e.g. gut, gall bladder.

32. Referred pain
•Definition: Pain is felt on a surface area originating from the
same dermatome of the diseased viscous or supplied by
the same dermatome.

33. Referred pain
Examples of referred pain:
1- Cardiac pain: is felt retrosternal, root of the neck, inner part of
the left arm & epigastrium.
2- Gall bladder pain: is felt in epigastrium & tip of right scapula
3- Gastric pain: is felt between the umbilicus & xiphoid process.
4- Appendix pain: is felt around the umbilicus.
5- Renal pain: is felt in the back, inguinal region & testicles

35. HEADACHE
•Type of referred pain to the surface of the head from deep
structures:
1- Headache of intracranial origin
•2- Headache of extracranial origin

36. 1- Headache of intracranial origin
•Intracranial pain sensitive structures include:
the venous sinuses,
the dural arteries and
the dura at the base of skull.
However the brain itself is insensitive to pain.

37. Causes of intracranial headache:
•Meningitis: due to inflammation of meninges.
•Meningeal trauma: due to meningeal irritation.
•Brain tumors: due to irritation of meninges.
•Migraine headache: due to marked dilatation of cerebral arteries.
•Low cerebrospinal fluid: removal of cerebrospinal fluid leads to
sinking of the brain due to its weight and causes meningeal traction.
•Constipation headache: due to irritation of meninges by toxins
absorbed from the rectum.

38. 2- Headache of extracranial origin:
Causes:
•Sinusitis, Allergic rhinitis.
•Errors of eye refraction, Glucoma.
•Dental abcess,Tooth ache.
•Otitis media.
•Spasm and traction of muscles.

40. •Threshold of pain: is the same of all people but
reactions differ.
•Reactions to pain:
(1) Motor reflexes: (Spinal reflexes)
 Cutaneous pain: flexor withdrawal reflex to remove
injurious part from painful stimuli.
 Visceral pain: Increased muscle tone above diseased area.
(2)Autonomic reactions:
 Cutaneous pain: sympathetic effects: Increase heart rate & ABP.
 Visceral pain: parasympathetic (depressor) effects:decrease heart rate & ABP.
(3)Emotional reactions:
 Acute pain: Crying & anxiety
 Chronic pain: Depression

41. Perception of pain signals:
•Fast pain is perceived in thalamus and cortex.
•Slow pain is perceived mainly in thalamus.
•Function of the cortex in pain perception is:
1- localization of pain: so sharp pain is very localized
2- Discrimination of pain (sharp, throbbing, dull aching,
spasmodic…..etc).
3- Modulation of pain by emotional and behavioral
factors.

42. • •SI: Receives different
sensations.
• •SII: Begin to give
meaning of pain.