Somatosensory System/sensory System And Descending Tracts

Question 1

S1=

Answer 1

Somatosensory cortex

Question 2

Areas 1,2,3

Answer 2

Post central gyrus

Question 3

Areas 5 and 7

Answer 3

Posterior parietal cortex

Question 4

Cortex devoted to each body is correlated to ….

Answer 4

The density of sensory input received from that body part

Question 5

Overview of skin

Answer 5

Important for tactile sensations
2 types, hairy and hairless (glaborus)
Home to several types of mechanoreceptor

Question 6

What are hair follicles

Answer 6

Embedded in the skin and innervated by nerve ending
Hair is bend resulting in mechanical deformation, activates the nerve ending

Question 7

What are pacinian corpuscles

Answer 7

Located in dermis of both skin types
Hands feet nipples mammary glands etc
Concentric lamellae of flattened cells
Spaces between lamellae fluid filled
Low threshold, rapidly adopting and sensitive to high vibration

Question 8

What are meissners corpuscles

Answer 8

Stacks of horizontally flattened epithelial cells in a connective tissue sheath
Beneath epidermis of in ridges of glaborus skin
Low threshold rapidly adapting sensitive to touch and vibration

Question 9

what are merkel’s discs

Answer 9

Located in epidermis of glaborus skin
Lips extremities external genetilia
Consist of nerve terminal and a flattened non-neuronal epithelial cell
Spaces between lamellae fluid filled
Low threshold, slowly adapting and sensitive pressure

Question 10

What are ruffini’s corpuscles

Answer 10

In epidermis of both skin types
Encapsulated bundles of collagen fibrils connected to fibrils of dermis
Low threshold, slowly adapting and sensitive to stretching of skin

Question 11

Describe conscious proprioception

Answer 11

• Joint capsules receptors provide sensory information to the cerebral cortex
• Used to generate conscious awareness of kinesthesia (joint position, direction and joint velocity).
• Receptors which are located in ligaments and joint capsules consist of free nerve endings and encapsulated receptors which are low threshold mechanoreceptors.
• Some of them are slowly adapting and provide information about the ability of an individual to judge the position of a joint without seeing it and without movement (static aspect of kinesthseia). Other receptors are rapidly adapting and provide information about the ability of an individual to perceive the movement of a joint and the judge the direction and velocity of its movement (dynamic aspect of kinesthesia).

Question 12

What does the dorsal funiculus/dorsal column contain

Answer 12

2 long ascending tracts

Question 13

What ascends the spinal cord in the ipsilateral dorsal columns

Answer 13

Axons mediating tactile sensation and proprioception

Question 14

Axons from the sacral region are located where in the dorsal column

Answer 14

Medially

Question 15

As axons are added at …… of the cord they …..

Answer 15

Higher levels
Occupy more lateral positions

Question 16

From T6 and above, 2 …..

Answer 16

Fascicles

Question 17

Gracile fascicle

Answer 17

Lower body

Question 18

Cuneate fascicle

Answer 18

Upper body

Question 19

Describe the fasciculus gracillis

Answer 19

Located medially\fivres from sacral , lumbar, and lower thoracic segments T6-T12 (the lower body)

Question 20

Describe the fasciculus gracillis in reference to 1st-3rd order neurons

Answer 20

1st order neurons - dorsal root ganglia peripheral processes innervate the Pacinian and Meissner’s corpuscles of the skin and proprioceptors
• Ascend ipsilaterally in the spinal cord
2nd order neurons in the ipsilateral nucleus gracilis in the medulla
• Travel as the internal accurate fibres
• Decussate at medial lemniscus
• Ascend as the medial lemniscus
Synapse with 3rd order neurons in the contralateral ventral posterolateral nucleus of the thalamus
• Terminate in medial aspect of sensorimotor cortex

Question 21

Trigeminal nerves give rise to :

Answer 21

Ophthalmic division
Maxillary division
Mandibular division

Question 22

Describe the trigeminal touch pathway

Answer 22

3 divisions
Axons from sensory receptors synapse with 2nd order neurons in the ipsilateral trigeminal nucleus
Decussate and project to VPN of thalamus
Sensory info relayed to somatosensory cortex

Question 23

Lesions of the dorsal column/ medial lemniscal pathway:

Answer 23

Loss of tactile sense (vibration, deep touch and two point discrimination) and kinaesthetic sense (position and movement) below the level of the lesion
The patient cannot perceive sensations such as touch or pressure and their movements are poorly co-ordinated and clumsy because of the loss of conscious proprioception of their position in space.
Patients unable to identify position of limbs when eyes are closed
If lesion affects cervical region of the spinal cord they cannot identify shape, size or texture of the object in hand ipsilateral to the lesion (asteregnosis)

Question 24

What is tabes dorsalis

Answer 24

Represents the late consequences of syphilitic infection - tertiary syphilis or neurosyphilis
In this syndrome the large diameter central processes of the dorsal root ganglion neurons degenerate, especially in the lower thoracic and lumbosacral segments
The fibres in the fasciculus gracilis degenerate and there is a loss of vibration sensation, two point discrimination and conscious proprioception
The loss of proprioception results in ataxia as the sufferer is deprived of sensory feedback signals that detect the position of the lower limbs at any given point.

