Ascending and Descending Pathways

Question 1

What are the main 3 properties of the spinothalamic pathway

Answer 1

Slow conduction

• pain
• temp
• crude/light touch

3 neurones

Question 2

Describe the pathway of the spinothalamic pathway

Answer 2

Sensory neurone (cell body in DRG) => SC DH synapse with 2nd order neurone

Decussates in SC ventrally => spinothalamic tract => VPL nucleus in thalamus synapse with 3rd order neurone

Enters internal capsule => thalamocortico tract => cortical neuron synapse in somatosensory cortex

Question 3

Internal capsule

• location
• importance
• consequences if damaged

Answer 3

Between lentiform nucleus and thalamus

Almost all tracts enter/leave cerebral cortex via here

Fanshaped

• ascending fibres radiate out => different cortical areas
• descending fibres collect at base => enter midbrain

V concentrated area, damage to capsule leads to

• paralysis
• anaesthesia
• loss of motor control
• cognitive and memory deficits

Question 4

Thalamus

• location
• structures here
• importance

Where do the somatosensory pathways synapse

• spinal cord input?
• trigeminal input?

Answer 4

Form lateral border of 3rd ventricle

23 nuclei here consisting of cell bodies
Nuclei grouped and separated into Y band of axons
-ant
-med
-lat

All sensory and other pathways synapse here before passing =>cortex (except olfactory)

Somatosensory pathways synapse in ventroposterior nuclei

• spinal cord input => VPL
• trigeminal input => VPM

Question 5

What are the 3 main properties of the medial leminiscus pathway

Answer 5

Fast conduction route

• discriminative touch
• proprioception
• vibration sense

3 neurones

Question 6

Describe the medial leminscus pathway

Answer 6

1st order neurone in dorsal column tract with cell body in dorsal root ganglion

1st synapse in either cuneate or gracile nucleus in dorsal medulla

Decussation of both in medial leminscus tract to ventral side of medulla

2nd synapse in lateral ventroposterior nucleus in thalamus

Both 3rd order neurones enter internal capsule

Cuneate tract goes laterally in somatosensory cortex
Gracile tract goes medially in somatosensory cortex

Both synapse in somatosensory cortex

Question 7

What is the difference between the cuneate tract and the gracile tract

Answer 7

Cuneate

• Upper trunk, upper limbs, neck, back of head
• Lateral to gracile tract in cervical region

Gracile tract

• Lower trunk, lower limbs
• Medial to cuneate tract in cervical region

Question 8

What are the 3 properties of the corticospinal pathway

Answer 8

Voluntary motor commands, controls spinal reflexes

Upper motor neurone terminates in spinal cord near/at its target

Lower motor neurone => effector muscle

Question 9

Describe the corticospinal pathway

Answer 9

From primary motor cortex, passes through internal capsule, midbrain and pons

85% of upper motor neurones decussate in ventral medulla (pyramidal decussation) => lateral corticospinal tract

15% of upper motor neurones decussate near termination => ant/ventral corticospinal tract

Both synapse at lower motor neurone => effector muscle

Question 10

Describe the 2 methods of termination in the lateral corticospinal tract
Why are they different and what are the uses of each method

Answer 10

No interneuron => precise movement and fractionating control
To muscles of hand and distal upper limb

Excitatory interneuron => many motor neurones cause broadly targetted muscle contraction

Inhibitory interneurones => inhibits antagonistic muscle activation by stretch reflexes
To muscles of body and lower limb

Question 11

What is the function of the primary motor cortex
What neurones are found here
What body parts are innervated by different regions of the primary motor cortex

Answer 11

Initiated voluntary muscle action

Contains upper motor neurones, axons => spinal cord to excite lower motor neurones in corticospinal tracts in ventral horns

Axons also => brainstem cranial nerve motor nuclei => activate lower motor neurones

Different parts are related to muscles of different body parts

• Face, most ventrolateral
• Lower parts, more medial

Question 12

Describe pyramidal decussation of the lateral corticospinal tract

Describe the effect of lesions above and below the point of decussation

Answer 12

Descends through the brainstem
Decussates from ventral medulla => lateral spinal cord

Above point of decussation => contralateral spastic paralysis

Below point of decussation => ipsilateral flaccid paralysis

Question 13

What is the function of the ventral/anterior corticospinal tract
Where does it extend down to
Describe the effect of lesions here

Answer 13

Smaller than lateral corticospinal tract
Decussates near terminations and synapses via interneurons
Does not extend much beyond cervical spinal cord

Lesions here do not result in obvious pathologies