Lecture 12: Ascending Pathways II

Question 1

Which cranial nerves carry somatic innervation to the head?

Answer 1

(a) Trigeminal Nerve - V1, V2 and V3 to the face
(b) Facial branch - The external Ear
(c) Glossopharyngeal - Posterior third of the tongue, pharynx and middle ear
(d) Vagus - Auditory canal, larynx, pharynx and oesophagus

Question 2

Which nucleus do the sensory fibres run through?

Answer 2

Trigeminal sensory nucleus.

Question 3

How is the trigeminal nuclei organised?

Answer 3

• The mesencephalic nucleus of V - located in the midbrain - receive proprioception
• The Chief/Pontine Nucleus of V - located in the pons - receives signals of discriminative touch
• The Spinal Nucleus of V - extends to the medulla - the more rostral part receives simple touch and pressure. The more caudal parts receive pain and temperature.

There is also a small motor nucleus of V more medially.

Question 4

Describe the trigeminal sensory pathway for proprioception.

Answer 4

The cell bodies of pronociceptive fibres in the face are in the mesencephalic nucleus directly. Here they make contact with another cell body of the second order nucleus to go to the VPM again via the trigemino-thalamic tract. They make contact with motor neurons in the trigeminal motor nucleus.

Question 5

Describe the trigeminal sensory pathway for discriminative touch.

Answer 5

The first order cell bodies in the trigeminal ganglion. They enter at the level of the pons .They then synapse here at the pontine nucleus/chief with the second order neuron. The second order neurons cross the midline and travel in to the VPM in the trigeminothalamic tract.

After the thalamus, the neurons will reach the areas in the face via the third order neurons.

Question 6

Describe the trigeminal sensory pathway for crude touch and pressure.

Answer 6

The first order cell bodies in the trigeminal ganglion. They enter at the level of the pons to move down into the more rostral part of the spinal nucleus of V via the spinal tract of the trigeminal. They then synapse here with the second order neuron. The second order neurons cross the midline and travel in the trigemninothalamic tract to the VPM.

After the thalamus, the neurons will reach the areas in the face via the third order neurons.

Question 7

Describe the trigeminal sensory pathway for pain and temperature.

Answer 7

The first order neurone has its cell bodies in the trigeminal nerve ganglion. The axons travel in the spinal tract of the trigeminal. They use the tract to travel caudally to contact the cell body of the second order neurone found in the caudal medulla. The second order neurone sends an axon that will cross the midline. This then travels in the trigemninothalamic tract to the thalamus - to the VPM - ventroposterior medial area.

Question 8

How would a lesion affecting the lateral left medulla affect sensation from the face and body?

Answer 8

The lesion is in the lateral medulla. The consequences of this would be:

	- The affect on the face: loss of discriminative touch, pain and pressure on the right. Any sensory information has its cell body in the trigeminal ganglion. It then enters the pons to go down to the spinal nucleus of V.  We are then preventing fibres from the right side of the face to travel further. Any information carrying information from the right sound of the face cannot travel to the left side - therefore loss of pain, temperature, touch and pressure on the ipsilateral side.
	- The affect on the body: spinothalamic tract is taken out this means that there will be issues with temperature sensation and analgesia on the left side of the face  Loss of pain, temperature, touch and pressure on the contralateral side.

Question 9

Which ganglion do the divisions of the trigeminal nerve have their cell bodies in?

Answer 9

Trigeminal ganglion (except those for proprioception - The first order cell bodies are in peripheral ganglia).

Question 10

How does the jaw jerk reflex work?

Answer 10

V3 fibres form this region carry proprioception all the way into the mesencephalic nucleus. Here we illicit the nucleus. A copy of that information is sent using a small collateral fibre to the motor nucleus of V in the pons. This send motor fibres to the muscles of mastication leading to the patient opening the jaw. The afferent limb of the jaw jerk reflex is carrier via V3 - sensory fibre. The efferent limb is V3 motor fibres.

They are again relayed by the thalamus into the region of the post central gyrus.