2017-11-05 08:21:08

Prelude (6:09)

Today's Topics

  • Wrap up on pain
  • The neuroscience of action

The Real Reason for Brains

The neuroscience of action

  • What types of actions are there?
  • How are they produced?
    • By the muscles
    • By the nervous system

Nervous system "output" includes

  • Movements
  • Autonomic responses
  • Endocrine responses

Types of actions

Types of actions

  • Reflexes
    • Simple, highly stereotyped, unlearned, rapid
  • vs. planned or voluntary actions
    • Complex, flexible, acquired, slower
  • Discrete (reaching) vs. rhythmic (walking)
  • Ballistic (no feedback) vs. controlled (feedback)

Multiple, parallel controllers

Key "nodes" in network

  • Primary motor cortex (M1)
  • Non-primary motor cortex
  • Basal ganglia
  • Brain stem
  • Cerebellum
  • Spinal cord

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

  • Motor neuron soma located in ventral horn of spinal cord
  • 'motor unit' = one motor neuron + all muscle fibers it connects with
  • Motor neurons create specialized synapse = neuromuscular junction
    • Releases ACh

From spinal cord to muscle

How skeletal muscles contract

  • Nicotinic ACh receptor (nAChR) binds ACh
    • Nicotine also binds to this receptor
    • nAChR's found in muscle (also in ANS and CNS)
  • Rate of motor neuron firing ~ force produced ('rate coding')

nAChR activation produces excitatory endplate potential

  • Na+ influx/K+ efflux
  • Muscle fibers depolarize
  • Depolarization spreads along fibers like an action potential
  • 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

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

Final thoughts

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

Next time…

  • Review for Exam 3

References

Ramirez-Zamora, Adolfo, Lucy Gee, James Boyd, and José Biller. 2016. “Treatment of Impulse Control Disorders in Parkinson’s Disease: Practical Considerations and Future Directions.” Expert Review of Neurotherapeutics 16 (4): 389–99. doi:10.1586/14737175.2016.1158103.