Spinal cord and eye anatomy, MS

Question 1

Anatomy of the spinal cord

Answer 1

Question 2

Types of neurons in the ascending pathway

Answer 2

Ascending pathway is made up of 3 different types of neurons:

1. First order neurons - receive sensory information from receptors and sends them to sensory neurons in posterior gray horn of spinal cord. Cell bodys are found within dorsal root ganglion.
2. Second order neurons - sensory nerons located in posterior gray horn synapse with fibers that form the ascending tract. They carry sensory information to different subcortical areas of the brain, such as the thalamus.
3. Third order neurons - reside in subcortical area and transmit information to the cerebral cortex.

Question 3

What are the various ascending tracts in the spinal cord?

Answer 3

• Dorsal column-medial lemniscal pathway (DCML)
• Anterolateral system
• Spinocerebellar tracts

Question 4

Dorsal column-medial lemniscal pathway (DCML)

Answer 4

Carries sensory information on fine touch, vibration, and proprioception

1. First order neurons - carry the sensory information from peripheral nerves to medulla oblongata via one of the two pathways
   
   • Signals from upper limbs (>T₆) - travels in fasciculus cuneatus (lateral part of dorsal column), then synapses in nucleaus cuneatus of medulla oblongata.
   • Signals from lower limbs (6) - travel in fasciculus gracilis (medial part of dorsal column), then synapse in nucleaus gracilis of medulla oblongata.
2. Second order neurons - start in cuneate or gracilis nucleaus. They take information from the first order neurons and pass it along to third order neurons in the thalamus. Within the medulla these fibers cross to the other side of the CNS where they then travel into the contralateral medial lemniscus to reach the thalamus.
3. Third order neurons - transmit sensory signals from thalamus to ipsilateral primary cortex of the brain. Ascend from ventral posterolateral nucleaus of the thalamus and terminate at sensory cortex.

Question 5

Anterolateral System

Answer 5

An ascending tract of the spinal cord.

1. First order neurons - Carry sensory information from peripheral nerves to the spinal cord. From here, they ascend 1-2 vertebral levels before synapsing at the tip of the dorsal horn (substantia gelatinosa).
2. Second order neurons - Carry the information from substantia gelatinosa to the thalamus. At the synapse between first and second order neruons, the fibers cross over and form two distinct tracts.
   
   • Anterior spinothalamic tract - carries sensory modalities of crude touch and pressure.
   • Lateral spinothalamic tract - carries sensory modalities of pain and temperature.
3. Third order neurons - Carry sensory signals from the thalamus to the ipsilateral primary sensory cortex of the brain.

Question 6

Spinocerebellar Tracts

Answer 6

A ascending tract of the spinal cord. It carries unconscious sensation. They help us co-ordinate and refine motor movements by transmitting information from the muscles to the cerebellum.

• Posterior spinocerebellar tract - carries proprioceptive information from lower limbs to ipsilateral cerebellum.
• Cuneocerebellar tract - carries proprioceptive information from upper limbs to ipsilateral cerebellum.
• Anterior spinocerebellar tracts - carries proprioceptive information from lower libms. Fibers cross over twice, so ultimately they terminate on ipsilateral side.
• Rostral spinocerebellar tracts - carries proprioceptive information from upper limbs to ipsilateral cerebellum.

Question 7

Descending Tracts of the spinal cord

Answer 7

• Pathways by which motor signals are sent from brain to lower motor neuron.
• There are no synpases within the descending pathways as the “termination” of the descending pathway is when it synapses with the lower motor neuron. All neurons within descending pathway are upper motor neurons. Cell bodies are found in cerebral cortex of brain stem, and axons remain in CNS.
• Includes the pyramidal tracts and the extrapyramidal tracts

Question 8

Pyramidal Tracts

Answer 8

Originate in cerebral cortex and carry motor fibers to spinal cord and brain stem. Responsbile for voluntary control of musculature of body and face. Can be further divided into 2 groups:

