Motor Systems - Descending tracts Flashcards
Describe the difference between an upper and lower motor neurone:
Upper:
- supraspinal neurones that arise above the decussation of
the pyramids - they innervate the LMNs of the brainstem and spinal cord
- brain’s command and control
- upper motor neurones descend and influence lower motor neurones
Lower:
- located in the motor nuclei of the brainstem + anterior horn of spinal cord
- spinal cord command + control
Describe the organisation of the motor systems (pyramidal and extrapyramidal):
Pyramidal tracts:
- corticospinal tracts
- corticobulbar tracts
Extra-pyramidal tracts:
- vestibulospinal tracts
- reticulospinal tracts
- rubrospinal tracts
- tectospinal tracts
Describe the corticospinal tracts (anterior + lateral):
Corticospinal tract:
- arise in primary & premotor areas of frontal lobes & post central gyri of parietal lobes
- travel with corticobulbar fibres through corona radiate
and converge in posterior limb of internal capsule - descend in contralateral lateral white column
Lateral Corticospinal:
- fibres terminate in anterior grey column of all spinal cord segments
- target neurones in lateral anterior horn
- responsible for innervation of distal muscle groups
- allows precise skilled movements
Anterior Corticospinal:
- fibres descend ipsilaterally and cross the midline at the segmental level which they terminate at
- terminate on LMN controlling trunk and proximal musculature
- below L2 the only innervations to lower motor neurons is from the lateral corticospinal tract
- important for bilateral postural adjustments
Describe the Corticobulbar / nuclear tract:
Arises from lateral aspect of primary motor cortex
UMNs to cranial nerve nuclei
Descend through the corona radiata and converge in the internal capsule
Synapse directly/indirectly with cranial nerves
Responsible for motor control of the face and most innervate neurones bilaterally
A subset of fibres terminate in reticular formation, superior
colliculus and red nucleus helping co-ordinate cortical and brainstem motor systems
Describe the rubrospinal tract:
Arises from the red nucleus in tegmentum of midbrain
Cross the midline in the ventral tegmentum decussation and descend to
enter lateral white column of spinal cord
Receive afferent input from 1O and premotor cortex and cerebellum
Humans rubrospinal tract facilitates neurons that innervate flexor muscles
of upper limb and inhibit extensor antigravity muscles
Both rubrospinal and corticospinal tracts activate muscles that act against
gravity are involved in the control of fine movement
In humans it plays back up more of an integrator and relay nucleus in
cerebellar circuits
The corticospinal and rubrospinal tracts lie in the lateral columns of the
spinal cord and terminate on the interneurons and motor neurones that
control the distal musculature of the limbs.
Both tracts are referred to as the lateral motor system of the spinal cord.
Describe the tectospinal tract:
Arises in superior colliculus of midbrain
Crosses over to the contralateral side in the dorsal tegmental decussation
Descends through the anterior white column close to the anterior median fissure to cervical levels of the spinal cord
Innervates motor neurone responsible for neck movements
It is thought to orientate the head and neck during eye movements in response to visual stimuli that reach the superior colliculus
Describe the medial/ pontine reticulospinal tract:
Arises from large cells in medial part of reticular formation within the pons
Descends through the anterior white column to all levels of the spinal cord
Facilitates voluntary or cortically controlled movements
Increases motor tone
Excites anti gravity muscles
Describe the lateral/ medullary reticulospinal tract:
Arises from cells in the medial two thirds of the medulla oblongata
Descends through lateral white column of the spinal cord
Opposes the pontine reticulospinal tract
Inhibits voluntary movement and decreases motor tone
Receives input from corticospinal tract, rubrospinal tract etc.
