Motor pathways and higher motor control Flashcards
What is the role of the spinal cord and brainstem in motor control?
Execution
What is the role of the motor cortex in motor control?
Generation and initiation
What is the role of the basal ganglia in motor control?
Select and release based on learning
What is the role of the cerebellum in motor control?
Calibrate and adjust
What do more lateral tracts supply?
More distal muscles
What do more medial tracts supply?
More proximal muscles
Is the cortex needed for most motor functions?
No
What and where are Betz cells?
Layer IV large pyramidal cells, in the primary motor cortex, essential for certain movements - grasping grabbing/ dexterity
What are the three functional types of skeletal muscle fibres?
Slow-contracting (Type 1)
Fast-contracting fatigue Resistant (Type 2A)
Fast-contracting, easily Fatigued (Type 2B)
What innervates the three skeletal muscle fibres?
Innervation by alpha motor neurons is subtly different
Type 1 - smallest alpha neurons
Type 2A - medium sized alpha motor neurons
Type 2B - largest alpha motor neurons
Effects of LMN lesions
Flaccid paralysis Muscle weakness or paralysis Hypotonia Hyporeflexia Initially fasciculations Long term muscle wasting
Why are there fasciculations early in an LMN lesion?
Lack of ACh signalling, so upregulation of ACh receptors, increased sensitivity of muscle to acetylcholine
What are the medial descending tracts?
Anterior corticospinal , Vestibulospinal, pontine reticulospinal, Tectospinal
What are the lateral descending tracts?
Lateral corticospinal, Rubrospinal (magnocellular), Medullary reticulospinal
What do medial descending tracts mainly supply?
Axial/proximal muscles
What do medial descending tracts mainly supply?
Mainly axial and proximal muscles
What do medial descending tracts end on?
Interneurons
What do lateral descending tracts end on?
Alpha motor neurons
Where do corticospinal fibres arise?
Premotor (broadmann area 6), motor (broadmann area 4) and sensory cortex (broadmann area 3,2 and 1).
What are the principle inputs to the primary motor cortex?
Primary somatosensory cortex (S1) area 3a for proprioception Premotor cortex (area 6)
Contralateral cerebellar hemisphere via VL nucleus of thalamus
Where do the corticospinal fibres pass?
Through the internal capsule
Fibres bundle in the pons (pontine nuclei), then gather into pyramids in the medulla.
What happens at the pyramidal decussation?
At the lower medulla, at the pyramidal decussation, 85-90% of the fibres cross the midline to form the lateral corticospinal tracts and the remaining 10-15% form the ventral corticospinal tract.
What do the lateral and ventral corticospinal tract control?
Lateral tract controls distal limb motor neurons (especially fine hand movements) while ventral tract controls the axial motor movements.
What proportions of corticospinal neurons come from each area of the cortex?
40% from M1, 20% from premotor and 40% from post central S1
What is the internal capsule?
White matter structure situated in the inferomedial part of each cerebral hemisphere of the brain.
Where does the internal capsule pass through?
It carries information past the basal ganglia, separating the caudate nucleus and the thalamus from the putamen and the globus pallidus.
What is the genu?
The bend in the V of the internal capsule
What is the anterior and posterior limb of the internal capsule?
The anterior limb is the part in front of the genu
The posterior limb is the part behind the genu between thalamus and lenticular nucleus
What is the retrolenticular portion of the internal capsule?
The retrolenticular portion is caudal to the lenticular nucleus and carries the optic radiation (from medial part of lateral geniculate nucleus) also known as the geniculocalcarine tract.
What is the sublenticular portion of the internal capsule?
The sublenticular portion is beneath the lenticular nucleus are tracts involved in the auditory pathway from the medial geniculate nucleus to the primary auditory cortex (Broadmann areas 41 and 42)
What fibres make up the genu?
Originating in the motor part of the cerebral cortex
Pass through the base of the cerebral peduncle with the cerebrospinal fibers, undergo decussation and end in the motor nuclei of the cranial nerves of the opposite side.
It contains the corticobulbar tract, which carries upper motor neurons from the motor cortex to cranial nerve nuclei that mainly govern motion of striated muscle in the head and face.
What is the corticobulbar tract?
Carries upper motor neurons from the motor cortex to cranial nerve nuclei that mainly govern motion of striated muscle in the head and face.
What contains the corticobulbar tract?
Genu
Why does lesion in the IC affect the facial nerve?
Supranuclear lesion of facial nerve (corticobulbar tract)
Why is the top portion of the face spared in IC facial nerve lesion?
Supranuclear, upper face receives innervation from ipsilateral corticobulbar tract
What fibres make up the anterior limb of the IC?
Fibers running from the thalamus to the frontal lobe
Fibers connecting the lentiform and caudate nuclei
Fibers connecting the cortex with the corpus striatum
Fibers passing from the frontal lobe through the medial fifth of the base of the cerebral peduncle to the nuclei pontis
Thalami pontine fibers
What fibres make up the posterior limb of the IC?