Question 25

Lesions in areas 5,3,2 and 1 leads to

Answer 25

Contralateral neglect

Question 26

What is contralateral neglect

Answer 26

Neglect for visual , auditory and somesthetic stimulation one side of the body or space
Lesion most often in the R inf parietal lobe (R intraparietal sulcus and the R angular gyrus)
Defective sensation or perception
Defective attention or orientation

Question 27

Lateral spinothalamic tract transmits:

Answer 27

Pain and temperature

Question 28

Anterior spinothalamic tract transmits:

Answer 28

Non discriminative touch

Question 29

Pain and temp sensations ascend in ..

Answer 29

The spinothalamic tract that occupies the ventrolateral quadrant of the spinal cord on the contralateral side to where the afferent axons enter
Due to the complexity of pathways modulating pain and its associated components which all ascend in the ventrolateral quadrant, they are collected known as the anterolateral system of ascending tracts

Question 30

What 2 pathways does the spinothalamic tract consist of

Answer 30

Direct and indirect

Question 31

Describe the direct pathway of the spinothalamic tract

Answer 31

Comprises of the neospinothalamic tract
Pain temp and simple tactile sensations
Lateral sensory-discriminative pathway

Question 32

Describe the indirect pathway of the spinothalamic tract

Answer 32

comprises paleospinothalamic, spinoreticular and spinomesencephalic tracts
Involved in the autonomic, endocrine, motor and arousal components of pain, temperature and crude touch
Involved in activation of pain inhibiting mechanisms
Ascend the cord bilaterally, poor somatotopic organisation
Connect with reticular formation, hypothalamus and limbic system
Mediate affective motivational pathways

Question 33

Describe how the direct pathway / neospinothalamic tract works

Answer 33

Axons from Ruffini’s corpuscles, noicecptors, thermoreceptors synapse with 1st order neurons in the dorsal root ganglia
Central axons enter spinal cord, branches travel up (tract of Lissauer)
Synapse with 2nd order neurons in proper sensory nucleus
Axons cross and enter to contralateral side
Ascend in lateral funiculus
Somatotopically arranged -Fibres from lower body ascend
dorsolaterally. Upper extremities & neck ascend ventromedially
Synapse 3rd order neurons in ventral posterolateral nucleus of thalamus
Project to sensorimotor cortex

Question 34

What is Brown-Sequard Syndrome a result of

Answer 34

Damage to the neospinothalamic tract and dorsal columns

Question 35

What occurs after damage to neospinothalamic tract and dorsal columns

Answer 35

Brown-Sequard Syndrome
Hemisection of the spinal cord leads to the patient experiencing a loss of discriminative touch, vibration and proprioception on the side ipsilateral to the lesion (due to disruption of the uncrossed dorsal columns).
Pain and temperature sensation will be lost on the side contralateral to the lesion due to disruption of the neospinothalamic tract
Transection of the corticospinal fibres will lead to hemiparesis on the side ipsilateral to the lesion

Question 36

Describe the paleospinothalamic tract

Answer 36

Sense non discriminative touch pain and temp
Located in dorsal horn and intermediate grey matter
Axons ascend bilaterally in the ventrolateral quadrant of the spinal cord
Make several synapses in the reticular formation of the brain stem
Finally projecting to the midline and intralaminar thalamic nuclei
These nucleus project to the cerebral cortex, especially the limbic regions such as cingulate gyrus

Question 37

Describe the spinoreticular tract

Answer 37

Sense pain
Neurons located in the dorsal horn and intermediate grey matter
1st order neurons from receptor to dorsal horn
2nd order neurons synapse in medullary and pontine reticular formations
Projections from the spinal cord to the brain are both crossed and uncrossed and these ascending fibres are believed to transmit sensory information to the reticular formation (which plays a part in influencing levels of conscious)
Activates the cerebral cortex through secondary and tertiary projections
via the midline and intralaminar thalamic nuclei
The thalamocortical projections are highly diffuse and influence wide areas of the cerebral cortex

Question 38

Describe the spinomesencephalic tract

Answer 38

located in the dorsal horn and intermediate grey matter
axons of these neurons ascend to the midbrain where they terminate in periaqueductal gray
It is thought that this pathway activates periaqueductal gray neurons which send descending fibres to the spinal cord to inhibit pain sensation.
Thought to transmit sensory information to the amygdala via the parabrachial nuclei.

Question 39

direct pathways of anterolateral system

Answer 39

Neospinothalamic tract

Question 40

Indirect pathways of anterolateral system

Answer 40

Paleospinothalamic tract
Spinoreticular tract
Spinomesencephalic tract