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Organization of Motor Functions 1. The Spinal Cord

Prof. Szabolcs Kéri

Prof. Gyula Sáry

Learning objectives 106-111.

Why is it important to study the motor system?

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Motor symptoms: the most important first-glance diagnostic features in

STROKE

85% 15%

- 15 million people/year worldwide suffer a stroke- 6 million die, 5 million permanently disabled

Transient ischemic attack (TIA): symptoms resolved within 24 hours

Myoelectronic control for robotic hand-assisted training in stroke

Lu Z et al. Front Neurol 2017;8:107.

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Cortex

Basalganglia

Cerebellum

Thalamus

Brainstem

Spinal cord MOVEMENTSTIMULI

Reflexes

Muscle tone and posture

Voluntary movements

Coordination of movementsSelection of movement patterns

REFLEX

- Stereotyped and inherited response to an adequate stimulus

- Elicited if the stimulus reaches a threshold

- Possible without consciousness

- But: voluntary modulation and variability for adaptive behavior

The reflex arc:

Receptor → afferent nerve → central processor (spinal cord, brain) →

efferent nerve → effectors (muscle)

Types:

Proprioceptive reflex: receptor and effector are in the same organ

(e.g., muscle spindle, Golgi tendon organ)

Exteroceptive reflex: receptor and effector are in different organs

(e.g., flexor reflex elicited by skin injury)

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Table support Hold cup

Table support

Hold cup

Avera

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nsor

Hold cup

Time (ms)

Reflexes are more complex than previously thought:modulation by task context and aim

excitation

inhibition

Basics of spinal cord functioning

1. Myotatic reflex: muscle spindle

2. Inverse myotatic reflex: Golgi tendon organ

3. Flexor – extensor reflex: skin receptors

4. Intersegmental integration

5. Dysfunctions of the spinal cord

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I. Reflexes of the spinal cord

Essential basics

• Skeleton as a framework for movement

– flexion, extension, adduction, abduction

• Muscle function and body movement

– isotonic and isometric contraction, agonists and antagonists

• Nervous system components

– α motor neurons as the final common path

• α and γ motoneurons, extra and intrafusal fibres, motor unit

– feedback system for continuous control

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Proprioceptors of the muscle 1.

MUSCLE SPINDLE

Location: fine, encapsulated intrafusal fibers parallel with extrafusal

fibers (working muscle fibers)

Task: detection of change in muscle length

Intrafusal muscle fibers (within the spindles)

1. nuclear bag fiber (static and dynamic)

2. nuclear chain fiber

Polar regions (edges) can contract – innervated by gamma motoneurons

Central region cannot contract (detection of length change)

Ia afferents (primary annulospiral nerve ending)

function: detection of dynamic changes (fast, short-duration activity)

II afferents – secondary nerve ending, only on static nuclear bag and chain fibers

function: detection of long-lasting and slow changes

central

polar

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Efferent (motor) fibers to muscle spindle: gamma-loop

Alpha - motoneurons: nerves to extrafusal fibers (working muscle)

(contraction → intrafusal tension↓)

Gamma - motoneurons: contraction in polar region of intrafusal fibers

→ tension↑ → compensation of tension reduction induced by the

contraction of working muscle, adjustment of sensitivity, maintenance of

muscle tone

1. Contracting

(shortening)

extrafusal muscle -

a-b distance will be

shorter

2. Passive tension in intrafusal

fibers↓ (slackening)

3. Contraction in polar regions

stretches intrafusal

fibers (pulls the central region from

both ends)

Proprioceptors of the muscle 2.: The gamma loop

http://www.angeltear.com/spindle/spindle.html

a

b

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Alpha-motoneuron Contraction of working muscle(extrafusal)

Collapse of tension sensing fibersof muscle spindle (intrafusal)Sensitivity↓

Gamma-motoneuron

Spindle fibers are stretched (polar regioncontraction)

Gamma-motoneuron’s intensive activity:spindle fibers are strongly stretched, their sensitivity is increased, the reflexcan be elicited easierBeta-motoneuron: innervating both extra- and intrafusal fibersBeta-activation = alpha-gamma co-activation

Ia, II

Interaction of alpha and gamma motoneurons in spinal cord:the gamma loop (fusimotor servo system)

GOLGI TENDON ORGAN

Location: - tendon-muscle boundary among collagen fibers

- serial connection with working muscle

Task: detection of muscle contraction and stretching

Ib afferent: activation during muscle contraction → further contraction↓

(negative feedback)

Proprioceptors of the muscle 3.

