Spinal & Brain Control of Movement PDF

Spinal & Brain Control of Movement Chapters 13 & 14  – Discuss the spinal cord and its control over a few movement reflexes: – Stretch reflex – Flexor reflex – Golgi tendon organ reflex Learning – Learn about the bra...

Spinal & Brain Control of Movement Chapters 13 & 14  – Discuss the spinal cord and its control over a few movement reflexes: – Stretch reflex – Flexor reflex – Golgi tendon organ reflex Learning – Learn about the brain’s contribution to movement planning objectives and execution, including the roles of: – Descending motor tracts – Movement association areas – Basal ganglia – M1 – Cerebellum  – Gray matter and spinal roots – Cross section of cord resembles butterfly or letter “H” – Three areas of gray matter are found on each side of center and are mirror images: – Dorsal horns: interneurons that receive somatic and visceral sensory input – Ventral horns: some interneurons; somatic motor neurons – Lateral horns (only in thoracic and superior lumbar regions): sympathetic neurons Spinal cord anatomy  – Gray matter and spinal roots (cont.) – Gray commissure: bridge of gray matter that connects masses of gray matter on either side – Encloses central canal – Ventral roots: bundle of motor neuron axons that exit the spinal cord – Dorsal roots: sensory input to cord – Dorsal root (spinal) ganglia: cell bodies of sensory neurons – Spinal nerves: formed by fusion of dorsal and ventral roots Spinal cord anatomy  – Gray matter divided into four groups based on of somatic or visceral innervation – Somatic sensory (SS), visceral sensory (VS), visceral (autonomic) motor (VM) and somatic motor (SM) Spinal cord anatomy  – White matter is divided into three white columns (funiculi) on each side – Dorsal (posterior) – Lateral – Ventral (anterior) – Each spinal tract is composed of axons with similar destinations and functions Spinal cord anatomy  – What is a reflex? – Definitions & function – What are the components of a reflex? Spinal – Anatomy reflexes – How do spinal reflexes work? – Pathways – What can modulate a reflex? – Environmental & cortical influences  Carry out: Are: Automatic actions like swallowing, sneezing, Automatic, subconscious responses to changes coughing, and vomiting What are within or outside of the body Maintain: reflexes? Regulate: Balance & posture, They: Homeostasis (autonomic especially spinal reflexes reflexes), including heart rate, blood pressure & Can involve: digestion Either or both the spinal cord and brain Spinal – We will focus on reflexes that are mediated by the spinal cord because they have a well-understood anatomy, circuitry, reflexes electrophysiological properties, and there is a growing knowledge of how they contribute to behavior.  The REFLEX Components of reflexes ARC Refers to the main components of a circuit required for executing a reflex.  MUSCLE Components SPINDLES Non-contractile of reflexes muscle fibers that serve as receptive surfaces and help regulate muscle tone  GOLGI TENDON Components of reflexes ORGANS Proprioceptors that constantly inform the brain about the state of the muscle  – Whereas muscle spindles are most sensitive to changes in muscle length, Golgi tendon organs are most sensitive to changes in muscle tension. Distinction between muscle spindles & Golgi tendon organs  Maintaining balance Regulating muscle Avoiding painful stimuli when standing & walking tension such as stepping on a nail (muscle spindles) to not damage muscles or insertion (cutaneous & nocioceptive points receptors) (golgi tendon organs) Functions of spinal reflexes  – Composed of a few intrafusal muscle fibers that lack actin & myosin in their central regions, making them non-contractile. Anatomy of Instead, they are receptive surfaces. the muscle – Wrapped by two types of afferent spindle endings: type Ia fibers (primary sensory endings) and type II fibers (secondary sensory endings) – These regions are innervated by gamma (γ) motor fibers. Let’s look at the muscle spindle in a little more detail & see how the components work together. Let’s look at – Stretching the muscle activates the the muscle muscle spindle, causing an spindle in a increased rate of action potentials on little more Ia fibers. detail & see Contracting the muscle reduces how the tension on the muscle spindle, components causing a decreased rate of work together. action potentials on Ia fibers Muscle spindle histology in rabbit  – Made up of strands of collagen