Lecture:

​

Somatic Sensation

1

Dr. Eyad M. Hussein

Ph.D. of Neurology

Consultant & Head of Neurology Department,

Nasser Hospital

الرجاء تحويل الجوال إلى وضع الصامت مع الشكر

​

2

The somatic sensations can be classified in two ways:

A. According to the Site of the Sensation:

1. Superficial (exteroceptive) sensations: the sensations from

the skin (pain, touch and temperature).

2. Deep sensations: the sensations from skeletal muscles,

tendons, joints, bones and ligaments:

a. Proprioceptive sensations: sense of position and the

sense of movement (kinesthetic sensation).

b. Pressure sense.

c. Muscle tension sense.

d. Muscle sense.

Sometimes the vibration sense is included in this group.

3. Combined or synthetic senses: stereognosis , tactile

discrimination and vibration sense.

​

Classification of the Somatic Sensation

3

B. According to the Modality (type) of the Sensation:

(More common classification)

I. Mechanoreceptive sensations: these include the:

• Touch sensations.
• Pressure sensations.
• Muscle tension sense.
• Vibration sensations.
• Tickle and itch sensations.
• Proprioceptive sensations.

II. Thermoreceptive sensations: heat and cold

sensations.

III. Pain sensation.

​

4

5

Sensation

1. Touch (Tactile) Sensation

There are 2 types of touch sensation:

a. Crude touch: this is a poorly-localized gross tactile

sensation.

• Receptors: free nerve endings and hair follicle endings.
• Afferent nerves: A-delta nerve fibers.
• Central pathway: ventral spinothalamic tract.
• Testing: by stroking the skin lightly with a piece of cotton.

b. Fine touch: this is a good-localized tactile sensation.

• Receptors: Meissner’s corpuscles and Merkel’s disks.
• Afferent nerves: A-beta nerve fibers.
• Central pathway: the Gracile and Cuneate tracts.

I. The Mechanoreceptive Sensations

6

• Tactile Localization (Topognosis): the ability to localize a touched skin point while the eyes are closed.
• Tactile Discrimination (2 point discrimination): the ability to distinguish 2 separated point of touch (minimal distance between them i.e. 5 mm in finger & 5 cm over back).
• Steriognosis: the ability to recognize the nature of object (a familiar object) by touch without using vision.
• The sense of texture of material: is a type of stereognosis. It is the sensation evoked by touching materials and is concerned with identification.

7

8

• Receptors: Pacinian corpuscles and Ruffini’s endings in the skin (light pressure) and subcutaneous tissues (deep pressure).
• Tested by: asking the patient to differentiate between various weights by place them in his hand without moving “supported” the limb or hand and with closed eyes.
• There are two types of pressure sensations:

a. Fine pressure sensation: Gracile and Cuneate tracts.

b. Crude pressure sensation: ventral spinothalamic

tract.

9

2. The Pressure Sensation

3. The Muscle Tension Sense

10

​

• Receptors: Golgi tendon organs.
• Pathway: Gracile and Cuneate tracts.
• Tested by: asking the person to differentiate between

various weight placed in his unsupported hand.

• Receptors:
• Meissner’s corpuscles (for vibration up to 80 Hz).
• Pacinian corpuscles (for vibration up to 800 Hz).
• Pathway: Gracile and Cuneate tracts.
• Vibration is closely related to proprioception.
• Tested by: place the tuning fork on a bony prominence.

11

4. The Vibration Sense

• Decreased in cases with DM,

tabes dorsalis, vit. B12

deficiency and posterior cord

syndrome.

• Tickle sensation: is a pleasurable sensation that result

from light tactile stimulation of the skin.

• Itch sensation: is annoying sensation that results from

skin irritation by either moving tactile stimuli or certain

chemical substances.

• Receptors:
• Rabidly-adapting free nerve endings.
• Afferent nerves: Unmyelinated type C nerve fibers.
• Pathway: Ventral spinothalamic tract.

12

5. The Tickle and Itch Sensations

• These sensations arise mainly from receptors in deep

structures (especially the muscles and joints).

• Receptors:
• In the muscle: muscle spindles.
• In the tendon: Golgi tendon organs.
• In the synovia of joint: Ruffini’s corpuscles.
• In the ligaments of joints: Pacinian corpuscles.
• Pathway: Gracile and Cuneate tracts.
• The proprioceptive sensation include two types:
1. Sense of position (static proprioception): perception of the position of different parts of the body.
2. Sense of movement (dynamic or kinesthetic proprioception): sensation of movement of joints.

​

13

6. The Proprioceptive Sensations

Joint Sense: "sense of movement".

