Descending Pathways Flashcards
What are the different classes of movement?
- Voluntary
- Reflexes
- Rhythmic motor patterns
What are the 3 phases of movement/motor control?
- Plan/strategy
- Programme/tactics
- Execution
What are the 5 areas of the brain which give rise to descending tracts?
- Corticospinal tract (pyramidal) (cerebral cortex)
- Reticular formation (brain stem + medulla)
- Vestibular nuclei (brainstem + medulla)
- Red nucleus (brainstem + medulla)
- Tectum (brainstem + medulla)
What are the extrapyramidal tracts?
- Reticulospinal (reticular formation)
- Vestibulospinal (vestibular nuclei)
- Rubrospinal (red nucleus)
- Tectospinal (tectum)
What percentage of corticospinal tract fibres originate in the motor cortex?
66%
What percentage of corticospinal tract fibres cross at the medulla (pyramidal decussation)?
80-90%
In what area are the arm fibres in comparison to the leg fibres located in the spinal tract?
The right (more lateral in spinal cord) (more medial in brainstem)
What do the corticospinal fibres which do not cross (remain ipsilateral) control?
Cervical and upper thoracic segments (control axial muscles)
In what lobe is the primary auditory complex found?
Temporal
What are the 3 areas recognised within the cerebral motor cortex?
- Primary motor cortex (M1 or Brodmann’s area 4)
- Premotor cortex (Brodmann’s area 6)
- Secondary motor cortex (supplementary motor cortex, M2 or Brodmann’s area 6 and 8)
What does Jacksonian epilepsy illustrate?
Primary and secondary motor cortices are somatotropically arranged
- Twitching begins at extremities then moves to hand then arm etc.
Does the secondary motor cortex control the muscles of the opposite side?
Can control muscles of both sides
Where do the rubrospinal tracts originate?
Red nucleus
Where do the fibres of the rubrosoinal tracts terminate?
Interneurons of ventral (motor) horn in contralateral spinal cord
Where do the rubrospinal tracts receive there input from?
Motor cortex area somatotropically
Where do tectospinal tracts originate?
Superior colliculus
Where do fibres of the tectospinal tract terminate?
Interneurons in the contralateral cervical region of the spinal cord
What is the tectospinal tract thought to control?
Head movements in response to visual and auditory input
Where do the vestibulospinal tracts originate?
Vestibular nuclei
Where do the vestibulospinal tract fibres receive inibitory and excitatory input from?
- Inhibitory - Cerebellum
- Excitatory - vestibular apparatus
Where do the vestibulospinal tract fibres terminate?
Lateral uncrossed, terminate on interneurons on ipsilateral spinal cord
Where do reticulospinal tracts originate?
Reticular formation
Where do reticulospinal tracts terminate?
Largely uncrossed, terminate on interneurons within spinal cord
WHat do reticulospinal tracts control?
- Muscles of trunk and proximal limbs
- Maintain posture and startle reactions
- Damage may lead to spasticity
What do the medullary lateral reticulospinal tracts inhibit?
Extensor spinal reflex activity and facilitates flexor activity
What do the Pontine (medial) reticulospinal tracts facilitate?
Extensor spinal reflex activity
What is the function of the medial motor pathways?
Maintenance of posture and startle reactions
What can damage to the medial motor pathways lead to?
Spasticity
What is the usual clinical presentation of a motor pathway lesion in the muscle itself (e.g myositis/muscular dystrophy)?
- Normal reflexes
- Weakness/wasting
What is the usual clinical presentation of a motor pathway lesion at the neuromuscular junction (e.g myasthenia gravis)?
- Fatigable weakness
- Normal reflexes
- Normal muscle bulk
Where can motor pathway lesions take place?
- Muscle
- Neuromuscular junction
- Motor neurons (UMN, LMN)
- Cerebellum
What afre the usual clinical presentations of an individual with a motor pathway lesion in the cerebellum?
Normal reflexes, strength, slight decrease in tone
What are the usual clinical presentations with an individual with a motor pathway lesion in the basal ganglia (e.g Parkinson’s, Huntington’s)?
Movement changes
Where are upper motor neuron’s cell bodies located?
In brain or brainstem and do not project outside CNS
Where are lower motor neuron’s cell bodies located?
Brainstem or spinal cord and project outside the CNS to muscle
What happens to muscle bulk in an UMN lesion compared to a LMN lesion?
UMN - normal
LMN - decreased
What ahppens to reflexes in UMN and LMN lesions?
UMN - increased
LMN - Absent
What is the effect of a UMN on power?
- Reduced extensors in arm
- Reduced flexors in leg
What is the effect of a LMN lesion on power?
Decreased
What happens in the Plantar response / Babinksi test if the patient has a UMN lesion?
Extension
What happens in the Plantar response / Babinksi test if the patient has a LMN lesion?
Absent if leg/foot involved in lesion
What does hemiplegia mean?
Paralysis of limbs on one side of the body
What are the patterns of weakness in an UMN lesion?
- Arm extensors weaker than flexors (flexors are stronger)
- Leg flexors weaker than extensors (extensors are stronger)
- Causes bent arm and walking on tip toes
More distal muscles’ motor neurons are situated where in the spinal cord?
More laterally
The neurons of extensors are situated where in comparison with flexors?
Extesnors - ventral
Flexors - dorsal
Where are alpa-motorneuron’s cell bodies located?
Clumps within ventral horn of spinal cord (LMNs)
What does each motorneuron activate?
A motor unit (6-1500 skeletal muscle fibres)
What are the interneurons which cause recurrent or feedback inhibition called?
Renshaw cells (suppress weakly firing motor neurons and dampening strongly firing ones, produces economical movement)
What is the overall purpose of Renshaw cells?
Helps make smooth, sustained movement, removes certain jerkiness
What fibres are involved in the myotatic (knee jerk) reflex?
Ia fibres - muscle spindles
What are muscle spindles?
Main proprioceptorsthat provide information about the state of musculature
Sensory muscle fibre
What are the muscle spindles motor input called?
Gamma input
What are the sensory muscle spindle fibres called?
Ia and II
Where are muscle spindles particularly numerous?
Fine motor control muscles
How long is a muscle spindle?
1cm
What is the purpose of the motor input (gamma) on the muscle spindles?
Allows muscle spindle to detect changes in length irrespective of what is happening to the muscle itself
What is the main role of muscle spindle?
Used as comparators for maintenance of muscle length during goal directed voluntary movements
What is alpha gamma coactivation?
Simultaneous activation of extrafusal fibres (alpha-motorneurons) and intrafusal fibres (gamma-motorneurons) (whenever muscle is contracted, muscle spindle also is)
What info does the golgi tendon provide?
Force of contraction in a muscle (tension generated in muscle)
Where is the golgi tendon situated?
Within the tendon
What is the purpose if the inverse myotactic (golgi tendon) reflex and how does it work?
- Contributes to maintenance of posture
- Increase in tension causes increase in firing to dorsal root - inhibits extensor and stimulates flexor
- Prevents overuse of force in muscle and ripping of muscle
- When tension is generated nerve fibre is squashed setting of signals
What does the withdrawal reflex and crossed extensor reflex allow for?
- Protective reflex of rapidly removing limb from damaging stimuli
- Simulation of withdrawal reflex, frequently elicits extension of the contralateral limb 250ms later
- Helps maintain posture and balance
What is the Central Pattern Generator (CPG)?
- 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)
