Lecture12-Movement - Movement • Reflexes Neuromuscular...

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Unformatted text preview: Movement • Reflexes Neuromuscular • Neuromuscular Junctions Junctions • Muscle spindles Pyramidal • Pyramidal and non-pyramidal systems systems Primary • Primary motor cortex cortex Movement • Movement disorders disorders Reflexes Neurons • Neurons that control muscle contraction are located in the ventral horn of the spinal cord Reflexes Charles • Charles Sherrington studied animals that had their spinal cords separated from their brains brains These • These ‘spinal’ animals could still withdrawal their limbs to a pinch their This • This withdrawal is a ‘reflex’ - an unlearned response to an environmental stimulus environmental Reflexes This • This pathway involves a short pathway linking dorsal and ventral roots of the spinal cord dorsal The neuroscience of movement Different • Different levels of organization (skeletal constraints, spinal cord and brainstem control, cortical and sub-cortical regulation) regulation) Skeletal • Skeletal system and associated skeletal muscles determine what movements are possible possible The skeletal system This • This system enables some movements and prevents others others Muscles generate force by Muscles shortening (contracting) shortening Some muscles maintain a Some consistent posture, some produce movement around a joint joint The rate and force of muscle The contraction influence the timing and pattern of behavior behavior • • • Relationships between muscles Muscles • Muscles are connected to • • • • bone by tendon bone Some muscles work Some together with other muscles – these are referred to as synergists referred Example, biceps and Example, triceps triceps When biceps contract, the When triceps extend triceps These muscles are also These referred to as antagonists referred Muscles: molecular machinery muscle • A muscle is composed of thousands of individual muscle fibers muscle Each muscle fiber Each contains two types of filaments, myosin and actin actin The interaction of myosin The and actin is responsible for muscle contraction for An increased overlap An between myosin and actin is the muscle contraction is • • • Actin Myosin Muscle types: smooth muscle Smooth • Smooth muscle are regulated by the autonomic nervous system system For • For example, muscle contractions in the gut are controlled by the vagus nerve vagus We • We cannot control these muscles voluntarily voluntarily Muscle types: striated muscle Striated • Striated muscles have a striped appearance striped These • These muscles are under voluntary control voluntary Since • Since different muscles need to contract at different speed and strength, there are different types different • Fast-twitch muscle fibers • Slow-twitch muscle fibers Breast = fast twitch Thigh = slow twitch Neuromuscular junction Neurons • Neurons that innervate muscles are called motoneurons motoneurons Each • Each motoneuron innervates many muscle fibers muscle Action • Action potentials in motoneurons release acetylcholine (ACh) acetylcholine ACh • ACh causes action potential in muscle Neuromuscular junction • Action potentials in muscle open voltage gated Na • + and Ca2+ channels Ca2+ This results in depolarization that causes molecular This changes in actin and myosin that produce contraction changes Neuromuscular Junction The • The neuromuscular junction is the region where the • motoneuron and the adjoining muscle fiber meet motoneuron All of these synapses are excitatory (ACh)… must inhibit All motoneuron to prevent a muscle from contracting motoneuron Neuromuscular Junction There • There are no inhibitory projections to muscle projections Muscle • Muscle contraction is prevented by inhibitory signals to the motoneuron in the ventral horn of the spinal cord spinal Motoneurons • Motoneurons innervate multiple fibers, motoneuron action potential contracts many fibers fibers Innervation ratios The • The innervation ratio determines how fine the control • • over the system is over One motoneuron innervates hundreds of fibers in the One arms and legs (lots of force… control not very precise) arms But to the eye, different motoneurons (from cranial But nerves III, IV, VI) innervate ~3 fibers (very fine control) nerves Integration at the motoneuron • Cell bodies in ventral horn Final common pathway Motoneurons • Motoneurons are referred to as a final common pathway because they are how the brain and spinal cord control muscle and thus behavioral output muscle Tremendous information processing determines if Tremendous motoneurons fire • Proprioception Proprioception • Proprioception (body sense); information about the position and movement of the body that is sent to the brain that The brain and spinal The cord must continuously gather information about the body in its environment environment Proprioception sent to Proprioception brain by large myelinated axons myelinated • • Muscle spindle­ informs brain about muscle • Small muscle fibers within the spindle (intrafusal) • Muscle fiber outside the spindle (extrafusal) Muscle spindle Muscle • Muscle spindle- a muscle receptor that lies parallel to a muscle… fires action potential to CNS when muscle is stretched muscle Consists of both afferent Consists and efferent elements and Afferent means information Afferent is traveling to… is Efferent means information Efferent is traveling away from… is • • • Muscle spindle • Motoneurons also called gamma efferents Muscle spindle Muscle • Muscle spindle contains two kinds of receptor endings: • primary sensory endings and secondary sensory endings primary Primary’s spiral around the center, Secondary’s terminate Primary’s toward the thin end toward Muscle spindle Spindle • Spindle activation: stretching muscle causes action potentials to fire in spindle afferents (i.e. the sensory nerve endings) sensory Upon initial stretch, the Upon primary’s show a robust response (dynamic response) response) The secondary’s show a The steady maintained response when the stretch is maintained (static indicator of muscle length) indicator • • Muscle spindle n • IIn addition to maintaining posture; muscle spindles also • coordinate movement coordinate Coordination significantly mediated by gamma and alpha Coordination motoneurons motoneurons Muscle spindle • Cell bodies of both in the ventral horn of the spinal cord Activation • Activation of gamma’s contracts the spindle, thus • increasing its stretch sensitivity increasing Alpha’s innervate extrafusal muscle