Motor Tracts Study Guide.docx

Lecture Two - Motor ❖ Sensory tracts Dorsal column medial lemniscus: "Larger, myelinated → faster" "Touch, vibration, joint sensation, pressure" Spinocerebellar tracts: "Proprioception" Spinothalamic tracts (anterolateral): "Technically 2 pathways" "Smaller, slower pain, temperat...

Lecture Two - Motor ❖ Sensory tracts Dorsal column medial lemniscus: "Larger, myelinated → faster" "Touch, vibration, joint sensation, pressure" Spinocerebellar tracts: "Proprioception" Spinothalamic tracts (anterolateral): "Technically 2 pathways" "Smaller, slower pain, temperature, crude touch, tickle, itch" ❖ Motor tracts Corticospinal tracts (pyramidal tracts) "Cortex → spine" Reticulospinal tract "Reticulo - means network" "Movement, posture, muscle control for walking, etc." "Extrapyramidal - think dopamine" "Extrapyramidal symptoms → think of symptoms of psychiatric meds; also neuroleptic malignant syndrome is dopamine related (s/sx similar to MH) ❖ Motor efferents CNS order of motor control: The primary motor cortex (precentral gyrus of the frontal lobe) determines the goal of movement and its consequences "Cortex involvement is needed for dancing aka complicated/coordinated movement" Basal ganglia provides the boost that turns intention into an actual movement - excitatory and inhibitory function (gateway) "Parkinson's → slow to get up, the intention is there but the transmission of the intention is the issue" Cerebellum determines the correct sequence of commands that will allow the goal to be achieved ❖ Intro: motor system Motor transmission is via a 2-neuron pathway Motor responses begin in the spinal cord (simple reflexes), brain stem (more complicated responses) and cerebrum (most complicated muscle skills) Upper motor neurons: transmit information from the brain → brainstem or spinal cord Neurotransmitter: glutamate Lower motor neurons: transmit information from the spinal cord → muscles and glands Neurotransmitter: acetylcholine Motor pathways are efferent (carrying impulses away from the CNS) ❖ Interneurons Present in all areas of cord gray matter (dorsal horns, anterior horns, areas between them) Small and highly excitable Can fire as rapidly as 1500x/second 30x more interneurons than anterior motor neurons Corticospinal tract consists primarily of interneurons ❖ Anterior motor neurons Innervate skeletal muscle Located in each segment of the anterior horns of the gray matter Largest (50-100% larger than most other neurons) Give rise to nerve fibers that exit spinal cord via anterior roots and directly innervate skeletal muscles Two types: Alpha motor neurons - innervate large skeletal muscles Gamma motor neurons - innervate intrafusal fibers of the muscle spindle "Even when you're sitting still your muscles have tone" ❖ Motor pathways Transmit information from the brain to: Voluntary (skeletal) muscles Smooth muscles Cardiac muscles Some glands Motor pathways are efferent (carrying impulses away from the CNS) Motor responses begin in the spinal cord (simple reflexes), brain stem (more complicated responses), and cerebrum (most complicated muscle skills) "Involuntary (due to constant tone) - heart rate, breathing, gag reflex, swallowing, contraction of muscle in response to stretch" "Humans have over 500 skeletal muscles - we need them all to be coordinated, can get out of whack due to many things" ❖ Motor functions Cerebellum - determines the correct sequence of commands that will allow the goal to be achieved Primary motor cortex - determines the goal of movement and its consequences Basal ganglia - provides the "boost" that turns intention into actual movement - excitatory or inhibitory (gateway) Corticospinal tract - voluntary muscles of the trunk and extremities Cranial nerves - supply voluntary muscles of head and neck ❖ Cerebral motor cortex Located anterior to the central cortical sulcus → "most complicated motor movements" Cerebral motor cortex occupies the posterior one-third of the frontal lobe Motor cortex consists of 3 areas: Primary motor cortex - muscles of hands, speech "Usually contraction of muscles" "more than half" "usually control a whole movement (arm extension and coordinating the multiple muscles it takes to do so) rather than just one muscle" Premotor area - patterns of movement, mirrored tasks "Mirroring someone is a different area than just doing it on your own" Supplementary motor - cortex bilateral movements "Think climbing, you are coordinating both sides" ❖ Specialized motor areas Broca's area: motor speech area; has to do with word formation "Muscles of lips, tongue, and throat to formulate speech" "Know what they want to say, but can't get it out - may get words out but not full sentences" Voluntary eye movement field: controls voluntary eye movement including blinking "Protective; automatically blink if something comes near the face" Head rotation area: stimulation in this area directs the head toward objects "If something flies past face you automatically follow it" Area for hand skills: damage to this area causes motor ataxia (difficulty with skilled movement) "A much bigger area of the brain than head rotation, eye movements, etc." ❖ Transmission of signals from the motor cortex → muscles Corticospinal tract (aka pyramidal tract) Most important output pathway! Originates from: Primary motor cortex (30%) Premotor and supplementary motor cortex (30%) Somatosensory area (40%) Indirect / alternative pathways: accessory pathways - "mesencephalon, corticospinal tract" Cranial nerves: supply voluntary muscles of head and neck "All parts if the body except for the face" "Brain → medulla → decussate → pyramids of medulla ❖ Corticobulbar tracts: CN - III, IV, V, VI, IX, X, XI, and XII Movements of eyes, tongue, chewing, expressions, and speech ❖ Motor pathways: lateral corticospinal tract (pyramidal)\* Originates in precentral gyrus of the frontal lobe Highly