that are connected at one end to the muscle fibers, and merge onto the actual tendon on the other end. Anatomy of – Each tendon organ is innervated by the Golgi a single afferent type Ib sensory nerve fiber (Aɑ fiber) that branches tendon organ and terminates as spiral endings around the collagen strands. – The Ib afferent axon is a large diameter, myelinated axon. Let’s look at the Golgi tendon organ in a little more detail & see how the components work together. Golgi tendon organ histology in rabbit  – Flexor – Muscle that decreases angle of joint when contracted – Extensor – Muscle that increases angle of joint when contracted – Agonist Physiology – Muscle that produces the primary/major force to complete a vocab review movement (bicep in a bicep curl) – Antagonist – Muscle that produces the opposite motion of the agonist – Synergist – Muscle that stabilizes a joint during movement, helping the agonist  Also known as the knee-jerk, or myotatic reflex. It is usually monosynaptic. The stretch reflex  Also known as the knee-jerk, or myotatic reflex. It is usually monosynaptic. The stretch reflex  – By sending commands to motor neurons, the brain is Key functions able to set a muscle’s length. The stretch reflex then makes of the stretch sure the muscle stays that length. This reflex is therefore reflex important for maintaining muscle tone and upright posture. The flexor Also known as the flexion, withdrawal, or the “ouch, I stepped on a nail!” reflex. reflex It is always polysynaptic.  – The flexor reflex helps you to avoid injury by withdrawing your limbs from painful stimuli. The flexor reflex The Golgi Also known as the autogenic inhibition, or inverse myotatic reflex. tendon reflex It is always polysynaptic.  – The Golgi tendon reflex helps you to avoid injury & tears to your tendons by automatically forcing you to drop loads that are too heavy for your muscles to handle The Golgi tendon reflex  – The Golgi tendon reflex helps you to avoid injury & tears to your tendons by automatically forcing you to drop loads that are too heavy for your muscles to handle The Golgi tendon reflex  – Cortical commands can modulate the stretch reflex response – During voluntary movement, specific motor cortex neurons with corticospinal projections are active – Corticospinal axons branch and synapse on both alpha motor neurons and opposing Ia interneurons Modification of reflex responses Modification – Depending on the context, the same of reflex reflex pathway can execute a different responses behavioral response.  – Suprasinal inputs can help “reverse” the Golgi tendon reflex – Descending pathways converge onto the Ib inhibitory interneurons along with input from the Ib afferents, and are able to Modification influence the Golgi tendon reflex – This can cause the reflex to have a net of reflex excitatory effect on the signaling muscle! responses  – Alpha-Gamma coactivation helps keep muscle spindles activatable – When a muscle is fully contracted, the spindle is completely lax and unable to sense any changes to Modification muscle length. of reflex – Gamma motor neurons help overcome this by responses shortening the muscle spindle, even when the muscle is contracted. – Cortical motor commands help achieve this co-activation!  – Tonic excitatory activity can modify spinal reflex strength Modification of reflex responses Increased gain brings the motor neuron closer to threshold for firing an action potential. Modification Cortical / Tonic Alpha- of reflex Supraspinal Excitatory Gamma Inputs Activity Coactivation responses  Stimulus Sensory Effect on Reflex (Clinical Response Receptor Synapses Muscle Other Effects Function Test) Stretch (Knee- Rapid stretch Stretched Muscle spindle Also excites Maintaining Ia: monosynaptic of muscle muscle primary (Ia) and Excites synergist & inhibits posture, Summary of Jerk, II: monosynaptic (such as tap contracts secondary (II) homonymous antagonist muscles countering Myotatic) & weakly on muscle rapidly (e.g. sensory (same muscle) (reciprocal sudden Reflex polysynaptic tendon) knee-jerk) neurons inhibition) loads spinal Golgi Tendon (Autogenic Large force on tendon (such Muscle tension Inhibits Also inhibits Protective, reflexes Inhibition, Inverse Myotatic) as pulling on muscle when resisted) decreases (e.g. drop a stack of books) Golgi tendon organ (Ib) Polysynaptic (via interneuron) homonymous (same muscle) synergist & excites antagonist muscles prevents tendon damage Reflex