2018-10-19 10:53:44

Prelude

Prelude

Today's Topics

  • The neuroscience of action
  • Quiz 2 now available; due next Friday

Muscle classes

  • Axial
    • Trunk, neck, hips
  • Proximal
    • Shoulder/elbow, pelvis/knee
  • Distal
    • Hands/fingers, feet/toes

Muscles

Muscle types

  • Smooth
    • Arteries, hair follicles, uterus, intestines
    • Regulated by ANS (involuntary)
  • Striated (striped)
    • Skeletal
    • Voluntary control, mostly connected to tendons and bones
  • Cardiac

Muscle types

How skeletal muscles contract

  • Motoneuron (ventral horn of spinal cord)
  • Neuromuscular junction
    • Releases ACh

From spinal cord to muscle

How skeletal muscles contract

  • Motor endplate
    • Nicotinic ACh receptor
  • Excitatory endplate potential
    • Muscle fibers depolarize
    • Depolarization spreads along fibers like an action potential
    • Sarcomeres are segments of fibers
    • Intramuscular stores release Ca++

Motor endplate

How skeletal muscles contract

  • Myofibrils (w/in sarcomere)
    • Actin & mysosin proteins
    • “Molecular gears”
  • Bind, move, unbind in presence of Ca++, ATP

Anatomy of muscle fibers

Anatomy of motor endplate

Muscle contraction

Agonist/antagonist muscle pairs

Meat preference?

Muscle fiber types

  • Fast twitch/fatiguing
    • Type II
    • White meat
  • Slow twitch/fatiguing
    • Type I
    • Red meat

Muscles are sensory organs, too!

Two muscle fiber types

Two muscle fiber types

  • Intrafusal fibers
    • Sense length/tension
    • Contain muscle spindles linked to Ia afferents
    • ennervated by gamma (\(\gamma\)) motor neurons
  • Extrafusal fibers
    • Generate force
    • ennervated by alpha (\(\alpha\)) motor neurons

Monosynaptic stretch (myotatic) reflex

  • Muscle stretched (length increases)
  • Muscle spindle in intrafusal fiber activates
  • Ia afferent sends signal to spinal cord
    • Activates alpha (\(\alpha\)) motor neuron
  • Muscle contracts, shortens length

Monosynaptic stetch (myotatic) reflex

  • Gamma (\(\gamma\)) motor neuron fires to take up intrafusal fiber slack

Monosynaptic stretch (myotatic) reflex

Why doesn't antagonist muscle respond?

Why doesn't antagonist muscle respond?

  • Polysynaptic inhibition of antagonist muscle
  • Prevents/dampens tremor

Brain gets fast(est) sensory info from spindles

How the brain controls the muscles

  • Pyramidal tracts
    • Pyramidal cells (Cerebral Cortex Layer 5) in primary motor cortex (M1)
    • Corticobulbar (cortex -> brainstem) tract
    • Corticospinal (cortex -> spinal cord) tract
  • Crossover (decussate) in medulla
    • L side of brain ennervates R side of body

Corticospinal tract

How the brain controls the muscles

  • Extrapyramidal system
    • Tectospinal tract
    • Vestibulospinal tract
    • Reticulospinal tract
  • Involuntary movements
    • Posture, balance, arousal

Extrapyramidal system

Disorders

  • Parkinson's
  • Huntington's

The Faces of Parkinson's

Parkinson's

  • Slow, absent movement, resting tremor
  • Cognitive deficits, depression
  • DA Neurons in substantia nigra degenerate
  • Treatments

Awakenings

Huntington's

Huntington's

  • Formerly Huntington’s Chorea
    • "Chorea" from Greek for "dance"
    • “Dance-like” pattern of involuntary movements
  • Cognitive decline
  • Genetic + environmental influences
  • Disturbance in striatum
  • No effective treatment

Huntington's

Remember

  • Control of movement determined by multiple sources
  • Cerebral cortex + basal ganglia + cerebellum + spinal circuits

Multiple, parallel controllers

Cerebellum as predictor of future sensory states? (Ito, 2008)

The Real Reason for Brains

What does motor cortex activity encode?

Dynamic systems perspective

  • Dynamics of
    • World events, \(\dot W\)
    • Extero- and interoceptive sensory systems, \(\dot S\)
    • Nervous system states, \(\dot N\)
    • Muscle states, \(\dot M\)
    • Body states, \(\dot B\)
    • Effects of muscles on world, \(\dot W = f(\dot M)\)

Next time…

  • Language

References

Ito, M. (2008). Control of mental activities by internal models in the cerebellum. Nat. Rev. Neurosci., 9(4), 304–313. https://doi.org/10.1038/nrn2332

Ramirez-Zamora, A., Gee, L., Boyd, J., & Biller, J. (2016). Treatment of impulse control disorders in parkinson’s disease: Practical considerations and future directions. Expert Rev. Neurother., 16(4), 389–399. https://doi.org/10.1586/14737175.2016.1158103