T7 (Fys) Fallet - F. Fisk

Question 0

What areas of the neocortex comprise the motor cortex?

Answer 0

The primary motor cortex (gyrus precentralis, Brodmann’s aera 4) and the premotor cortex (including divisions of Brodmann’s area 6).

Question 1

Outline the different motor subsystems of the body.

Answer 1

1. Spinal chord and brainstem circuits: local circuit neurons (lower motor neuron integration) and motor neuron pools (lower motor neurons)
2. Descending systems (upper motor neurons): motor cortex (planning, initiating and directing voluntary movements) and brainstem centers (basic movements and postural control)
3. Cerebellum (sensory motor coordination of ongoing movement)
4. Basal ganglia (gating proper initiation of movement)

Question 2

How are lower motor neurons clustered in the spinal chord?

Answer 2

They form cylindrical shapes and distinct distributions in the ipsilateral ventral horn of the spinal chord. Areas medial to lateral correspond with muscles proximal to distal of e.g. the upper or lower limb.

Question 3

What are the purpose of local circuit neurons and how are they categorized?

Answer 3

Local circuit neurons serve to connect neurons along the longitudinal axisnof the spinal chord. There are two distinct patterns.

Medial local circuit neurons: supply lower motor neurons of the medial ventral horn, may span even cervical-lumbar distance, synapses contralaterally, controls axial movement

Lateral local circuit neurons: ipsilateral, controls finer distal movements ( e.g. hands in primates)

Question 4

How do the tasks of γ- and α-motor neurons compare?

Answer 4

γ-motor neurons supply the intrafusal muscle fibers of the muscle spindle, whilst α-motor neurons supply extrafusal, force-generating muscle fibers.

Question 5

What is the arrangement of muscle fibers in a muscle that belong to the same motor unit?

Answer 5

Widespread and random.

Question 6

What types of motor units are there?

Answer 6

S: slow motor units, serve enduring tasks such as posture maintenance, fatigue slowly
FF: fast fatigable motor units, produce the most force but fatigue quickly
FR: fast fatigue-resistant motor units, generate twice the force of S units but are fatigue resistant

Question 7

What plastic reactions take place in an α-motor neuron with increased motor unit size?

Answer 7

Increased: cell body size, dendritic complexity, short-term EPSP potentiation with repeated activation, axonal diameter, number of axonal branches

Decreased: inout resistance, excitability, Ia EPSP amplitude, PSP delay constant, duration of after-hyperpolarization

Question 8

What is the ‘size principle’ in muscle force regulation?

Answer 8

Motor units are recruited in order of increasing size when muscles produce increasing force. Recruitment progresses from S to FR to FF motor units.

Question 9

How can a movement be smooth despite it resulting from 8 contractions/s in the (few) recruited motor units?

Answer 9

This is a result of multiple motor units, though few, contracting asynchronously, leading to a smooth movement.

Question 10

What type of muscle spindle fibers are there and how do they differ?

Answer 10

Nuclear bag fibers (dynamic and static, the belly of the spindles) and nuclear chain fibers (the ends of the spindles). They differ in location of the nuclei, the intrinsic arcitechture of their myofibrils and their dynamic sensitivity to stretch. Usually a muscle spindle contains 2-3 nuclear bag fibers and twice as many nuclear chain fibers.

Question 11

How are muscle spindles contacted by afferent axons and what does each signal to the CNS?

Answer 11

They are contacted by the largest afferent axons of the peripheral NS. Group Ia form annulospirals around the dynamic nuclear bag fibers whilst the group II axons contact the static nuclear bag fibers and the nuclear chain fibers through flower-spray endings.

Question 12

What stimulus do Ia afferents and II afferents respond to respectively?

Answer 12

Group Ia afferents: phasically to small stretches following dynamic bag type muscle fiber action

Group II: sustained fiber stretch through tonic firing where the frequency correlates with the degree of stretch

Question 13

Outline the connections on which ‘reciprocal innervation’ is based on.

Answer 13

The afferents from the muscle spindles form excitatory synapses with α-motor neurons of the same muscle in the ventral horn of the spinal chord, and inhibitory synapses with the α-motor neurons of the antagonistic muscle. This makes for a rapid and efficient muscle contraction.

