Ascending Sensory Pathways

Question 1

What is the function of somaesthetic pathways?

Answer 1

• Carry info of different types/modalities from skin, mucus membranes, joints, muscles to the brain
• 2 modalities: 1=essential to survive 2=increase detail

Question 2

What modalities are essential to survive? How are they carried?

Answer 2

• Pain, temperature, some touch & pressure
• Carried via thin, poorly myelinated or unmyelinated fibres
• Conduction relatively slow

Question 3

What modalities increase detail? How

Answer 3

• Proprioception
• Fast conduction
• Carried via large diameter, heavily myelinated fibres
• Discriminative touch: 2-point discrimination & vibration

Question 4

Describe a pseudounipolar neuron

Answer 4

• Also referred as a unipolar neuron
• Single stalk splits to give off 2 processes
• Derived from a bipolar neuron

Question 5

What is the general sensory pathway comprised of?

Answer 5

3 neurons: 1st, 2nd, 3rd order

Question 6

Where do the cell bodies of 1st, 2nd and 3rd order neutrons reside?

Answer 6

1st= From PNS to CNS .dorsal root ganglia (PNS)
2nd= CNS ipsilateral grey matter
3rd= thalamus &axons project to somatosensory cortex (post-central gyrus)

Question 7

What route do axons of 2nd order neurons take?

Answer 7

Cross midline & ascend to ventral posterolateral nucleus of the thalamus

Question 8

Describe the synaptic transmission from 1st, 2nd & 3rd order neurons

Answer 8

Not a simple relay, can be modified by other inputs- descending pathways, convergence/divergence

Question 9

How is discriminative touch & proprioception sensed by the brain?

Answer 9

• Received by 1st order neuron cell body in DRG

- Passed to 2nd order fibres ascending in one of ipsilateral dorsal columns

Question 10

What does the medulla receive?

Answer 10

Ascending sensory tracts from the spinal cord part of the dorsal column pathway (discriminatory touch & position sense)

Question 11

Describe the course of fibres carrying pain & temperature

Answer 11

• Spinothalamic pathway
• Travel contra laterally in the cord
• Cross at level of spinal cord entry
• Use 3 neurons

Question 12

Describe the course of fibres carrying discriminative touch & proprioception

Answer 12

• Dorsal column pathway
• Travel ipsilaterally in the cord
• Cross the level of closed medulla
• Use 3 neurons

Question 13

What determines whether fibres run in the gracile or cuneate fascicle?

Answer 13

Vertebral level at which info is given
GF= T7 and below
CF= T6 and above

Question 14

How does Herpes Zoster virus affect specific dermatomes?

Answer 14

• After chickenpox infection it resides in sensory roots of specific spinal nerves
• Reactivation leads to shingles
• As virus is restricted to specific segmental spinal nerve blisters are only distributed in that dermatome

Question 15

What do cranial nerve nuclei receive & give rise to?

Answer 15

• Sensory: Receive info from fibres entering brainstem in cranial nerves
• Motor: Give rise to motor fibres that leave the brainstem in cranial nerves

Question 16

In which cranial nerves is general sensation from the head carried? What pathway do these nerves travel along?

Answer 16

• Trigeminal: Face, nose, scalp, dura
• Facial: External ear
• Glossopharyngeal: Post 1/3 tongue, pharynx, middle ear
• Vagus: Auditory canal, larynx, pharynx, oesophagus
• All info travels via the Trigeminal sensory pathway-within the trigeminal nucleus to the thalamus

Question 17

Where do motor nuclei of trigeminal run in relation to sensory nuclei?

Answer 17

-Motor nuclei lie more medial

Question 18

In the trigeminal nuclei how are the sensory nuclei divided?

Answer 18

• Mesencephalic nucleus
• Pontine/chief sensory nucleus
• Spinal nucleus
• Present on both sides

Question 19

What modalities does the spinal nucleus receive?

Answer 19

• Caudal part= pain & temperature

- Rostral part= simple touch & pressure

Question 20

How does pain & temperature reach the thalamus? (Related to trigeminothalamic tract)

Answer 20

1) Travels along thin, slow fibres
2) Reaches first order neuron cell bodies in trigeminal ganglion
3) Enters the pons and runs inferiorly to medulla
4) Fibres run caudally in spinal tract of trigeminal
5) Synapse with second order neutrons in spinal nucleus
6) Cross midline and ascend up to the thalamus in trigeminothalamic tract

Question 21

How does touch & pressure reaching the thalamus differ in its pathway to temperature & pain?

Answer 21

Fibres synapse more rostrally in spinal nucleus

Question 22

Where are discriminative touch & proprioception received in the trigeminal sensory nucleus?

Answer 22

• Discriminative touch= Pontine/chief sensory nucleus

- Proprioception= Mesencephalic nucleus

Question 23

How does discriminative touch reach the thalamus?

Answer 23

1) Fast, fat fibres
2) Reach first order neutrons cell bodies in trigeminal ganglion
3) Enter pons
4) Synapse with second order neurons in pontine nucleus
5) cross midline and ascend via trigeminothalamic tract to thalamus

Question 24

Describe the proprioception pathway in the trigeminal nucleus

Answer 24

1) Most axons travel in mandibular division of trigeminal nerve
2) First order neurons in the mesencephalic nucleus
3) Synapse alongside mesencephalic nucleus
4) Axons cross midline & ascend to thalamus in trigeminothalamic tract
5) Other fibres contact motor neurons in trigeminal motor nucleus
6) Synapse in pontine/chief motor nucleus
7) Motor fibres to muscles of mastication

Question 25

What unite to form a spinal nerve and where?

Answer 25

Dorsal & ventral nerve roots in intervertebral foramen

Question 26

What do dorsal & ventral rami of spinal nerves supply?

Answer 26

D= Muscles of posterior trunk
V= Anterolateral parts of trunk & limbs