The anterior two-thirds of the occipital part contains fibers of the corticospinal tract, which arise in the motor area of the cerebral cortex and, are continued into the pyramids of the medulla oblongata.
The posterior third of the occipital part contains:
Sensory fibers, largely derived from the thalamus, though some may be continued upward from the medial lemniscus
The fibers of optic radiation, from the lower visual centers to the cortex of the occipital lobe
Acoustic fibers, from the lateral lemniscus to the temporal lobe
Fibers that pass from the occipital and temporal lobes to the pontine nuclei
Blood supply to the IC Anterior limb
Lenticulostriate branches of middle cerebral artery (superior half) and recurrent artery of Heubner of the anterior cerebral artery (inferior half)
Blood supply to the IC Genu
Lenticulostriate branches of middle cerebral artery (Charcot)
Blood supply to the IC posterior limb
Lenticulostriate branches of middle cerebral artery (superior half) and anterior choroidal artery branch of the internal carotid artery (inferior half)
IC lesion symptoms
Contralateral spastic paralysis in upper and lower limbs
Asymmetrical hemiplegic gait
Contralateral lower face weakness: CNVII nuclei only receives contralateral innervation whereas the upper face receives bilateral innervation.
ACA lesion symptom
Lower limb contralateral UMN lesion
MCA lesion symptom
Upper limb contralateral UMN lesion
Paraplegia
Paralysis of lower half of body with involvement of both legs
Hemiplegia
Paralysis of one side of the body
Hemiparesis
Muscular weakness or partial paralysis restricted to one side of the body
Spastic paraplegia
Paraplegia where paralysis is spastic, not flaccid.
Tetraplegia or quadriplegia
All four limbs are affected by paralysis
Monoplegia
Only one limb is affected
Why is UMN resultant in spastic paralysis?
Removal of inhibitory signal on the lower motor neuron leads to spasticity.
Rigid also due to the reflex being maintained.
What determines if a lesion produces a flexion/extension?
If above red nucleus then flexion response as rubrospinal is still intact - but abnormal.
If below red nucleus - pure extension response.
Reticulospinal tract functions
Controls muscle tone, gain of spinal reflexes especially axial and proximal limb muscles
Recent evidence for direct reticulospinal input to distal limb muscles including hand in primates
Origins of the reticulospinal tract
Medial reticulospinal tract from pontine, lateral reticulospinal tract from medullary reticular formation
Inputs to the reticulospinal tract
From ascending tracts, motor/premotor cortex, vestibular apparatus, tectum
Course of reticulospinal tract
Pontine RF projects ipsilaterally in ventral funiculus - medial reticulospinal tract.
Medullary RF projects contralaterally in anterolateral funiculus - lateral reticulospinal tract
What are the funiculi of the spinal cord?
Lateral, ventral, posterior white matter regions
Effects and importance of the reticulospinal tract
Modulates reflex action during ongoing movements (postural readjustment)
Pontine facilitate axial and proximal reflexes
Medullary end on both α and γ motorneurons - inhibit proximal limb muscles
Cortical input to reticular formation important to suppress reflexes during voluntary movements
Vestibulospinal tract functions
Antigravity actions
Vestibulospinal tract origin
From lateral and medial & inferior vestibular nuclei
Vestibulospinal tract inputs
From otoliths and cerebellum to lateral vestibular nucleus
Semicircular canals and neck proprioceptors to medial vestibular nucleus
Vestibulospinal tract course
Both medial and lateral tracts run ipsilaterally in ventral funiculus of the spinal cord
Lateral vestibulospinal tract effects and importance
Lateral vestibulospinal tract facilitates antigravity (extensor) motor neurones (especially axial) for posture
What inputs to the lateral vestibular nucleus?
Otoliths and cerebellum
What inputs to the medial vestibular nucleus?
Semicircular canals and neck proprioceptors
Medial vestibulospinal tract effects and importance
Medial (continuous with MLF) to axial muscles of neck/trunk and propriospinal neurons
Tectospinal tract functions
Control of neck/eye coordination
Tectospinal tract origin
Superior colliculus (tectum)
Tectospinal tract inputs
Retina via optic nerve, from visual cortex, from auditory inputs
Tectospinal tract course
Fibres cross and descend in contralateral ventral funiculus of upper spinal cord
Tectospinal tract acts on
Axial muscles of neck/upper thorax – moves the neck and thus the direction of gaze
Rubrospinal tract origin
Large neurones of red nucleus, topgraphically arranged
Rubrospinal tract inputs
Motor cerebral cortex, cerebellum, globus pallidus, reticular formation, ascending from cord.
Rubrospinal tract course
Cross in midbrain, descend through pons, medulla, lateral funiculus of cord
Rubrospinal tract actions
Inter neurones controlling distal limb muscles.
Facilitates flexors.
