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Lecture 8

Psych 2220A Lecture 8

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Psychology 2220A/B
Scott Mac Dougall- Shackleton

Psych 2220A Lecture 8 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 – Asingle motor neuron and all of the muscle fibers that it innervates – 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 Muscles • Muscles can only contract to generate force • Two types of fibres – Fast twitch (white meat) – Slow twitch (dark meat) – Both present in a muscle, but vary in proportion • Flexors and extensors act in antagonistic pairs • Isometric and dynamic contraction • Movement and action require coordinated movement • Depends on multiple sources of feedback from the musculature and sensorimotor control Receptor Organs of Tendons and Muscles • 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 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 Reflexes • Stretch Reflex: monosynaptic, serves to maintain limb stability – e.g. Patellar tendon reflex is monosynaptic • Withdrawal Reflex is NOT monosynaptic • Reciprocal Innervation – antagonistic muscles interact so that movements are smooth – flexors are excited while extensors are inhibited Psych 2220A Lecture 8 • Recurrent Collateral Inhibition – feedback loop through Renshaw cells that gives muscle fiber a rest after every contraction 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 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 • Cerebellum and basal ganglia then serve to coordinate the various programs Central Sensorimotor ProgramsAre Capable of Motor Equivalence •Agiven movement can be accomplished various ways, using different muscles • Central sensorimotor programs must be stored at a level higher than the muscle (as different muscles can do the same task) • 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 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 The Development of Central Sensorimotor Programs • Practice can also generate and modify programs – Response Chunking • Practice combines the central programs controlling individual response – Shifting Control to Lower Levels • Frees up higher levels to do more complex tasks • Permits greater speed Psych 2220A Lecture 8 Development of the Nervous System • Neural development consists of a series of processes • Continue into adulthood and result in neural plasticity • However, highest rates are earlier in development and through childhood/ puberty Neural Development • Disruption or alteration of development can result in lifelong impairments • Depends critically on sensory input to drive development – e.g. case of “Genie”; or development of ocular dominance columns Phases of Development • Induction of the neural plate – Neural proliferation • Migration and aggregation •Axon growth and synapse formation • Neuron death and synapse rearrangement Induction of the Neural Plate • Apatch of tissue on the dorsal surface of the embryo becomes the neural plate • Development induced by chemical signals from the mesoderm (the “organizer”) • Visible three weeks after conception • Three layers of embryonic cells – Ectoderm (outermost) – Mesoderm (middle) – Endoderm (innermost) • Neural plate cells: embryonic stem cells – Have unlimited capacity for self renewal • Can become any kind of mature cell – Totipotent – earliest cells have the ability to become any type of body cell – Multipotent – with development, neural plate cells are limited to becoming one of the range of mature nervous system cells Neural tube • Eventually develops into the nervous system • Failure of tube to fully close can result in neural tube defects Neural Proliferation • Neural plate folds to form the neural groove, which then fuses to form the neural tube • Inside will be the cerebral ventricles and neural tube • Neural tube cells proliferate in species-specific ways: three swellings at the anterior end in humans will become the forebrain, midbrain, and hindbrain • Proliferation is chemically guided by the organizer areas – the roof plate and the floor plate Psych 2220A Lecture 8 Migration • Once cells have been created through cell division in the ventricular zone of the neural tube, they migrate • Migrating cells are immature, lacking axons and dendrites • Two types of neural tube migration – Radial migration (moving out) – usually by moving along radial glial cells – Tangential migration (moving up) • Two methods of migration – Somal – an extension develops that leads migration, cell body follows – Glial-mediated migration – cell moves along a radial glial network • Most cells engage in both types of migration Neural Crest •Astructure dorsal to the neural tube and formed from neural tube cells • Develops into the cells of the peripheral nervous system • Cells migrate long distances Aggregation • After migration, cells align themselves with others cells and form structures • Cell-adhesion molecules (CAMs) –Aid both migration and aggregation – CAMs recognize and adhere to molecules • Gap junctions pass cytoplasm between cells – Prevalent in brain development – May play a role in aggregation and other processes Axon Growth and Synapse Formation • Once migration is complete and structures have formed (aggregation), axons and dendrites begin to grow • Growth cone – at the growing tip of each extension, extends and retracts filopodia as if feeling its way • Chemoaffinity hypothesis – postsynaptic targets release a chemical that guides axonal growth, but this does not explain the often circuitous routes often observed • Mechanisms underlying axonal growth are the same across species •Aseries of chemical signals exist along the way – attracting and repelling • Such guidance molecules are often released by glia •Adjacent growing axons also provide signals • Pioneer growth cones – the first to travel a route, interact with guidance molecules • Fasciculation – the tendency of developing axons to grow along the paths established by preceding axons • Topographic gradient hypothesis – seeks to explain topographic maps Psych 2220A Lecture 8 Synapse Formation • Formation of new synapses • Depends on the presence of glial cells – especially astrocytes • High levels of cholesterol are needed – supplied by astrocytes • Chemical signal exchange between pre- and postsynaptic neurons is needed Neuron Death and Synapse Rearrangement • ~50% more neurons than are needed are produced – death is normal • Neurons die due to failure to compete for chemicals provided by targets – The more targets, the fewer cell deaths – Destroying some cells increases survival rate of remaining cells – Increasing number of innervating axons decreases the proportion that survives Life-Preserving Chemicals • Neurotrophins – promote growth and survival, guide axons, stimulate synaptogenesis – Nerve growth factor (NGF) • Cell death during development is usually programmed: apoptosis, not passive: necrosis Synapse Rearrangement • Neurons that fail to establish correct connections are particularly likely to die • Space left after apoptosis is filled by sprouting axon terminals of surviving neurons • Ultimately leads to increased selectivity of transmission Neural Production • Involves overproduction of neurons and connections • Then selective attrition Postnatal Cerebral Development in Human Infants • Postnatal neural development is a result of – Synaptogenesis – Myelination – sensory areas and then motor areas. Myelination of prefrontal cortex continues into adolescence – Increased dendritic branches • Overproduction of synapses may underlie the greater plasticity of the young brain Development of the Prefrontal Cortex • Believed to underlie age-related changes in cognitive function • No single theory explains the function of this area • Prefrontal cortex plays a role in working memory, planning and carrying out sequences of actions, and inhibiting inappropriate responses (executive function) Psych 2220A Lecture 8 Effects of Early Experience • Permissive experiences: those that are
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