Cranial nerves > ACCESSORY > Flashcards

ACCESSORY Flashcards

(59 cards)

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1
Q

What does “origin” mean for cranial nerve XI?

A

For CN XI, origin means:
• 🧠 Which nuclei give rise to its fibres
• 📍 Where those nuclei are located
• ➡️ Where the nerve fibres emerge to form the nerve

🧠 CN XI is unique because it has TWO anatomical origins.

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2
Q

Why is CN XI unusual compared with other cranial nerves?

A

Because CN XI has a dual origin 🔥:
• 🧠 Cranial (medullary) origin
• 🧠 Spinal origin (cervical spinal cord)

👉 Most cranial nerves arise only from the brainstem.

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3
Q

What are the TWO components of the accessory nerve (CN XI)?

A

CN XI has:
1️⃣ Cranial part
2️⃣ Spinal part 🔥

🧠 The spinal part is the main functional component in modern anatomy.

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4
Q

Where does the cranial part of CN XI originate?

A

👉 From the nucleus ambiguus 🧠

• Located in the medulla
• Shared with CN IX and CN X

🧠 This part is often considered functionally part of the vagus nerve (CN X).

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5
Q

Where is the nucleus ambiguus located exactly?

A

📍 Lateral medulla
• Deep in the reticular formation
• Ventral (anterior) to the nucleus solitarius
• Medial to the spinal trigeminal nucleus

🧠 Not visible on the surface of the brainstem.

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6
Q

What type of fibres arise from the cranial part of CN XI?

A

👉 Branchial motor fibres

🧠 These fibres:
• Quickly join the vagus nerve
• Do NOT innervate SCM or trapezius

⚠️ This is why the cranial part is often ignored clinically.

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7
Q

Where does the spinal part of CN XI originate?

A

👉 From the spinal accessory nucleus 🔥

📍 Location:
• Anterior horn of the spinal cord
• Levels C1–C5 (sometimes C6)

🧠 This is the key nucleus for CN XI.

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8
Q

What type of nucleus is the spinal accessory nucleus?

A

👉 A somatic motor nucleus

🧠 Meaning:
• Supplies skeletal muscle
• Equivalent to anterior horn motor neurons

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9
Q

Why is the spinal accessory nucleus considered part of a cranial nerve?

A

Because its fibres:
• Ascend into the skull
• Enter via the foramen magnum
• Exit via a cranial foramen
• Supply head and neck muscles

🧠 CN XI = cranial nerve with spinal roots.

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10
Q

How do spinal accessory fibres reach the skull (origin only)?

A

Step-by-step:
1️⃣ Arise from C1–C5 spinal accessory nucleus
2️⃣ Exit the spinal cord
3️⃣ Ascend in the vertebral canal
4️⃣ Enter the skull via the foramen magnum

🧠 Course beyond this comes later.

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11
Q

Where does CN XI emerge relative to CN IX and CN X?

A

CN XI emerges from the:
👉 Post-olivary sulcus

🧠 Order from top to bottom:
• CN IX
• CN X
• CN XI (most inferior)

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12
Q

Which part of CN XI is clinically important for muscle innervation?

A

👉 The SPINAL PART 🔥

• Origin: spinal accessory nucleus (C1–C5)
• Cranial part joins CN X and does not act independently.

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13
Q

Can you summarise the origin and nuclei of CN XI in one sentence?

A

The accessory nerve has a dual origin, with a cranial component from the nucleus ambiguus in the lateral medulla (functionally joining the vagus nerve) and a spinal component from the spinal accessory nucleus in the anterior horn of the cervical spinal cord at C1–C5, with the spinal component forming the clinically important motor nerve.

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14
Q

What does “CN XI tracts & course” mean?

A

It means describing:
• 🧠 Where CN XI fibres start
• ➡️ The exact route they travel (inside + outside the skull)
• 🎯 Where they end (target muscles)

🧠 CN XI has TWO components:
• Spinal part (main functional part)
• Cranial part (briefly joins CN X).

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15
Q

Where do the SPINAL accessory fibres originate?

A

👉 From the spinal accessory nucleus 🔥

📍 Location:
• Cervical spinal cord
• Levels C1–C5 (± C6)

🧠 This is the main origin of functional CN XI.

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16
Q

How do spinal accessory fibres leave the spinal cord?

A

Step-by-step:
1️⃣ Fibres arise from C1–C5 spinal accessory nucleus
2️⃣ Exit the spinal cord as multiple rootlets
3️⃣ Rootlets emerge from the lateral aspect of the upper cervical cord
4️⃣ Rootlets unite to form the spinal accessory nerve

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17
Q

How does the spinal accessory nerve enter the skull?

A

After forming in the neck:
1️⃣ CN XI ascends upward inside the vertebral canal
2️⃣ Enters the cranial cavity through the foramen magnum 🔥

🧠 This upward entry is a key anatomical oddity.

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18
Q

What happens to CN XI after it enters the skull via the foramen magnum?