Counterbalances signals from the pontine reticulospinal tract
Describe the medial vestibulospinal tracts:
Also known as descending medial longitudinal fasciculus
Descends bilaterally through the brainstem and travels in the anterior white column of spinal cord to cervical and upper thoracic regions
Activate LMNs associated with spinal accessory nerve
Influences rotation and lifting of the head and rotation of the
shoulder
Important in changes to maintain posture and balance, keeping the head stable while walking
Describe the lateral vestibulospinal tract:
Arises from lateral vestibular nucleus
Descends ipsilaterally in brainstem and projects to all levels of spinal cord
Facilitates α and γ motor neurons that innervate extensor muscles and inhibits activity of flexor muscles
Modulated by activation of vestibular apparatus (nuclei) and cerebellum
Mediates postural adjustments to compensate for movements and changes in position of the body and co-ordinate the orientation of the head and body
Describe the sensory feedback from the muscle spindle, Golgi tendon organ system and how they function:
Muscle spindle is a stretch receptor consisting of several specialised muscle fibres in a fibrous capsule deep within skeletal muscle
Primary (Type Ia) Afferent Fibers: Wrap around the central region of intrafusal fibers and are highly sensitive to both the rate and magnitude of muscle length changes. They provide rapid feedback on dynamic muscle stretch
Secondary (Type II) Afferent Fibers: Attach to the ends of intrafusal fibers and primarily detect static muscle length, offering information about the muscle’s sustained position
The activation of alpha and gamma motor neurons has opposite effects on Ia output; alpha activation alone decreases Ia activity while gamma activation alone increases Ia activity
Golgi tendon organs also provide proprioceptive inputs from the muscles as they act like sensitive strain gauges as they monitor muscle tension or force of contraction
located at the junction of the muscle and the tendon and are innervated by Ib sensory axons that entwine among collagen fibrils
GTOs monitor and regulate muscle tension, preventing potential damage from excessive force by inhibiting the contracting muscle and stimulating its antagonist
Describe the relative position of the motor tracts within the white matter of the spinal cord
Lateral corticospinal - posterior part of the lateral funiculus
Anterior corticospinal - medial part of the anterior funiculus
Tectospinal - medial position in the anterior funiculus
Rubrospinal - Situated in the lateral funiculus, ventral to the lateral corticospinal tract
Medial Reticulospinal - anterior funiculus
Lateral reticulospinal - lateral funiculus
Lateral vestibulospinal - Descends along the periphery of the anterior funiculus
Medial vestibulospinal - anterior funiculus
Describe the relative positions within the spinal cord gray matter responsible for the control of the axial, proximal and distal skeletal muscles:
Medial Region of the Anterior Horn:
Function: Innervates axial muscles responsible for movements of the trunk and head.
Location: Situated in the medial portion of the anterior horn.
Intermediate Region of the Anterior Horn:
Function: Controls proximal limb muscles, facilitating movements of the shoulders, upper arms, hips, and thighs.
Location: Positioned between the medial and lateral regions of the anterior horn
Lateral Region of the Anterior Horn:
Function: Manages distal limb muscles, enabling fine motor skills in the forearms, hands, lower legs, and feet
Location: Found in the lateral portion of the anterior horn
Describe the Dorsal (posterior) spinocerebellar tract:
1st order neurons of this tract innervate muscle spindles (lesser extent GT organs)
Synapse with 2nd order neurons in the nucleus dorsalis of Clarke (T1/C8-L2)
Ascend ipsilaterally to the inferior cerebellar peduncle
Terminate in ipsilateral
cerebellar vermis
This tract transmits signals
form ipsilateral caudal body
and lower limbs (non conscious proprioception)
Cerebellum coordinates and
integrates neural signals to control movement
Damage to this tract results in the loss of nonconscious proprioception and coordination ipsilateral to the lesion
Describe the cuneocerebellar tract:
Above C8 ascend ipsilaterally in the fasciculus cuneatus
Project accessory cuneate nucleus
Give rise to the cuneocerebellar tract
Conveys nonconscious proprioception from the upper limb .
Enter the cerebellum through the inferior cerebellar peduncle (posterior external arcuate fibres)
Terminate in the ipsilateral cerebellar cortex and convey nonconscious proprioception information about the upper limb.
Describe the ventral (anterior) spinocerebellar tract:
Peripheral processes of tract innervate the Golgi tendon organs of the lower limb
and trunk
➢ 1st order neurons are in the dorsal root ganglia
➢ Project to 2nd order neurons in nucleus
dorsalis
➢ 2nd order neurons cross the spinal cord and
ascend in contralateral white column
➢ Join the superior cerebellar peduncle and
cross midline again to enter cerebellum
➢ Terminate in the vermis of the anterior lobe of
cerebellum
➢ Transmits information about whole limb and
postural movement
➢ Damage results in the loss of nonconscious proprioception and co-ordination of the
lower limb
Describe the rostral spinocerebellar tract:
Its afferent axons are from the
Golgi tendon organs located in the upper limb
The tract travels ipsilaterally and it enters the cerebellum via the inferior cerebellar peduncle
Damage to this tract results in the loss of nonconscious proprioception and coordination in the
ipsilateral upper limb
Describe the reticulospinal tracts:
Both sets of fibres enter anterior grey columns of spinal cord
Facilitate or inhibit activity of α and γ motor neurons
May influence voluntary movements and reflex activity
Thought to provide a pathway for hypothalamus to control
sympathetic and sacral parasympathetic outflow
Neurons in reticular formation of medulla oblongata project to respiratory muscles
Influence the motor control of breathing
Receive input from limbic system and influence emotional motor system
