Motor system Flashcards by Unknown Unknown

4 Systems Control Movement

1 - Local SC & brainstem circuits: located in grey matter of Sc & tegmentum of brainstem
2 - Descending control centers in the cerebral cortex & brainstem
3 - Cerebellum
4 - Basal ganglia

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1 - SC & Brainstem circuits = Local level - cells include:

1 - Lower motor neurons: send axons out of brainstem & Sc to innervate skeletal muscles of head & body
2 - Local circuit neurons - major source of synaptic input to all lower motor neurons

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2 - Descending Systems = Middle level; consists of:

1 - UMN (upper motor neurons) from cortex - planning & initiation of voluntary movement of muscles
2 - UMN from brainstem - regulating muscle tone & orienting eyes, head & body with respect to vestibular, somatic, auditory, visual sensory info

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3 & 4 - Cerebellum & Basal Ganglia = Middle level

massive complex neural circuits with output pathways that have no direct access to either LCN or LMN
Control movement indirectly by regulating activity of UMNs

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Function of cerebellum vs basal ganglia

Cerebellum - co-ordination of ongoing movement

Basal ganglia - prevents UMN from initiating unwanted movements & prepares motor circuits for initiation of movements

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Which motor neurons located most medially in ventral horn of SC:

Motor neurons that innervate the axial musculature (postural muscles of the trunk)
LMN innervate the proximal muscles

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Which motor neurons located more laterally in ventral horn of SC:

Motor neurons that innervate the muscles of the shoulders

LMN that innervate distal muscles

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Define motor unit

a single motor neuron & all of the muscle fibres it innervates
(each alpha motor neuron synapses w multiple fibers in the muscle)

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Types of motor units

1 - Slow motor units : resistant to fatigue, contract slowly
2 - Fast fatigable (FF) motor units: little mitochondria (easily fatigued)
3 - Fast Fatigue resistant (FR) motor units: not as fast as FF; generate twice the force & fatigue resistant

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5 Components of a reflex arc:

1 - Sensory receptor
2 - Sensory neuron
3 - Information processing in CNS
4 - motor neuron
5 - effector

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Reflexes by early development:

Innate reflexes: Basic neural reflexes formed before birth, genetically programmed, e.g. withdrawal to pain, chewing, visual tracking
Acquired Reflexes: Rapid, automatic learned motor patterns, repetition enhances, e.g. braking car in emergency

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Reflexes by motor response:

Somatic Reflexes: control skeletal muscle contractions, superficial reflexes (skin/mucous membranes), immediate, eg. pattellar (knee jerk)
Visceral reflexes (autonomic reflexes): control other effectors - smooth muscle, cardiac muscle, glands

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Reflexes by complexity of neural circuit:

Monosynaptic reflex: Single synapse - simplest reflex arc; sensory neuron synapses directly w motor neuron; fast response e.g. stretch reflex
Polysynaptic reflex: at least 1 interneuron between sensory neuron & motor neuron (most common); slower response; intersegmental reflex arcs: many spinal cord segment interact (variable response), e.g.Flexor/Crossed Extensor reflex

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What cells do the lateral premotor cortex contain:

Mirror cells

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What are mirror neurons

Mirror neurons are neurons that fire both when an animal acts and when the animal observes the same action performed by another

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Which part of the brainstem is important in feedforward control of posture:

Reticular formation

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Name the two tracts from the corticospinal tract

Lateral Corticospinal tract & Anterior corticospinal tract

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What are considered pyramidal tracts?

