3: Anatomy - Ascending and descending tracts Flashcards by Ross Wilson

How many spinal nerves are given off at each of the five spinal levels?

C - 8

T - 12

L - 5

S - 5

C - 1

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At which level does the spinal cord terminate?

L1/2

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What are the two large swellings found in the spinal cord?

Cervical and lumbar enlargements

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What are the swellings found where the posterior rootlets enter the spinal cord?

Posterior root ganglia

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What are groups of nerve bodies called when they are found in the

a) CNS
b) PNS?

a) nucleus

b) ganglion

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What nerve fibres are found in

a) posterior rootlets
b) anterior rootlets?

a) Sensory

b) Motor (including autonomic)

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What is the end of the spinal cord called?

Conus medullaris

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What are the three layers of the meninges?

Dura mater

Arachnoid mater

Pia mater

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Does the spinal cord have meninges?

Yes

Continuous with the brain

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Which ligaments attach each spinal level to the arachnoid mater?

Denticulate ligaments

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In the spinal cord, what is white matter arranged into?

Anterior, posterior and lateral columns

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What connects the fourth ventricle to the spinal cord?

Central canal

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What is found in

a) white matter
b) grey matter?

a) Axons

b) Nerve bodies, glial cells and blood vessels

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What is found at the

a) Posterior horn
b) Anterior horn

of the spinal cord?

a) Sensory nerves

b) Motor nerves

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T1 - L2 are special in that they have lateral horns of white matter.

What is found in the lateral horns of the spinal cord?

Pre-synaptic sympathetic fibres

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At which spinal nerves are sympathetic nerves given off?

T1 - L2

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What are the three main types of blood vessel supplying the spine?

Longitudinal arteries (run vertically)

Segmental arteries (branches of arteries like the intercostal and lumbar, pass through intervertebral foramen and supply spinal cord)

Radicular arteries (branches of the segmental arteries which supply the individual anterior and posterior roots)

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How many arteries run vertically on the

a) anterior
b) posterior spinal cord?

Which arteries do they arise from?

a) 1

b) 2

Vertebral arteries

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What is a notable segmental spinal artery which arises from a posterior intercostal artery between the 9th and 12th ribs?

Artery of Adamkiewicz

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Which space exists between the spinal cord and dura mater posteriorly and allows for injection of anaesthetics?

Epidural space

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Where is the primary somatosensory cortex i.e the first place where information is received by the brain?

Post-central gyrus of the parietal lobe, posterior to central sulcus

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What happens to the proportions of white and grey matter as you go down the spinal cord?

White matter decreases

Grey matter increases

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What sensations are carried by the Dorsal Column / Medial Lemniscus pathway?

Fine touch

Vibration

Proprioception (knowing where your limbs are in space without being able to see them)

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What side of the brain processes information from the right?

Left side

and vice versa

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In which specific part of the brainstem do nerve fibres from the **Dorsal Column / Medial Lemniscus pathway** cross over?

**Medial lemniscus of the medulla**

Once they have arrived at the medulla and crossed over, fibres from the DC/ML system synapse at nuclei (named for arm/leg). Where do they go next?

**Thalamus**

The **DC/ML pathway** decussates (**crosses over**) in the **medulla.** At which structures do fibres from the a) **lower body** b) **upper body** synapse before crossing over?

**a) Lower body** \> **Nucleus gracilis** (medial structure) **b) Upper body** \> **Nucleus cuneatus** (lateral structure)

What really important thing do nerve fibres from the DC/ML system do before ascending from the **medulla** to the **thalamus**?

**Cross over** - **"decussate"** At the **medial lemniscus of the medulla**

Through which **tract of white matter** do fibres from the DC/ML system travel from the **thalamus** to the **primary somatosensory cortex**?

**Internal capsule**

Which sensations is the **Spinothalamic tract** responsible for?

**Pain** **Temperature** **Deep pressure**

Where do sensory fibres from the **spinothalamic tract** synapse first?

**Spinal cord** happens quick

What happens after fibres from the **spinothalamic tract** synapse in the spinal cord?

**They immediately decussate** (cross over) Compare this to the DC/ML fibres, which ascend to medulla on the ipsilateral side before crossing over

The **main sensory pathways** (DC/ML pathway and spinothalamic tract) are both described as having **first, second and third order neurons**. Where do these neurons **synapse** in each pathway?

**DC/ML system** - dorsal root to medulla; medulla to thalamus; thalamus to post-central gyrus **Spinothalamic tract** - dorsal root to spinal cord; spinal cord to thalamus; thalamus to post-central gyrus

Where do the sensory tracts **decussate** in the a) DC/ML pathway b) spinothalamic tract?

**a) Medial lemniscus of the medulla** **b) Spinal cord**

Where exactly is the **pre-central gyrus**, or **motor strip**, found?

**Frontal lobe, anterior to the central sulcus**

Sensory neurons **\_\_\_** from the spinal cord to the brain.

**ascend**

Motor neurons **\_\_\_** from the brain to the spinal cord.

**descend**

What are the a) sensory b) motor pathways we need to know about?

