12 Nervous System III Sensory & Motor Pathways PDF

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Lecture 12...

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The Chinese University of Hong Kong Faculty of Medicine School of Biomedical Sciences

Anatomy SBMS 1102 (2016-2017)

Nervous System (Sensory and Motor Pathways)

Maria SM Wai Room 612F Choh-Ming Li, BMSB Office Tel.: 3943 5768 Email: [email protected] Date: 13th March 2017

The learning outcomes Upon completion of this lecture, the students should be able to: • • • • • •

Describe the locations of sensory and motor tracts in the spinal cord. Draw and describe the paths of the sensory & motor tracts along the spinal cord and the brain. Describe the possible cause of syringomyelia and its influence to the body sensation. Describe the effect of dorsal column lesions on body sensation and on the manner of walking (i.e. gait). Describe how hemisection brings the loss of different sensation on left and right sides of the body. Compare the signs and symptoms of upper and lower motor lesions.

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A spinal cord segment Central canal

• ventricle continuous withbrain. the 4th of the • contains cerebrospinal fluid. • gradually occluded from lower cord to upper cord as one ages.

Question 1: Where is the 4th ventricle? Question 2: Where does the cerebrospinal fluid come from?

• Composed mainly of myelinated nerve fibers.

White matter

• Composed of neuronal cells bodies & their synapses.

Grey matter

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White matter of the spinal cord • Contains bundles of ascending tracts. • Contains different groups of ascending & descending tracts

Dorsal column

Lateral column

• Contains bundles of descending & ascending tracts

Ventral column

Notes on nerve tract • A tract = a bundle of nerve fibers passing from one site to another in the central nerve system. • Ascending tracts convey sensory signals from the spinal cord to the brainstem, cerebellum, or cerebrum. • Descending tracts convey motor signals from the cerebrum to the brainstem, cerebellum, or spinal cord. 4

Grey matter of the spinal cord Dorsal horn

Lateral horn Ventral horn Notes on spinal cord grey horns: • Dor Dorsal sal ho horns rns consist of cell bodies of sensory neurons for processing incoming sensory signals Ven tral ho horns rns consist of cell bodies of motor neurons for controlling motor activities. • Ventral Lateral horns • are only present in the region between T1 and L2/3 & they contain cell bodies of sympathetic neurons which control activities of internal organs and glands.

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Dorsal & ventral roots Question 3: What structures form the dorsal root ganglion?

Spinal nerve

*Dorsal root ganglion

Dorsal rootlets

Dorsal root

* Ventral root

Ventral rootlets Question 4: What kind of functional neurons are mostly found in ventral roots?

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Ascending & descending tracts of the spinal cord “Sensation”

“Motor action”

Ascending tracts

Descending tracts

Fasciculus gracilis Fasciculus cuneatus Dorsal Spinocerebellar tr. Ventral Spinocerebellar tr.

Spinothalamic tr.

Lateral corticospinal tr. Rubrospinal tr. Medullary reticulospinal tr. Lateral vestibulospinal tr.

Pontine reticulospinal tr. Tectospinal tr. Ventral corticospinal tr.

Remarks: the ascending and descending tracts in the spinal cord are symmetrically located on both sides. [Don’t be misled by the above diagram]

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Ascending sensory pathways Bundles of nerve fibers ascend from spinal cord to higher centers of the brain

1) a) Lateral spinothalamic tract ◦ Pain & temperature sensation b) Anterior spinothalamic tract ◦ Touch & pressure sensation

Anterolateral System

2) Dorsal column tract ◦ Vibration ◦ Stereognosis - recognizes the shape and form of objects

◦ Discriminative touch - recognizes the two separated points of touch when they are of a certain minimal distance (two-point discrimination)

◦ Conscious proprioception - recognizes self positioning of the body both at rest & during movement 8

Spinothalamic pathway The sensory cortex has a topographical organization for different body parts.

Sensory cortex (lower limb)

Sensory cortex

Cerebral hemisphere

(upper limb)

Internal capsule Thalamus

Red nucleus

Midbrain

Crus cerebri

Spinal lemniscus

Middle Cerebellar peduncle Fascicles of corticospinal fibres

Pons

Question 5: Where is the site of crossing over (decussation) in this pathway?