Pass through medullary pyramids –> descend onto LMN in SC

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Lateral corticospinal tract

90% of fibers from corticospinal tract decussate at medullary pyramid & descend in lateral SC (Lateral CST)
Detailed movements of distal muscles

Anterior corticospinal tract

10% of CST fibers descend ipsilaterally in ventral SC (anterior CST) (do not decussate)
Axial & proximal muscles

Collaterals of CST

Collaterals extend to red nucleus & reticular formation

Corticobulbar Tract

Brain –> spinal nerves
Control facial, neck muscles
Pathway: 1 motor cortex - through corona radiata —> midbrain –> exit brainstem at appropriate level to synapse on LMN of cranial nerves
ONLY nerves controlling muscles of lower face decussate

DAMGE to corticobulbar tract:

Unilateral - only involves face

Bilateral - pseudobulbar pasly (inability to control facial muscles)

Extra-Pyramidal pathways

Motor neurons from motor cortex that don’t pass through pyramids of medulla - tracts run through pons & medulla
Damage - various types of dyskinesias (involuntary movement disorders)

RUBROSPINAL TRACT

``` Not sure if exists in humans Originates in red nucleus projects to SC Runs adjacent to CST Projection participates together with direct pathway from motor cortex control of arms Damage - temporary slowness of movement ```

Vestibulospinal Tract

Lateral & Medial Vestibulospinal tract Lateral - originates in lateral vestibular nucleus medial - originates medial vestibular nucleus Activates extensors; inhibits flexors Maintains upright balance & posture Receives input form cerebellum

Reticulospinal Tract

Coordinates autonomic locomotion & posture movements Pons (pontine) medial reticulospinal tract Medullary (lateral) reticulospinal tract both originate from reticular formation in corresponding areas (pons & medulla)

Somatotopic organisation in motor cortex

not as finely represented as in the ssx More overlapping, multiple representations, no strict somatotopy: - A single corticospinal axon may supply multiple columns of LMN in ventral horn - A single spike in corticospinal axon may activate 4 different muscles in the forelimb More movement representation than muscle representation

Babinski Reflex

Sole of foot is stimulated Normal plantar response - toes down (Flexion) Extensory plantar response - fanning of toes (babinski sign - sign of pathology)

Basal Ganglia Structures:

Striatum (caudate nucleus & putamen) Globus Pallidus (external & internal segments) Substantia nigra Subthalamic nuclei

Two distinct regions of substantia nigra

``` Pars compacta (Snc) - dopaminergic Pars retiuclata (Snr) - GABAergic ```

Disorders of Substantia Nigra

Parkinson's Disease | Loss of dopaminergic neurons of substantia nigra (pars compacta (Snc)

Affect of degeneration of pars compacta

Reduce inhibition via indirect pathway, reduce excitation via direct pathway

Treatment for Parkinson's

``` Aim to increase dopamine levels L-DOPA (dopamine precursor) Dopamine Agonist (bromocriptine) binds to postsynaptic dopaminergic receptors MAO-B inhibitors: inhibit dopamine breakdown by MAO ```

Huntington's disease

Disease caused by destruction of striatal, cortical, cholinergic neurons, inhibitory GABAergic neurons Presents with chorea (writhing movements), dementia

Located in precentral gyrus of frontal lobe | Function: control skeletal muscle movement

Primary motor cortex

Located anterior precentral gyrus | Function: controls learned motor skills & coordinates movement of several muscle groups

Premotor cortex

true or false: motor neurons are scattered haphazardly all over the SC

FALSE

Lateral descending tracts are associated with the function of a) and medial tracts are associated with the function of b)

a) distal | b) proximal

A reflex are includes:

At least 2 sequential neurons

T/F: Reflex actions include the inhibition of sensory receptors

false

Patellar reflex is an example of a) reflex and the flexor reflex is an example of a b) reflex

a) monosynaptic | b) polysynaptic

Class = monosynaptic; stimulus = muscle stretch, effect = muscle contraction

Stretch reflex

Class = polysynaptic, stimulus = pain, effect = flexion of afferent limb

Flexor/withdrawal reflex

In the flexor withdrawal reflex, contraction of flexor muscles is associated with reciprocal inhibition of:

Ipsilateral extensor muscles

Descending motor tracts:

Corticospinal tract Corticobulbar tract Reticulospinal tract

Corticospinal tract originate from

a) premotor area in frontal lobe b) supplementary motor area in frontal lobe c) somatosensory area in parietal lobe