1. Cortiospinal tracts
2. Corticobulbar tracts

Question 9

Corticospinal Tracts

Answer 9

Pyramidal tract that supply musculature of the body. Begins in cerebral cortex and recieves inputs from primary motor cortex, premotor cortex, and supplementrary motor area. From here, neurons converge and descend through internal capsule (white matter pathway between thalamus and basal gangilia), neurons then pass through crus cerebri of midbrain, pons, and medulla. At the lowest part of the medulla the tract divides into the:

• Lateral corticospinal tract - crosses over to the other side of the CNS before terminating in the ventral horn. From here, lower motor neurons supply the limb muscles of the body.
• Anterior corticospinal tract - stays ipsilateral into the spinal cord before crossing over to synpase with lower motor neurons. Terminates in ventral horn of cervial and upper thoracic segment levels. Supply trunk (axial) muscles.

Question 10

Corticobulbary Tracts

Answer 10

• Pyramidal tract that supplies the musculature of the head and neck.
• Arises from lateral aspect of the primary motor cortex and receives same inputs as corticospinal tracts. Fibers then converage and pass through internal capsule to the brainstem. The neurons terminate on the motor nuclei of the cranial nerves, where they synpase with lower motor neurons that carry the motor signals to the muscles of the face and neck.
• Many of these fibers innervate the motor neruons bilaterally, with a few exceptions:
  
  • Upper motor neurons for the facial nerve (CN VII) have a contralateral innervation - only affects muscles below the eye.
  • Upper motor neurons for the hypoglossal (CN XII) nerve have contralateral innervation.

Question 11

Extrapyramidal Tracts

Answer 11

• A descending tract of the spinal cord. It orginates in the brainstem and carries motor fibers to the spinal cord.
• Responsbile for involuntary and automatic control of all musculature - tone, balance, posture, and locomotion.
• There are four extrapyramidal tracts:
  
  • Vestibulospinal tracts
  • Reticulospinal tracts
  • Rubrospinal tracts
  • Tectospinal tracts

Question 12

Vestibulospinal Tracts

Answer 12

• An extrapyramidal tract
• There are two vestibulospinal tracts; medial and lateral. They arise from vestibular nuclei, which receive input from organs of balance. Tracts brings the information to the spinal cord ipsilaterally.
• Fibers control balance and posture by innervating flexors of arm and extensors of leg (“anti-gravity” muscles) via lower motor neurons.

Question 13

Reticulospinal Tracts

Answer 13

• An extrapyramidal tract
• There are two reticulospinal tracts

1. Medial - arises from pones and facilitates voluntary movements and increases muscle tone.
2. Lateral - arises from medulla. It inhibits voluntary movements and reduces muscle tone.

Question 14

Rubrospinal Tact

Answer 14

• An extrapyramidal tract
• Orginates from red nucleus in midbrain. From here, fibers cross over to the other side of the CNS and descend into the spinal cord (contralateral). Thought to play a role in fine control of hand.

Question 15

Tectospinal Tracts

Answer 15

• An extrapyramidal tract
• Originates from superior colliculus of the midbrain (structure that receives input from optic nerve). The neurons then cross over before entering the spinal cord. This tract terminates at cervical level.
• Coordinates movement of head in relation to vision stimuli.

Question 16

Multiple Sclerosis Define

Answer 16

• Most common immune-mediated inflammatory demyelinating disease of CNS

Question 17

Multiple Sclerosis Etiology

Answer 17

• Main pathologic mechanisms are inflammation, demyelination and axonal degereration.
• Thought that MS begins as an inflammatory immune-mediated disorder characterized by autoreactive lymphocytes.
• Potential risk factors:
  
  • Genetic suscpetibility
  • Environmenta factors - viral infections geographic latitude, place of birth, sunlight exposure, etc.