Musclefibers

Ib afferent

Capsule

Collagenfiber

Tendon

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1. Reflex of the muscle spindle:stretch (myotatic) reflex

Reflex of m. quadriceps femoris – patellar „knee-jerk” reflex (L4)

Tap on tendon → stretch of muscle spindle → spindle Ia afferent →

ventral horn (alpha – motoneuron) → quadriceps contraction

Reciprocal inhibition: inhibition of antagonist muscle (flexor) via an

interneuron

Hoffmann (H) reflex: myotatic reflex elicited by electric stimulation

of Ia afferents, muscle activity registered by electromyography

Tap on tendon:musclestretch

ExtensorSpindle

Scheme of myotatic (stretch) reflex

Sensorynerve

Extensor

moto-

neuron

activated

Flexor moto-neuron

inhibited

Monosynaptic reflex

Disynapticreflex

Ganglion

Dorsal horn

Ventral horn

Rexed laminae

Bell - Magendie rule:nerve impulse goes in at dorsal horn, leaves at ventral horn

Modified after Prof. Benedek (2009)

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Enhancement of the knee-jerk reflex:

- Jendrassik maneuver (crossing

fingers and pulling hands)

- Arousal, anxiety

- Hyperactive reflex because of the

loss of descending inhibition

- Myoclonic reaction (repetitive,

rhythmic contraction as a form of

hyperactivity)

- Often with increased muscle tone

Mechanism:

- Gamma-system↑

- Increased background alpha-activity

- Increased Ia activity

Achilles reflex (S1)

Biceps reflex (C5-6) Triceps reflex (C7)

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2. Reflex of the Golgi tendon organ: inverse myotatic reflex

Muscle contraction → Ib afferent → dorsal horn → interneuron (disynaptic

reflex) → contracted muscle’s alpha–motoneuron is inhibited

Sensory neuron

Extensor (antagonist muscle)

Flexor

Golgi tendonorgan

Biceps inhibited

Extensor activatedModified after Prof. Benedek (2009)

Convergence on Ib interneurons

Action of Ib afferents

Intracellular recording from

ankle extensor motor neuronIb afferent

Joint afferent

Cutaneous afferentDescendingpathways

Ib inhibitoryinterneuron

Resting

Locomotion

Joint receptor

Flexor muscleGolgi

tendonorgan

Extensor muscle

State-dependent reflex reversal

Animal in rest: Ib stimulation inhibitsmotor neuron (classic Golgi tendonorgan reflex)

Walking: Ib inhibitory interneurons areinhibited by other afferents – the reflex is reversed

Reconceptualization of Golgi tendonorgan

Classic role:Protection of muscle from overstretching(activation when the tension in themuscle is too high)

New roles:- monitoring rapid changes in tension- precise control of muscle force (softgrasping) – together with other Ib afferents!

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3. Reflexes of exteroceptors: flexor - extensor reflex

- Stimulus:

Aβ: stretch, deformation (skin pressure, touch)

Aδ, C: injuring stimuli (skin and muscle pain - nociceptors)

- Type: multisynaptic; (i) more than one spinal cord segments, (ii)

multisensory, (iii) irradiation, (iv) habituation

- Process:

stimulus side:

flexion (moving away from painful stimulus)

opposite site:

extension (maintaining balance and posture)

Crossed flexor-extensor reflex(flexion-withdrawal)

Other

exteroceptive

(cutaneous)

reflexes:

- Plantar reflex

- Abdomen

skin reflex

- Cremaster reflexIpsilateralextensorsinhibited

Flexorsactivated

Contralateralflexorsinhibited

Contralateralextensorsactivated

Modified after Prof. Benedek (2009)

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Normal flexion in plantar (sole) reflex

Babinski sign

Injury or immaturation of the descending control(pyramid tract): the Babinski sign

Rapid stimulation of abdominal skin (from lateral to

medial)

Muscle contraction,umbilicus

moves toward the site of stimulus

Abdominal skin reflex (Th7-12)

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II. Intersegmental integration in the spinal cord

Multisegmental and integrative functions of the spinal cord

Interneurons and motoneurons - complex interactions

Functions of the interneurons:

1. Receiving afferent pathway information (e.g., reflex arc, Ia inhibitory

interneurons)

2. Receiving descending controls pathways (e.g., pyramid tract)

3. Intrinsic cyclic activity

- Central Pattern Generation population of cells

- LDP [locomotor drive potential] - activation and

coordination of flexor/extensor muscles and limbs (e.g.,

walking)

4. Renshaw-cell: inhibitory interneuron of motoneurons (transmitter:

GABA or glycine)

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Topography of motoneurons:

A. Flexor: dorsalExtensor: ventral

B. Proximal - distalarrangement

Flexormotoneuron’saxon

Extensormotoneuron’saxon

Iainhibitoryinterneuron

Renshaw-cell (RC)

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Intersegmental integration:

- Scratch reflex in animals

- Locomotion: decerebrated animals (e.g., feline) - central pattern

generator in spinal cord elicits walking-like motor pattern;

drive: „walking center in the mesencephalon

- Humans: cephalization - complex movement requires cerebral cortex

CPG = central

pattern generator

FRA = flexor reflex

afferent

IN = interneuron

MN = motoneuron

Electromyography:

spontaneous reciprocal

flexor - extensor

activity

stimuli may interrupt it

Pattern generation within the interneuron group

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Strengthening spinal automatism in rehabilitation

Black: patients with wheelchair

Wering A et al. Eur J Neurosci 1995;7:823.

With robotic orthotics

Colombo G et al. JRDD 2000, 2008

III. Dysfunctions of the spinal cord

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Disorders of motoneurons in humans

Lower motoneuron: innervating

muscle (alpha motoneurons)

Upper motoneuron: cortex

Peripheral palsy (paresis):

- Injury: lower motoneuron

- Muscle: loss of tone (flaccid)

- Decreased reflexes

- Muscle fasciculation

- Rapid decline of muscle tissue

(atrophy)

Central paresis:

- Injury: upper motoneuron

- Spastic muscle (tone↑)

- Increased reflexes

- Slow atrophy due to

inactivation

- Babinski sign

Stephen Hawking

Degeneration of motoneurons in amyotrophic lateral sclerosis

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Consequences of spinal cord injury in humans 1.

Partial or total injury induced by: trauma, tumor, infection, hypoxia,

discus hernia

Severe injury: spinal shock with motor, sensory, and vegetative

symptoms

Phases:

1. Loss of all reflexes below the level of transsection (~ 24 hours) –

(disconnection of descending facilitation)

2. Polysynaptic reflexes regenerate (~ 72 hours)

3. Tendon reflexes return + hyperreflexia (~ 1 week - 1 year) –

- growth of moto- and interneuron’s projections

- genesis of new synapses

Not restored after spinal shock in humans:

- Voluntary movements

- Standing and walking

- Muscle tone’s adequate regulation

Paraplegia: paralysis of the lowerpart of the body

Tetraplegia: paralysis of all 4extremities

Hemiplegia: paralysis of one sideof the body (e.g., left arm and leg

after right-sided cerebral lesion,

no spinal shock)

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Brown – Séquard syndrome: one-sided spinal cord lesion

Posterior ascending system (fasciculus gracilis/cuneatus)

Corticospinal tract

Anterior and lateral spinothalamic tract

- In the segment of the lesion: flaccid paresis and reflex loss

- Below the segment of the lesion:

1. same side spastic paresis – disconnection of corticospinal tract

2. same side epicritic sensory loss (fine touch, vibration, position) – disconnection

of posterior ascending system

3. opposite side protopathic sensory loss (heat, pain, coarse touch) –

disconnection of spinothalamic tract

Lesion above thoracic 6 segment:

- loss of sympathetic tone (blood pressure↓, hear rate↓)

- loss of urination and defecation reflexes (initial phase of spinal shock)

Vegetative hyperactvity (3-6 weeks):

- hypertension, sweat, headache

- return of urination/defecation reflexes (can be hyperactive) but the

voluntary controls is lost

n. IntermediolateralisLesion in the segment:

red and dry skin

Consequences of spinal cord injury in humans 2.Vegetative functions

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Human tetraplegy, cervical 7 lesion, stimulation of sacral ventral root

Urinary bladder pressure

Sphincter pressure

Blood pressure

Heart rate

Schurch B et al., 2010