Alpha’s Golgi tendon organs Golgi • Golgi tendon organs are sensory receptors in the tendons that fire action potentials to the CNS when muscle is contracted muscle Sensitive to muscle tension Minimal response to a low Minimal level stretch… but robust response to a severe stretch of the tendon stretch In this case, the golgi In tendon organs fire action potentials that lead to inhibition of motoneurons inhibition • • • Golgi tendon organs Brain pathways and movement • Some muscles are controlled directly by the brain Cranial • Cranial motor nuclei of the brainstem send their axons to innervate muscles in the head and neck innervate Brain pathways and movement For • For all other muscles, the brain has to send commands to the spinal cord and then the spinal cord controls the muscles controls The brain The communicates to the spinal cord via two pathways: the pyramidal system and the extrapyramidal motor system motor • Pyramidal system Also • Also called the corticospinal system consists of neuronal cell bodies within the cerebral cortex and their axons axons These axons pass These through the brainstem and form the ‘pyramidal’ tract to the spinal cord tract In the medulla, the In pyramidal tract crosses the midline (decussating) • • Primary motor cortex Many • Many of the axons of the pyramidal tract originate from neurons in layer V of the primary motor cortex (M1) (the precentral gyrus just anterior to the central sulcus) precentral Primary motor cortex Damage • Damage to primary motor cortex interferes with specific • movements on contralateral side movements Disproportionately large regions of M1 are devoted to Disproportionately body parts associated with complex movement M1 to spinal cord Cell • Cell bodies in M1 project to spinal cord and directly (but not exclusively) control motoneuron control of muscle muscle Electrophysiological • Electrophysiological recording of M1 neurons show that neurons encode different aspects of movement aspects Measuring action potentials in M1 • Monkey trained to move arm in 8 different directions single • A single M1 neurons responds different depending on the direction the arm is moved the Measuring action potentials in M1 Many • Many cells involved in movement… an individual M1 neuron encodes only partial information Nonprimary motor cortex Supplementary • Supplementary motor area (SMA) and premotor cortex are also important for motor control for Patients with lesions of Patients the premotor cortex retain fine motor control of the fingers but have impaired hand coordination hand Patients with bilateral Patients lesions of SMA cannot initiate movement initiate Supplementary motor area Primary motor area • • Mirror neurons in premotor cortex There • There is a population of neurons in monkey premotor cortex that not only fire when the monkey performs an action, but also when the monkey sees the action performed (by another monkey or a person) person) Called • Called mirror neurons Mirror neurons in premotor cortex Mirror • Mirror neurons fire less when something from the action is missing the Extrapyramidal systems and movement Brain • Brain systems outside of the corticospinal system important for movement important Extrapyramidal • Extrapyramidal systems engage the spinal cord through the reticulospinal tract or the rubrospinal tract tract The • The rubrospinal tract originates in the red nucleus nucleus Reticular formation The • The reticulospinal tract of axons originates in the reticular formation reticular Reticular • Reticular formation is an extensive region of brainstem (contains both inhibitory and excitatory projections) projections) Poorly • Poorly defined differentiated area of the brain stem and midbrain brain Basal Ganglia and movement Basal • Basal ganglia includes the caudate and putamen (dorsal striatum), globus pallidus, subthalamic nucleus, and substantia nigra nigra Receives dense innervation Receives from cortex and projects back to cortex through the thalamus thalamus Basal ganglia influences Basal the amplitude and direction of movement (not action initiation) • • Basal Ganglia: motor learning Cerebellum Cerebellum • Cerebellum also critical for movement for Larger • Larger in highly mobile fish vs fish that move less… and also larger in birds that fly vs those that do not that ayered • 3 llayered cerebellar cortex cortex Major • Major output cells are purkinje cells (inhibitory projections) Cerebellum Purkinje • Purkinje cells send inhibitory projections to deep • cerebellar nuclei cerebellar Cerebellum also important for learned motor sequences Disorders of movement Muscular • Muscular dystrophyassociated with degeneration of muscle degeneration Degeneration • Degeneration of motoneurons in amyotrophic lateral sclerosis (ALS) (Lou Gehrig’s disease) Gehrig’s Damage • Damage to spinal cord leads to paralysis leads Stroke in cortex Paralysis • Paralysis is also called plegia plegia Stroke • Stroke in cortex will most often result in impairment of contralateral movement of Paralysis • Paralysis on one side of the body is hemiplegia the Apraxia• Apraxia- the inability to carry out purposeful action (stroke in parietal or frontal cortex) or Parkinson’s disease Parkinson’s • Parkinson’s patients show very few spontaneous actions actions • Resting hands tremor Symptoms • Symptoms caused by degeneration of dopamine containing neurons in the substantia nigra substantia Patients • Patients given a precursor to dopamine (L-dopa) show decreased tremor and increased movement and Parkinson’s disease L-dopa • L-dopa is believed to loose its effectiveness because dopamine terminals are also lost dopamine Huntington’s disease Also • Also a disease of basal ganglia, but Huntington’s patients present with excessive movement excessive Inherited disease, transmitted Inherited by a single dominant gene (child has 50% chance of getting Huntington's if parent has it) has Involuntary movement starts Involuntary out small (finger twitches) but then gets much worse but Intellect is also impacted and Intellect patients get depression patients • • • Huntington’s disease Symptoms • Symptoms present starting at • • • • • ~30 years of age ~30 The gene for Huntington's The disease contains a trinucleotide repeat (CAG) trinucleotide Less than 30 repeats then no Less symptom symptom More than 38 then you have More the disease the The more repeats, the earlier The symptoms present symptoms Many cells produce the Many defective protein (called huntingtin) huntingtin) ...
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