Hold the big toe from each side (or the index finger in U.L.) and dorsiflexion or plantar flexion.

Ask the patient about which finger and direction of movement (with closed eyes).

​

Muscle Sense: done by pinching the calf or abdominal muscles.

​

14

15

II. The Thermoreceptive Sensations

• There are two types of thermoreceptors:
• Internal thermoreceptors: located in the hypothalamus.
• External thermoreceptors:

- Cold receptors.

- Heat receptors.

- Thermosensitive pain receptors.

• Pathway: lateral spinothalamic tract.
• The warmth receptors:
• They are free nerve endings.
• Respond to temperature from about 30 ºC to 45 ºC.
• The cold receptors:
• They are free nerve endings.
• Respond to temperature from about 10 ºC to 38 ºC.
• The thermosensitive pain receptors: respond to temperature

below 10 ºC and above than 45 ºC.

16

III. Pain Sensations

​

• Pain Receptors(Nociceptors): these are specific

free endings (3 types: thermo, chemo and

mechano-sensitive pain receptors).

• According to its site, pain is classified into:
• Cutaneous pain.
• Deep pain.
• Visceral pain.
• It is more frequently divided into:
• Fast pain.
• Slow pain.

​

• Pathway: lateral spinothalamic tract.

17

Fast and Slow Pain Sensations

18

Nociceptive Fibers

19

Central Perception of Pain Sensations

• Pain sensation is transmitted to higher centers along the

lateral spinothalamic tract which consists of two parts:

• Paleospinothalamic tract: transmits slow pain and terminates subcortically (at reticular formation and thalamus).
• Neospinothalamic tract: transmits fast pain and relay in the thalamus and terminate in the cortical sensory areas.
2. Removal of the cortical somatic sensory areas does not

abolish perception of slow pain.

• Slow pain is generally perceived at a subcortical level.
• The cortical centers are for localization and

discrimination of the quality of pain.

20

Cutaneous Pain

• It may be fast or slow pain.
• It is associated with:
1. Somatic effect: the withdrawal reflex.
2. Autonomic effect: usually sympathetic (vasocnstriction, tachycardia and increase the BP), but parasympathetic effect may occur in severe pain (Nausea, vomiting, vasodilatation, bradycardia and hypotension).
3. Emotional effect: anxiety and restless syndrome.
4. Hyperalgesia: This is a condition of hypersensitivity to pain.

21

Deep Pain

• This originates from muscles, joints, periosteum and

other deep structures and is characterized by:

• It is slow pain that is conducted by type C nerve fibers.
• It is diffuse and dull.
• Associated with muscle tenderness.
• Deep pain may result from trauma to deep structures,

bone fractures, inflammation, arthritis, muscle

injury and ischemia.

22

Ischemic Pain

• One type of deep pain.
• This occur due to either a severe muscle spasm “cramp”

or an occlusive vascular disease due to accumulation of

certain metabolite known as Lewis P factor (is most

probably potassium) and lactic acid.

• At rest muscle ischemia decreased, so ischemic pain is

usually absent during rest.

• Example of ischemia pain:
1. Angina pectoris.
2. Intermittent claudication (severe pain in the muscles of lower limbs during walking due to a vascular occlusive disease.

23

Visceral Pain

• Pain is almost the only sensation produced from the

viscera.

• Some viscera are pain insensitive (e.g. the liver tissue,

lung, alveoli) but the parietal layers of these membranes

are very painful.

• Characters of visceral pain:
• Slow pain and usually dull or spasmodic (colic pain).
• Frequently referred to specific area in the skin.
• Associated with nausea and parasympathetic effects.
• Associated with some somatic effect e.g. contraction

of the near abdominal muscles.

​

1. Autonomic Nerves: by sympathetic and parasympathetic nerves.
2. Somatic Nerves: pain from the gall bladder is transmitted by the phrenic nerves. Also pain from the parietal layers of the pleura, peritoneum and pericardium is transmitted by somatic spinal nerves.

Causes of the visceral pain:

• Visceral ischemia.
• Inflammation.
• Mechanical stimuli: over distension and spasm of certain viscera
• Chemical stimuli: e.g. irritation of the parietal peritoneum by the gastric juice from a perforated peptic ulcer).

Visceral Afferent Nerves

24

25

Referred Pain

• Referred pain is pain perceived in an area of the body that is not directly innervated by the neurons exposed to the noxious stimulus.
• This is pain is felt away from its original site.
• It is most common with visceral pain.
• Pain is usually referred to a somatic structure or a dermatome that developed from the same embryonic segment in which pain originates “the spinal cord levels of the sympathetic fibers that supply those viscera”.

