Motor System & Descending Tracts

Question 1

What is a motor unit?

Answer 1

• a unit consisting of a single motor neuron and all the muscle fibers it innervates
• (motor units of eyes involve few muscle fibers for each motor neuron; those involved in posture involve thousands)

Question 2

What is a motoneuron pool?

Answer 2

• the set of motoneurons innervating the fibers of a single muscle

Question 3

What are the two types of motoneurons?

Answer 3

• alpha motoneurons: innervate extrafusal skeletal muscle fibers (via neuromuscular junctions); generate contraction
• gamma motoneurons: innervate intrafusal muscle fibers; adjust the sensitivity of muscle spindles

Question 4

What are muscle spindles?

Answer 4

• muscle spindles are made up of intrafusal muscle fibers; they run parallel with the extrafusal skeletal muscle fibers
• they have contractile ends innervated by gamma motoneurons, and non-contractile centers innervated by sensory afferent fibers
• they are involved in detecting muscle length/stretch (they are stretch receptors that function to return muscle to its resting length)
• two types: nuclear bag and nuclear chain

Question 5

What are the two types of intrafusal muscle fibers that make up a muscle spindle?

Answer 5

• nuclear bag fibers: more numerous, larger, nuclei are in a central (bag) region; detect velocity of stretch (via type Ia fibers and annulospiral endings)
• nuclear chain fibers: fewer, smaller, nuclei are arranged in rows (chain); detect magnitude of stretch (via type II fibers and flower-spray endings)
• (dynamic nuclear bag fibers detect velocity, static nuclear bag fibers and nuclear chain fibers detect magnitude)

Question 6

Which afferent nerve types innervate the muscle spindles?

Answer 6

• each muscle spindle is innervated by a single group Ia afferent nerve fiber and by multiple group II afferents
• group Ia innervate the central region of nuclear bag fibers; they detect the velocity of the length change (dynamic nuclear bag) and the magnitude of stretch (static nuclear bag)
• group II innervate mainly only nuclear chains; they detect the magnitude of the length change

Question 7

Gamma motoneurons innervate the muscle spindles, what are the two types?

Answer 7

• dynamic gamma motoneurons: synapse on dynamic nuclear bags via “plate endings”
• static gamma motoneurons: synapse on static nuclear bags and nuclear chains via “trail endings”

Question 8

What are the basic steps involved in the reflex arc?

Answer 8

• sensory receptors –> afferent nerves –> interneurons in spinal cord –> efferent motoneurons –> contraction/relaxation response

Question 9

What is the stretch reflex?

Answer 9

• (AKA the myotatic reflex) ex: knee jerk
• the simplest of the spinal reflexes, involving only one synapse between Ia afferents of the muscle spindle and alpha motoneurons to the extrafusal muscle fibers (Ia afferents activate alpha motoneurons)
• stretched muscle is reflexively contracted

Question 10

What is the Golgi tendon reflex?

Answer 10

• (AKA the inverse myotatic reflex)
• spinal reflex of intermediate complexity that involves two synapses between Ib afferents of the Golgi tendon organ and the alpha motoneurons to extrafusal fibers (Ib afferents inhibits alpha motoneurons)
• group Ib afferents –> synapse 1 –> stimulation of inhibitory interneurons –> synapse 2 –> inhibition of alpha motoneurons
• contracted muscle is reflexively relaxed

Question 11

What is the Golgi tendon organ?

Answer 11

• a stretch receptor in tendons that senses contraction of muscle
• contains type Ib afferent nerve fibers
• lies IN SERIES with extrafusal muscle fibers, usually in the junction between muscle fibers and tendon (vs. the parallel muscle spindle stretch receptors)

Question 12

What is the extensor-withdrawal reflex?

Answer 12

• the most complex spinal cord reflex, involving many synapses between afferents (types II, III, or IV) and alpha motoneurons
• stimulus results in withdrawal (flexion) of the affected side and simultaneous extension of the contralateral side (to maintain balance); stimulation of ipsilateral flexors and contralateral extensors, and inhibition of ipsilateral extensors and contralateral flexors

Question 13

Where are the cell bodies of the motor neurons? More specifically, where are the cell bodies of the nerves innervating the limbs? Of the nerves innervating the axial muscles?

Answer 13

• in the grey matter (ventral horn) of the spinal cord (except for those that supply the muscles of the head; they’re in the brain stem motor nucleus)
• limb muscle innervation: cell bodies are more laterally located in the ventral horn
• axial muscle innervation: cell bodies are more medially located in the the ventral horn

Question 14

How many ganglionic synapses are involved in somatic pathways?

