Ascending Sensory Pathways Flashcards
What is the function of somaesthetic pathways?
- Carry info of different types/modalities from skin, mucus membranes, joints, muscles to the brain
- 2 modalities: 1=essential to survive 2=increase detail
What modalities are essential to survive? How are they carried?
- Pain, temperature, some touch & pressure
- Carried via thin, poorly myelinated or unmyelinated fibres
- Conduction relatively slow
What modalities increase detail? How
- Proprioception
- Fast conduction
- Carried via large diameter, heavily myelinated fibres
- Discriminative touch: 2-point discrimination & vibration
Describe a pseudounipolar neuron
- Also referred as a unipolar neuron
- Single stalk splits to give off 2 processes
- Derived from a bipolar neuron
What is the general sensory pathway comprised of?
3 neurons: 1st, 2nd, 3rd order
Where do the cell bodies of 1st, 2nd and 3rd order neutrons reside?
1st= From PNS to CNS .dorsal root ganglia (PNS) 2nd= CNS ipsilateral grey matter 3rd= thalamus &axons project to somatosensory cortex (post-central gyrus)
What route do axons of 2nd order neurons take?
Cross midline & ascend to ventral posterolateral nucleus of the thalamus
Describe the synaptic transmission from 1st, 2nd & 3rd order neurons
Not a simple relay, can be modified by other inputs- descending pathways, convergence/divergence
How is discriminative touch & proprioception sensed by the brain?
- Received by 1st order neuron cell body in DRG
- Passed to 2nd order fibres ascending in one of ipsilateral dorsal columns
What does the medulla receive?
Ascending sensory tracts from the spinal cord part of the dorsal column pathway (discriminatory touch & position sense)
Describe the course of fibres carrying pain & temperature
- Spinothalamic pathway
- Travel contra laterally in the cord
- Cross at level of spinal cord entry
- Use 3 neurons
Describe the course of fibres carrying discriminative touch & proprioception
- Dorsal column pathway
- Travel ipsilaterally in the cord
- Cross the level of closed medulla
- Use 3 neurons
What determines whether fibres run in the gracile or cuneate fascicle?
Vertebral level at which info is given
GF= T7 and below
CF= T6 and above
How does Herpes Zoster virus affect specific dermatomes?
- 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
What do cranial nerve nuclei receive & give rise to?
- Sensory: Receive info from fibres entering brainstem in cranial nerves
- Motor: Give rise to motor fibres that leave the brainstem in cranial nerves
In which cranial nerves is general sensation from the head carried? What pathway do these nerves travel along?
- 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
Where do motor nuclei of trigeminal run in relation to sensory nuclei?
-Motor nuclei lie more medial
In the trigeminal nuclei how are the sensory nuclei divided?
- Mesencephalic nucleus
- Pontine/chief sensory nucleus
- Spinal nucleus
- Present on both sides
What modalities does the spinal nucleus receive?
- Caudal part= pain & temperature
- Rostral part= simple touch & pressure
How does pain & temperature reach the thalamus? (Related to trigeminothalamic tract)
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
How does touch & pressure reaching the thalamus differ in its pathway to temperature & pain?
Fibres synapse more rostrally in spinal nucleus
Where are discriminative touch & proprioception received in the trigeminal sensory nucleus?
- Discriminative touch= Pontine/chief sensory nucleus
- Proprioception= Mesencephalic nucleus
How does discriminative touch reach the thalamus?
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
Describe the proprioception pathway in the trigeminal nucleus
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
What unite to form a spinal nerve and where?
Dorsal & ventral nerve roots in intervertebral foramen
What do dorsal & ventral rami of spinal nerves supply?
D= Muscles of posterior trunk V= Anterolateral parts of trunk & limbs
