motor system Flashcards
- What is the difference between lower and upper motor neurons?
Lower Motor Neurons
• Alpha and gamma motor neurons
• Brain stem (cranial nerves)
• Ventral horn of the spinal cord (spinal
nerves)
• Final common pathway for motor commands.
Upper motor neurons
• Cerebral cortex, reticular formation and
vestibular nuclei
• Project down to lower motor neurons in spinal
cord and cranial nerve nuclei.
- What is a muscle spindle and how does it respond to changing muscle length?
Intrafusal muscle fibers, parallel to extrafusal, surrounded by CT
• Spindles convey changes in muscle length to spinal cord.
• Ia (primary) & II (secondary) afferents detect changes in length and velocity
• Stretch spindle → Ia & II afferents → α-motor neuron → contract same & synergistic muscles
-Na/Ca entry upon mechanically opening channels generates a depolarizing generator potential which in turn initiates action potentials in the axon.
- Describe the myotatic reflex.
Maintain muscle length: Increase in muscle length triggers
homeostatic, negative feedback to maintain limb in constant position.
• Stretch spindle → Ia & II afferents → α-motor neuron → contract same & synergistic muscles
• Reciprocal innervation: Spindle afferents stimulate inhibitory interneurons to relax antagonist muscles.
- Patellar tap tests integrity of myotatic reflex circuit
- Jendrassik maneuver (clenching hands)
How does gamma motor neurons activity affect myotatic reflexes?
Regulate spindle sensitivity during movement
• Gamma motor neurons innervate muscle ends of spindles.
• Tautening of spindle by the gamma neuron increases sensitivity of Ia and II afferent neurons to muscle stretch.
-Gamma motor neurons controlled by brain/brain stem
- What is a Golgi tendon organ and how does it respond to muscle tension?
Muscle tension regulated by Golgi tendon organs, GTO
• GTO located in muscle tendons
• Not innervated by gamma motor neurons
Is it regulated by gamma motor neurons?
Not innervated by gamma motor neurons
- Describe propriospinal neurons and their action on spinal reflexes.
Propriospinal neurons interconnect spinal levels for
complex actions and regulating reflexes
• Located in intermediate spinal grey matter.
• Axons in peripheral border of the ventral horn.
Medial propriospinal neurons
• Axons extend over whole spinal cord
-proximal muscles, whole body posture
Lateral propriospinal neurons
• Extend fewer spinal segment
• Regulate distal muscles for independent control of
fine movements.
-distal muscles, fine movements
- What are some general functions of upper motor neurons?
•Control Posture, Balance & Movements • Brain stem pathways maintain posture and balance • Motor cortex pathways regulate fine movements in extremities.
- Describe the motor control functions of the vestibulospinal and reticulospinal tracts.
Motor related tracts in white matter (descending):
Vestibulospinal tract: nuclei in medulla relay head movement activity from semicircular duct, utricle and saccule receptors to spinal cord.
• Activates extensors of lower limb and flexors of upper limb.
• Maintain posture and balance.
Reticulospinal tracts (medialand lateral: Regulate muscle tone and sensitivity of flexor responses • Integrate information to coordinate complex actions, such as orienting, stretching, and maintaining a complex posture.
What is the antigravity posture and what produces it?
medial reticulospinal tract…“Anti-Gravity” Posture: Extension of the lower limbs & flexion of the upper limbs.
due to loss of cortical control over reticulo- and
vestibulospinal tracts.
- Describe the three parts of the corticospinal tract system (corticospinal, corticonuclear corticoreticular) and what they do.
Descending motor pathways–>Motor cortex (primary, premotor, supplementary) forms 3 tracts:
- Corticospinal tracts to the ventral and lateral horn of spinal cord: •Internal capsules, cerebral pedncles, pyramids in medulla, projects ti alpha snd gamma neurons in spinal cord•
2.Corticonuclear tract to cranial nerve motor nuclei: Controls muscles of the face, head and neck
• Project to the motor neurons of the cranial nerves.
3• Corticoreticular tract to pontine and medullary reticular formation: Primary motor & premotor cortices project to pontine and medullary reticular formation
• Smoothes out general movements by limiting inhibition among extensor muscles of the lower limb.
• Breaks up stereotypic patterns generated in the reticular formation.
- What is the significance of the internal capsule? The medullary decussation?
Internal capsule:in corticospinal tract…Bundle of axons in cerebral hemispheres is a common site of strokes
medullary decussation: Pyramids in medulla
• Decussation forms lateral and ventral (anterior) corticospinal tracts
- How do the anterior and later corticospinal tracts differ in structure and function?
LATERAL CORTICOSPINAL TRACT:
• 90% crossed in medulla descends in lateral funiculus
• Fine control of movement
• From large homuncular areas for face, hands
VENTRAL (ANTERIOR) CORTICOSPINAL TRACT:
• 10% uncrossed, but descends bilaterally in ventral funiculus
• Posture of neck and trunk
- Compare lower motor neuron and upper motor neuron disease.
