Neuro: Descending Tracts
Terms in this set (80)
Descending consist of two things:
An upper and lower motor neuron pathway
Upper Motor Neuron
Originates in the cortex or brainstem and ends in the CNS, including on lower motor neurons
Lower Motor Neuron
Alpha motor neurons or "final common pathyways"
One motor neuron and the muscle fibers it innervates
Range of Motor Neurons to Amount of Muscle Fibers Innervated
Stapedius: 1 motor neuron to 3 muscle fibers
Gastroc: 1: 1000
Type I Muscle Fibers
Red, slow twitch
Type II a
White, fast twitch, fatigue resistant
Type II b
white, fast twitch, fatiguable
Feed Forward Control
Anticipatory use of sensory information to prepare for movement
Use of sensory information during or after movement to make corrections
What are the three key sensory sources that subconscious forward-feedback loops are dependent on?
Visual input (sometimes auditory input)
Loss of one of the three key sensory sources causes what?
Some degree of movement decomposition (accuracy, time, smoothness/efficiency)
Movement routine rises to conscious control
Covariates with Motor Programs include:
Intact nervous system
Skeletal muscle integrity and strength
Experience with task/skill
Presence of well learned movement patterns that need to be replaced
Disrupted nervous systems that need to be accommodated or healed
Cardiovascular system to support practice
Motivation (unconscious or conscious)
Categories of Descending Tracts
Postural/gross movement: nuclei located in the medial spinal cord
Fine movement and limb flexion: nuclei located in the lateral spinal cord
Nonspecific UMN tracts: located throughout the ventral horn
Postural/gross movement Tracts
Located in the medial spinal cord
Medial and lateral vestibulospinal
Fine movement and limb flexion Tracts
Located in the lateral spinal cord
Nonspecific UMN Tracts
Located throughout the ventral horn
Descending Postural motor Control Cortical and Brainstem Tracts: Anatomy and Function
- Medial corticospinal
- Medial reticulospinal
- Medial and lateral vestibulospinal
Function: core control
Anatomy of the TectoSpinal Tract
Originates in the superior colliculus (tectum)
Descends in the contralateral anterior funiculus
Synapses in the cervical levels of gray
Function of the TectoSpinal Tract
Assists in head turning to coordinate with visual stimuli and auditory stimuli
Recticulospinal Formation Anatomy
3 Zones that stretch through the pons and medulla:
Raphe nuclei (most medial)
Medial zone (intermediate, gives rise to efferent fibers; Gigantocellular portion is most rostral)
Lateral zone (synapses with cranial nuclei)
Reticulospinal Tract Anatomy
Medial: pontine fibers descned in ipsilateral MLF via anterior funiculus
Lateral: medullary fibers descend bilaterally in anterior lateral funiculus
Reticulospinal Tract Function
Direct stimulation of alpha motor neurons for posture and limb extension
Regulation of spinal reflex arc sensitivity
Control over multi-limb reflex patterns of movement
Pain suppression through presynaptic inhibition of 1st order spinothalamic neurons
Vestibulospinal Tract Anatomy
Arise in lateral and medial vestibular nuclei. Ipsilateral, lateral, and bilateral medial descending tracts leave from each vestibular nucleus.
The lateral vestibulospinal tract descneds via which funiculus?
Through the anterolateral funiculus- ipsilateral antigravity
Medial vestibulospinal tract descends via which funiculus?
Through bilateral coordination of head stability on the body and head-eye coordination
What is the function of the vestibulospinal tract?
Righting, balance integrated output from vestibular, visual and proprioception input
Medial/Anterior Corticospinal Tract Anatomy
15% fibers from ipsilateral cortex via the internal capsule
Travel in anterior funiculus near ant. median fissure
The Median/Anterior Corticospinal Tract synapses where?
Synapses with interneurons and alpha motor neurons in the cervical and thoracic levels, primarily to axial muscles and neck and shoulder muscles
Some fibers cross in the anterior white commisue to synapse with axial muscles.
What are the fine motor/limb flexion tracts?
Corticobulbar (face, throat, shoulders)
Lateral Corticospinal/pyramidal Tract composition
85% fibers from the contralateral cortex (motor, premotor and supplemental motor cortex)
via the posterior limb of the internal capsule
Where does the lateral corticospinal tract/pyramidal tract decussate?
Cross below the pyramids (pyramidal decussation)
Travel in lateral funiculus, medial to spinocerebellar
Terminate in the anterior horn or intermediate gray
What lower motor neurons does the lateral corticospinal/pyramidal tract synapse with?
Synapses with the interneurons, alpha and gamma mn
What is the function of the lateral corticospinal tract/pyramidal tract?
