Voluntary Movement

Ch. 14: Supplemental Images

Skeletal Motor Unit: The basics• Upper motor neuron: Neurons that supply input to lower motor neurons.• Lower motor neuron: neuron that innervates muscles, including skeletal muscle fibers

involved in movement.1. Alpha motor neuron: Aα fiber innervates extrafusal muscle fibers2. Gamma motor neurons: Aγ fibers innervates intrafusal muscle fibers

• Motor Unit: one lower motor neuron and all muscle fibers that it innervates.1. Innervation ratio: Extraocular =1:10; Hand = 1:100; Gastrocemius in knee = 1:20002. Proportional to size of muscle3. Trade off between control and total force

• Extrafusal muscle fibers:1. Workhorse fibers – all of the force of muscle2. ACh at neuromuscular endplate: endplate potential that leads to “All-or-none” fiber contraction.3. Sherrington (1925) “ Final common pathway”

a. Lower motor neuron influenced by many sourcesb. Sums influences on neuron to determine if Action Potential occurs, releasing ACh at endplate

• Grading a muscle contraction1. Number of motor units activated2. Rate of action potentials of a motor neuron (rate modulation): higher rate = more force

Hierarchy of Movement• Motor Unit

• Reflexes:– Monosynaptic reflex arc (e.g. patellar)– Polysynaptic reflexes (e.g. cross-extensor reflex)

• Rhythmic Motor Patterns:– once initiated, the sequence of relatively stereotyped repetitive movements

can continue in automatic manner (e.g., walking)– combines characteristics of reflexes with voluntary (initiation,

termination) movement.– central pattern generators (Ch 13)

• Voluntary movement: – purposeful, goal-drected movement– Often learned movements which improve with practice and requires less

attention, e.g., combing hair, swinging a bat, driving a car, swimming, using eating utensils, etc, etc, etc….

– Cortical control over movement.

Voluntary movementOrganization of voluntary movement – general sequence

• Planning, strategy: – Purpose/motivation– Order of movement (each set of movement prepares body for next set)

•Direction•Rate of movement•Strength•Timing

–One movement relative to another–Duration of each movement–Phasic (transient, discrete movements)–Static (e.g., stabilize joints)

• Initiate (start) movement• Postural adjustments (+ feedback)• Medial limb movement (+ feedback)• Distal limb (+feedback)• Adjustments at EVERY point• Stop movement

Voluntary movement (cont.): hierarchical organization

2

3

4

3 3

5, 6ProprioceptiveSensory input

Non-proprioceptive sensory input1 1

7 7

8

3.5

3.5

General Organization of Spinal CordAscending pathways

Descending pathways

Lateral spinothalamic tract

14.2

Primary Motor Cortex: M1/Area 4/Precentral gyrus• Amount of force

– Different cells for extensors, flexors– Cells are active BEFORE contraction– AP rate increases and decreases as load

increases or decreases.• Direction of movement depends on

population of cells– Individual cells have directional preference– Population vectors– Requires feedback for ongoing corrections

• Minor discrepancies –modifies cells within population

• Major discrepancies - changes population of cells

• ALL cells are modified by– Other cortices– Basal ganglia– Cerebellum

14.7

14.14

Premotor Motor Cortex: Area 6

• Proximal and axial muscles – initial phases of orienting body and limbs– Set-related neurons

• Preparation of motor response: cells activated when subject is given information to perform a task with specific location

• Relatively simple tasks.• Cells active before execution of movement,

continues until movement is complete.• Does NOT appear to encode fine detailed

movement• Stimulus triggered movement

– Spontaneous movement

• Cell AP responses are modified by– Other motor cortices– Basal ganglia– Cerebellum

14.7

Descending pathways: Reticulospinal pathways

• Posture and antigravity muscles of limbs– Pontine: Enhances antigravity reflexes by facilitating extensors of legs to

maintain length and tension of muscles (e.g., vestibular).– Medullary: Inhibits antigravity reflexes (opposite effect) to allow change as

needed (e.g., to begin movement).– Cortex keeps these two antagonistic systems in balance.

14.6

14.5

Descending pathways: Vestibulospinal and tectospinal

– Vestibulospinal: Several vestibular nuclei in brain stem keep head balanced during movement and turns head as needed.

– Tectospinal: Helps coordinate eye movement (superior colliculus function) with head movement (tectospinal).

14.4

•Posture of head and neck

Descending pathways: Corticospinal and rubrospinal tracts

– Corticospinal (pyramidal): Contralateral Areas 4 and 6. 80-85% decussate (cross) at posterior medulla. Terminate on alpha motor neurons and interneurons – mostly flexor activation.

– Rubrospinal: Complements corticospinal pathways.

14.3

Fine Movement of Arms and Fingers

Basal Ganglia Terminology• “Basal Ganglia” includes all of the

following -– Caudate nucleus– Putamen– Globus pallidus (palidum) - external,

internal– Subthalamus– Substantia nigra

• Subgroupings– “Striatum” = caudate nucleus + putamen– “Lenticular nucleus” = putamen + globus

pallidus– “Corpus striatum” = caudate n. +

lenticular n. + internal capsule (cross-hatch of gray and white fibers)

• Direct Pathway

• Indirect Pathways

Direct Pathway (Motor Loop)

• Flow of information through basal ganglia for point-to-point adjustments in execution of ongoing motor signals

Motor cortices → putamen → globus pallidus → VA\VL → motor cortices

No Movement no AP no AP high AP = tonic inh. no AP unaltered

Ongoing movement

Topographic increase in AP

Specific +AP

specific - AP = release of inh.

Specific +AP

Specific +AP

+-

Selected excitation that modifies motor signals

Selected release of inhibition- - + - - -

+

Enhanced or modified movement

Other Input

Basal Ganglia: Indirect Motor Loops

Ventrolateral n.

14.12

Substantia Nigra Subthalamus

Combined +/-

Excites selected cells of GP which INCREASESinhibition of VL n. of thalamus (and decreases behavior)

Posterior and Anterior Spinocerebellar Pathways

Human upright position

Cerebellum (Latin="little brain")• Functions

– Coordinating reflexive and voluntary movement, particularly if• Movements are rapid• Require accurate aiming and timing• Automatic, not at level of awareness

– Maintain equilibrium– Generating and planning movements– Some motor learning (Classical conditioning; well-practiced, automatic responses)– Cognitive functions

• Behavioral Examples– Playing instruments– Athletic skills– Typing– Writing– Speaking

Cortical Zones (Nolte, 1993)

Medial Zone:Vermis (“worm”-spinocerebellum)Vestibular input/postural controlDeep nuclei

Lateral & Intermediate zonesSimilar but not same circuitry.Output through deep nucleiSuperior cerebellar peduncles (brachium conjunctivum)

VL/VA n. of thalamusMotor cortices

Flocculonodular node“Vestibulocerebellum”Vestibular input/eye movementDeep nuclei

Deep nuclei (Nolte, 1999)

• All output from cerebellar cortex goes through deep nuclei.

• Nuclei:– Fastigial– Interposed nuclei

• Globose n.• Emboliform n.

– Dentate

Principal Cerebellar - Cortical Relationship

• Per caption: principal circuit by which deep nuclei influence cortically controlled movement.

• PD: Pyramidal Decussation

• RST: Rubrospinal tract