Question 14

What is special about the afferent of the muscle spindle contacting the α-motor neuron directly in the ventral horn of the spinal chord? What reflexes are of this type?

Answer 14

It makes for a monosynaptic reflex arch (rare). This is known as the stretch, deep tendon or myotatic reflex (e.g. patella, bicep and akilles reflex).

Question 15

What is the relationship between deep tendon reflexes and muscle tone?

Answer 15

They’re both based on the reciprocal innervation mechanism (muscle mediated mainly by group II afferents). Changes in musclentone can be determined by examining the monosynaptic relfexes.

Question 16

How is the stretch reflex a negative feedback loop?

Answer 16

Stretching causes excitation of the afferent, which synapses with the α-motor neuron. The motor neuron initiates contraction which restores the muscle length to previous, decreasing the excitation of the afferent.

Question 17

How do γ motor neuron functions correlate with fibers and afferents of the muscle spindles?

Answer 17

There are also dynamic and static versions of the γ motor neurons. Firing of one class elicits higher responses in the correlating class of afferents.

Question 18

How does afferent response of the muscle spindle and golgi tendon organ differ with passive stretching of the muscle, and active contraction fo the muscle?

Answer 18

Passive stretch: muscle spindle afferent fires much more than the golgi tendon organ

Active contraction: only the golgi tendon organ afferent firing increases

Question 19

How is the golgi tendon organ innervated and through which oath does it affect α motor neurons of the same muscle?

Answer 19

By Single group Ib sensory axons. It synapses with Ib inhibitive interneurons that inhibit α motor neurons (parallel with upper motor neurons, cutaneous receptors, muscle spindles etc.).

Question 20

What is the basic principle of flexion reflexes?

Answer 20

Ipsilateral flexor activation and extensor inhibition. Contralateral flexor inhibition and extendor activation (for balance).

Question 21

What does the complex interconnection of the flexion reflex pathway result in?

Answer 21

Diverse modulation fo the reflex response (instead of flexion from noxious stimuli simply squeezing the limb might elicit the reflex).

Question 22

Where is the rhythmic pattern of limb movement elicited from?

Answer 22

From local centers in the limb itself.

Question 23

What symptoms are part of the the lower motor neuron syndrome and what does it follow?

Answer 23

Lower motor neuron syndrome follows damage to the lower motor neurons of the brainstem and spinal chord.

• paralysis
• paresis
• muscle atrophy
• fibrillations: twitches of denervated muscle fibers
• fasciculations: twitches of denervated motor units

Question 24

Outline the general routes of the upper motor neurons to the muscles respectively for the motor cortex and the brainstem.

Answer 24

Upper motor neurons in cerebral cortex - contralateral white matter of spinal chord - lower motor neurons in medial ventral horn - distal limb muscles (skilled movements)

Upper motor neurons of brainstem - ipsilateral anterior-medial white matter of spinal chord -

1) ipsilateral lower motor neurons in medial ventral horn - axial and proximal limb muscles (posture and balance)
2) contralateral motor neurons in medial ventral horn - axial and proximal limb kuscles (posture and balance)

Question 25

What is Brodmann’s area 4?

Answer 25

The primary motor cortex.

Question 26

Outline the path of the upper motor neuron from the primary motor cortex to the spinal chord.

Answer 26

1. Cortex layer 5
2. Corticobulbar or corticospinal tract
3. Posterior limb of capsula interna
4. Pedunculus cerebri (base of diencephalon)
5. Pass through base of pons and scetting through transverse pontine fibers and nuclei of the basal pontine gray matter
6. Coalesce into the medullary pyramids (innervation of cranial nuclei and reticular system at appropriate level)
7. Dessucation (90%) at caudal end of medulla forms lateral corticospinal tract
8. Remaining 10% constitute the ventral (anterior) corticospinal tract which terminates either ipsi- or bilaterally (primarily from doral and medial regions of area 4, serving axial and proximal limb muscles)

Question 27

How do upper motor neurons contact lower motor neurons?

