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Lecture 8

Lecture 8 Motor and Neural Development.pdf

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Western University
Psychology 2220A/B
Beth Mac Dougall- Shackleton

Lecture 8 March-15-12 2:33 PM Psychology 2220B Week 9 March 15, 2012 Mirror Neurons • Active when performing an action or watching another perform the same action • First seen in primates • In monkey studies, mirror neurons fired while – grasping or watching another grasp a particular object but not other objects – grasping or watching another grasp an object for a specific purpose but not for another purpose • Possible neural basis of social cognition(knowledge of others’ mental processes – e.g., intentions) – You want to know what the person is going to do through their actions • Likely to be found in humans – Indirect evidence from functional brain-imaging studies • High level perceptual response Primary Motor Cortex • Located in precentral gyrus of the of the frontal lobe • Receives sensory feedback from muscles and joints • Has somatotopic organization – Diff regions are responsive for diff locations – somatotopic organization – motor homunculus • initial studies by Penfield used brief low-intensity stimulation (initially documented somatopic organization) • elicited simple motor reflexes • however, longer more realistic stimulation elicit more complex and naturalistic movements • The motor mapping - such as controlling specific muscles happens lower down • In the primary motor cortex, focuses on specific movement • firing of neurons is often correlated with end goal of movements, even if different trajectory taken – Fires when an action occurs despite how you got to the action – When picking up a cup - do you pick it up right away, or move around something • recording from primary motor cortex can be used to drive prosthetics and robots Primary Motor Cortex Lesions • Small lesions often with minimal effects • Large lesions may disrupt a patient’s ability to move one body part independently of others (move a finger separately) • Do no eliminate voluntary movement – b/c motor paths exist that go directly from secondary cortex to lower circuits w/out passing primary motor cortex • Large lesions may also produce stereognosia – deficit in stereognosia (ability to identify an object by touch) Cerebellum and Basal Ganglia • Interact with different levels of the sensorimotor hierarchy • Coordinate and modulate actions – See the cup, smell the tea and then say, ok I'm going to pick up the tea • May permit maintenance of visually guided responses despite cortical damage Cerebellum • 10% of brain mass, but more than 50% of its neurons – High density of gray matter to white matter • Complex but highly organized structure - in lobes, columns and layers • Input from primary and secondary motor cortexes • Input from brain stem motor nuclei • Feedback from motor responses • Involved in timing, fine-tuning, and motor learning • May also do the same for cognitive responses – Cognitive activities may draw on timing and sequencing methods – Cerebellum seems to be playing more of a role in these activities After Midterm 1 Page 1 – Cerebellum seems to be playing more of a role in these activities Basal Ganglia • A heterogeneous collection of interconnected nuclei- a lot more complex than cerebellum • Part of neural loops that receive cortical input and send output back via the thalamus • Modulate motor output and cognitive functions including learning • Speaking is more of a complicated muscular activity that a cognitive-- coordinating tongue, face etc Descending Motor Paths • multiple paths project from primary motor cortex to motor neurons of the spinal cord • act together to control voluntary movement Sensorimotor Spinal Circuits • Motor units are the smallest unit of motor activity – a single motor neuron and all of the muscle fibers that it innervates (controls) – Motor neuron makes multiple synaptic connections to muscle fibers – When motor neurons sends potential down, it releases acetylcholine and all of the fibers contract at the same time – all fibers contract together when neuron fires – acetylcholine is the neurotransmitter released at the neuromuscular junction • motor pool – all of the motor neurons that innervate the fibers of a given muscle – Multiple motor neurons driving all of those fibers Muscles • muscles can only contract to generate force • two types of fibers – fast twitch (white meat) • Turkey and chicken - white meat - can't fly that well - fast power outlets, fast burnout- low endurance • Controls the flapping – slow twitch (dark meat) • In the drumsticks - so the legs – both present in a muscle, but vary in proportion • Ratio of fast to slow twitch varies per individual- endurance runner vs. sprinter • flexors and extensors act in antagonistic pairs • Muscles can only contract - so to be antagonistic, contract diff set of muscles • Biceps - closes/ flexes arm • Triceps - open/ extend arm • Require both to be coordinated so movement is smooth Muscles • isometric and dynamic contraction • Isometric - maintaining arm position despite force of gravity • Dynamic - actually move arm - throw something across the room • Walking required both - isometric: to maintain posture; and