Question 18

Disease onset and pattern MS

Answer 18

• MS is charaterized as relapsing and remitting and progressive disease
• The pattern and course of MS can be further categorized into clincial subtypes:
  
  • Clinically isolated syndrome - representating the first attack of MS
  • Relapsing and remitting MS
  • Secondary progressive MS
  • Primary Progressive MS

Question 19

MS clinically isolated syndrome

Answer 19

First clinical episode suggestive of MS. Characterized by:

• Episode of patient reported symptoms and objective findings that reflect a focal or multifocal inflammatory demyelinating event in CNS
• Develops acutely or subacutely, with duration of at least 24 hours - tend to develop over hours-days and gradually remit over week-months.
• Absence of fever or infection
• Resembles typicaly MS relapse, but patient in not known to have MS

Question 20

Typical pattern of first clinical MS episode

Answer 20

• Unilateral optic neuritis - painful, monocular visual loss, visual blurring, or scotoma
• Painless diplopia (double vision) due to internuclear ophthalmoplegia or sixth nerve palsy.
• Brainstem or cerebellular syndrome - diplipia, ataxia with nystagmus, vertigo, facial numbness, or paroxysmal epiosdes of dysarthria or vertigo.
• Partial trasverse myelitis, usually with prominant sensory symptoms - Partial Brown-Sequard syndrome, sphincter symptoms (bladder > bowel)

Question 21

MS Patterns

1. Relasping-remitting
2. Secondary progressive
3. Primary progressive

Answer 21

1. Characterized by clearly defined attacks with full recovery or with sequelae and residual deficit upon recovery. There is little disease progression in between relapses.
2. Characterized by an initial relapsing-remitting MS followed by gradual worsening with or without occasionaly relapses, minor remissions, and pleatues. Transitions from relasping remitting to secondary progressive tends to occur 10-20 years after onset
3. Characterized by progressive accumulation of disability from disease onset with occasionaly plateaus, temporary minor improvements or acute relapses

Question 22

Characteristic features of MS

Answer 22

• Relapses and remissions
• Onset between 15-50
• Optic neuritis
• Lhermitte sign - electric shock-like sensations that run down back and/or limbs upon neck flexion
• Internuclear ophthalmoplegia
• Fatigue
• Heat sensitivity
• Motor symptoms - paraparesis or paraplegia - more common than isolated upper extremity weakness due to frequent occurence of lesions in descending motor tracts. Severe spasticity can also occur.
• Sensory symptoms - affects majority of patients, and is a common initial feature. Includes - numbness, tingling, paresthesia, coldness, swelling of limbs and trunk.

Question 23

Investigations MS

Answer 23

• MRI - demyelinating plaques appear as hyperintense lesions on T2 weighted MRI
• CSF - oligoclonal bands in 90%, increase in IgG concentration.
• Evoked potentials (visual/auditory/somatosensory) - delayed but well presevered wave forms.

Question 24

Treatment MS

Answer 24

• Acute treatment - methylprednisolone 1,000 mg IV daily X 3-7days - no taper requried.
• Disease modifying therapy (DMT)
  
  • Teriflunomide, interferon-B, glatiramer acetate, BG-12
  • Natalizumbal, fingolimod
• Symptomatic treatment:
  
  • Spasticity - baclofen, tizanidine, dentrolene, benzodiazepine, botulinum toxin
  • Bladder dysfunction - oxybutynin
  • Pain -TCA, carbamazepine, gabapentin
  • Fatigue - amantacline, modafinil, methylphenidate
  • Depression - antidepressants, lithium
  • Constipation - increase fibers, stool softeners, laxatives
  • Sexual dysfunction - Viagara, Cialis, Levitra

Question 25

Fibrous Layer of the Eye

Answer 25

Outermost layer of the eye consisting of the sclera and cornea, which function to support the deeper structures and provide shape to the eye.

• Sclera - makes up the majority of the fibrous layer (white part of eye); provides attachment to extraocular muscles (muscles responsible for movement of eye).
• Cornea - Positioned centrally at front of the eye. It refracts light entering the eye.

Question 26

Vascular Layer of the Eye

Answer 26

Lies underneath the fibrous layer. Consists of:

• Choroid - Layer of connective tissue and blood vessels that provides nutrients to outer layer of retina.
• Ciliary body - made up of 2 parts that control the shape of the lens and contributes to formation of aqueous humour.
  
  • Ciliary muscles - collection of smooth muscle fibers that attach to lens
  • Ciliary processes - attach the ciliary muscles to the lens
• Iris - circular structure with an opening in the centre to allow light through (pupil).
  