LOWER MOTOR NEURON DISEASE:
• Lesion of alpha motor neurons interrupts neural input to the muscles.
• Flaccid paralysis and atrophy of muscle, eg polio
UPPER MOTOR NEURON DISEASE
Spasticity & other conditions
STROKE
1. Decrease in fine control of extremities due to disruption of the lateral corticospinal tract.
What are the aspects of spasticity?
Spasticity (=spastic paralysis) involves active, but inappropriate, contraction of muscles. due to disruption of cortical projection to reticular formation
- Compare the functions of the primary, supplementary and pre-motor cortices.
PRIMARY MOTOR CORTEX
• Encodes force, direction, extent and speed of movements
• Activates small groups of muscles for discrete movements.
• Motor somatotopy in a homunculus similar to that of
somatosensory cortex
• Somatotopy not fixed
SUPPLEMENTAL
motor association areas
Ensures correct motor sequences independent of external conditions
involved in the transformation of kinematic to dynamic information
PREMOTOR CORTEX
• Integrate spatial and sensory information, abstract rules in the planning & preparation of movement
• Premotor neurons encode intention to perform a particular movement
• Involved in the selection of movements based on external or internal (memory) events.
• Broca’s area: part of premotor cortex that controls motor preparation for speech.
• Sensitive to the behavioral context of a particular movement
- What is efference copy? How does it play a role in self awareness?
motor cortex sends “copies” of motor programs back to
somatosensory cortex (efference copy)
–> integrate with incoming sensory information to predict the expected sensation that will occur
–> assess present and future states of a limb in action
- What are mirror neurons and where are they located?
• Mirror neurons unify action perception and action execution
• Neurons in mirror regions respond to both observed and executed actions
–>dorsal sector of inferior prefrontal cortex (pars opercularis)
- Describe the pathway from cerebellar cortex to motor cortex and back.
tactile and proprioception Sensory input innervates granule
cells in cerebellar cortex –> Local interneurons transform activity into meaningful response–>Purkinje cells transmit output to local deep cerebellar nuclei–>Nuclei send their information to thalamus–>motor cortex
- Compare the location and functions of the vestibulocerebellum, spinocerebellum, and cerebrocerebellum.
VESTIBULOCEREBELLUM – vestibular nuclei: Receives input from the vestibular system
• Controls posture, balance & eye movements
-enhances VOR
SPINOCEREBELLUM: provides feedback control of
ongoing movements
fastigial & interposed nuclei
•
• Cerebrocerebellum -
designed to initiate voluntary movement by projecting anticipatory information to the motor cortex (feed forward)
Learning: movements to become more rapid, precise and automatic with practice.
How do the vestibulocerebellum, spinocerebellum, and cerebrocerebellum differ in their connections with the motor cortex and spinal cord?
VESTIBULOCEREBELLUM:• Projects to vestibular nuclei in the medulla
SPINOCEREBELLUM:Vermis and intermediate areas of cerebellum project to fastigial and interposed nuclei. Nuclei project to thalamus-cortex – • Receive input from:
• Spinal cord
• Cerebral cortex via pontine nuclei
Cerebrocerebellum: dentate nuclei: Cerebellar hemispheres –> dentate nuclei –> thalamus-cortex
• Cerebral cortex (wide areas)–>pontine nuclei –> cerebellar cortex
How do the vestibulocerebellum, spinocerebellum, and cerebrocerebellum participate in motor learning?
Spinocerebellum: provides feedback control of ongoing movements. It compares inputs from spinal cord and motor
cortex to detect disparities between internal and external representations of world.
VESTIBULOCEREBELLUM:Interconnects with vestibular and oculomotor nuclei to regulate head movements and eye coordination
Cerebrocerebellum:Continued communication between
cerebellum and motor cortex enables movements to become more rapid, precise and automatic with practice.
• Learned motor patterns enable the lateral cerebellar hemispheres to predict the speed, force and direction of a limb prior to movement execution.
- What are dysmetria
Inability to control range of movement. Placement falls short of or extends beyond the initial goal, as in the finger to nose test
ataxia
Ataxia is the lack of smoothly coordinated movements. It is the combined result of dysmetria and decomposition of movement. Movements are imprecise, halting, awkward, and clumsy.
Dysarthria
Inability to articulate words correctly, with slurring and inappropriate phrasing
Dysdiadochokinesia
: Inability to perform rapid alternating movements
“intention tremor”?
Rhythmic, alternating, oscillatory movement of a limb as it approaches a target
- What roles does the cerebellum play in cognition and visceral functions?
cognitive–>Posterior part of cerebellum: lateral hemisphere and vermis are involved in cognitive regulation.
Prefrontal and association cortices project cognitive patterns to the pons to be mapped onto the cerebellar cortex
Somato-visceral functions of cerebellum
• Cerebellar cortex, via all deep nuclei, project to both brain stem (reticular formation, vestibular nuclei) and hypothalamus to regulate various visceral systems
Vermis dysfunctions can lead to bradycardia…