Answer 27

Mainly through interneurons. Some muscles of the hands and arms are innervated by lower motor neurons that are synapsed upon directly by upper motor neurons.

Question 28

Do all components of the corticobulbar and corticospinal projections participate in upper control of lower motor neurons?

Answer 28

No. The projections also carry axons from layer 5 neurons in the somatic sensory regions, terminating near the sensory trigeminal nuclei and dorsal column nuclei of the brainstem, and in the dorsal horn of the spinal chord (monitor proprioception).

Question 29

How does the functional organization of the primary motor cortex differ from that of the somatic sensory cortex?

Answer 29

Areas on the primary motor cortex are organized, rather than according to single motor units in a similar topographic order to that of the somatic sensory cortex, according to:

• movement patterns
• proportionally according to how fine the movement is
• movement direction
• muscle fields (one upper motor neuron per many lower motor neurons, control of movementS as opposed to one movement, discovered through ‘spike triggered averaging’)

Question 30

What does the activation of the motor cortex shortly prior before execution of a small movement suggest?

Answer 30

That the motor cortex is involved in recruiting lower motor neurons for movements (rather than performing the movement?).

Question 31

What could be a general idea for movement pattern concerning movements of the same limb but in opposite directions?

Answer 31

Neuron firing patterns activate in movements of one direction and inhibit in kovements of the opposite direction.

Question 32

Through what paths does the premotor cortex influence movement?

Answer 32

Indirectly through intricate interconnections with the primary motor cortex. Directly through the corticobulbar and corticospinal (30% of pathways concists of premotor cortex axons) pathways, influencing interneurons and lower motor neurons of the brainstem and spinal chord.

Question 33

How do primary motor cortex - lower motor neuron, and premotor cortex - lower motor neuron synapses generally differ (quantitatively)?

Answer 33

Primary motor cortex synapses have a higher rate of monosynaptic configuration.

Question 34

Describe a theory of the function of the premotor cortex.

Answer 34

Intention. When stimulated by repeated visual cues, appropriate premotor cortex neurons start activating before and in anticipation of a certain movement being required in response to the visual cue.

Also the understanding of intention and “behaviorally relevant actions of others” is probably relayed via the premotor cortex, through what is called the “mirror motor neurons” (lie in proximation to the neurons coding the same intention of the animal itself and fire when one sees someone else perform the same movement).

Question 35

How do the functions of the lateral and medial premotor cortex compare?

Answer 35

Lateral: initiation of movement following outer stimulus (visual, audio)
Medial: initiation of movement following inner stimulus (e.g. memory)

Question 36

How is head position reflexively maintained?

Answer 36

By neurons from the medial vestibular nucleus, traveling through the medial vestibulospinal tract (terminates bilaterally in the medial ventral horn of the spinal chord). The tract activity is regulated by afferents from the semicircular canals.

Question 37

What is the task of the lateral vestibulospinal tract? Where does it run in the spinal chord and where does it terminate?

Answer 37

The lateral vestibulospinal tract activates limb extensors in response to signals from the otolith organs. It runs in the anterior white matter of the spinal chord, laterally to the medial vestibulospinal tract, terminating “among medial lower motor neuronal pools”.

Question 38

What other pathways project from the vestibular nuclei?

Answer 38

Eye muscle controlling paths (maintain eye position while the head is turned).

Question 39

How do the descending motor control pathways of the vestibular nuclei and reticular formation resemble each other?

Answer 39

Both terminate “primarily in the medial part of the gray matter where they influence the local circuit neurons that coordinate axial and proximal limb muscles”.

Question 40

How does the reticular formation differ from other basal ganglia of the midbrain?

Answer 40

It comprises a scattered gathering of neurons in the tegmentum, stretching through the midbrain down to the middle medulla, instead of occupying a fairly limited location.

Question 41

How does vestibular and reticular pathway mediated postural modulation differ?

Answer 41

Whilst ventricular nuclei mediated postural modulation is direct feedback to the spinal chord from the CN VIII (reflex reaction), the reticular formation functions through feedforward adjustments (anticipated movement).

Question 42

What is the indirect path from the motor cortex to the spinal chord and what is its function?