What are all the cranial nerve motor nuclei
(III, IV, VI, V3, VII, IX, X, XI, XII).
Examples of diffuse descending tracts
Locus coeruleus
Raphe nuclei
What NT does the locus coeruleus supply and what is its role?
Locus coeruleus (noradrenaline) facilitates locomotor and sympathetic activity and inhibits nociception
What NT does the raphe nuclei supply and what is its role?
Raphe nuclei (5-HT/serotonin) inhibits spinal neuronal activity, promotes sleep and inhibits nociception.
What element of the reflex can be modified by the cortex?
Long latency M2 component of the reflex
Inputs to the PFC
Limbic system (via dorsomedial & anterior thalamic nuclei)
Parietal & occipital cortex: visuo-spatial signals (magnocellular-dominated dorsal visual stream)
Inferotemporal cortex (ventral stream object recognition)
Outputs from the PFC
Frontal eye fields
Pre- & supplementary motor cortex
Back to posterior parietal cortex
Caudate & cerebellum
What can PFC neurons encode
PFC neurons can encode delay, short-term memory, necessary for subsequent motor response.
Lateral prefrontal cortex roles
Inhibitory control, executive function, task switching, working memory
Orbitofrontal PFC roles
Motivated decision, emotional & autonomic responses: taste, smell, reward (medial), loss chasing behaviour (lateral).
What is the role of the posterior parietal cortex?
Integration of visual, proprioceptive, auditory, motor and motivational signals
Active touch - sensorimotor integration for the representation of the body image (morpho-synthesis)
Active sight - representation of peripersonal and teleceptive space
Coordinate conversions: e.g. from retinotopic (visual map) to egocentric by direction of attention, e.g. for preparation of eye movements.
Connections of the posterior parietal cortex
Prefrontal & supplementary motor cortex
What do lesions of the PPC cause?
rhs Lesions cause neglect, misdirection of movement, lack of body image, dressing apraxia, anosagnosia
lhs languange focused
Pre motor cortex Inputs
Prefrontal, posterior parietal, cerebellum (via VL thalamus)
Pre motor cortex outputs
Motor cortex, prefrontal cortex, posterior parietal cortex, cerebellum, corticospinal tract
Pre motor cortex function
Sensory guidance of movement e.g. directing reach
When does the pre motor cortex fire?
In advance of movement, can show direction selective firing - shows ‘planning’ function
What are the two distinct parieto-premotor channels?
Dorsal pathway for reaching (direction and extent)
Ventral for grasping (shape dimension of object)
Where are mirror neurons?
Lateral ventral premotor area
What are mirror neurons?
Neuron fires when other monkey/human performs action (i.e precision grip)
Does not fire when pliers used for picking up food
What is the role of mirror neurons? How does this work? And what is their importance?
Mirror neurones might help us to imitate and learn new movements, such as speaking. Encode abstract representation of motor task.
When monkey observes an action, he starts automatically to prepare the same action. In this way he becomes able to perform it fast, thus prevailing on possible competitors
What are mirror neurons importantly NOT associated with?
Motor preparation
Mirror neurons cease firing when the food is moved toward the animal and becomes available to him. If the firing of mirror neurons were related to motor preparation, the neuron activity should have increased and not decreased in the phase that precedes movement execution
What is the supplementary motor area function?
Used for internally generated movements, mental rehearsal sequential movements - readiness potential
SMA Inputs
Prefrontal cortex, basal ganglia
SMA outputs
Motor cortex, corticospinal tract
Primary motor cortex inputs
Cortico-cortical: sensory cortex behind (3a particularly), supplementary (medial 6) and premotor (lateral 6) cortex
Thalamic: dorsal columns via (VPL), cerebellum (VL)
Non-specific: from intralaminar thalamus & ascending aminergic systems (NA, 5-HT, DA)
Primary motor cortex outputs
Provide 40% of the corticospinal (pyramidal) tract axons, 20% of these monosynaptic to distal muscle motoneurones
Putamen (input to neostriatum of basal ganglia)
Pons (corticopontine tract: input to intermediate cerebellum)
Cranial nerve nuclei (corticobulbar)
Red nucleus (input to rubrospinal tract)
Brainstem reticular formation (descending control of reflex activity)
What is the role of M1 revealed by stimulation?
Not about specific muscle but about coordinated behaviour - initiation of these movements
What do populations of M1 neurons code for?
Direction and force
Which areas would you NOT expect to be active when subjects mentally rehearse the movement without moving the fingers?
Motor cortex (area 4) and somatosensory motor cortex (area 1)
Supplementary motor cortex - area 6 - to be active only
The area where a cortical lesion would paralyse the right arm
Left hemisphere, medial M1
The principal relay site from cerebral cortex to cerebellar cortex
Thalamus
The area where a cortical lesion would paralyse the left leg
Right hemisphere, most medial M1 along the longitudinal fissure
The area responsible for planning spontaneous movement
PMA