Sharp, painful Also inhibits Protective, Flexor stimulus Limb is rapidly Cutaneous extensor muscle of withdrawal Polysynaptic (via Excites flexor (Withdrawal) (such as withdrawn (skin) & pain same limb & from interneuron) muscle Reflex stepping on a from stimulus receptors excites on opposite painful nail) limb stimuli  – The brain influences motor activity of the spinal cord. – Initiates voluntary movements – Hierarchy of controls – Highest level: strategy Brain control – Middle level: tactics of movement – Lowest level: execution – Sensorimotor system – Sensory information used by all levels of the motor system  – Deliver efferent impulses from brain to spinal cord – Two groups Descending – Direct pathways: pyramidal tracts – Indirect pathways: all others pathways of – Motor pathways involve two neurons: the nervous – Upper motor neurons system – Pyramidal cells in primary motor cortex – Lower motor neurons – Ventral horn motor neurons – Innervate skeletal muscles  – Corticospinal / Direct (pyramidal) pathways – Impulses from pyramidal neurons in Descending precentral gyri pass through pyramidal (lateral and ventral corticospinal) tracts pathways of – Descend directly without synapsing until the nervous axon reaches end of tract in spinal cord – In spinal cord, axons synapse with system interneurons (lateral tract) or ventral horn motor neurons (ventral tract) – Direct pathway regulates fast and fine (skilled) movements  – Indirect pathways – Also referred to as multineuronal pathways – Complex and multisynaptic – Includes brain stem motor nuclei and all motor pathways except pyramidal pathways Descending – These pathways regulate: – Axial muscles, maintaining balance and posture pathways of – Muscles controlling coarse limb movements – Head, neck, and eye movements that follow the nervous objects in visual field – Consist of four major pathways: system – Reticulospinal and vestibulospinal tracts: – maintain balance by varying tone of postural muscles – Rubrospinal tracts: control flexor muscles – Tectospinal tracts: originate from superior colliculi and mediate head movements in response to visual stimuli  – Indirect pathways – Also referred to as multineuronal pathways – Complex and multisynaptic – Includes brain stem motor nuclei and all motor pathways except pyramidal pathways Descending – These pathways regulate: – Axial muscles, maintaining balance and posture pathways of – Muscles controlling coarse limb movements – Head, neck, and eye movements that follow the nervous objects in visual field – Consist of four major pathways: system – Reticulospinal and vestibulospinal tracts: – maintain balance by varying tone of postural muscles – Rubrospinal tracts: control flexor muscles – Tectospinal tracts: originate from superior colliculi and mediate head movements in response to visual stimuli  – Corticospinal tract (direct) – Reticulospinal and vestibulospinal tracts: Descending – maintain balance by varying tone pathways of of postural muscles – Rubrospinal tracts (red the nervous nucleus): control flexor muscles system – Tectospinal tracts: originate from superior colliculi and mediate head movements in response to visual stimuli  – Motor cortex: areas 4 and 6 of the frontal lobe Planning of – Area 4: primary motor cortex, or M1 – Area 6: “higher” motor area (Penfield) movement by – Lateral region à premotor area the cerebral (PMA) – Medial region à supplementary cortex motor area (SMA) – Motor maps in PMA and SMA – Similar functions, different groups of muscles innervated Somatotopic motor map of precentral gyrus Posterior – Represent highest levels of motor parietal & control prefrontal – Decisions made about actions and their outcome cortex – Area 5: inputs from areas 3, 1, and 2 contributions – Area 7: inputs from higher order visual cortical areas such as MT to motor control Posterior – Anterior frontal lobes: abstract thought, decision making, and anticipating parietal & consequences of action prefrontal – Area 6: Actions converted into signals specifying how actions will be cortex performed. contributions – Per Roland à monitored cortical activation accompanying voluntary to motor movement (PET) – Results supported view of higher control order motor planning Neuronal – Edward Evarts demonstrated importance correlates of of area 6 in planning movement. – “Ready”—parietal and frontal lobes motor – “Set”— supplementary and premotor areas planning – “Go”—area 6  – Some neurons in cortical area 6 respond