​

​

26

Example of Referred Pain

1. Inflammation of gall bladder (cholecystitis): is transmitted by phrenic nerve fibers (due to irritation of the diaphragm) to the 3^rd , 4^th cervical spinal segments (usually to the tip of the right shoulder).
2. Cardiac pain: is usually referred to the left shoulder and inner side of the left arm and less frequently to epigastrium.
3. Gastric pain: is referred to the abdominal surface above the umbilicus.
4. Pain from kidney and urethra is referred to the inguinal and testicular regions.
5. Pain of appendicitis is referred to the umbilical region.

​

27

Referred Pain

28

• A sensory pathway includes:
1. Receptors.
2. Afferent (sensory) nerve that transmits the signals to the CNS.
3. Transmitting ascending tract.
4. Cortical sensory areas.
• All somatic sensation (superficial and deep), pass through three order neurons, from receptors in the skin and tendons, to reach the cortical sensory area.

The Sensory Pathway or Axis

29

Anterolateral System

30

The anterolateral system:

• Carries pain and temperature, and nondiscriminative forms of touch sensations through the lateral spinothalamic and ventral (anterior) spinothalamic tracts.
• Transmitted through the slightly myelinated A-delta fibers or unmyelinated C fibers.

​

Pathway of Pain and Temperature sensations

1. The first order neuron:

Is the cell of the posterior root ganglion and its process.

This process divided into:

• The lateral branch (Dendrite): goes peripherally forming the afferent sensory nerve (the ends of this nerve are receptors of the skin).
• The medial branch (Axon): enters the spinal cord, forming Lissauer’s tract and relays in the cells of the Substantia Gelatinosa of Rolandi (S.G.R.) in the posterior horn of the gray matter.

Pathway of the Superficial Sensation

“Anterolateral System”

31

2. The second neuron:

Is the cells of Substantia Gelatinosa of Rolandi and its axon. This axon crosses to the opposite side cross the midline in the ventral white commissure, usually within two to three segments above the level of entry of the peripheral fibers and ascends in the lateral column of the SC as the lateral spinothalamic tract → medulla oblongata → the pons → the mid brain → to relay the sensory impulse at the thalamus.

​

3. The third order neuron:

Starts in the cells of the posterolateral ventral nucleus (PLVN) of the thalamus, its axon ascends to pass through the posterior limb of the internal capsule conducting the impulse to the cortical somatic sensory area (1, 2, 3) in the upper 2/3^rd of the postcentral gyrus of the parietal lobe.

Pathway of the Superficial Sensation

32

33

Principles of Peripheral Sensitization

34

35

36

37

38

The Somatotopic Representation of the Body and Face in Primary Somatosensory Gyrus

1. Crude Touch: has the same pathway as pain and temperature but the second order neuron is main sensory nucleus in posterior horn cells. The axons crosses to the opposite side and ascends in the ventral column of the spinal cord as the Ventral Spinothalamic Tract.

​

• Fine Touch: ascends in the Gracile and Cuneate tracts within the posterior column of the SC, with the fibers carrying deep sensation.

​

N.B. The spinal lemniscus= lateral spinothalamic tract + ventral

spinothalamic tract

​

​

Pathway of the Touch Sensation

40

41

• Lesions to the posterior horn cells: Ipsilateral at the same level loss of pain and temperature, and nondiscriminative forms of touch sensations.
• Lesions to the anterolateral spinothalamic tract: Contralateral loss of pain and temperature, and nondiscriminative forms of touch sensations below the level of lesion.
• Lesions thalamus and postcentral gyrus: Contralateral loss of pain and temperature, and nondiscriminative forms of touch sensations.

Lesions of the Anterolateral System

Posterior Column Medial Lemniscus System

42

• The posterior column-medial lemniscus system carries general somatic afferent (general sensory) information about discriminative touch, pressure, vibration and conscious proprioception.
• Transmitted through the fast-conducting large-diameter fibers (A-Beta fibers).

1. The first order neuron:

Is the cells of the posterior root ganglion and its process. This process divided into:

• The lateral branch (Dendrite): goes peripherally forming the afferent sensory nerve (the ends of this nerve are receptors of the tendon).
• The medial branch (Axon): enters the SC and ascends directly in the Gracile and Cuneate tracts within the posterior column on the same side to relay in the Gracile and Cuneate nuclei of the medulla.

​

​

Pathway of Deep Sensation

Posterior Column Medial Lemniscus System

43

2. The second neuron:

Is the cells of the Gracile and Cuneate nuclei in the medulla→ decussating (crossing) to the opposite side of the medulla oblongata ascends as Medial Lemniscus through → the pons → the mid brain → to relay in the thalamus.

