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Bi/CNS 150 Lecture 19

Monday November 11, 2013

Motor SystemsChapter 14, p 309 (ALS); chapter 34, 35,

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Henry Lester, based on Ralph Adolphs’s lectures

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Today:

Motor cortex

Corticospinal tract

Motor neurons

Reflexes

Basal ganglia

Higher motor functions

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1. Transduction

2. Perception (early)

3. Recognition (late perception)

4. Memory (association)

5. Judgment (valuation, preference)

6. Planning (goal formation)

7. Action

Stages of Processing

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Sensory & Motor Aspects of Behavior Account for Roughly Equal Times

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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)

Examples of motor output

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Motor output at different levels

Reflexes --spinal --central

"Fixed action patterns"Emotional reactionsActionsLong-term plans

Stimulus-coupled

Stimulus-decoupled

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Primary Motor CortexBA 4

Premotor/supplementaryMotor cortex

BA 6

Frontal Eye FieldsBA 8

Broca’s Area(left side)BA 44, 45

Prefrontal Cortex(Frontal

Association Areas)

Motor Areas of Cortex

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Structure of Motor Cortex vs Sensory Cortex have striking differences

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Motor System Hierarchy

ganglia

Motor System Hierarchy

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Key Motor Tracts

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Decussation in hindbrain

• 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

• Recruitment: adding muscle units to increase force of contraction

Some Spinal Cord Motor Concepts

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Fewer Myelinated Fibers in Lower Spinal Cord

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The Motor Unit

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Myelin

Dorsal HornSensory

Ventral HornMotor

Ventral RootMotor

Motoneuron in Typical Spinal Cord Cross Section

Motoneuron

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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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• 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)

Motor Unit Size & Physiology

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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

97% of spinal cord neurons are interneurons.

Reflexes must be coordinated; this is complex

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Tracts Groups of interneurons

Ipsilateral part of the crossed extensor reflex:

Interneurons inhibit extensors when the flexors are

commanded, and vice-versa

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A Feedback Loop Controls Muscle Function

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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

Sensory information goes to spinal cord segment, dorsal column nuclei (proprioception), and cerebellum

1. Sensory Organs in Muscle Participate in the Feedback Loop

Extrafusal fibers 22

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

2. Gamma motoneurons in muscle participate in the feedback loop

Extrafusal fibers 23

Damage to Motoneuron (Cell body or axon)Example: Amyotrophic lateral sclerosis (ALS)

“Lou Gehrig’s Disease”

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“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

The Basal Ganglia and ventral midbrain: Most Nuclei are GABAergic

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“striatum”

Dopaminergic.Future lecture on

Parkinson’s disease

Glutamatergic

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The Basal Ganglia: Major inputs

“striatum”

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The Basal Ganglia: Projections among nuclei

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, bradykinesiaHuntington’s: chorea,

hyperkinesia

Ipsi deficit in spinal cordContra deficit above decussation;

Ipsi deficit below decussationContra

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Stimulation in human motor cortex.An array is implanted . . .

to localize an epileptic focus

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Anterior Cingulate CortexLesions 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?

Mirror Neurons

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Links Between Perception and Action:Mirror Neurons

End of Lecture 19