Descending Pathways

Question 1

What are the three main classes of movement?

Answer 1

• Voluntary
• Reflexes
• Rhythmical motor patterns

Question 2

Describe voluntary movement

Answer 2

• Complex actions (reading, writing)
• Purposeful goal directed
• Learned

Question 3

Describe the movement involved in reflexes

Answer 3

Involuntary, rapid, stereotyped (knee jerk, eye blink)

Question 4

Describe the movement of rhythmical motor patterns

Answer 4

• Combines voluntary and reflexive acts (chewing, walking, running)
• Initiation and termination voluntary
• Once initiated repetitive and reflexive

Question 5

What are the functions of motor control systems and what are they guided by?

Answer 5

• posture and balance
• goal-directed movements
• communication

Guided by sensory systems

Question 6

Draw a diagram highlighting the pathways of the brain that:

• plan/strategy of movement
• program/tactics of movement
• execution of movement

Answer 6

Question 7

What are the 5 areas of the brain that give rise to descending tracts?

Answer 7

• Cerebral cortex
  
  • Corticospinal tract (pyramidal)
• 4 in brain stem and medulla
  
  • Reticular formation
  • Vestibular nuclei
  • Red nucleus
  • Tectum

Question 8

Name the 4 extrapyramidal tracts

Answer 8

reticulospinal, vestibulospinal, rubrospinal, tectospinal

Question 9

What are the 2 main lateral motor pathways?

Answer 9

• Corticospinal tract (direct pathway)
• Rubrospinal tract (indirect pathway)

Question 10

Where do the majority of the corticospinal tracts originate and where do the majority of fibres cross the spinal cord?

Answer 10

• 2/3rds originate in motor cortex
• 80-90% of corticospinal tract fibres cross at medulla (pyramidal decussation) run length of spinal cord

Question 11

What is the main functions of the corticospinal tract?

Answer 11

• control of voluntary fine movements of distal muscles, particularly flexors
• control of axial muscles (posture)

Question 12

What happens to the fibres in the corticospinal tract that do not cross at the medulla?

Answer 12

remain ipsilateral abd run down the anterior corticospinal tract to cervical and upper thoracic segments

Question 13

Answer 13

Question 14

What are the three recognised areas within the cerebral motor cortex?

Answer 14

• Primary motor cortex (M1 or Broadmann’s area 4)
• Premotor cortex (Broadmann’s area 6)
• Secondary motor cortex (supplementary motor cortex, M2 or Broadmann’s area 6 or 8)

Question 15

How are the primary and secondary motor cortices arranged?

Answer 15

• somatotrophically
• Areas of the body with refined or complex motor abilities have disproportionately larger cortical areas

Question 16

What does the primary motor cortex control? And how does this differ from the secondary motor cortex?

Answer 16

Muscles of opposite side of the body

Secondary motor cortex can control muscles on both sides of the body

Question 17

Where does the rubrospinal tract originate and where does it recieve input from?

Answer 17

• Originates in red nucleus
• Input from motor cortex area somatotopically

Question 18

Where does the rubrospinal tract cross the spinal cord? And where does it terminate?

Answer 18

• Crosses at midbrain (ventral tegmental decussation)
• Terminate in interneurons of ventral (motor) horn in contralateral spinal cord

Question 19

What are the functions of the rubrospinal tracts?

Answer 19

• FAcilitates flexor motor neurons and inhibits extensor motor muscles
• Voluntary movements impaired following lesions (e.g. tremor, ataxia)
• Relatively small in humans

Question 20

Where do tectospinal tracts originate and where do they terminate?

Answer 20

• Originate in superior colliculus
• Terminate on interneurons in contralateral cervical region of spinal cord

Question 21

What is the function of tectospinal tracts?

Answer 21

Little is known about this pathway – BUT thought to control head movements in response to visual and auditory input

Question 22

Where do the Vestibulospinal tracts originate? And where fo they recieve input from

Answer 22

• Originate on vestibular nuclei - 2 parts (medial and lateral)
• recieve inhibitory input from cerebellum
• recieve excitatory input from vestibular apparatus

Question 23

What is the function of the lateral vestibulospinal tracts?

Answer 23

• Lateral uncrossed, terminate in interneurons on ipsilateral spinal cord
• Control extensor muscle motor tone of proximal limbs
• Maintenance of erect posture

Question 24

What is the function of the medial vestibulospinal tracts?

Answer 24

• medial bilateral, terminate on interneurons in the cervical region
• control neck muscles
• keep head upright when body moved

Question 25

What is the pathway of recticulospinal tracts?

Answer 25

largely uncrossed, terminate in interneurons within the spinal cord

Question 26

What is the general function of reticulospinal tracts?

Answer 26

* control muscles of the trunk and proximal limbs * maintenance of posture and stratle reactions

Question 27

What is the action of medullary (lateral) reticulsopinal tracts?

Answer 27

* inhibits extensor spinal reflex activity and facilitates flexor activity

Question 28

What is the action of medial (pontine) reticulospinal tracts?

Answer 28

facilitate extensor spinal activity

Question 29

What are the 5 main locations a motor pathway lesion can occur?

Answer 29

* Muscle * Neuromuscular junction * motor neurons * cerebellum * basal ganglia

Question 30

What is an upper motor neuron?

Answer 30

Cell bodies are in brain or brainstem and do not project outside the CNS Orchestrate complex directed movements

Question 31

What is a lower motor neuron?

