2017-04-03 11:30:37

Prelude

Prelude

Today's Topics

  • 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

Remember

  • 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

  • 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
In practice, this doesn't happen because there is a polysynaptic connection from the stretch receptor that inhibits the antagonist muscle. So, the bicep's stretch receptor inhibits the extrafusal fibers in the tricep and vice versa.

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…

  • 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.