3. The third order neuron:

Starts in the cells of the posterolateral ventral nucleus (PLVN) of the thalamus, its axon ascends to pass through the posterior limb of the internal capsule conducting the impulse to the cortical somatic sensory area (1, 2, 3) in the upper 2/3^rd of the postcentral gyrus of the parietal lobe.

Pathway of Deep Sensation

44

45

46

49

​

• The Gracile Tract: lies more medially and received impulses from the lower part of the body (the lower limbs and lower half of the trunk).
• Cuneate Tract: lies more laterally and received

impulses from the upper part of the body (the upper limbs

and upper half of the trunk).

• From medial to lateral side: sacral nerve fibers → lumbar nerve fibers → thoracic nerve fibers → cervical nerve fibers.

Lamination of the Gracile and Cuneate Tracts

50

51

• Lesions to the posterior columns: Ipsilateral loss of discriminative touch, pressure, vibration and conscious proprioception “Sensitive Ataxia”.
• Lesions to the medial lemniscus, thalamus and postcentral gyrus: Contralateral loss of discriminative touch, pressure, vibration and conscious proprioception “Sensitive Ataxia”.

​

Lesions of the Posterior Column Medial Lemniscus System

Below the level of lesion:

• Ipsilateral loss of proprioception & fine touch below the level of the lesion (due to damage of the dorsal column → cuneate and Gracile Tract).
• Ipsilateral upper motor neuron paralysis below the level of the lesion( due to damage of the lateral column → corticospinal tract ).
• Contralateral loss of pain, temperature and crude touch sensations below the level of lesion (due to damage of lateral and ventral spinothalamic tract, which has decussated).

Brown-Sequard Syndrome

53

1. The first order neuron:

It is formed by the cells of trigeminal ganglion and its process divided into:

• The peripheral or lateral processes (Dendrite): of the cells of the trigeminal ganglion form of the ophthalmic, maxillary and sensory part of the mandibular nerves. These nerves carry pain, temperature and touch sensations from the face, cornea, mouth and greater part of the scalp and deep sensation.
• The central or medial processes (Axon): of the cells of the trigeminal ganglion form the sensory root of the trigeminal nerve which enters the pons and midbrain:

a. The Spinal (Descending) nucleus of trigeminal N.:

for pain and temperature sensations.

b. The chief or main sensory nucleus: for discriminative

touch, vibration and conscious proprioception sensations.

c. Mesencephalic nucleus: for nonconscious

proprioception from muscles of mastication.

Pathway of Sensations from the Face and Scalp

“Trigeminal Pathway”

54

2. The second neuron:

• The axons of the cells of the spinal descending nucleus and the main sensory nucleus cross to the opposite side and join each other in the pons and ascend as trigeminal lemniscus in the pons → the mid brain → to relay the sensory impulse to posteromedial ventral nucleus (PMVN) of the thalamus.
• The axons of the cells of mesencephalic nucleus travel to the reticular formation, cerebellum and motor nucleus of trigeminal nerve.

3. The third order neuron:

The axons of the cells of the PMVN of the thalamus ascend to pass through the posterior limb of the internal capsule conducting the impulse to the cortical somatic sensory area (1, 2, 3) in the lower 1/3 of postcentral gyrus of the parietal lobe.

55

Pathway of Sensations from the Face and Scalp

56

Trigeminal Nuclei in the Brainstem

57

Modalities and Functions of the Trigeminal Nerve

58

59

Trigeminal Nuclei in the Brainstem

60

Comparison of Spinal Cord Structures to trigeminal Nuclear Complex

• This is a painful sensation at the head.
• Its causes are either intracranial or extracranial in origin.
• Extracranial Causes:
• Eye disease: e.g. glaucoma.
• Teeth disease.
• Sinusitis.
• Ear inflammation: e.g. otitis media.
• Mastoiditis.
• Temporal arteritis.
• Tension “psychogenic headache”.
• Certain systemic diseases: e.g. anemia.

61

Headache

II. Intracranial Causes:

• Meningeal irritation: menigitis, subarachnoid hemorrhage.
• Decrease of CSF pressure: e.g. post LP headache.
• Increase of CSF pressure: e.g. brain SOL.
• Head trauma.
• Distention of the intracranial arteries: hypertension, fever, migraine.
• Venous sinus thrombosis.

62

Distribution of the Sensory Spinal Roots on the Surface of the Body

63

64

Distribution of the Sensory Spinal Roots on the Surface of the Body

65

66

67

68

69

70

71