Descending Pathways Flashcards
What are the three main classes of movement?
- Voluntary
- Reflexes
- Rhythmical motor patterns
Describe voluntary movement
- Complex actions (reading, writing)
- Purposeful goal directed
- Learned
Describe the movement involved in reflexes
Involuntary, rapid, stereotyped (knee jerk, eye blink)
Describe the movement of rhythmical motor patterns
- Combines voluntary and reflexive acts (chewing, walking, running)
- Initiation and termination voluntary
- Once initiated repetitive and reflexive
What are the functions of motor control systems and what are they guided by?
- posture and balance
- goal-directed movements
- communication
Guided by sensory systems
Draw a diagram highlighting the pathways of the brain that:
- plan/strategy of movement
- program/tactics of movement
- execution of movement
What are the 5 areas of the brain that give rise to descending tracts?
- Cerebral cortex
- Corticospinal tract (pyramidal)
- 4 in brain stem and medulla
- Reticular formation
- Vestibular nuclei
- Red nucleus
- Tectum
Name the 4 extrapyramidal tracts
reticulospinal, vestibulospinal, rubrospinal, tectospinal
What are the 2 main lateral motor pathways?
- Corticospinal tract (direct pathway)
- Rubrospinal tract (indirect pathway)
Where do the majority of the corticospinal tracts originate and where do the majority of fibres cross the spinal cord?
- 2/3rds originate in motor cortex
- 80-90% of corticospinal tract fibres cross at medulla (pyramidal decussation) run length of spinal cord
What is the main functions of the corticospinal tract?
- control of voluntary fine movements of distal muscles, particularly flexors
- control of axial muscles (posture)
What happens to the fibres in the corticospinal tract that do not cross at the medulla?
remain ipsilateral abd run down the anterior corticospinal tract to cervical and upper thoracic segments
What are the three recognised areas within the cerebral motor cortex?
- 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)
How are the primary and secondary motor cortices arranged?
- somatotrophically
- Areas of the body with refined or complex motor abilities have disproportionately larger cortical areas
What does the primary motor cortex control? And how does this differ from the secondary motor cortex?
Muscles of opposite side of the body
Secondary motor cortex can control muscles on both sides of the body
Where does the rubrospinal tract originate and where does it recieve input from?
- Originates in red nucleus
- Input from motor cortex area somatotopically
Where does the rubrospinal tract cross the spinal cord? And where does it terminate?
- Crosses at midbrain (ventral tegmental decussation)
- Terminate in interneurons of ventral (motor) horn in contralateral spinal cord
What are the functions of the rubrospinal tracts?
- FAcilitates flexor motor neurons and inhibits extensor motor muscles
- Voluntary movements impaired following lesions (e.g. tremor, ataxia)
- Relatively small in humans
Where do tectospinal tracts originate and where do they terminate?
- Originate in superior colliculus
- Terminate on interneurons in contralateral cervical region of spinal cord
What is the function of tectospinal tracts?
Little is known about this pathway – BUT thought to control head movements in response to visual and auditory input
Where do the Vestibulospinal tracts originate? And where fo they recieve input from
- Originate on vestibular nuclei - 2 parts (medial and lateral)
- recieve inhibitory input from cerebellum
- recieve excitatory input from vestibular apparatus
What is the function of the lateral vestibulospinal tracts?
- Lateral uncrossed, terminate in interneurons on ipsilateral spinal cord
- Control extensor muscle motor tone of proximal limbs
- Maintenance of erect posture
What is the function of the medial vestibulospinal tracts?
- medial bilateral, terminate on interneurons in the cervical region
- control neck muscles
- keep head upright when body moved
What is the pathway of recticulospinal tracts?
largely uncrossed, terminate in interneurons within the spinal cord
What is the general function of reticulospinal tracts?
- control muscles of the trunk and proximal limbs
- maintenance of posture and stratle reactions
What is the action of medullary (lateral) reticulsopinal tracts?
- inhibits extensor spinal reflex activity and facilitates flexor activity
What is the action of medial (pontine) reticulospinal tracts?
facilitate extensor spinal activity
What are the 5 main locations a motor pathway lesion can occur?
- Muscle
- Neuromuscular junction
- motor neurons
- cerebellum
- basal ganglia
What is an upper motor neuron?
Cell bodies are in brain or brainstem and do not project outside the CNS
Orchestrate complex directed movements
What is a lower motor neuron?
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
What is the function of interneurons?
Co-ordinate groups of muscles
Describe the
- muscle bulk
- reflexes
- tone
- power
- coordination
- plantar response
of a UMN lesion
- muscle bulk = normal
- reflexes = increased
- tone = increased
- power = Reduced
- Extensors in arm
- Flexors in leg
- coordination = normal
- plantar response = extensor
Describe the
- muscle bulk
- reflexes
- power
- coordination
- plantar response
of a LMN lesion
- muscle bulk = decreased
- reflexes = absent
- power = decreased
- coordination = normal
- plantar response = absent if leg/foot is involved in the lesion
Describe the pattern of weakness in an upper motor neuron lesion
o Arm extensors weaker than flexors (flexors are stronger)
o Leg flexors weaker than extensors (extensors are stronger)
What is the function of lamina I-VI in the spinal cord?
Terminations for primary afferent sensory neurons and neurons of reflex circuits
What is the function of lamina VI in the spinal cord?
sensory input from joints and muscles
What is the function of lamina VIII and IX in the spinal cord?
cell bodies of motor neurons
How are motor neurons arranged in the spinal cord?
- 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
Where do α-motor neurons lie in the spinal cord?
-motoneurons’ cell bodies lie in clumps within ventral horn of spinal cord (lower motor neurons)
What si the function of each motor neuron?
activates a motor unit (6-1500 skeletal muscle fibres)
What is the action of Renshaw Cells?
- Suppresses weakly firing motor neurons and dampening strongly firing ones
- Produces economical movement
Give an example of a monosynaptic stretch reflex
Myotactic (knee jerk) reflex - tap of patellar tendon stretches quadriceps muscle and stimulates dynamic nuclear bag receptors of muscle spindle
What leads to the contraction of the quadriceps muscle in a knee jerk reflex?
- 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
What is the inverse myotactic (Golgi Tendon) Reflex?
- 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
What are muscle spindles and how do they function?
- 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
What is the main function of muscle spindles?
- 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
What is the Golgi Tendon Organ and how does it function?
- 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)
What is the withdrawal reflex and crossed extensor reflex?
- 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
What are the two main phases in the step cycle of locomotion?
- 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
What is the Central Pattern Generator and how does it function?
- 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
Describe the Central Pattern Generator for walking
- 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)
