Tuesday, May 23, 2017

Motor nuclei in the brainstem : An overview

Hi everyone. Just thought of doing an overview of the various motor nuclei of cranial nerves in the brain stem.

So we can classify the motor nuclei into 3 groups -
1. Somatic motor efferent - 4
2. Branchial motor efferent - 4
3. Visceral motor efferent - 4
Now how are these classified ?

1. Somatic Motor Efferent

- In the embryological stage , there are certain precursors to muscle and skin segment groups called 'Somites'. These are processes of the paraxial mesoderm.
- Sach somite gives rise to a particular set of muscles called its myotome. 
- There 4 such important somite groups -->

A. Pre otic somites = 3.
So this is simple.
There are 3 pre otic somites giving rise to distinct groups of extraocular muscles supplied by their own cranial nerve.

Somite 1  =
Muscles -
All Extra ocular muscles except Lateral Rectus and Superior oblique.
Nerve -
Oculomotor nerve (III)
Nucleus -
Oculomotor nucleus in the Upper Midbrain.

Somite 2  =
Muscles -
Superior oblique.
Nerve -
Trochlear nerve (IV)
Nucleus -
Trochlear motor nucleus in the Lower Midbrain.

Somite 3  =
Muscles -
Lateral Rectus.
Nerve -
Abducent nerve (VI)
Nucleus -
Abducent motor nucleus in the Pons.

(I'm sure you remember the popular mnemonic - LR6 SO4)

B. Occipital somites
Muscles -
All muscles of the tongue except Palatoglossus
Nerve -
Hypoglossal I'm nerve (XII)
Nucleus -
Hypoglossal nucleus in the Medulla.

Since these nuclei represent the motor innervation to the derivatives of Somites , they're called Somatic Motor or General Somatic Efferent (GSE) Fibres. 

2. Branchial Motor Efferent - 

- In the embryological stage , there are various branchial or Pharyngeal arches that give rise to muscles , bones and cartilage supplied by a particular nerve of that arch.

- Each nucleus that supplies the muscles from such a Branchial arch is called Branchiomotor Efferent or Special Visceral Efferent. (SVE) 
- There are 4 such important arches - 

A. 1st Pharyngeal arch (mandibular arch)
Muscles -
All muscles of mastication + TT (Tensor tympani + Tensor veli Palatini) + Digastric anterior belly. ( And Meckel cartilage)
Nerve -
Mandibular branch of Trigeminal
Nucleus - 
Trigeminal motor nucleus in Pons 

B. 2nd Pharyngeal arch (hyoid arch) 
Muscles -
All muscles of facial expressions + Stapedius + Digastric posterior belly.  ( And Reichter cartilage)
Nerve -
Facial nerve (VII)
Nucleus - 
Facial motor nucleus in Pons 

C. 3rd Pharyngeal arch
Muscles -
(And the hyoid bone funnily.)
Nerve -
Glossopharyngeal nerve (IX)
Nucleus - 
Nucleus Ambiguus in Medulla

D. 4th and 6th Pharyngeal arches
Muscles -
- All muscles of  Soft palate ( except Tensor veli which is up in the 1st arch) by the 4th. + cricothyroid muscle of Larynx. 
- All muscles of Larynx by the 6th except cricothyroid which is by the 4th. 
(All laryngeal cartilage as well)
Nerve -
4th arch - Superior laryngeal nerve of the Vagus.(X)
6th arch - Recurrent laryngeal nerve of the Vagus (X)
Nucleus - 
Nucleus Ambiguus of Medulla 

Now there's another Motor nucleus - The Accessory nerve. It supplies Trapezius and Sternomastoid muscles but it's doubtful if it's Branchial or Somatic. 

3. Visceral Motor Efferent - General

- These nuclei are parasympathetic and stimulate a particular gland to secrete or a ganglion to function. 
- These are called Secretomotor or General Visceral Efferent Fibres 

Again , there are 4 of these. 

A. Ciliary ganglion 
Function mediated by - 
Sphincter pupillae - Constricts pupil 
(Mnemonic = Remember C and C - Cholinergic Constricts )
Nerve -
Oculomotor nerve
Nucleus - 
Edinger Westphal in Midbrain 

B. Pterygopalatine ganglion 
Function mediated by - 
Lacrimal glands, nasal mucosal, sinuses mucosal glands and pharynx mucosal - Secretomotor. 
Nerve -
Facial nerve  (Greater Petrosal)
Nucleus - 
Superior salivatory nucleus - Pons. 

C. Submandibular ganglion 
Function mediated by - 
Submandibular glands , sublingual glands - Secretomotor.
Nerve -
Facial nerve  (Chorda tympani)
Nucleus - 
Superior salivatory nucleus - Pons. 

D. Otic ganglion 
Function mediated by - 
Parotid gland
Nerve -
Glosspharyngeal nerve  (Lesser Petrosal)
Nucleus - 
Inferior salivatory nucleus - Pons. 

The Vagus nerve has the largest parasympathetic discharge and supplies a lot of visceral with this input in the guy as well.

Hope this helps you to re-orient yourself to neuroanatomy and grasp the roles of various brainstem structures ! 
Happy studying ! 
~ A.P.Burkholderia

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