Lecture 7 -- Motor Systems 2

What is the functional unit of the nervous system?

A reflex.

Are reflexes innate or learned?

Innate; present from birth.

Does a reflex require input from upper motor neurons (UMN)?

No.

What is a response (in contrast to a reflex)?

A learned behavior.

Name 3 characteristics of somatic reflexes.

1. Monosynaptic or polysynaptic

2. Ipsilateral or contralateral

3. Intrasegmental or intersegmental.

What type of reflex is the patellar reflex?

Monosynaptic reflex

Describe the process of patellar reflex.

Tapping on patellar ligament cause a bit of stretching of muscle → Send a sensory neuron via the femoral nerve → Goes through the dorsal root into the dorsal horn in the spinal cord → Directly synapses on a motor neurone in the spinal cord → Leave the ventral horn through ventral roots → back to the femoral nerve → instructs the quadriceps muscle to contract → Knee joint extend

BUT in oder o have contraction of quadriceps, the antagonistic muscle (hamstrings) should be inhibited → After tapping the patellar ligament, it also synapse the inhibitory interneurone within the spinal cord → Inhibit muscle via ischiatic nerve

Which nerve carries the sensory input for the patellar reflex?

Femoral Nerve.

Why can't the patellar reflex be properly tested when a dog is standing?

Because the dog is bearing weight.

Is the flexor withdrawal reflex monosynaptic or polysynaptic?

Polysynaptic.

What is the function of the flexor withdrawal reflex?

To remove the limb away from a noxious stimulus.

Describe the process of withdrawal reflex.

Send signal via sensory neurone in the spinal nerve → Into the dorsal horn → NO direct synapsing on the motor neurone BUT interact with motor neurone via interneuron → Motor neurone then travels out the ventral horn → Ventral root through spinal nerve back to the muscles → Contraction of hamstring + Inhibitation of quadriceps → Flexion fo stifle joint

It is also a bilateral reflex → The fibres decussates to the opposite side of the spinal cord → One leg is lifting up; the other leg is braced → Able to balance

How does the brain interact with the flexor withdrawal reflex?

The brain receives impulses and becomes aware, but it is not needed to cause the reflex.

Is the flexor withdrawal reflex bilateral?

Yes, it impacts both left and right sides of the body.

What two types of fibers travel along the muscles?

Intrafusal and extrafusal muscle fibers.

Where are the muscle spindles located?

Muscle spindles are located within the belly of skeletal muscles, surrounding the intrafusal muscle fibres

Where is the Golgi tendon organ located?

It sits at the interface between the tendon and muscle

What is the function of the Golgi tendon organ?

1. Detects contraction → Prevent muscle pulling off the tendon too hard

2. Stimulate 1b afferent fibre → Signal signal to the dorsal root of the spinal horn → Inhibit 1a motor neurons → Calm down the contraction of the muscle

What happens to the muscle spindle when the muscle shortens?

Muscle contracts and shortens → Muscle spindle wold go slack

BUT we dont want the spindle to “turn off” → Need to sense stretch and maintain tone

→ UMN, coming from the motor cortex, stimulate the gamma motor neurons → Contract the intramural fibre of muscle spindle → Once muscle spindle is activate, they send 1a afferent signal to spinal cord → Send the motor neurone back to the muscle via alpha lower motor neurone → Contract extrafusal fibre

How is the sensitivity of the muscle spindle maintained during muscle contraction?

Gamma motor neurons fire to contract the muscle within the muscle spindle (intrafusal fibers)

What type of afferent fiber is associated with the muscle spindle?

1a afferent fiber

What type of afferent fiber is associated with the Golgi tendon organ?

1b afferent fiber

What is the effect of stimulating gamma motor neurons on intrafusal muscle fibers?

Re-primes those muscle spindles to still able to detect stretch despite that shortening of overall muscle length.

List 5 major upper motor neuron (UMN) tracts.

Corticospinal, rubrospinal, tectospinal, vestibulospinal, and reticulospinal tracts.

What is the main function of the corticospinal tract? Where is it originate and decussate?

Fine, voluntary motor skills.

Originate from the motor cortex of the cerebrum → Decussate in the spinal cord as lateral corticospinal tract

What is the function of the rubrospinal tract? Where is it originate?

Regulation of skilled movements and muscle tone

Originate from the red nucleus at midbrain → Decussates in the midbrain → Travel down the spinal cord and goes to the lateral funiculus

What is the function of the tectospinal tract? Where is it originate?

Orientates head & eyes in response to sights and sounds.

Originate from the midbrain

What is the function of the vestibulospinal tract? Where is it originate?

Maintenance of posture and balance

Originate from the medulla oblongata

What is the function of the reticulospinal tract?

Where is it originate?

Stabilizes body against gravity

Originate from the medulla oblongata

What is another name for the corticospinal tract? What is the main function?

Pyramidal tract.

Function: Fine, voluntary movement

What type of movements do extrapyramidal tracts mainly control?

Posture and subconscious, rhythmic movements.

(Although rubrospinal tract does generate fine skilled movement in non-primate mammals)

How do the pyramidal and extrapyramidal UMNs distribute according to different species?

Human: pyramidal tract + extrapyramidal tract present all the way from the head to the toe as we need fine movement, as well as communication skilled movement with hands and walking

Horses: They have little pyramidal tract only in the head as fine movement of lip are controlled by pyramidal tract but the rest of the body is large, rhythmic movement

Cat: Pyramidal tract travels all the way down BUT more dominant at their head became their predatory behaviour e.g. chasing mice = fine movement

What are the names of the basal nuclei? What are the function of basal nuclei?

Corpus striatum

Function: Motor planning and learning = Development of habitual motor responses

What is the main function of the cerebellum?

Coordinates and integrates sensory input and input from the cerebral cortex, refining movements.

What happen if the cerebellar dysfunction?

Loss of coordination

What happens to synapses with practice?

Synapses become more efficient, particularly important for rapid/complex movements.

What motor deficits might you expect to see from a lesion in the left motor cortex of a dog?

As the left corticospinal upper motor neuron tract on the motor cortex decussate to the right at the level of spinal cord → Problem in the right hand side