Answer 14

• none! somatic pathways are single neuron chains
• (but don’t forget that their is a synapse between upper motor neurons of the CNS and the “lower motor neurons” that leave the CNS)

Question 15

Motoneuron fibers release which neurotransmitter? What affect does this neurotransmitter have on the effector skeletal muscle?

Answer 15

• motoneurons release ACh
• on skeletal muscle, ACh is ENTIRELY stimulatory (meaning, motoneurons can only stimulate skeletal muscle - they can’t inhibit it)

Question 16

What type of receptors are used at the postsynaptic cell (muscle fiber) in the neuromuscular junction?

Answer 16

• nicotinic ACh receptors
• these are stimulated by nicotine, ACh, and carbachol; inhibited by curare, but NOT by hexamethonium (this only antagonizes the ganglia nicotinic receptors)

Question 17

Which laminae do the motoneurons project to?

Answer 17

• alpha and gamma motoneurons project to interneurons in lamina IX (in the ventral horn)

Question 18

Motoneurons are also called “lower motor neurons,” so what is an upper motor neuron?

Answer 18

• upper motor neurons are the neurons that synapse with the “lower motor neurons” in the CNS; they are the descending tracts, carrying higher level output
• upper motor neuron cell bodies are in the cerebral cortex, and they have long descending tacts axons

Question 19

alpha motoneurons are larger motoneurons, but still come in a variety of sizes. Which muscle fibers do small, medium, and large alpha motoneurons innervate?

Answer 19

• small: type I extrafusal fibers (slow-twitch, oxidative); small alpha motoneurons tend to have small motor units
• medium: type IIa extrafusal fibers (fast-twitch, oxidative)
• large: type IIb extrafusal fibers (fast-twitch, glycolytic); large alpha motoneurons tend to have large motor units

Question 20

What other reflex is simultaneously triggered with the stretch (myotatic) reflex?

Answer 20

• the reciprocal inhibition reflex, leading to the inhibition of motoneurons from stimulating ipsilateral antagonist muscles
• this reflex involves 2 synapses (so there is an interneuron involved, specifically an inhibitory interneuron)

Question 21

What roles involved in movement is the spinal cord responsible for?

Answer 21

• spinal reflexes and central pattern generators (CPGs)
• CPGs are semi-automatic rhythmic motor activities, involving coordinated muscle activity in different muscle groups (examples: walking, chewing, breathing, etc.)

Question 22

What is the major role of the brain stem that relates to movement? What are the major tracts involved?

Answer 22

• brain stem’s major role is in the control of posture and movement
• pyramidal tracts (corticospinal tract and corticobulbar tract) that pass through the medullary pyramids
• extrapyramidal tracts (rubrospinal tract, reticulospinal tracts, vestibulospinal tracts, and tectospinal tract)

Question 23

What is the corticospinal tract? What is its major function? What part of the spinal cord contains these descending nerve fibers?

Answer 23

• a pyramidal tract (meaning it passes through the medullary pyramids before descending to the spinal cord) of the brain stem
• controls fine and skilled conscious movements, sending excitatory inputs to motoneurons
• 90% of axons here cross the mid-line in the medulla to descend in the lateral white matter of the spinal cord (contralateral side)
• 10% of axons here don’t cross the mid-line, and descend in the ventral white matter (ipsilateral side)

Question 24

What are the reticulospinal tracts? What are their major functions?

Answer 24

• extrapyramidal tract of the brain stem that originate in the reticular formation
• coordinates automatic postural reflexes, locomotion, and other simple movements that don’t require dexterity
• 2 divisions: the pontine (medial) reticulospinal tract and the medullary (lateral)reticulospinal tract
• even mix of crossed and uncrossed axons
• (medial) pontine tract: stimulates anti-gravity extensors (facilitates spinal reflexes), axons project down ventromedial white matter
• (lateral) medullary tract: inhibits anti-gravity extensors (suppresses spinal reflexes), axons project down lateral white matter

Question 25

What is the vestibulospinal tract? What is its major function?

Answer 25

• extrapyramidal tracts of the brain stem that originate in the vestibular nuclei
• receive info from labyrinths (in inner ear) and cerebellum to maintain balance, posture, and head-and-neck orientation
• stimulates extensors (anti-gravity muscles) and inhibits flexors
• 2 divisions: medial vestibulospinal tract (descend via ventromedial white matter onto the cervical segments) and lateral vestibulospinal tract (descend via ventrolateral white matter onto lower segments)
• axons don’t cross and therefore innervate the ipsilateral side

Question 26

What is the rubrospinal tract? What is its major function?