A

Inside the posterior cranial fossa:
• The spinal part ascends briefly
• It meets the cranial part of CN XI (variable)
• Together they head toward the jugular foramen

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19
Q

How does CN XI leave the skull?

A

👉 CN XI exits the skull via the jugular foramen 🕳️

🧠 It exits together with:
• CN IX
• CN X

🔥 Shared exit is crucial for localisation anatomy.

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20
Q

What happens to the CRANIAL part of CN XI after leaving the skull?

A

The cranial part:
• Immediately joins CN X (vagus)
• Continues as part of vagal motor branches

🧠 It does NOT supply SCM or trapezius independently.

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21
Q

What is the early neck course of the SPINAL accessory nerve after the jugular foramen?

A

Step-by-step:
1️⃣ Leaves the jugular foramen
2️⃣ Descends a short distance in the upper neck
3️⃣ Runs deep to or within sternocleidomastoid (SCM)
4️⃣ Gives motor branches to SCM

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22
Q

What is the course of CN XI after supplying the sternocleidomastoid?

A

After innervating SCM:
1️⃣ CN XI emerges at the posterior border of SCM
2️⃣ Enters the posterior triangle of the neck
3️⃣ Travels superficially across the posterior triangle
4️⃣ Enters and supplies the trapezius

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23
Q

Why is the posterior triangle an important part of the CN XI course?

A

Because in the posterior triangle CN XI is:
• Superficial
• Easily identifiable
• Traveling between SCM and trapezius

🧠 Anatomically vulnerable due to its exposed course.

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24
Q

What are the FINAL TARGET MUSCLES of the spinal accessory nerve?

Study These Flashcards
A

👉 Sternocleidomastoid (SCM)
👉 Trapezius

🧠 These are the only muscles supplied by the spinal part of CN XI.