  • Pupil diameter is controlled by smooth muscle fibers in iris that are innervated by autonomic nervous system.

Question 27

Inner Layer of the Eye

Answer 27

• Retina - light detecting part of eye. Composed of two layers:
  
  • Neural layer - Innermost layer of retina. Contains the photoreceptors (rods and cods)
  • Pigmented layers - outer layer of retina. Attached to choroid layer and acts to support neural layers.
    
    • Macula - center of retina containing depression called the fovea, which has a high concentration of light detecting cell.
    • Optic disc - area where optic nerve enters the retina. Contrains no light detecting cells.

Question 28

Anterior chamber of eye

Answer 28

• Located between cornea and iris. The chambers are filled with aqueous humour - clear plasma like-fluid that neurishes and protects eye. It is constantly produced and drained via trabecular mesh work - area of tissue in the eye located around cornea and cililary body.
• Obstructive drainage = glaucoma

Question 29

Eye Vasculature

Answer 29

• Arterial supply - Eye receives arterial blood from ophthalmic artery (branch of internal carotid). Ophthalmic artery gives rise to many branches which supply different components of the eye.
• Venous drainage - superior and inferior ophthalmic veins which drain into cavernous sinus.

Question 30

Optic Nerve (CN II)

Answer 30

• Responsible for transmitting sensory information for sight
• Does not join with the brainstem - arises from optic vesicle (out-pouching from forebrain) and thus entirety of CN II can be considered part of CNS.
  
  • Why examination of optic nerve can be used to assess intra-cranial health.
  • Surrounded by cranial meninges not epi-, per-, and endoneurium.

Question 31

Extracranial Optic Pathway

Answer 31

• Optic nerve is formed by convergence of axons from retinal ganglion cells, which receive impulses from the rodes and cones.
• After the opitc nerve is formed, it leaves the bony orbit via the optic canal and enters cranial cavity running along the surface of midle cranial fossa.

Question 32

Intercranial Visual Pathway

Answer 32

• Within middle cranial fossa the optic nerve from each eye units to form the optic chiasm. At the chiasm, fibres from medial half of each retina cross over, forming optic tracts.
  
  • Left optic tract - contains fibres from left temporal (lateral) retina, and right nasal (medial) retina.
  • Right optic tract - contains fibres from right temportal retina, and left nasal retina.
• Each optic tract travels to its corresponding cerebral hemisphere to the Lateral Geniculate Nucleus (LGN) in the thalamus. Axons from the LGN will then carry information via the optic radiation pathways:
  
  • Upper optic radiation - carries fibres from superior retinal quadrants (corresponds with inferior visual quadrants). Travels through parietal lobe to visual cortex.
  • Lower optic radiation - carries fibers from inferior retinal quadrants (superior visual quadrants). Travels through temporal lobe via Myer’s loop to reach visual cortex.

Question 33

How visual fields are mapped onto the eye

Answer 33

Question 34

Light Reflex Afferent Pathway

Answer 34

• Light enters pupil → stimulates retina → retinal ganglion cells transmit signal to optic nerve → optic nerve enters optic chiasm (nasal fibers proceed contralaterally; temporal fibers proceed ipsilaterally) → fibers synpase in pretectal nuclei in dorsal midbrain → pretectal nuclei project fibers to ipsilateral and contralateral Edinger- Westphal nucleaus via posterior commissure (interconnects pretectal nuclei, mediating consensual pupillary light reflex).

Question 35

Light Reflex Efferent Pathway

Answer 35

• Edinger-Westphal nucleus projects pre-ganglionic parasympathetic fibers → fibers exit midbrain and travel along oculomotor nerve (CN III) and synpase on post-ganglionic parasympathetic fibers in the ciliary ganglion → Ciliary ganglion post-ganglionic parasympathetic fibers innervate sphincter muscles of pupils → pupillary constrction

Question 36

What should happen when light is shined in one eye?

Answer 36

Light shined in one eye will constrict:

• The ipsilateral pupil (direct pupillary light reflex)
• The contralateral pupil (consensual pupillary light reflex)