L3 Central Somatosensory Pathways & Thalamus

Question 1

Sensory information about internal & external environment is carried to CNS in __________ nerve fibres running in cranial & spinal nerves

Answer 1

Sensory information about internal & external environment is carried to CNS in afferent nerve fibres running in cranial & spinal nerves

Question 2

Sensory information carried in Cranial nerves (5)

Answer 2

Special Senses

• Olfaction
• Vision
• Taste
• Hearing
• Balance

Question 3

How are Somatic Senses carried from the trunk/limbs and from the head?

Answer 3

Peripheral information carried from trunk/limbs in spinal nerves, from the head predominantly in one cranial nerve (trigeminal, V)

Question 4

Modalities of the Somatosensory System? (4)

Answer 4

• Touch (discriminative (Fine), non-discriminative (Crude))
• Temperature
• Proprioception (body position: proprioceptors embedded in muscle fibers sense contraction of muscles => relays to cerebellum coordinating movement)
• Nociception (potentially injurious (‘noxious’) stimuli)

Question 5

Somatosensory System reacts to diverse stimuli through receptors, causing impulses to flow via ____________nerves, through _________ in the spinal cord, and into the brain for processing

Answer 5

Somatosensory System reacts to diverse stimuli through receptors, causing impulses to flow via sensory (afferent) nerves, through tracts in the spinal cord, and into the brain for processing

Question 6

What is carried along with discriminative (fine) touch afferents?

Answer 6

Vibration sense & conscious proprioception

Question 7

What is carried along with pain/temperature afferents?

Answer 7

Non-discriminative (Course) touch & pressure

Question 8

General principal of CNS organisation: pathways conveying sensory information to a conscious level as well as descending motor pathways _____________ from one side of CNS to other

Answer 8

General principal of CNS organisation: pathways conveying sensory information to a conscious level as well as descending motor pathways “cross over” ( decussate ) from one side of CNS to other

Question 9

First Order (Primary) Neuron sends axon into ______ or _______; cell body lies in a ______

Answer 9

First Order (Primary) Neuron sends axon into cord or brainstem; cell body lies in a ganglion

Question 10

Second order (secondary) neuron is located in _______ and its axon projects to ___________ (_________lateral)

Answer 10

Second order (secondary) neuron is located in CNS and its axon projects to the thalamus (Contralateral)

Question 11

Third order (tertiary) neurone’s cell body is located in a specific __________ nucleus; axon projects to a specific area of _______lateral cerebral cortex (_______________________)

Answer 11

Third order (tertiary) neurone’s cell body is located in a specific thalamic nucleus ; axon projects to a specific area of ipsilateral cerebral cortex (primary somatosensory cortex)

Question 12

Small vs. Large Mylenated First Order Neurons

Answer 12

Question 13

Answer 13

Question 14

Right & left Thalami control the flow of sensory information to the cortex and are across midline by the ________________ forming the lateral walls of the _________

Answer 14

Right & left Thalami control flow of sensory information to the cortex and are joined across midline by the Interthalamic Adhesion forming the lateral walls of the Third Ventricle

Question 15

The Thalamus is the largest part of the ____________ (between brainstem/ cerebral hemispheres)

Answer 15

The Thalamus is the largest part of the Diencephalon (between brainstem/ cerebral hemispheres)

Question 16

What is known as the Sensory Relay Station?

Answer 16

Thalamus

Question 17

All sensory information to cerebral cortex synapses in the _________ before going to the cortex (_______ is a partial exception)

Answer 17

All sensory information to cerebral cortex synapses in the thalamus before going to the cortex (smell is a partial exception)

Question 18

__________________ connects directly to the thalamus while other cranial nerves send fibres to the thalamus from lower (brainstem) nuclei

Answer 18

Cranial nerve II (optic) connects directly to the thalamus while other cranial nerves send fibres to the thalamus from lower (brainstem) nuclei

Question 19

The projection from the thalamus to the cortex uses the ______________

Answer 19

The projection from the thalamus to the cortex uses the Internal Capsule

Question 20

______ horns receive axons of sensory (afferent) neurons

Answer 20

Dorsal horns receive axons of sensory (afferent) neurons

Question 21

______ horns send out the axons of motor (efferent) neurons

Answer 21

Ventral horns send out the axons of motor (efferent) neurons

Question 22

Spinal nerve roots are functionally distinct :

• ____________ sensory
• _____________ motor

Answer 22

Spinal nerve roots are functionally distinct

• Posterior (dorsal) sensory
• Anterior (ventral) motor

Question 23

Organisation of Spinal Cord White Matter?

