Descending Pathways Flashcards
What are the classes of movement?
• Voluntary:
- complex actions e.g. reading and writing
- purposeful, goal directed
- learned
- Reflexes
- Rhythmic motor patterns
- combines voluntary and reflexive acts
- initiation and termination is voluntary
- once initiated, repetitive and reflexive
What are the functions of motor control systems?
- Posture and balance
- Goal directed movements
- Communication
What areas of the brain give rise to descending tracts?
• Corticospinal tract from cerebral cortex From brainstem and medulla: • Reticulospinal • Vestibulospinal • Rubrospinal • Tectospinal
Where does the corticospinal tract originate?
2/3 originate in the motor cortex
Where do the fibres of the corticospinal tract cross?
- Lateral corticospinal crosses at the medulla (pyramidal decussation)
- Anterior corticospinal remains ipsilaterally to cervical and upper thoracic segments
What is the function of the lateral corticospinal tract
- Runs the length of the spinal cord
* Control voluntary fine movements of distal muscles, particularly flexors
What is the function of the anterior corticospinal tract?
Control axial muscles
What does Jacksonian epilepsy highlight?
- Twitching begins at the extremities then movement of the hand and then of the arm
- Reflects the spread of excitation over the cortex from epileptic focus
- Shows the somatotropic arrangement
What is the difference in which muscles can be controlled by the primary and secondary motor cortices?
- Primary controls muscles of opposite side of the body
* Secondary controls muscles on both sides
Where does the rubrospinal tract originate?
In the red nucleus in the midbrain
Where do the fibres of the rubrospinal tract cross?
Midbrain - ventral tegmental decussation
Where do the fibres of the rubrospinal tract terminate?
On interneurones of ventral (motor) horn in the contralateral spinal cord
What is the function of the rubrospinal tract?
Facilitates flexor motor neurones and inhibits extensor motor muscles
Where does the tectospinal tract originate?
In the superior colliculus
Where do the fibres in the tectospinal tract terminate?
In interneurones in the contralateral cervical region of the spinal cord
What is the function of the tectospinal tract?
Controls head movements in response to visual and auditory input
Where does the vestibulospinal tract originate?
In vestibular nuclei
What inputs does the vestibulospinal tract receive?
- Inhibitory input from the cerebellum
* Excitatory from the vestibular apparatus
Lateral vestibulospinal tract
- Fibres don’t cross, terminate on interneurones in ipsilateral spinal cord
- Control extensor muscle motor tone of the proximal limbs
- Maintenance of erect posture
Medial vestibulospinal tract
- Bilateral
- Terminate on interneurones in the cervical region
- control the neck muscles
- Keep head upright when body moves
Where does the reticulospinal tract originate?
Pontine reticular formation
Where do the fibres of the reticulospinal tract terminate?
Largely uncrossed, terminate on interneurons within the spinal cord
What is the function of the reticulospinal tract?
Control muscles of the trunk and proximal limbs
• Medullary (lateral) tracts inhibit extensor spinal reflex activity and facilitates flexor activity
• Medial (pontine) tracts facilitate
• Maintenance of posture and startle reactions
What may be caused by damage to the reticulospinal tract?
Spasticity
Muscle motor pathway lesion
- Myositis, muscular dystrophy
- Normal reflexes
- Weakness/ wasting
Neuomuscular junciton motor pathway lesion
- Myasthenia gravis
- Fatigueable weakness
- Normal reflexes
- Normal muscle bulk
Motor neurone motor pathway lesion
- UMN: CVA, spinal cord trauma
* LMN: motor neurone disease, neuropathy
Cerebellum motor pathway lesion
- Incoordinaiton, multiple sclerosis
- Normal reflexes and strength
- Slight change in tone
Basal ganglia motor pathway leison
- Parkinsons, Huntington’s
* Movement changes
Describe upper motor neurones
- Cell bodies are in the brain or brainstem and do not project outside of the CNS
- orchestrate complex directed movements
Describe lower motor neurones
- 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 the spinal cord or motor nuclei of the brainstem
Muscle bulk in UMN vs LMN lesion
- UMN - normal
* LMN - decreased
Reflexes in UMN lesion vs LMN lesion
- UMN - increased
* LMN- absent
Tone in UMN lesion vs LMN lesion
- UMN - increased
* LMN - decreased
Power in UMN lesion vs LMN lesion
- UMN - Reduced extensors in arm, flexors in leg
* LMN - decreased
Co-ordination in UMN vs LMN lesion
- UMN - normal
* LMN - normal
Plantar response in UMN lesion vs LMN lesion
- UMN - extensor (normal is a flexed response)
* LMN - absent if leg/foot involved in lesion
What is a normal plantar response - Babinski
Toes go down (flexion)
Lamina I-VI
Terminations for primary afferent sensory neurones and neurones of reflex circuits
Lamina VI
Sensory input from joints and muscles
Lamina VII and IX
Cell bodies of motor neurones
Describe the location of motor neurones supplying the trunk vs those supplying distal muscles
- Those supplying the trunk are situated medially
* Those supplying the distal muscles are situated laterally
Describe the location of motor neurones that flex the limbs vs those that extend the limbs
Motor neurones that flex the limbs lie dorsal to those that extend the limbs
What are Renshaw cells and what do they do?
- Interneurones
* Suppresses weakly firing motor neurones and dampens strongly firing ones to produce economical movement
What reflects the importance of Renshaw cells?
- Strychnine poisoning
- Disables Renshaw cells
- Leads to convulsions
Describe the Myotatic reflex (knee jerk)
- Monosynaptic stretch reflex
- Tap of the patellar tenon stretches the quadriceps muscle
- Stimualtes nuclear bag receptors of the muscle spindle
- Increases the rate of firing of group IA afferent fibres leading to the contraction of the quadriceps muscle
- The IA fibres also stimulate inhibitory interneurones which inhibit antagonistic (flexor) muscles of the knee joint
When is the knee jerk reflex lost?
If there is damage to the lower lumbar dorsal roots of the spinal cord
What is the step cycle
• Stance phase:
- foot touching the ground, flexion of the 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)
• Alternates between the swing and stance phase
What is the central pattern generator
- Located in the spinal cord
- Capable of autonomous signals
- Modulated by proprioception input
- Thought to be initiated by mesencephalic locomotor region - output thought to be through reticular nuclei and reticulospinal tracts
