Motor Systems

Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of fibers (e.g. digits) To thousands of fibers (e.g. trunk and major limb segments) Smaller motor units yield more refined control a motor “fovea”

sarcomere

Excitation of a Muscle Fiber An action potential in an alpha motoneuron axon. ACh is released by the axon terminal at the neuromuscular junction (synapse between motoneuron and muscle. Nicotinic receptor channels open, Na+ enters and the membrane depolarizes (EPSP). Voltage-gated Na+ channels open and an action potential is propagated down the muscle fiber. Depolarization produces Ca2+ release from intracellular stores.

Contraction of a Muscle Fiber Ca2+ binds to tropinin. Myosin binding sites are exposed. Myosin heads bind actin. Myosin cross-bridges pivot. Myosin heads disengage at the expense (ATP-dependent). Recycle as long as Ca2+ and ATP are present.

Relaxation of a Muscle Fiber As EPSPs end, membrane returns to resting potential. Free Ca2+ is sequestered by metabolically driven pump. Myosin binding sites covered.

Co-activation of α-motoneurons and γ-motoneurons at rest α-motoneuron activation α-, γ-motoneuron co-activation

Meissner’sMerkel’s PacinianRuffini’s Free nerve ending

Monosynaptic stretch reflex Golgi tendon organ reflex

somatosensory afferents motor afferents

(A δ ) (C)

Muscle Spindles (intrafusal fibers) Golgi Tendon Organs respond to muscle length, or stretch insensitive to tension 1a afferents for dynamic spindles Group II afferents for postural spindles monosynatic stretch reflex (excitatory) respond to muscle tension insensitive to muscle length, or stretch 1b afferents disynaptic reflex (inhibitory)

Problem of Locomotion Degrees of freedom problem (Bernstein) Dozens of joints controlled by hundreds of muscles Muscles can contract in any combination, in any order, for any duration, and with any force. Possibilities are virtually limitless, making control of complex movements at the level of individual muscles impossible

Reciprocal inhibition between motor units of antagonistic muscles

Crossed extensor reflex

oscillators = “legos” of movement CPGs = what you can build with legos

Central Pattern Generators vs. Reflexes Central Pattern Generators Fast Act as “clocks” Flexible--<100 oscillators necessary Reflexes Refine timing Respond to the unexpected Acquisition of new skills

Descending motor pathways Lateral and ventromedial pathways Pyramidal and extra-pyramidal systems

Supplementary Motor Area (SMA) important for instituting motor programs activated prior to voluntary movement activated during mental rehearsal Premotor cortex sensory guided movement cooperatively with basal ganglia mirror neurons Pre-frontal cortex decisions for actions long-term planning

Cortex Cerebellum Substantia Nigra Caudate- putamen (striatum) Globus Pallidus Thalamus and STN Reticular Formation Red Nucleus (nucleus ruber) Reticulospinal tract Rubrospinal tract

Cortex Cerebellum Substantia Nigra Caudate- putamen (striatum) Globus Pallidus Thalamus and STN Pontine nuclei and Reticular Formation Red Nucleus (nucleus ruber) Reticulospinal tract Rubrospinal tract cortex pontine nuclei cerebellumthalamus

Cortex Cerebellum Substantia Nigra Caudate- putamen (striatum) Globus Pallidus Thalamus and STN Pontine nuclei and Reticular Formation Red Nucleus (nucleus ruber) Reticulospinal tract Rubrospinal tract cortex striatum globus pallidusthalamus

Cortex Cerebellum Substantia Nigra Caudate- putamen (striatum) Globus Pallidus Thalamus and STN Pontine nuclei and Reticular Formation Red Nucleus (nucleus ruber) Reticulospinal tract Rubrospinal tract cortex striatum substantia nigrathalamus

Cortex Cerebellum Substantia Nigra Caudate- putamen Globus Pallidus Thalamus and STN Reticular Formation Red Nucleus (nucleus ruber) Reticulospinal tract Rubrospinal tract degenerate in Huntington’s disease

Cortex Cerebellum Substantia Nigra Caudate- putamen Globus Pallidus Thalamus and STN Reticular Formation Red Nucleus (nucleus ruber) Reticulospinal tract Rubrospinal tract degenerates in Parkinson’s disease Treatments: 1. l-DOPA to compensate for lost DA projections 2. Deep brain stimulation of striato-pallidal projections 3. Surgical destruction of cells in medial pallidum

Subthalamus  pedunculopontine nucleus (PPN)  reticulospinal tract  spinal CPGs

ALS: Upper motoneuron disease Polio: Spinal motoneuron disease Apraxias: Frontal cortical damage