L8 Descending Pathways

Question 1

Descending Pathways

-pathway by which the brain controls lower motor neurons

Answer 1

High order level of control of movement:

• Supplementary motor cortex
• Premotor Cortex
• Motor cortex

Then either

-> Corticospinal tract -> Spinal cord

OR

->Brain stem -> Rubrospinal/Reticulospinal/Vestibulospinal/Tectospinal tract -> Spinal Cord

There are several different mechanisms to control movement!

Question 2

Pyramidal

Answer 2

In the corticospinal tract

Question 3

Extrapyramdial

Answer 3

Everything else NOT in the corticospinal tract

Question 4

Extrapyramidal systems

Answer 4

Basal Ganglia and Cerebellum

can sometimes be referred to as extrapyramidal systems

2 key groups involved in motor control

send projections to the cortex to modulate motor output

Question 5

Basal Ganglia

Answer 5

• Loss of this area can result in Parkinson’s disease
• very important in modulating movement control
  
  • prevents unwanted movement

Question 6

Cerebellum

Answer 6

Seen as a comparator

Provides fine tuning to movement

N.B. cortex also controls fine tuning

Question 7

Corticospinal Tract

Pyramidal system

Answer 7

Direct route

from cortex to lower motor neurons of the:

• Brainstem (corticobulbar)
• Spinal cord (corticospinal)

FAST PATHWAY to the lower motor neurons

only present in mammals

Question 8

Other Indirect pathways

• have lots of synapses
• therefore relatively slow

Answer 8

• Rubrospinal tract
• Reticulospinal tract
• Vestibulospinal tract
• Tectospinal tract

Question 9

Descending Pathway order

Answer 9

Cortex -> 1st order - upper motor neuron -> 2nd order (interneuron)-> 3rd order - lower motor neuron -> Muscle

Most upper motor neurons come from the primary motor cortex

In some cases the upper motor neuron synapses directly to the lower motor neuron

i.e for hand movements - need to be fast hence fewer synapses

Question 10

Corticospinal tract

Upper & lower motor neuron

Answer 10

Upper motor neuron:

• pyramidal neuron in motor cortex that ends on lower motor neuron (or on interneuron)

Lower motor neuron:

• motor neuron in ventral horn of spinal cord that contacts striated muscle

Question 11

Corticospinal Tract

info

Answer 11

• Corticospinal tract originates mainly from motor cortex, but also from somatosensory, premotor and supplementary motor cortex
• Corticospinal tract travels via the internal capsule and brainsterm
• At the pyramidal decussation, 85% of fibres cross to make the lateral corticospinal tract
• 15% remain ipsilateral to make the anterior corticospinal tract
• some corticospinal axons synapse directly on lower motor neurons, others synapse first on an interneuron

Question 12

Lateral corticospinl tracts

Answer 12

85% of fibres that did cross

Question 13

Anterior corticospinal tracts

Answer 13

15% of fibres that did not cross

Question 14

Effect of vascular lesion on the artery that supplies the motor cortex

Answer 14

Paralysis of the opposite side of the body

Question 15

Red nucleus

Answer 15

Involved in Motor Coordination

Controls Gait

Question 16

Effect of UPPER MOTOR NEURON lesions

= net loss of inhibition

no voluntary control of the muscle

Answer 16

Spastic paralysis syndrome:

• if lesion is above decussation:
  
  • paralysis of contralateral side of body below lesion – HEMIPLEGIA
• if lesion is below decussation:
  
  • paralysis of ipsilateral side of body below lesion
• hypertonia – fixed postures of limbs
• hyperreflexia – increased resistance to manipulation of limbs - clasp knife reflex

Question 17

Effects of a LOWER MOTOR NEURON Lesion

= either in MND or lesion to (the ventral horns esp of ) the spinal cord

Answer 17

• paralysis of muscles ipsilateral to the lesion at the affected level
• hypotonia – diminished resistance to manipulation of limbs
• atrophy – muscle wasting after several weeks (due to absence of muscle activity)