Answer 26

• extrapyramidal tract of the brain stem that originates in the red nucleus
• stimulates flexors and inhibits extensors
• axons here decussate in the midbrain in the ventral tegmental decussation before descending to the contralateral tract
• only descends into the cervical and thoracic segments, not the lumbosacral region, so it deals with the upper trunk and upper limbs

Question 27

What is the tectospinal tract? What is its major function?

Answer 27

• extrapyramidal tract of the brain stem that originates in the superior colliculus (of the tectum)
• these fibers decussate in the midbrain in the dorsal tegmental decussation before descending to the contralateral tract
• controls the neck muscles (for visuospinal reflex)
• axons project mainly to the cervical spinal cord

Question 28

What is decerebrate rigidity?

Answer 28

• a resulting increase in tonic activity (increased tone, hyperreflexia, etc.) when the connections between the brain stem tracts/motor centers and the cerebral cortex is severed (the cerebral cortex normally inhibits and controls the output of these tracts)

Question 29

What are the two general mechanisms involved in postural reflexes mediated by the brain stem motor centers?

Answer 29

• feed forward: anticipatory, directive, scale of response can be refined; ex) walking up the stairs
• feed back: compensatory, reactive, scale can’t be refined; ex) thinking there’s one more step, but not falling forward/down

Question 30

Which two motor centers indirectly modulate movement by communicating with the cerebral cortex?

Answer 30

• the basal ganglia and the cerebellum send input to the cerebral cortex via the thalamus
• (when a movement is generated in the cerebral cortex, it is sent down to the spinal cord to the proper muscles AND down to the basal ganglia and cerebellum, which monitor, correct, and fine-tune the movements by sending input back to the cerebral cortex via the thalamus)

Question 31

Which major cerebral cortices are involved in movement?

Answer 31

• primary motor cortex (precentral gryus): triggers the actual movement
• premotor area (anterior to primary motor cortex): plans the movement
• supplementary motor area (superior to premotor area): deals with primitive movements (involves axial muscles)
• primary somatosensory cortex is also important

Question 32

What and where is Broca’s area?

Answer 32

• this is located in the dominant premotor area and deals with speech
• lesions here result in expressive/Broca’s aphasia (where the patient knows what he wants to say, but can’t; patient is aware that he can’t speak properly)

Question 33

What lies just above Broca’s area in the premotor cortex?

Answer 33

• the frontal eye field, which is concerned with eye movement
• stimulating a frontal eye field results in movement of the eyes to the contralateral side (bilateral stimulation results in looking forward)
• a lesion to a frontal eye field will result in the eyes looking towards the side containing the lesion

Question 34

Explain the layout of the motor homunculus in the primary motor cortex.

Answer 34

• progressing laterally to medially along the cortex: larynx and pharynx, tongue, face, eye, neck, hand, elbow, shoulder, trunk, hip, thigh, knee, foot, genitals

Question 35

From which cortices does the corticospinal (pyramidal) tract arise?

Answer 35

• from the primary motor cortex (30%), premotor area + supplementary motor area (30% total), and the primary somatosensory cortex (40%)

Question 36

Follow the path of a corticospinal tract from start to finish.

Answer 36

• originates in the cortices, projects towards the ipsilateral internal capsule via the corona radiata, and densely converges into the posterior limb of the internal capsule
• continues into the midbrain into the ipsilateral crus cerebri (anterior part of midbrain) to converge into the ipsilateral cerebral peduncle, emerges from the peduncle and disperses into the ipsilateral pons before converging again into the ipsilateral medullary pyramid
• half-way through the pyramid, 90% of the fibers will decussate to the contralateral side, becoming lateral corticospinal tracts (the remaining 10% continue ipsilaterally, becoming anterior/ventral corticospinal tracts)
• fibers then descend and project onto LMNs in the contralateral anterior horn of the spinal cord (note that the ventral corticospinal tracts decussate to the contralateral horn at this level)

Question 37

What are cells of Betz?

Answer 37

• these are special large neurons in the primary motor cortex that give off very large, heavily myelinated axons that contribute to the corticospinal tracts
• in the anterior horn of the spinal cord, these axons project directly onto the LMNs (the axons from normal cells use interneurons)

Question 38

Lateral vs. Ventral Corticospinal Tracts

Answer 38

• lateral tracts decussate at the pyramids, ventral tracts decussate at the spinal cord level
• lateral tracts supply appendicular muscle LMNs, ventral tracts supply axial muscle LMNs
• (axial muscles are involved in primitive movements, so the ventral tracts are largely originated from the supplementary motor area!)