Answer 42

The path runs from the premotor cortex via the basal ganglia where they synapse with upper motor neurons which ultimately terminate bilaterally. These coordinate basic movement patterns and control countermovement. This is calle the cortico-reticulospinal pathway.

Question 43

What functions are attached to the superior colliculus and rubrospinal tract?

Answer 43

Superior colliculus: head orientation in relation to stimulus, pathway synapses with reticular formation before leading to the spinal chord

Rubrospinal tract: in animals control of forepaws, in humans mainly synapses with the inferior olive (central in learning)

Question 44

Compare upper motor neuron syndrome with lower motor neuron syndrome.

Answer 44

Upper motor neuron syndrome:

• weakness (spinal shock)
• spasticity (increased tonus, hyperactive deep reflexes, clonus (oscillating flexing and relaxing followin passive stretching))
• babinski positive
• loss of fine voluntary movements

Lower motor neuron syndrome:

• paresthesia or paralysis
• decreased superficial reflexes
• hypoactive deep reflexes
• decreased tone
• fasciculations and fribrillations
• severe muscle atrophy

Question 45

What is decerebrate rigidity and what does it result from?

Answer 45

The excessive spasticity of the muscles of patients suffering cortical lesions under the red nucleus but above the vestibular nucleus. This isna rsult of excessive excitatory pathways being preserved, ehilst the inhibitory cortical pathways are mostly severed.

Question 46

What muscle sensory organs are each responsible for the upper motor neuron syndrome symptoms spasticity and clonus?

Answer 46

Spasticity: golgi tendon organ (also jack knife phenomenon)
Clonus: muscle spindles

Question 47

What categories can the cerebellum gray matter roughly be divided into?

Answer 47

Surface: laminated cerebellar cortex

White matter: deep cerebellar nuclei

Question 48

What function is each part of the cerebellum concerned with?

Answer 48

Cerebrocerebellum: planning and executing of complicated movement series (including speech)
Spinocerebellum: only part that receives direct input from the spinal chord
- vermis: movement of proximal muscles, certain eye movements
- vestibulocerebellum: vestibulo-ocular reflex, posture and equilibrium

Question 49

What are the major pathways in and out of the cerebellum?

Answer 49

Superior cerebellar peduncle (brachium conjunctivum): almost entirely efferent pathways from deep cerebellar nuclei, projections to motor nuclei of dorsal thalamus and motor neurons of superior colliculus

Middle cerebellar peduncle (brachium pontis): afferent pathway to cerebellum, mostly from the pontine nuclei

Inferior cerebellar peduncle (restiform body): smallest and most complex, afferent pathways (from vestibular nuclei, spinal chord, regions of brainstem tegmentum) and efferent pathways (to vestibular nuclei and the reticular formation)

Question 50

What is the major input for the cerebellar cortex and what path does it take?

Answer 50

The cerebral cortex which’ efferents synapse with mid-pontine nuclei that form transverse pontine fibers (cerebral peduncle) that synapse in the contralateral cerebellar hemishpere’s cortex after passing through the middle cerebellar peduncle.

Question 51

What sensory input is sent to the cerebellum and through what pathways?

Answer 51

Information about head acceleration: trigeminal nerve to vestibulocerebellum

Proprioception from the face: from the mesencephalic nucleus of the trigeminal complex to the spinocerebellum

Relay neurons (for proprioception): to spinocerebellum from dorsal nucleus of Clarke and externsl cuneatus nucleus

Visual and auditory: brainstem nuclei

Question 52

How is the sensory input represented in the spinocerebellum?

Answer 52

In repeated somatotopographocic maps, with ipsilateral representation of the vestibular and spinal inputs.

Question 53

What is the relationship between the inferior olivary nucleus (inferior olive) and the cerebellum?

Answer 53

The inferior olivary nucleus sends modulative input to the cerebellum, itself receiving signals from amongst other from relays of the parvovellular pathway from the red nucleus.

Question 54

Outline the outputs of the cerebellum.