dynamic to actually move • Movement and action require coordinated movement • Coordination requires feedback – Ex: he drank w eyes closed – How did he do it w/out visual feedback? – Uses feedback from muscles themselves, hearing, etc • depends on multiple sources of feedback from the musculature and sensorimotor control Receptor Organs of Tendons and Muscles Following are list of organelles that provide feedback • Golgi tendon organs – Embedded in tendons – Tendons connect muscle to bone – Detect muscle tension • Muscle spindles – Embedded in muscle tissue – Detect changes in muscle length After Midterm 1 Page 2 – Detect changes in muscle length Muscle Spindle Feedback Circuit – Intrafusal muscle within each muscle spindle innervated by its own intrafusal motor neuron • Keeps tension on the middle, stretch-sensitive portion of the muscle spindle to keep it responsive to changes in the length of the extrafusal muscle – Intrafusal is on the inside - and is control differently – In order for muscle length changes to be detected - it needs to maintain tension - to notice a change in length • Without intrafusal motor neurons, the spindles of a skeletal muscle would become slack and unresponsive to stretch during a muscle contraction • The intrafusal motor neurons adjust the length of the intrafusal muscles, thus maintaining tension on the muscle spindles – Read over this part**** Reflexes • Stretch Reflex: monosynaptic, serves to maintain limb stability • A reflex elicited by a sudden external stretching force on the muscle – Muscle take part in isometric contraction- that’s the main goal of the stretch reflex, to keep external forces from altering intended body position – Can be overridden by external input- the knee knocking thing • Rap on knee tendon stretches spindles of thigh muscle • This initiates action potentials carried from stretch receptor to the spinal cord byspindle afferent neurons • These neurons enter spinal cord through dorsal root, motor neurons in ventral root are excited • Causes thigh muscle to contract and leg to extend – Sends something to golgi organs – Occurs through monosynaptic feedback • afferent neurons coming in through dorsal root synapse on motor neurons leaving through ventral – e.g. Patellar tendon reflex is monosynaptic • Withdrawal Reflex is NOT monosynaptic – Involves only one interneuron synapse – Sensory neuron carries information to interneurons- one excites the bicep motor neurons causing them to contract; other inhibits the triceps, causing them to relax • Reciprocal Innervation – antagonistic muscles interact so that movements are smooth – flexors are excited while extensors are inhibited • Recurrent Collateral Inhibition – feedback loop through Renshaw cells that gives muscle fiber a rest after every contraction – The other synaptic connection reduces a delay in the line – Renshaw cells: small inhibitory interneurons Walking • requires a complex program of reflexes • integrates visual, somatosensory, and balance information • produces integrated series of limb movements and posture changes • despite complexity, can be coordinated by spinal cord in many species – In humans though, the descending motor pathway plays an important role too Central Sensorimotor Programs • Perhaps all but the highest levels of the sensorimotor system have patterns of activity programmed into them, and complex movements are produced by activating these programs – Once activated, these programs can function without further higher input • Cerebellum and basal ganglia then serve to coordinate the various programs Central Sensorimotor Programs Are Capable of Motor Equivalence • A given movement can be accomplished various ways, using different muscles = motor equivalence – Signing your name w hand or w toe • Central sensorimotor programs must be stored at a level higher than the muscle (as different muscles can do the same task) – Indicates neural plasticity • Sensorimotor programs may be stored in secondary motor cortex Sensory Information That Controls Central Sensorimotor Programs Is Not Necessarily Conscious • Evidence that patients could respond to visual stimuli of which they had no conscious awareness • Evidence that patients could not effectively interact with objects that they consciously perceived • Ebbinghaus Illusion: Conscious perception of disk size differs from motor response After Midterm 1 Page 3 The Development of Central Sensorimotor Programs • Central sensorimotor programs may be hierarchically organized and capable of using sensory feedback without direct control at higher levels • Programs for many species-specific behaviors established without practice – Grooming movement of mice was still displayed in mice w no limbs The Development of Central Sensorimotor Programs • Practice can also generate and modify programs – Ex playing the piano - typing, not usually things evolved to do, but can still learn how to – Response Chunking • Practice combines the central programs controlling individual response • Individual motor movements become groups together into bigger pieces- ex: know how to type a bunch of words, but when told to type a new word, become a lot slower – Shifting Control to Lower Levels of the hierarchy • Frees
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