Reflex & Voluntary Control of Movement

Csilla Egri, KIN 306, Spring 2012

Keyboard cat, a fine example of voluntary control of movement in the animal kingdom.

Outline

Locomotion reflexes Central pattern generators (CPGs)

Descending tracts Pyramidal tracts Extrapyramidal tracts

Cortical control of movement Motor cortex

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Motor Control3

There are three levels in the hierarchy of motor control: Spinal cord

Locomotion reflexes CPGs

Brain stem Postural reflexes Locomotor regions Voluntary control of movement

Cortical motor areas Voluntary control of movement

These different areas are highly interdependent and movements result from the coordinated action of these three regions.

Talked about these during the vestibular and visual system lectures

Spinal cord: locomotion reflex example

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Stepping reflex in newborns Disappears around 6 weeks, gradually replaced by voluntary walking behaviour

Simplified half-center model for alternating rhythm generationB&B Figure 14-8

Spinal cord: central pattern generators (CPGs)

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Neuronal network capable of generating a rhythmic pattern or motor activity in the absence of sensory input Walking, swimming, respiration Simplest CPGs contain spontaneously bursting or

reciprocally innervated neurons Basic firing pattern modified by sensory or

descending inputs

Spinal cord: CPG example6

Transect afferent input, and decerebrate cat: With support, walking motion is reproducible with stimulation of brainstem Each limb is controlled by its own CPG

Spinal Cord: organization of motor tracts

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Motor neurons of ventral horn organized topographically Descending upper motor neurons control lower motor neuron

firing Also influenced by the activity of interneurons & peripheral

sensory receptorsB&L Figure 9-12

Spinal Cord: organization of motor tracts

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Medial reticulospinal tract

Direct (pyramidal) pathway lateral and anterior corticospinal tract corticobulbar tract

Indirect (extrapyramidal) pathway rubrospinal tract tectospinal tract vestibulospinal tract reticulospinal tract (lateral & medial)

Direct (pyramidal) pathways

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Pyramidal tracts originate primarily in motor, premotor and supplementary motor areas of cortex

~ 90% of descending axons cross in the medulla and descend in lateral columns of spinal cord (lateral corticospinal tract)

Control distal muscles; precise, agile, skilled movements

~ 10% cross over in spinal cord (anterior corticospinal tract)

Control proximal trunk muscles; coordinate movements of axial skeleton

Corticolbulbar tracts also originate in motor cortex, descend to brainstem

Axons terminate in motor nuclei of cranial nerves

Control precise, voluntary movement of head, neck and tongue

Indirect (extrapyramidal) pathways

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Complex, polysynaptic circuits involving motor cortex, basal ganglia, thalamus, cerebellum, and reticular formation

Rubrospinal: from red nucleus to contralateral muscles controlling precise movements of distal parts of upper limbs

Tectospinal: from superior colliculus to contralateral muscles controlling reflexive movements of head and neck to auditory or visual stimuli

Medial reticulospinal tract

Vestibulospinal: from vestibular nucleus to ispsilateral skeletal muscles of trunk and proximal parts limbs for maintaining posture and balance

Medial and lateral reticulospinal: from reticular formation to ipsilateral skeletal muscles of trunk and proximal parts of limbs for maintaining posture and regulating muscle tone in response to body movements.

Motor areas of cerebral cortex11

Primary motor cortex Main motor area involved in executing

voluntary movements Movement elicited with least amount of

electrical stimulation Pre motor cortex

Sensory guidance of movement (receive input from posterior parietal cortex

Contributes to extrapyramidal pathways Lesion impairs ability to develop strategy

for movement Supplementary motor cortex

Involved in planning of complex and two handed movements

Coordinates posture

Motor areas of cerebral cortex: somatotopy12

Specialized cortical motor areas 13

Broca’s area (area 44, 45) Coordinated movements of

tongue and vocal cords for word formation

Lesion results in expressive aphasia

Voluntary eye movement field Also controls voluntary blinking

Area for hand skill Lesions result in motor apraxia:

uncoordinated, nonpurposeful hand movements

Guyton Figure 55-3

Voluntary movement: simplified linear sequence of events

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Command must be organized by the brain1. Identify target in space

a) objective is identified in posterior parietal cortex which receives input from somatosensory, visual, vestibular and auditory systems

b) Sense of body position in relation to target also required2. info transmitted to supplementary & premotor areas

where the motor plan is developeda) Choice of muscles, sequence of contractions, required

force and trajectory computed3. Motor plan transmitted to primary motor cortex and

down descending pathways to interneurons & motor neurons

Motor Plan15

Sensory feedback provided through ascending afferent pathways Transmitted to motor cortex either directly from

thalamus or indirectly thru connections between somatosensory & visual cortex

Motor cortex has bidirectional connections with thalamus, cerebellum & basal ganglia Important in planning & execution of movement

(more in next lecture)

Objectives

After this lecture you should be able to: Give an example of a locomotion reflex and an activity

governed by a CPG Discuss the organization of the spinal cord and how it

relates to voluntary control of movement Include both pyramidal and extrapyramidal descending tracts

Describe the organization of the motor cortices Outline the sequence of events involved in initiation of

voluntary movement

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1. A lesion to Broca’s area results in _____________________

2. Precise, voluntary movements of the head, neck and tongue are controlled by descending inputs via the __________________________ tract

3. Motor neurons descending in the pyramidal tracts originate in ________________________ whereas motor neurons descending in the extrapyramidal tracts originate in _______________________________________________.

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