Answer 54

1. Cerebrocerebellum-dentate nucleus-pathways to upper motor neurons-premotor cortex (motor planning)
2. Spinocerebellum-interposed and fastigisl nuclei-pathways to upper motor neurons-motor cortex and brainstem (motor execution)
3. Vestibulocerebellum-vestibular nuclei-lower motor neurons in spinal cord and brainstem (balance and vestibulo-ocular regulation)

Question 55

What happens at the decussation of the superior cerebellar peduncle?

Answer 55

Axons from the dentate nuclei cross sides to ascend to the thalamus. Along with this, collaterals are sent to the red nucleus (P-cell layer, leads to feeback to cerebellar input)

Question 56

What is the function of the ‘closed loop’ routes of the cerebellum?

Answer 56

A means of modulating itself, used both in movement coordination as well as possibly problem solving in the prefrontal cortex.

Question 57

Outline the ipsi- and contralateral pathways of output from the cerebellum.

Answer 57

Contralateral: cerebellar cortex-deep cerebellar nuclei (dentate/interposed)-via superior cerebellar peduncle-superior colliculus-reticular formation-via anterior-medial white matter of spinal cord-lower motor neurons in medial ventral horn

Ipsilateral:
1. cerebellar cortex-inferior cerebellar peduncle-vestibular nuclei-anterior-medial white matter of spinal cord-lower motor neurons in medial ventral horn
2. cerebellar cortex-deep cerebellar nuclei (fastigial)-inferior cerebellar peduncle-
/superior colliculus-reticular formation-anterior-medial white matter of spinal cord-lower motor neuron in medial ventral horn
/reticular formation-anterior-medial white matter of spinal cord-lower motor neurons in medial ventral horn

Question 58

What pathway does the efferent message of sackadic eye motion travel?

Answer 58

lateral portion of cerebellar cortex - dentate-interposed nuclei - superior pedunculus - dessucation - contralateral superior colliculus - sackade movement of eye towards original side (of cerebellum)

Question 59

What is the route of the vestibulocerebellum efferents?

Answer 59

inferior cerebellar peduncle - nuclei of vestibular complex in brainstem

Question 60

What are the layers and associated cells of the cerebellwr cortex?

Answer 60

Molecular layer: Parallell (granule cell) fibers, purkinje cell dendrites, climbing fiber synapses, stellate cells, basket cells

Purkinje cell layer: purkinje cell somas

Granule cell layer: granule cell somas, golgi cells, mossy cell-granule cell synapses, purkinje cell axons

Question 61

What does the purkinje cells being GABAergic mean?

Answer 61

That output from the cerebellar cortex is wholly inhibitory.

Question 62

What are the roles of the cells of the cerebrocerebellum?

Answer 62

Purkinje cell: gather stimulus from granule cells’ parallel fibers, climbing fibers from inferior olive, and inhibit deep cerebellar nuclear cells

Deep cerebellar nuclear cell: the only efferent cell type of the cerebellar cortex, inhibited by purkinje cell, stimulated by climbing fibers and mossy fibers

Climbing fibers: afferents from inferior olive, modulate purkinje dendrite sensitivity to parallell fiber stimulus, stimulate deep cerebellar nuclei

Mossy fibers: axons of pontine nuclei

Basket cell: inhibitory input to purkinje cells

Golgi cell: input from parallell fiber, inhibits the source of the parallell fiber (granule cell)

Question 63

Outline how the deep excitatory loop works together eith the cortical inhibitory loop to result in the computational unit of the cerebellar cortex. Ehat is the role here of the golgi, stellate and basket cells?

Answer 63

Deep excitatory loop: mossy fiber excites granule cell whilst also exciting deep cerebellar nuclear cell

Cortical inhibitory loop: excited granule cell excites purkinje cell, whoch in turn inhibits deep cerebellar nuclear cell (modulating the initial feed from thr deep excitatory loop

The golgi (inhibitory feedback circuit controls gain of input of granular on to purkinje), stellate and basket (lateral inhibition, modulates multiple purkinje cells) cells control the flow of information through the cerebellar cortex

Question 64

What are the symptoms of cerebellar lesions?

Answer 64

Inability to perform smooth synchronized movements.

• nystagmus
• dysdiadochokinesia (difficulty oerforming rapid alternating movements)
• dysmetria (over- and underreaching)
• action/intention tremors accompany dysmetria