Bi/CNS 150 Lecture 19
Monday November 11, 2013
Motor Systems
Chapter 14, p 309 (ALS); chapter 34, 35, 37, 38
Henry Lester, based on Ralph Adolphs’s lectures
1
Today:
Motor cortex
Corticospinal tract
Motor neurons
Reflexes
Basal ganglia
Higher motor functions
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Stages of Processing
1.
2.
3.
4.
5.
6.
7.
Transduction
Perception (early)
Recognition (late perception)
Memory (association)
Judgment (valuation, preference)
Planning (goal formation)
Action
3
Sensory & Motor Aspects of Behavior Account for Roughly Equal Times
44
Examples of motor output
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Spinal reflexes and motor units
Posture and muscle tone
Locomotion
Control of distal extremities
Breathing
Eye movements
Speech
Emotions
Autonomic Nervous System (visceromotor)
5
Motor output at different levels
Reflexes
--spinal
--central
"Fixed action patterns"
Emotional reactions
Actions
Long-term plans
Stimulus-coupled
Stimulus-decoupled
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Motor Areas of Cortex
Frontal Eye Fields
BA 8
Premotor/supplementary
Motor cortex
BA 6
Primary Motor Cortex
BA 4
Prefrontal Cortex
(Frontal
Association Areas)
Broca’s Area
(left side)
BA 44, 45
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Structure of Motor Cortex vs Sensory Cortex have striking differences
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MotorSystem
SystemHierarchy
Hierarchy
Motor
ganglia
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Key Motor Tracts
Decussation in hindbrain
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Some Spinal Cord Motor Concepts
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Motor unit: motoneuron and all innervated muscle fibers; variable
number of fibers, depending on force required
Alpha-motoneuron: final common pathway
Motoneuron terminals, endplates, muscle action potentials,
muscle contraction
•
When MN fires, all muscle fibers contract
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Recruitment: adding muscle units to increase force of contraction
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Fewer Myelinated Fibers in Lower Spinal Cord
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The Motor Unit
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Motoneuron in Typical Spinal Cord Cross Section
Dorsal Horn
Sensory
Motoneuron
Myelin
Ventral Horn
Motor
Ventral Root
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Motor
Electrophysiology of the Motor Neuron and Muscle Fiber
Previous Lectures
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Herniated Disks Compress Nerve Roots
(L5 most common)
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Motor Unit Size & Physiology
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Force increased by recruiting motor units
Motoneurons of different sizes: small MNS to small, slow motor
units; large MNs to large, fast motor units
Size principle: smallest motor units (and smallest force) first; then
larger motor units
Muscle fibers: slow (red); fatigue resistant (intermediate); fast,
fatigue (white)
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97% of spinal cord neurons are interneurons.
Reflexes must be coordinated; this is complex
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Sensorimotor integration in absence of supraspinal input
Motoneurons get input from sensory fibers, interneurons and descending fibers
Stretch reflexes
Flexion-withdrawal reflex
Crossed extensor reflex
Tracts
Groups of interneurons
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Ipsilateral part of the crossed
extensor reflex:
Interneurons inhibit extensors
when the flexors are
commanded, and vice-versa
Figure 35-2B
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A Feedback Loop Controls Muscle Function
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1. Sensory Organs in Muscle Participate in the Feedback Loop
Intrafusal fibers in parallel with extrafusal muscle fibers
Two types of sensory fibers – primary (Group Ia fibers)
and secondary (Group II fibers) spindle afferents
Group Ia – change in length (dynamic)
Group II – length (static)
Golgi tendon organ measures tension of muscle
contraction
Extrafusal
fibers
Sensory information goes to spinal cord segment,
dorsal column nuclei (proprioception), and cerebellum
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2. Gamma motoneurons in muscle participate in the feedback loop
Small MNs that project out ventral roots to
intrafusal fibers
Activity in gamma-MNs contracts the intrafusal
muscles and makes the spindle apparatus more
sensitive
In turn, the group Ia and II fibers become more
active
Gamma-bias impacts muscle tone
Extrafusal
fibers
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Damage to Motoneuron (Cell body or axon)
Example: Amyotrophic lateral sclerosis (ALS)
“Lou Gehrig’s Disease”
“Upper”
motoneurons
also
degenerate
Loss of motor unit innervation leads to weakness or
paralysis of muscle
Fasciculations (spontaneous contractions of muscle
fibers); detected with electromyography (EMG)
Atrophy of muscles, due to loss of trophic factors
from motoneuron
Hyporeflexia or areflexia
Average time from diagnosis to death ~ 3 yr
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The Basal Ganglia and ventral midbrain: Most Nuclei are GABAergic
“striatum”
Glutamatergic
Dopaminergic.
Future lecture on
Parkinson’s disease
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The Basal Ganglia: Major inputs
“striatum”
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The Basal Ganglia: Projections among nuclei
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Behaviors in Basal Ganglia Diseases
• Three common characteristics:
• tremor and other involuntary movements
• changes in posture and muscle tone
• slowness of movement without paralysis
• Cause either excess or diminished movement
• Cognitive changes (via caudate nucleus)
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Damage in the Motor System
Lower Motor Neuron
Upper Motor Neuron
Basal Ganglia
Paralysis
Paresis (weakness)
No paralysis
Muscle atrophy
No atrophy
No atrophy
Areflexia & atonia
Hyperreflexia, hypertonia, spasticity
Parkinson’s: rigidity, resting
tremor, bradykinesia
Huntington’s: chorea,
hyperkinesia
Ipsi deficit in spinal cord
Contra deficit above decussation;
Ipsi deficit below decussation
Contra
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Stimulation in human motor cortex.
An array is implanted . . .
to localize an epileptic focus
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Anterior Cingulate Cortex
Lesions in this region cause impairment in one of the hierarchically highest levels
of the motor system: the will to act .
Patients with lesions to ACC can exhibit "akinetic mutism": they are not paralyzed and are
conscious but respond poorly to their surroundings.
They sometimes respond to very automatic things, like picking up a phone that rings next to
their bedside (but then say nothing).
They often recover, and then explain that while in this state, they were fully conscious but just
lacked motivation to do anything and so did not respond or act on their surroundings.
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Links Between Perception and Action:
Why Can’t You Tickle Yourself?
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Links Between Perception and Action:
Neurons
Mirror Mirror
Neurons
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End of Lecture 19