8 Flashcards
Sensation from the head
•The anterior part → trigeminal nerve → trigeminal pathway.
•The posterior part → sensory roots of cervical nerves 2 & 3 → anterolateral & dorsal column lemniscal system.
Pathway of Sensations from the face
•First order neuron: Unipolar neurons in trigeminal ganglion (similar to DRG)
trigeminal nerve has 3 branches ophthalmic, maxillary and mandibular
•Second order neuron
–Main sensory trigeminal nucleus the for fine touch
–Mesencephalic nucleus for proprioceptive sensation
–Spinal trigeminal nucleus. for pain and temperature
Axons of second order neurons cross the midline and ascend as trigeminal lemniscus to end on PVMT.
•Third order neuron:Arises from the PVMT ascend in the posterior limb of the internal capsule to reach the lower part of sensory area of the cerebral cortex.
Lesions in the trigeminal pathway:
•Trigeminal Anesthesia:
- Due to injury or damage of Trigeminal (Gasserian) ganglion.
- Ipsilateral loss of sensation of area supplied by trigeminal.
- Loss of sensation from the eye → inflammation of cornea & conjunctiva → corneal ulcer.
•Trigeminal Neuralgia:
- Sever lancinating pain in the area of the face supplied by trigeminal nerve of unknown reason.
- Pain may last form few seconds to continuous pain.
Treated by cutting peripheral nerve of hypersensitive area.
The human cerebral cortex, is divided into about 50 distinct areas called
Brodmann’s areas based on histological structural differences.
Layers of the Somatosensory Cortex and Their Function
•Layers I and II receive diffuse, nonspecific input signals from lower brain centers
•layers II and III send axons to related portions of the cerebral cortex on the opposite side
•incoming sensory signal excites neuronal layer IV
•layers V and VI send axons to the deeper parts of the nervous system
•From layer VI, especially large numbers of axons extend to the thalamus,
Functions of Somatosensory Area I
•Bilateral excision of somatosensory area I causes the following:
1.The person is unable to localize discretely the different sensations in the different parts of the body.
2.The person is unable to judge critical degrees of pressure against the body.
3.The person is unable to judge the weights of objects.
4.The person is unable to judge shapes or forms of objects. This is called astereognosis.
5.The person is unable to judge texture of materials
Functions and characteristics of Somatic sensory area II
•Located in the superior wall of the Sylvain fissure,
•The representation of the body parts is not as complete or detailed as it is in the postcentral gyrus.
•This receives connections from the primary sensory cortex and also less specific thalamic nuclei
•Little is known about its function.
•This area responds to sensory stimuli bilaterally,
although with much less precision than the
primary cortex.
Somatosensory Association Areas
•Bradman’s areas 5 and 7 of the cerebral cortex.
•Electrical stimulation → complex body sensation
•It receives signals from
(1) Somatosensory area I
(2) Ventrobasal nuclei of the thalamus
(3) Other areas of the thalamus
(4) Visual and auditory cortices.
•Function: interprets all information to find out the meaning of somatic sensory information.
Effect of Removing the Somatosensory Association Area on one side
Amorphosynthesis
Hyperalgesia Hypersensitivity to Pain
Hyperalgesia—Hypersensitivity to Pain
•Pain nervous pathways sometimes become excessively excitable, leading to hyperalgesia ( Exaggerated pain sensation).
•Possible causes of hyperalgesia are:
1.Excessive sensitivity of the pain receptors, which is called primary hyperalgesia, and e.g. in cases of skin injuries (due to histamine and prostaglandins release)
2.Facilitation of sensory transmission, which is called secondary hyperalgesia. e.g. due to cns lesions
Occasionally herpesvirus infects a
Herpes Zoster, or “Shingles
This infection causes severe pain in the dermatomal segment sub-served by the ganglion, thus eliciting a segmental type of pain that circles halfway around the body.
•The disease is called herpes zoster, or “shingles,” because ofa skin eruption that often ensues.
Tabes Dorsalis
Causes
Causes: syphilis → degeneration of dorsal roots.
Manifestation:
•First, irritations of nerve fibers → sever pain in somatic structure served by affected dorsal roots.
•Later, degeneration of pain fiber → loss of pain sensation.
•Degeneration of fibers that pass directly in dorsal column directly in dorsal
column loss of fine touch, position sense &
vibration sense. Affected site: Lumbosacral segments: loss of sensation in lower
limb & around the anus. Cervicothoracic segments: loss of sensation in
upper chest & ulnar borders of the upper limbs.
Syringomyelia Causes:
cavities formation in the grey matter around the central canal in spinal cord → damage of crossing fibers of pain, temperature & crude touch → loss of these sensation on both sides of the body at the level of affected segment.
Hemisection of the spinal cord (Brown-Sequard Syndrome)
- On the same side:
•Above the level of lesion:
Zone of hyperesthesia due to irritation of the dorsal roots (skin is more sensitive to touch, thermal & pain sensation).
•At the level of lesion:
Loss of all sensation.
Paralysis of muscles supplied by destroyed segment (LMNL). rata
Thalamic Syndrome
•Cause: Usually result from thrombosis of the arterial supply leading to destruction of (PVNT) and (VLNT).
•Manifestations
1- Sensory defect:
–loss of all sensations in opposite side.
–After few months crude sensations reappear as slow pain.
2- Motor defect:
–Muscle weakness, decreased muscle tone and ataxia
