Lecture 9: Descending/Motor Spinal Pathways

Elicited by cutaneous, nociceptive, or thermoreceptive activity

Monosynaptic reflexes

Because of antagonism between opposing muscles

Polysynaptic reflexes

This is a lower motor neuron (LMN)

No reflexes=LMN disease (no brain needed)

Final common pathway

Alpha motor neuron

Motor cortices in frontal lobe found here:

• Primary motor cortex (pre-central gyrus)

• Premotor areas ( association areas) – discussed in depth in future lecture

Basal ganglia (sub-cortical) found here

Forebrain

What 5 structures are found in the brainstem?

Acronym: Create Red Veggies Ready Soon

1. Cerebellum (does not have its own tract, works through vestibular)

2. Red nucleus (midbrain, deep in thalamus)

3. Vestibular nuclei (balance)

4. Reticular formation (reticulospinal tract)

5. Superior colliculus/tectum (tecto-spinal tract)

What cortex is involved in voluntary movement?

Motor cortex

• Primary (pre-central gyrus)

Originate mostly from precentral, primary motor cortex and travel to spinal cord levels WITHOUT intervening synapses.

May synapse directly onto alpha motoneurons OR interneurons which then synapse onto alpha

• Typically purposive, manipulative movements (voluntary)

• Typically distal musculature and fine movements

Corticospinal - Pyramidal

Originate from brainstem nuclei (but these have input from motor cortex)

Automatic, ‘preconscious’ movements

Typically postural or ‘in-flight’ corrections

Typically proximal * musculature and more gross movements.

Other - Extrapyramidal

Descending motor axons terminate onto these neurons which have direct activation of a specific motor unit and are critical in fine motor control

Alpha motoneurons

Descending motor axons terminate onto these neurons which excite alpha motoneurons, affecting movement indirectly, or can allow for activation of a larger number of motor units or even entire muscles

Interneurons

Organized at the spinal cord level

Can be activated by descending tracts synapsing onto just a few interneurons rather than thousands of individual alpha motoneurons.

Reflexive movement ‘programs’

Axons from the corticospinal tracts (lateral + anterior) originate mostly from this cortex then travel caudally though the forebrain via the corona radiata, then ____a______

Then, in the midbrain via ___b____

In the pons - axons lie posterior to pontine fibers

Travel in medulla as the __c___

• ____d_____ will cross at the pyramids

• Most of the ___e___ will stay ipsilateral

Axons then descend in the spinal cord in the positions indicated by their names (lateral or anterior/ventral)

a. internal capsule

b. crus cerebri

c. pyramids

d. Lateral corticospinal

e. anterior corticospinal tracts

This is the main motor pathway in humans.

• Dexterous movements, fractionation (FINE MOTOR CONTROL - finger, hand movement)

• 90% cross in the MEDULLA

• Synapses onto interneurons or alpha motoneurons of DISTAL musculature

• Note – NO intervening synapses between motor cortex and termination in spinal cord

Lateral Corticospinal tract (mostly contralateral control)

PROXIMAL tract - upper muscles (neck, shoulder, upper trunk)

• Gross motor control

• DOES NOT cross in medulla - remains ipsilateral

• Descends WITHOUT intervening synapses

• Synapses onto ipsilateral interneurons in the spinal cord which will form bilateral synapses with alpha motoneurons

Anterior/ventral Corticospinal Tract (Bilateral control)

What type of lesion is this?

Contralateral loss of dexterous movements

Gross movements remain

Spasticity and hyperactive reflexes

Lateral corticospinal unilateral lesion

What type of lesion is this?

Bilateral loss of gross movements in neck, shoulder and upper trunk

Anterior corticospinal unilateral lesion

Mostly gross movement, little or no fine muscle control.

One set of tracts terminate onto proximal muscles (postural adjustments)

Another set of tracts terminate onto more distal muscles (gross locomotion)

Extrapyramidal

What are 2 tracts that terminate onto proximal muscles and are extrapyramidal?

Vestibulospinal tracts - ipsilateral

Tectospinal

Includes input from the vestibular system

Balance

Controls all proximal muscles including facilitation of extensor muscles in the upper leg

Vestibulospinal tract

Includes the superior colliculus

Upper trunk and neck

Coordinates head and eye movements

Tectospinal tract

What 2 tracts terminate onto distal muscles (extrapyramidal)?

R.R

Reticulospinal

Rubrospinal

What aspect of the reticulospinal tract facilitates extensor muscles (inhibits flexor)?

What aspect facilitates flexor muscles (inhibits extensor)?

Medial (pons)

Lateral (medulla)

What tract includes the red nucleus (midbrain), upper limbs (gross control), and mostly flexion (no fractionation)?

Rubrospinal tract

This tract is typically inhibitory to ‘lower’ reflexes, whether brainstem or spinal cord

Corticospinal tract

Damage to cortical processes

Decorticate

With decorticate, which system is now unopposed/free?

Rubrospinal (flexion)

Disruption of motor pathways at brainstem level- typically pons- below red nucleus

Rigidity due to unopposed gamma activity, unopposed extensor activity (loss of flexor activity from Red nucleus)

Not much hope for function

Decerebrate

Term for “weakness”

Paresis

Term for “paralysis”

Plegia

Lateral half of the body

Hemi-

Lower part of the body

Para-

Four limb involvement

Tetra- or quadri-

The degree of muscle tension during resting postural states

Muscle tone

Resting state resistance during passive motion

Tone

Force generated by a muscle

Strength

With flaccidity, it is dysfunction of what?

*Think of “L” in flaccidity

LMN

With spasticity, it is dysfunction of what?

UMN

What issue is mainly related to flaccidity?

Hypermobility

Hyperactivity of muscles can include what 4 things?

Clowns Fake Some Creations

1. Clonus

2. Fasciculation

3. Spasm/spasticity

4. Contracture