Question 39

Which fibers descend with the corticospinal tracts, but terminate at the pons?

Answer 39

• corticopontine tracts; these terminate onto the pontine nuclei, which will then project to the cerebellum to keep it updated

Question 40

Corticobulbar Tracts

Answer 40

• these are the equivalent of the corticospinal tracts for the head and neck (these project onto the LMNs of the head and neck - namely, the motor nerves of the cranial nerves)
• while these don’t cross the medullary pyramids, they are still considered pyramidal tracts (note that they DO cross to innervate the contralateral LMNs)

Question 41

What are the four major extra-pyramidal tracts?

Answer 41

• (note that these all have sub-cortical origin)
• tectospinal tract (from the tectum of the posterior midbrain); deals with visuospinal and auditospinal reflexes via the superior and inferior colliculus, respectively
• rubrospinal tract (from the midbrain’s red nucleus); deals with the tone of flexor muscles
• vestibulospinal tract (from the vestibular nuclei in the brainstem); deals with the tone of extensor AKA anti-gravity muscles
• reticulospinal tract (from the reticular formation); the medullary reticular nuclei assist in flexor tone, the pontine reticular nuclei assist in extensor tone

Question 42

Where are the corticospinal tracts located in the spinal cord?

Answer 42

• lateral corticopsinal tract is located lateral to the dorsal horn and posterior to the lateral spinothalamic tract
• anterior corticospinal tract is located at the anterior of the cord just lateral to the ventral fissure

Question 43

What percentage of corticospinal fibers terminate at the cervical levels? At the thoracic levels? At the lumbosacral levels?

Answer 43

• cervical: 55%
• thoracic: 20%
• lumbosacral: 25%

Question 44

What is degenerative motor neuron disease? What causes it? When does it usually begin?

Answer 44

• this is a group of disease that lead to progressive weakness and paralysis, without any sensory loss and without any autonomic dysfunction
• these are idiopathic (other than a genetic basis in some familial cases)
• usually begins between 30-60 years of age

Question 45

What are the five varieties of motor neuron disease?

Answer 45

• progressive bulbar palsy: affects the motor nuclei of CNs (LMNs)
• pseudobulbar palsy: affects the corticobulbar tracts (UMNs)
• progressive spinal muscular atrophy: affects anterior horn cells of the limbs (LMNs)
• primary lateral sclerosis: affects the corticospinal tracts (UMNs)
• amyotrophic lateral sclerosis (ALS): affects LMNs in the spinal cord and brain stem + death of the UMNs (mainly Betz cells) (combo of LMNs and UMNs)
• (60% of patients also develop cognitive impairment)

Question 46

What is the prognosis of Amyotrophic Lateral Sclerosis? What are the classic early warning signs? What is the most common cause of morbidity in these patients?

Answer 46

• prognosis is very poor: 2-5 years
• early signs: atrophy and weakness of hands (patient will drop things, etc.)
• most patients die from pulmonary infections due to weakened respiratory muscles
• (note that a small percent of cases are familial - NOT sporadic - and involve a mutated superoxide dismutase, SOD, gene)

Question 47

How do we treat motor neuron disease?

Answer 47

• management is largely supportive care (liquid diet, intubation, etc.) and drugs are mainly use for symptomatic relief of excess saliva, dyspnea, constipation, cramps, spasms, depression, sensation of choking, etc.
• non-invasive ventilatory support greatly improves quality of life and can prolong survival up to 1 year
• Riluzole may slow the progression of the disease by reducing the presynaptic release of glutamate; however, it is only 20-30% effective (increasing life expectancy by only 3-6 months)

Question 48

What is Werdnig-Hoffman disease? How do patients classically present? What should you suspect in an older patient presenting with similar symptoms?

Answer 48

• Werdnig-Hoffman disease (AKA spinal muscular atrophy) is an inherited (A.R.) degeneration disorder of the anterior horns, resulting in LMN lesions
• it presents immediately at birth; the neonate is a “floppy baby” and death usually arises within 1st year
• (suspect poliomyelitis in older patients who are presenting with signs of infection and only LMN lesions; the poliovirus destroys the anterior horns)

Question 49

What is tabes dorsalis? What causes it? How do patients present?

Answer 49

• tabes dorsalis is targeted demyelination of the dorsal columns and roots
• it is caused by syphilis
• patients will present with impaired sensation of fine touch, vibration, and proprioception