Pyramid

Medulla

Spinothalamic tract

Ventral white commissure

Cervical cord Lumbar cord

Third-order neurons Second-order neurons First-order neurons

*Note the sites of synapses

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Spinothalamic tract lesions e.g. Syringomyelia

Loss of pain & temperature sensation in upper limbs, chest & shoulder

Notes on syringomyelia • A cyst (syrinx) develops within the central canal of the spinal cord • Two possible causes: 1) prolapse of cerebellum 2) pressure differences between the central canal and subarachnoid space outside the spinal cord • More common in upper spinal cord  weakness of the arms, stiffness of the back and shoulder, lose the ability to feel hot or cold

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e.g. Syringomyelia

x

x

Cyst Ventral white commissure

Upper cervical spinal cord

x

x

Lower cervical spinal cord

Thoracic spinal cord

Compression of second-order neurons of the spinothalamic tract at the ventral white commissure  affect both sides (bilateral) 11

Dorsal column pathway The sensory cortex has a topographical organization for different body parts.

Somatosensory cortex (lower limb)

Somatosensory cortex

Cerebral hemisphere

(upper limb)

Internal capsule Thalamus

Medial lemniscus Red nucleus Crus cerebri

Midbrain

Middle Cerebellar peduncle

Medial lemniscus

Nucleus gracilis Nucleus cuneatus Internal arcuate fibres

Pons

Medulla Medial lemniscus

Question 6: Where is the site of crossing over (decussation) in this pathway?

Third-order neurons

Pyramid

Fasciculus gracilis Fasciculus cuneatus Cervical cord Lumbar cord

Second-order neurons First-order neurons

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Lesions of the dorsal column e.g. Tabes dorsalis

Dorsal column

Notes on tabes dorsalis • It is a late sign of synphilitic infection of the central nervous system which mainly affects the dorsal spinal roots and dorsal column of the spinal cord at lumbosacral level. • The loss of proprioception causes inability to maintain a steady posture with the feet close together when eyes are closed ( body sways and falls)  Romberg’s signs

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Hemisection of spinal cord Brow quard syndrome

Normal sensation Spinal cord lesion Complete loss of sensation

Reduced sensation of temperature & pain Question 7: Which sensory pathway is affected?

Reduced sensation of proprioception, vibration, & stereognosis Question 8: Which sensory pathway is affected?

Notes: pathway will result in a loss of sensory • Lesion of the spinothalamic modalities on the opposite side below the lesion & at the lesion site. dorsal al co column lumn in the spinal cord will cause a loss of sensory • Lesion of the dors modalities on the same side below the lesion & at the lesion site. However, if the lesion occurs after the crossing in the lower medulla, the opposite side of the body below the lesion is affected. • Hemisection does not only affect sensory pathways, but also motor pathways. • The left lower limb (purple) in the above case is paralyzed (i.e. unable to move) 14

Descending motor pathway Bundles of nerve fibers descend from cerebral cortex or brainstem to muscle and joints

1) Corticospinal tract (pyramidal tract) ◦ Provides voluntary, discrete & skilled movements

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Corticospinal pathway Motor cortex (lower limb)

Motor cortex (upper limb)

Cerebral hemisphere

Internal capsule

Midbrain Crus cerebri Middle cerebellar peduncle

Fascicles of corticospinal fibres

Pons

Medulla Pyramid

Crossing over Lateral corticospinal tr. Question 9: Where are the sites of crossing over (decussation) in these two tracts?

Ventral corticospinal tr. Cervical cord Lumbar cord

Upper motor neuron Lower motor neuron

Question 10: Which of the two tracts is dominant? 16

Effects of motor neuron lesions Effects Eff ects

Upper neuron Up per motor neu ron lesion les ion

Lower Low er muscle neuron lesion les ion

Muscle tone

Hypertonia

Hypotonia

Paralysis

Spastic type

Flaccid type

Wastage of muscle

No wastage

Wastage of muscle

Superficial reflexes

Lost

Lost

Deep reflex

Exaggerated

Lost

Plantar reflex

Babinski’s sign

Plantar reflex absent

Clonus

Present

Lost

Muscles affected

Group of muscles are affected

Individual muscles are affected

https://www.youtube.com/watch?v=4SrhgjGIZ30

Babinski’s sign

The End

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