Answer 31

Cell bodies are in the brainstem or spinal cord and project outside the CNS to muscle Single muscle innervation, cell bodies in the ventral horn of spinal cord or motor nuclei of the brain stem

Question 32

What is the function of interneurons?

Answer 32

Co-ordinate groups of muscles

Question 33

Describe the * muscle bulk * reflexes * tone * power * coordination * plantar response of a UMN lesion

Answer 33

* muscle bulk = normal * reflexes = increased * tone = increased * power = Reduced * Extensors in arm * Flexors in leg * coordination = normal * plantar response = extensor

Question 34

Describe the * muscle bulk * reflexes * power * coordination * plantar response of a LMN lesion

Answer 34

* muscle bulk = decreased * reflexes = absent * power = decreased * coordination = normal * plantar response = absent if leg/foot is involved in the lesion

Question 35

Describe the pattern of weakness in an upper motor neuron lesion

Answer 35

o Arm extensors weaker than flexors (flexors are stronger) o Leg flexors weaker than extensors (extensors are stronger)

Question 36

What is the function of lamina I-VI in the spinal cord?

Answer 36

Terminations for primary afferent sensory neurons and neurons of reflex circuits

Question 37

What is the function of lamina VI in the spinal cord?

Answer 37

sensory input from joints and muscles

Question 38

What is the function of lamina VIII and IX in the spinal cord?

Answer 38

cell bodies of motor neurons

Question 39

How are motor neurons arranged in the spinal cord?

Answer 39

* Motor neurons supplying muscle of trunk situated medially while those supplying distal muscles situated laterally * Muscles that flex limbs lie dorsal to those that extend limbs

Question 40

Where do α-motor neurons lie in the spinal cord?

Answer 40

-motoneurons’ cell bodies lie in clumps within ventral horn of spinal cord (lower motor neurons)

Question 41

What si the function of each motor neuron?

Answer 41

activates a motor unit (6-1500 skeletal muscle fibres)

Question 42

What is the action of Renshaw Cells?

Answer 42

* Suppresses weakly firing motor neurons and dampening strongly firing ones * Produces economical movement

Question 43

Give an example of a monosynaptic stretch reflex

Answer 43

Myotactic (knee jerk) reflex - tap of patellar tendon stretches quadriceps muscle and stimulates dynamic nuclear bag receptors of muscle spindle

Question 44

What leads to the contraction of the quadriceps muscle in a knee jerk reflex?

Answer 44

* Increase in rate of firing of group Ia afferents leads to contraction of quadriceps muscle * Ia fibres also stimulate inhibitory interneurons which inhibit antagonistic (flexor) muscle of knee joint

Question 45

What is the inverse myotactic (Golgi Tendon) Reflex?

Answer 45

* Contributes to maintenance of posture * During maintained posture (e.g. standing), quadriceps muscle will start to fatigue * Force in patellar tendon will decline, thus activity in afferent Ib fibres will decline * Normal inhibition of motoneurons supplying quadriceps will be removed * Muscle will contract more strongly, so increasing force in patellar tendon

Question 46

What are muscle spindles and how do they function?

Answer 46

* Main proprioceptors that provide information about the state of musculature * Muscle spindles lie within muscles in parallel with skeletal muscle fibres * Particularly numerous in fine motor control muscles (e.g. eyes, hands) * Innervated by gamma-motoneurons (efferents) and group Ia and II afferent fibres * Afferents respond to muscle stretch while gamma-efferent activity regulates the sensitivity of the spindle

Question 47

What is the main function of muscle spindles?

Answer 47

* Muscle spindles play important role as comparators for maintenance of muscle length during goal directed voluntary movements * Voluntary changes in muscle length initiated by motor areas of brain, orders include changes to set point of muscle spindle * Simultaneous activation of extrafusal fibres (-motoneurons) and intrafusal fibres (-motoneurons) is called alpha-gamma co- activation * This process readjusts sensitivity of muscle spindle continuously as muscle shortens

Question 48

What is the Golgi Tendon Organ and how does it function?

Answer 48

* Main proprioceptors that provide information about the state of musculature * Golgi tendon organs lie within tendons in series with contractile fibres * Respond to degree of tension within the muscle * Group Ib afferent fibres relay information to CNS (particularly spinal cord and cerebellum)

Question 49

What is the withdrawal reflex and crossed extensor reflex?

Answer 49

* Protective reflex of rapidly removing limb from damaging stimuli * Stimulation of withdrawal reflex, frequently elicits extension of the contralateral limb 250ms later * Helps maintain posture and balance

Question 50

What are the two main phases in the step cycle of locomotion?

Answer 50

* Stance phase * Foot touching ground, flexion of knee and ankle * Finishes with extension about all joints for forward movement * Swing phase * Bending (flexion) of hip, knee and ankle followed by knee and ankle straightening (extension) * Alternate swing and stance that are out of phase

Question 51

What is the Central Pattern Generator and how does it function?

Answer 51

* Located in spinal cord * Capable of autonomous signals * Modulated by proprioception input * Thought to be initiated by mesencephalic locomotor region * Output through reticular nuclei and reticulospinal tracts

Question 52

Describe the Central Pattern Generator for walking

Answer 52

* 2 half centres which activate flexors and extensors respectively and which mutually inhibit each other * Can be modelled using inhibitory 1a interneurons and Renshaw cells (RC)