Motor Tracts - PDF

TRACTS-Part 2 Motor Tracts • 2 groups of descending motor tracts 1. Pyramidal – voluntary, originate in the cortex CORTICOSPINAL CORTICOBULBAR 2. Extrapyramidal- involuntary, originate in the brainstem Vestibulospinal Tectospinal Reticulospinal Rubrospinal Olivospinal Motor Pathways ➢Body mo...

TRACTS-Part 2 Motor Tracts • 2 groups of descending motor tracts 1. Pyramidal – voluntary, originate in the cortex CORTICOSPINAL CORTICOBULBAR 2. Extrapyramidal- involuntary, originate in the brainstem Vestibulospinal Tectospinal Reticulospinal Rubrospinal Olivospinal Motor Pathways ➢Body movement ➢Reflex movements ➢Automatic rhythmic ➢Voluntary/purposeful ➢Control of body movement ➢motor portions of cerebral cortex ➢Primary, premotor, supplementary ➢initiate and control precise movements ➢basal ganglia help establish muscle tone and integrate semi-voluntary automatic movements ➢cerebellum helps make movements smooth and helps maintain posture and balance ➢Somatic motor pathways ➢direct pathway from cerebral cortex to spinal cord and out to muscles ➢indirect pathway includes synapses in basal ganglia, thalamus, reticular formation and cerebellum Upper and Lower Motor Neurons Lower Motor Neuron “Final Common Pathway” Corticospinal tract ➢Only in mammals (variable); best developed in humans ➢Part of pyramidal system ➢Originates in motor (precentral gyrus), premotor, supplementary motor cortices, postcentral gyrus, paracentral lobule ➢Functions ⚫Provides skill, precision, fractionation, speed, and agility during voluntary movements ➢ Upper motor neuron in the primary motor cortex Primary motor cortex ➢Muscles represented unequally (according to the number of motor units) ➢More cortical area is needed if number of motor units in a muscle is high ⚫vocal cords, tongue, lips, fingers & thumb ➢Axons descend in corona radiata, internal capsule, crus cerebri, basilar pons, and pyramids ➢85% cross in the pyramidal decussation - lateral CST ➢15% descend uncrossed as the ventral CST but cross (anterior commissure) before terminating in the gray matter of the cervical and upper thoracic segments] ➢Terminate on alpha and gamma motor neurons and on interneurons (integrate excitatory and inhibitory input) • Somatotopically organized • Cervical “leave” first Motor Tracts – Ventromedial Pathway Controls Posture/Axial Muscles Ventromedial Motor Tracts – Posture/Involuntary motor Cortex Basal Nuclei Cerebellum Have influence on the Ventromedial Tracts Rubrospinal tract • Originates in the red nucleus of the midbrain • Crosses as the ventral tegmental decussation • (dorsal tegmental decussation is tectospinal tract) • Extends to cervical cord in humans; length of cord in lower mammals • Runs in lateral white with lateral corticospinal tract • Facilitates flexor and inhibits extensor motor neurons • Receives input from cerebellum and motor cortex MOTOR CORTEX + + RED NUCLEUS RETICULAR N. Rubrospinal tract Stimulates flexors TST VESTIBULAR N. VST PONTINE RS Medullary RS SUPERIOR COLLICULUS Stimulates anti-gravity muscles Spinal Cord Rubrospinal tract can be voluntary but in humans mainly involuntary to upper extremity flexors Tectospinal tract ➢Originates in the superior colliculus of midbrain ➢Crosses in the dorsal tegmental decussation ➢Terminates in cervical cord segments ➢Reflex head movements in response to visual, auditory, cutaneous stimuli ➢Head movements for tracking a moving visual stimulus Reticulospinal tracts (ReSpT) Pontine (Medial) Reticulospinal Tract Stimulates extensors (extensor bias), inhibits flexors) ipsilateral Medullary (Lateral) Reticulospinal Tract Stimulates flexors (flexor bias), inhibits extensors, bilateral ➢RF receives input from motor cortex, cerebellum, and brain stem nuclei. Functions: ⚫Control and coordinate automatic movements by modulating activity in cord locomotor pattern generators ⚫Influences posture,which is: • Position of body parts held between movements • Stabilization of proximal limb joints so voluntary movements of distal limbs can occur Vestibulospinal tracts Lateral Vestibulospinal Tract From vestibular nuclei to anterior horn length of cord, ipsilateal Stimulates extensors (antigravity muscles) Medial Vestibulospinal Tract (descending MLF) From vestibular nuclei to anterior horn Cervical level, bilateral Coordinates neck and head movement for visual tracking ➢Other descending tracts ⚫Descending autonomic fibers • Originate from hypothalmus and brain stem nuclei • Hypothalamospinal “tract” • Terminate on preganglionic neurons ⚫Aminergic pathways (e.g., raphespinal) • Originate in reticular formation (PAG influences) • Terminate in dorsal horn • Endogenous analgesic system ⚫Solitariospinal tract • Originate in nucleus of the tractus solitarius and medullary reticular formation • Terminate on phrenic motor nucleus and motor neurons to intercostal muscles • Respiration The Hypothalamus controls the Autonomic Nervous System- both parasympathetic and sympathetic (intermediolateral cell column) Control of glands, smooth muscle, and cardiac muscle These neurons synapse on the preganglionic autonomic neurons Hypothalamospinal Hypothalamobulbar Tracts Upper Motor Neurons ➢Neurons that influence LMNs descending tracts ➢Originate in motor cortex and brain stem nuclei ➢Lesions of UMNs produce characteristic signs seen in the muscles innervated by motor neurons that have not been lesioned (typically these motor neurons are located below the level of the lesion). These signs are: ⚫Spasticity or hypertonicity ⚫Hyper-reflexia ⚫Decreased/absent superficial reflexes ⚫Pathological reflexes (Babinski) Lower Motor Neurons ➢Final common pathway to skeletal muscle ➢Alpha & gamma motor neurons located in cranial nerve motor nuclei and ventral horn ➢Lower motor neurons receive signals from both direct and indirect upper motor neurons ➢Sum total of all inhibitory & excitatory signals determines the final response of the lower motor neuron and the skeletal muscles ➢Lower motor neuron lesion signs ⚫ Fasciculations ⚫ Neurogenic atrophy ⚫ A or hypo reflexia ⚫ Flaccid paralysis Stretch Reflex Monosynaptic, deep tendon reflex, myotactic reflex, spindle reflex Reflex Testing An easy way to screen test the nerve and/or cord Can be done on most muscles but typical ones are checked clinically Biceps – C5 Patellar – L4 Brachioradialis – C6 Achilles – S1 Triceps – C7 Hyporeflexia – suspect lower motor lesion Hyperreflexia – suspect upper motor lesion Testing for lesions (motor) Weakness can be caused by lesions or dysfunction at any level in the motor system Identify the correct motor system level, side & specific structures affected Motor exam: Observation (twitching, tremors), Inspection (posture, wasting), Palpation, Muscle tone testing (resistance?), Functional testing (drift), Fine movements, Strength testing, Reflexes – deep tendon and pathological Muscle Testing Both Upper Motor Neuron and Lower Motor Neuron Lesions result in weakness Pathological Signs If Clonus or Babinski are present suspect upper motor lesion If abdominal reflex is absent, suspect UMN lesion The reason for the pathological signs is loss of inhibition Loss of abdominal reflex or cremasteric reflex is suggestive of UMN lesion Abdominal- stroke towards umbilicus, abdominal muscles should contract and draw the umbilicus towards the stimulus Cremasteric- stroke along ipsilateral medial thigh, cremasteric muscle should contract and draw testicle upwards Upper Motor Neuron Lesion Signs of upper motor neuron lesions include Weakness Hyperreflexia (increased reflexes) Spastic paralysis Increased tone Pathological reflexes ***The corticospinal tract plays a large INHIBITORY role on alpha motor neurons Corticospinal Tracts R L A lesion above the decussation leads to contralateral weakness except in head, usually only contralateral lower face, maybe tongue A lesion below the decussation produces ipsilateral weakness Anterior corticospinal tracts do not cross until they reach the level of the spinal cord where they synapse Lower Motor Neuron Lesion Injury to LMNs interrupts final path Results in: - Flaccid paralysis - Decreased or absent reflexes (hyporeflexia) - Possibly twitches/fasciculation (due to aberrant action potential as neuron is dying) - Decrease in muscle bulk/atrophy (over time) Paralysis and atrophy are segmental Damage to a LMN could be in CNS or PNS Lower motor neuron syndrome Disorders/Lesions Examples of Lower Motor Neuron Lesions • Polio – virus that affects the alpha motor neurons, children • Amyotrophic Lateral Sclerosis (ALS) – cause unknown, 50s alpha motor neurons • Nerve Root – herniated disc • Peripheral Nerve – Carpal tunnel syndrome, median nerve Expect “nerves” to be more localized than lower motor neuron lesions involving several levels of the cord Examples of Upper Motor Neuron Lesion • Brain or Brainstem infarct, tumor – contralateral upper motor neuron signs • Cord infarct, trauma – ipsilateral upper motor neuron signs