organized (neurons supplying specific areas of the body are grouped together) Motor fibers for all parts of the body except the face Axon travel from cortex → through the internal capsule → midbrain (basis pedunculi) → medulla → lateral corticospinal tract (aka pyramidal tract) → spinal cord Most fibers decussate in lower medulla Those that do not cross at medulla travel via ventral corticospinal tracts and cross in neck/thorax Motor nerves exit the spinal cord via the anterior horn ❖ Intrinsic muscle control Muscle spindles - "balance, prevent excessive stretch - if running and you overstretch a muscle you'll get a pain signal" Found in belly of skeletal muscles Send sensory info to the brain about muscle length or rate of change of length 2 sensory fibers to differentiate between slow and fast stretch "Static → slow increase in length/tension" "Dynamic → fast increase in length/tension" Excited by small gamma motor nerve fibers - originate from type A-gamma motor neurons in anterior horn the spinal cord "Underlying constant muscle tone → due to spindles" Golgi tendon organs Located in the muscle tendons Transmit information about tendon torsion or rate of change of tension Consist of an encapsulated sensory receptor through which muscle fibers must pass Organ is stimulated when the bundle of fibers is tensed ❖ Reflex responses Flexor reflex: Stimulation of pain endings (nociceptive reflexes, pain reflex) causes that part of the body to be withdrawn from stimulus Flexor reflex pathways pass directly from neuronal pool in spinal cord to motor fibers → "don't have to go all the way to brain for motor response" Activates diverging circuits to spread impulse to: Necessary muscles for withdrawal → "coordination of all muscles needed for reflex/response" Circuits to inhibit opposing muscles (reciprocal inhibition circuits) Circuits to cause after-discharge (lasts a few seconds after stimulus is over) ◆ After-discharge duration depends on intensity of initial stimulus ◆ This after-discharge holds the irritated part away from the stimulus for 0.1-3 seconds after irritation is over- "don't do anything for a millisecond after, delay until message gets to the brain → protective" Crossed extensor reflex: Extension of the opposite limb to "push away" the entire body from the spinal stimulus Video on this ❖ Clinical relevance: disorders of the motor system Upper motor neuron disease: Increased DTRs Increased muscle tone Positive Babinski sign Spastic paralysis Lower motor neuron disease: Decreased DTRs Decreased muscle tone Negative Babinski sign Flaccid paralysis / muscle atrophy Where is injury in relation to decussation? Above decussation, paralysis on opposite side of body Below decussation, paralysis is on the same side of the body "Pt with strokes → injury above the level → contralateral symptoms" "Disorders are classified on if upper, lower, or both and the level of the injury" "Most cross at the medulla → not all" "Upper motor neuron - brain → spinal cord" "Reflexes intact" "Brain, cortex" "Injury: thrombosis; aneurysm; brainstem (demyelination (MS), trauma, degenerative disease (parkinsons), ALS, cancers/neoplasms; spinal cord (neoplasms, demyelination, ALS) "Lower motor neuron - spinal cord → effector" "No reflexes, no constant tonicity, flaccid paralysis" "Dr. S's husband → occlusion myositis → hands are atrophied, disrupted impulse between brain and hands" ❖ Clinical relevance: movement disorder terminology Ataxia - inability to coordinate muscle activity Athetosis - involuntary mvmts of flexion/extension, pronation/supination of hands, toes, and feet; slow; writhing-type mvmts Ballismus - jerking, swinging, sweeping motions of the proximal limbs Bradykinesia / hypokinesia - decrease in spontaneity and movement Chorea - irregular, spasmodic, involuntary movements of limbs or facial muscles, often with hypotonia Cogwheel rigidity - resistance to movement; rigidity decreasing to stiffness after movement begins Dystonia - abnormal tonicity; difficulty maintaining posture Hyperkinesia - excessive motor activity Tic - repeated, habitual muscle contractions; movements that can be voluntarily suppressed for short periods Tremor - oscillating, repetitive movements of whole muscles; irregular, involuntary contractions of opposing muscles ❖ Clinical relevance: cerebral palsy Caused by damage to upper motor neurons from an event that occurs prenatal, perinatal or postnatal (cerebral anoxia, hemorrhage, and other neurologic insult) Classified by the type of motor dysfunction: Spastic - inability of muscles to relax Hemiplegia - involving one arm and one leg on the same side of the body Diplegia - involving both legs Quadriplegia - involving all four extremities, the trunk, and neck muscles Athetoid or dyskinetic - inability to control muscle movement Ataxic - inability to control balance and coordination Clinical manifestations: Altered body movements and muscle coordination Developmental delays Not a progressive disease ❖ Clinical relevance: ALS (amyotrophic lateral sclerosis) Degenerative disease of motor neurons Exact cause unknown (glutamate excitotoxicity and/or oxidative stress?) No known cure Affects both upper and lower motor neurons Leads to the destruction of NMJ Rapid progression, fatal disease Symptoms appear first in arms, hands, legs, and swallowing muscles → muscles gradually weaken, waste away, and twitch ❖ Clinical relevance: spinal cord injury SCI impairs the transduction of afferent and/or efferent neural impulses Damage to the gray matter of the central cord may result in loss of motor neurons and interneurons In complete injury, sensation and motor function below the level of injury are lost Categorization of PARTIAL cord transections Central cord syndrome Anterior cord syndrome Brown-Sequard syndrome - "test on the exam" "Partial transection - loss of motor/proprioception on one side and pain/temp on the other"