when movement is only imagined. – Giacomo Rizzolatti’s research with monkeys – Each cell has very specific movement Mirror preferences. neurons – Very likely that humans also have mirror neurons – May be part of extensive brain system for understanding actions and intentions of others  – The basal ganglia are crucial for the selection and initiation of willed movements – You can think of them as quality control to make sure unwanted movements are suppressed (the brain does random things sometimes! Ever had an intrusive thought?) The basal ganglia  – Basal ganglia – Project to the ventral lateral (VLo) nucleus The motor – Provides major input to area 6 loop – Cortex – Projects back to basal ganglia – Forms a “loop”  – Excitatory connection from cortex to putamen – Cortical activation – Excites putamen – Inhibits globus pallidus – Release VLo from inhibition – Activity in VLo boosts activity in SMA (supplementary motor area) The direct motor loop  – We won’t get into the complexities (you could take a whole class on just the basal ganglia…) but be Direct/indirect aware that there are multiple pathways through the basal (“go/stop” ganglia that allow it to fine-tune selection of movements pathways – Not only the direct ”go” pathway, through the but we have indirect “stop” pathways as well basal ganglia) – At the end of the day everything is looping between BG, cortex, thalamus  – Parkinson’s disease: trouble initiating willed movements due to increased inhibition of the thalamus by basal ganglia – Symptoms: hypokinesia, bradykinesia, akinesia, rigidity, and tremors of hand and jaw – Organic basis: degeneration of dopaminergic substantia nigra inputs to striatum Basal ganglia – L-dopa treatment: facilitates production of dopamine to alleviate some symptoms disorders – Huntington’s disease – Symptoms: chorea, hyperkinesia, dyskinesias, dementia (impaired cognitive disability), personality disorder – Loss of neurons in caudate nucleus, putamen, globus pallidus – Consequent loss of inhibitory output to the thalamus – Cortical degeneration primarily responsible for dementia and personality changes  – Electrical stimulation of area 4 causes contraction of small group of muscles. – The input–output organization of M1 Primary – Betz cells: pyramidal cells in cortical layer 5 motor cortex – Two sources of input to Betz cells initiation of – Cortical areas – Thalamus movement – Coding movement in M1 – Many broadly tuned neurons in M1 encode force and direction of movement. M1 movement coding – Recordings from a single cell in M1 illustrating direction vector  – Movement of direction encoded by collective activity of neurons – Motor cortex: many cells active for every movement – Activity of each cell represents a single “vote”. – Direction of movement: determined by a tally (and averaging) of votes M1 – Population vectors movement coding  – Microstimulation of M1 cortex normally elicits whisker movement. Malleable – Cut nerve that supplies whisker muscles motor maps – Microstimulation now causes forelimb or eye movements.  – Key function: planning out the correct sequence and timing of muscle contractions – Cerebellar lesions – Ataxia: uncoordinated and inaccurate movements – Dyssynergia: decomposition of synergistic multijoint movements – Dysmetria: overshoot or undershoot target Cerebellum!  – Proper execution of planned, voluntary, multijoint movements – Pontine nuclei – Axons from layer V pyramidal cells in the sensorimotor cortex Motor loop form massive projections to pons. through – Corticopontocerebellar projection lateral – 20 times larger than pyramidal tract cerebellum – Cerebellum – “small brain within the brain” – Motor learning – New motor programs created to ensure smooth movement.  – Example of a baseball pitcher – Walking: ventromedial pathways – Ready to pitch – Neocortex engaged, ventromedial pathways Summary of maintain posture – Pitch signs and strategy brain motor – Sensory information engages parietal and prefrontal cortex and area 6. control – Winds and throws pathways & – Increased basal ganglia activity (initiation) – SMA activity à M1 activation loops – Corticopontocerebellar pathways à cerebellum – Cortical input to reticular formation à release of antigravity muscles – Lateral pathway à engages motor neurons à action  – 40 MC – bring pencil for Scantron Exam 2 info – 2 essay questions – you will be asked to draw pathways of sensory systems and describe the functions of each system Questions?