25
What is the course of the CRANIAL part of CN XI (summary)?
Step-by-step: 1️⃣ Arises from the **medulla** (nucleus ambiguus region) 2️⃣ Rootlets emerge from the **post-olivary sulcus** 3️⃣ Exit skull via **jugular foramen** 4️⃣ **Join CN X (vagus) immediately** 5️⃣ Continue to pharynx/larynx via vagal branches
26
Can you summarise the entire CN XI course in one clean chain?
**Spinal part (main):** C1–C5 rootlets → ascend → **foramen magnum** → cranial cavity → **jugular foramen** → SCM → **posterior triangle** → trapezius **Cranial part:** medulla rootlets → **jugular foramen** → joins **CN X**
27
Front
Back
28
What does “innervation” mean for CN XI?
Innervation means **which muscles the accessory nerve supplies and controls**. 👉 **CN XI is a PURE MOTOR nerve** 🔥 • No sensory fibres • No parasympathetic fibres
29
How many muscles does CN XI innervate?
👉 **TWO muscles ONLY** 🔢: 1️⃣ **Sternocleidomastoid (SCM)** 2️⃣ **Trapezius** 🧠 Both are **skeletal muscles of the neck and shoulder**.
30
Which muscle does CN XI innervate first?
👉 **Sternocleidomastoid (SCM)** 🧠 CN XI supplies SCM **before** crossing the posterior triangle of the neck.
31
Where is the sternocleidomastoid (SCM) muscle located?
SCM runs from: • **Sternum + clavicle** → to • **Mastoid process** of the skull 🧠 It is the **large, prominent neck muscle**.
32
What does the sternocleidomastoid muscle do?
SCM actions: • 🔄 **Rotates the head to the opposite side** • ↘️ **Tilts the head to the same side** • 💨 Assists deep inspiration (minor role) 🧠 Example: • Right SCM contracts → head turns **left**.
33
How is sternocleidomastoid (SCM) function tested?
Ask the patient to: • Turn their head **against resistance** 🧠 Weakness suggests **CN XI involvement**.
34
Which muscle does CN XI innervate after SCM?
👉 **Trapezius** 🧠 After supplying SCM, CN XI: • Crosses the **posterior triangle** • Enters and supplies the trapezius.
35
Where is the trapezius muscle located?
Trapezius is a **large, flat muscle** covering: • Upper back • Neck • Shoulder region 🧠 Extends from skull & spine → shoulder girdle.
36
What does the trapezius muscle do?
Trapezius actions: • ⬆️ **Elevates the shoulder** (shrugging) • 🔄 Rotates the scapula • 🧱 Stabilises the shoulder girdle 🧠 Essential for arm elevation above shoulder level.
37
How is trapezius function tested clinically?
Ask the patient to: • **Shrug the shoulders against resistance** 🧠 Weakness suggests **CN XI involvement**.
38
Does CN XI innervate pharyngeal or laryngeal muscles?
❌ **NO** 🔥 • Pharynx & larynx → **CN X (vagus)** • Stylopharyngeus → **CN IX** 🧠 CN XI innervates **ONLY SCM and trapezius**.
39
Does the cranial part of CN XI innervate SCM or trapezius?
❌ **NO** • Cranial part joins **CN X** • SCM & trapezius are supplied by the **spinal part only** 🧠 Very high-yield exam distinction.
40
One-line summary of CN XI innervation
**CN XI is a pure motor nerve** that innervates only two skeletal muscles: the **sternocleidomastoid**, which rotates the head to the opposite side and tilts it to the same side, and the **trapezius**, which elevates and stabilises the shoulder and enables arm elevation.
41
What does “localisation” mean for CN XI lesions?
Localisation means identifying **WHERE along the CN XI pathway** the damage has occurred, based on the **pattern of muscle weakness** and **associated nerve involvement**. 🧠 For CN XI, think of **THREE key levels**: 1️⃣ **Jugular foramen** 2️⃣ **Posterior triangle of the neck** 3️⃣ **Upper motor neuron (central) lesion**
42
Which muscles are affected in ALL CN XI lesions?
👉 CN XI innervates **ONLY TWO muscles** 🔥: • **Sternocleidomastoid (SCM)** • **Trapezius** 🧠 All localisation depends on how **these two muscles** are affected.
43
Why does a jugular foramen lesion affect CN XI?
Because CN XI exits the skull via the **jugular foramen** 🕳️ **together with**: • **CN IX** • **CN X** 🧠 A lesion here affects **ALL THREE nerves**.
44
What is the classic nerve pattern in a jugular foramen lesion?
👉 **CN IX + CN X + CN XI palsy** 🔥 🧠 This combination is a **major exam localisation clue**.
45
What are the CN XI motor signs in a jugular foramen lesion?
Due to CN XI involvement: • Weak **head rotation away from the lesion** (SCM weakness) • **Ipsilateral shoulder droop** (trapezius weakness) 🧠 These signs do **not occur in isolation** at the jugular foramen.
46
What additional features suggest a jugular foramen lesion?
Because **CN IX and CN X** are also involved: • Hoarse voice • Dysphagia • Palate droop • Uvula deviation • Loss of gag reflex (afferent limb) 🧠 Shoulder weakness **plus voice/swallowing problems** = jugular foramen.
47
Why is the posterior triangle a common site of CN XI injury?
In the posterior triangle, CN XI is: • **Superficial** • **Unprotected** • Running between **SCM and trapezius** 🧠 Vulnerable to surgery, trauma, or lymph node biopsy.
48
Which muscle is MOST affected in posterior triangle CN XI lesions?
👉 **Trapezius** 🔥 Why: • CN XI has already supplied **SCM** before entering the posterior triangle 🧠 SCM is often normal or only mildly weak.
49
What are the key features of a posterior triangle CN XI lesion?
Classic features: • Ipsilateral **shoulder droop** • Inability to **shrug the shoulder** • Difficulty abducting arm above shoulder level • Scapular winging (downward/lateral) 🧠 **Voice and swallowing are normal**.
50
How do posterior triangle lesions differ from jugular foramen lesions?
Key comparison: Jugular foramen: • CN IX + X + XI involved • Voice & swallowing affected • SCM + trapezius weak Posterior triangle: • CN XI only • Voice & swallowing normal • Trapezius weak, SCM usually spared
51
What is an upper motor neuron (UMN) lesion affecting CN XI?
A UMN lesion damages **corticobulbar fibres** from the motor cortex to the spinal accessory nucleus. 🧠 The nerve itself is intact, but **central control is lost**.
52
How does corticobulbar innervation to CN XI work?
Important rule: • **SCM** receives **bilateral cortical input** • **Trapezius** receives **predominantly contralateral cortical input** 🧠 This explains the UMN pattern.
53
What happens to the SCM in a UMN CN XI lesion?
👉 **SCM is relatively preserved** ✅ Why: • It has **bilateral cortical innervation** 🧠 Head rotation is often only mildly affected.
54
What happens to the trapezius in a UMN CN XI lesion?
👉 **Contralateral trapezius weakness** 🔥 Example: • Right hemispheric stroke • Left shoulder droop 🧠 Classic exam-tested pattern.
55
How do UMN and LMN CN XI lesions differ?
UMN lesion: • No wasting or fasciculations • SCM relatively spared • **Contralateral trapezius weakness** LMN lesion: • Muscle wasting • Fasciculations • **Ipsilateral SCM and/or trapezius weakness**
56
CN XI weakness + hoarse voice + dysphagia localises where?
👉 **Jugular foramen** 🕳️ 🧠 Combined IX–X–XI involvement.
57
Isolated shoulder droop with normal voice and swallowing localises where?
👉 **Posterior triangle of the neck** 🌿 🧠 Isolated CN XI (trapezius) involvement.
58
Shoulder droop opposite to a hemispheric stroke localises where?
👉 **Upper motor neuron (central) lesion** 🧠 🧠 Contralateral trapezius weakness.
59
One-line summary of CN XI lesions & localisation
Accessory nerve lesions localise by pattern: **jugular foramen lesions** cause combined IX–X–XI palsies with shoulder droop and head-turn weakness, **posterior triangle lesions** cause isolated trapezius weakness with preserved voice and swallowing, and **UMN lesions** cause contralateral trapezius weakness with relative sparing of the sternocleidomastoid due to bilateral cortical innervation.
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