Answer 23

Question 24

Sensory modalities carried by the Dorsal Column Pathway? (3)

Answer 24

• Discriminative Touch
• Vibratory Sense
• CONSCIOUS Proprioception

Question 25

**Pathway of Dorsal Columns** * **First Order** fibers enter the cord and ascend *_________* in the _________________ (Lower limb/trunk) and __________________ (Upper Limb/trunk) to the __________________and synapse (Nucleus __________ or ___________) * **Second Order** axons decussate in ____________________ and ascend as *______________* ________________ (aka. Trigemnial Lemnuscus) to the Thalamus * **Third Order** neurons project from the Thalamus to the ____________________ "Somatotopically"

Answer 25

**Pathway of Dorsal Columns** * **First Order** fibers enter the cord and ascend *Ipsilaterally* in the _GRACILE FASCICULUS_ (Lower limb/trunk) and _CUNEATE FASCICULUS_ (Upper Limb/trunk) to the _MEDULLA OBLONGATA_ and synapse (NUCLEUS GRACILIS or CUNEATUS) * **Second Order** axons decussate in _MEDULLA_ and ascend as *Contralateral* Medial Lemnisus (aka. Trigemnial Lemnuscus) to the THALAMUS * **Third Order** neurons project from the Thalamus to the POSTCENTRAL GYRUS "**Somatotopically**"

Question 26

Primary Somatosensory area (receptive area for touch) of the cortex?

Answer 26

Postcentral Gyrus of the Parietal Lobe

Question 27

What effect do lesions of the Doral Columns have? What is the common sign of this? What Can cause these lesions? (4)

Answer 27

Lesions result in _loss of fine tactile sensation_ and _interference with proprioception_ e.g. **Sensory Ataxia** (ataxia = loss of coordination **Rommberg's Sign** _Causes:_ 1. Vit B12 Deficiecny 2. Multiple Sceleroris 3. Diseases of Peripheral Nerves 4. Tertiary Syphillis

Question 28

Identify these parts of the Dorsal Column

Answer 28

Question 29

Sensory modalities carried by the Spinothalamic Pathway? (4)

Answer 29

* Pain Localization * Temperature * Non-Discriminative Touch * Pressure

Question 30

Two types of Pain Localization/Temperature Fibers? What pathway are they carried on?

Answer 30

Carried on Spinothalamic Pathway

Question 31

Arrangement of Dorsal Columns vs. Spinothalamic Tracts?

Answer 31

Question 32

General Rules for the effect of Spinothalamic Pathways lesions?

Answer 32

* **Pain Localization/Temperature**: cross immediately => Lesion leads to *Contralateral* loss of pain/temperature a few segments below * **Proprioceptive/Fine Touch**: ascend ipsilaterally, cross in MEDULA => Lesion leads to *Ipsilateral* loss of tactile and limb position sense at lesion level and below * Lesions of SPINAL CORD and LOWER BRAINSTEM can damage one pathway preferentially and spare the other * Lesions of HIGHER BRAINSTEM or CEREBRAL HEMISPHERE lead to loss of all sensation on *Contralateral* side

Question 33

Sensory modalities carried by the Spinocerebellar Pathways? What is special about the way this is conveyed?

Answer 33

**Unconscious Proprioception** * Do not project to the thalamus/cerebral cortex, and hence DOES NOT REACH CONSCIOUS AWARENESS * Most proprioceptive information (tendons, muscles and joints) is transmitted directly to the cerebellum (_Two-Neuron Pathway_) * Provides the cerebellum with minute-by-minute feedback on what the muscles are doing **Spinocerebellar pathways remain IPSILATERAL!** * Posterior tract does not cross * Anterior tract ‘double-crosses’!

Question 34

Spinocerebellar pathways IPSILATERAL or CONTRALATERAL Consequences of Lesion?

Answer 34

Spinocerebellar pathways are IPSILATERAL? * Posterior tract does not cross * Anterior tract ‘double-crosses’! **Cerebellar Lesions** => IPSILATERAL Malfunctioning **Cerebral Lesions** => CONTRALATERAL Malfunctioning

Question 35

What is Romberg's Sign and what is it indicative of?

Answer 35

**Romberg's Sign**: can maintain balance with the eyes open (Begins to totter when asked to close eyes)=> indicative of DORSAL COLUMN LESION

Question 36

Dorsal Columns vs. Spinothalamic Tracts? Pathways? Sensory Information?

Answer 36

**Dorsal columns**: carry information related to _proprioception_, _touch_, and _vibration sensations_ from skin and muscle to brain. * **Fasciculus Gracilis**: lower extremities * **Fasciculus Cuneatus**: upper extremities. **Spinothalamic tracts** information related to _pain_, _temperature_, and _crude touch_ from the skin/internal organs to brain. * **Lateral spinothalamic tract**: pain and temperature * **Anterior spinothalamic tract**: crude touch.