Primary limb innervation
What is the anatomy of the rubrospinal tract?
Begins in red nucleus
Descends in lateral brainstem
Descends in lateral sc column
with lateral corticospinal tract.
What is the function of the rubrospinal tract?
Assists with movement control
What is the anatomy of the lateral/medullary reticulospinal tract?
Origin: lateral reticular
formation in medulla
Travels in lateral funiculus
Synapses in anterolateral
What is the function of the lateral/medullary reticulospinal tract?
Facilitates flexors and
inhibits extensors except
during walking, when
What is the anatomy of the corticobulbar tracts?
Termination: Cranial nerve nuclei for motor
control of face, tongue, pharynx, larynx,
trapezius, and sternocleidomastoid muscles.
What is the function of the corticobulbar tracts?
Cranial nerve motor innervation
Lower face - contralateral corticobulbar fibers.
Upper face - bilateral control
Corticobulbar Tracts contain fibers from where?
primary motor cortex
to reticular formation
and selected cranial
(Not III-occulomotor, IVtrochlear,or
What is the order in which motor units are recruited?
Small to large
S-Slow twitch first
FR -Fast twitch, fatigue
FF-Fast twitch, fatigable last.
What does recruiting motor units small to large do?
Yields smooth recruitment of force
Simple reflex arcs
Minimal descending input
Stimulate afferent > efferent loop> ascending input
for cortical awareness.
Recriprocal reflex patterns
Stimulate afferent>efferent loop + reciprocal
innervation of opposite muscles contralaterally
(Flexion withdrawal and extensor patterns) with
ascending input for cortical awareness.
Reciprocal movement patterns
subcortical control unless disrupted
CPGs - complex intralimb movements
Central/Stepping Pattern Generators
Spinal cord circuitry that
movements, e.g. walking,
arm swinging, running
Linked contralaterally and between UEs and LEs
Conscious control of simple reflexes
Submit to stimulus.
Cortical override of desire to withdraw finger from stimulus.
Examples of conscious overriding of simple reflexes
receiving an inoculation
running finger over a flame
picking up a hot hors d'oerve
walking on coals
Complex reflexes, synergies and automatic motor routines
Series of coordinated movements
Unlearned complex reflexes
Falling - UE protective extension
Fear - Increased flexion withdrawal
Learned complex reflexes
Basketball lay up
Finger patterns on a musical instrument
Overriding unlearned complex reflexes
Learning to dive (head tucked) rather than belly flop
Learning to fall without extending arms.
Leaning down the hill when skiing.
Playing 'blink' and not responding to hands in your face.
Upper Motor Neuron Damage
Damage of neurons leaving the cortex
Examples of Upper Motor Damage
Spasticity or hypertonia
partial loss of voluntary motor control
partial loss of voluntary contraction on one side
of the body
complete loss of voluntary motor control
Spasticity or hypertonia
increase in stretch reflex, with increased resting tone
resistance to passive stretch regardless of
speed of force.
Types of Rigidity
Midbrain damage resulting in UE and
Damage above the midbrain level
results in UE flexion and LE extension
Low tone due to pervasive brain damage,
genetic disorders, or developmental delays.
Lower Motor Neuron Injury
damage of alpha mn cell
body or peripheral axons
Examples of Lower Motor Neuron Injury
Paralysis or flaccidity
Loss of reflexes
Paralysis or Flaccidity
damage of alpha mn cell
body or peripheral axons
Loss of muscle bulk
due to nerve damage
due to lack of use
What is polio?
A virus that destroys alpha motor neurons
What is the cause of a stroke/CVA?
Interrupted blood supply to motor cortex
Sensory loss with stroke/CVA
Motor loss with stroke/CVA
Motor losses mostly
contralateral to lesion
What is the cause of Amyotrophic Lateral Sclerosis?
Usually unknown cause, with small percent having
a genetic component.
Bilateral destruction of UMNs,
brainstem & spinal cord LMNs.
Presentation of ALS
Hyper-reflexia, Babinski's sign,
Difficulty swallowing, breathing, speaking due to CN involvement
What is the cause of Cerebral Palsy?
Cause: not always clear, but includes any of
- Abnormal neuronal development, migration and
- Abnormal supraspinal regulation of spinal MNs.
- Intra or periventricular hemorrhages (IVH/PVH),
cerebral infarcts, periventricular leukomalacia
Presentation of Cerebral Palsy
hyper or hypotonia,
What is the pathology of Brown-Sequard Syndrome?
Hemisection of the spinal cord
Symptoms of Brown-Sequard Syndrome
- Ipsilateral spastic paralysis below level of lesion
- Ipsilateral disruption of tactile, vibration and
- Contralateral disruption of pain and temperature
1-2 levels below lesion
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