**a) Dorsal column / medial lemnsicus pathway** and **spinothalamic tract** **b) Corticospinal tract** (or **Pyramidal tract)**

Where exactly in the brain do **motor fibres** descend to form the **corticospinal tract**?

**Primary somatomotor cortex**, found in the **pre-central gyrus** of the **frontal lobe**

What is the function of the **corticospinal tract**?

**VOLUNTARY control of FINE MOTOR movement**

What are the functions of the a) **DC/ML pathway** b) **spinothalamic tract** c) **corticospinal tract**?

**a) Fine touch, vibration, proprioception** **b) Firm touch, temperature, pain** **c) Fine movement**

Motor fibres from the **corticospinal tract** also decussate to supply the contralateral muscles. Where does this happen?

**Pyramids** of the medulla

What percentage of motor fibres decussate at the pyramids of the medulla? Which tract do they form?

**85%** **LATERAL corticospinal tract**

What percentage of motor fibres **don't decussate** at the pyramids of the medulla? Which tract do they form?

**15%** **Ventral / Anterior corticospinal tract**

After they (mostly) decussate at the pyramids of the medulla, the corticospinal tracts **(ascend / descend****)**.

**descend** to supply the muscles on the contralateral side

What happens to the **ventral corticospinal tract** (i.e the motor fibres which DON'T decussate at the pyramids of the medulla)?

**They descend to the spinal cord segment and decussate there** So the corticospinal tract still supplies the contralateral muscles, like the sensory pathways

Is the **corticospinal tract** the only example of a **motor pathway**?

No, there are **accessory pathways** e.g tectospinal tract, reticulospinal tract, vestibulospinal tract...

Which **descending pathway** contains the **upper** **motor neurons** for a number of the **cranial nerves?**

**Corticobulbar pathway**

Is the **corticobulbar pathway** involved in **voluntary** or **involuntary control?**

**Voluntary control** So it's a **pyramidal tract**

The **UMNs** of **which cranial nerves** are contained in the **corticobulbar tract?**

**CN V** **_CN VII_** **CN XI** **CN XII**

What is special about the **innervation of most of the UMNs in the corticobulbar tract**?

**Bilateral** **innervation** Meaning if an UMN is compromised on one side, the other side should take over

The **corticobulbar tract** contains the **UMNs of CN VII.** What is the **motor function** of **CN VII?**

**Muscles of facial expression** Some misc muscles like **stapedius**

The **corticobulbar tract** carries the UMNs of CN VII, which innervate the **muscles of facial expression bilaterally.** **Which muscle of facial expression is found in the forehead area?**

**Frontalis**

Which pathologies can cause **UMN damage** re: the corticobulbar tract?

**Stroke** **Subdural haematoma** (usually secondary to a head injury) **Tumour**

Which pathologies can cause damage to the **LMNs of CN VII?** ## Footnote **What is the name of the presentation caused by LMN damage to CN VII?**

**Infection** **Trauma** Bell's palsy

What is the presentation of **Bell's palsy?** ## Footnote **What specifically causes it?**

**Facial paralysis WITHOUT FOREHEAD SPARING** Reversible damage to the LMNs of CN VII; thought to be viral in origin

What is the presentation of damage to the UMNs of CN VII?

**Facial paralysis WITH FOREHEAD SPARING**

Why does an **UMN CN VII lesion** e.g stroke cause **forehead sparing** while a **LMN** (Bell's palsy) **does not?**

**UMN innervation is bilateral** **LMN innervation is unilateral** This gets asked about all the time in exams

Name **two important extra-pyramidal tracts** and their functions. What is meant by **extra-pyramidal?**

**Rubrospinal tract** - excites upper body flexors and inhibits upper body extensors **Reticulospinal tract** - excites upper body extensors Not passing through the pyramids of the medulla; **involuntary movements**

Where do the a) **corticospinal** b) **rubrospinal** c) **reticulospinal** tracts originate?

**a) Pre-central gyrus** of cerebral cortex **b) Red nucleus of midbrain** **c) Pons/medulla** Sort of like a tower

The **corticospinal tract** controls **voluntary fine movements.** If it is compromised, which **descending tract** takes over? What **posture** does this result in?

**Rubrospinal tract** **Decorticate posture** - legs adducted and extended; upper limbs, wrists and fingers flexed against the chest "Decorticate" because the cerebral cortex isn't involved

If the **corticospinal tract** is compromised, the **rubrospinal tract** takes over and the patient adopts a **decorticate posture.** What happens if the **rubrospinal tract** is compromised?

**Reticulospinal tract** (bottom of the tower) **has to take over** Patient adopts a **decerebrate posture** - legs extended and adducted; arms extended "Decerebrate" because the cerebrum itself is no longer involved

Describe a) **decorticate** b) **decerebrate posturing.** Which **descending motor tracts** are responsible for these postures?

**a) Decorticate posturing** - legs extended and adducted; arms, wrists and fingers flexed against the chest; **rubrospinal tract** **b) Decerebrate posturing** - legs extended and adducted; arms extended; **reticulospinal tract**

What are some **causes of decorticate and decerebrate posturing?**

**Serious brain / brainstem pathology**